The etymological origin of the term blastula is found in the Greek word blastós (which can be translated as “germ”), together with the diminutive suffix -ŭla. This concept passed into scientific Latin as blastula and then came to our language as blastula.
A blastula is called one of the stages of embryogenesis of animals (that is, of their embryonic development). The blastula, which succeeds the morula and precedes the gastrula, is the second stage of this development.
The reproduction of sexual begins with fertilization resulting in the zygote a cell resulting from the union of the ovum (the female gamete) and espermatozozide (the male gamete). This zygote, within the framework of embryogenesis, stars in a process known as segmentation, where a mass of cells called morula (made up of cells known as blastomeres) is first produced.
A fluid creates the cavity of the blastocyst and causes the blastomeres to mobilize. At the end of the segmentation process, the blastula is formed, a state that implies that the body already has more than 64 cells.
According to DigoPaul, the embryo, in the blastula phase, has different types of cells (also called, in this context, blastulae). According to different characteristics, it is possible to refer to stereoblastules, periblastules, celloblastules or discoblastules.
This classification is based on the differences that each type of blastula has in its content, as well as in the way in which the yolk of the egg is distributed, something also related to the class of segmentation. Let’s look at each of these four types of blastula in detail:
* Stereoblastules: sometimes, the blastocele is virtual because it is occupied by voluminous blastomeres whose pole is vegetative, and in these cases it is possible to speak of stereoblastules. On the mass of macromers of these blastulas we can see a cap of micromeres;
* Periblastules: they are originated by the centrolecitic eggs (type of zygote with abundant yolk and the cytoplasm surrounding the central mass; its segmentation is superficial and partial) of the arthropods and they exhibit a virtual blastocele, with blastomeres that surround the mass of non-segmented yolk;
* Celloblastules: the unequal or equal total segmentation that takes place if the egg has little yolk and is distributed in granulations evenly throughout the cytoplasm leads to the emergence of the celloblastula, a blastula whose blastocele has a regular layer of cells around it. At first, it is correct to say that your blastomeres are all the same; however, the blastocele moves toward the animal pole and thus ends up being irregular. This occurs in heterolecitic eggs because the blastomeres of the vegetative pole are richer in yolk and are larger in size;
* Discoblastules: originate in the telocytic eggs of reptiles and fish through partial discoidal segmentation. We can see a cap of blastomeres above the unsegmented yolk if we look at the animal pole of the egg.
During embryogenesis, the volume of the blastula grows until this structure, little by little, breaks down. About five days after fertilization, the blastula disappears and the gastrula arises in the framework of a restructuring that is called gastrulation.
This term also appears frequently in the field of assisted reproduction, where it can be used to refer to the ectoderm, the outermost layer of the embryo. In total there are three, and the other two are called endoderm (the proximal) and mesoderm (the intermediate). During embryonic development, the outer layer is the first to be generated, precisely along the blastula phase; later, it allows the emergence of the remaining two, when it reaches gastrulation.