A Practical Approach, Second Edition=Ronald D. Ho.pdf

POSTNATAL DEVELOPMENTAL MILESTONES 1029APPENDIX C-4*DEVELOPMENT AND MATURATIONOF THE MALE REPRODUCTIVE SYSTEMM. Sue Marty, 1,5 Robert E. Chapin, 2 Louise G. Parks, 3 and Bjorn A. Thorsrud 41Dow Chemical Company, Midland, MI 48674, USA2Pfizer Global Research & Development, Groton, CT 06340, USA3Merck & Company, West Point, PA 19486, USA4Springborn Laboratories, Inc., Spencerville, OH 45887, USA5Correspondence to: Dr. M. Sue Marty, Dow Chemical Company, Toxicology Research Laboratory, 1803 Building,Midland, MI 48674, 517-636-6653, Fax 517-638-9863, Email: martym@dow.comIntroductionThis review briefly describes some of the key events in the postnatal development of the malereproductive system in humans, non-human primates, rats and dogs. Topics discussed includedevelopment of the testes, epididymides, the blood-testis barrier, anogenital distance, testiculardescent, preputial separation, accessory sex glands (prostate and seminal vesicles), and the neuroendocrinecontrol of the reproductive system. The objective of this work is merely to allow thereader to make initial comparisons of the developmental processes and timing of these events inhuman versus animal models. This review is not intended to be comprehensive, but merely providesan initial overview of these processes. In some cases, information was not available for all species.Available information is summarized in Table 1.1 Reproductive Organs1.1 Testes1.1.1 HumanIn the human, spermatogenesis does not begin until puberty; however, the prenatal, early postnataland prepubescent testis plays a critical role in hormone production. In the early postnatal testis,immature Sertoli cells are the most common cell type 1,2 with limited numbers of germ cells in arelatively undifferentiated state. According to Cortes et al. 2 total Sertoli cell number increases fromthe fetal period through childhood, puberty and early adulthood. In contrast, Lemasters et al. 3reported that Sertoli cells proliferate after birth, ceasing at 6 months when the adult number ofSertoli cells are achieved. Sertoli cells secrete inhibin B until 2-4 years of age and anti-Müllerianhormone during the entire prepubescent period. 4 In humans, there are three known testosteronesurges, one from 4-6 weeks of gestation, one from 4 months of gestation to 3 months of age, andthe last from 12 to 14 years of age. 5 Consistent with the early postnatal increase in testosterone,there is a biphasic increase in Leydig cell number, which includes an early increase in Leydig cells,followed by a decrease to the lowest level at 1.5 years of age, 6 then a continuous increase in Leydigcells until their numbers plateau in adulthood. In infant boys, an increase in total germ cell numberoccurs with peak numbers achieved between 50 and 150 days of age, followed by a decrease inolder boys. 7 During the early proliferative period, there is an overall decrease in germ cell density* Source: Marty, M. S., Chapin, R. E., Parks, L. G., and Thorsrud, B. A., Development and maturation of the malereproductive system, Birth Defects Research, Part B: Developmental and Reproductive Toxicology, 68, 125-136, 2003.© 2006 by Taylor & Francis Group, LLC