The growth and development after birth may be categorized into five stages:
1. NEONATAL PERIOD
The 28 days post birth delineated by rapid growth and adaption to extra uterine environment has been designated as neonatal period. The new born is detached from the mother’s placenta and now relies on their own circulatory, digestive and excretory system. They maintain their body temperature within very narrow limit. Inadequate growth and development during prenatal period disposes the new-born to vulnerability especially for the duration of 24 hours after birth and is the pioneer factor contributing to high neonatal mortality rate.
2 INFANCY
Rapid velocity of growth followed by steep deceleration makes infancy, the life’. Stage of most rapidly changing rate of growth than any other post natal stage. During these first three year of human postnatal life Infant attains more than 50 percent of birth length (about 25cm) and 200 percent birth weight (about 3.4kg). This phase has been known as nutritional dependent phase as mother provides all or some nourishment to her offspring via lactation. The growth of infants of normal birth weight, from a wide variety of ethnic and socio-economic classes, in both the developed and developing nations, is remarkably similar during the first six months of life. Perhaps breast feeding, which supplies the nutrient, immunity and psychological needs of the infant, overrides the effect of variations in other aspects of the environment (Bogin, 2001).
The development of the skeleton, musculature, and the nervous system accounts for motor and cognitive advancements. The human brain grows more rapidly during infancy than any other tissue or organ of the body. Rapid motor skills development is mostly intended for the development of upright posture and independent locomotion. The gradual attainment of control over head, upper trunk and upper extremities is allied with accomplishment of sitting posture and the proficiency in walking skills by the age of 4 years.
Emergence of deciduous dentition allows the infants to switch from dependence on breast/infant food feeding to eating appropriate weaning foods. Infants will erupt five deciduous teeth in each quadrant of their mouth, the central incisor, lateral incisor, canine, first molar, and second molar. The emergence of last deciduous teeth, usually M2 signals the end of infancy. This is noteworthy that deciduous teeth of boys emerge about one month earlier than girls, as in other aspects of physical growth and maturation girls are, on an average ahead of boys. The hypothalamus, a centre of neurological and endocrine control produces high levels of gonadotropin-releasing hormone (GnRH) which indirectly through a pathway stimulate release of gonadal hormones, These hormones are responsible for rapid rate of growth during infancy. By late infancy, however, the GnRH secretion is virtually inhibited and level of sex hormones decline, which suspend reproductive maturation (Bogin, 2001).
3. CHILDHOOD
Childhood, a post weaning dependency period with relative growth stability may be defined by its own pattern of growth, feeding behavior, motor development and cognitive maturation, categorizing the phase into three sub-stages:
- Early childhood : The rapid decline in growth rate during infancy ends with the beginning of childhood, leading to slow and stable growth during this period. The constraints of immature dentition, small digestive system and rapid growth of brain among children necessitate specially prepared foods. Susceptibility to disease and procurement of food incline them to be dependent on elder people . The neurological development enhances psycho-motor skills and by the end of this phase, child is able to comprehend language accompanied by social, physic and cultural norms. Slow physical and cognitive growth facilitates efficiency of learning . It is observed that with regard to the physical growth the females outgrow males and are relatively taller than their counterparts.
- Mid-childhood: This is the most stable period characterized by differential rate of growth in different body parts and changes in body proportion by the age of 9 years. For instance, the ratio of sitting height to stature declines with age revealing rapid growth of lower extremities than the trunk. In other words, sitting height contributes less to the stature with age. Replacement of milk teeth by permanent teeth begins with the eruption of 1st (permanent) molar which takes place on an average around 6 years in most human population. By the end of this phase most children exhibit the four first molars with the exception of 2nd molar which erupts at 12 year of age (Bogin, 2007). Besides these the permanent incisors and canines also erupt during this period. The child dentally becomes capable of processing adult-type diet. By the age of ten years a child attains 96 percent of growth and only four percent growth is required till the age of twenty years (Tanner, 1962). It is also noticed that in physical growth the females continue to remain ahead in most of the body dimension than the males till around 11 years of age where after the males over grow them and remain ahead throughout. According to Gesell, maturation alone shapes motor development and is linked particularly to the development of central nervous system and also to muscular development. With the acquisition of proficiency in locomotive skills, child is able to walk adult type gait efficiently by the age of 7 years. The mid growth spurt, a short-term acceleration in height and weight is experienced by children several years before the onset of adolescent growth spurt. It has also been observed in other body dimension but with considerable variability in intensity of spurt. The mid-growth spurt usually occurs between 6.5-8.5 years. A sex difference in the timing of maximum velocity of the spurt is not apparent, but it occurs more frequently in boys than in girls (Malina and Beunen, 2007). Researchers have linked the spurt in childhood with ‘adrenarche’, an endocrine event associated with onset 23 of androgen secretion (e.g. testosterone) from adrenal gland (e.g. testes). Adrenarche involves psychological maturation in the context of increasing complexity and importance of social-environmental information during this period of increasing child independence, It is followed by gonadarche that is maturation of gonads, and marks the end of mid childhood. Mid growth spurt is not a universal phenomenon hence, reflect the biological variation among individuals.
- Late childhood: Late childhood designated as pre-pubertal stage is a period of rapid linear growth. Under the influence of growth and sex hormone development of reproductive system and secondary sexual characteristic begins to appear. It marks the beginning of adolescent growth spurt and the sexual dimorphism in body size and proportion during pre-puberty stage. . Thus, important physical development milestones of childhood according to Begin; (2001) are: The replacement of the deciduous teeth by permanent teeth and Completion of growth of the brain in weight. The onset of adult style locomotion, along with the eruption of permanent teeth and end of the brain growth are all indicators that the physically dependent child is moving on to independence.
4. ADOLESCENCE
Adolescence marks a rather abrupt transition in the human life from an immature and actively growing organism to a physically mature and potentially reproductive organism. It is initiated by pubertal reactivation of the hypothalamic-pituitary gonadal axis and a dramatic increase in secretion of sex hormones, which has been inactive in terms of sexual development since late infancy (Bogin, 2001). This change over extends over a period of 5 to 8 years and its primary characteristics as a life-history stage include:
- The pubertal growth spurt in height and weight ii)’ development of secondary sexual traits, including sex-specific alterations to fat and muscle distribution
- Completion of permanent tooth eruption (except third molar, which erupts’ between 17 to 25 years of age)
- psychological and behavioural changes related to sexual and social maturation.
The maturation is signaled by a rapid acceleration in the growth velocity of virtually all skeletal tissues-the adolescent growth spurt. An increase in growth velocity marks the commencement of adolescent spurt. The growth rate which eventually reaches’ a maximum, articulated as peak height velocity (PRV) and gradually decline. The rate of growth and age at PHV indicate the intensity and timing of adolescent spurt ‘ respectively. The rise to PHV is relatively slower than fall after peak. Age at peak height velocity is an individual characteristic and ‘is commonly used as maturity indicator. ,,’ ‘
The boys gain their adolescent growth spurt on an average between 12.5 and 15.5 years of age with PHV between ages 13.5- 14.5 years. In contrast, females enter puberty 2 years earlier than males and attain PHV between ages 11,5 to 12.5 years. ‘ The linear growth at peak height velocity is slightly higher among boys (8.2 ‘to 10.3 ” cm) than girls (7.1 to 9.1 cm). Consequently, though the females are taller than their male counterparts up to eleven years the adolescent boys are typically taller than girls.
The adolescent spurt in muscle mass in boys is usually accompanied by an increase in bone density, an increase in cardio-pulmonary function, larger blood volume, and greater density of red blood cells. The increase in each biological and physiological parameter among girls is relatively and absolutely lower than boys (Bogin; ~001). Greater width of shoulder it! Male and hip in female during ‘adulthood is due to stimulation of cartilage tissue at differential rate during adolescence. Quite unexpectedly adolescence spurt did not influence the size of the pelvis. Rather it follows its own pattern of growth which continues for several years even after adult stature is achieved. Thus, the immaturities of pelvis inlet among the adolescent girl under the age 17, who become pregnant, risk their own as well as fetal, health. The evident sex differences in body shape and composition are the consequence of differential rate of growth and pattern during adolescence. The four aspects which lead to sexual dimorphism are as:
- Boy’s greater amount of growth prior to adolescence,
- Boy’s delay in onset of adolescence, ‘
- Boy’s greater intensity of the spurt, and
- Girl’s longer duration of growth following the spurt
The interplay of various factors regulates the harmony .of growth during adolescence. Boas (1930) found that the age at which adolescent growth begins is inversely correlated with the size of the spurt. The early maturing children have higher peak height with the slow growth prior to puberty will tend to have a longer-lasting growth spurt during adolescence than a child who achieves a greater pre-pubertal percentage of adult height. ‘
Where chronic under nutrition, disease and child labor are endemic, height at every age is reduced compared with less stressed populations. However the’ total span of the growth period is prolonged, upto age 25 or 26 years, so that a greater adult height may beachieved. The size of the spurt and the age at peak height velocity is not related to finaladult height.In fact, some normal slow maturing individuals and people with certain endocrine disorders do not have a growth spurt but may reach normal adult height. This fact makes ‘the otherwise universal nature of the adolescent growth spurt an even more striking human characteristic (Bogin, 2001).
Sexual development
Gonadarche is preceded by an increase in the amplitude of luteinizing hormone (LH) and follicle-stimulating hormone (SH) secretion. A rise in the level of these hormones facilitates the sexual maturation which involves development of secondary sex characteristics. Gonadal steroid hormones enhance bone’ mineral accrual and affect adult height by promoting epiphyseal fusion through direct effects on the” growth plate (Attie et al., 1990).
The onset of puberty follows the appearance of the, secondary sex characteristics. In boys these include changes in size of the penis and scrotum, the growth of pubic, axillary and facial hair, the ‘breaking of voice’, and seminal emission. In girls, the secondary sexual characteristic includes growth of the breasts, appearance of pubic and axillary hair onset of menarche and development of uterus, vagina, and vulva to their mature size and appearance. Menarche experienced after PHV is an unambiguous external signal of internal reproductive system development. Over the past century, age at menarche has declined, the adolescent growth spurt has occurred “”at younger ages, and peak height growth velocity has increased. In the assessment of sexual maturation, the literature is dominated by ‘Tanner scale’ or ‘Tanner stage'(Tanner, 1962). The criteria described are used for breast, genital and pubic hair maturation and is appropriate to describe the stages specific to each characteristic. Assessing maturity by using different body size measurements is not possible because body size by itself is not an indicator of maturity, though somatic maturation is visually the most obvious expression of biological maturation of the child (Row land, 1996). However, the use of physique measurements as indicators of maturity status requires longitudinal data (Malina and Beunen, 1996; Rowland, 1996). If longitudinal data that span adolescence are available, specifically for height, the inflection in the growth curve that marks the adolescent growth spurt can be used to derive indicators of maturity such as age at the onset of growth spurt and age at maximum rate of growth during the spurt (Malina and Beunen 1996).
Breast development
- Stage 1 Preadolescent: elevation of papilla only. Stage
- 2 Breast bud stage: elevation of breast and papilla as small mound. Enlargement of areolar diameter.
- Stage 3 Further enlargement and elevation of breast and areola, with no separation of their contours.
- Stage 4 Projection of areola and papilla to form a secondary mound above the level of the breast.
- Stage 5 Mature stage: projection of papilla only, due to recession of the areola to the general contour of the breast
Genitalia development
- Stage 1 Preadolescent: testes, scrotum and penis are of about the same size and proportion as in early childhood.
- Stage 2 Enlargement of scrotum and testes: the skin of the scrotum reddens and changes in texture. There is little or no enlargement of penis at this stage.
- Stage 3 Enlargement of penis: this occurs first mainly in length. Further growth of testes and scrotum.
- Stage 4 Increased size of the penis with growth in breadth and development of glans. Further enlargement of testes and scrotum; increased darkening of scrotal skin.
- Stage 5 Genitalia adult in size and shape.
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.
5. ADULTHOOD
The attainment of adult stature and sexual maturation are the hallmarks depicting the transition from adolescence to adulthood. Young women and men of the middle to upper socio-economic status reach adult height at about 18 years of age and 21 years of age respectively. Height stops when the long bones of the skeleton (eg. femur, tibia etc.) lose their ability to increase in length (Bogin, 2001) represents the attainment of skeletal maturity.
However among children without gonads or whose gonads a e not functional, never e perience epiphyseal union but they do stop growing, signify that the change in the sensitivity to growth stimuli of cartilage and bone tissue in the growth plate region leads to the loss of hyperplastic growth potential in cells .
In context of sexual maturation, the production of viable spermatozoa in boys and oocytes in girls is achieved during adolescence, but these events mark only the early stages, not the completion of reproductive maturation. In girls, menarche is usually followed by a period of 1-3 years of adolescence sterility. The menstrual cycles, which are often irregular in length, are anovulatory. Reproductive maturity is reached at the end of the adolescent stage of life, which occurs, on an average at 19 years (Bogin, 2001).
Thus, transition to adulthood is marked by dramatic events such as the cessation of growth in height and attainment of full reproductive maturity. However, the most striking feature of the adulthood stage of life is its stability, or homeostasis and its resistance to pathological influences, such as infectious disease and psychological stress (Timiras, ]972).
6. 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.