13 topics with concise reviews: definitions, key facts, and class questions.
This section covers the first and most dynamic weeks of human life, when a single cell develops into a multilayered embryo with the rudiments of future organ systems. This is the foundation of medical embryology, essential for understanding both the mechanisms of histogenesis and the causes of congenital malformations.
We will examine the entire pathway step by step: the characteristics of the egg cell, gamete transport, and the influence of genomic imprinting. Next, we will move on to zygote cleavage, blastocyst formation, and the critical stage of implantation. The central part of the section is dedicated to the two phases of gastrulation, the formation of extraembryonic organs, and the body folding. The module concludes with the differentiation of the mesoderm and ectoderm, as well as the transition to early organogenesis.
In seminars and exams, instructors most frequently demand a precise understanding of the timing of implantation, the tissue sources of extraembryonic organs (amnion, chorion, yolk sac), and the strict correlation of derivative tissues with specific germ layers. Pay special attention to these aspects.
Why is the term "egg cell" considered conditional in mammals?
There is no true mature egg cell in the classical biological sense during the ovulatory cycle. This term typically refers to either a secondary oocyte released into the uterine tube or a primary oocyte within an ovarian follicle.
Why do spermatozoa move slowly in the vagina?
Their motility is suppressed due to the acidic environment of the vagina, which is unfavorable for the active movement of male gametes.
In which cells does the methylation status remain unchanged after fertilization?
The methylation status is stably preserved in all somatic cells of the new organism throughout its life.
Why does the total volume of the embryo not increase during cleavage?
A dense fertilization membrane persists around the embryo, which physically constrains growth and blocks the uptake of substances from the outside. The cells simply divide the existing volume of cytoplasm, becoming progressively smaller.
On which day does the embryo become a blastocyst?
The embryo reaches the blastocyst stage on day 5, when a cavity (blastocoele) appears inside it.
What type of nutrition is characteristic of the embryo immediately after implantation?
During this period, the histiotrophic type of nutrition predominates. The embryo receives all necessary substances from the breakdown products of destroyed cells and the intercellular substance of the endometrium.
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