The human body is composed of roughly 75 × 1012 cells (= 75000 billion cells), of which as many as 25 × 1012 (25000 billion) occur as erythrocytes in the blood and which therefore constitute the commonest type of cell. Of the remaining cells, 100 × 109 (= 100 billion) are part of the nervous system. Since the number of cells is so great, each individual building block must be microscopically small. The size of each cell varies in the human body between 5 μm (e. g., single connective tissue cells) and 150 μm (the ovum of the female). When cell processes are included, however, some cells can reach considerable lengths; for example, nerve cells that run from the brain to the spinal cord attain lengths of up to 1m. The shapes of the various cells also vary considerably. Ova are round, connective tissue cells form processes, and other cells are spindleshaped (muscle cells), flat, cuboid, or highly prismatic (epithelial cells). Size and shape are often closely linked to a cell’s specific properties.
Properties
All cells have a number of basic properties in common, even if they are differentiated to carry out specific tasks.
Metabolism and the Generation of Energy
Every cell possesses a metabolism, by which absorbed substances are changed into compounds that serve the organization of the cell and are discharged in the form of end products. Therefore, in order to maintain the normal functions necessary for life, cells require nutrients fromwhich they acquire the energy for their tasks. The chemical processes that take place during the transformation of nutrients (fats, proteins, and carbohydrates) to generate energy are basically the same in all cells, as also is the release of end products into the fluids surrounding the cells.
Reproduction and Life Expectancy.
With few exceptions, almost all cells have the capacity to reproduce themselves by dividing. This property is often retained throughout life and is the prerequisite for the replacement of dead cells and the regeneration (restoration) of tissues and organs after injury. The human bone marrow, for instance, creates about 160 million red blood cells per minute, and in the male the testes create about 85 million sperm cells daily. Another instance of a high rate of cell division is given by the cells of the mucous membrane of the small intestine, which have an average life expectancy of only a few days (30− 100 hours). Yet other cells divide only in certain phases of development and subsequently survive for life, e. g., nerve and muscle cells.
Sensitivity to Stimulation and Response to Stimulation.
Almost all cells are connected to their immediate environment by specific structures on their surfaces (e. g., receptors) and can sense, evaluate, and respond to distinct stimuli.
Besides these basic properties, certain cells possess specific properties. These may include mobility (e. g., histiocytes in connective tissue; male sperm in the female genital tract), the assimilation and elimination of substances (e. g., assimilation of cell debris by defense cells; secretion by glandular cells), or the development of specific surface differentiations (e. g., cilia on the mucous membrane cells of the respiratory tract; brush border of the mucous membrane cells of the small intestine).
