Expression of the VENTX homeobox gene identifies precursors of human primordial germ cells

The VENTX gene is the single human homologue of the Ventx homeobox family of genes, first identified in Xenopus laevis. Ventx genes represent a class of non-clustered homeobox transcription factors expressed during early vertebrate embryogenesis that are involved in the ventral patterning of mesoderm. Although Ventx homologues have been identified in many vertebrate species, phylogenetic analysis has revealed the loss of the Ventx locus in rodents, which has limited their study in mammals. Analyses of the specific roles of the VENTX gene in early human differentiation have not been performed. In an effort to explore the expression and function of VENTX in human development, we generated human embryonic stem cell (hESC) lines in which fluorescent reporter genes were targeted to the VENTX locus by homologous recombination. In these cell lines, expression of the fluorescent protein faithfully reported VENTX expression. My studies, reported in this thesis, have revealed that expression of the human VENTX gene marks committed primordial germ cells (PGCs) prior to their migration and colonisation of the gonads. Using the VENTX hESC reporter lines, a VENTX differentiation protocol was developed for the generation of VENTX-expressing cells using an embryoid body differentiation system complemented with a defined, serum-free medium. Interestingly, the growth factors that induced VENTX expression, the combination of bone morphogenetic protein-4 (BMP4), basic fibroblast growth factor (bFGF; FGF2), stem cell factor (SCF) and retinoic acid (RA), were very similar to factors supporting the generation of mouse and human PGCs. Through their expression of the fluorescent reporter gene, viable VENTX-expressing cells were isolated and their gene expression profile analysed. VENTX-expressing cells expressed markers of early PGCs, positioning VENTX as a marker of committed pre-migratory PGCs. A co-culture system was developed which enabled the propagation of VENTX-expressing cells and permitted prolonged culture during which VENTX-expressing cells exhibited features suggestive of evolution towards an embryonic germ-like cell.