Cytology
Cytology is the science about a cell.
The cell
The cell is the basic structural and functional unit of all multicellular organisms, limited to an active cell membrane and consisting of cytoplasm and a nucleus. The cell is open, automatically adjusting and self reproductive system.
The cytoplasm is a complex colloidal system. The cytoplasm of a cell consists of organelles, inclusions and hyaloplasm. The structure of hyaloplasm includes water, proteins, nucleinic acids, different polysaccharides and a lot of enzymes. The colloid system of hyaloplasm can be in a liquid or gel consistence.
Plasma membrane or plasmalemma
Figure 1. Diagram of a plasma membrane
The
cell membrane (or plasma membrane or plasmalemma) is an universal
system
which form many structures of a cell. Each cell is bounded by a cell membrane. The structure of each membrane includes proteins, lipids and carbohydrates. Proteins make 50-60% of its weight, lipids 30-40 % and carbohydrates 5-10%. Cell membrane is about 7, 5 nm thick and appears as a trilaminar structure, of two thin, dense lines and light area in the middle. The inner and the outer dense line composed of a single layer of phospholipids, between them settle some proteins. Carbohydrates are present at the surface of the plasma membrane. This layer is referred to as the cell coat or glycocalyx.
The plasmalemma and other membranes of a cell carry out some of the important functions:
Barrier. The plasmalemma separates a cell from an environment and other cells; the nucleus is separated from cytoplasm, membranes organelles from hyaloplasm.
Receptor on a surface of plasmalemma. There are special structures-receptors due to which the cell finds out different chemical substances, physical factors, other cells, hormones and antigens.
Transport-through a membrane of cell freely passes water, salts and substances with low molecular weight. Such transport is called passive. The transport against a gradient of concentration is called active.
Intercellular connections
1. Simple contact – membranes of two cells are on distance of 10-12 nm in such manner that glycocalyx one cell adjoins with glycocalyx of another cell. The basic function is metabolism and information interchange between cells.
2. Zonulae occludentes — also called tight junctions. Zonula occludens are located between adjacent plasma membranes most typically near the apices of epithelial cells. They form a "belt-like" junction that encircles the entire circumference of the cell. These junctions act as barriers that prevent the movement of molecules into the intercellular spaces.
3. Zonular adherentes are band-like adhesion. This device surrounds the cell and joins it to its neighbors.
4. Desmosomes (Maculae adherens). This is the most common type of tight junction between adjoining cells. A desmosome is a small circumscribed area of attachment – attachment plaques. At the side of a desmosome the plasma membrane (of each cell) is thickened because of the presence of dense layer of protein on inner surface. Desmosomes are serving to attach the basal cell membrane to the basal lamina.
5. Gap junctions, also called communicating junctions, are regions of intercellular communication.
They are widespread in epithelial tissues, in cardiac muscle smooth muscle cells and neurons. Gap junctions are built by six closely packed transmembrane proteins connexins that assemble to from structures called connexons. The two connexons fuse, forming the functional intercellular communication channel. The hydrophilic channel permits the passage of ions, small molecules and hormones.
6. Plasma membrane enfoldings of the basal plasma membrane increase the surface area available for transport. The basal surface of some epithelia, especially those involved in ion transport, possesses multiple enfoldings of the basal plasma membrane. These enfoldings partition the basal cytoplasm and many mitochondria into the finger-like enfoldings.
7. Synapse - type of contact between two nervous cells or between a nervous cell and a muscle. Through synapses pass nervous impulses.
The cytoplasm contains organelles and inclusions in a cytoplasmic matrix.
Organelles constantly present in a cell.
Inclusions are changeable ingredients of cytoplasm.
Organelles are described as membranous and nonmembranous.
The membranous organelles include:
-endoplasmic reticulum
-mitochondria
-Golgi apparatus
-lysosomes
-peroxisomes
The nonmembranous organelles include:
-microtubules
-filaments (different varieties)
-centrioles
-ribosomes
Membranous organelles are the most part of organelles of a cell.