Cell Biology

# Introduction to histology and epithelial tissue This section introduces histology as the microscopic study of tissues and delves into the fundamental characteristics, functions, and specialized junctions of epithelial tissue. ## 1. Introduction to histology and epithelial tissue Histology is the study of tissues at the microscopic level, focusing on their structure and function. Tissues are classified into four basic types: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. ### 1.1 Epithelial tissue Epithelial tissue, also known as covering or lining tissue, forms the outer surfaces of the body (like the skin) and lines internal surfaces and cavities. It also constitutes glands, which are derived from epithelial cells and specialized for secretion. #### 1.1.1 Characteristics of epithelial tissue * **Closely packed cells:** Epithelial cells are densely packed, forming continuous sheets with minimal intercellular space. * **Basal membrane attachment:** Epithelial tissues are attached to underlying connective tissue by a basement membrane, a thin, non-cellular layer. * **Avascularity:** Epithelial tissues lack blood vessels, meaning they are avascular. Nutrients are supplied by diffusion from underlying connective tissues. * **Regeneration:** Epithelial cells have a high capacity for regeneration and are continuously replaced if damaged or lost. * **Surface specializations:** Epithelial surfaces can exhibit specialized structures. * **Microvilli:** These finger-like projections increase surface area for absorption and secretion. They are found in the digestive tract and kidney tubules. * **Cilia:** These hair-like structures beat rhythmically to move substances along the epithelial surface. They are found in the respiratory tract and oviducts. #### 1.1.2 Functions of epithelial tissue * **Physical protection:** Provides a barrier against mechanical injury, dehydration, and pathogens. * **Regulated permeability:** Controls the passage of substances into and out of the body or internal organs. * **Sensory reception:** Contains sensory receptors for touch, temperature, and pain. * **Secretion of glandular products:** Specialized epithelial cells form glands that produce and secrete substances for internal or external use. #### 1.1.3 Glandular epithelium Glands are specialized structures derived from epithelial tissue. They are classified based on their secretion method: * **Endocrine glands:** Secrete hormones directly into the bloodstream or surrounding tissue fluid. They lack ducts. * **Example:** Glands producing hormones like insulin or thyroid hormone. * **Exocrine glands:** Secrete products through ducts onto an epithelial surface or into a body cavity. * **Example:** Salivary glands, sweat glands, mammary glands. > **Tip:** It is crucial to understand the different types of glands and their secretion mechanisms for exam preparation. ##### 1.1.3.1 Modes of secretion for exocrine glands * **Merocrine secretion:** Products are released via exocytosis from vesicles. The cell remains intact. * **Example:** Salivary glands, sweat glands. * **Apocrine secretion:** A portion of the apical cytoplasm, containing the secretory product, is shed from the cell. The cell regenerates. * **Example:** Mammary glands, some sweat glands. * **Holocrine secretion:** The entire cell disintegrates and is released along with its secretory product. New cells are formed to replace those lost. * **Example:** Sebaceous glands of the skin. #### 1.1.4 Epithelial junctions Epithelial cells are connected by various specialized junctions that provide structural integrity and regulate transport: * **Tight junctions:** Formed by the fusion of the outer layers of adjacent plasma membranes. They create a seal that prevents the passage of substances between cells, maintaining control over what enters and leaves the tissue. * **Function:** Prevent leakage and regulate permeability. * **Examples:** Cells lining the intestines and kidney tubules. * **Adherens junctions (also referred to as fusion plates in the text):** Connect the actin microfilaments of one cell to those of another, acting like "staples" to hold cells firmly together. They provide structural support and can limit passage between cells, but are not as impermeable as tight junctions. * **Function:** Mechanical strength and structural support. * **Gap junctions:** Form channels (connexons) that directly connect the cytoplasm of adjacent cells. They allow small molecules and ions to pass rapidly between cells. * **Function:** Rapid communication and transport of ions and small molecules. * **Example:** Cardiac muscle, where they facilitate synchronized contraction. * **Desmosomes:** Provide strong mechanical adhesion between cells, holding them firmly together. They are spot-like junctions that connect intermediate filaments of the cytoskeleton. * **Function:** Enhance mechanical strength and cohesion. * **Hemidesmosomes:** Anchor epithelial cells to the basement membrane and underlying extracellular structures. * **Function:** Attach cells to the extracellular matrix. #### 1.1.5 Classification of epithelial tissue Epithelial tissues are classified based on two main criteria: 1. **Number of cell layers:** * **Simple epithelium:** A single layer of cells. Typically involved in absorption, secretion, filtration, and diffusion. * **Stratified epithelium:** Two or more layers of cells. Primarily involved in protection in high-abrasion areas. 2. **Cell shape:** * **Squamous epithelium:** Flattened, scale-like cells. * **Cuboidal epithelium:** Cube-shaped cells with a height similar to their width. * **Columnar epithelium:** Tall, column-shaped cells with a height greater than their width. ##### 1.1.5.1 Types of epithelial tissue (with functions and locations) * **Simple squamous epithelium:** * **Function:** Diffusion, filtration, secretion of lubricating fluids. * **Location:** Alveoli of lungs, lining of heart, blood vessels, lymphatic vessels, lining of ventral body cavity. * **Simple cuboidal epithelium:** * **Function:** Secretion and absorption. * **Location:** Kidney tubules, ducts of small glands, ovary surface. * **Simple columnar epithelium:** * **Function:** Absorption and secretion of mucus and enzymes. * **Location:** Lining of digestive tract, gall bladder, some glands. * **Pseudostratified columnar epithelium:** * **Description:** Appears stratified but is actually a single layer of cells of varying heights, with nuclei at different levels. Often ciliated. * **Function:** Secretion (especially of mucus) and propulsion of mucus. * **Location:** Trachea and upper respiratory tract. * **Stratified squamous epithelium:** * **Function:** Protection against abrasion. * **Location:** Esophagus, mouth, skin (keratinized). * **Stratified cuboidal epithelium:** * **Function:** Protection and limited secretion/absorption. * **Location:** Ducts of some glands (e.g., sweat glands, mammary glands). * **Stratified columnar epithelium:** * **Function:** Protection and secretion. * **Location:** Rare; found in small amounts in the pharynx, male urethra, and lining of some glandular ducts. * **Transitional epithelium:** * **Description:** Stretchy, stratified epithelium that can change shape. * **Function:** Ability to distend and recoil. * **Location:** Lining of the urinary bladder, ureters, and part of the urethra. > **Tip:** For exams, focus on understanding the specific functions and locations of each epithelial tissue type. ### 1.2 Connective tissue Connective tissue is the most abundant and widely distributed tissue type, serving to support, connect, or separate different types of tissues and organs. It is characterized by having few cells embedded in an abundant extracellular matrix. #### 1.2.1 Components of connective tissue * **Specialized cells:** Various cell types, depending on the specific connective tissue. * **Extracellular protein fibers:** Collagen fibers (strength), elastic fibers (recoil), and reticular fibers (supportive network). * **Ground substance:** An amorphous, gel-like material that fills the space between cells and fibers, composed of proteoglycans and glycoproteins. The fibers and ground substance together form the matrix. #### 1.2.2 Functions of connective tissue * **Support and protection:** Provides structural framework and protects organs. * **Binding and connection:** Connects tissues and organs together (e.g., tendons, ligaments). * **Transport:** Blood, a type of connective tissue, transports gases, nutrients, and waste products. * **Energy storage:** Adipose (fat) tissue stores energy reserves. * **Defense:** Immune cells within connective tissue protect against pathogens. #### 1.2.3 Types of connective tissue Connective tissues are broadly classified into three major types: 1. **Connective tissue proper:** * **Areolar (loose) connective tissue:** Contains all three fiber types in a loose, irregular network. Found beneath epithelia, acting as packing material and providing support. * **Adipose tissue:** Dominated by adipocytes (fat cells). Stores energy, insulates, and cushions organs. * **Dense connective tissue:** Contains a high proportion of collagen fibers. * **Dense regular connective tissue:** Fibers are parallel, providing strong resistance to tension in one direction (e.g., tendons, ligaments). * **Dense irregular connective tissue:** Fibers are interwoven, providing resistance to tension from multiple directions (e.g., dermis of skin). 2. **Fluid connective tissue:** * **Blood:** Composed of specialized cells (red blood cells, white blood cells, platelets) suspended in a fluid matrix (plasma). Functions in transport and defense. * **Lymph:** Similar to blood but with fewer proteins and primarily lymphocytes. Involved in immune function and fluid balance. 3. **Supportive connective tissue:** * **Cartilage:** A tough, flexible tissue with a firm matrix. Avascular. * **Hyaline cartilage:** Most common type, found in joints, ribs, trachea, and nose. Provides smooth surfaces and flexibility. * **Elastic cartilage:** Contains abundant elastic fibers, providing great flexibility and resilience (e.g., external ear). * **Fibrocartilage:** Contains thick bundles of collagen fibers, providing strength and shock absorption (e.g., intervertebral discs, menisci of the knee). * **Bone (osseous tissue):** A rigid tissue with a mineralized matrix composed of collagen fibers and calcium salts. Provides structural support, protects organs, and stores calcium. #### 1.2.4 Cells of connective tissue proper * **Fibroblasts:** The most common cell type, responsible for synthesizing the extracellular matrix and collagen. * **Fibrocytes:** Mature fibroblasts that maintain the matrix. * **Macrophages:** Phagocytic cells that engulf debris, bacteria, and foreign matter. * **Adipocytes:** Fat cells that store lipids. * **Mast cells:** Involved in inflammatory responses and allergic reactions. ### 1.3 Membranes Membranes are physical barriers composed of an epithelial layer supported by a connective tissue layer. #### 1.3.1 Types of membranes * **Mucous membranes:** Line body cavities that open to the exterior (e.g., digestive, respiratory, urinary, reproductive tracts). They secrete mucus to protect and lubricate. * **Serous membranes:** Line internal body cavities (thoracic and abdominal cavities) and cover organs. They secrete serous fluid for lubrication. Examples include pleura, peritoneum, and pericardium. * **Cutaneous membrane (skin):** The outer covering of the body, composed of stratified squamous epithelium and underlying connective tissue. * **Synovial membranes:** Line joint cavities and produce synovial fluid to lubricate joints. ### 1.4 Tissue damage and repair When tissues are injured, they undergo coordinated responses to restore homeostasis. This involves inflammation, which isolates the damaged area and removes debris, and regeneration, where damaged tissue is replaced or repaired, potentially leading to scar tissue formation (fibrosis). The effectiveness of regeneration varies significantly between tissue types. ### 1.5 Effects of aging on tissues With aging, tissues undergo changes, including slower repair, altered chemical composition, and reduced cell renewal. Epithelial and connective tissues tend to become thinner and more vulnerable, increasing the risk of disease. --- # Connective tissue types and components Connective tissues are the most diverse and abundant basic tissues, providing structural support, protection, and transport throughout the body. ### 2.1 Components of connective tissue Connective tissue is characterized by its extracellular matrix, which consists of specialized cells, extracellular protein fibers, and ground substance. #### 2.1.1 Specialized cells Connective tissues contain various cell types, each with specific functions: * **Fibroblasts:** These are the principal cells of connective tissue proper, responsible for synthesizing the ground substance and extracellular fibers. They are permanent residents of connective tissue. * **Fibrocytes:** These are differentiated fibroblasts that maintain the connective tissue fibers. * **Macrophages:** Also known as "big eaters," these are phagocytic cells that engulf damaged cells, pathogens, and foreign materials. They can be either free (wandering) or fixed. * **Adipocytes (Fat cells):** These cells store energy reserves in the form of lipids. They are considered permanent residents. * **Mast cells (Mastocytes):** Small, mobile cells found near blood vessels, involved in defending the body against pathogens. #### 2.1.2 Extracellular protein fibers These fibers provide strength and support to the connective tissue: * **Collagen fibers:** Long, straight, and unbranched fibers that are strong and resistant to stretching. * **Elastic fibers:** Fibers containing the protein elastin, which are branched and wavy. They can stretch and recoil to their original length after deformation. * **Reticular fibers:** Thin, branched fibers that form a delicate, interwoven network (stroma) that supports soft organs and helps bind connective tissue to other tissues. #### 2.1.3 Ground substance This is an amorphous, gel-like material that fills the spaces between cells and surrounds the extracellular fibers. It is composed of proteoglycans and glycoproteins and plays a role in diffusion and transport. #### 2.1.4 Matrix The combination of extracellular fibers and ground substance forms the connective tissue matrix. ### 2.2 Classification of connective tissue Connective tissues are broadly classified into three main categories based on their physical properties and function: connective tissue proper, fluid connective tissues, and supportive connective tissues. #### 2.2.1 Connective tissue proper This category includes tissues with a wide variety of cell types and a matrix of fibers and ground substance. It is further divided into loose and dense connective tissues. ##### 2.2.1.1 Loose connective tissue These tissues have a loose network of fibers, allowing for flexibility and movement. * **Areolar connective tissue:** Found beneath epithelia and surrounding organs, it provides cushioning and elasticity. It contains all three fiber types and numerous cell types. * **Adipose tissue:** Primarily composed of adipocytes, it serves as an energy reserve, insulation, and shock absorber. ##### 2.2.1.2 Dense connective tissue These tissues are characterized by a high density of fibers, primarily collagen, providing great strength and resistance to stretching. * **Dense regular connective tissue:** Fibers are arranged in parallel bundles, providing strong, unidirectional resistance. Examples include tendons (muscle to bone) and ligaments (bone to bone). * **Dense irregular connective tissue:** Fibers are interwoven into a meshwork, providing multidirectional strength. This is found in the dermis of the skin. #### 2.2.2 Fluid connective tissues These tissues consist of specialized cells suspended in a watery ground substance. * **Blood:** Composed of red blood cells, white blood cells, platelets, and plasma. It transports substances throughout the body. * **Lymph:** Contains lymphocytes and other white blood cells, playing a role in the immune system and fluid balance. #### 2.2.3 Supportive connective tissues These tissues provide structural support and protection to the body. * **Cartilage:** A flexible yet strong tissue with a firm matrix. It is avascular and consists of chondrocytes located in lacunae. * **Hyaline cartilage:** Tough and flexible, found in joints, the rib cage, and the respiratory tract. * **Elastic cartilage:** Contains abundant elastic fibers, providing flexibility and resilience, found in the ear and epiglottis. * **Fibrocartilage:** Dense with collagen fibers, making it durable and shock-absorbent, found in intervertebral discs and menisci of the knee. * **Bone (Osseous tissue):** A hard, calcified matrix composed of collagen fibers and mineral salts. It provides support, protection, and is involved in blood cell formation. Osteocytes are embedded in lacunae, and nutrients are supplied via canaliculi. The outer layer is covered by the periosteum. ### 2.3 Epithelial membranes Membranes are physical barriers made up of an epithelial layer supported by a connective tissue layer. They cover and protect body surfaces and cavities. * **Mucous membranes:** Line cavities that open to the exterior (e.g., digestive, respiratory, reproductive tracts). They secrete mucus to protect and lubricate. * **Serous membranes:** Line internal body cavities (thoracic, abdominal, pelvic) and cover organs. They consist of simple squamous epithelium and loose connective tissue, secreting serous fluid for lubrication. Examples include pleura, peritoneum, and pericardium. * **Cutaneous membrane (Skin):** The outer covering of the body, composed of stratified squamous epithelium (epidermis) and areolar and dense irregular connective tissue (dermis). * **Synovial membranes:** Line joint cavities and are composed of loose connective tissue and an incomplete layer of epithelial-like cells, secreting synovial fluid. ### 2.4 Glandular epithelium Glandular epithelium is specialized for secretion. * **Endocrine glands:** Secrete hormones directly into the bloodstream or surrounding tissue fluid; they lack ducts. * **Exocrine glands:** Secrete their products onto body surfaces or into body cavities through ducts. * **Merocrine secretion:** Products are released by exocytosis, leaving the cell intact (e.g., salivary glands, sweat glands). * **Apocrine secretion:** A portion of the apical cytoplasm is shed with the secretory product (e.g., mammary glands). * **Holocrine secretion:** The entire cell disintegrates to release its product (e.g., sebaceous glands). --- # Membranes, muscle tissue, and nervous tissue This section details the various types of membranes, the characteristics of muscle tissues, and the fundamental components of nervous tissue, crucial for understanding the structural and functional organization of the body. ### 3.1 Membranes Membranes are physical barriers composed of epithelial tissue and underlying connective tissue. They serve to cover and protect body surfaces and cavities. #### 3.1.1 Types of membranes * **Mucous membranes:** These line hollow cavities that open to the external environment, such as the mouth, nasal passages, and reproductive tracts. They secrete mucus to trap pathogens and are found in the digestive, respiratory, urinary, and reproductive systems. * **Serous membranes:** These line internal body cavities that are not open to the exterior, including the thoracic (pleura), abdominal (peritoneum), and pericardial cavities. They consist of a layer of simple squamous epithelium supported by areolar connective tissue. Serous membranes secrete serous fluid, which lubricates surfaces and reduces friction between organs. * **Cutaneous membrane (skin):** This is the external covering of the body. It is a dry membrane composed of stratified squamous epithelium (epidermis) and underlying areolar and dense irregular connective tissue (dermis). * **Synovial membranes:** These line the cavities of freely movable joints. They are composed of areolar connective tissue and incomplete layers of squamous or cuboidal epithelial cells. Synovial membranes produce synovial fluid, which lubricates and nourishes cartilage within the joint. ### 3.2 Muscle tissue Muscle tissue is specialized for contraction, enabling movement and generating heat. It consists of cells that contain actin and myosin filaments, which interact to produce contractile force. #### 3.2.1 Skeletal muscle tissue * **Characteristics:** Striated (striped appearance under a microscope), voluntary control, multinucleated cells (fibers). * **Function:** Produces body movement, maintains posture, generates heat. * **Location:** Attached to bones by tendons. #### 3.2.2 Cardiac muscle tissue * **Characteristics:** Striated, involuntary control, typically uninucleated cells, interconnected by intercalated discs with gap junctions, contains pacemaker cells. * **Function:** Pumps blood throughout the body, maintains blood pressure. * **Location:** Walls of the heart. #### 3.2.3 Smooth muscle tissue * **Characteristics:** Non-striated, involuntary control, uninucleated, spindle-shaped cells. * **Function:** Propels substances through internal passageways (e.g., food through the digestive tract, urine through the urinary tract). * **Location:** Walls of hollow organs such as the digestive tract, bladder, uterus, and blood vessels. ### 3.3 Nervous tissue Nervous tissue is responsible for transmitting and processing information throughout the body. It comprises two main cell types: neurons and neuroglia. #### 3.3.1 Neurons * **Characteristics:** Specialized cells that transmit electrical and chemical signals. They are responsible for thought, sensation, movement, and response. Neurons consist of a cell body, dendrites (receive signals), and an axon (transmits signals). #### 3.3.2 Neuroglia (glial cells) * **Characteristics:** Support cells that assist neurons in their functions. They do not transmit signals themselves but provide structural support, insulation, nourishment, waste removal, and protection to neurons. #### 3.3.3 Tissue damage and repair When tissues are damaged, the body initiates a coordinated response involving inflammation and regeneration. * **Inflammation:** An essential process that isolates the damaged area, removes debris, and prepares the tissue for repair. Key signs include swelling, pain, heat, and redness. * **Regeneration:** The process of replacing or repairing damaged tissue. The extent of regeneration can vary, leading to complete, partial, or no recovery of tissue function. Fibroblasts play a crucial role by producing scar tissue (fibrosis) during repair. #### 3.3.4 Effects of aging on tissues As individuals age, tissues undergo changes that affect their repair and maintenance capabilities. These changes include slower and less effective tissue repair, alterations in chemical composition, and a decrease in cell renewal. Epithelial and connective tissues become thinner and more fragile, potentially increasing susceptibility to diseases like cancer. --- # Tissue damage, repair, and aging effects This section details the body's responses to tissue injury, the mechanisms of repair, and the impact of aging on tissue integrity and function. ### 4.1 Tissue damage and repair When tissues are injured, they exhibit coordinated reactions to restore homeostasis. These responses primarily involve inflammation and regeneration. #### 4.1.1 Inflammation Inflammation is a localized protective response that isolates the injured area and removes damaged tissue. The four cardinal signs of inflammation are swelling, pain, heat, and redness. #### 4.1.2 Regeneration Regeneration is the process of repairing or replacing damaged tissue, aiming to restore normal function. The extent of regeneration can vary: * **Complete regeneration:** The damaged tissue is fully restored to its original state and function. * **Partial regeneration:** Some tissue is repaired, but the function may be compromised. * **No regeneration:** The damaged tissue is not replaced, leading to permanent loss of function. ##### 4.1.2.1 Scar tissue formation In cases where complete regeneration is not possible, fibroblasts are activated to produce scar tissue, a process known as tissue fibrosis. This scar tissue, while providing structural support, often impairs the original tissue's function. ### 4.2 Effects of aging on tissues Aging affects all tissues in the body, leading to slower and less effective repair and maintenance processes. Key changes associated with aging include: * **Altered chemical composition:** The chemical makeup of many tissues changes over time. * **Decreased cell renewal:** The rate at which cells are replaced diminishes. * **Thinning and increased vulnerability:** Epithelial and connective tissues become thinner and more susceptible to damage. This structural alteration can increase the risk of diseases such as cancer. > **Tip:** Understanding the cellular mechanisms of repair and the cumulative effects of aging is crucial for comprehending various age-related diseases and optimizing treatment strategies. --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Histology | The microscopic study of the structure and function of tissues. | | Epithelial tissue | A type of tissue that covers body surfaces, lines body cavities, and forms glands. It is characterized by tightly packed cells. | | Basement membrane | A thin, fibrous layer that connects epithelial tissue to underlying connective tissue. | | Avascular | Lacking blood vessels; epithelial tissue is typically avascular. | | Tight junction | A junction between cells that prevents leakage by fusing the outer surfaces of adjacent plasma membranes, controlling passage of substances. | | Adherens junction | A junction that connects the actin cytoskeletons of cells, providing mechanical strength and structure. | | Gap junction | Channels that connect adjacent cells, allowing small molecules and ions to pass directly from one cell to another, facilitating rapid communication. | | Desmosome | A junction that provides mechanical strength and adhesion between cells, anchoring them firmly together. | | Hemidesmosome | A junction that anchors a cell to extracellular structures, such as the basal membrane. | | Microvilli | Finger-like projections on the surface of some epithelial cells that increase the surface area for absorption. | | Cilia | Hair-like projections on the surface of some epithelial cells that are involved in transport, movement, and coordination. | | Endocrine secretion | The release of substances (like hormones) directly into the bloodstream or internal environment. | | Exocrine secretion | The release of substances through a duct to an external surface or into a body cavity. | | Connective tissue | A type of tissue that supports, connects, or separates different types of tissues and organs in the body. It consists of specialized cells, fibers, and ground substance. | | Extracellular matrix | The non-cellular component of connective tissue, consisting of protein fibers and ground substance. | | Ground substance | The gel-like material that fills the space between cells and fibers in connective tissue. | | Fibroblast | A cell responsible for the production of ground substance and connective tissue fibers. | | Fibrocyte | A mature fibroblast that maintains the connective tissue fibers. | | Macrophage | A large phagocytic cell that engulfs damaged cells and pathogens. | | Mast cell | A mobile connective tissue cell involved in defense against invaders, releasing substances like histamine. | | Collagenous fibers | Strong, unbranched fibers that provide tensile strength to connective tissue. | | Elastic fibers | Fibers containing elastin that allow tissues to stretch and recoil to their original shape. | | Reticular fibers | Thin, branched fibers that form a supportive framework in various organs. | | Hyaline cartilage | A type of cartilage with a smooth matrix, found in joints, the rib cage, and the respiratory tract, providing support and flexibility. | | Elastic cartilage | A type of cartilage containing abundant elastic fibers, providing flexibility and resilience, found in the ear and epiglottis. | | Fibrocartilage | A type of cartilage with a dense matrix of collagenous fibers, providing strength and shock absorption, found in intervertebral discs and menisci. | | Osteocyte | A mature bone cell responsible for maintaining bone tissue, located in lacunae. | | Periosteum | A thin membrane covering the outer surface of bones, containing blood vessels and nerves. | | Membrane | A physical barrier at the tissue level, typically composed of epithelial and connective tissue. | | Mucous membrane | A membrane lining cavities that open to the exterior, secreting mucus for protection and lubrication. | | Serous membrane | A membrane lining internal body cavities, secreting serous fluid for lubrication. | | Cutaneous membrane | The skin, a multilayered epithelial and connective tissue covering of the body. | | Synovial membrane | A membrane lining joint capsules, producing synovial fluid for lubrication. | | Skeletal muscle tissue | Striated muscle tissue that is under voluntary control, responsible for body movement. | | Cardiac muscle tissue | Striated muscle tissue found in the heart, responsible for pumping blood; it is involuntary. | | Smooth muscle tissue | Non-striated muscle tissue found in the walls of internal organs, responsible for involuntary movements like digestion. | | Neuron | A specialized cell of the nervous system that transmits electrical and chemical signals. | | Neuroglia | Supporting cells of the nervous system that assist neurons in their functions. | | Inflammation | A localized response to injury or infection, characterized by swelling, pain, heat, and redness. | | Regeneration | The process of restoring damaged tissue to its original state. | | Fibrosis | The formation of scar tissue, often occurring when regeneration is incomplete. |

# The human microbiome and holobionts The human microbiome and its symbiotic relationship with the host represent a co-evolved entity known as a holobiont, fundamentally reshaping our understanding of evolution and health [5](#page=5). ### 1.1 Defining the human microbiome The human microbiome refers to the vast and diverse population of microorganisms, predominantly prokaryotes, residing within the human body. Even seemingly healthy individuals host an immense variety of bacteria, with prokaryotic cells constituting approximately 90% of human cells [3](#page=3). ### 1.2 The dynamic nature of the microbiome Crucially, microbiomes are not static entities. Their composition changes over time in response to environmental factors, including alterations in diet, the use of medications, and atmospheric conditions. This dynamic interplay underscores the intricate relationship between the host and its microbial inhabitants [4](#page=4). ### 1.3 Symbiotic relationship and the concept of holobionts The relationship between humans and their resident bacteria is considered symbiotic rather than infectious, as these microbes are essential for maintaining human health. This co-evolution has led to the formation of a eukaryotic-prokaryotic consortium termed the holobiont [4](#page=4) [5](#page=5). > **Tip:** Understanding the microbiome as an integral part of our being, rather than just a collection of external organisms, is key to grasping the concept of holobionts. ### 1.4 The holobiont as a "superorganism" and the hologenome The recognition of holobionts has revolutionized scientific perspectives on evolution. The human host and its microbiome can now be viewed as a "superorganism" that possesses the capacity to evolve as a single genetic unit, collectively referred to as the hologenome. This hologenome encompasses the genetic material from both the host and its associated microbes [6](#page=6). ### 1.5 Genetic contributions of the microbiome The human microbiome significantly contributes to the genetic diversity of this superorganism. While human cells possess approximately 22,000 different genes, with remarkable genetic similarity among individuals (99.9%), current estimates suggest that the microbiome harbors around 10 million genes. Humans effectively contain genetic material detectable in bacteria and even viruses, highlighting the profound genetic contribution of the microbiome [6](#page=6) [7](#page=7). > **Example:** Imagine the human genome as a single book, and the microbiome's genome as a vast library containing millions of unique volumes. The holobiont's capabilities are a combination of both. ### 1.6 The microbiome's role in health and disease Changes in the microbiome are increasingly implicated in human health and disease. A growing body of evidence suggests that a variety of illnesses, including inflammatory and autoimmune disorders, may be linked to the loss of specific components within our microbiome [8](#page=8). ### 1.7 Microbiome influence during pregnancy and early life The influence of the microbiome is evident from early stages of life. For instance, during pregnancy, bacteria like *Lactobacillus* colonize the vaginal tract, creating a highly acidic environment that prevents the entry of potentially pathogenic microorganisms [9](#page=9). During childbirth, newborns are exposed to microbes that help establish their initial microbial colonies. This initial colonization can be altered in infants born via Cesarean section, who may miss out on this early exposure through the vaginal canal. Furthermore, breastfeeding mothers can provide nutrients in their milk that support the growth of specific beneficial bacteria in the infant's gut, thereby preventing the colonization by harmful pathogens [10](#page=10) [11](#page=11). ### 1.8 Therapeutic applications: Fecal Microbiota Transplant (FMT) The understanding of the microbiome's impact has led to novel therapeutic interventions. Physicians are now leveraging the concept of "transplanting" microbes from one individual to another to treat diseases [12](#page=12). #### 1.8.1 Clostridium difficile infection (CDI) A prime example of this therapeutic approach is the treatment of *Clostridium difficile* infection (CDI). *Clostridium difficile* is a common bacterium in the human digestive tract, usually harmless. However, under certain conditions, it can cause CDI, characterized by symptoms such as frequent watery diarrhea, severe abdominal pain, nausea, loss of appetite, fever, and potentially blood or pus in stool, which can be life-threatening [13](#page=13). #### 1.8.2 FMT as a treatment for CDI While oral antibiotics are the standard treatment for CDI, they can sometimes be ineffective. In such cases, a more complex treatment called Fecal Microbiota Transplant (FMT) may be employed. FMT involves transferring a fecal sample from a healthy donor to a patient with CDI, typically via colonoscopy or an orally administered capsule. This procedure has proven to be highly effective, with over 95% success rates in treating CDI [14](#page=14) [15](#page=15). > **Tip:** Researching FMT, CDI, and the role of fecal microbial flora provides valuable insights into how manipulating the microbiome can restore health [19](#page=19). ### 1.9 Reasons for CDI susceptibility Despite *Clostridium difficile* being common in the human digestive tract, only a small number of individuals develop CDI. This is primarily because a healthy, diverse gut microbiome typically keeps *C. difficile* populations in check, preventing them from proliferating and causing disease. The most common causes of CDI are disruptions to this balance, often triggered by the use of broad-spectrum antibiotics, which can kill off beneficial bacteria, allowing *C. difficile* to overgrow. Other factors, such as advanced age, compromised immune systems, and prolonged hospitalization, can also increase susceptibility [19](#page=19). --- # Prokaryotic and eukaryotic cell structures Prokaryotic and eukaryotic cells represent fundamental distinctions in cellular organization, with prokaryotes being simpler, unicellular organisms and eukaryotes exhibiting greater complexity through compartmentalization by organelles. ### 2.1 Prokaryotic cells Prokaryotes are characterized by their simplicity and adaptability, capable of thriving in extreme environments due to their evolutionary history as descendants of early Earth cells [25](#page=25). #### 2.1.1 Basic structure * Prokaryotic cells possess only a single membrane, the cytoplasmic membrane. All intracellular chemical reactions occur within this single compartment, the cytosol, limiting their specialization [20](#page=20). * While generally lacking complex internal membranes, some prokaryotes have evolved elaborate modifications of their cytoplasmic membrane, forming stacks of membranous folds that provide a degree of compartmentalization [22](#page=22). * The prokaryotic cytosol, despite the cell's simplicity, appears heterogeneous under electron microscopy, suggesting some level of internal organization [22](#page=22). #### 2.1.2 Unicellular nature and division * All prokaryotic organisms are unicellular. They do not form multicellular organisms, although some may aggregate into large symbiotic structures called biofilms [23](#page=23). * Prokaryotes do not undergo mitotic division; instead, they divide by binary fission [24](#page=24). * The majority of genetic information in prokaryotic cells is contained in a single circular DNA molecule, in contrast to the numerous linear chromosomes found in eukaryotic DNA [24](#page=24). #### 2.1.3 Environmental resilience and abundance * Despite their relatively simple structure, prokaryotes inhabit some of Earth's most hostile environments, including extreme temperatures, high atmospheric pressure, oxygen-deficient areas, and a wide pH range (2 to 12) [25](#page=25). * This resilience stems from their origin as early Earth cells adapted to vastly different conditions [26](#page=26). * Due to their adaptability, prokaryotes are the most abundant organisms on Earth [26](#page=26). #### 2.1.4 Protective cell wall * Most prokaryotes are protected by an additional layer outside the cell membrane, known as a cell wall, typically composed of peptidoglycan [27](#page=27). * The cell wall is primarily made of sugar molecules linked together in a dense mesh [27](#page=27). * Beyond protection from physical damage, the cell wall helps retain water, ensuring proper cell hydration [27](#page=27). ### 2.2 Eukaryotic cells Eukaryotic cells are significantly more complex, characterized by their organized cytoplasm and the presence of membrane-bound subcellular organelles [29](#page=29). #### 2.2.1 Compartmentalization and organelles * The most striking feature of eukaryotic cells under microscopic observation is their highly organized cytoplasm [29](#page=29). * The presence of membranes allows eukaryotic cells to create specialized compartments within the cytoplasm dedicated to specific cellular tasks under optimal conditions [30](#page=30). * These internal membranes, like the plasma membrane, are selectively permeable, contributing to a unique internal environment suitable for the contained molecules [30](#page=30). * A **subcellular organelle** is defined as a microscopic cellular structure that performs specific functions within a cell [31](#page=31). * Organelles are often enclosed by their own membranes, dividing the cell into multiple compartments for different biochemical reactions [31](#page=31). * Subcellular organelles are responsible for a wide range of duties, from energy production to controlling cell growth and reproduction [31](#page=31). #### 2.2.2 Key subcellular organelles ##### 2.2.2.1 Nucleus * The nucleus is the cellular organelle that houses the chromosomes [32](#page=32) [33](#page=33). * It features a nuclear membrane with pores that selectively allow certain molecules, such as proteins and nucleic acids, to pass in and out [33](#page=33). * The nucleolus, located within the nucleus, is the site of ribosome production [36](#page=36). * **Example:** In humans and all mammals, mature red blood cells lack a nucleus, allowing for more space to store oxygen-binding hemoglobin, thus increasing oxygen transport capacity. These cells also have a biconcave shape to maximize surface area for oxygen diffusion. In non-mammalian vertebrates, mature red blood cells do possess a nucleus [37](#page=37). ##### 2.2.2.2 Mitochondria * Mitochondria are known as the "powerhouses" of the cell, converting energy from food into adenosine triphosphate (ATP), the body's primary energy currency [39](#page=39) [40](#page=40). * They are composed of distinct compartments: an outer membrane, an inner membrane that folds into cristae, and an intermembrane space between them [42](#page=42). * The outer membrane contains porins, protein complexes that allow free diffusion of small to medium-sized molecules [43](#page=43). * The intermembrane space generally resembles the cytosol, except for larger molecules like proteins that cannot pass through the outer membrane [43](#page=43). * Mitochondrial DNA is found within the matrix, the innermost compartment [43](#page=43). * Mitochondria are considered semi-autonomous organelles [43](#page=43). * **Origin hypotheses:** * **Endosymbiotic hypothesis:** Suggests mitochondria were once prokaryotic cells engulfed by an early eukaryotic cell approximately 2 billion years ago [44](#page=44) [46](#page=46). * **Autogenous hypothesis:** Proposes that DNA from a progenitor eukaryote fragmented and exited the nucleus [44](#page=44). * **Evidence supporting the endosymbiotic hypothesis:** 1. Mitochondria are similar in size to average bacteria [47](#page=47). 2. Mitochondria possess their own DNA, can synthesize some RNA, ribosomes, and proteins. Mitochondrial DNA sequences show closer kinship to bacteria than to eukaryotes. This DNA is organized into multiple copies of identical, circular chromosomes, unlike the linear chromosomes of eukaryotes [47](#page=47) [48](#page=48). 3. Mitochondria have a double membrane; the inner membrane is believed to be from the original prokaryote, and the outer membrane is thought to be from the host eukaryote's engulfing membrane [49](#page=49). 4. The lipid composition of the inner mitochondrial membrane resembles that of bacteria, while the outer membrane's lipid composition is more similar to eukaryotes [49](#page=49). * Mitochondria vary in size (0.5 to 3 micrometers), shape, and number depending on the cell type's energy requirements. Cells with high energy demands, like muscle, liver, and brain cells, contain more mitochondria. Hepatocytes can have 1,000-2,000 mitochondria, while heart, sperm, and muscle cells may have around 5,000 per cell [50](#page=50). * Mitochondria are sensitive indicators of cell health, and metabolic disturbances can lead to morphological changes, including an increase in size to become megamitochondria [51](#page=51). ##### 2.2.2.3 Ribosomes * Ribosomes are described as molecular protein production factories [53](#page=53). * They are composed of protein and ribonucleic acid (RNA) and are the site of translation, the process of protein synthesis [54](#page=54). * All eukaryotic ribosomes are 80S ribosomes, consisting of a small 40S subunit and a large 60S subunit. The "S" refers to Svedberg units, a measure of a particle's sedimentation coefficient [54](#page=54). * During protein synthesis, a ribosome attaches to an mRNA template and reads its code. Transfer RNA (tRNA) molecules carrying amino acids bind to the ribosome, leaving the amino acid behind as the ribosome moves along the mRNA, thus building the protein chain [55](#page=55). ##### 2.2.2.4 Endoplasmic reticulum (ER) * The ER is a network of interconnected sacs involved in processing newly synthesized secreted and membrane proteins (rough ER) and in lipid production and toxin detoxification (smooth ER) [59](#page=59). * Rough ER (RER) appears as flat membrane sacs called cisternae with ribosomes attached to the cytoplasmic face; its quantity reflects the cell's protein synthesis level [57](#page=57). * Smooth ER (SER) is involved in lipid synthesis and detoxification [58](#page=58). ##### 2.2.2.5 Vesicles * Vesicles are organelles containing fluid enclosed by a phospholipid bilayer [60](#page=60). * They perform various functions, including transporting substances into and out of the cell, and serving as storage units [60](#page=60). * During endocytosis, the cell membrane actively takes in larger molecules, forming pockets of the phospholipid bilayer that pinch off to become vesicles [62](#page=62). ##### 2.2.2.6 Vacuoles * Vacuoles are specialized vesicles primarily containing water [63](#page=63). * In most plant cells, vacuoles maintain osmotic balance and store nutrients [63](#page=63). * They can also store pigments in colorful plant organs like petals and store proteins and lipids in plant seeds, serving as an energy source for developing seeds [63](#page=63). ##### 2.2.2.7 Golgi apparatus * The Golgi apparatus receives proteins and lipids from the ER, packages them, and sends them to their destinations within or outside the cell [65](#page=65). * It is a collection of flattened membranes called cisternae where synthesized proteins are further processed and modified [67](#page=67). * Proteins are tagged with recognition labels to ensure proper delivery [68](#page=68). * The Golgi apparatus can be visualized as a cellular post office, sorting, packaging, and dispatching proteins to their correct locations. It was first identified by Camillo Golgi in 1898 [69](#page=69) [70](#page=70). ##### 2.2.2.8 Lysosomes * Lysosomes are specialized vesicles involved in cellular digestion [73](#page=73) [74](#page=74). * They contain acid hydrolase enzymes to break down waste materials, cellular debris, and ingested food particles [72](#page=72) [74](#page=74). * During phagocytosis, vesicles containing food merge with lysosomes, releasing digestive enzymes that break down biomolecules into their fundamental monomers [74](#page=74). * These monomers can be recycled by the cell to synthesize new biomolecules and organelles, a process known as autophagy [75](#page=75). * Lysosomes are also believed to be involved in programmed cell death (apoptosis) in multicellular organisms [76](#page=76). * **Lysosomal storage diseases (LSDs)** are inherited metabolic disorders characterized by the accumulation of substrates within cells due to faulty lysosomal function, leading to organ dysfunction. Most LSDs are caused by mutations in genes encoding lysosomal enzymes, specifically hydrolases, which break down chemical bonds using water [77](#page=77) [78](#page=78). ##### 2.2.2.9 Peroxisomes * Peroxisomes are organelles that use molecular oxygen and hydrogen peroxide to perform oxidative reactions [81](#page=81). * They contain oxidative enzymes, such as catalase, which breaks down hydrogen peroxide into water and oxygen [81](#page=81). ##### 2.2.2.10 Cytoskeleton * The cytoskeleton is a structure that helps cells maintain their shape and internal organization, providing mechanical support for essential functions like division and movement [83](#page=83). * It is an interconnected network of protein filaments present in all cells, including prokaryotes and eukaryotes [85](#page=85). * Its primary function is to resist compression, allowing the cell to maintain its overall shape. In multicellular organisms, it stabilizes tissues and provides structural integrity [85](#page=85). * The cytoskeleton is also involved in cell movement. Its protein filaments can contract and relax, changing cell shape to facilitate locomotion. In muscle cells, actin filaments contract to shorten the cell [86](#page=86). * In some cells, the cytoskeleton extends beyond the cell membrane to form structures like cilia and flagella [86](#page=86). * **Cilia** are hair-like projections extending from the cell surface, acting as cellular antennae to receive signals from the extracellular environment. Motile cilia are found in various cell types and can facilitate movement, such as the cilia in human fallopian tubes moving an egg towards the uterus [87](#page=87) [91](#page=91). * **Flagella** are typically longer, thicker, and fewer than cilia. These whip-like extensions create efficient locomotion [88](#page=88). * In eukaryotes, the cytoskeleton also facilitates the movement of vesicles within the cell, guiding them along protein filaments to their destinations. It plays a role in endocytosis and organelle transport as well [90](#page=90). * The cytoskeleton is composed of three main types of protein filaments: * Microtubules * Microfilaments (actin filaments) * Intermediate filaments [84](#page=84). --- # Pathogen-host interactions and cellular mechanisms This topic explores the intricate ways pathogens interact with host cells, focusing on how intracellular pathogens exploit cellular machinery for their survival and propagation [93](#page=93) [94](#page=94) [95](#page=95) [96](#page=96) [97](#page=97). ### 3.1 Exploiting the host cytoskeleton Certain intracellular bacteria leverage the host cell's actin cytoskeleton to create their own transport systems. A prime example is *Listeria monocytogenes*, which utilizes a protein called ActA. ActA hijacks the host's actin polymerization machinery to build an actin tail behind the bacterium. This actin tail propels the bacterium and allows it to push against the host cell membrane, forming protrusions that can invade neighboring cells [93](#page=93). ### 3.2 Interacting with host membrane compartments Some intracellular bacteria, such as *Legionella pneumophila*, establish residence within host cell membrane-bound compartments. Upon entry, these bacteria engage with host membranes and secrete effector proteins that facilitate their control. *Legionella*, specifically, interacts with the Golgi apparatus and endoplasmic reticulum. It 'steals' proteins from these organelles and redirects vesicular traffic for its own purposes [94](#page=94). ### 3.3 Lysosome interplay with intracellular pathogens Lysosomal homeostasis within eukaryotic cells is frequently disrupted by various pathogens. Infections with *Mycobacterium tuberculosis* (Mtb) and *Staphylococcus aureus* lead to an increase in host cellular lysosomes compared to uninfected conditions. In contrast, *Salmonella* infections induce the formation of filamentous structures derived from late endosomes/lysosomes. In *Trypanosoma* infections, lysosomes migrate towards the cell periphery at the site of pathogen entry and fuse with the plasma membrane, thereby facilitating pathogen entry into the host cell. Under these infectious conditions (Mtb, Salmonella, Staphylococcus, and Trypanosoma), the transcription factor EB (TFEB) is translocated to the nucleus, a phenomenon not observed in uninfected states [95](#page=95) [96](#page=96). > **Tip:** Understanding how pathogens manipulate host organelles like lysosomes is crucial for developing effective therapeutic strategies that can restore cellular balance or disrupt pathogen survival mechanisms. ### 3.4 Bacterial reprogramming of host cells Bacteria can fundamentally alter host cell behavior. *Mycobacterium leprae*, the causative agent of leprosy, induces extreme cellular reprogramming. It can revert host Schwann cells to a state resembling stem cells. These reprogrammed cells can then differentiate into other cell types, such as muscle cells, potentially aiding the bacterium's spread to different tissues. Furthermore, these reprogrammed cells can transmit the infection to macrophages [97](#page=97). --- # The cell theory and cell types The cell theory provides fundamental principles for understanding life at its most basic structural and functional level, while the diversity of specialized cell types in multicellular organisms highlights the complexity and sophistication of biological organization [100](#page=100) [99](#page=99). ### 4.1 The cell theory Scientific theories are well-substantiated explanations of the natural world, acquired through the scientific method and extensively tested and confirmed. The cell theory is one of the two major scientific theories in biology. It is governed by three core principles [100](#page=100) [99](#page=99): 1. The cell is the fundamental unit of life [100](#page=100). 2. All living organisms are composed of cells [100](#page=100). 3. All cells arise from pre-existing cells [100](#page=100). > **Tip:** Understanding these three tenets is crucial for grasping the foundational concepts of biology, as they underpin all discussions of life at the cellular level. ### 4.2 Diversity of cell types in multicellular organisms Multicellular organisms exhibit a remarkable diversity of specialized cell types, each adapted for specific functions. This specialization allows for the complex organization and efficient operation of tissues, organs, and organ systems . #### 4.2.1 Embryonic stem cells Embryonic stem cells (ESCs) are pluripotent cells, meaning they possess the ability to differentiate into any cell type. The specific cell type an ESC matures into is determined by the biochemical signals it receives. This characteristic makes ESCs a promising source for regenerative medicine, potentially used to repair damaged tissues in conditions such as Parkinson's disease and insulin-dependent diabetes . > **Example:** A biochemical signal indicating a need for muscle repair might trigger an ESC to differentiate into a cardiac muscle cell. #### 4.2.2 Cardiac muscle cells Cardiac muscle is one of the three types of muscle tissue found in the human body, alongside skeletal and smooth muscle. It is exclusively located in the heart. Cardiac muscle cells are responsible for the coordinated contractions that enable the heart to pump blood throughout the circulatory system . #### 4.2.3 Bone tissue Bone tissue, particularly cancellous (spongy) bone, is found in the interior of bones. It provides structural support within the skeletal system . #### 4.2.4 Sperm cells (spermatozoa) Sperm cells, also known as spermatozoa, are the male reproductive cells. These small cells are produced in the testes and are essential for the fertilization of the female egg (ovum). Each sperm cell has a head containing the male hereditary material (DNA). During ejaculation, approximately 300 million sperm are released, whereas a female typically produces one large egg at a time . #### 4.2.5 Ova (egg cells) The ovum, or egg cell, is the female reproductive gamete. Mature ova are released from the Graafian follicle during ovulation . #### 4.2.6 Nerve cells (neurons) Nerve cells, or neurons, are specialized cells that form the basis of the nervous system. They are responsible for transmitting electrical and chemical signals throughout the body, enabling communication, sensation, thought, and movement . #### 4.2.7 Fat cells (adipocytes) Fat cells, or adipocytes, are among the largest cells in the human body, with diameters typically ranging from 100 to 120 microns. These cells are surrounded by fine strands of supportive connective tissue and are responsible for storing energy in the form of fat . > **Tip:** The specialization observed in these diverse cell types is a direct consequence of gene expression, where specific genes are activated or silenced to produce proteins that dictate a cell's structure and function. This process is fundamental to differentiation and the development of complex organisms. --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Microbiome | The collection of microorganisms, including bacteria, archaea, viruses, and fungi, that live in and on a particular environment, such as the human body. | | Holobiont | A biological unit consisting of a host and its symbiotic microorganisms, considered as a single ecological and evolutionary entity. | | Superorganism | An entity composed of multiple organisms that functions as a single unit, where the collective behavior or characteristics are distinct from those of the individual components. | | Ologenome | The complete set of genes contributed by a host and its associated microbiota, forming a collective genetic repertoire for the holobiont. | | Prokaryote | A single-celled organism that lacks a membrane-bound nucleus and other membrane-bound organelles. Their genetic material is typically found in a circular DNA molecule in the cytoplasm. | | Eukaryote | An organism whose cells contain a nucleus and other membrane-bound organelles. These cells are typically larger and more complex than prokaryotic cells. | | Cytoplasmic membrane | The selective barrier that encloses the cytoplasm of a cell, regulating the passage of substances into and out of the cell. | | Cytoplasm | The material or protoplasm within a living cell, excluding the nucleus. It comprises the cytosol and the organelles suspended within it. | | Mitosis | A type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth. | | Binary fission | A form of asexual reproduction and cell division used by prokaryotic organisms and some eukaryotic organelles. The cell divides into two equal or near-equal parts. | | Peptidoglycan cell wall | A rigid layer found outside the plasma membrane of most bacteria, composed of a polymer of N-acetylglucosamine and N-acetylmuramic acid, which provides structural support and protection. | | Subcellular organelle | A specialized subunit within a cell that has a specific function, often enclosed by its own membrane. Examples include the nucleus, mitochondria, and ribosomes. | | Nucleus | A membrane-bound organelle found in eukaryotic cells that contains the cell's genetic material in the form of chromosomes. | | Mitochondrion | An organelle found in large numbers in most cells, in which the biochemical processes of respiration and energy production occur. It has a double membrane, its own DNA, and ribosomes. | | Endosymbiotic theory | A theory proposing that certain organelles of eukaryotic cells, such as mitochondria and chloroplasts, originated as free-living prokaryotes that were engulfed by an ancestral eukaryotic cell and established a symbiotic relationship. | | Ribosome | A cellular particle made of ribosomal RNA and protein that serves as the site of protein synthesis in the cell. It translates messenger RNA into protein. | | Endoplasmic reticulum (ER) | A network of membranes found throughout the cytoplasm of eukaryotic cells, involved in protein and lipid synthesis. Rough ER has ribosomes attached and is involved in protein synthesis, while smooth ER is involved in lipid synthesis and detoxification. | | Vesicle | A small sac or bladder-like structure within a cell, enclosed by a membrane, that can transport or store materials. | | Vacuole | A membrane-bound organelle found in eukaryotic cells, typically containing fluid. In plant cells, they are large and involved in maintaining turgor pressure and storage. | | Golgi apparatus | An organelle in eukaryotic cells that receives proteins and lipids from the endoplasmic reticulum, modifies, sorts, and packages them for secretion or delivery to other organelles. | | Lysosome | A membrane-bound organelle in eukaryotic cells containing digestive enzymes that break down waste materials and cellular debris. | | Peroxisome | A small organelle present in the cytoplasm of eukaryotic cells, containing a variety of enzymes, including those involved in the breakdown of fatty acids and detoxification. | | Cytoskeleton | A network of protein filaments and tubules in the cytoplasm of many living cells, giving them shape and internal organization, and enabling movement. | | Cytoplasmic inclusions | Granules or droplets found within the cytoplasm of cells that store various substances, such as glycogen, lipids, or pigments. | | Endocytosis | The process by which cells absorb molecules from outside the cell by engulfing them with their cell membrane. | | Exocytosis | The process by which cells transport molecules (e.g., neurotransmitters or hormones) out of the cell (exo- + cytosis). | | Cell theory | A fundamental biological theory stating that all living organisms are composed of cells, that cells are the basic unit of life, and that all cells arise from pre-existing cells. | | Pluripotent | A stem cell that has the potential to differentiate into any type of cell in the body. | | Cytosol | The aqueous component of the cytoplasm of a cell, within which various organelles and particles are suspended. | | Homeostasis | The maintenance of a stable internal environment in an organism, despite changes in external conditions. | | Transcription factor EB (TFEB) | A master regulator of lysosomal biogenesis and function, and autophagy. It translocates to the nucleus to activate genes involved in these processes. | | Schwann cell | A type of glial cell that forms the myelin sheath around nerve axons in the peripheral nervous system. | | Macrophage | A large phagocytic cell found in stationary form in the tissues or as a mobile white blood cell, especially at sites of infection. |