Tissues of the Human Body

• Organs consist of four basic tissue types: epithelial, connective, muscular, and nervous.
• Each tissue is a group of specialized cells performing a specific function.
• Basic tissues contain extracellular matrix (ECM) and cells, associating in varying proportions and morphologies.
• Connective tissue features cells producing abundant ECM.
• Muscle tissue consists of elongated cells specialized for contraction and movement.
• Nervous tissue comprises cells with fine processes specialized for receiving, generating, and transmitting impulses.
• Most organs have parenchyma (cells performing specialized functions) and stroma (supporting cells, usually connective tissue except in the brain and spinal cord).
• Epithelial tissues consist of closely packed polyhedral cells adhering to each other and a thin layer of ECM.
• Epithelial tissues form cellular sheets lining organ cavities and covering the body surface.
• Epithelium lines all body surfaces; entering or leaving substances must cross this tissue.
• Epithelial tissues function includes covering, protecting, absorption, and secretion.
• Some epithelial cells may be contractile (myoepithelial) or sensory

Characteristic Features of Epithelial Cells

• Dimensions and shapes vary from columnar to cuboidal to squamous, with function dictating their morphology.
• Nuclei shapes vary (elliptic, spherical, or flattened).
• Columnar cells have elongated nuclei; squamous cells have flattened nuclei.
• Cuboidal or pyramidal cells have spherical nuclei.
• Nuclei number and shape indicate cell shape and density.
• Nuclei determine the number of cell layers, a key classification criterion.
• Most epithelia are adjacent to connective tissue containing blood vessels, which provide nutrients and O2.
• Epithelia lack blood vessels.
• The lamina propria is connective tissue underlying the epithelia of the digestive, respiratory, and urinary systems.
• Papillae (evaginations) project from connective tissue into the epithelium, increasing the contact area between the two tissues.
• Epithelial cells show polarity, with uneven distribution of organelles and membrane proteins within the cell.
• The basal pole contacts the ECM and connective tissue; the apical pole faces a space.
• Lateral surfaces adjoin neighboring cells and have folds, increasing area and function.

Basement Membranes

• The basal surface rests on the basement membrane, a thin extracellular sheet of macromolecules, acting as a semipermeable filter.
• Basement membranes are made of glycoproteins, which can be stained and visualized with a light microscope
• TEM resolves two parts: the basal lamina (thin, electron-dense layer) and the reticular lamina (more diffuse and fibrous).
• The basal lamina is nearest to the epithelial cells.
• The basement membrane and basal lamina terms are sometimes used interchangeably, but basal lamina usually refers to the ultrastructural layer, and basement membrane refers to the entire structure.
• The macromolecules includes type IV collagen, laminin, nidogen, and perlecan.
• Type IV collagen forms a 2D network of evenly spaced subunits.
• Laminin are glycoproteins attaching to transmembrane integrin proteins and project type IV collagen.
• Nidogen and perlecan cross-link laminins to the type IV collagen network, to provide structure and determine molecule size able to filter through
• Basal/external laminae also surround muscle, nerves, and fat-storing cells, regulating macromolecular exchange.
• The reticular lamina contains type III collagen bound to the basal lamina by anchoring fibrils of type VII collagen.
• Basement membrane functions include support, attachment, maintenance of cell polarity, localization of activities, cell-to-cell interactions, and allowing repair and regeneration.

Intercellular Adhesion & Other Junctions

• Membrane-associated structures provide adhesion and communication.
• Epithelial are strong due to neighboring cells and basal laminae.
• Lateral surfaces of epithelial cells have specialized intercellular junctions.
• Tight or occluding junctions form a seal between adjacent cells.
• Adherent or anchoring junctions are sites of strong cell adhesion.
• Gap junctions are channels for communication

Tight Junctions

• Tight junctions, also called zonulae occludens, are most apical
• "Zonula" indicates a band completely encircling each cell
• Adjacent membranes appear fused/tightly apposed.
• Interactions between transmembrane proteins (claudin and occludin) form a seal.
• After cryofracture, tight junctions appear as a band of branching strands around each cell's apical end.
• Tight junctions ensure that molecules crossing an epithelium go through (transcellular) rather than between the cells (paracellular).
• Epithelia with one or few sealing strands are more permeable than epithelia with many fused strands.
• Tight junctions restrict movement of membrane lipids and proteins, maintaining distinct apical and basolateral membrane domains.
• Apical membranes are part of the luminal compartment, while basolateral domains are part of a basal compartment.

Adherens Junctions

• The second type of junction is the adherent junction/zonula adherens, encircling the cell below the tight junction and anchoring cells
• Cell adhesion is mediated by cadherins
• Cadherins bind catenins that link to actin filaments.
• Actin filaments linked to adherens junctions form part of the "terminal web."
• Tight and adherent junctions function together: holding cells together

Desmosomes

• Another anchoring junction is the desmosome/macula adherens, resembling a "spot-weld" instead of a belt.
• Desmosomes are disc-shaped structures matched with structures at an adjacent cell surface.
• Desmosomes have desmogleins and desmocollins.
• Cytoplasmic ends of proteins bind plakoglobins and link to desmoplakins.
• Desmoplakins bind intermediate filament proteins.
• Epithelial desmosomes attach to cytokeratin filaments.
• The proteins are strong provide cellular adhesion and strength, allowing tight adhesion between tissue and cell.

Gap junctions

• Gap junctions mediate intercellular communication rather than adhesion/occlusion.
• Abundant in epithelia, these are functional in mammalian tissues.
• Cryofracture shows aggregated transmembrane protein complexes that form circular patches.
• Transmembrane gap junction proteins (connexins) form complexes called connexons, each with a hydrophilic pore.
• Connexons in adjacent membranes move laterally and align
• These junctions permit intercellular exchange of molecule sizes less than 1.5 nm in diameter.
• Molecules mediating signal transduction move rapidly.

Hemidesmosomes and Focal adhesions

• On the basal epithelial surface, cells attach to the basal lamina by hemidesmosomes.
• Ultrastructurally resemble half-desmosomes; transmembrane proteins linking to keratin are integrins.
• Integrins of hemidesmosomes primarily bind to laminin.
• Another basal anchoring junction is the focal adhesion or contact found in moving cells. Focal adhesions are smaller/more numerous and consist of integrins linked to bundled actin filaments.
• Integrins of focal adhesions link to focal adhesion kinase, affecting cell adhesion, mobility, and expression.

Specializations of the Apical Cell Surface

• Apical ends may have structures projecting for better absorption/movement of substances.
• Many cells have cytoplasmic projections moved by actin filaments.
• In epithelia specialized for absorption, apical surfaces are filled with projecting microvilli.

Microvilli

• For absorption the apical cell is filled with various microvilli
• Densely packed microvilli are visible as a brush/striated border
• An average microvillus is about 1 µm long and 0.1 µm, creating large SA
• The thick glycocalyx covering the border includes enzymes for digestion.
• Each microvillus has bundled actin filaments capped and bound to the membrane by actin-binding proteins.
• Although microvilli are stable, microfilament arrays are dynamic, which help channel, receptor, and protein production.
• The Filaments insert into the terminal web of cortical microfilaments.

Stereocilia

• Stereocilia are a less common processes seen in the male reproductive system increasing the cells' surface area. More specialized stereocilia function for detection of inner ears
• They also contain microfilamnets with actin-binding protein, and contain cell's terminal web
• Stereocilia tend to be larger more branched

Cilia

• Cilia are elongated surfaces and apicals
• Cilica is on most cell if not all
• Contains primary clillium, they also are with recperots adn signal transduction used to detect different stimulus Motile cilia are abundant.
• Typical cilia are 5-10 µm long and 0.2 µm in diameter
• Arranged in A 9 + 2
• Also had basal bodies that contian protein,
• The dyniein in the cilia helps beat it