CELL. — In biology, the term *cellula* (cell) designates the smallest vital unit capable of independent life and reproduction in metazoans and metaphytes, i.e., multicellular animals and plants. Many microscopic organisms, such as the Protists (Protozoa and Protophyta), are unicellular, though in reality they possess a far more complex organization than a simple cell. Cells are masses of protoplasm (v.), mostly microscopic in size (ranging from a few microns, or thousandths of a millimeter, to about a hundred), and of elementary shape—spherical, subspherical, or polyhedral—equipped with an external limiting membrane (primary membrane), which may be more or less thick and distinct, or reduced to a film of surface tension. They also contain a nucleus, usually spherical, and various organelles. In some cases (plant cells, egg cells), the cell is enclosed by a shell or secondary membrane (cellulose in plant cells and the zona pellucida in eggs). The protoplasm surrounding the nucleus is called cytoplasm, while that within the nucleus is termed karyoplasm.
The cell was discovered in 1665 by Hooke in cork and observed in animals and plants by Leeuwenhoek (1632–1723). In 1675, Malpighi referred to cells as "utricles" and considered them the fundamental constituents of plants.
In 1831, Brown identified the nucleus as a fundamental component of cells, and in 1850, Mohl observed cell division.
The structure of the nucleus gained significant importance when its role in hereditary phenomena—particularly through its constituents, the chromosomes (v.)—was recognized. These small bodies, usually rod-shaped or V-shaped, are clearly visible in the nucleus during cell division. They are not observed in the resting nucleus, though some consider the axial filament or chromonema to be present, as this condition is deemed highly significant by some for explaining the transmission of hereditary traits. The number of chromosomes is constant in all cells of individuals of a given animal or plant species, whether in a simple (haploid) or doubled (diploid) state.
From a purely structural standpoint, in the resting nucleus, chromatin clumps can be seen immersed in the nuclear sap. Chemically, chromatin consists of nucleoproteins and ribo- and deoxyribonucleic (or thymonucleic) acids. The cytoplasm contains other organelles and structures: the chondriome, composed of granules or filaments made of phospholipoproteins, which plays a crucial role in many cellular functions, including secretion processing and the formation of differentiated structures; the ergastoplasm, the densest part of the cytoplasm, considered by some authors as "superior cytoplasm" (Garnier); the sphere apparatus, which during cell division divides into two parts that move to opposite poles. The Golgi apparatus, the lacunoma, paranuclei, and other formations are also of great importance in the expression of the cell’s vital phenomena.
The structure of the primary cell membrane is of particular significance because, despite possessing the physical attributes of membranes, it exhibits special properties—best described as vital—which include selective permeability, a capacity no physical membrane possesses.
Chemically, the fundamental cytoplasm is a complex of proteins and lipids (lecithins and sterols) imbibed with water and containing mineral salts.
All cells, by virtue of being living entities, possess fundamental characteristic properties, including assimilation—the ability to transform heterogeneous substances into those similar to their own constitution.
of fabricating, that is, new living substance proper: disassimilation, i.e., the property of breaking down synthesized organic compounds and releasing energy; reproduction, i.e., the ability to divide into two daughter cells after a period of growth; excitability, i.e., the capacity to perceive stimuli from the external world (sensitivity) and to react in various ways (reactivity) through movements, secretions, production of light, color, etc. Through division of labor, individual functions are enhanced in various cell groups that become specifically modified, or differentiated; in this way, the various tissues are formed. It is the task of histology to study tissues. Through histological differentiation, cells assume the form and structure best suited to their specific function, losing the subservient and polyhedral appearance of undifferentiated embryonic elements.
Among differentiated cells, the following deserve mention: red blood cells, which are hydrodynamic in shape and capable of moving through the circulatory stream; in mammals, to increase the surface area for gas exchange, they assume a biconcave disk shape and lose their nucleus to accommodate a greater quantity of respiratory pigment, hemoglobin; nerve cells, which produce extremely long extensions capable of reaching, through various tissues, peripheral sensory or receptor organs, or reactive or effector organs such as muscles and glands; smooth muscle fibers, which are extremely elongated in the direction of contraction and in whose cytoplasm contractile myofibrils differentiate. Often, to achieve functional unity, the individuality of cells is lost through the fusion of various cells into multinucleated syncytia: such are, for example, the striated muscle fibers of skeletal muscles and the heart, many connective elements, etc. With the acquisition of specific differentiation, cells lose the ability to reproduce and are therefore sooner or later destined to be worn out by work and destroyed, as in the case of epidermal cells (desquamation) and blood cells. It is therefore necessary that in these tissues young reserves remain to replace cells that die: such is the germinal layer for the epidermis and the bone marrow for blood cells. These tissues have the same significance as cambial tissues in plants.
However, not all tissues of the organism have such intense physiological wear. To these labile tissues (according to Bizzozero's classification) correspond stable-element tissues and permanent-element tissues. In the former, reproduction continues until the end of body growth and then stops (e.g., in the liver); in the latter, reproductive capacity stops in the early stages of embryonic development, and the same cells accompany the organism throughout life. A typical permanent-element tissue is nervous tissue, the tissue in which differentiation reaches the highest degree so that individual nerve cells with specific structure assume a specific function of characteristic physiological significance. The nerve cells of higher forms, and especially of man, are repositories of the highest psychic manifestations, and it is well understood how conditions of education and memory can only be reconciled with the permanent condition of these cells. On the other hand, it is well known how the destruction of particular nervous territories nullifies certain specific activities while other nervous functions remain intact; and if nerve cells are destroyed, the functions linked to them can no longer be restored.