Progress towards human primordial germ cell specification in vitro.

Primordial germ cells (PGCs) have long been considered the link between one generation and the next. PGC specification begins in the early embryo as a result of a highly orchestrated combination of transcriptional and epigenetic mechanisms. Understanding the molecular events that lead to proper PGC development will facilitate the development of new treatments for human infertility as well as species conservation. This article describes the latest, most relevant findings about the mechanisms of PGC formation, emphasizing human PGC. It also discusses our own laboratory's progress in using transdifferentiation protocols to derive human PGCs (hPGCs). Our preliminary results arose from our pursuit of a sequential hPGC induction strategy that starts with the repression of lineage-specific factors in the somatic cell, followed by the reactivation of germ cell-related genes using specific master regulators, which can indeed reactivate germ cell-specific genes in somatic cells. While it is still premature to assume that fully functional human gametes can be obtained in a dish, our results, together with those recently published by others, provide strong evidence that generating their precursors, PGCs, is within reach.