Describe the cellular and molecular features of tight junctions, adherens junctions, desmosomes, and gap junctions in epithelial tissue.

What are the cellular and molecular features of tight junctions?

Encircle the apical region of epithelial cells. The lipid portions of two membranes become tightly bound by interlocking membrane proteins. Inferior (beneath) to this is the adhesion belt. They prevent water and solutes passing between the cells and isolate the contents of the lumen from the basolateral surface of the cell.

What are the cellular and molecular features of adherens junctions?

Typically found close to the apical surface (though can be near the basal membrane). Cadherins (transmembrane receptors) form bridges between cells through homophilic interactions. The inner membrane associates with actin microfilaments, which form a plaque (like an anchor or spring toggle). The entire structure forms an adhesion belt, joining two cells together.

What are the cellular and molecular features of desmosomes?

Very strong; resist stretching and twisting, acting as a 'spot weld'. Disk-shaped structures that connect to filaments in the cytoskeleton. Spot desmosomes: two disks connected to bands of intermediate filaments which stabilize the shape of the cell; one on each cell, with cell adhesion molecules interacting with the membranes of the neighbouring cell. Hemidesmosomes: only one disk, attaching the cell to extracellular filaments in the basal surface, down to the basement membrane.

What are the cellular and molecular features of gap junctions?

Two cells are held together by connexons — transmembrane proteins made of six smaller proteins called connexins. Two aligned connexons form a narrow passageway between cells, allowing diffusion of ions and small molecules between them. Voltage, pH, Na+ concentration, modifications to connexins, and other changes in the cellular environment influence how open the channel is, controlling this diffusion.

What is the role of tight junctions in maintaining epithelial integrity and homeostasis?

They prevent water and solutes passing between the cells, and isolate the contents of the lumen from the basolateral surface of the cell.

What is the role of adherens junctions in maintaining epithelial integrity and homeostasis?

They join two cells together via an adhesion belt, providing cell-to-cell adhesion that maintains the integrity of the protective epithelial layer.

What is the role of desmosomes in maintaining epithelial integrity and homeostasis?

They provide very strong adhesion, resisting stretching and twisting ('spot welds'), and connect to intermediate filaments to stabilize the shape of the cell.

What is the role of gap junctions in maintaining epithelial integrity and homeostasis?

They form a narrow passageway allowing diffusion of ions and small molecules directly between cells, with channel openness controlled by the cellular environment.