150 THE DISTRIBUTION PATTERN OF PRIMORDIAL GERM CELLS IN EARLY CHICK EMBRYOS

Abstract

In all vertebrates, primordial germ cells (PGCs) appear during early stages of development in extragonadal sites, then they migrate to the gonad and give rise to ova or spermatozoa. Unlike in other species, however, in avian and reptile embryos, PGCs use the vascular system as a vehicle to transport them to the future gonadal region where they leave the blood vessels. The present study was carried out to know the details of this unique migration pathway and the proliferation of endogenous PGCs in chicken embryos. Whole of the chicken embryos during stages X [Roman numerals refer to the staging system of Eyal-Giladi and Kochav (1976 Dev. Biol. 49, 321–327) to 17 (Arabic numerals refer to the staging system of Hamburger and Hamilton (1951 J. Morphol. 88, 49-82))] or embryonic blood during stages 12 to 17 were immunohistochemical stained using specific antibody raised against chicken vasa homolog (CVH), which could be recognized as a marker for chicken PGCs. The distribution patterns and populations of PGCs in embryos were observed under a stereomicroscope. The numbers of PGCs were presented mean and standard deviation (mean ± SD). Anti-CVH staining revealed the distribution and population of chicken PGCs in early chick embryos. PGCs existed mainly in the area pellucida and concentrated in the central zone at stage X. The mean number of PGCs per embryo at this stage was 130.4 ± 31.9. With the formation of primitive streak, PGCs were carried anteriorly to the edge of the blastoderm. The PGCs scattered anteriorly began to concentrate to the anterior point of the head on the dorsal side of stage 10 embryos. The average number of PGCs per embryo at stage 10 was 439.3 ± 93.6. The mean numbers of PGCs per embryo during stages X to 10 increased gradually as development progressed to stage 10. We found the entrance point of PGCs from anterior edge of the blastoderm to the vascular network during stages 10 to 11. In the blood, PGCs could be detected from all of the samples during stages 12 to 17. In contrast, no PGC was recognized in the future gonadal region before stage 14, and then they began to appear in the same region at stage 15. The mean numbers of PGCs that located in the future gonadal region during stages 15 to 17 increased intensively and were 97.3 ± 57.3, 200.3 ± 113.5, and 327.6 ± 102.4, respectively. Interestingly, the numbers of PGCs within future gonadal region during stages 15 to 17 were consistently and significantly different (P < 0.05) between the left and right side of the region. The results suggest that chicken PGCs move from extraembryonic area to the vascular network during stages 10 to 11, circulate in the blood stream, and finally, they begin to leave the blood vessels actively and migrate to the future gonadal region at stage 15.