Testis

Immunohistochemical Profiling of Germ Cells Within the Human Fetal Testis: Identification of Three Subpopulations

Abstract
In the human fetal testis, germ cells that have migrated to the genital ridges become enclosed within testicular cords by 8 weeks gestation. Most papers refer to all types of germ cells as being 'gonocytes' or 'pre-spermatogonia' giving the impression that they are identical although detailed morphological studies have suggested that there is a heterogeneous population. We have used single, double and triple immunohistochemistry to evaluate the differentiation of cells within fetal testes recovered during the first (7-9 weeks) and second (14 to 19 weeks) trimester. In the first trimester differentiation of Sertoli cells preceded the formation of testicular cords and the differentiation of interstitial (Leydig, peritubular myoid) cells. Immunostaining for CHK2, C-KIT, placental alkaline phosphatase (PLAP), PCTAIRE-1 and MAGE-A4 revealed that the proportion of germ cells expressing each of these proteins was correlated with gestational age. Expression of the pluripotency marker OCT4 was restricted to a population of small round germ cells. Three types of germ cell were identified and we propose these should be known as gonocytes [OCT4^pos/C-KIT^pos/MAGE-A4^neg], intermediate germ cells [OCT4^low/neg/C-KIT^neg/MAGE-A4^neg] and pre-spermatogonia [OCT4^neg/C-KIT^neg/MAGE-A4^pos]. In the first trimester, most germ cells had a gonocyte phenotype however, from 18 weeks gestation pre-spermatogonia were the most abundant cell type. These data provide evidence of functional differentiation of human testicular germ cells during the second trimester of pregnancy and argue against them being considered as a homogeneous population as in rodents.

Key words: Gamete Biology • Male Reproductive Tract • Testis • Developmental biology • Sertoli cells

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