Human Body Tissues

• Organs within the human body consist of four basic tissue types: epithelial, connective, muscular, and nervous tissues.
• Tissues are comprised of dedicated and specialized cells, working together to perform specific functions.
• Both cells and extracellular matrix (ECM) are found in basic tissues.

Tissue Classifications

• Connective tissue features cells which produce an abundant ECM.
• Elongated cells specialized for contraction and movement makes up muscle tissue .
• Nerve impulses are sent through cells specialized with fine processes in nervous tissue.
• Most organs have parenchyma for specialized functional cells, and the stroma which supports.
• Stroma is connective tissue, except in the brain and spinal cord.
• Epithelial tissues consist of closely packed polyhedral cells, adhering strongly to one another and to a thin ECM layer.
• Epithelial tissues are cellular sheets that line organ cavities and the body surface.
• Epithelia line all the body's external and internal surfaces; substances entering or leaving an organ pass through this tissue type.

Epithelial Tissue Functions

• Covering surfaces (epidermis)
• Absorption (intestinal lining)
• Secretion (parenchymal gland cells)
• Specialized epithelia cells include contractile myoepithelial cells, and sensory cells like those in taste buds or olfactory epithelium.

Epithelial Cells

• Epithelial cells can be tall columnar to cuboidal to low squamous in shape and size.
• Function generally dictates epithelial cells size and overall morphology.
• Shapes of the cell's nuclei match to the cell itself (elongated, flattened, spherical, etc.)
• Columnar cells usually have elongated nuclei with squamous cells having flattened nuclei, and cuboidal or pyramidal cells have more spherical nuclei.
• The number and shape of stained nuclei becomes critical because lipid-rich membranes are regularly indistinguishable via light microscopy.
• The number of cell layers in an epithelium is a primary morphologic criteria for classifying epithelia.
• Epithelia is typically adjacent to connective tissue with blood vessels, allowing the epithelial cells to receive nutrients and O2.
• The epithelia lining the digestive, respiratory, and urinary systems connects to the lamina propria.
• Evaginations called papillae project from the connective tissue into the epithelium, making the contact area between tissues greater.
• Tissues prone to friction such as skin and tongue coverings frequently contain papillae.
• Epithelial cells exhibit polarity, with uneven distribution of organelles and membrane proteins in cells.
• The region contacting the ECM and connective tissue is the basal pole, while the opposite end is the apical pole.
• The basal and apical poles differ greatly in structure and function.
• The lateral surfaces of cuboidal or columnar cells connect to neighboring cells.
• Numerous folds on cell membranes increase the surface area and functional capacity of the surface on the lateral membrane.

Basement Membranes

• All epithelia rests on the the basal surface, a thin ECM sheet of macromolecules.
• This sheet acts like a semipermeable filter for substances reaching epithelial cells from below.
• Glycoproteins and other components can be stained and visualized via light microscopy.
• The transmission electron microscope (TEM) can resolve two parts of the basement membrane.
• Basal lamina sits nearest the epithelial cells and is a thin, electron-dense layer of fine fibrils.
• Beneath the basal lamina you will find the reticular lamina: a fibrous layer.(Figure 4-3a).
• Basal lamina refers to the fine extracellular layer seen ultrastructurally and basement membrane is the entire structure beneath epithelial cells visible with a light microscope.

Basal Lamina Components

• Type IV collagen monomers assemble into a 2D network.
• Large glycoproteins called Laminin attach to transmembrane integrin proteins in the basal cell membrane and through the mesh formed by type IV collagen.
• Protein Nidogen and proteoglycan Perlecan cross-link, to help aid the basal lamina's 3D structure, attach epithelium, determine porosity and molecule size.
• External laminae serve as semipermeable barriers regulating macromolecular exchange between enclosed cells and connective tissue.
• The reticular lamina contains type III collagen which binds to the basal lamina via anchoring fibrils of type VII collagen, produced via connective tissue cells.

Basement Membrane Functions

• Basement membranes support epithelial cells and connect epithelia to underlying connective tissue.
• Basal lamina components help organize integrins and other proteins in epithelial cell plasma membranes.
• Maintains cell polarity
• Localizes endocytosis.
• Transduction of Signals
• Proteins in the basement membrane proteins have specific roles in cell-to-cell interactions.
• ECM serves as the building blocks for repair of epithelia

Cell Adhesion and Communication

• Membrane associated structures provide adhesion and communication among cells.
• Epithelial cells connect strongly to neighboring cells and basal laminae, particularly in epithelia which undergo friction or other types of mechanical force.
• Lateral epithelial cell surfaces contain specialized intercellular junctions with different functions.
• Tight or occluding junctions form a seal between adjacent cells.

Junction Types

• Adherent or anchoring junctions create strong area of cell adhesion together with cells nearby.

• Gap junctions function as intercellular channels by providing communication between cells.

• In epithelia, junctions usually sit in a specific order at the cells' apical end.

• Tight junctions are also called zonulae occludens, and are most apical of the junctions.

• Zonula - band completely encircling each cell

• Adjacent membranes tightly appose, and they appear fused in TEM images.

• Molecular interactions between proteins claudin and occludin create the seal between two cell membranes.

• Banded strands occur around each cell's apical end via tight junctions seen on a membrane.

• Tight junctions ensure molecules entering/exiting do so through cells via the transcellular path, instead of the paracellular pathway.

• The intercellular seal of tight junctions enables transport and absorption through cells.

• The proximal renal tubule has epithelium that contains fewer sealing strands for greater permeability to water and solutes rather than epithelia lined urinary bladder

• Continuous cell membrane zones within the cell function as fences, restricting movements or preventing leakage.

• T tight junctions maintain distinct cell membrane domains, like apical and basolateral, with specific functions.

• Proteins of tight junctions are targets for bacteria.

• Enterotoxins such as Clostridium perfringens disrupts claudin molecules, prevents insertion of proteins, causes tissue fluid losses into intestinal lumen per paracellular pathway.

• Helicobacter pylori binds to tight-junction of stomach cells, inserts targets protein ZO-1, and causes signaling disruptions from the junction.

• Cadherins mediate cell adhesion via transmembrane glycoproteins which is adherent cell function; requires calcium

• Cadherins (transmembrane glycoproteins) mediate adheren junctions in prescence of calcium

Cell Surface

• Cadherins attach to actin filaments with actin binding proteins.
• Tight and adherent apical cell junctions function like plastic bands to secure canned drinks.
• Desmosomes are macula adherens "spot-weld" junctions, rather than encircling the cell.
• Desmosomes are disc-shaped structures on the surface of one cell to match structures on an adjacent cell surface.
• This disc shaped surface contains cadherin proteins that can also be described as desmogleins and desmocollins.
• Cytoplasmic ends bind to plakoglobins and catenin-like proteins to create desmoplakin in a dense plaque.
• Intermediate filament proteins bind desmoplakins instead of actins.
• Epithelial desmosomes will attach to cytokeratin (tonofilaments) which aids adhesion and strength in the whole of the epithelium.
• Gap junctions mediate intercellular communication, not adhesion or occlusion.
• Abundant in epithelia and functionally significant in nearly all mammalian tissues.
• Cryofracture shows that gap junctions have aggregated transmembrane protein complexes.
• Transmembrane gap junction proteins, connexins,make hexameric connexons (1.5 nm diameter pores.)
• Connexins in adjacent membranes align producing intercellular channels that are molecule selective.
• Cyclic nucleotides and ions in gap junctions allow cells in coordinated action rather than individually.
• Cardiac and visceral muscles facilitate gap junctions for heart and visceral contractions.
• Hemidesmosomes are anchoring junctions that attach cells to basal lamina (TEM visualized).
• Hemidesmosome adhesive structures are half-desmosomes.
• Integrins are indirect linkers to keratin not cadherins.
• Hemidesmosome integrins primarily bind to laminin molecules in the basal lamina.
• Focal adhesions and contacts are basal anchoring junctions found in moving cells during epithelial repair and reorganization.
• Integrins are linked to bundled actin filaments to creat numerous cellular connections.
• Important integrins within proteins initiate intracellular protein phosphorylation.
• These initiations affect mobility adhesion and gene expression.

Apical Cell Surface Specializations

• The columnar/cuboidal apical ends have structures projecting: to increase surface area for absorption, or move substances across surface.
• Microvilli are are cytoplasmic projections via electron microscopy (temporary, variable number) for absorption and projecting microvilli.
• Absorptive epithelia apical cell surfaces are regularly filled with many microvilli of a uniform length.
• Densely packed microvilli (small intestine) are visible as a brush border; each microvillus is about 1 µm long and 0.1 µm wide, but surface area increases 20- or 30-fold.
• Microvilli are covered with glycocalyx containing enzymes for digestion.
• Each contains bundled actin filaments capped and linked to the membrane by actin-binding proteins.
• Bundled microfilament arrays are dynamic via myosin-based movements.
• Filaments attach to the base by the terminal web of cortical microfilaments.

Stereocilia and Cilia

• Stereocilia are apical type processes mainly seen on epithelial cells for absorption and motion detection.
• Microfilaments containing actin-binding proteins connects stereocilia to a cells terminal web.
• Stereocilia resembles diameter and and cellular connections of microvilli.
• Stereocilia are longer and less motile.
• Cilia are long, motile structures larger than microvilli (also contain microtubules instead of microfilaments).
• Primary Cilium are not motile but enriched with signal transducing; detecting motion, light, liquid, odors.
• Mobile Cilia is in cuboidal or columnar epithilia with typical cilia about 5-10 um long.
• Figure 4-10 shows the cilium axoneme, made up of nine peripheral microtubule doublets.
• The 9 + 2 assembly of microtubules is called an axoneme.
• Kinesin and cytoplasmic dynein motors allows cilia to move to transport through the structures.
• Axonemes microtubules are linked to Basal bodies (structure is similar to centrioles).
• Complex accessory molecules create rapid beating patterns by moving across the epithelium,
• Axonal dynein complexes bound to one microtubule creates a sliding movement.
• A long flagellum extend for sperm cells with structures similar than cilia, so they allow to move with mechanism.

Types of Epithelia Tissue Classifications

• Epithelia cells in two groups, covering and lining (covering epithelia)

• Secretory or gland forming (glandular epithelia).

• Lining epithelia cells are in layers covering inner cavities and the surface of an organ.

• Epithelia are classified by cell layers, and other is outer layer of cells.

• One layer is Simple, and more than one is stratified epithelia.

• Shapes categorize simple epithelia (squamous, cuboidal, or columnar)

• Stratified epithelia are categorized with cell shape.

• The thin layer of epithelium can also be found within what is known as Keratinized (packed with keratin filaments), or nonkeratinized (with few filaments)

• Dry keratinized squamous is mainly epidermus of skin prevents dehydration.

• The nonkeratinized moist variety lines organs to prevent water loss (surface cell nuclei can be seen in moist areas).

• The layer of surface cells helps prevents water loss in this epithelium.

• Stratified cuboidal epithelium - Sweat glands

• Unique Transitional Lines or Urothelium - Urinary Tract.

• (Fig.4.16) Umbrella layer- protects tissues from toxins from urine.

• Puseostratified columnar epithelium-(FIG. 4.17) Nuclei at different levels and not all cells extend to the free surface

• The respiratory tract is pseudostratified columnar.

• Goblet cells secrete mucus in intestinal lining.

Explore the different types of tissues found in the human body. Learn about epithelial, connective, muscle, and nervous tissues, as well as their respective roles in organs and systems. Understand the structure and function of each tissue type.