CELLULA. - In biology, this is the name given to the smallest vital unit, capable of autonomous life and of reproduction of the metozoa and metaphyta, animals and multicellular plants. Many microscopic organisms consisting of a single undivided mass of living matter, the Protists (Protozoa and Protophyta), are considered unicellular, but in reality such organisms have a much more complex organization than that of a cell. Cells are small masses of protoplasm (v.) mostly microscopic in size (from a few microns, or thousandths of a millimeter, to about a hundred) and of elementary spherical, subspherical or polyhedral shape, provided with an external limiting membrane (primary membrane), which may be more or less thick and evident to the point of being reduced to a film of surface tension, with a nucleus, mostly spherical, within, and with organelles. Sometimes (plant cells, egg cells), the cell is surrounded by a shell or secondary membrane (cellulose of plant cells and pellucid membrane of the egg). The protoplasm surrounding the nucleus is called cytoplasm and that of the nucleus karyoplasm.
The cell was discovered in 1665 by Hooke in cork and seen in animals and plants by Leeuwenhoek (1632-1723); Malpighi in 1675 considers the cells, which he calls utricles, as the fundamental constituents of plants.
In 1831 Brown considers the nucleus a fundamental constituent of cells; Mohl in 1850 observes cell division.
The structure of the nucleus has assumed great importance ever since its role in hereditary phenomena was seen, especially through the action of its constituents: the chromosomes (v.). These small bodies, mostly in the form of straight or V-shaped rods, are observed very well in the nucleus at the moment when the cell is engaged in division. They are not observed in the nucleus at at rest, although some wish to consider the axial filament or chromonema as present, since this is a condition, for some a very important one, for explaining the transmission of hereditary characters. The number of chromosomes is constant in all the c. of the individuals of a given animal or plant species, in simple (haploid) or doubled (diploid) complement.
CELLULA. - Some types of differentiated c.: 1. ovum (egg cell); 2. amoebocyte (leukocyte); 3. cubical epithelium; 4. squamous epithelium; 5. erythrocyte; 6. nerve cell; 7. spermatozoon; 8. connective tissue cell; 9. " fetchpriority="high" />From a purely structural point of view, in the resting nucleus one sees clumps of chromatin immersed in the nuclear sap. Chemically, chromatin is formed of nucleoproteins and of ribo-and deoxyribonucleic (or thymonucleic) acid. In the cytoplasm are found other organelles and structures: the chondriome, formed of granules or filaments made up of phospholipoproteins, which is of great importance in many manifestations of cells, among them the elaboration of secretions and the formation of differentiative structures; the ergastoplasm, the denser part of the cytoplasm considered by some authors as superior cytoplasm (Garnier); the sphere apparatus, of radiate form, which during cell division divides into two parts that move to the poles. The reticulum or Golgi apparatus, the lacunome, the paranuclei, etc. represent other formations of great importance for the manifestation of the vital phenomena of the cell. Of particular importance is the structure of the primary cell membrane because, although it has the physical attributes of physical membranes, it has special properties, which we can only define as vital, such as the power of choice that no physical membrane possesses.
From a chemical point of view, the fundamental cytoplasm is a complex of protids and lipids (lecithins and sterols) imbibed with water and containing mineral salts.
All c., by the very fact of being living, have fundamental characteristic properties. They are: assimilation, that is, the capacity to transform heterogeneous substances into substances similar to one's own constitution, that is, of building new living substance of its own; disassimilation, that is, the property of breaking down the organic compounds that have been synthesized and of releasing energy; reproduction, that is, the capacity to divide into two daughter cells after a period of growth; excitability, that is, the capacity to perceive stimuli from the external world (sensibility) and to react in various ways (reactivity) with movements, secretions, production of light, color, etc. Through the division of labor the individual functions are heightened in various groups of cells that modify themselves in a specific way, that is, they differentiate; thus the various tissues come to be constituted. The study of tissues is the task of histology. Through histological differentiation the cells come to assume the form and structure best suited to their specific work, losing the subspherical and polygonal appearance of undifferentiated embryonic elements.
CELLULA. - Undifferentiated plant cell (1) and animal cell (2), m. cellulose membrane; c. chloroplasts; e. vacuoles; n. nucleus; d. deutoplasm (reserve substance of the egg); ci. cytoplasm; co. chondriome; ce. centro" loading="lazy" />Among the differentiated cells, mention should be made of: the red blood corpuscles, of hydrodynamic form, suited to running in the circulatory stream which, in order to increase the surface for gaseous exchanges in mammals, assume the form of biconcave discs and lose the nucleus so as to be able to contain a greater quantity of respiratory pigment, hemoglobin; the nerve cells, which produce processes, some of them very long, suited to reaching, through the various tissues, the peripheral sensory organs or receptors, or the reactive organs or effectors, such as the muscles and the glands; the smooth muscle fibers, greatly elongated in the direction of contraction, in whose cytoplasm contractile myofibrils differentiate. Often, for the attainment of functional units, the individuality of the cells is lost through the fusion of various cells into multinucleate syncytia: such are, for example, the striated muscle fibers of the skeletal muscles and of the heart, many connective elements, etc. With the acquisition of specific differentiation the cells lose the property of reproducing and are therefore destined sooner or later to be worn out by work and to destroy themselves, as in the case of the epidermal cells (desquamation) and of the blood corpuscles. It is therefore necessary that in these tissues there remain young reserves so as to replace the cells that die one after another: such is the germinative layer for the epidermis and the bone marrow for the blood cells. These tissues have the same significance as the cambial tissues of plants.
But not all the tissues of the organism have such an intense physiological wear. These labile tissues (according to the classification of Bizzozero) are matched by tissues with stable elements and tissues with perennial elements. In the former, reproduction continues until the end of bodily growth and then stops (for example, in the liver); in the latter, the reproductive capacity stops in the early stages of embryonic development and the same cells accompany the organism throughout its life. A typical tissue with perennial elements is nervous tissue, the tissue in which differentiation is carried out to the highest degree, so that individual nerve cells with a specific structure assume a specific function of characteristic physiological significance. The nerve cells of the higher forms and above all of man are the depositaries of the highest psychic manifestations, and it is easy to understand how the conditions of education and memory can be reconciled only with the perennial condition of these cells. It is known, on the other hand, how the destruction of particular nervous territories annuls certain particular activities while the other nervous functions remain unharmed; and if the nerve cells are destroyed, the functions bound to them can no longer be restored.