Preparation for fertilization and the zygote stage
Development is preceded by ovarian follicle maturation, which is completed by days 13–14 of the menstrual cycle. The mature follicle has a fluid-filled cavity, a multi-layered coat (the theca), and an inner layer of follicular cells that nourish the oocyte and produce estrogens.
By the time of ovulation, the follicle ruptures, and the secondary oocyte enters the abdominal cavity, from which it proceeds into the uterine (fallopian) tubes. The oocyte is surrounded by two coats:
- Zona pellucida (zona pellucida).
- Corona radiata (corona radiata), consisting of follicular cells.
Upon sperm penetration, the egg is activated, forming the zygote—a short-lived single-celled embryo stage.
Cleavage and blastocyst formation
Immediately following the zygote stage, cleavage begins—a series of mitotic divisions without growth of the resulting cells (blastomeres). In humans, this process has specific features:
- Asynchrony: the first two blastomeres do not divide simultaneously. Stages with an odd number of cells (3, 5, 7, etc.) are possible.
- Unevenness: one of the first two blastomeres is larger than the other.
- The plane of the first division is meridional, and upon division of the smaller blastomere, the pair of new cells rotates by 90 degrees.
The embryo gradually turns into a morula. At the stage of approximately 58 blastomeres, fluid accumulates inside and a cavity—the blastocoele—forms. The embryo becomes a blastocyst. Already at the 12-blastomere stage, cells differentiate into two populations:
- Trophoblast (outer dark cells)—will provide nutrition and implantation.
- Embryoblast (inner light cells, inner cell mass)—will give rise to the embryo itself and some extraembryonic membranes.
Movement through the fallopian tubes and implantation
All cleavage stages occur while the embryo moves through the fallopian tube. During this period, the zona pellucida prevents the embryo from sticking to the walls. If attachment occurs prematurely, an ectopic ("tubal") pregnancy will develop—a dangerous pathology threatening tubal rupture and hemorrhage.
Having reached the uterus (around day 4), the blastocyst sheds its transparent coat. On days 6–9, implantation begins:
- Trophoblast cells secrete enzymes.
- The blastocyst embeds into the mucous membrane of the uterus (endometrium).
- The endometrium responds with local swelling and intensive development of blood vessels (vascularization).
From day 7, the embryo begins to feed on the secretion of the uterine glands.
Gastrulation and provisional organs
After implantation, gastrulation begins within the embryoblast. In the second week, by delamination, the embryonic node splits into two layers, and in the third week, through cell immigration, the formation of germ layers is completed:
- Epiblast (upper layer)—the source of the ectoderm, mesoderm, endoderm, and amnion.
- Hypoblast (lower layer)—the source of the yolk sac.
Concurrently, provisional (temporary) organs are formed:
- Amnion (amniotic sac)—filled with fluid, creates an environment and protects the embryo.
- Chorion (villous membrane)—forms from the trophoblast starting from day 14. Its villi deeply invade the uterine tissues.
- Allantois—performs the function of the embryonic urinary bladder, is closely connected with the chorion, and excretes toxic metabolic products.
- Yolk sac—poorly developed due to the small amount of yolk in mammals.
Placental formation and fetal development
The fusion of the chorion with the uterine wall leads to the formation of the placenta—a disc-shaped organ of permanent nutrition. Substance exchange occurs via diffusion across the villous membranes. Oxygen and nutrients come from the mother, while urea and carbon dioxide come from the fetus. The placental barrier reliably separates the bloodstreams, but remains permeable to certain antibodies, viruses (e.g., cytomegalovirus and rubella), and parasites (Toxoplasma).
Having passed the stage of histo- and organogenesis, after 9 weeks the embryo acquires the status of a fetus, which develops right up until birth.
Briefly: answers to topical questions
From which germ layers does the allantois form?
The wall of the allantois is formed from two extraembryonic primordia: the endoderm and the mesoderm.
In the histological structure of this finger-like outgrowth, two layers are distinguished:
- Extraembryonic endoderm—lines the organ from the inside, forming as an outgrowth of the intestinal endoderm.
- Extraembryonic mesoderm—covers the structure from the outside.
In reptiles and birds, the outer layer is also called the splanchnopleure.
Which pathogenic microorganisms and viruses are capable of crossing the placental barrier?
Certain parasites and viruses are capable of penetrating through the placental barrier.
Among the pathogenic agents are:
- Viruses—rubella virus and cytomegalovirus.
- Parasites—Toxoplasma.
What hormones does the placenta produce to maintain pregnancy?
The placenta acts as a powerful endocrine organ, synthesizing protein and steroid hormones.
Hormones produced by the placenta include:
- Human chorionic gonadotropin—stimulates the function of the corpus luteum.
- Placental lactogen—participates in maternal metabolism.
- Progesterone and estrogens—compensate for ovarian inactivity.
- Relaxin—relaxes the myometrium and prepares the pelvis for childbirth.
- Prolactin, ACTH, chorionic somatomammotropin.
- Hypothalamic factors (releasing hormones and inhibiting factors).
What tissue derivatives are formed from the mesoderm during organogenesis?
During embryogenesis, the mesoderm differentiates into somites, nephrogonotomes, and splanchnotomes, giving rise to a number of structures.
Main derivatives of the mesoderm:
- Sclerotomes — vertebrae, ribs, scapulae.
- Myotomes — striated skeletal musculature.
- Dermatomes — dermis of the skin.
- Nephrogonotomes — excretory organs and gonads.
- Visceral layer of the splanchnotome — myocardium, epicardium, mesentery, lung parenchyma.
- Parietal layer of the splanchnotome — mesothelium of serous membranes (peritoneum, pleura, pericardium).
It is easy to remember blastocyst differentiation: Trophoblast is responsible for "trophics" (nutrition) and forms the outer chorion, while the embryoblast lies inside and builds the embryo itself.
Class and exam questions
When does the embryonic period begin and how long does it last?
The embryonic period lasts from fertilization up to 8 weeks of development. After the 9th week, once the main organs are formed, the embryo is referred to as a fetus.
Why is tubal pregnancy dangerous?
It occurs when the embryo prematurely implants in the fallopian tube. This leads to rupture of the tube, severe internal bleeding, and poses a life-threatening risk to the woman.
Do maternal and fetal blood mix?
No, the fetal chorionic vessels are not connected to the maternal vessels. All gas and nutrient exchange occurs exclusively via diffusion across the placental cell membranes.
What else to study on this topic
- Structure and functions of the zona pellucida
- Stages of gastrulation: delamination and cell immigration
- Mechanism of endometrial vascularization during implantation
- Placental barrier permeability in Rh-incompatibility
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