NOTES OF EMBRIOLOGY
Embryology studies the sequence of forms of development from the zygote to the organism with all its organs and systems.
It is good to remember, in this regard, the distinction between development (succession of structural and organizational phases of increasing complexity) and growth, understood above all in a quantitative sense.
In vertebrate metazoas we witness, ascending in the evolutionary series up to man (through cyclostomes, fish, amphibians, reptiles, birds and mammals), to the appearance of adult forms of increasing complexity, for which the greater is the complication of embryonic development phases.
At the beginning, the zygote, always equipped with reserve material, is subdivided (by successive mitosis) into 2, then 4, then 8, etc. cells called blastomeres, without growth, until reaching the normal nucleus / cytoplasmic relationship of the species.
This initial segmentation can follow different patterns, depending on the quantity and distribution of the deutoplasm.
At the beginning the deutoplasm is scarce ("oligolecitic eggs"), for which the segmentation is total and gives rise to slightly different blastomeres. As the complexity of the embryo grows, more time and material is needed before its development allows it to begin independent living. For this reason, an increase in deutoplasm ("telolecitic eggs") is needed, which tends to be placed in a part of the zygote. This causes a growing "anisotropy", which is linked to modifications of the segmentation, regulated by two general principles:
- Hertwig's law says that, in mitosis, the achromatic spindle (whose equator determines the plane of division of daughter cells) tends to dispose in the sense of greater length of the cytoplasm;
- Balfour's law says that the speed of segmentation is inversely proportional to the amount of deutoplasm.
We see then that already in the cyclostomes and in the fish the segmentation is unequal, with a rapidly segmented animal pole (which will give the upper structures of the embryo) and a little calf pole that will contain most of the reserve material. Even greater is this anisotropic tendency in amphibians (in which it is necessary to predispose the organs in charge of aerial respiration), in which the calf pole, although slowly segmented, remains relatively inert and ends up being covered by cells derived from the rapidly segmented animal pole. Up to this evolutionary step the succession of the main embryonic stages includes: zygote, blastomeres, morula (blastomere cluster similar to a blackberry), blastula (morula with regressed internal cells), gastrula (blastula in which the cells of one side are invaginated) ), in which the primitive cavity of the organism appears, with an external cellular layer (ectoderm, from which the nervous system will derive first) and an internal one (entoderm), between which a third layer will then interpose (mesoderm). From these layers or "embryonic leaflets" then, in orderly sequence, all the tissues, organs and apparatuses will derive.
In the more advanced species, the increase in deutoplasm (or "calf") is such that it cannot even segment. Thus we see that in birds the segmentation affects only a thin superficial disc, leading to a "discoblastula" and to a series of phenomena that guarantee the formation of the embryo in a different way from the one mentioned above.
A further increase in deutoplasm would probably not have been more efficient, so that in the Mammals the development and growth up to the capacity of independent living are obtained with another system. We note in fact in the Mammals that the deutoplasma serves only for the earliest stages of development; then the embryo establishes metabolic relationships with the maternal organism (with the appearance of the placenta) and no longer uses the deutoplasma, whose excess is eliminated. At this point the eggs go back to being oligolecitiche and the segmentation can go back to being total (and therefore in the first stages it is similar to that of the anfiosso), but after the morula the embryogenesis continues according to the most evolved scheme of the birds, with a "Blastocyst" followed by implantation on the uterine wall, so that the metabolism of the embryo is ensured by the maternal organism (via the placenta) rather than by the deutoplasm.
EMBRYONIC DIFFERENTIATION
When the segmentation of the zygote has brought the nucleus / cytoplasmic relationship to the norm of the species, it is necessary that it starts, in parallel with the development, also the growth. For this reason the metabolism starts, with the appearance of nucleoli and protein synthesis. The protein synthesis thus started is due to the genes in charge of the first phases of embryonic development. These genes are derepressed by the substances present in the different blastomeres of the animal pole and calf. In turn, the products of these initial genes can derepresent the operons of the genes in charge of the subsequent stages. The products of this second series of genes will be able to act both in the sense of constructing new embryonic structures, and in the sense of repressing the previous operons and derepronouncing the subsequent ones, in an orderly sequence that leads to the construction of the new organism, thanks to the accumulated genetic information from the genome through the millennia in ever more evolved species.
The famous expression of Haeckel "ontogeny recapitulates phylogenesis" actually expresses precisely the fact that higher species repeat, in the stages of embryonic development, the succession that is already found in evolutionarily earlier species.
The initial stages of the embryo tend to be similar in vertebrates, particularly until the gills appear.
In the species that pass to the aerial respiration, the gills are then reabsorbed and reused (for example for the formation of endocrine glands), but the genetic information related to the formation of the gills is conserved also in humans. This is clearly an example of embryonic structural genes that are present in the genome of all vertebrates and must remain repressed after having worked in their ontogenetic moment.
The interpretation of embryogenesis in the sense of regulation of gene action allows us to unify the complex traditional experiences of experimental embryology.
TWINS
The zygote and the first blastomeres, until the protein synthesis begins, are totipotent, that is capable of giving life to an entire organism. To this are connected the experiments of Spemann, who obtained two embryos from the throttling of an amphibian zygote. A similar phenomenon appears to be at the basis of the phenomenon of identical twins in humans, which for this very reason are called monozygotic (MZ). Spemann's experimental twins were half the normal size, while in humans they are perfectly normal. This is explained because in the amphibians the two embryos had to divide the only yolk already received, while in humans the embryos can receive, through the placenta, all that is necessary for their development and growth.
It is good to remember that in man two thirds of the cases of twins have another origin: they derive from the occasional contemporary maturation of two follicles, with release of two eggs which, fertilized, give two zygotes; in this case we speak of dizygotic twins (DZ).
Since the MZ twins, divided by mitosis from the only zygote, have the same genome, the differences between them must be of environmental origin. Instead the genome of two DZ twins resembles only as much as that of any two brothers. The twin method is based on this principle, widely used in human genetics and also in the field of sport.
In humans, where certain ethical reasons would prohibit experimentation, it can be ascertained that any character is regulated by hereditary factors: in fact, strictly inherited characters (such as blood groups) are always concordant in the MZ twins only; as the concordance of one character in the MZs approaches that of the DZs, it can be deduced that the environmental factors prevail over the hereditary ones in determining that phenotypic character.
Edited by: Lorenzo Boscariol
