Histology Tissues and Cell Functions

• The organs of the human body consist of only four basic tissue types: epithelial, connective, muscular, and nervous.

Basic Tissue Types

• Epithelial tissue aggregates closely packed polyhedral cells connected to each other and to the ECM.
• Connective tissue features cells creating abundant ECM.
• Muscle tissue comprises elongated cells specialized for contraction and movement.
• Nervous tissue consists of cells with long, fine processes specialized to generate and transmit nerve impulses.

Epithelial Tissue Functions

• Epithelia cover, line, and protect surfaces, like the epidermis.
• Epithelia absorb substances, such as the intestinal lining.
• Epithelia secrete fluids, as seen in parenchymal cells of glands.
• Specific epithelial cells can be contractile (myoepithelial cells) or specialized sensory cells (taste buds).

Characteristic Features of Epithelial Cells

• Shapes and dimensions of epithelial cells vary from tall columnar to cuboidal to low squamous, the cell nuclei shape corresponds to cell shape.
• The number and shape of stained nuclei indicates cell shape and density, determines cell layers' number.
• Epithelia are adjacent to connective tissue containing blood vessels, from which the epithelial cells receive nutrients and O2; the connective tissue under epithelia lining digestive, respiratory, and urinary systems is the lamina propria.
• Papillae are evaginations projecting from the connective tissue into the epithelium, occurring in tissues with friction.
• Epithelial cells show polarity, with organelles and membrane proteins distributed unevenly within the cell.
• The basal pole contacts the ECM and connective tissue, and the apical pole faces a space.

Basement Membranes

• A basement membrane is a thin extracellular sheet of macromolecules acting as a semipermeable filter.
• The basal lamina consists of a thin, electron-dense layer nearest to the epithelial cells.
• The reticular lamina consists of a diffuse and fibrous layer beneath the basal lamina with type III collagen.
• Basal lamina macromolecules include Type IV collagen, laminin, and Nidogen and perlecan.
• Basal laminae also exist as thin sleeves surrounding muscle cells and nerves.
• Functions of basement membranes include supporting epithelial cells and attaching epithelia to underlying connective tissue; basal lamina components maintain cell polarity and localize endocytosis.
• Basement membranes serve as scaffolds for epithelial repair and regeneration.

Intercellular Adhesion and Other Junctions

• Epithelial cells adhere strongly to neighboring cells and basal laminae through membrane-associated structures.
• Tight junctions (occluding junctions) form seals between adjacent cells.
• Adherens or anchoring junctions provide strong cell adhesion.
• Gap junctions are intercellular communication channels.

Tight Junctions

• Tight junctions (zonulae occludens) are the most apical junctions, with fused adjacent membranes.
• The tight seal between membranes is due to transmembrane proteins such as claudin and occludin.
• Intercellular seals ensure molecules cross epithelia through cells (transcellular path) and not between them (paracellular pathway).
• Tight junctions restrict movements of lipids and proteins at the apical cell surface, maintaining distinct membrane domains.

Adherens Junctions

• Adherens junctions/zonula adherens firmly anchor a cell to its neighbors.
• Cell adhesion is mediated by cadherins for cell adhesion; at their cytoplasmic ends, cadherins link to actin filaments with actin-binding proteins.
• Tight and adherent junctions function like plastic bands securing cells together.

Desmosomes

• Desmosomes/macula adherens are spot-weld-like anchoring junctions, containing cadherin family and proteins such as desmoplakins.
• Epithelial desmosomes attach to cable-like filaments of cytokeratin and provide cellular adhesion and strength.

Gap Junctions

• Gap junctions consist of transmembrane connexins forming connexons, permitting intercellular exchange of molecules.
• Molecules mediating signal transduction move rapidly to allow coordinated action rather than independent units.

Hemidesmosomes and Focal Adhesions

• Hemidesmosomes, on the basal epithelial surface, attach cells to the basal lamina, linking integrins to laminin molecules.
• Focal adhesions link integrins indirectly to bundled actin filaments, affecting cell adhesion, mobility, and gene expression.

Specializations of The Apical Cell Surface

• Apical ends of columnar and cuboidal epithelial cells feature specialized structures for absorption or movement of substances.

Microvilli

• Epithelial cells’ microvilli are cytoplasmic projections that appear as a brush or striated border.
• Each microvillus contains bundled actin filaments capped and bound to the plasma membrane by actin-binding proteins.
• Dynamic microfilament arrays and myosin-based movements maintain optimal conditions for absorption.

Stereocilia

• Stereocilia increase cells' surface area in regions such as the male reproductive system, contain microfilaments and actin-binding proteins, and can be motion-detecting.
• Stereocilia are typically longer and less motile than microvilli, and they sometimes branch distally.

Cilia

• Cilia are motile apical structures containing microtubules (not microfilaments).
• Short projection enriched with receptors and signal transduction complexes called a primary cilium.
• Each cilium contains nine peripheral microtubule doublets around two central microtubules called an axoneme, that connect to basal bodies.
• Ciliary motion occurs through changes in the axoneme, in which dynein-powered sliding doublets bends the axoneme.

Types of Epithelia

• Epithelia are separated into covering (or lining) epithelia and secretory (glandular) epithelia.

Covering and Lining Epithelia

• Cells of covering epithelia, organized into one or more layers, are classified by the number of cell layers and the cell morphology in the outer layer.
• Simple epithelia contain one cell layer.
• Stratified epithelia contain two or more layers.
• Pseudostratified columnar epithelium features tall, irregular cells attached to the basement membrane but with nuclei at varying levels.

Simple Epithelia

• Simple squamous epithelia facilitate movement of the viscera, active transport by pinocytosis, and secretion.
• Simple cuboidal epithelia function in covering and secretion.
• Simple columnar epithelia function in protection, lubrication, absorption, and secretion.

Stratified Epithelia

• Stratified squamous keratinized epithelium protects against water loss, and stratified squamous nonkeratinized epithelium protects against water loss.
• Stratified cuboidal epithelium is rare but occurs in excretory ducts, while stratified columnar epithelium occurs in the conjunctiva lining the eyelids.
• Unique transitional epithelium or urothelium comprises superficial layer of umbrella cells to protect against urine.

Secretory Epithelia And Glands

• Epithelial cells function mainly to produce and secrete macromolecules, in epithelia with other functions, or glands.
• Secretory cells may synthesize, store, and release proteins, lipids, or carbohydrates and proteins; epithelia of mammary glands secrete all three substances.
• Glands develop from covering epithelia by growth into connective tissue, differentiating into exocrine glands or endocrine glands.
• Exocrine glands remain connected with the surface epithelium via ducts.
• Endocrine glands lose the connection and lack ducts.

Glandular Epithelia Organization

• Epithelia of exocrine glands organize as a continuous system of secretory portions and ducts supported by connective tissue stroma.
• Glands are simple (ducts not branched) or compound (ducts with two or more branches).
• Secretory portions can be tubular or acinar; either type of secretory unit may be branched, can have multiple tubular, acinar, or tubuloacinar secretory portions.

Mechanisms for Releasing Products

• Merocrine secretion: common method involving typical exocytosis.
• Holocrine secretion: cells accumulate product and undergo complete disruption to release product and cell debris.
• Apocrine secretion: product accumulates at the cells' apical ends, which pinch off to release the product surrounded by cytoplasm and membrane.

Exocrine Gland Category

• Exocrine glands with merocrine secretion can be serous (proteins that are mostly not glycosylated, digestive enzymes), mucous (heavily glycosylated proteins called mucins), or mixed seromucous glands. , mucous tubules containing watery mucus combined with digestive enzyme
• Epithelia of many exocrine glands contain contractile myoepithelial cells at the basal ends, aiding in secretion.

Transport Across Epithelia

• Epithelial cells transport certain ions involving the protein Na+/K+-ATPase, actively. Ions are then balanced with osmotic balance that occurs from the transfer of chloride ions.
• Transcellular transport occurs using some epithelial cells in the transfer of ions.

Renewal of Epithelial Cells

• Epithelial tissues are renewed continuously by mitotic activity and rate of renewal varies.
• Stem cells and mitosis occur only within the basal layer.
• During cancerous growth, epithelia are prone to abnormal groth and dysplasia which can progress on to more problematic effects such as benign and malignant tumors.
• Sometimes some epithelial cells may transform into other tissue types from metaplasia.

This lesson explores the role of cells within organs, focusing on the functions of epithelial, connective, and muscle tissues. It also covers the composition and function of the basement membrane, the extracellular matrix, nidogen, perlecan, and laminins.