What is growth? |

Process and Effects

The human body grows from conception until adult size is reached. Adult size is reached in females around the age of eighteen and in males around twenty or twenty-one, but there is considerable variation in either direction. (Nearly all numerical measurements of growth and development are subject to much variation.) On the average, males end up with a somewhat larger body size than females because of these two or three extra years of growth.

Growth begins after conception. The first phase of growth, including approximately the first month after conception, is called embryonic growth, and the growing organism is called an embryo. During embryonic growth, the most important developmental process is differentiation, the formation of the various organs and tissues. After the organs and tissues are formed, the rest of prenatal growth is called fetal growth and the developing organism is called a fetus. Respiratory movements begin around the eighteenth week of gestation, during the fetal stage; limb movements (such as kicking) begin to be felt by the mother around the twenty-fourth week, with a considerable range of variation. At birth, the average infant weighs about 3.4 kilograms (7.5 pounds) and measures about 50 centimeters (20 inches) in length.

Growth continues after birth and throughout childhood and adolescence. From the perspective of developmental biology, childhood is defined as the period from birth to puberty, which generally begins at twelve years of age, and adolescence continues from that point to the cessation of skeletal growth at around the age of eighteen in females and twenty or twenty-one in males. The long period of adulthood that follows is marked by a stable body size, with little or no growth except for the repair and maintenance of the body, including the healing of wounds. After about age sixty, there may be a slight decline in body height and in a few other dimensions.

By one year of age, the average baby is seventy-five centimeters (thirty inches) long and weighs ten kilograms (tewnty-two pounds). (There is actually a slight decline in weight in the first week of postnatal life, but this is usually regained by age three weeks.) For ages one to six, the average weight (in kilograms) can be approximated by the equation“weight = age 2 + 8.” For ages seven to twelve, growth takes place more rapidly: Average weight (in kilograms) can be approximated by “weight = age 3.5 - 2.5,” while average height (in centimeters) can be approximated for ages two to twelve by the equation “height = age 6 + 77.” Head circumference has a median value of about 34.5 centimeters at birth, 46.3 centimeters at an age of one year, 48.6 centimeters at age two, and 49.9 centimeters at age three. All these figures are about one centimeter larger in boys than in girls, with considerable individual variation. Median heights and weights, when differentiated by sex, reveal that boys and girls are generally similar until age fourteen, after which boys continue to gain in both dimensions.

Growth of the
teeth takes place episodically. In most children, the first teeth erupt between five and nine months of age, beginning with the central incisors, the lower pair generally preceding the upper pair. The lateral incisors (with the upper pair first), the first premolars, the canines, and the second premolars follow, in that order. All these teeth are deciduous teeth (“baby teeth”) that will eventually be shed, to be replaced during late childhood by the permanent teeth. At one year of age, most children have between six and eight teeth.

Growth takes place in several directions. Growth at the same rate in all directions is called isometric growth, which maintains similar proportions throughout the growth process. Isometric growth occurs in nautilus shells and a variety of other invertebrates. Most of human growth, however, is allometric growth, which takes place at different rates in different directions. Allometric growth results in changes in shape as growth proceeds. Moreover, different parts of the body grow at different rates and in different directions. During fetal development, for example, the head develops in advance of the fore and hind limbs, and the fetus at about six months of age has a head which is about half its length. The head of a newborn baby is about one-third of its body length, compared to about one-seventh for an adult. In contrast, the legs make up only a small part of the body length in either the six-month-old fetus or the newborn baby, and their absolute length and proportionate length both increase throughout childhood and adolescence.

Growth of the skeleton sets the pace for growth of the majority of the body, except for the nervous system and reproductive organs. Most parts of the skeleton begin as fast-growing cartilage. The process in which cartilage tissue turns into bone tissue is called ossification, which begins at various centers in the bone. The first center of ossification within each bone is called the diaphysis; in long bones, this ossification usually takes place in the center of the bone, forming the shaft. Secondary centers of ossification form at the ends of long bones and at certain other specified places; each secondary center of ossification is called an epiphysis. In a typical long bone, two epiphyses form, one at either end. Capping the end of the bone, beyond the epiphysis, lies an articular cartilage. Between the epiphysis and the diaphysis, the cartilage that persists is called the epiphyseal cartilage; this becomes the most rapidly growing region of the bone. During most of the growth of a long bone, the increase in width occurs by accretion, a
gradual process in which material is added at a slow rate only along a surface. In the case of a bone shaft, increase in width takes place only at the surface, beneath the surrounding membrane known as the periosteum. By contrast, the epiphyseal cartilage grows much more rapidly, and it also grows by interstitial growth, meaning that growth takes place throughout the growing tissue in all directions at once. As the epiphyseal cartilage grows, parts of it slowly become bony, and those bony portions grow more slowly.

During the first seven or eight years of postnatal life, the growth of the epiphyseal cartilage takes place faster than its replacement by bone tissue, causing the size of the epiphyseal cartilage to increase. Starting around age seven, the interstitial growth of the epiphyseal cartilage slows down, while the replacement of cartilage by bone speeds up, so that the epiphyseal cartilage is not growing as fast as it turns into bone tissue; the size of the epiphyseal cartilage therefore starts to decrease. At the time of puberty, the hormonal influences create an adolescent growth spurt during which the individual’s bone growth increases for about a one-year period. In girls, the adolescent growth spurt takes place about two years earlier than it does in boys—the average age is around twelve in girls, versus about fifteen in boys—but there are tremendous individual variations both in the extent of the growth spurt and in its timing. By age fourteen, most girls have already experienced most of their adolescent growth spurt, while most boys are barely beginning theirs. Consequently, the average fourteen-year-old girl is a bit taller than the average fourteen-year-old boy.

At around eighteen years of age in females and twenty or twenty-one years of age in males, the replacement of the epiphyseal cartilage by bone is finally complete, and bone growth ceases. The age at which this occurs and the resulting adult size both vary considerably from one individual to another. For the rest of adult life, the skeleton remains more or less constant in size, diminishing only slightly in old age.

Most of the other organs of the body grow in harmonious proportion with the growth of the skeleton, reaching a maximum growth rate during the growth spurt of early adolescence and reaching a stable adult size at around age eighteen in women and age twenty or twenty-one in men. The nervous system and reproductive system, however, constitute major exceptions to this rule. The nervous system and brain grow faster at an earlier age, reaching about 90 to 95 percent of their adult size by one year of age. The shape of the head, including the shape of the skull, keeps pace with the development of the brain and nervous system. For this reason, babies and young children have heads that constitute a larger proportion of their body size than do the heads of adults.

The growth of the
reproductive system also follows its own pattern. Most reproductive development is delayed until puberty. The reproductive organs of the embryo form slowly and remain small. The reproductive organs of children, though present, do not reach their mature size until adolescence. These organs, both the internal ones and the external ones, remain small throughout childhood. Their period of most rapid growth marks the time of puberty, which spans ages eleven through thirteen, with a wide range of variation. At this time, the
pituitary gland begins secreting increased amounts of the follicle-stimulating hormone (FSH), which stimulates the growth and maturation of the gonads (the ovaries of females and the testes of males). The ovaries or testes then respond by producing increased amounts of the sex hormones testosterone (in males) or estrogen (in females), which stimulate the further development of both primary and secondary sexual characteristics. Primary sexual characteristics are those which are functionally necessary for reproduction, such as the presence of a uterus and ovaries in females or the presence of testes and sperm ducts in males. Secondary sexual characteristics are those which distinguish one sex from another, but which are not functionally necessary for reproduction. Examples of secondary sexual characteristics include the growth of breasts or the widening of the hips in females, the growth of the beard and deepening of the voice in males, and the growth of hair in the armpits and pubic regions of both sexes.

Growth takes place psychologically and socially as well as physically. Newborn babies, though able to respond to changes in their environment, seem to pay attention to such stimuli only on occasion. At a few weeks of age, the baby will respond to social stimuli (such as the sound of the mother’s voice) by smiling. Babies usually can grasp objects by five months of age, depending on the size and shape of the object. By six months, most babies will show definite signs of pleasure in response to social stimulation; this may include an open-mouth giggle or laugh. At seven months of age, most babies will respond to adult facial expressions and will show different responses to familiar adults as opposed to strangers. The age at which babies learn to crawl varies greatly, but most infants learn the technique by nine or ten months of age. Social imitation begins late in the first year of life. Also, by this time, children learn object permanence, meaning that they will search for a missing object if they have watched it being hidden. Walking generally develops around eighteen months of age, but the time of development varies greatly.

Jean Piaget
(1896–1980) was a pioneer in the study of the social and cognitive development of children. Piaget identified four stages of cognitive and social growth, which he called sensorimotor, preoperational, concrete operational, and formal operational. In the sensorimotor stage, from birth to about two years of age, infants begin with reflexes such as sucking or finger curling (in response to touching their palms). Starting with these reflexes, they gradually learn to understand their senses and apply the resulting information in order to acquire important adaptive motor skills that can be used to manipulate the world (as in picking up things) or to navigate about and explore the world (as in walking). Socially, infants develop ways to make desirable stimuli last by such acts as smiling. In the preoperational stage, which lasts from about two to six years of age, children acquire a functional use of their native language. Their imagination flourishes, and pretending becomes an important and frequent activity. Most of the thinking at the preoperational stage is egocentric, however, which means that the child perceives the world only from his or her own point of view and has difficulty seeing other points of view.

The concrete operational stage spans the years from about seven to eleven years of age. This is the stage at which children learn to apply logic to concrete objects. For example, they realize that liquid does not change volume when poured into a taller glass, and they develop the ability to arrange objects in order (for example, by size) or to classify them into groups (for example, by color or shape). The final stage is called the formal operational stage, beginning around age twelve. This is the stage of adolescence and adulthood, when the person learns to manipulate abstract concepts in such areas as ethical, legal, or mathematical reasoning. This is also the stage at which people develop the ability to construct hypothetical situations and to use them in arguments.

Complications and Disorders

Disorders of growth include dwarfism, gigantism, and several other disorders such as achondroplasia (chondrodystrophy). Dwarfism often results from an insufficiency of the pituitary growth hormone, also called somatostatin or somatotrophic hormone. Some short-statured individuals are normally proportioned, while others have proportions differing from those of most other people. An overabundance of growth hormone causes gigantism, a condition marked by unusually rapid growth, especially during adolescence. In some individuals, the amount of growth hormone remains normal during childhood but increases to excessive amounts during the teenage years; these individuals are marked by acromegaly, a greater than normal growth which affects primarily the hands, feet, and face.

Achondroplasia, also called chondrodystrophy, is a genetically controlled condition caused by a dominant gene. In people having this condition, the epiphyseal cartilages of the body’s long bones turn bony too soon, so that growth ceases before it should. Those exhibiting chondrodystrophy therefore have short stature and childlike proportions but rugged faces that look older than they really are.

Inadequate growth can often result from childhood malnutrition, particularly from insufficient amounts of protein. If a child is considerably shorter or skinnier than those of the same age, that child’s diet should be examined for the presence of malnutrition. Intentional malnutrition is one of the characteristic features of anorexia nervosa. The opposite problem, overeating, can lead to obesity, although obesity can also result from many other causes, including diabetes and other metabolic problems.

By the late twentieth century, human growth hormone, a drug used since the 1950s to help very short children grow, was being used for “off-label” purposes that included antiaging and body building. In 2002, researchers suggested an apparent link between the use of the hormone and cancer, specifically Hodgkin’s disease and colorectal cancer.

Perspective and Prospects

As a phenomenon, growth of both wild and domestic animals was well known to ancient peoples. Hippocrates (ca. 460–ca. 370 BCE), considered the founder of medicine, wrote a treatise on embryological growth, and Aristotle (384–322 BCE) wrote a longer and more complete work on the subject. During the Renaissance, Galileo Galilei (1564–1642) studied growth mathematically and distinguished between isometric and allometric forms of growth, arguing that the bones of giants would be too weak to support their weight.

The most important era in the study of human embryonic development was ushered in by the Estonian naturalist Karl Ernst von Baer (1792–1876), who discovered the human ovum. From this point on, detailed studies of human embryonic and postnatal development proceeded at a rapid pace, especially in Germany. Much of the modern understanding of growth in more general or mathematical terms derives from the classic studies of the British anatomist D’Arcy Wentworth Thomson (1860–1948). In the twentieth century, Piaget became a leader in the study of childhood social and cognitive growth phases.

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