In the Divulgation Blog we have talked about the origin and evolution of the different organs that can be found in modern organisms, based on three elements that interact with each other: organisms meteorization, pronutrients influence on gene expression and natural selection.

Meteorization is an anatomic characteristic of annelids and invertebrates. This feature is very visible in annelids because of their body division in metamers or segments, separated by partitions, while in vertebrates it is less visible externally, but not less real, due to posterior modifications and metamer fusions. However, meteorization is visible in vertebrates during embryo maturation so that we can observe the different fases corresponding to the different species until the final embryo is formed.

Pronutrients influence in gene expression has been proved by the continuous interaction and joint evolution between animal and vegetable cells. We can quote the influence of iodine and calcitriol contained within Laminaria, Fucus and Macrocistis algae cells on genes Hoxd 10-13 expression for extremities formation.

Finally, natural selection influences the ability to survive and increase the presence on the planet of those successful models in relation to environmental circumstances imposed by geological changes and competitive pressure of other species.

In Divulgation we have dedicated several blogs to the origin of lots of organs (nº4 for intestine, nº8 for cardiac muscle, nº9 for stomach, nº10 for kidney, nº13 for the eye and central nervous system, nº14 for liver, nº15 for pancreas, nº16 for hypophysis, nº17 for brainstem nodes chain, nº19 for skeletal system, nº22 for the relation between nervous and immunitary system, nº28 for brain phylogeny and regeneration, and nº35 for the eye origin as an example of the phylogenetic dating.

The present blog is inteded to focus on the origin of the current endocrine system based on animal meteorization. To do so we must remember that in bilateral symmetry coelomates (annelides, arthropods, vertebrates and molluscs) each segment is formed by a dermis in contact with the exterior, a coelom or cavity which is not connected with the outside but contains organs connected with the outside through excretory ducts and the circulatory system light.

Among the organs contained in each metamer, some constitute chains that connect internally each metamer organ with their homologues in the following metamer, like the intestine and the nervous system nodes chain, while other organs, which exist in each metamer, are not connected with their equivalents of the neighbour metamers, like nephridium and gonads. Thus in each metamer we find gonads with the capacity to segregate hormones and to produce meiotic cells intended to sexual reproduction which are not anatomically connected with other gonads from neighbour metamers.

These primitive metameric gonads may have evolved and specialized to become the current endocrine system formed by the pineal gland, the thyroid, the parathyroid, the thymus, the adrenal glands, the ovaries and the testicles.

Even though they seem to have lost their meiotic capacity except for the ovary and the testicle, they all preserve their secretory ability, some of them sharing the secretion of the same substances like it happens with the adrenal glands, which produce identical testosterone and estrogens than those produced in testicles and ovaries.

Specific information is available relating the first metamers with the thyroid, that comes from brachial arches I and II, with the parathyroid, which comes from arches III (superior parathyroid) and IV (inferior parathyroid), and with thymus, coming from brachial arch III.

About pineal and adrenal glands embryonic origin there isn’t unanimity, but the secretory coincidence of this last one with testicles and ovaries approaches it to a gonadal origin of an inferior metamer.  

Subsequent anatomical and histological studies will be necessary to confirm this approach in a way that it would be set or not, definitely, the metameric gonadal origin of the glands set that constitute the current endocrine system. These studies may be based in the comparison between hormonal synthesis genetic mechanisms and meiosis allowing or inhibiting genetic mechanisms.