Primordial germ cell proliferation is impaired in Fused Toes mutant embryos

Abstract

Over the first 4 days of their life, primordial germ cells invade the endoderm, migrate into and through the developing hindgut, and traverse to the genital ridge where they cluster and ultimately inhabit the nascent gonad. Specific signal-receptor combinations between primordial germ cells and their immediate environment establish successful migration and colonization. Here we demonstrate that disruption of a cluster of six genes on murine chromosome 8, as exemplified by the Fused Toes (Ft) mutant mouse model, results in severely decreased numbers of primordial germ cells within the early gonad. Primordial germ cell migration appeared normal within Ft mutant embryos; however, germ cell counts progressively decreased during this time. Although no difference in apoptosis was detected, we report a critical decrease in primordial germ cell proliferation by E12.5. The six genes within the Ft locus include the IrxB cluster (Irx3, -5, -6), Fts, Ftm, and Fto, of which only Ftm, Fto, and Fts are expressed in primordial germ cells of the early gonad. From these studies, we have discovered that the Ft locus on mouse chromosome 8 is associated with cell cycle deficits within the primordial germ cell population that initiates just before translocation into the genital ridge.

The Fused Toes Locus Is Essential for Somatic-Germ Cell Interactions That Foster Germ Cell Maturation in Developing Gonads in Mice1

“…Few germ cells were clustered within cysts or cords of female or male gonads, respectively, supporting previous findings (Fig. 2, B and D) [16]. In developing testes, cord formation and Sertoli cell maturation appeared no different than wild-type littermates as assessed by immunohistochemistry of laminin, a basement membrane marker that outlines testis cords (data not shown), and a Sertoli cell marker, Müllerian inhibitory substance (MIS; Fig.…”

“…Although six genes-Irx3, Irx5, Irx6, Ftm, Fto, and Fts-are expressed in gonads of both sexes, Irx3, Irx5, Ftm, and Fts transcripts were present at higher levels in female gonads. Ft mutant testes appeared relatively normal up to 2-wk posttransplantation other than a low germ cell count [16]. However, by the third week after transplantation (equivalent to approximately P14), obvious defects were observed in the basement membrane, peritubular myoid, and Sertoli cells.…”

“…These include decreased germ cell numbers [16], disrupted basement membranes around germ cell units, and dysfunction of both somatic and germ cell components. Testis formation can occur in the absence of communication between somatic and germ cells; it is not until after birth that their interactions become essential for formation of the spermatogonia stem cell niche [6,9,44].…”

“…Previously, we reported defects within primordial germ cells during early development of Ft mutant mice [16]. In those studies, we discovered that primordial germ cell migration to the developing gonad was normal; however, their numbers were significantly decreased due to proliferation defects.…”

“…However, Irx5 À/À mice are fertile, suggesting that these factors likely compensate for each other and imply that optimal assessment of the function of these genes requires elimination of the entire IrxB cluster [16]. To assess the loss of the IrxB cluster, we investigated gonad development using the fused toes (Ft) mutant mouse model.…”