The increase in mass or dimensions of an organism with time. It is one of the basic characteristics of living things and represents the visible result of a complex and interrelated series of metabolic and developmental events. One of the unique features of biological growth is that the organism changes in size and shape, and to some extent in chemical composition, although it still retains its integrity and its individuality. This is true because growth fundamentally involves synthesis by the organism of more materials like itself.
Growth occurs by two main processes: increase in the number of cells and increase in the size of cells. Living cells, removed from the body and placed in an appropriate culture medium, display growth in its most uncomplicated form. They grow by synthesizing new protoplasm, dividing into smaller cells, and then repeating the process over and over as long as essential nutrients are supplied and accumulation of deleterious waste products is prevented.
In the intact animal, growth begins at, or soon after, the initiation of development. It involves cell division and synthesis of new protoplasm from raw material, either contained in the egg or derived from the environment. However, the process is much more complex than the growth of cells in tissue culture. Except in early embryonic development, growth is normally never dissociated from such processes as differentiation (diversification of cell structure and function) and morphogenesis (change in the form and pattern of the embryo). In later development, growth by cell enlargement is the predominant process. See also Animal morphogenesis;
The vast majority of animals, including most protozoa, require already elaborated organic molecules in order to synthesize new living material during growth. For animals in general, the chemical requirements for growth are (1) water, (2) inorganic substances, (3) organic substances, especially certain amino acids, and fatty acids, and (4) accessory factors or vitamins. See also Axenic culture.
Water is important to all living cells as the solvent and vehicle through which essential substances enter and metabolic waste products leave. The cells composing the growing embryo are no exception. The embryos of animals that develop in an aquatic medium, both fresh-water and marine, take up considerable amounts of water as the nonliving, stored components of the egg (yolk) become hydrolyzed and synthesized into the living and nonliving components of the embryo. See also Absorption (biology).
Inorganic substances are needed in large quantities for the formation of skeletal and other supporting structures. In both invertebrates and vertebrates, silicon, calcium, magnesium, carbon as carbonate, and phosphorus as phosphate are extremely important. Salts of sodium, potassium, calcium, and magnesium are essential components of body fluids. They contribute to the osmotic properties of the body fluids and provide a milieu in which cells, tissues, and organs can function properly. Some trace elements, found in protoplasm in only minute amounts, are essential for growth. Many are probably involved in the formation and action of enzymes and enzyme cofactors. See also Enzyme; Osmoregulatory mechanisms.
The organic compounds used by animals as raw materials for growth are carbohydrates, proteins, and fats, or their breakdown products. Carbohydrate apparently is not essential; laboratory animals can grow in the complete absence of carbohydrate. Fat, as such, is probably also nonessential, although some animals need certain unsaturated fatty acids to sustain growth. However, carbohydrates and fats are major sources of energy during the process of growth. See also Carbohydrate; Lipid.
Proteins represent the chief organic constituent of living tissues and are the most important raw material for growth. During growth, the proteins stored in the egg or provided as food from outside are digested into their constituent amino acids. These are then resynthesized into the substance of the living cells. The proteins synthesized by the organism have specific characteristics which depend upon the kinds and numbers of amino acids they contain; therefore not all proteins are equally capable of supporting growth. Proteins that fail to induce growth are deficient in one or more essential amino acids (amino acids needed by the organism but which it cannot synthesize). See also Amino acids; Nutrition; Protein.
In addition to the materials used to synthesize the bulk of the protoplasmic system, animal organisms also require certain accessory substances in order to grow. Some of these are now known as vitamins. See also Vitamin.
The growth of vertebrates is influenced by a specific growth hormone produced by the anterior lobe of the pituitary gland. Secretions from the thyroid gland, the adrenal cortex, and, in some instances, the gonads also affect growth, but perhaps less directly. Growth in various arthropods is controlled by hormone action. Molting, an essential prelude to growth in insects and crustaceans, is under hormonal control. See also Endocrine system (invertebrate); Endocrine system (vertebrate).
Growth determines not only the size of the animal but also its shape and form. As long as an animal grows at the same rate along all its dimensions, it will not change in bodily proportions. However, when the growth rate in certain directions is different from that along others, or when one or more parts of the growing organism develop more rapidly or more slowly than others, progressive changes in form result. Such relative growth is known as allometric growth, heterogony, or heterauxesis.
The form changes produced by alterations in body proportion are well illustrated in human development. At the second month of fetal life, the head and neck account for almost one-half the total volume of the fetus; at birth the relative size of the head is only 32% of the body; at maturity it is 10%. Conversely, at the same three stages, the legs comprise, respectively, 2%, 16%, and 29% of the total body volume. During this developmental span the relative size of the trunk remains constant at approximately 50% of body volume. Obviously, the growth of the head during prenatal development after the second fetal month is relatively smaller and that of the legs relatively larger than growth of the body as a whole.
There are many factors involved in the regulation of growth, some intrinsic, some environmental. One important factor is the histological differentiation of the cells of the organism; as differentiation proceeds, the rate of growth declines. Heredity is important in determining the limit and the rate of growth. Tall parents give rise to tall children, and the offspring of toy terriers and mastiffs grow to characteristic size. Hormonal influences also affect growth, as do such environmental factors as nutritive level, temperature, and degree of crowding. In most organisms, growth ceases at maturity, but in some, growth continues throughout life. Complete cessation of growth when the adult stage has been reached occurs mainly in terrestrial animals.
