7 topics with concise reviews: definitions, key facts, and class questions.
In this section, we analyze the very earliest stages of new life at the cellular and tissue levels. You will study the morphology of male and female germ cells (spermatozoa and oocytes), as well as the sequential stages of development: from gamete fusion during fertilization to cleavage and blastula formation.
A logical continuation will be the analysis of gastrulation—the process of germ layer formation, establishment of the axial organ primordia, and tissue differentiation during embryogenesis. This foundational knowledge is absolutely essential for understanding histogenesis and the developmental sources of all human body tissues.
During colloquiums and exams, instructors most frequently ask about the classification of oocytes, types of cleavage depending on the amount and distribution of yolk, and the phases of gastrulation. Pay special attention to the acrosomal and cortical reactions during fertilization, and be ready to name without hesitation the tissue derivatives of each of the three germ layers.
Why does the DNA of germ cells not accumulate as many mutations as somatic cell DNA?
In gametogenesis, strict quality control mechanisms operate: meiosis promotes efficient error correction, and cells with irreparable defects are eliminated via apoptosis.
Does the egg cell have a cell center?
No. After the first meiotic division, centrioles disappear in the oocyte. For the second division and subsequent cleavage of the zygote, centrioles that the spermatozoon introduces during fertilization are required.
At what stage of meiosis is the oocyte at the moment of fertilization?
At the moment of fertilization, it is a secondary oocyte arrested at metaphase of the second meiotic division.
Does the embryo grow during cleavage?
No, this is the main feature of the stage. The cells divide mitotically, but there is no growth between divisions, so the total volume of the zygote remains unchanged.
What is the essence of gastrulation?
The essence of the process is the transformation of the blastula into the gastrula with the formation of three germ layers: the outer (ectoderm), middle (mesoderm), and inner (endoderm).
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