Tissues and Their Classifications

Tissues and Their Classification

• Tissues consist of cells with a shared embryonic origin that work together to perform specific tasks
• Histology is the study of tissues
• The four major tissue types are epithelial, connective, muscular, and nervous tissue
• Epithelial tissue interacts with the environment, lines surfaces and forms glands
• Connective tissue supports the body, stores energy, and provides immunity
• Muscular tissue contracts, generates force, and produces heat
• Nervous tissue detects changes and responds with nerve impulses

Cell Junctions

• Cell junctions are contact points between the plasma membranes of tissue cells
• Tight junctions seal adjacent cells together, preventing leakage and found in epithelial tissue lining organs like the stomach and bladder
• Adherens junctions use cadherins to join cells, forming adhesion belts in epithelial cells to resist separation during contractile activities
• Desmosomes use cadherins and intermediate filaments to link cells, providing stability in tissues like the epidermis and cardiac muscle
• Hemidesmosomes anchor cells to the basement membrane using integrins and laminin
• Gap junctions use connexons to create fluid-filled tunnels between cells, allowing the passage of ions and small molecules for cell communication

Epithelial Tissue Features

• Epithelial tissue consists of cells in continuous sheets with little intercellular space, held together by cell junctions
• The Apical (free) surface faces a body surface while the Lateral surfaces face adjacent cells
• The Basal surface is opposite the apical surface, it adheres to the basement membrane
• The Basement membrane is a thin extracellular layer between epithelium and connective tissue, is made of the basal lamina and reticular lamina
• The Basal lamina contains laminin, collagen, glycoproteins, and proteoglycans, attaching to integrins in hemidesmosomes
• The Reticular lamina contains collagen produced by fibroblasts
• The Basement membrane anchors epithelium, guides cell migration, restricts molecule passage, and is involved in kidney filtration
• Epithelial tissue lacks blood vessels but has nerve supply
• A high cell division rate allows for renewal and repair
• Functions include protection, filtration, secretion, absorption, and excretion
• Two types exist: covering/lining epithelium and glandular epithelium
• Covering/lining epithelium forms outer coverings and inner linings
• Glandular epithelium forms secreting portions of glands

Epithelial Tissue Classification

• Arrangement of cells in layers can be simple, pseudostratified, or stratified
• Simple epithelium is a single layer for diffusion, osmosis, filtration, secretion, and absorption
• Pseudostratified epithelium appears multilayered but is a single layer, has varying nuclei levels, and may contain cilia or goblet cells
• Stratified epithelium consists of two or more layers; for protection
• Cell shapes can be squamous, cuboidal, columnar, or transitional
• Squamous cells are thin, flat, for rapid passage
• Cuboidal cells are cube-shaped and function in secretion or absorption
• Columnar cells are taller than wide, protect underlying tissues, and specialize in secretion/absorption
• Transitional cells change shape as organs stretch

Covering and Lining Epithelium Types

• Simple squamous epithelium is a single layer of flat cells; it lines the cardiovascular and lymphatic systems (endothelium), forms serous membranes (mesothelium), and found in air sacs of lungs, glomerular capsule of kidneys, inner eardrum
• Simple cuboidal epithelium, with cube-shaped cells, covers the ovary surface, lines lens capsules and kidney tubules, and forms smaller ducts of glands; its function is secretion and absorption
• Nonciliated simple columnar epithelium lines the GI tract and gallbladder, containing absorptive cells and goblet cells; it functions in secretion and absorption and secreting mucus that lubricates linings
• Ciliated simple columnar epithelium lines some bronchioles, uterine tubes, uterus, sinuses, and brain ventricles, cilia moves mucus
• Pseudostratified columnar epithelium, both ciliated and nonciliated, has cells at varying levels; The ciliated type lines airways secreting mucus and sweeping it away, while the nonciliated type lines epididymis and male urethra for absorption and secretion
• Stratified squamous epithelium is either keratinized (skin surface) or nonkeratinized (wet surfaces); it protects against abrasion, water loss, UV radiation, and foreign invasion
• Stratified cuboidal epithelium, with cube-shaped apical cells, is found in adult sweat glands and male urethra for protection
• Stratified columnar epithelium which is uncommon, lines urethra and some gland ducts; for protection and secretion
• Transitional epithelium (urothelium) lines the urinary bladder providing stretchability and protection

Glands

• Glandular epithelium produces secretion
• Glands are epithelium that secrete substances into ducts, onto a surface, or into the blood
• Endocrine glands secrete hormones into interstitial fluid then to the bloodstream, regulating activities for homeostasis
• Exocrine glands secrete products into ducts onto a surface, producing sweat, oil, earwax, saliva, and digestive enzymes/fluids
• Unicellular glands are single cells
• Multicellular glands are composed of many cells forming a structure, with classifications based on duct branching and secretory portion shape
• Merocrine glands secrete via exocytosis (e.g., salivary glands, pancreas)
• Apocrine glands accumulate secretory product at the apical surface, which then pinches off (e.g., mammary glands)
• Holocrine glands accumulate secretory product in the cytosol, then the cell ruptures (e.g., sebaceous glands)

Components of Connective Tissue

• Connective tissue contains extracellular matrix and cells, supplied with nerves and blood vessels (except cartilage and tendons)
• The extracellular matrix consists of protein fibers and ground substance determining tissue qualities such as how hard or firm the tissue is
• Ground substance is a material between cells and fibers and can be fluid, semifluid, gelatinous, or calcified, and it supports and binds cells, stores water and is a medium for exchange
• Glycosaminoglycans (GAGs) like hyaluronic acid, chondroitin sulfate, dermatan sulfate, and keratan sulfate, trap water, making ground substance jellylike
• Hyaluronic acid is viscous, and slippery, and lubricates joints
• Chondroitin sulfate provides support and adhesiveness
• Dermatan sulfate is found in tendons and blood vessels
• Keratan sulfate is found in bone and cartilage
• Adhesion proteins like fibronectin link ground substance components and cells
• Fibers provide strength and support and are secreted by connective tissue cells
• Collagen fibers are strong, flexible, resist pulling, and add tensile strength
• Elastic fibers branch and stretch easily, returning to their original shape
• Reticular fibers form branching networks for support and strength in soft organs

Connective Tissue Cells

• Connective tissue cells originate from mesenchymal cells
• Immature cells (-blast) secrete matrix, with examples being fibroblasts, chondroblasts, and osteoblasts
• Mature cells (-cyte) maintain matrix, with examples being fibrocytes, chondrocytes, and osteocytes
• Fibroblasts secrete fibers and ground substance
• Macrophages phagocytize, residing in tissues (fixed) or moving to infection sites (wandering)
• Plasma cells secrete antibodies, they mostly reside in the GI and respiratory tracts
• Mast cells release histamine, involved in dilating blood vessels during inflammation
• Adipocytes store triglycerides
• Leukocytes migrate from blood during infection or allergic reactions

Embryonic Connective Tissues

• Embryonic connective tissues are present in the embryo and fetus and differentiate into all other connective tissue types
• Mesenchyme has irregularly shaped cells in semifluid ground substance with reticular fibers, almost exclusively found under the skin, and along developing bones
• Mucous connective tissue has fibroblasts in viscous ground substance with collagen fibers, found in the umbilical cord of the fetus
• Mucous connective tissue supports what it surrounds

Mature Connective Tissues

• Mature connective tissues are present at birth and persist throughout life
• Connective tissue proper have flexible, viscous ground substance, rich in fibers
• Loose connective tissues include areolar, adipose, and reticular tissues; they have more ground substance and cells compared to fibers, making them flexible
• Areolar connective tissue is contains collagen, elastic, and reticular fibers, and various cells in semifluid ground substance, acts as packing around organs and under the skin, provides strength, elasticity, and support
• Adipose tissue stores triglycerides in adipocytes; brown adipose tissue (BAT) is mainly in fetus/infants, contains a rich blood supply, and generates heat
• Adipose tissue is found around organs and under the skin, and it reduces heat loss, stores energy, and protects organs
• Reticular connective tissue is a network of reticular fibers and cells, found in the stroma of organs and around blood vessels, and it supports organs, binds cells, and filters blood cells
• Dense connective tissues include dense regular, dense irregular, and elastic tissues; they have more fibers and less ground substance and cells, making them stronger
• Dense regular connective tissue contains collagen fibers in parallel bundles, forming tendons and ligaments, providing strong attachment along one axis
• Dense irregular connective tissue contains irregularly arranged collagen fibers, forming sheets like fasciae and dermis, providing tensile strength in many directions
• Elastic connective tissue has predominantly elastic fibers allowing stretching and recoil, found in lung tissue and artery walls

Supporting Connective Tissue

• Supporting connective tissues protect and support soft tissues of the body
• Cartilage contains collagen/elastic fibers in chondroitin sulfate gel, lacks blood vessels/nerves, and heals poorly
• Chondrocytes reside in lacunae within the matrix, and are surrounded by perichondrium that provide new cells
• Cartilage provides support, flexibility, and smooth surfaces for joints providing support
• Hyaline cartilage has a bluish-white, shiny look
• Fibrocartilage has chondrocytes among thick collagen bundles and lacks perichondrium, supporting and joining structures
• Elastic cartilage has chondrocytes in a network of elastic fibers, maintaining shape and strength

Bone Tissue

• Bone tissue has mineral salts for hardness and collagen for tensile strength and it supports, protects, stores minerals, and enables movement
• Compact bone's basic unit is an osteon (haversian system), made of concentric rings of calcified matrix with collagen and mineral salts
• Osteocytes reside in lacunae between lamellae
• Canaliculi connect lacunae, providing routes for nutrients/wastes
• A central canal contains blood vessels and nerves
• Spongy bone lacks osteons and consists of trabeculae containing lamellae, osteocytes, and canaliculi, filled with red bone marrow which produces blood cells
• Bone's function is support, protection, storage, and housing blood-forming tissue

Liquid Connective Tissue

• Liquid connective tissue has a liquid extracellular matrix
• Blood's liquid matrix is plasma, containing RBCs, WBCs, and platelets, dissolved water and substances
• Blood transports gases, functions in immunity, and clotting
• Lymph has a clear liquid matrix (similar to plasma but with a higher or lower protein content depending on certain variables) it is within lymphatic vessels, involved in immunity and lipid absorption

Membranes

• Membranes are flat sheets of pliable tissue that cover or line the body
• Epithelial membranes have an epithelial layer and connective tissue layer; mucous, serous, and cutaneous

Mucous Membrane

• Mucous membranes (mucosa) consist of an epithelial layer overlying areolar connective tissue, with the epithelium varying by location
• Mucous membranes line cavities opening to the exterior of the body, such as that of the GI and respiratory systems
• Mucous membranes provide a barrier and defense against microbes, traps particles, lubricates food, secrets digestive enzymes, etc

Serous Membrane

• Serous membranes (serosa) consist of areolar connective tissue covered by mesothelium
• There are two layers: the parietal which lines a cavity wall and the visceral which overs organs
• Serous fluid is secreted which is watery
• Serous membranes line cavities not opening to the exterior of the body, which includes the heart, lungs, and abdominal organs
• The serous membranes serve the function of allowing organs to easily glide using the secreted lubricating fluid

Cutaneous Membrane

• A cutaneous membrane or skin consists of superficial epidermis (keratinized stratified squamous epithelium)
• It also consists of a deeper dermis (dense irregular and areolar connective tissue)
• The cutaneous membrane overs the entire body surface
• A Cutaneous membrane's function is to protect

Synovial Membrane

• Synovial Membrane lacks epithelium and is composed of a discontinuous layer of synoviocytes overlying areolar and adipose connective tissue
• They line the cavities of freely movable joints
• Synoviocytes secrete the components of synovial fluid and lubricate/nourish the joints and removes debris/microbes

Muscular Tissue

• Muscular tissue consists of cells with elongated cells (muscle fibers/myocytes), specialized for contraction using ATP
• Muscular tissue produces body movements, maintains posture, generates heat, and provides protection
• Skeletal muscle has long, cylindrical, striated fibers with multiple nuclei located at the periphery, attached to bones with voluntary control
• Cardiac muscle has branched, striated fibers (usually one nucleus), interconnected by intercalated discs in the heart wall with involuntary control
• Smooth muscle tissue is made of nonstriated fibers (smooth), with a spindle-shape with a single central located nucleus in hollow organ structures; mostly under involuntary control

Nervous Tissue

• Nervous tissue consists of neurons and neuroglia cells
• Neurons respond to stimuli by converting them into electrical signals like nerve and action potentials, exhibiting electrical excitability
• Neurons contain a Cell body, dendrites (branched to receive a signal), and single, thin, cylindrical axon
• Neuroglia support neuron cells
• Nervous tissue exhibits sensitivity to stimuli, converts stimuli into nerve impulses, and conveys impulses to other neurons by releasing neurotransmitters allowing for communication
• This tissue comprises the nervous system
• Neurons (and muscle fibers) exhibit electrical excitability, are able to respond to stimuli by producing electrical signals (action potentials), which then propagate along the plasma membrane