Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Key stages of individual human development
Postnatal ontogenesis

Immediately after birth, the neonatal period begins. This stage is defined by the fact that the newborn is fed colostrum for 8–10 days. During the initial period of adaptation to extrauterine life, newborns are classified by their level of maturity into full-term and pre-term. Intrauterine development lasts 39–40 weeks for full-term infants and 28–38 weeks for pre-term infants. When determining maturity, not only these timeframes are taken into account, but also birth weight and body length.

Newborns with a body weight of at least 2500 g (and a body length of at least 45 cm) are considered full-term, while those with a body weight of less than 2500 g are considered pre-term. In addition to weight and length, other measurements are also considered, such as chest circumference in relation to body length and HEAD circumference in relation to chest circumference. It is generally accepted that chest circumference at the nipple line should exceed 0.5 of the body length by 9–10 cm, while head circumference should exceed chest circumference by no more than 1–2 cm.

The next stage—infancy—lasts up to one year. The beginning of this period is marked by the transition to feeding with "mature" milk. Infancy exhibits the highest growth intensity compared to all other periods of extrauterine life. Body length increases by 1.5 times from birth to one year of age, and body weight triples. Milk Teeth begin to erupt at 6 months of age. Uneven body growth is particularly pronounced during infancy: infants grow faster in the first six months of life than In the second. New developmental milestones appear every month of the first year of life. In the first month, the infant begins to smile in response to adults speaking to them; at 4 months, they make persistent attempts to stand on their feet (with support); at 6 months, they try to crawl on all fours; at 8 months, they make attempts to walk; and by one year of age, the child usually walks independently.

The early childhood period lasts from 1 to 4 years of age. Tooth eruption is completed by the end of the second year of life. After 2 years of age, both the absolute and relative values of annual body size increments decrease rapidly.

The first childhood period begins at 4 years of age and ends at 7 years. Starting from age 6, the first permanent teeth appear: the first molar and the medial incisor in the lower jaw.

The age span from 1 to 7 years is also referred to as the period of neutral childhood, since boys and girls show almost no differences in body size and shape.

The second childhood period lasts from 8 to 12 years for boys and from 8 to 11 years for girls. During this period, Sexual Dimorphism in body size and shape becomes apparent, and an accelerated increase in body length begins. Girls experience higher growth rates than boys because Puberty begins, on average, two years earlier in girls. The increased secretion of Sex Hormones (especially in girls) triggers The Development of secondary sex characteristics. The sequence in which secondary sex characteristics appear is fairly consistent. In girls, the Mammary Glands develop first, followed by the appearance of pubic Hair, and subsequently axillary hair. The Uterus and Vagina develop concurrently with The formation of the mammary glands. The process of puberty is much less pronounced in boys at this stage; only towards the end of this period do they experience accelerated growth of the Testes and Scrotum, followed by the Penis.

The next stage—adolescence—is also referred to as the puberty period. It lasts from 13 to 16 years for boys and from 12 to 15 years for girls. During this time, a further increase in growth rates is observed—the pubertal growth spurt—which affects all body dimensions. Girls experience the greatest gains in body length between 11 and 12 years of age, and in body weight between 12 and 13 years. In boys, the peak increase in length occurs between 13 and 14 years of age, while the peak increase in weight occurs between 14 and 15 years. The velocity of linear body growth is particularly high in boys, As a result of which they surpass girls in body length by 13.5–14 years of age. Secondary sex characteristics form due to increased activity of the hypothalamic-pituitary system. In girls, the Development of the mammary glands continues, and pubic and axillary hair growth is observed. The clearest indicator of sexual maturation in the female body is menarche (the first menstruation).

Intensive sexual maturation of boys takes place during adolescence. By the age of 13, their voice breaks (mutates) and pubic hair appears, while axillary hair appears at age 14. At 14–15 years of age, boys experience their first nocturnal emissions (involuntary seminal discharges).

Compared to girls, boys have a longer pubertal period and a more pronounced pubertal growth spurt.

Youth (adolescent transition) age lasts from 18 to 21 years for young men and from 17 to 20 years for young women. During this period, the processes of growth and body formation are largely completed, and all major body dimensions reach their definitive (final) size.

In youth, the Formation of the Reproductive System and the maturation of reproductive function are completed. Ovulatory cycles in women, as well as the rhythmicity of testosterone secretion and The production of mature sperm in men, become definitively established.

In adulthood, body shape and Structure change very little. Between the ages of 30 and 50, body length remains constant, after which it begins to decrease. Gradual involutionary changes occur in the body during the elderly and senile ages.

Individual variations in the process of GROWTH AND DEVELOPMENT can fluctuate within wide ranges. The existence of individual differences in growth and development processes provided the basis for introducing METABOLISM/2.html">THE CONCEPT OF biological age, or developmental age (as opposed to chronological/passport age).

The main criteria of biological age are considered to be:

1) "skeletal maturity" (the sequence and timing of skeletal ossification);

2) "dental maturity" (the timing of eruption of deciduous and permanent teeth);

3) the degree of development of secondary sex characteristics. For each of these biological age criteria—"external" (integumentary), "dental," and "skeletal"—evaluation scales and normative tables have been developed, allowing chronological (passport) age to be determined based on morphological features.

Factors influencing individual development (ontogeny) are subdivided into hereditary and environmental (environmental influences).

The degree of hereditary (genetic) influence varies across different stages of growth and development. The impact of hereditary factors on total body dimensions intensifies from the neonatal period to second childhood, followed by a weakening towards 12–15 years of age.

The Influence of Environmental factors on the morphofunctional maturation processes of the Organism is clearly observed in the timing of menarche (menstruation). Studies of growth processes in children and adolescents across various geographical zones have shown that climatic factors have almost no effect on growth and development unless living conditions are extreme. Adaptation to extreme conditions causes such profound alterations in the functioning of the entire organism that it inevitably impacts growth processes.

Body dimensions are divided into total (from French *total* — whole) and partial (from Latin *pars* — part). Total (overall) body dimensions are the primary indicators of human physical development. These include body length and weight, as well as chest circumference. Partial body dimensions are components of total size and characterize the magnitude of individual body parts.

Body dimensions are determined through anthropometric examinations of various population groups.

The majority of anthropometric indicators exhibit significant individual variations. Table 2 presents some averaged anthropometric indicators in postnatal ontogeny.

Body proportions depend on a person's AGE AND SEX (Fig. 4). Body length and its age-related changes are generally individual. For example, variations in the birth length of newborns born at full-term Pregnancy typically fall within the range of 49–54 cm. The greatest increase in children's body length is observed During the first year of life and averages 23.5 cm. In the period from 1 to 10 years, this indicator gradually decreases by an average of 10.5 to 5 cm per year. From the age of 9, sex differences in growth rate begin to appear. For most people, body weight gradually increases from the first days of life up to approximately 25 years of age, and then remains unchanged. After 60 years of age, body weight generally begins to decrease gradually, mainly as a result of atrophy in Tissues and a reduction in their Water content. Total body mass is composed of several components: the mass of the Skeleton, musculature, adipose tissue, Internal Organs, and Skin. The average body weight is 52–75 kg for men and 47–70 kg for women.

In the elderly and senile age groups, distinct changes are observed not only in body size and weight, but also in body composition; these changes are studied by a specialized science called gerontology (from gerontos meaning old man).

It should be especially emphasized that an active lifestyle and regular Physical Exercise significantly slow down the Aging process.

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Fig. 4. Changes in body proportions during human growth. KM is the midline. Numbers on the right show the body part ratios for children and adults, and numbers at the bottom indicate age

Table 2. Body length, weight, and surface area in postnatal ontogeny

Note. Sex: M — male, F — female. Data sourced from the books "Human: Morphobiological Data" (1977) and "Human Morphology", edited by B.A. Nikityuk and V.P. Chtetsov (1990).

It is worth noting that over the past 100–150 years, there has been a marked acceleration in the somatic development and physiological maturation of children and adolescents—a phenomenon known as acceleration (from the Latin acceleratio, meaning acceleration). Another term used to describe this trend is "secular shift." Acceleration is characterized by a complex combination of interrelated morphological, physiological, and psychological phenomena. To date, specific morphological indicators of acceleration have been identified.

For instance, over the past 100–150 years, the birth length of newborns has increased on average by 0.5–1 cm, and body weight by 100–300 g. During this same period, maternal placental weight has also increased. Furthermore, an earlier equalization of chest and head circumference ratios has been observed (occurring between the 2nd and 3rd month of life). Modern one-year-old infants are 5 cm taller and 1.5–2 kg heavier than their peers in the 19th century.

Over the past century, the height of preschool children has increased by 10–12 cm, and that of school-aged children by 10–15 cm.

In addition to increases in body length and weight, acceleration is characterized by the enlargement of individual body parts (limb segments, skinfold thickness, etc.). For example, the increase in chest circumference relative to body height has been relatively modest. Puberty in modern adolescents sets in approximately two years earlier. Motor Functions have also accelerated: modern teenagers run faster, jump further in the standing long jump, and perform more pull-ups on the horizontal bar.

The secular shift (acceleration) affects all stages of human life, from birth to death. For example, the body height of adults is also increasing, though to a lesser extent than in children and adolescents. Specifically, among 20–25-year-olds, the average height of men has increased by about 8 cm.

Acceleration affects the entire organism, influencing body size, organ and skeletal growth, as well as the maturation of the Gonads and the Skeletal System. These developmental changes are generally more pronounced in males than in females.

Men and women are distinguished by sex characteristics. These include primary characteristics (reproductive organs) and secondary characteristics (such as pubic hair distribution, breast development, voice changes, etc.), as well as distinct features of physique and body proportions.

Human body proportions are calculated as percentages based on longitudinal and transverse measurements taken between reference landmarks established on various skeletal prominences.

The harmony of body proportions is one of the key criteria in assessing a person's health status. Disproportionate body structure may indicate a disruption in growth processes and underlying causes (endocrine, chromosomal, etc.). Based on body proportion calculations, anatomy distinguishes three main human somatotypes: mesomorphic, brachymorphic, and dolichomorphic. The mesomorphic somatotype (normosthenics) includes individuals whose anatomical traits closely approach standardized average parameters (accounting for age, sex, etc.). Individuals with a brachymorphic somatotype (hypersthenics) are characterized by predominant transverse dimensions, well-developed musculature, and relatively short stature. Their Heart lies in a more transverse position due to a high Diaphragm. In hypersthenics, the Lungs are shorter and wider, and the loops of the Small Intestine are arranged primarily horizontally. Individuals with a dolichomorphic somatotype (asthenics) feature a predominance of longitudinal dimensions, relatively longer limbs, poorly developed Muscles, a thin layer of subcutaneous fat, and narrow bones. Their diaphragm is positioned lower, resulting in longer lungs, and The Heart is oriented almost vertically. Table 3 presents the relative dimensions of body parts across different somatotypes.

Table 3. Body proportions (according to P.N. Bashkirov)