Question

What types of cell junctions are found in epithelial tissue?

Answer 1

Tight Junctions, Adhering Junctions and Gap Junctions

Explanation:

Tight Junctions : Tight Junctions are when the borders of two cells are fused together along a system of surface ridges, thus creating a belt around the cell perimeter. The regions are often so tightly together that the plasma membranes are sealed. This tightness is created when proteins called occludin interact with each other in membranes of adjacent cells.

The function is to completely stop macromaterials and heavily limit water small molecules such as water and electrolytes from passing across the epithileum. A complete tight junction across the whole cell is a zonula occludens, while a tight junction on only parts of the cells is called a fascia occludens.

Adhering Junctions: There are two, zonula adherens and macula adherens. Zonular adherens lie just below the Tight Junctions, and are themselves a strong, belt-like junction around the perimeter of the cells. In the intercellular gap (`~~`15-20nanometres), proteins called cadherins interact with adjacent cells to "zip up" the gap. Inside the cell, microfilaments join up the adhesion junctions in circumferential "margin" bands.

Macular adherens, or desmosome, is a spot junction below the zonular adherens. It is a series of junctions spread unevenly around the cell perimeter. The opposed membranes hold electron-dense plaques, which serve as the attachment points for bundles of tono or intermediate filaments. Between the two plaques is a fine electron-dense line, which represents transmembrane linkers made of cadherins extending across the intercellular gap. Additionally, Hemidesmosomes (half desmosomes) connect the basal surface of of epitheal cells to the underlying basal lamina.

Gap Junctions: Also called nexuses, gap junctions are essentially small, circular communication junctions that are opposed cell membranes with small gaps between. To bridge those gaps a group of protein molecules called connexins form a structure called a connexon, which are shaped like small tubes. When one cell connexon aligns with another cells connexon, it creates a continous channel. This channel permits direct transfer of low molecular weight nutrients and intracellular messengers like cAMP (smaller than 1000kDa), but not proteins, sugars or nucleic acids.

http://www.iupui.edu/~anatd502/lecture.f04/epithelia.f04/Epithelial%20Tissue%2007.htm

http://www.histology.leeds.ac.uk/tissue_types/epithelia/epi_cell_junctions.php

I hope I helped!