Animal cell coats

In plants, fungi, and bacteria, the cell wall is en¬tirely separate from the plasma membrane, as seen under digital compound microscopes. If the cell shrinks in a hypertonic medium, the membrane separates from the much more rigid wall. By contrast, the cell coat of an animal cell is not an independent entity. The carbohydrates of which it is composed are covalently bonded to protein or lipid molecules in the plasma membrane. The resulting complex molecules, made up of carbohydrate and protein or carbohydrate and lipid components, are termed glycoproteins or glycolipids, and the cell coat itself is often called the glycocalyx.

According to recent research and using the latest digital compound microscope, it is the glycocalyx that provides the recognition sites on the surface of the cell enabling it to interact with other cells. For example, if individual liver and kidney cells are mixed in a culture medium, the liver cells will recognize one another and reassociate; similarly, the kidney cells will seek out their own kind and reassociate. Apparently some property of the glycocalyx enables the cells to distinguish liver from kidney cells in such a situation. Cell recognition in the process of embryonic development doubtless also depends, at least in part, on the glycocalyx. And the same is probably true for the control of cell growth. When normal cells grown in tissue culture touch each other, they cease moving and their growth slows down or stops altogether, as seen under digital compound microscopes. This phenomenon of contact inhibition ap¬pears to be absent in cancer cells, which continue growing without restraint, most likely because they have an abnormal glycocalyx that does not permit them to interact properly.

The glycocalyx, as studied under digital compound microscopes, also provides recognition sites for interaction be¬tween the cell and important molecules in the extracellular medium. For example, certain white blood cells in the vertebrate body are ca¬pable of recognizing cells foreign to the organism. It is thought that the carbohydrates projecting from the glycocalyx allow the invading cell to be recognized as foreign. The recognition of a host cell by an invading virus probably also depends on the carbohydrates of the glycocalyx.