General observations suggest that most body measurements follow approximately the growth curves as described for height. Majority of skeletal and muscular dimensions grow in this manner, and internal organs such as liver, spleen and kidney also follow similar path. But then the brain and skull, the reproductive organs, the lymphoid tissues of the tonsils, adenoids and intestines, and the subcutaneous fat are exceptions to it. In Figure 1.7 these differences are shown, using the size attained by various tissues as a percentage of the birth-to-maturity increment. Height follows the ‘general’ curve. The reproductive organs, internal and external, have a slow pre-pubescent growth, followed by a very large adolescent spurt.
The brain, along with the skull, eyes and ears develops earlier than any other part of the body and thus has a characteristic postnatal curve. The brain experiences a very small adolescent spurt, which is if it occurs. Head length and breadth experience a small but sure spurt, which can actually be attributed to thickening of the skull bones and the scalp, and development of the air sinuses. The dimension of the face is similar to the general curve. There is a significant adolescent spurt, particularly in the mandible, ensuing in the jaw’s becoming longer and more projecting, the profile straighter, and the chin more pointed. Nevertheless, substantial individual differences, to the extent that a few children have no detectable spurt at all in some face measurements is also reported.
Eye witnesses a slight adolescent spurt. Though myopia escalates constantly from at least age 6 to maturity, a particularly speedy rate of change happens at about 11 to 12 in girls and 13 to 14 in boys, as would be probable if there was a somewhat greater spurt in the axial dimension of the eye than in its vertical dimension.
It is observed that unlike the rest of the body, the lymphoid tissue has quite a different growth curve from the rest of the body. Generally, it reaches its maximum amount before adolescence and then, possibly under the direct influence of sex hormones, declines to its adult value.
The subcutaneous fat layer also has its particular curve which is somewhat complex. The methods used to measure subcutaneous fat are x-rays or by picking up a fold of skin and fat between the thumb and forefinger and measuring its thickness with a special, constant-pressure caliper referred to as skinfold thickness. Figure 1.8 shows the distance curves of skinfolds taken half-way down the back of the arm (triceps) and at the back of the chest, just below the shoulder blade (subscapular). Subcutaneous fat begins to be laid down in the foetus at about 34 weeks postmenstrual age, and increases continuously, to reach a peak at about 9 months after birth. Usually this is true for an average child. The peak may be reached as early as 6 months or as late as 12 or 15. After 9 months, the skinfolds decrease until age 6 to 8 when they begin to rise again. Girls have a little more total fat than boys at birth, and the difference becomes gradually more marked during childhood. From 8 years on, the curves for girls and boys diverge more radically, as do the curves for limb and body fat. At adolescence the limb fat in boys on average decreases (see triceps, Figure 1.8); the body fat (subscapular) shows a temporary slowing down of gain, but no loss. In girls there is a slight halting of the limb-fat gain at adolescence, but no loss; the trunk-fat shows only a steady rise until adulthood (Tanner, 1978).
Post-adolescent Growth:
Growth, even of the skeleton, does not entirely cease at the end of the adolescent period. The limb bones stop increasing in length, but the vertebral column continues to grow until about age 30 years, by apposition of bone to the tops and bottoms of the vertebral bodies. Thus height increases by a small amount, on average 3 to 5 millimeters. From about 30 to 45 years height remains stationary, and then it begins to decline. Head length, head breadth and facial diameters increase slightly throughout life. The widths of the bones in the leg and in the hand, in both sexes also increase. For practical purposes, however, it is useful to have an age at which we may say that growth in stature virtually ceases, i.e. after which only some 2% is added. At present in the developed nations such as North America and north-west Europe, the average boy stops growing, in this sense, at 17·5 years and the average girl at 15·5 years. There is a normal range of variation amongst individuals, amounting to about two years, on either side of these averages.
Senescence: The one thing in life that is certain to occur is – death. It may be sooner or later, and the manner in which it occurs may vary considerably. As one grows older the chances are that death will be preceded by a varying period during which the physical or mental faculties, or both, become gradually reduced. It is these processes followed by death, which is called senescence. Senescence could also be defined as including those effects which lead to a decreased expectation of life as the age increases. We can measure senescence by finding the death rate in a population. Senescence can be influenced by genetic as well as environmental factors. The genetic component of senescence can be studied by inheritance of longevity, which may be due to absence or presence of predisposition to disease. The classical method of genetic analysis using twins also gives interesting information. The difference in age at death between monozygotic (identical) twins is only half of that between dizygotic (nonidentical) twins. But the correlation between ages at death of siblings is twice that between parent and child. This suggests that environmental factors may also be of importance. For example, lung cancer is a senescent disease, whether caused by excessive smoking or atmospheric pollution, it is largely environmentally.
Sample Questions
- 1) What does the study of ‘growth’ and ‘development’deal with? Explain the effects of uterine environment on prenatal growth.
- 2) Write an essay on various stages of postnatal growth of children.
- 3) Describe adolescent growth in children highlighting sex differences.
- 4) Describe growth curves of different tissues and parts of the body.
- 5) Short Notes: (i) Growth and Development (ii) Embryo (iii) Premature babies (iv) Small-for- date babies. (v) Adolescent growth spurt (vi) Postadolescent growth (vii) Senescence.