Characterization, isolation and culture of primordial germ cells in domestic animals: recent progress and insights from the ovine species

Primordial germ cell (PGC) allocation, characterization, lineage restriction, and differentiation have been extensively studied in the mouse. Murine PGC can be easily identified using markers as alkaline phosphatase content or the expression of pluripotent markers such as Pou5f1, Nanog, Sox2, Kit, SSEA1, and SSEA4. These tools allowed us to clarify certain aspects of the complex interactions of somatic and germinal cells in the establishment of the germ cell lineage, its segregation from the neighbouring somatic tissue, and the guidance mechanisms during migration that direct most of the germ cells into the genital ridges. Few data are available from other domestic animals and here we reported our preliminary studies on the isolation, characterization, and in vitro culture of sheep PGCs. Sheep PGCs can be identified with the markers previously used in mouse, but, in some cases, these markers are not coherently expressed in the same cell depending on the grade of differentiation and on technical problems related to commercial antibodies used. Pluripotency of PGCs in culture (EGCs) from domestic animals also needs further evaluation even though the derivation of embryonic pluripotent cell lines from large mammals may be an advantage as they are more physiologically similar to the human and perhaps more relevant for clinical translation studies. Comprehensive epigenetic reprogramming of the genome in early germ cells, and derived EGCs including extensive erasure of epigenetic modifications, may be relevant for gaining insight into events that lead to reprogramming and establishment of totipotency. EGCs can differentiate in vitro in a various range of tissues, form embryonic bodies, but in many cases failed to generate tumours when transplanted into immunodeficient mice and are not able to generate germline chimeric animals after their transfer. Such incomplete information clearly indicates the urge to improve the studies on derivation of stem cells in farm animals and shows the need for a multidisciplinary investigation in order to create farm animal models to set up suitable ethical and technical systems for cell regenerative therapies in humans