Cell Junctions and Epithelial Functions

Epithelial tissues cover surfaces and line cavities of the body. They are responsible for protection, secretion, absorption, filtration and excretion.

Epithelial tissues are classified based on the number of layers of cells and the shape of the cells in the top (apical) layer. Simple epithelia have one layer and stratified epithelia have multiple layers.

Epithelial cells have polarity, meaning they have distinct regions with different functions. The apical domain faces the lumen or external environment, the lateral domain interacts with adjacent cells, and the basal domain interacts with the basement membrane.

The basement membrane is a thin, specialized structure that supports and anchors epithelial tissues. It's composed of two layers: basal lamina and fibrous reticular lamina.

The basal lamina is composed of type IV collagen, laminin, nidogen, and perlecan. These molecules provide structural support and regulate epithelial cell behavior.

Tight junctions, adhering junctions, and communicating junctions connect epithelial cells. Tight junctions prevent leakage, adhering junctions provide strong attachment, and communicating junctions allow communication between cells.

Tight junctions are the most apical junctions. They are composed of transmembrane proteins called claudin, which create a seal to prevent leakage between cells.

Adhering junctions, specifically zonula adherens and desmosomes, provide strong attachment between epithelial cells. They are composed of various proteins, including cadherins.

A type of cell junction that forms a tight seal between adjacent cells, restricting fluid movement. It involves integral proteins like claudin and occludin that fuse together, creating a continuous belt-like structure.

A type of cell junction that encircles epithelial cells, anchoring them firmly to their neighbors. It lies below tight junctions and involves cadherin proteins interacting with calcium ions (Ca2+) to 'zip' the cells together.

Cadherin proteins in adherens junctions are linked to actin filaments via catenin and actin-binding proteins, forming electron-dense plaques on the cytoplasmic side.

Disk-shaped structures at the surface of adjacent cells, connected via cadherin proteins. They form spot-like adhesions on cell membranes and bind intermediate filaments, providing strength to tissues.

Transmembrane proteins, mainly integrins, that bind to extracellular molecules like laminin and collagen type IV in the basal lamina, connecting epithelial cells to the underlying tissue.

A network of interconnected protein filaments that provides structural support and helps maintain cell shape. It is composed of various types like actin filaments, intermediate filaments, and microtubules.

Passageways between adjacent cells that allow the transfer of small molecules between adjacent cells. They are formed by pairs of abutting particles (connexons), which are each composed of six protein subunits (connexins) that span the lipid bilayer of each cell membrane.

Small finger-like projections found on the luminal surface of many epithelia. They are shorter and narrower than cilia.

Proteins that bind to actin filaments, helping to link them to other components of the cell, such as cadherins in adherens junctions.

A structure formed by microvilli on the apical surface of some epithelial cells, such as the small intestine.

Specific locations where cells are attached to each other or to the extracellular matrix, playing crucial roles in tissue organization and function.

A type of microvilli that are immotile and longer than microvilli. They are found in the epididymis, the proximal part of the ductus deferens of the male reproductive system, and the sensory (hair) cells of the inner ear.

Long, highly motile, hair-like extensions of apical plasma membrane. They are larger than microvilli and are made up of microtubules.

Cilia are classified into three basic categories.

Loss of microvilli brush border of absorptive cells. It is an immune reaction against gluten.

Each cilium contains nine pairs of microtubules forming the outside of a ring, and two central microtubules ('9+2').