Tissue Types and Junctions Quiz

Basic Tissue Types

• The human body's organs are composed of four basic tissue types: epithelial, connective, muscular, and nervous tissues.
• Each tissue type is a group of specialized cells performing specific functions, containing extracellular matrix (ECM) as well as cells.
• These tissues associate in varying proportions and morphologies characteristic of each organ.

Connective, Muscle, and Nervous Tissues

• Connective tissue is characterized by cells producing an abundant ECM.
• Muscle tissue is composed of elongated cells specialized for contraction and movement.
• Nervous tissue comprises cells with long, fine processes specialized for receiving, generating, and transmitting nerve impulses.

Parenchyma and Stroma

• Organs are divided into parenchyma and stroma.
• Parenchyma consists of cells responsible for the organ's specialized functions.
• Stroma, primarily composed of connective tissue (except in the brain and spinal cord), supports the parenchyma.

Epithelial Tissues

• Epithelial tissues are composed of closely grouped polyhedral cells adhering strongly to one another with a thin layer of ECM.
• They form cellular sheets that line the cavities of organs and cover the body's surface, lining all internal and external surfaces.
• All substances entering or leaving an organ must cross this tissue type.

Principal Functions of Epithelial Tissues

• Covering, lining, and protecting surfaces (e.g., epidermis of skin).
• Absorption (e.g., intestinal lining).
• Secretion (e.g., parenchymal cells of glands).
• Certain epithelial cells may be contractile (myoepithelial cells) or specialized as sensory cells (e.g., taste buds or olfactory epithelium).

Characteristics of Epithelial Cells

• Epithelial cells vary in shape and dimensions, ranging from tall columnar to low squamous cells.
• Cell shape is generally dictated by function.
• Cell nuclei shapes correspond to the cell shape: columnar cells have elongated nuclei, squamous cells have flattened nuclei, and cuboidal or pyramidal cells have more spherical nuclei.

Epithelial Tissue Adjacency to Connective Tissue

• Most epithelia are adjacent to connective tissue containing blood vessels, supplying nutrients and oxygen to the epithelial cells.
• Even thick epithelia typically do not contain their own blood vessels.
• The connective tissue under the epithelia in the digestive, respiratory, and urinary systems is known as the lamina propria.
• Epithelial-connective tissue contact is facilitated by papillae, small evaginations.

Polarity and Structural Aspects of Epithelial Cells

• Epithelial cells display polarity, having organelles and membrane proteins distributed unevenly across the cell.
• Basal pole contacts the ECM and connective tissue, while the apical pole faces a space.
• Cuboidal or columnar cells have lateral surfaces adjoining neighboring cells with membrane folds increasing surface area and functional capacity.

Basement Membranes

• The basal surface of epithelia rests on a basement membrane consisting of macromolecules.
• The basement membrane acts as a semipermeable filter for substances reaching epithelial cells from below.
• It contains glycoproteins and other components, often visible with a light microscope.

Basal Lamina and Reticular Lamina

• The basement membrane consists of the basal lamina and the reticular lamina.
• Basal Lamina is a thin sheet-like layer of fibrils close to epithelial cells.
• Reticular Lamina is a more diffuse layer beneath the basal lamina.

Components of the Basal Lamina

• Type IV collagen forms a two-dimensional network structure similar to a window screen.
• Laminin is a large glycoprotein that attaches to integrins.
• Nidogen and perlecan contribute to basal lamina structure and are proteins and proteoglycans, respectively.

Intercellular Adhesion and Junctions

• Epithelial cells adhere strongly to neighboring cells and basal lamina, especially in areas subject to friction.
• Lateral surfaces of epithelial cells feature intercellular junctions with distinct functions.
• Tight junctions create a seal between adjacent cells.
• Adherent junctions anchor cells together.
• Gap junctions form channels for communication.

Desmosomes

• Desmosomes (or macula adherens) are disc-shaped anchoring junctions resembling spot-welds, rather than belts.
• Desmosomes consist of cadherins which link to plakoglobins (catenin-like proteins) that are linked to desmoplakins.

Gap Junctions

• Gap junctions connect cells mediating intercellular communication, and are common in nearly all mammalian tissues.
• Cryofracture preparations show transmembrane protein complexes that form circular patches in the plasma membrane known as connexons.

Hemidesmosomes

• Hemidesmosomes are anchoring junctions on the basal surface of epithelial cells that link to laminin of the basal lamina.

Focal Adhesions

• Focal adhesions are basal anchoring junctions that link integrins to bundled actin filaments, rather than intermediate filaments.
• Focal adhesions are involved in epithelial repair and reorganization.
• Integrins in focal adhesions connect to paxillin and focal adhesion kinase (FAK), a signaling protein that affects cell mobility and gene expression.

Specializations of Apical Surface

• The apical ends of many columnar and cuboidal epithelial cells feature specialized structures like microvilli and stereocilia for enhancing absorption or moving substances along the surface, observed in some absorptive epithelial cells.

Microvilli

• Microvilli are cytoplasmic projections formed by bundled actin filaments, found in large numbers in epithelial cells specialized for absorption.
• They dramatically enhance the cell's surface area.

Stereocilia

• Stereocilia are even longer microvilli found in the male reproductive system for absorption.

Cilia

• Cilia are long, motile structures containing internal microtubules.
• Cilia are often found in epithelial cells lining the upper respiratory tract or in other specialized epithelial cells (e.g., those of the inner ear).
• The rhythmic beating patterns of cilia move fluid along the epithelial surface.

Basal Bodies and Axoneme Continuity

• The microtubules of cilia are continuous with basal bodies, which are similar to centrioles.

Types of Epithelia

• Epithelia are classified as covering/lining or secretory (glandular).

Covering Epithelia

• Covering epithelia are single-layered or multiple-layered cells that cover surfaces or line cavities of organs.
• They are classified based on the number and shape of cell layers.
  • Simple (single layer) — squamous, cuboidal, columnar
  • Stratified (multiple layers) — squamous, cuboidal, columnar

Stratified Squamous Epithelia

• These epithelia have thin surface cells and can be either keratinized or nonkeratinized
  • Keratinized (epidermis)
  • Nonkeratinized (moist areas like mouth, esophagus)

Transitional Epithelium

• Also known as urothelium and lines the urinary tract.
• It can distend as the urinary bladder fills, a unique morphological feature.

Secretory Epithelia

• Secretory epithelia are primarily involved in producing and releasing macromolecules.
  • They are also part of specialized organs called glands.
• Cells in glands synthesize, store, and release either proteins (e.g., pancreas), lipids (e.g., adrenal glands), or carbohydrate-protein complexes, (e.g., salivary glands).

Mechanisms of Secretion

• Merocrine: Secretion via exocytosis (most common).
• Holocrine: Secretory product released with cell debris.
• Apocrine: Product released with cell membrane.

Serous and Mucous Cells (Merocrine Secretion)

• Serous cells synthesize protein secretions typically non-glycosylated (e.g., digestive enzymes), with a well-developed endoplasmic and Golgi apparatus, filled with secretory granules, stained with acidophilic stains.
• Mucous cells synthesize and secrete protective secretions that become mucous upon hydration (e.g., goblet cells).

Myoepithelial Cells

• Myoepithelial cells are contractile cells found in many secretory glands (sweat, lacrimal, salivary, mammary)

Endocrine Glands

• Endocrine glands lose their connection to the original epithelium (ductless) and release hormones directly into the blood, lacking myoepithelial cells.
• Protein hormones are released by exocytosis, whereas lipophilic steroid hormones diffuse through the cell membrane.

Ion Transport and Transcellular Transport

• Active ion transport moves ions against concentration/electrical gradients (e.g., Na+/K+-ATPase).
• Transcellular transport involves ion pumps and water channels, through aquaporins, to cross the epithelium.
• Tight junctions prevent paracellular diffusion between cells.

Renewal of Epithelial Tissues

• Epithelial tissues renew constantly via mitotic activity and stem cells, with rates varying across tissues.

Repair and Replacement in Epithelial Tissues

• Epithelial cells have a high capacity for rapid repair and replacement, with cell division in target areas after significant damage.
• Stem cells may facilitate epithelial regeneration in specific locations.

Test your knowledge on the roles and characteristics of various tissue types, focusing on adherens junctions, desmosomes, gap junctions, and epithelial tissues. This quiz covers their functions, structural differences, and associated cell types, providing a comprehensive overview of histology.