Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - Sapin M.R., Sivoglazov V.I. 2002

Key stages of human development

Every individual has unique personal characteristics determined by two main factors: heredity—traits inherited from parents—and environmental influences related to how a person grows, develops, learns, and works.

Individual development, or ontogeny, spans all periods of life from conception to death. Human ontogeny (derived from the Greek on, genitive ontos meaning "being") is divided into two major stages: prenatal (before birth) and postnatal (after birth). During the prenatal period, from conception until birth, the embryo develops within the mother's body. The first 8 weeks involve the primary formation of Organs and body parts; this phase is called the Embryonic period, and the developing Organism is an embryo. From the 9th week onward, once the main external human features become recognizable, the organism is referred to as a fetus, and the period as the fetal period.

Following Fertilization (the fusion of a spermatozoon and an oocyte), which typically takes place in the fallopian tube, a single-celled zygote is formed. Over the course of 3 to 4 days, the zygote undergoes Cleavage (Cell Division), resulting in a multicellular fluid-filled vesicle known as a blastocyst. The walls of this vesicle are composed of Two Types of Cells: large and small. The small cells form the outer wall, or trophoblast, which later gives rise to the outer layers of the embryonic membranes. The larger cells (blastomeres) form a cluster called the embryoblast (the embryonic germ disc), located inside the trophoblast (Fig. 1). This cell cluster gives rise to the embryo itself and its associated extraembryonic structures (excluding the trophoblast). On the 6th to 7th day of gestation, the vesicular embryo implants into the uterine mucosa. During the second week of development, the embryoblast splits into two distinct layers.

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Fig. 1. Position of the embryo and extraembryonic membranes at various stages of Human Development:

A — 2–3 weeks; B — 4 weeks; 1 — amniotic cavity, 2 — body of the embryo, 3 — yolk sac, 4 — trophoblast; C — 6 weeks; D — fetus at 4–5 months: 1 — body of the embryo (fetus), 2 — amnion, 3 — yolk sac, 4 — chorion, 5 — umbilical cord

The outer layer, adjacent to the trophoblast, is called the ectoderm (outer germ layer). The inner layer, facing the blastocyst cavity, constitutes the endoderm (inner germ layer). The edges of the inner germ layer expand outward, fold, and form the yolk sac, while the ectoderm forms the amniotic vesicle. Within the trophoblast cavity, surrounding the yolk and amniotic vesicles, extraembryonic mesoderm—an embryonic Connective Tissue—is loosely distributed. Where the yolk and amniotic vesicles come into contact, they form a two-layered plate known as the embryonic disc. The layer adjacent to the amnion forms the outer part of the embryonic disc (ectoderm), whereas the layer facing the yolk sac serves as the embryonic (intestinal) endoderm. The latter gives rise to the epithelial lining of the digestive tract and respiratory passages, as well as digestive and certain other glands, including The Liver and Pancreas.

The trophoblast together with the extraembryonic mesoderm forms the villous membrane of the embryo, known as the chorion. The chorion participates in forming the Placenta (afterbirth), through which the embryo receives nourishment from the maternal organism.

By the 3rd week of Pregnancy (days 15–17 of Embryogenesis), the embryo develops a three-layered Structure and its axial organs begin to form. Cells of the outer (ectodermal) layer of the embryonic disc migrate toward its posterior end, producing a localized thickening known as the primitive streak, which is oriented cranially. The anterior (cranial) part of the primitive streak features a slight elevation called Hensen's node (primitive node). Cells of the outer layer located anteriorly to the primitive node migrate inward between the ectodermal and endodermal layers to form the notochordal process, which ultimately develops into the notochord (Spinal Cord precursor). Cells of the primitive streak proliferating laterally and between the embryonic layers on either side of the notochord form the mesoderm, the middle germ layer, rendering the embryo trilaminar. During the 3rd week of development, the neural tube also begins to form from the ectoderm.

From the posterior region of the endodermal layer, an outpouching called the allantois extends into the extraembryonic mesoderm (the connecting stalk or amniotic stalk). Blood Vessels (umbilical vessels) grow alongside the allantois from the embryo through the amniotic stalk into the chorionic villi, subsequently forming the core of the umbilical cord.

During the 3rd and 4th weeks of development, the body of the embryo (the embryonic disc) gradually separates from the extraembryonic organs (yolk sac, allantois, and connecting stalk). The embryonic disc bends, and a deep furrow—the trunk fold—forms along its margins, demarcating the edges of the germ layers from the amnion. The embryonic body transitions from a flat disc into a three-dimensional structure, with the ectoderm eventually covering the embryo on all sides.

The endoderm enclosed within the embryonic body rolls into a tube, creating the primordium of the future gut. A narrow opening connecting the embryonic gut to the yolk sac later transforms into the umbilical ring. The endoderm gives rise to the epithelium and Glands of the gastrointestinal and respiratory tracts. Meanwhile, the ectoderm forms The Nervous system, the epidermis and its derivatives, the epithelial lining of the Oral Cavity, the anal canal, the Vagina, and other organs.

Initially, the embryonic (primitive) gut is closed at both its anterior and posterior ends. Ectodermal invaginations—the oral pit (future oral cavity) and the anal pit—appear at the cranial and caudal ends of the embryonic body, respectively. Between the primitive gut cavity and the oral pit lies a two-layered (ectoderm and endoderm) buccopharyngeal (oropharyngeal) membrane. Similarly, a two-layered cloacal membrane separates the gut from the anal pit. The buccopharyngeal membrane ruptures during the 3rd to 4th weeks of development, while the cloacal membrane breaks down in the 3rd month. The amnion, filled with Amniotic Fluid, surrounds and protects the embryo from various mechanical injuries and shocks. The growth of the yolk sac gradually slows down, leading to its regression.

Differentiation of the mesoderm begins at the end of the 3rd week of development, giving rise to mesenchyme. The dorsal portion of the mesoderm, located on either side of the notochord, segments into 43–44 pairs of body somites. Each somite differentiates into three regions: the anteromedial sclerotome, which develops into the bones and Cartilage of the Skeleton; the myotome, located more laterally, which forms the striated skeletal musculature; and the outermost dermatome, which gives rise to the dermis of the Skin.

The anterior (ventral) unsegmented mesoderm (splanchnotome) forms two layers. One of these (the medial, visceral layer) adheres to the primitive gut and is termed the splanchnopleure, while the other (the lateral, outer layer) adheres to the body wall and ectoderm, forming the somatopleure. These layers give rise to the Peritoneum and Pleura (serous membranes), and the space between them develops into the peritoneal, pleural, and pericardial cavities. The mesenchyme of the ventral unsegmented mesoderm also produces non-striated Cytology/cytology/32.html">Smooth Muscle tissue, connective tissue, Blood and Lymphatic vessels, and Blood Cells. Furthermore, mesenchyme from the splanchnotomes contributes to The formation of The Heart, Kidneys, adrenal cortex, Gonads, and other structures.

By the end of the first month of intrauterine development, the Formation of the embryo's main organs is complete, and it reaches a length of 6.5 mm.

During the 5th to 8th weeks, the embryo develops paddle-like limb buds—first for the upper extremities and subsequently for the lower ones—in the form of skin folds into which the precursors of bones, Muscles, vessels, and nerves later grow.

The primordia of the External ear appear in the 6th week, while fingers and toes begin to form during the 6th to 7th weeks. Organogenesis concludes by the 8th week. From the 3rd month of development onward, the embryo takes on a distinctly human appearance and is referred to as a fetus. Birth takes place in the 10th month.

Throughout the fetal period, continuous growth and further maturation of already formed organs and Tissues take place. Differentiation of the external genitalia begins, and fingernails start to form. Eyebrows and eyelashes appear by the end of the 5th month. In the 7th month, the eyelids open, and subcutaneous adipose tissue begins to accumulate. Following birth, the infant grows rapidly, accompanied by increases in body mass, body length, and body surface area (Table 1). Human physical growth continues throughout the first two decades of life. In males, an increase in body length typically ceases around 20–22 years of age, and in females, around 18–20 years. Stature remains largely unchanged until ages 60–65. However, in elderly and senile individuals (after 60–70 years), body height decreases by 1–1.5 mm annually due to increased spinal curvatures, changes in posture, thinning of the intervertebral discs, and flattening of the FOOT arches.

During the first year of postnatal life, an infant's height increases by 21–25 cm.

During early childhood and the first childhood period (ages 1 to 7 years), the growth rate rapidly declines; at THE START OF the second childhood period (ages 8–12), the growth velocity is 4.5–5.5 cm per year, after which it increases again. During adolescence (ages 12–16), the annual increase in body height averages about 5.8 cm in boys and approximately 5.7 cm in girls.

Table 1 Body length, body mass, and body surface area across various age periods of postnatal ontogeny

Note: Numerical data are drawn from the books Human: Morphobiological Data (1977) and Human Morphology, edited by B.A. Nikityuk and V.P. Chtetsov (1990).

Girls experience their most intensive growth spurt between the ages of 10 and 13, whereas boys experience theirs during adolescence. Following this, growth decelerates.

Body weight doubles by 5—6 months of age. It triples by the end of the first year and increases approximately 4-fold by two years of age. Body length and weight increase at roughly the same rate. The maximum annual increase in body weight is observed in adolescents: at age 13 in girls and age 15 in boys. Body weight continues to increase up to 20—25 years, after which it stabilizes. A stable body weight is typically maintained until 40—46 years of age. It is considered important and physiologically sound to maintain the body weight of one's 19—20-year-old self for the rest of one's life.

Over the past 100—150 years, there has been an acceleration in the morphofunctional development and maturation of the entire organism in children and adolescents (acceleration), which is most pronounced in economically developed countries. For instance, over the past century, the birth weight of newborns has increased by an average of 100—300 g, and that of one-year-olds by 1,500—2,000 g. Body length has also increased by 5 cm. The body length of children during the second childhood and adolescence has increased by 10—15 cm, and that of adult men by 6—8 cm. The time span during which human body length increases has shortened. At the end of the 19th century, linear growth continued until 23—26 years of age. By the end of the 20th century, linear body growth in men continues up to 20—22 years, and in women up to 18—20 years. Eruption of primary and permanent Teeth has accelerated. Mental development and Puberty occur more rapidly. By the end of the 20th century, compared to its beginning, the average age of menarche in girls decreased from 16.5 to 12—13 years, while the age of onset of menopause increased from 43—45 to 48—50 years.

Following birth, during the period of ongoing human growth, each age stage has its own distinct morphofunctional characteristics.

In a newborn infant, the HEAD is rounded and large, the neck and chest are short, the abdomen is long, the legs are short, and the arms are long (Fig. 2). The head circumference exceeds the chest circumference by 1—2 cm, and the cerebral cranium is relatively larger than the facial cranium. The shape of the rib cage is barrel-like. The spine lacks curvatures, with only a very faint sacral promontory. The bones forming the pelvic bone are not fused together. Internal Organs are relatively larger than in an adult. For example, the liver mass of a newborn accounts for ¹/₂₀ of the body weight, whereas in an adult it is ¹/₅₀. The length of the intestine is twice the body length, compared to 4—4.5 times in an adult. The Brain mass of a newborn makes up 13—14% of body weight, whereas in an adult it is only about 2%. The Adrenal Glands and Thymus are notably large in size.

Fig. 2. Changes in body proportions during growth.

CM — midline. The numbers at the top indicate the fraction of the total body height represented by the head. The divisions marked by numbers on the right correspond to the body proportions of children and adults; the numbers at the bottom indicate age

During infancy (10 days — 1 year), the child's body grows most rapidly. Eruption of primary teeth begins at approximately 6 months of age. Over the first year of life, the dimensions of several organs and systems reach adult-sized proportions (the eyes, Inner ear, Central Nervous System). During the first years of life, the musculoskeletal, digestive, and respiratory systems grow and develop rapidly.

During early childhood (1—3 years), all primary teeth erupt, and the first "rounding" occurs—meaning that the increase in body weight outpaces linear body growth. The child's mental development, speech, and memory progress rapidly. The child begins to navigate space. Over the 2nd and 3rd years of life, linear growth predominates over weight gain. Towards the end of this period, permanent teeth begin to erupt. Due to the rapid Development of the brain—whose mass reaches 1,100—1,200 g by the end of this period—mental capacities and causal thinking develop rapidly, and the capacity for recognition, orientation in time, and knowing the days of the week is retained for long periods.

In early and first childhood (4—7 years), Sexual Dimorphism (aside from primary sex characteristics) is barely noticeable.

During the second childhood period (8—12 years), lateral growth (broadening) predominates once again; however, puberty begins during this time, and towards the end of the period, linear body growth accelerates, with a faster rate in girls.

Children's mental development progresses. Orientation regarding months and calendar days improves. Puberty begins, occurring earlier in girls due to an increase in the secretion of Female Sex Hormones. In girls aged 8—9, the pelvis begins to widen and the hips to round out, the secretion of Sebaceous Glands increases, and pubic Hair appears. In boys aged 10—11, the Larynx, Testes, and Penis begin to grow, with the latter increasing by 0.5—0.7 cm by age 12.

During adolescence (12—16 years), the sex organs grow and develop rapidly, and secondary sex characteristics become more pronounced. In girls, pubic hair density increases, underarm hair appears, and the size of the genitalia and Mammary Glands increases; the alkaline reaction of the vaginal secretions becomes acidic, menstruation begins, and the pelvis widens. In boys, the testes and penis grow rapidly; initially, pubic hair develops in a female distribution pattern, and the mammary glands swell. Towards the end of adolescence (15—16 years), facial, body, and underarm hair appears, pubic hair develops in a male distribution pattern, the skin of the Scrotum becomes pigmented, the genitalia enlarge further, and the first ejaculations (involuntary emissions of semen) occur.

Mechanical and verbal-logical memory develop during adolescence.

Youth/adolescence (16—21 years) coincides with the period of maturation. At this age, the GROWTH AND DEVELOPMENT of the organism are largely completed, and all Organ Systems virtually reach morphofunctional maturity.

Body structure changes little during adulthood (22—60 years), whereas old age (61—74 years) and senility (75—90 years) exhibit characteristic age-related reorganizations studied by a specialized science known as gerontology. The temporal boundaries of Aging vary widely among different individuals. Senescence involves a decline in the body's adaptive capabilities and alterations in the morphofunctional parameters of all organ systems, among which the immune, nervous, and circulatory systems play a critical role.

An active lifestyle and regular Physical Exercise slow down the aging process. However, this is only possible within the limits determined by hereditary factors.

Men and women are distinguished by their sex characteristics (Table 2). These are divided into primary (reproductive organs) and secondary (development of pubic hair, breast development, voice changes, etc.).

In anatomy, there are concepts regarding body types (somatotypes). Body type is determined by genetic (hereditary) factors, environmental influences, and social conditions. Three main human body types are distinguished: mesomorphic, brachymorphic, and dolichomorphic. In the mesomorphic (from Greek *mesos* — middle, *morphe* — form, shape) body type (normosthenics), the Anatomical Features of body structure approximate average normal values (adjusted for AGE AND SEX). Individuals with a brachymorphic (from Greek *brachys* — short) body type (hypersthenics) are short in stature, have a broad torso, and are prone to corpulence. Their Diaphragm is positioned high, the heart lies almost transversely upon it, the Lungs are short, and muscles are well developed. Individuals with a dolichomorphic (from Greek *dolichos* — long) body type are tall and have long limbs. Musculature is poorly developed. The diaphragm is positioned low, the lungs are long, and the heart is oriented almost vertically.

Table 2 Certain sexual differences between men (m) and women (f)

Human anatomy studies The structure of a normal (average) human being, which is why this branch of anatomy is termed Normal Anatomy. To facilitate The Study of the positions of organs and body parts, three mutually perpendicular planes are used. The sagittal plane (from Greek *sagitta* — arrow) vertically bisects the body from front to back. The frontal plane (from Latin *frons* — forehead) lies perpendicular to the sagittal plane and is oriented from right to left. The horizontal plane is perpendicular to the first two and separates the upper part of the body from the lower.

A multitude of such planes can be passed through The Human Body. The sagittal plane that separates the right half of the body from the left is called the median plane. The frontal plane separates the anterior part of the body from the posterior.

In anatomy, the terms medial (closer to the median plane) and lateral (situated further away from the median plane) are used. To designate PARTS OF THE upper and lower limbs, the terms proximal (situated closer to the Water/144.html">Origin of the limb) and distal (situated further from the torso) are applied.

When studying anatomy, terms such as right and left, Major and minor, superficial and deep are employed.

When determining THE POSITION OF internal organs in a living human and projecting their boundaries onto the body surface, vertical lines drawn through specific anatomical landmarks are used. The anterior median line runs down the middle of the anterior surface of the body. The posterior median line runs along the spinous processes of the vertebrae. Both of these lines divide the body into right and left halves. The right and left sternal (parasternal) lines run along the respective borders of the Sternum. The midclavicular line passes vertically through the midpoint of the clavicle. The axillary (anterior, middle, and posterior) lines are drawn through the apex and the respective margins of the axillary fossa. The scapular line passes through the inferior angle of the scapula. The paravertebral line runs parallel to the spine through the costotransverse joints.

Review and Self-Assessment Questions:

1. WHAT IS A zygote? How and where is it formed?

2. From which embryonic structures do the ectoderm and endoderm develop? What organs subsequently arise from them?

3. When and from what source is the middle germ layer formed?

4. What parts are distinguished in the somites and the splanchnotome?

5. What factors influence embryonic development?

6. What anatomical features are characteristic of a newborn?

7. Which organ systems and apparatuses show the most rapid growth and development in children, adolescents, and young adults?

8. Name the body types familiar to you and describe their distinct characteristics.