Escort-like somatic cells mediate early mouse fetal ovarian development but surface-derived LGR5+ cells support primordial follicles

What are the somatic cells that support follicle formation and development in mammals and how do they relate to the better studied ovarian somatic cells in in Drosophila? We show that all mouse germ cells arriving at the genital ridge interact initially with a common type of somatic cell we term "escort-like cells (ELCs)." Our gene expression data argues that these cells are the "bipotential" somatic cells postulated by MacLaren and others that develop into either granulosa cells or Sertoli cells. Furthermore, ELCs strongly resemble Drosophila escort cells, as they resemble them physically, participate in the same steps in germ cell development, and express similar key signaling genes such as Wnt4 and BMP2.