Growth
From Encyclopædia
Growth in a single-celled organism may be due either to an increase in the size of the
cell or to an increased
number of
cells in one location (population growth). Thus we may speak of a bacterial
cell enlarging or that the bacterial colony is in the growth phase--an increase in
numbers. In an organism with many
cells--vertebrates, insects, and higher
plants--growth is an increase in the size of the entire organism or of one of its
organs or tissues. This growth is due either to an increase in the size of individual
cells (hypertrophy), to increased
numbers of
cells (hyperplasia), to increased production of materials between
cells whether mineral (as in bone), fibers (as in tendon or scar tissue), or the gelatinous ground substance, or any combination of increase in
cell size,
cell number, or extracellular products. Growth depends on intake of food, which is metabolically converted to suitable chemical building components. Animal
cells are not surrounded by the rigid cellulose wall found in
plants and thus retain the ability to grow and divide as well as to change position. In
plants these properties are restricted to specific growing
cells.In general, in animals, growth is chiefly due to hyperplasia (new
cells), although the increased muscle
Mass from
exercise is the result of hypertrophy.
plants, however, typically produce very small
cells which then massively enlarge, the visible growth thus being due to hypertrophy. Animal and
plant growth differ in another striking way: in most animals, the growth period ends with maturity (completed development), whereas
plants maintain embryonic tissues, called meristems, throughout their lives, and growth may occur at any time.
cell GrowthGrowth involving the production of new
cells may occur by fission, budding, or filamentous growth. In fission, the parent
cell divides into two smaller but equivalent
cells. In budding, which is typical of certain
fungi such as yeast, a small new
cell (the bud) is pinched off from the parent
cell and then grows into a duplicate of its parent. Filamentous growth, seen in certain algae, occurs when hairlike filaments of two or more algal
cells join end to end; enlargement of the tip of the filament forms new
cells.Most animal and
plant cells undergo a form of fission termed MITOSIS in which each daughter
cell receives at least one copy of each gene present in the parent, plus approximately one-half of the cellular structures (organelles) and materials of the parent. In germ
cells that form the egg and sperm,
cells undergo a process termed MEIOSIS in which the genes are divided between the two daughter
cells. Growth occurs in two phases: immediately after mitosis, the new
cell grows to adult size and remains at this size until shortly before it is to undergo mitosis. At this time, it duplicates its DNA, which contains most of the
cell's genetic information, and increases in
Mass to less than twice normal size.The first period of growth is the most variable phase of the mitotic cycle. The cytoplasm of the
cell must have a specific size relative to the size of the
nucleus before
cell division can occur, and the growth period must be long enough to achieve the proper relative proportions.
cell division can be prevented indefinitely by periodically cutting away portions of the cytoplasm or by starving the
cell.DifferentiationGrowth in animals and
plants must produce an unfolding of the shapes and structures characteristic of the organism, as well as increase its size or
Mass. The development of shapes and structures, called morphogenesis, is brought about by the coordinated action of growth and differentiation, the process by which
cells develop into such specialized types as muscle, nerve,
skin, leaf, or root
cells. In a developing organism, different centers of growth are active at different times and proceed at different rates. This pattern of growth is called differential growth and results in the "sculpturing" of various kinds of body parts. For example, when the rate and amount of growth varies in different directions, new shapes can arise, such as elongations and altering contours; a solid
Mass can become hollow if the outer layers of the
Mass grow faster than the core; and a hollow structure can become solid if the inner layers of the rind grow faster than the outer.Differential, or relative, growth in the
embryo is responsible for forming the vertebrate eye from a hollow ball of ectoderm (outermost of the three primary tissue layers of an
embryo) and for forming the heart from a spongy sheet of mesoderm (middle layer of embryonic tissue). In
plants, changes in shape result almost entirely from differential growth; the petiole (stalk) of a leaf, for example, develops at a rate different from that of the blade (see DEVELOPMENT).Differential growth stops when a certain size, proportional to the whole organism, is reached. In
mammalS, growth in overall length ends after ossification (
conversion to bone) of the epiphyseal plate (an area near the ends of a bone where new bone is made). At the same time, other
organs and tissues cease growing, indicating that the total supply of body
cells (approximately 100 trillion in adult humans) is limited by a
Central regulatory system. The constancy of size of
organs or whole organisms merely indicates that there is no net gain or loss of
cells: the death of
cells is balanced by the production of new
cells.Some animal tissues are capable of growth throughout life, whereas others lose this capacity after formation in the
embryo. In humans, red blood
cells die and are replaced by blood-forming tissue in the bone marrow at the rate of 3 million
cells per second.
cells in the
skin and in the lining of the digestive tract continually die and are renewed by growth. If a portion of the
liver is surgically removed,
cells of the remaining part multiply until the original
Mass is restored, and then growth ceases. Most nerve
cells are formed before or soon after birth and cease multiplying soon after birth.
organs or tissues forced to function at higher than normal levels may grow to compensate for the increased demands placed on them. For example, when one kidney is lost, the other undergoes compensatory growth and enlarges;
constant stress may cause enlargement (hypertrophy) of the adrenal gland, and the heart often enlarges by hypertrophy when the heart must work harder.The factors resulting in balanced growth and morphogenesis have not yet been identified, nor is it known how the different parts of a fully grown adult retain correct proportions and shapes. In a healthy body, no group of
cells dominates; an effect called contact inhibition, in addition to other factors, slows or stops the multiplication of
cells when they are crowded together and the group has reached its proper size. CANCER is a term for a class of diseases in which growth escapes normal
Controls limiting
cell multiplication; the uncontrolled growth results in tumors (masses of
cells).Allometric GrowthThe proportionate growth of different parts of the animal body is sometimes called allometric growth. The role of allometric growth in evolution was first described by the Scottish biologist d'Arcy W. Thompson in On Growth and Form (1915). Thompson found that the varied body shapes of animals belonging to the same family could be represented by ?roportionate distortions of a single basic shape, much like the stretching of a figure drawn on a rubber sheet. The distortions, or transformations, could be expressed by simple mathematical formulas. Among the
primates, for example, Thompson found that the differing
skull shapes of humans, gorillas,
chimpanzees, and
baboons, if drawn on an imaginary rubber sheet, are simply different proportionate distortions of the same basic
skull. This finding indicates, according to Thompson, that the modifications of jaws, braincase, and the regions between are regulated by a center in the body coordinating their relative growth.RegenerationAll
plants and animals have some ability to survive injury by regenerating lost tissue to heal wounds. Some animals, however, have the extraordinary ability to restore lost body parts. (See REGENERATION.) Certain lizards can regenerate entire limbs and tails, a new hydra can grow from a fragment of its original tissue, and flatworms and starfish can do the same.
plants can repair or replace many lost parts, and sometimes new
plants can grow from cuttings.Little is known about how regeneration occurs. Experiments with the salamander and
axolotl have shown that regeneration of an amputated limb occurs only if a critical
Mass of the leg nerve is present in the regenerating stump. Similar experiments with
mammals are more difficult; reportedly, however, some regrowth of the amputated hind limb of the opossum is possible under experimental conditions. It is conjectured that such experiments could eventually provide knowledge enabling medical scientists to induce regeneration of lost limbs and other structures in humans.Animal GrowthAt birth or hatching, the vertebrate is usually far from mature. The final
stages of development involve the growth of tissues and
organs generated in the
embryo. Young
mammals, for example, possess the elements of a reproductive system, but the system is not fully developed until the onset of puberty and the accompanying maturation of these existing structures (see
REPRODUCTIVE SYSTEM, HUMAN).Hormonal
Control.Animal growth requires the coordinated action of various kinds of HORMONES. The hormones active in regulating growth in higher animals are produced by glands in the ENDOCRINE SYSTEM. Two animal groups are known to possess specialized glands that synthesize and release growth factors--the insects and the vertebrates.
