Cell Cycle and Metabolic Features
The process starts in the lumen of the uterine tube and finishes when the embryo enters the uterine cavity. In the early stages, a thick zona pellucida (fertilization envelope) remains around the embryo. It acts as a rigid barrier: it prevents the overall volume from increasing and blocks the influx of external nutrients. All vital activity relies exclusively on internal reserves.
Because the total volume is fixed, daughter cells become smaller with each new cycle. The cell cycle practically lacks the $G_1$ phase (the growth phase prior to DNA synthesis). Meanwhile, the number of nuclei grows, along with the mass of DNA and chromosomal proteins. Conversely, the cytoplasmic fraction per cell drops sharply. The biochemical meaning of this phenomenon is that maternal cytoplasmic components are actively recycled into nucleotides for DNA construction and amino acids for histone synthesis.
Human Cleavage Specifics
Human embryonic cleavage is characterized by three key features:
- Holoblastic (complete): absolutely all cells of the embryo undergo cleavage.
- Uneven (unequal): the resulting blastomeres vary in size (forming larger and smaller cells).
- Asynchronous: cells do not divide simultaneously. Due to varying division rates, intermediate stages with an odd number of blastomeres (e.g., 3 or 5) may occur.
Shortly after the process begins, the cells separate into two pools with different functions. Dark, large blastomeres divide slowly and become precursors to the embryoblast (from which the body of the fetus will form). Light, small blastomeres divide very rapidly and form the trophoblast, the cell layer responsible for uterine implantation and subsequent nutrition.
Chronology of Development: From Zygote to Blastocyst
The increase in cell number is strictly tied to the time elapsed since fertilization:
- First two days (early stage): the first division (two-blastomere stage) is completed in 30–32 hours. By 40 hours, the embryo consists of four cells.
- Day 3 (72 hours): the embryo counts 12 to 16 cells.
- Day 4: the morula is formed. This is a dense cellular mass lacking an internal cavity, resembling a mulberry. It consists of 32 blastomeres: roughly 3–4 are slow dark cells (having undergone 2 division cycles), and the remaining 27–28 are fast light cells (having undergone about 5 cycles).
- Days 4 to 6 (late stage): at the 4–4.5 day mark (58 cells) and by 5.5 days (107 cells), the blastocyst forms—an embryonic vesicle containing a fluid-filled cavity.
Biological Potency of Cells
During the first two to three divisions, blastomeres retain totipotency; each possesses equal potential and can give rise to an entire organism. If cells dissociate (split) at this stage, it leads to the birth of monozygotic twins.
However, at the 8–16 blastomere stage, totipotency is lost. Synthetic processes activate within the cells, they begin to visually diverge, and differentiation is triggered. Under natural conditions, totipotency is retained exclusively by the precursor cell of primordial germ cells.
In experimental medicine, totipotency can be induced artificially. To achieve this, specific protein transcription factors are introduced into differentiated tissue cells. The genetic potential can also be realized through cloning by transplanting somatic cell nuclei into an enucleated zygote (stripped of its own nucleus). Success rates are highest when the nuclear donors are stem cells rather than regular somatic cells.