The problem of the origin of primordial germ cells (PGCs) in vertebrates: historical review and a possible solution

Abstract

A concise review of the articles about the origin of primordial germ cells (PGCs) in vertebrates is provided. Differences among various taxa concerning the origin of PGCs, not easily understandable on the base of traditional knowledge, are pointed out. All those differences can be explained taking into consideration the recent "theory of the endoderm as secondary layer". That theory allows us to understand that those differences are only apparent, being related to modifications of stages of the consequent embryogeny, overall, to a different amount of yolk in the egg. Eggs very rich in yolk became meroblastic, and the portion of primordial ectomesenchyme destined to give rise to a part of the mesoderm and the PGCs separates early from the part destined to give rise to the rest of the mesoderm and to the digestive endoderm in order to form the vitelline hypoblast lamina. To this lamina, in contrast to the traditional interpretation, a mesodermal, not endodermal, origin must be attributed. With the misunderstanding regarding the origin of this lamina clarified, all the differences about the origin of PGCs disappears. Furthermore, in taxa where PGCs were considered to be of endodermal origin, they too have a mesodermal origin. Considering that a mesodermal origin of PGCs has been demonstrated in all sponges and cnidarians, as well, a unique, mesodermal origin of germinal cells in all pluricellular animals results.

Biology of primordial germ cells in vertebrates with emphasis in urodeles amphibians

“…In this extraembryonic region, the PGCs could locate in several places, variating their location even in spaces from the same taxonomic group. The PGCs could locate at the anterior region, at the posterior region, or all around the blastodisc (Pilato et al, 2013). In this regard, in the decade of the 70s, Hubert reported that in the Vipera aspis snake, the PGCs derive from the epiblast cells and migrate from the central region of the area pellucida to a crescent‐shaped zone in the hypoblast at the anterior border of the area pellucida.…”

Committing the primordial germ cell: An updated molecular perspective

“…From an evolutionary developmental perspective, what could be a likely reason behind alteration of the early pro‐PGC gene regulatory network? A school of thought in PGC development holds that PGCs segregate very early from nascent mesoderm, and hence may be mesodermal derivatives (Extavour & Akam, ; Johnson et al, ; Pilato et al, ). Molecularly, this may be supported by the observation of T and EOMES expression preceding canonical pro‐PGC factors, where T and EOMES are traditionally considered to be mesodermal genes and reported as such (Guo et al, ; Irie et al, ; Sasaki et al, ; Sugawa et al, ; Yokobayashi et al, ).…”

Cellular remodeling and JAK inhibition promote zygotic gene expression in the Ciona germline

“…In various animal species, PGCs associate with endodermal progenitors (Ying et al, 2002 ; Pilato et al, 2013 ). In C. elegans for instance, intestinal precursors actively phagocytose the germline lobes (Abdu et al, 2016 ).…”