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# 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. |