Cell surface performs various physiological activities such as absorption, secretion, transportation etc. To perform such specialized function certain modification are inevitable in the plasma membrane of such cells. Cell surface differentiations include invaginations, microvilli, basement membrane and cell to cell interconnection junctions.
Certain cells such as cells of kidney perform active transportation and contain many invaginations or infoldings of the plasma membrane. At the base of these folds there develops a septa an thus narrow compartments of basal cytoplasm are formed. These infoldings contain many mitochondria. These mitochondria along with the enzymes of plasma membrane provide energy rich compound ATP , which help in active transport of the solutes.
Microvilli are finger like projections on the plasma membrane which are found in mesothelial cells, hepatic cells, epithelial cells of intestine, gall bladder, uterus, growing oocyte and yolk sac. Microvilli increases the surface area which helps in having an higher absorption rate. A single epithelial cell has 3000 microvilli. Microvilli are 0.6 to 0.8 micrometer long and 0.1 micrometer in diameter. Narrow spaces between them form a valleys through which substances may pass during the absorption process.
Within the cytoplasmic core of the microvillus fine microfilaments are observed which in the underlying cytoplasm form a a terminal web. The microfilament contains actin and are attached to tips of microvilli by alpha actinin. Their function is to produce contractions in microvilli.
Interface between all epithelial and underlying connective tissue is marked by a non cellular structure called basement membrane. The basement membrane has two basic layers
- Basal lamina- which is in contact with the epithelial basal plasma membrane and is composed of fine work of fibrils of collagen (type IV) that are embedded in a amorphous matrix but is secreted by the epithelial cells.
- Reticular layer – Exists beneath the basal lamina and is composed of fine reticular fiber of reticulum proteins. The reticular layer is synthesized by underlying connective tissue into which it is merged . The basement membrane provides structural support for epithelia and also constitute an important barrier to the passage of materials between the epithelial and connective tissue compartments.
TIGHT JUNCTIONS (ZONULA OCCLUDENS)
Junctions that prevent or reduce the flow of even small molecules between the lateral surface of adjacent cells are known as tight junctions. in higher animals they are termed as tight junctions and in invertebrates as septate junctions.
Tight junctions are situated between the apical border of the epithelial cells and act as permiability barriers. Thus all nutrients are absorbed from the intestine into one side of the epithelial cell and then released from the other side into the blood beacuse tight intercellular junctions do not allow small molecules to diffuse directly. Also tight junctions help prevent leakage of pancreatic proteins and digestive enzymes into blood.
In 3D , tight junctions appear as network of ridges on the cytoplasmic half of the membrane with complementary grooves in the outer half. The ridges appear to be composed of 2 rows of protein particles as in zipper, each one belonging to the adjacent cells. The lines of these particles produce the sealing and for this reason have been named sealing strands.
In invertebrates septate junction perform the function similar to tight junctions. They differ from the tight junctions in that the proteins that straddle the gaps occur in parallel rows or septate.
Desmosomes are abundantly found in tissues that have to withstand severe mechanical stress such as skin epithelia, bladder, cardiac muscle, the neck of uterus and vagina. their presence in such tissues allows the tissue to function as elastic sheets without the individual cells being torn one from another. desmosomes are of following types
- Belt desmosomes (Zonula adherens)
They are generally found at the interface between the columnar cells just below the region of the tight junctions. They form a band that forms a gridle around the inner surface of the plasma membrane. This band contains a web of 6 to 7 nm actin microfilaments and another group of interwoven intermediate filaments of 10 nm. Actin microfilaments are contractile and intermediate filaments play a structural roe. at belt desmosome, The plasma membrane adjacent are parallel , thicker than usual and 15-0 nm apart, the intercellular space between them is filled with an amorphous material.
2. Spot desmosomes (Macula adherens)
The spot desmosomes act like spot weld to hold epithelial cells together at points of contact. they represent localized circular area of contact about 0.5 micrometer in diameter in which the plasma membrane of two adjacent cells are separated by a distance of 30-50 nm.
The intercellular or central stratum between the 2 membranes consist of specific desmosomal material rich in proteins and monopolysacchrides. Under each facing plasma membrane of the spot desmosome there is a discoidal intracellular plaque, 15-20 nm thick, having non glycosylated protein such as desmoplakins I, II and III. Numerous 10 nm thick intermediate filaments of keratin protein called tonofilaments, form a loop in the wide arc and course back into the cytoplasm. In addition, there are thinner filaments that arise from each dense plaque and transverse the plasma membrane to form ” transmemebrane linkers” in the intercellular space. These linkers provide mechanical coupling and chemically are made of glycosylate proteins, called desmogleins I and II with the carbohydrate moiety exposed towards the intercellular space. two filaments provide the intracellular mechnical support, the cellular adhesion at desmosome depend on extracellular coating materials.
They are half desmosomes which resemble spot desmosomes but join the basal surface of an epithelial cell to a basal lamina. They anchor extracellular proteins such as collagen and other proteins to the cell.
4. Gap junctions
Many cells of the tissues of higher animals are coupled together by interconnecting gap junctions, nexus or communicating unctions. This presence of gap junctions explains the ionic or electronic connection between adjacent cells, which means that there are some cells which are electrically coupled and have regions of low resistance in the membrane through which there is a rather free flow of electrical current carried by ions. In adult tissues , it is usually found in epithelia, cardiac cells and liver cells.
Gap junctions permit molecules such as inorganic ions, sugars, aminoacids, nucleotides and vitamins to pass with comparative freedom between one cell and another within the tissue, but they prevent larger molecules such as proteins, nucleic acids and polysacchrides from being transferred. This observation also explains the phenomenon of metabolic coupling between the cells.
A gap junction appears as a plaque like contact in which the plasma membrane of adjacent cells are in close position, separated by a space of only 2 to 4 nm structurally, gap junctions consist of hollow channels around which a series of six proteins ( a macromolecular unit, called connexon) of 27000 daltons has been isolated from rat liver preparation consisting of almost pure gap junction material. A connexon appears as a annulus of 6 subunits surrounding the channels. Sliding of subunits cause the channels to open and close , which is regulated by calcium ions.