Growth

Growth implies development, from the time of emergence or birth to the time of maturity and for many species, beyond maturity to eventual senescence or death. Growth also implies increase in size resulting from cell multiplication and cell expansion, as well as maturation of tissues. However, growth, while accentuating increased cell number and size, also necessitates programmed cell death, leading to the production of the final body form. Thus, growth is an incredibly complex phenomenon, which involves changes in body form, metabolism , and body processes.

Patterns of Growth

In most animals, the growth pattern follows an S-shaped curve. Slow early growth occurs from first emergence, or birth, which is followed by a long phase of rapid increase in body mass and maturation of organs, especially structural or somatic tissue that support the individual, up to about the time of puberty or reproductive maturity. Finally, growth slows, and in some species stops altogether after reproductive maturation. In many animals and most plants, however, growth continues throughout life, so that the oldest individuals in the population are generally the largest.

In many animals, young emerge looking like miniature adults, and gradually enlarge throughout their lifetime, going through alternating stages of rapid growth and plateaus. In contrast, in some vertebrate as well as many invertebrate species, the young emerge looking completely different from the adults and spend their early lives acquiring body mass as a larva, then go through a metamorphosis (complete rearrangement of body pattern) to emerge in the adult form. This is typical of some insects, such as butterflies and moths, and some amphibians, such as frogs.

In birds and mammals, young generally emerge looking vaguely like adults, but the body proportions are very different, characterized by an enlarged head and reduced supportive limb elements. During the rapid growth phase of these individuals, the head grows much less than the body, limbs elongate, and skin maturation results in the typical adult feather or fur patterns. Since young birds and mammals are usually dependent on their parents for a time after birth, the incomplete development at birth is not a disadvantage.

The pattern of human growth provides a good example of the change in body proportions throughout development from birth to adult (the ultimate size of the individual). At two months past conception, the head of the embryo makes up approximately 50 percent of its total length, and the limbs less than 25 percent. At birth, the head size makes up about 25 percent of the total length and the limbs approximately 37 percent. Throughout childhood, the head size to limb length ratio continues to decrease toward the adult pattern of head size about 12 percent of body length and limb size over 50 percent of body length.

Bone Growth

The increase in body size is supported by increased skeletal structure in vertebrates, as a soft and pliable cartilage matrix becomes invested with hard and resistive bone. In the early newborn, the cartilage model of the eventual skeletal structure serves as the template for bone deposition. Bone-forming cells called osteoblasts lay down a "collar" of calcium and phosphate crystals in a lattice matrix around the shaft of the cartilage. This provides the strength for the bone to bear weight. At the same time, the terminal ends of the cartilaginous model also develop centers of osteoblastic activity, called epiphyses (singular, epiphysis). As the bone elongates, the collar elongates and the epiphyses in the ends of the bones continue to deposit calcium and phosphate. Eventually, the cartilage between these two bony centers of ossification, called the epiphyseal plate, is completely replaced with the bony matrix, and growth (limb elongation) ceases.

The epiphyseal plate is maintained under the influence of a hormone from the pituitary gland (the master endocrine gland at the base of the brain) called growth hormone (GH). However, at puberty, the hormones associated with reproductive maturity (estrogen and testosterone) cause an initial surge in GH release and in elongation of limbs, and then cause closure of the epiphyseal plate, causing growth to cease. This "growth spurt" tends to happen earlier in human females than in human males.

Hormonal Control

Growth hormone is essential to normal growth and development. It is regulated by two hormones released from the brain (in the hypothalamus) which cause daily peaks of GH in the blood. The peaks are most closely associated with the sleep cycle, large peaks appearing right after going to sleep and right before waking. Since growth hormone is associated not only with growth and differentiation but also tissue maintenance and repair, it makes sense that the peak of GH activity would occur during the nonactive period. In fact, the hypothalamic hormone that induces the release of GH (GH-releasing hormone) is a sleep inducer. Some researchers have suggested that the disappearance of deep sleep as we age and associated reduction of GH release may contribute to the physical decline that humans experience in old age.

GH represents about one-half the total hormone content of the anterior pituitary gland. GH stimulates the absorption of amino acids and protein synthesis necessary for development of skeletal muscle; stimulates breakdown of fat for energy utilization by cells of the body; stimulates the formation and maintenance of the epiphyseal plate in bone, and encourages lengthening of the long bones by stimulation of osteoblast cellular deposition of bone; and it stimulates the liver to make growth stimulating proteins, called insulin-like growth factors (IGF), which then affect the cellular metabolism of all cells in the body.

Growth Disorders

Abnormal secretion of GH can lead to growth disorders. Oversecretion of GH can lead to gigantism, marked by extreme limb elongation especially in the terminal elements (hands and feet) and enlargement of the face, especially the chin, nose, and ears, a condition called acromegaly . This condition can occur either because of a tumor of specific cells that manufacture GH or GH-like proteins or because of insufficient regulation by the hypothalamic releasing factors that control GH release. Not only are body proportions distorted with acromegaly, but hypersecretion of GH causes excessive sweating and secretion by the skin, enlargement of the heart, and sometimes high blood pressure. As a result of the many physiological effects of excessive GH secretion, life expectancy is shortened.

In contrast, lack of sufficient GH, especially during early years of development, can produce short stature or dwarfism. However, short stature with normal body proportions can be found throughout the human population and is probably associated with deficient production of IGF from the liver. For example, African pygmies are short, but normally proportioned people who have normal GH levels, but exhibit low levels of one form of IGF. Low GH release after birth can result in retarded growth, and these individuals are at risk for hypoglycemia (low blood sugar) as well. This condition severely impairs normal development, and these individuals are not only short but exhibit greatly retarded maturation of all tissues.

see also Bone; Development; Fetal Development, Human; Hypothalamus; Insect; Pituitary Gland; Scaling

Susan B. Chaplin

Bibliography

Tanner, James M. Foetus into Man: Physical Growth from Conception to Maturity. Cambridge, MA: Harvard University Press, 1990.

Vlijasek, Stanley J., et al., eds. The Cambridge Encyclopedia of Human Growth and Development. New York: Cambridge University Press, 1998.

Growth

Growth, as used as a term to discuss the human body, has a number of meanings. In general terms, growth is the process towards full development and physical maturity. At a biological organism level, growth represents the change in a cell from a simple to a more complicated structure. All human cells are formed with mechanisms that foster growth, and all cell creation taken together combines to define the growth of the human body.

Growth in the human body is regulated by a remarkably complicated group of internal components that are linked to the particular hereditary traits of the individual, which are compartmentalized in the genetic structure of every person, and coded in the DNA contained within each gene. These internal components, both with and in response to external factors such as the environment and exercise, regulate human growth processes through the release of the growth hormone, known as somatotropin.

Hormones are chemicals produced by the body to regulate its own cellular processes and organ function. Many hormones are commonly the subject of interest in a sport science context; erythropoietin (EPO) is the substance secreted by the kidneys that signals increased production of erythrocytes (red blood cells) for increased oxygen transport in the bloodstream; adrenaline is the hormone produced by the adrenal gland, located near the kidney, during periods of excitement or stimulation to increase circulation and muscular activity.

The growth hormone is produced by the anterior pituitary gland, a small structure located inside the skull. The main endocrine gland is the pituitary gland, a structure that permits the flow of a hormone directly into the bloodstream. It is often referred to as the "master gland" of the body, as it regulates the performance of other glands. The release of growth hormone by the pituitary gland is the most important regulator of physical development, as this chemical will determine the rate of cell growth, sexual development, and physical traits.

The process initiated by the release of growth hormone from the pituitary gland sets into motion a complicated series of biochemical relationships. The growth hormone is a compound known as a polypeptide, which comprises 191 different amino acids, the formation material from which proteins, the building blocks of muscle and tissue formation, are made. The sequence of the amino acids within each polypeptide is determined by the individual genetic code of the person, which is contained in the DNA. Increases in the release of growth hormone by the pituitary gland boost the synthesis of protein, and correspondingly impact both the speed of cell growth and metabolism. Conversely, natural release of the growth hormone declines from a peak at age 20; by age 40, the typical adult produces only 40% of the growth hormone available at peak. For this reason, supplement forms of human growth hormone have attracted significant attention in the athletic community as well as with the general population.

While the growth mechanisms of the body may be set into motion and regulated by the hereditary impulses determined by the genetic structure of every human, factors external to the body are as important to growth. Diet, including foods, vitamins, and minerals, is the most important of these growth-impacting factors. If a dietary deficiency exists with respect to a particular aspect of growth, the impulses sent to the body to increase cell formation through hormonal release will be ineffective. An example of this contrast is found in bone growth. Calcium, in combination with vitamin D, is essential to the creation of osteoblasts, the building blocks of skeletal cells. The release of growth hormone from the pituitary gland may send a signal to the bones to create more cells, but if the raw materials for construction are inadequately supplied, there will be insufficient growth. The existing structure will also experience a reduced capacity to repair itself.

Environmental factors may also affect growth. The exposure of young children to toxins such as air pollution has a proven inhibiting effect upon the growth of the entire physical structure, including the cardiorespiratory system and skin.

The long bones of the body such as the femur (thigh bone) and the humerus (upper arm) are designed to grow to mature size over a period of years. The increases in length, circumference, and density of such bones are facilitated in part by the physis (growth plate), the portion made of developing tissue located near each end of these long bones; the epiphysis is the head or the extremity of the bone. The growth plate function is to regulate the pattern of shape and development of the mature, fully formed bone, a maturation process that begins at birth and is generally completed by age 20. As the growth plate is the last portion of the bone to harden into maturity (a process known as ossification), it is more vulnerable to injury. Damage to the growth plate of a youth that is not properly treated can result in shortened or deformed bones in adulthood.

When the human growth hormone became the subject of intense scientific research in relation to its potential use as a nutritional supplement, particular emphasis was placed upon those sports where physical size and strength are at a premium. The growth hormone has been synthesized, and is widely distributed under the acronym HGH; human growth hormone has also been extracted from the pituitary glands of cadavers for this purpose. A wide range of scientific studies has confirmed that in its natural as well as in its synthetic form, human growth hormone will tend to permit the increase in physical strength and endurance when combined with a focused training program. The hormone generally stimulates the liver to produce a chemical known as IGF-1, which has the dual effect of stimulating the production of myoblasts, a key to the development of muscle growth, and facilitating cartilage and bone cell growth. Additional human growth hormone appears to generally increase the metabolism of fats, proteins, and carbohydrates.

The period within which human growth hormone has been examined scientifically is relatively short; definitive conclusions regarding the safety of its use as a supplement are not yet available. However, it is clearly a potential performance-enhancing substance, and it is a prohibited substance in elite athletics of all types. Human growth hormones in supplement form are also listed as a prohibited substance by the World Anti-Doping Agency (WADA).

see also Diet; Growth hormones; Health; Muscle mass and strength.

191. Growth

See also 74. CHANGE ; 147. EVOLUTION

auxesis

growth, especially owing to an increase in cell size. Cf. merisis. —auxetic, adj.

auxology

Obsolete. Medicine. the science of growth, especially applied to micro-organisms. Also called auxanology.

bathmism

a hypothetical vital force, thought to control growth and the function of nutrition.

chemotropism

growth or motion in response to a chemical stimulus. —chemotropic, adj.

culturist

a cultivator or a person who grows things.

embryogeny

the formation and growth of an embryo. —embryogenic, embryogenetic, adj.

endogeny

development or growth from within. —endogenicity, n. —endogenous, adj.

epiboly

the growth of part of an organism in such a way that it overlays or surrounds another. —epibolic, adj.

galvanotropism

growth or moveinent of an organism in response to an electric current. —galvanotropic, adj.

histogenesis, histogeny

the growth of organic tissues. —histogenic, histogenetic, adj.

merisis

any form of growth, especially as a product of cell division. Cf. auxesis.

neoplasia

the growth or formation of a neoplasm. —neoplastic, adj.

neoplasm

any abnormal formation or growth of tissue such as a tumor. —neoplastic, adj.

physis

1. the principle or concept of growth and change in nature.

2. nature considered as the source of growth and change.

3. something that grows or develops.

plagiotropism

the tendency of some plants to diverge from the vertical in their growth. —plagiotropic, adj.

polyeidism

the passing of an organism through several different forms in the growth process.

stereotaxis

orientation or movement of an organism in response to the stimulus of a solid object. Cf. stereotropism. —stereotactic, adj.

stereotropism

growth or movement determined by contact with a solid. Cf. stereotaxis. —stereotropic, adj.

teratology

Biology. the study of malformations or abnormal growth in animals or vegetables. —teratologist, n. —teratological, adj.

thigmotropism

stereotropism. —thigmotropic, adj.

growth / grō[unvoicedth]/ • n. 1. the process of increasing in physical size: the upward growth of plants the growth of the city affects the local climate. ∎  the process of developing or maturing physically, mentally, or spiritually: keeping a journal can be a vital step in our personal growth. ∎  the increase in number and spread of small or microscopic organisms: some additives slow down the growth of microorganisms. ∎  the process of increasing in amount, value, or importance: the rates of population growth are lowest in the north. ∎  increase in economic value or activity: the government aims to get growth back into the economy.2. something that has grown or is growing: a day's growth of unshaven stubble on his chin. ∎  Med. & Biol. a tumor or other abnormal formation.3. a vineyard or crop of grapes of a specified classification of quality, or a wine from it.

growth An increase in the dry weight or volume of an organism through cell division and cell enlargement. Growth may continue throughout the life of the organism, as occurs in woody plants, or it may cease at maturity, as in humans and other mammals. See also allometric growth; exponential growth.