Growth and Development
Beginning at the moment of conception, the human organism depends upon adequate nutrition for growth, development, and survival. In the first week from the fertilization, the zygote produces a series of blastomeres and, because of further cellular divisions, the morula, that contains about 10-30 cells. The morula stage proceeds to the formation of a fluid filled cavity, the early blastocyst. Inside the blastocyst is an inner cell mass or embryoblast (future embryo), and the outer cell mass or trophoectoderm (future placenta).
Embryogenesis (three to eight weeks) involves three major processes: morphogenesis (generation of shape), pattern formation (biologic-spatial cell organization), and differentiation (specialization in specific phenotypes). Morphogenesis and pattern formation are regulated by bone morphogenetic proteins and homeobox genes. During embryogenesis tissues and organs develop. The most important events of the embryonic period can be resumed in: (1) gastrulation, a process able to convert the bilaminar embryonic disk into the three primary embryonic germ layers; (2) formation of the dorsal mesoderm, (notochord and paraxial mesodermal cords), somites, intermediate and lateral mesoderm; (3) neural tube formation from ectoderm that will produce the nervous system and skin; (4) evolution of endoderm, that essentially develops into the digestive apparatus, respiratory apparatus, some parts of the urogenital system and branchial pouches (part of the branchial apparatus). By the end of eight weeks, the embryo is about one inch long.
The fetal period goes from nine to 38 weeks. It is characterized by rapid body growth. In this period the embryo, now termed the fetus, is starting to grow. Length growth velocity has a peak at 20 weeks about 10 cm/ 4 weeks. Weight growth velocity peaks at 30-34 weeks. At nine weeks, half the fetus' overall size is its head. From 13 to 16 weeks, the head is relatively smaller and the limbs (legs) are longer. At the fifth month (17-20 weeks) growth slows down, but the fetus is longer and the limbs reach their final relative proportions. Parameters used for monitoring fetal growth include the embryonic crown-rump length, biparietal diameter (BPD), head circumference, femur length, and abdominal circumference. Ultrasound technology is often used to evaluate fetal growth.
After birth, the primary hormonal regulators of growth are growth hormone (GH) and the GH-dependent growth factors, insulin-like growth factor-I (IGF-I). This contrasts with the fetal period, where the fetal endocrine system is believed to play a relatively minor role in growth. Postnatal growth also is influenced by a variety of factors. Normal changes in the rate of linear growth occur frequently during infancy, allowing some infants to "cross percentiles" (up or down) during the first months of life. More than 50% of babies experience an upward shift of growth during the first three months of postnatal life. Breast milk is the optimal food for growth because it provides immunologic and antibacterial factors, hormones, enzymes, and opoid peptides not present in alternative feedings. Newborns require approximately 100-120 cal/kg/day for adequate growth and development.
After infancy, growth speed slows down in the toddler years. After age two, toddlers gain about 5 lb. (2.3 kg) in weight and 2.5 in. (6.4 cm) in height each year. In comparison, head circumference increases by about 1 in. (2.5 cm) from two to twelve years of age. Growth does not increase steadily. A toddler's weight can remain the same for some weeks. Increases in height result primarily from growth of the lower extremities and, secondarily, to a lesser extent elongation of the trunk. Body proportions change, with upper-to-lower segment ratios ranging from 1.40 at age two years, to 1.15-1.20 at age five years. By age two, toddlers of both gender will stand about 34 in. tall (86 cm) and weigh about 27-28 lb. (12.2-13.1 kg) on average. Growth charts are very useful at this time in order to detect abnormal growth that can be a manifestation of clinical disorders.
Child growth is the period from the post-toddler years through pre-adolescence. In the post-toddler period, nutrition continues to be critically important. At this time, growth patterns are largely determined by genetics and under hormonal control. It is a very complex process, and requires the coordinated action of several hormones. Normal growth, supported by good nutrition adequate sleep and regular exercise, is one of the best overall indicators of good health. A significantly malnourished child may be pushed off the "natural" genetically determined growth curve. Severe iponutrition (inadequate nutrition), enough to affect a child's growth rate, is uncommon today in the United States and other developed countries unless the child has an associated chronic illness or disorder. Extra food or greater than recommended amounts of vitamins, minerals, or other nutrients will not increase the height of a growing child.
Adolescence defines the time between the beginning of sexual maturation (puberty from the Latin pubertas, meaning adult) and adulthood. Roughly, adolescence spans 13-19 years of age. Adolescence includes physical growth and emotional, psychological, and mental change. Adolescence's first signal stems from higher concentrations of leptin, a protein produced by adipocytes of the fat tissue. The hypothalamus stimulates the hypophysis to secrete hormones able to promote the overall growth of the body to maturate the gonads, as well as adrenal cortex and thyroid. Normal growth is categorized in a range used by pediatricians to gauge how a child is growing. The following are some average ranges of weight and height, based on growth charts developed by the Centers for Disease Control and Prevention (CDC): briefly, at 12 years of age, a male should be 54-63.5 in. (137-160 cm) tall and a female 55-64 in. (140-163 cm). The weight should be 66-130 lb. (29.9-58.9 kg) and 68-136 lb. ( 30-61.6 kg), respectively. Throughout childhood, a child's body becomes more proportional to other parts of his or her body. Growth is complete between the ages of 16 and 18, at which time the growing ends of bones fuse. Although a child may be growing, his or her growth pattern may deviate from the normal. Ultimately, the child should grow to normal height by adulthood. A number of hormonal conditions can lead to excessive or diminished growth. Dwarfism (very small stature) can be due to underproduction of GH, or a pathologic feedback of the GH production, included a flaw in target tissue response to growth hormones. Overproduction of GH or an exaggerated response can lead to gigantism. Gigantism is the result of GH overproduction in early childhood leading to a skeletal height up to 8 ft. (2.5 m) or more.
Development of primary sex characteristics includes the further maturing of the gonads, the testis in boys and the ovaries in girls. In both sexes, hormonal regulation of reproduction is regulated by the brain. Until eight weeks of gestation, the brain is organized in female direction irrespectively with the gender of the fetus. Successively, testosterone, for example, organizes the male brain in patterns of behavior, many of which may not appear until much later. Secondary sex characteristics can be considered traits that give an individual an advantage over its rivals in courtship. During puberty, changing hormonal levels play a role in activating the development of secondary sex characteristics. These include: (1) growth of pubic hair (pubarche); (2) Growth of the breasts (thelarche) (for girls) (3) menarche (first menstrual period for girls) or penis growth (for boys); (4) voice changes (for boys); (5) growth of underarm hair; (6) facial hair growth (for boys); (7) nighttime ejaculations (nocturnal emissions; "wet dreams" for boys) and (8) increased production of oil, increased sweat gland activity, and the beginning of acne.
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