Subject of embryology and periodization of development
Embryology is a branch of biology that studies the early stages of organism development, including progenesis, embryogenesis, and the early postnatal period. Ontogenesis is the individual development of an organism, consisting of the embryonic and postnatal periods. Progenesis combines gametogenesis and fertilization, while embryogenesis encompasses zygote formation, cleavage, gastrulation, histogenesis, and organogenesis.
In obstetric practice, embryonic development is divided into three periods:
- Initial — the first week of development;
- Embryonic — from the 2nd to the 8th week;
- Fetal — from the 9th week until birth.
Germ cells and gametogenesis
Germ cells have a haploid set of chromosomes, are incapable of division, are highly differentiated, and possess an altered nuclear-cytoplasmic ratio.
Spermatozoon, about 70 µm long, consists of a head, neck, and tail. The head contains the nucleus and an acrosome with enzymes that digest the egg membranes. The neck contains two centrioles, with the distal one participating in the formation of the axial filament. The tail is divided into intermediate (with mitochondria), principal, and end pieces; movement is driven by the protein dynein, which converts ATP energy.
Ovum in humans is spherical, about 130 µm, isolecithal, and oligoecithal (low-yolk). It is surrounded by the oolemma, zona pellucida, and corona radiata.
Spermatogenesis occurs in the seminiferous tubules and includes the periods of proliferation, growth, maturation (two meiotic divisions without an interphase), and spermiogenesis. The full cycle takes 60–65 days. Oogenesis takes place in the ovaries: proliferation of oogonia ends during embryogenesis; growth (small and large) lasts until menopause; maturation involves two unequal divisions resulting in a secondary oocyte and polar bodies, with the second division completing in the uterine tube after sperm penetration.
Fertilization and conditions for its occurrence
Fertilization is the fusion of male and female gametes to form a zygote. It includes three phases:
- Distant interaction — capacitation, rheotaxis, chemotaxis;
- Contact interaction — acrosomal reaction, breakdown of the corona radiata and zona pellucida, fusion of oolemma and sperm plasma membrane;
- Penetration — entry of the spermatozoon and formation of the fertilization membrane, which blocks polyspermy.
Required conditions: an ejaculate volume containing at least 150 million spermatozoa (≥60 million/ml), patency of the female genital tract, normal uterine position, normal body temperature, and an alkaline environment. The zygote exists for up to 24 hours.
Cleavage, blastocyst, and implantation
In humans, cleavage is holoblastic, unequal, and asynchronous. Blastomeres divide into dark, large ones (forming the embryoblast) and light, small ones (forming the trophoblast). The first cleavage division begins 30 hours after fertilization. On days 3–4, a morula is formed, and from day 4 onwards, a blastocyst with an embryoblast, trophoblast, and blastocoele develops.
On the 5th day, the blastocyst reaches the uterus; implantation into the endometrium begins on the 7th day, consisting of adhesion and invasion phases and lasting about 40 hours. The trophoblast secretes proteolytic enzymes, ensuring the transition from histiotrophic to hematotrophic nutrition. By the 8th day, the embryo is fully embedded in the endometrium.
Gastrulation and extraembryonic organs
Gastrulation proceeds in two phases. The first (days 7–8) occurs via delamination, forming the epiblast and hypoblast. Between the phases (days 9–14), extraembryonic mesenchyme and three extraembryonic organs develop: chorion, amnion, and yolk sac. The second phase (days 14–17) leads to the formation of the three germ layers.
The trophoblast becomes bilaminar (cytotrophoblast and syncytiotrophoblast), and after invasion by extraembryonic mesenchyme, it becomes trilaminar and is termed the chorion. Chorionic villi progress through primary, secondary, and tertiary stages (containing blood vessels). The chorion is divided into the smooth chorion and the villous chorion; the placenta develops from the latter.
- Amnion — a wall composed of extraembryonic ectoderm and mesenchyme; produces amniotic fluid and performs a protective function;
- Yolk sac — extraembryonic endoderm and mesenchyme; provides hematopoiesis and the formation of gonocytes;
- Allantois — an outpocketing of the embryonic endoderm into the connecting stalk, serving as a pathway for umbilical vessels.
During the second and third weeks, the axial organs are laid down: notochord, neural tube, and gut tube.
Placenta and differentiation of germ layers
Placenta consists of maternal (basal decidua) and fetal (villous chorion) parts. Functions: metabolism, antibody transfer, endocrine (progesterone, human chorionic somatomammotropin, relaxin, prolactin), detoxification, and acting as a placental barrier. The structural and functional unit is the cotyledon; by the 140th day, 10–12 large, 40–50 small, and up to 150 rudimentary cotyledons are formed.
Mesoderm is divided into somites, nephrotomes, paramesonephric ducts, and splanchnotomes. A somite includes the dermatome (dermis), myotome (skeletal muscle tissue), and sclerotome (cartilage and bone). The nephrotome gives rise to the epithelium of the kidneys and ductus deferens; the paramesonephric duct gives rise to the epithelium of the uterine tubes and uterus; the splanchnotome gives rise to the mesothelium, adrenal cortex, and myocardium; the mesenchyme of the splanchnotome gives rise to blood cells, connective tissue, blood vessels, smooth myocytes, and microglia.
Ectoderm is subdivided into neuroectoderm (neural tube, neural crest), surface ectoderm (epidermis), prechordal plate derivatives (epithelium of the esophagus, trachea, bronchi), and placodes (otic, lens). Endoderm is divided into intestinal and yolk endoderm; the intestinal endoderm gives rise to the epithelium and glands of the stomach and intestines, liver, and pancreas.
Critical periods of development
Critical periods are the stages of highest susceptibility of the organism to damaging factors:
- Gametogenesis;
- Fertilization;
- Implantation;
- Placentation and formation of axial complexes;
- Accelerated brain growth (weeks 15–20);
- Formation of the reproductive system;
- Childbirth and the neonatal period up to one year;
- Puberty.
Key points
- Human embryogenesis is divided into initial, embryonic, and fetal periods.
- Spermatogenesis takes 60–65 days, while oogenesis is completed only after fertilization in the uterine tube.
- Fertilization goes through the phases of distant interaction, contact interaction, and penetration, resulting in the formation of the fertilization membrane.
- Human cleavage is holoblastic, unequal, and asynchronous; a blastocyst is formed by day 4.
- Implantation on days 7–8 is accompanied by a transition from histiotrophic to hematotrophic nutrition.
- Gastrulation occurs in two phases; concurrently, the chorion, amnion, yolk sac, and allantois develop.
- The placenta is a hemochorial organ with cotyledons, performing metabolic, endocrine, and barrier functions.
- Derivatives of somites, nephrotomes, and splanchnotomes determine the formation of skin tissues, the musculoskeletal system, and the urogenital and cardiovascular systems.