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Cell and its Components

With the introduction of Microscope for the study of biochemical materials, the importance of cell as the basic building unit of body has been recognized. All cells engage in a number of activities that are designated as vital functions by classical cytologists. Cells use similar mechanisms to synthesize proteins, to transform energy, to move the essential substances into them, utilize the same kind of molecules to engage in contraction and they duplicate their genetic materials in the same manner.

All cells are built upon a similar basic plan, though different cells of the body possess striking features which readily distinguishes them from one another.

Each cell is remarkably independent. Each receives a supply of food stuff and oxygen from blood with which it produces its own structural components. The secretions released, form energy which is required for mechanical, chemical or electrical work. Each cell possesses an outer limiting membrane, plasmalemma and within its protoplasm there is another limiting membrane which encloses the nuclear material i.e. nuclear membrane.

Nucleus:

It contains the genetic material of cell.

Size:

3-10 um in diameter

Position:

Most cells possess a single central nucleus. The basal nuclei of some columnar epithelial cells and eccentric nucleus of plasma cell are obvious exceptions. Cells with more than one nucleus are called giant or multinuclear cells e.g. osteoclast.

Number:

Usually only one per cells but certain cells may be binucleate (e.g. some liver cells and some superficial cells of transitional epithelium) or multinucleate (e.g. skeletal muscle cells). In humans the only non-nucleated cells are platelets and erythrocytes. Both have limited existence and neither is capable of reproduction.

Structure:

It is surrounded by a membrane called nuclear membrane or envelope. Protoplasm of nucleus is called nucleoplasm containing the chromosomes in the form of a basophilic material known as chromatin, and one or more darkly staining rounded bodies called nucleoli.

Nuclear Envelops:

Under electron microscope it consist of two closely opposed unit membranes, about 40 nm thick, Small openings in the envelop called nuclear pores are present at various points (60 nm in diameter) which serve as channels in and out of nucleus especially for large molecules of RNA and proteins.

Chromatin:

Under routine hematoxyline and eosin (H&E) staining, chromatin appears as basophilic (purple) fine to coasre granules. Chromatin is nothing but chromosomes.

Heterochromatin:

Heterochromatic represents parts of chromosomes which are folded and coiled and form little masses.

Euchromatin:

Those parts of chromosomes which are uncoiled, are invisible under light microscope and constitute what is called extended chromatin or euchromatin. The euchromatin is active in Ribronucleic acid (RNA) synthesis while hetrochromatin is not.

Nucleolus:

Nucleoli are the sites of ribosomal RNA synthesis and ribosomal assembly.

Cytoplasm:

The structural components of the cytoplasm have been traditionally classified as organelles, inclusions and cytoskeletal components.

“Organelles”, the little organs of the cell, are metabolically active internal structures, carrying out essential specific function within a cell.

“Inclusions” are metabolically inert accumulations of metabolites and cell products such as carbohydrates, proteins, lipids, various crystals, pigment deposits and secretory granules etc. (storage components of the cell). In contrast to organelles, inclusions are dispensable and often are temporary constituents of cells.

Cytoskeletal components are regarded as fibrillar elements of cytoplasmic matrix that escaped detection with light microscope (as organelles and inclusions).

The portion of the cytoplasm which surrounds the organelles and inclusions is referred to as cytosol, the cytoplasmic ground substance or cytoplasmic matrix.

Many of the organelles are membrane limited structures. In addition to membrane limited organelles, the cell also contains organelles that are not surrounded by membranes.

Plasma Membrane (Cell Membrane):

Electron microscope shows that all the cells are surrounded by a plasma membrane, which is 7.5nm in width. It is invisible by light microscope. It may be seen, if sectioned obliquely, thus increasing its thickness or if its staining is enhanced by the presence of associated materials on its surface.

All membranes including intracytoplasmic membranes as well as the cell membranes are composed mainly of lipids and proteins with a small amount of carbohydrates. The thickness varies a little by cell type and internal membranes are usually somewhat thinner than the plasmalemma.

With high magnification of suitably prepared material, cell membrane can be resolved into two electron dense laminae of 2.5nm thickness, separated by a clear space (Lucent intermediate layer) of about 2.5nm (Figure 2.3). This trilaminar structure first described by Robertson is known as unit membrane and it appears to have no pores that have been postulated to explain the observed permeability of the cell membrane.

In summary, the cell membrane is a complex lipoprotein structure. It can consume energy and change its shape in response to stimuli. It contains protein molecules which are heterogeneous, some being responsible for receptor sites, other for antigenicity and enzyme activity etc.

The cell membrane differs from other membranes in that its external surface is covered with glycoprotein, the cell coat or glycocalyx. It varies greatly in thickness but can be demonstrated in some cells by Periodic Acid-Schiff (PAS) technique e.g on the luminal surface of the intestinal epithelium where it is associated with small finger like projections of cell surface called microvilli.

Functions

Cell membrane is a very important structure since it forms an interface between the cell cytoplasm and the interstitial fluid. Its functions are as under:

•     Movement:

Amoeboid movements and phagocytic activity of white blood corpuscles is well known. Folds of membranes have been observed to entrap droplets of fluid by pinocytosis.

•     Cell recognition:

Cell membranes are associated with antigens by which the body is able to recognize its own cell and tolerate them. Cells from another individual are regarded as aliens and are attacked by immune response, which they provoke.

•     Receptor function:

Many agents act on a cell at specialized points or cell receptors. The presence of specialized receptors on a particular cell is indeed the explanation of how agents such as hormones and drugs act only on their target cells and non other. The influenza viruses attach themselves to specific receptors on red cell envelope.

•      Cell Growth

The phenomenon of contact inhibition is a function of cell surface e.g. treatment with proteolytic enzymes appear to alter the cell membrane and also remove components involved in the control of cell growth.

•      Cell adhesion

The cell membrane is concerned with adhesiveness which is a factor that induces cells of like constituency to stick together. If the cells of an embryo are separated from each other and are then allowed coming together again, they aggregate to forms organs or tissues.

•      Transfer functions

All substances that enter or leave the cell protoplasm must cross the cell membrane and the properties of this membrane are responsible for the peculiar chemical composition of the cytoplasm. Chemicals soluble in organic solvents enter cell much easier than do those which are water soluble.