Human Anatomy (with the Fundamentals of Dynamic and Sports Morphology) - Ivanitsky M. F. 2008

Introduction to Anatomy
Development of the Organism

Throughout its development, the Organism sequentially passes through a series of stages, starting with the fertilized egg, followed by the prenatal and postnatal periods. Once full maturity is reached, a period of decline sets in, eventually culminating in death. This entire developmental cycle is known as ontogeny, or the individual development of an organism.

The science that investigates the laws of embryo formation and development is called Embryology. The term "embryo" typically refers to the developing organism up to the third month of Pregnancy, during which the initial primordia of all Organs are laid down. From the third month until the end of prenatal life, the embryo is referred to as a fetus. The branch of science dedicated to The final stage of life associated with Aging is known as gerontology.

Germ Cells develop in the Gonads (see pp. 270, 275): the female germ Cell (ovum) in the Ovaries, and the male germ cell (spermatozoon) in the Testes. The ovum reaches up to 1/3 mm in diameter and lacks independent motility. The spermatozoon consists of a HEAD and a tail, the movements of which provide active locomotion. The forward swimming speed of a spermatozoon can reach 5 mm/min. It is significantly smaller than the female germ cell, measuring 60–70 µm in length including the tail.

Upon entering the Vagina, spermatozoa independently travel through the uterine cavity into the fallopian tubes, where Fertilization typically takes place—the union of a single egg with a single spermatozoon. Driven by the peristaltic contractions of the fallopian tubes and the beating of the cilia lining their walls, the fertilized egg (embryo) is transported into the uterine cavity, where it implants into the mucosal lining of the uterine wall. The fertilized egg begins dividing while still traversing the fallopian tube, forming a cluster of cells that successively passes through the morula stage (a solid ball of cells) and then the blastula stage (a hollow sphere of cells where the cells arrange themselves peripherally around a fluid-filled cavity) (Fig. 11, A, B). By the tenth day of development, two distinct cavities form—the amniotic cavity and the yolk sac—with the embryonic disc situated between them (Fig. 11, D). Subsequently, three primary germ layers arise within the embryo: the outer ectoderm, the inner endoderm, and the middle mesoderm (Fig. 12).

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Fig. 11. Diagram of the Cytology/cytology/16.html">Early stages of Human embryonic development:

A — blastocyst; B — 8-day embryo; C — 14–15-day embryo;

1 — inner cell mass; 2 — blastocoele; 3 — trophoblast; 4 — trophoectoderm; 5 — amniotic cavity; 6 — endoderm cells; 7 — amnion; 8 — embryo; 9 — yolk sac; 10 — mesoderm cells; 11 — body stalk; 12 — extraembryonic coelom; 13 — chorionic villi; 14 — extraembryonic mesoderm; 15 — endoderm; 16 — outer layer of the embryonic disc (after E.L. Shubnikova)

Human embryonic development exhibits several distinctive features, particularly concerning extraembryonic Materials known as provisional organs.

The embryo implants into the uterine mucosa with the participation of the trophoblast, which develops from the superficial Cells of the blastula and performs a nutritive function.

A cord-like Structure known as the notochord separates from the middle germ layer, the mesoderm (see Fig. 12), serving as the initial foundation for the body's supporting structures. The vertebral bodies subsequently form from the mesoderm surrounding the notochord. The notochord itself regresses during further development, leaving behind gelatinous nuclei within the intervertebral discs as its human remnants. A significant portion of the epithelium of the Digestive System develops from the inner germ layer, the endoderm. The middle germ layer, or mesoderm, gives rise to mesenchymal cells—the embryonic Connective Tissue that forms the circulatory and lymphatic systems, as well as Smooth Muscle tissue. Dorsally, the mesoderm divides into discrete segments called somites, which further differentiate into dermatomes (contributing to the dermis of the Skin), sclerotomes (forming the skeletal framework), and myotomes (which develop into skeletal musculature). The ventral portion of the mesoderm remains unsegmented and splits into the somatopleure and splanchnopleure, which give rise to the parietal and visceral layers of the serous membranes lining the thoracic and abdominal cavities.

Fig. 12. Cross-sectional diagram of a vertebrate trunk

Further embryonic development proceeds through uneven growth and The formation of outgrowths that transform into independent organs. For example, the Respiratory system—including the Larynx, Trachea, Bronchi, and Lungs—develops as an outgrowth from the anterior wall of the foregut; The Liver and Pancreas form as outgrowths of the duodenal mucosa; and nerves arise from the extension of nerve fibers originating in the cells of the neural tube. The limbs begin to form toward the end of the third week of embryonic development as projections from the trunk. Six pairs of visceral arches develop in the Head and Neck region: the first is termed the mandibular arch, the second the hyoid arch, and the remaining four are known as branchial (pharyngeal) arches. Outgrowths from the pharyngeal pouches between these arches give rise to the thyroid, parathyroid, and Thymus glands.

The visceral arches contribute to the Formation of the facial Skeleton, notably the upper and lower jaws, the Hyoid bone, and the auditory ossicles. The clefts between these arches transform into the auditory tube and the tympanic cavity of the Middle ear. The Arteries developing within the region of the branchial arches subsequently form the aortic arch, the carotid arteries, and several smaller vascular branches.

During prenatal life, the fetus receives nourishment through the Placenta (afterbirth), to which it is connected by the umbilical cord. Two umbilical arteries run from the fetus to the placenta, while a single umbilical vein carries Blood from the placenta to the fetus. Through the blood flowing via the umbilical vein, the fetus obtains the oxygen and nutrients essential for life and growth, while releasing carbon dioxide and Metabolic waste products back into the maternal Circulation via the placenta. Following birth, the umbilical cord is severed, and the placenta is expelled shortly after the fetus. A series of physiological adjustments takes place in the newborn's body, driven by the onset of pulmonary respiration, the independent functioning of the digestive system (to supply nutrients), and significant reorganization within The Cardiovascular system.

Following birth, the postnatal stage of human life begins. This stage is further divided into several distinct periods, which are described in detail in Chapter 9.