(Anthropology paper I)

Syllabus Section: 10. Concept of Human Growth and Development

 

Introduction

The study of human physical growth and development has been an important part of physical anthropology and human biology since the founding days of these disciplines. Needless to say, this topic has a rich historical tradition of continuous research advancements and accumulation of huge amount of data on physical growth and development of children from human populations inhabiting the globe in diverse physical environments and geographical locations experiencing enormous cultural and socio-economic diversity. Some estimates suggest that systematic studies on growth and maturation dates back to mid- 1860s. During that time European anthropology considered the science of body measurements to be a valid tool to assess body dimension and in American Anthropology in the late 19^th century, studies on human growth became evident. Since then, the practitioners of physical anthropology in many countries including India, collected data on growth and development of children in time and space dimensions for assessment of growth patterns of children and evaluate factors responsible for such patterns.

Human growth and development is a biocultural phenomenon. It is an important biological process that incorporates a complex interaction among the biological endowment of our species, the physical environment in which this species lives, and the social, economic and political environments that are created by human cultures. Since the basic pattern of human growth and development is common for all human groups, it is considered that this basic pattern is the outcome of the evolutionary history of the hominids, the living humans and our fossil ancestors. The fact mentioned above justifies the interest of the biological anthropologists and the human biologists to undertake research on human growth and development. Furthermore, study of growth and development has an applied anthropological and human biological value in terms of measuring child health and nutrition.

Growth refers to increase in physical dimensions of an individual, e.g., increase in height, weight, etc. or various parts and organs of the body. It implies increase in size or general bodily growth. The increase is limited by hereditary factors, and influenced by environmental factors, such as, ethnicity, climate, diet and many other factors.

Development refers to enhancement in the functioning of the human body. It entails changes in structure, form, or shape. It takes into account the increase in dimensions, change in proportions and adjustment of parts. Development is evaluated by improvement in the performance of the human body. Development is always gradual, progressive and diversified in form according to different periods of growth.

STAGES OF GROWTH – PRE-NATAL, NATAL, INFANT, CHILDHOOD, ADOLESCENCE, MATURITY, SENESCENCE

Prenatal and Postnatal Growth

The entire period of growth can be divided into two major phases, i.e., Prenatal (before birth) and Postnatal (after birth). The prenatal phase is further divided in three distinct stages – the fertilized egg (ovum) or Zygote, the Embryo and the Foetus. The stages of postnatal phase are – infancy, childhood, adolescence, maturity and senescence.

Prenatal Growth

The period of prenatal growth is significantly important to the child’s future well-being; the fact remains that it is the period about which, certainly, we do not have much knowledge. For the first and second trimesters of pregnancy we have to depend on cross-sectional studies. In the second trimester we also have to rely almost completely on foetuses expelled from the uterus because one or the other was abnormal, whereas during the earliest weeks of pregnancy we have mostly normal products of social abortions. For later foetal life we can study infants born prematurely, making the conjecture that these children have grown before birth and will grow after it in exactly the same way as children who remain in the uterus the average length of time, which is in normal cases.

Period of Egg: Fertilized Egg (ovum) or Zygote (first 2 weeks): We do not know what forces are responsible for selecting one out of millions of sperm (i.e. male gamete) which fertilizes the ovum (i.e. female gamete). Fertilization takes place in one of the tubes which lead from the ovaries to the uterus. The fertilized egg spends some four to five days drifting down the tube and floating in the uterine cavity before it implants into the wall of the uterus (the female organ in which embryo/foetus develops). During this time the cells divide steadily so that at the time of implantation, the blastocyst as it is called consists of around 150 cells. After implantation, the outer layer of the blastocyst undergoes a series of changes which culminate in the formation of the placenta which gives nourishment to embryo/foetus. A small proportion of the inner layer develops into the embryo.

Period of the Embryo (2 to 8 weeks): The period of the embryo is considered to begin 2 weeks after fertilization and ends 8 weeks after fertilization. The child now is recognizably human, with arms and legs, a heart that beats, and a nervous system that shows the beginning of reflex responses to tactile stimuli, is called a foetus. At this stage it is about 3 cm long.

This is a hazardous period, and many more ova are fertilized than come to fruition. It is estimated that 10% fail to implant and of those that implant and become embryos half of them are spontaneously aborted, usually without the mother’s knowledge. Such abortion is in most cases due to developmental abnormalities, either of the embryo or of its protective and nutritive surrounding structures. 5% to 10% of fertilized ova have abnormalities of the chromosomes but amongst newborns it is only 0.5%. Spontaneous abortions take care of this situation when 90% to 95% of all conceptions with these abnormalities are rejected.

The counting of age in the prenatal period remains problematic. Traditionally, and because we have no better way, age tends to be counted from the first day of the last menstrual period. This occurs on an average 2 weeks before fertilization. Thus the most frequent age at birth is 280 days or 40 weeks, reckoned as ‘postmenstrual age’, but this represents only 38 weeks of true foetal age. However, there are difficulties in individual cases. The interval from menstruation to fertilization varies considerably; and again, menstrual bleeding may continue in some women for 1 or even 2 months after fertilization.

The velocity is not pronounced in the embryonic period. Initially, during the first 2 months differentiation of the originally homogeneous whole into regions, such as head, arms take place. Histogenesis which is the differentiation of cells into specialised tissues such as muscle and nerve also occurs same time. Each region transforms into a definite shape, by differential growth of cells or by cell migration due to the  process called morphogenesis. This carries on until adulthood and in some parts of the body, into old age, though the major part of it is completed by the 8^th postmenstrual week.

Period of the Foetus (9 to 40 weeks): As has already been explained reliable growth curves of the foetus are hard to come by. There are reliable data available for body length of foetuses from about 10 to 18 weeks of postmenstrual age, and for infants born prematurely from about 28 weeks onwards. No useful data is available for 18 and 28 weeks. Figure 1.1 shows the distance and velocity curves of body length in prenatal life, and for the first year after birth. The peak velocity is experienced at about 4 month’s postmenstrual age. The solid lines shows the actual length and length velocity; the interrupted lines interpret the theoretical curve which is expected if no uterine restriction takes place in the last weeks of pregnancy, followed by a compensating catch-up after birth.

Due to the continuing cellular multiplication the high rate of growth of the foetus takes place compared with that of the child. As the foetus gets older, the proportion of cells undergoing division in any tissue becomes gradually less and it is normally few new nerve cells and only a small proportion of new muscle cells appear after 30 postmenstrual weeks. By this time the velocity in linear dimensions drops sharply. There is however considerable difference in appearance of muscle and nerve cells of the foetus as compared those of the child or adult, due to the fact that early in development there is little cytoplasm around the nuclei. Great deal of intracellular substance and a much higher proportion of water compared to mature muscle are found in foetal muscles, while the later foetal and postnatal growth of muscle comprises primarily of building up the cytoplasm of the muscle cells. Then again salts are incorporated and the contractile proteins are formed as a result the cells become bigger in size, the intracellular substance mainly disappears and the concentration of water drops. This continues fairly vigorously till 3 years and slows down subsequently. It briefly speeds up again in adolescence, particularly in boys, under the influence of androgenic (male determining) hormones. In the foetal nerve cells cytoplasm is added, and the cell processes grow. Postnatal growth for most tissues is significant as a period of development and enlargement of existing cells, while in early foetal life cell division and the addition of new cells takes place.

It is also to be noted that growth in weight in the foetus follows the same general pattern as that of height, except that the peak velocity is reached later, usually at the 34^th postmenstrual week. During the last 10 weeks in the uterus, the foetus stores considerable amounts of energy in the form of fat. Up till about 26 weeks postmenstrual age, most of the increase in foetal weight is due to accumulation of protein as the main cells of the body are built up. From then on fat begins to accumulate, both deep in the body and subcutaneously. It has been found that from about 30 to 40 postmenstrual weeks’ fat increases from nearly 30g to 430g. Since fat contains much more energy than protein or carbohydrate per unit volume this represents a large reserve of energy available for the first, perhaps critical, period after birth. Conversely, the creation of such a store represents a considerable drain on the energy resources of the mother in the last weeks of pregnancy.

The Effect of the Uterine Environment on Prenatal Growth: By the time the foetus is 34 to 36 weeks the growth slows down due to the influence of the uterus, whose available space is by then becoming fully occupied. This facilitates a child to be successfully delivered. As a result of poor environmental situations, especially of nutrition, lower birth weight is observed.

So-called ‘Premature’ Babies: Measured from the first day of the last menstrual period, the average length of gestation is 280 days or 40 weeks. Term babies born within 38 and 41 completed weeks are babies less than 2,500g at birth ‘Low birth weight’ babies (earlier known as ‘premature’ babies). The prognosis for a small child born after a normal-length gestation is very different from the prognosis for an equally small child born after a shortened gestation. Leaving the uterus early is not in itself harmful, but then growing less than normally during a full uterine stay reflects pathology of foetus, placenta or mother

Postnatal Growth (birth to maturity)

The amount of growth achieved obviously depends on the time for which growth proceeds and on the speed of growth per unit time. Measurements taken on a single individual at intervals can be plotted against time to produce a graph of progress, whether they are derived from the whole body (e.g. height) or from one of its components (e.g. leg length).

The Adolescent Growth Spurt: The adolescent growth spurt is a constant phenomenon and occurs in all children, though it varies in intensity and duration from one child to another. The difference in size in men and women is to a large degree due to differences in timing and intensity of the adolescent spurt. Before adolescent spurt boys and girls differ only by about 2% in height, but after it by an average of about 8%. The difference is partly because of later occurrence of the male spurt allowing an extra period of growth, and partly because of a greater intensity of the spurt (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 northwest 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 (non- identical) 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 determined.