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

Main stages of individual human development
Prenatal ontogenesis

To understand the individual features of Human Body Structure, one must first examine The Development of the human Organism during the prenatal period. The fact is that every person possesses unique characteristics of external appearance and internal structure, the presence of which is determined by two main factors. These are heredity—traits inherited from parents—and the Influence of the external environment in which a person grows, develops, learns, and works.

During the prenatal period, spanning from conception to birth over the course of 280 days (9 calendar months), the embryo (or fetus) develops inside the mother's body (from Fertilization until birth). The first 8 weeks are marked by the Primary processes of organ and body part formation. This stage is designated as the Embryonic period, and the developing human organism is called an embryo. From the 9th week onward, when the main external human features begin to take shape, the organism is referred to as a fetus, and the period as the fetal stage (from the Greek fetus — offspring).

The development of a new organism begins with fertilization (the fusion of a sperm and an oocyte), which typically takes place in the fallopian tube. The united Germ Cells form a qualitatively new unicellular embryo—the zygote—possessing all the combined properties of both parent cells. This exact moment marks the beginning of the development of a new (daughter) organism.

Optimal conditions for sperm-egg interaction are usually established within 12 hours following ovulation. The union of the sperm Nucleus with the egg nucleus results in the formation within the unicellular organism (zygote) of the human diploid chromosome set (46). The sex of the future child is determined by the chromosome combination within the zygote and depends on the father's sex Chromosomes. If the oocyte is fertilized by a sperm bearing an X sex chromosome, the resulting diploid set will contain two X chromosomes, characteristic of the female organism. Conversely, fertilization by a sperm bearing a Y sex chromosome produces an XY combination of sex chromosomes in the zygote, which is characteristic of the male organism.

The first week of embryonic development is the period of Cleavage (division) of the zygote into daughter cells (Fig. 1, see color insert). Immediately following fertilization and throughout the first 3–4 days, the zygote divides while simultaneously migrating down the fallopian tube toward the uterine cavity. As a result of zygote division, a multicellular fluid-filled vesicle—the blastocyst—is formed (from the Greek blastos — sprout). The walls of this vesicle are composed of Two Types of cells: large and small. The outer layer of small cells forms the vesicle wall, known as the trophoblast. Subsequently, trophoblast cells form the outer layer of the embryonic membranes. Larger, darker cells (blastomeres) form a cluster called the embryoblast (inner Cell mass, or embryonic germ disc), which is located internal to the trophoblast. The embryo and its associated extraembryonic structures (excluding the trophoblast) develop from this cell cluster (embryoblast). A small amount of fluid accumulates between the surface layer (trophoblast) and the embryonic knot. By the end of the 1st week of development (days 6–7 of gestation), the embryo reaches the Uterus and embeds (implants) into its mucous membrane; implantation takes about 40 hours. The superficial Cells of the embryo that form the vesicle—the trophoblast (from the Greek trophe — Nutrition)—secrete an enzyme that loosens the Superficial layer of the uterine mucosa, which is prepared for embryo implantation. The developing trophoblast villi (projections) come into direct contact with the Blood Vessels of the maternal organism. Numerous trophoblast villi increase the surface area of contact with the uterine mucosal Tissues. The trophoblast transforms into the nutritive membrane of the embryo, known as the chorion. Initially, the chorion features villi on all sides; later, these villi persist only on the side facing the uterine wall. At this site, a new organ develops from the chorion and the adjacent uterine mucosa—the Placenta (afterbirth). The placenta is the organ that connects the maternal organism with the embryo and ensures its nourishment.

The second week of embryonic life is the stage when embryoblast cells divide into two layers (two plates), from which two vesicles are formed (Fig. 2). The outer layer of cells adjacent to the trophoblast forms the ectoblastic (amniotic) vesicle. The inner layer of cells (the embryonic rudiment, or embryoblast) forms the endoblastic (yolk) vesicle. The primordium (the "body") of the embryo is located where the amniotic vesicle comes into contact with the yolk vesicle. At this stage, the embryo is a bilayered disc consisting of two germ layers: the outer germ layer (ectoderm) and the inner germ layer (endoderm). The ectoderm faces the amniotic vesicle, while the endoderm is adjacent to the yolk vesicle. At this stage, the surfaces of the embryo can be distinguished: the dorsal surface is adjacent to the amniotic vesicle, and the ventral surface faces the yolk vesicle. The trophoblast cavity surrounding the amniotic and yolk vesicles is loosely filled with cords of extraembryonic mesenchyme cells. By the end of the 2nd week, the length of the embryo is only 1.5 mm. During this period, the embryonic disc thickens in its posterior (caudal) region, where the axial Organs (notochord, neural tube) subsequently begin to develop.

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Fig. 2. 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 third week of embryonic life is the period of Formation of the trilaminar disc (embryo). Cells of the outer ectodermal plate of the embryonic disc shift toward its posterior end. As a result, a cellular ridge (primitive streak) is formed, elongated along the longitudinal axis of the embryo. In the cranial (anterior) part of the primitive streak, cells grow and multiply more rapidly, resulting in a small elevation—the primitive node (Hensen's node). The Location OF THE primitive node marks the cranial (HEAD) end of the embryonic body.

Multiplying rapidly, the cells of the primitive streak and primitive node proliferate laterally between the ectoderm and endoderm, thereby forming the middle germ layer—the mesoderm. Mesoderm cells located between the layers of the disc are called intraembryonic mesoderm, while those that migrate beyond its boundaries are termed extraembryonic mesoderm.

A portion of the mesoderm cells within the region of the primitive node grows forward particularly actively from the head and tail ends of the embryo, penetrates between the outer and inner layers, and forms a cellular cord—the notochord. At the end of the 3rd week of development, active cell proliferation occurs in the anterior part of the outer germ layer, forming the neural plate. This plate soon bends, creating a longitudinal depression known as the neural groove. The edges of the groove thicken, approach each other, and fuse, closing the neural groove into the neural tube. Subsequently, the entire Nervous system develops from the neural tube. The ectoderm closes over the newly formed neural tube and loses its connection with it.

During this same period, a finger-like protrusion—the allantois—grows from the posterior part of the endodermal plate of the embryonic disc into the extraembryonic mesenchyme (into the so-called amniotic stalk); in humans, this structure performs no specific Functions. Along the path of the allantois, embryonic umbilical (placental) blood vessels grow from the embryo toward the chorionic villi. The vascular cord connecting the embryo with the extraembryonic membranes (placenta) forms the body stalk.

Thus, by the end of the 3rd week of development, the human embryo appears as a trilaminar plate, or trilaminar disc. In the region of the outer germ layer, the neural tube is visible, and deeper lies the notochord; in other words, the axial Organs of the human embryo make their appearance. By the end of the third week of development, the length of the embryo is 2–3 mm.

During the fourth week of life, the embryo, having the appearance of a trilaminar disc, begins to fold in both transverse and longitudinal directions. The embryonic disc becomes convex, and its margins are demarcated from the surrounding amnion by a deep furrow—the trunk fold. The body of the embryo transforms from a flat disc into a volumetric structure, with the ectoderm covering the embryonic body on all sides.

The ectoderm subsequently gives rise to The Nervous System, the epidermis of The Cytology/cytology/66.html">Skin and its Derivatives, as well as the epithelial lining of the Oral Cavity, the anal portion of the rectum, and the Vagina. The mesoderm gives rise to Internal Organs (except for endodermal derivatives), The Cardiovascular system, the musculoskeletal apparatus (bones, joints, Muscles), and the dermis proper.

The endoderm, now enclosed within the body of the human embryo, rolls into a tube and forms the embryonic primordium of the future gut. A narrow opening connecting the embryonic gut to the yolk sac later transforms into the umbilical ring. The endoderm forms the epithelium and all the Glands of the digestive and respiratory systems.

The embryonic (primitive) gut is initially closed at both its anterior and posterior ends. In the anterior and posterior Regions of the embryonic body, ectodermal invaginations appear—the oral pit (future oral cavity) and the anal pit. Between the primitive gut cavity and the oral pit lies a bilayered (ectoderm and endoderm) anterior (oropharyngeal) plate (membrane). Between the gut and the anal pit lies the cloacal (anal) plate (membrane), which is also bilayered. The anterior (oropharyngeal) membrane ruptures during the 4th week of development, while the posterior (anal) membrane ruptures during the 3rd month.

As a result of this folding, the body of the embryo becomes surrounded by the Contents of the amnion—Amniotic Fluid—which serves as a protective medium shielding the embryo from damage, primarily mechanical shocks and jolts.

The yolk sac lags in growth, appearing as a small sac by the 2nd month of prenatal development, and subsequently regresses (disappears) completely. The body stalk elongates, becomes relatively slender, and is later referred to as the umbilical cord.

During the 4th week of embryonic development, the Differentiation of the mesoderm—which began in the 3rd week—continues. The dorsal part of the mesoderm, located along the sides of the notochord, forms paired thickened protrusions known as somites. The somites become segmented, meaning they divide into metameric units; hence, the dorsal part of the mesoderm is termed segmented. Somite segmentation occurs progressively in a craniocaudal direction (from front to back). On the 20th day of development, the 3rd pair of somites is formed; by the 30th day, there are already 30 pairs, and by the 35th day, 43–44 pairs. The ventral part of the mesoderm is not divided into segments, forming two plates (the unsegmented mesoderm) on either side. The medial (visceral) plate lies adjacent to the endoderm (primitive gut) and is called the splanchnopleure. The lateral (outer) plate lies adjacent to the body wall of the embryo and the ectoderm, and is named the somatopleure (Fig. 3, see color insert). The epithelial lining of the serous membranes (mesothelium), as well as the lamina propria and subserous layer of the serous membranes, develop from the splanchnopleure and somatopleure. The mesenchyme of the splanchnopleure also contributes to The formation of all layers of the digestive tube, except for the epithelium and glands, which form from the endoderm. The space between the plates of the unsegmented mesoderm transforms into the embryonic body cavity, which is subdivided into the peritoneal, pleural, and pericardial cavities.

At the boundary between the somites and the splanchnopleure, the mesoderm forms nephrotomes (segmental stalks), from which the tubules of the pronephros (primitive Kidney) and the Gonads develop. Three primordia arise from the dorsal part of the mesoderm—the somites. The anteromedial region of the somites (sclerotome) contributes to skeletal tissue, giving rise to the Cartilage and BONES OF THE Axial Skeleton—THE Vertebral Column. Lying lateral to it is the myotome, from which skeletal musculature develops. In the posterolateral part of the somite lies the dermatome, from the tissue of which the Connective Tissue foundation of the skin—the dermis—is formed.

In the head region of the embryo, on each side, ectodermal primordia of the Inner ear (initially otic pits, then otic vesicles) and the future lens of the eye are formed during the 4th week. At the same time, the visceral regions of the head undergo remodeling, forming the frontal and maxillary processes around the oral bay (stomodeum). Caudal to these processes, the contours of the mandibular and hyoid visceral arches are visible.

On the anterior surface of the embryonic trunk, prominences are visible: the cardiac prominence, followed by the hepatic prominence. The depression between these elevations marks the site of formation of the septum transversum—one of the primordia of the Diaphragm. Caudal to the hepatic prominence lies the body stalk (umbilical cord), which contains large blood vessels and connects the embryo to the placenta. By the end of the 4th week, the length of the embryo is 4–5 mm.

During the period from the 5th to the 8th week of embryonic life, the formation of organs (Organogenesis) and tissues (histogenesis) continues. This is a time of Early Development of The Heart and Lungs, increased structural complexity of the intestinal tube, formation of the visceral arches, and Development of the sensory organ capsules. The neural tube closes completely and expands in the cranial region (forming the future Brain). At an age of approximately 31–32 days (5th week), the length of the embryo is

7.5 mm. At the level of the lower cervical and 1st thoracic segments, fin-like arm bud primordia begin to appear. By the 40th day, leg buds are formed.

At week 6 (embryonic crown-rump length — 12–13 mm), the primordia of the External ear become noticeable; from the end of week 6 to 7, the primordia of the fingers appear, followed by the toes.

By the end of week 7 (embryo length — 19–20 mm), the eyelids begin to form, giving the eyes a more defined outline. By week 8 (embryo length 28–30 mm), the formation of embryonic organs is complete. From week 9, i.e., the beginning of the 3rd month, the embryo (crown-rump length 39–41 mm) acquires a human appearance and is referred to as a fetus.

Starting from the third month and throughout the entire fetal period, further GROWTH AND DEVELOPMENT of the formed organs and body parts take place. At this time, the differentiation of the external genitalia also begins. Fingernails and toenails begin to develop. From the end of the 5th month (length 24.3 cm), eyebrows and eyelashes become visible. In the 7th month (length 37.1 cm), the eyelids open, and subcutaneous fat begins to accumulate. In the 10th month (length 51 cm), the fetus is born.

Critical Periods of ontogenesis. Throughout individual development, there are critical periods characterized by increased sensitivity of the developing organism to damaging environmental and internal factors. Several critical periods of development are distinguished. The most hazardous periods include: 1) the time of germ cell development — oogenesis and Spermatogenesis; 2) the moment of gamete fusion — fertilization; 3) embryo implantation (days 4–8 of Embryogenesis); 4) the formation of axial organ primordia (brain, Spinal Cord, vertebral column, primitive gut) and placenta formation (weeks 3–8 of development); 5) the stage of accelerated brain growth (weeks 15–20); 6) the formation of functional body systems and differentiation of the Urogenital apparatus (weeks 20–24 of the prenatal period); 7) the moment of birth and the neonatal period — transition to extrauterine life, along with metabolic and functional adaptation; 8) early and first childhood (ages 2 to 7), when the formation of interrelationships between organs, systems, and organ apparatuses is completed; 9) adolescence (Puberty — in boys from 13 to 16 years, in girls from 12 to 15 years). Concurrently with the rapid growth of the reproductive organs, emotional activity becomes heightened.

Review and Self-Assessment Questions:

1. Name the periods of human prenatal development.

2. What determines the sex of the unborn child?

3. When and from which cells do the embryoblast and trophoblast form?

4. What do these structures represent?

5. Which structure is referred to as the unsegmented part of the mesoderm? What develops from it?

6. Which organs develop from the endoderm, mesoderm, and ectoderm?

7. Which embryonic organs are called axial? When and from what do they form?

8. Name the timing of the critical periods of embryonic development.