BLASTOMERES: FORMATION, DEVELOPMENT OF THE EMBRYO - BIOLOGY - 2024

The blastomeres are the cells resulting from the first mitotic divisions of the zygote, which is the product of fertilization or fusion of gametic cells (the egg and sperm in animals and plants) of two individuals of the same species.

Gametes are specialized cells used by many living organisms during sexual reproduction, in which two different individuals (or the same individual) "mix" half of each other's genetic material to form a new cell: the zygote.

Embryogenic stages of Hyla crepitans (Source: Internet Archive Book Images, via Wikimedia Commons)

These sex cells are produced through a special type of cell division known as meiosis, characterized in genetic terms as a reductional process, in which the chromosomal load of each individual decreases by half (in the first instance they separate into different cells homologous chromosomes and then sister chromatids).

Some authors consider that the zygote (the fertilized ovum) is a totipotent cell, since it has the capacity to give rise to all the cell types that characterize the living being that will be formed in the future.

Blastomeres, the cells that result from the division of this totipotent zygote, form approximately 30 hours after fertilization, although these times may vary slightly between species.

Blastomere formation

The process by which these cells originate is known as "cleavage," "cleavage," or "fragmentation." It is a period of intense DNA replication and cell division in which the daughter cells do not increase in size, but rather get smaller with each division, since the resulting multicellular embryo remains the same size.

When the zygote goes through these mitotic events, the first thing that happens is the multiplication of the nuclei within the cytosol. Cytosolic division occurs later, resulting in the formation of new identical cells (the blastomeres) that are partially independent.

In mammals, the divisions of the zygote that give rise to the blastomeres (cleavage) begin when it passes through the fallopian tubes towards the uterus and when it is covered by the “zona pellucida”.

The first division of the zygote gives rise to two cells that, in turn, divide, forming a tetracellular embryo. The number of blastomeres increases with each mitotic division and when 32 cells have been reached, what embryologists have called "morula" is formed.

The blastomeres of the morula continue to divide, thereby forming the "blastula," from 64 to more than 100 blastomeres. The blastula is a hollow sphere, inside which is a liquid known as the blastocele, which marks the end of the "cleavage" process.

The divisions of the zygote

It is important to mention that the different divisions of the zygote occur in specific senses or directions depending on the type of organism considered, since these patterns will subsequently determine, for example, the positions of the mouth and anus in animals.

Furthermore, cleavage is a carefully regulated process, not only by the "physical" characteristics of the initial zygotes, but also by the determinants of development that exert direct actions on the divisions.

Appearance of blastomeres during zygote divisions

At the beginning of cell divisions, the blastomeres formed have the appearance of a "mass of soap bubbles" and these initial cells only undergo changes in numbers, not in size.

When the number of cells is around 8 or 9, the blastomeres change their shape and align closely to form the morula, which looks like a compact "ball" of rounded cells.

This process is known as compaction and is thought to be facilitated by the presence of adhesion glycoproteins on the surface of each blastomere. Morulation occurs when the dividing zygote reaches the uterus, approximately 3 days after fertilization.

Curious fact

For many animal species, the size and shape of the blastomeres is uniform during the cleavage process, but their morphology can be compromised by chemical or physical stressors.

This has been exploited from an aquaculture point of view, as the "abnormal" morphology of blastomeres has been linked to the non-viability of the eggs of many commercially important fish species.

Different studies have determined that the presence of contaminating agents, for example, can lead to the production of eggs with morphologically aberrant blastomeres, and that this can mean the inability of the zygotes to complete the embryogenic process.

The morphological "aberrations" of the blastomeres in the fish species studied are very often related to asymmetries or irregular spatial interactions, unequal cell sizes, incomplete cell margins, and so on.

Development of the embryo

As already mentioned, the consecutive division of the zygote leads to the production of numerous cells known as blastomeres that, eventually, begin to organize to form different transitory structures.

The first structure, mentioned earlier, is the morula, which is made up of 12 to 32 closely arranged blastomeres and begins to form once the dividing zygote reaches the uterine cavity (in mammals).

Shortly after, a fluid-filled cavity, the blastocystic cavity, begins to form inside the morula, which acquires fluid from the uterus through the zona pellucida that covers the zygote.

This process marks a division between the blastomeres, forming a thin layer on the outside: the trophoblast (in charge of nutrition and which gives rise to the embryonic placenta); and a layer or group of internal blastomeres, the embryoblast, which will later represent the embryo per se.

At this point, the resulting structure is known as a blastula or blastocyst, which joins the endometrial epithelium to achieve the proliferation of the trophoblastic layer, which is divided into two additional layers: an internal one called a cytotrophoblast and an external one known as a syncytiotrophoblast.

The blastocyst is implanted in the endometrial cavity through the syncytiotrophoblast and continues its subsequent development until the formation of the amniotic cavity, the embryonic disc and the umbilical vesicle.

Gastrulation, the event that follows after blastulation, is when three layers known as ectoderm, mesoderm and endoderm form in the primary embryo, from which the main structures of the developing fetus will be formed.

References

1. Edgar, LG (1995). Blastomere Culture and Analysis. Methods in Cell Biology, 48 (C), 303-321.
2. Hickman, CP, Roberts, LS, & Larson, A. (1994). Integrated Principles of Zoology (9th ed.). The McGraw-Hill Companies.
3. Moore, K., Persaud, T., & Torchia, M. (2016). The Developing Human. Clinically Oriented Embryology (10th ed.). Philadelphia, Pennsylvania: Elsevier.
4. Setti, AS, Cássia, R., Figueira, S., Paes, D., Ferreira, DA, Jr, I., & Jr, EB (2018). Blastomere nucleation: Predictive factors and influence of blastomere with no apparent nuclei on blastocyst development and implantation. JBRA Assisted Reproduction, 22 (2), 102–107.
5. Shields, R., Brown, N., & Bromage, N. (1997). Blastomere morphology as a predictive measure of fish egg viability. Aquaculture, 155, 1–12.
6. Solomon, E., Berg, L., & Martin, D. (1999). Biology (5th ed.). Philadelphia, Pennsylvania: Saunders College Publishing.