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

1040 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITION2.2 Seminal Vesicles2.2.1 HumanThe seminal vesicles and the ductus deferens are present by the 6th month of fetal developmentin an arrangement similar to that seen in adults. The seminal vesicles have a larger lumen andthicker, stronger muscular wall than the ductus deferens. Prenatal development of the muscularwall is mediated by estrogenic stimulation. By the 7th month of gestation, the seminal vesicleshave attained their adult form, although it is not until term that the mucous membrane surroundingthe lumen begins to arrange itself in folds. 42 The seminal vesicles continue to grow slowly untilpuberty. 42Secretory activity is present in the seminal vesicles by the 7th month of gestation and slowlyincreases thereafter, persisting for a considerable time after birth (detected at 17 months). At 4years of age, seminal vesicle secretion was no longer detected. 74 Secretion by the seminal vesiclesis androgen dependent. 952.2.2 DogThe dog has neither seminal vesicles nor bulbourethral glands. 732.2.3 RatIn rats, the basic pattern for seminal vesicle formation is present at postnatal day 10. Lumenformation occurs over a relatively protracted period from postnatal days 2-15. Secretory granulesbecome evident at postnatal day 16. The seminal vesicles markedly increase in size betweenpostnatal days 11-24, and continues to grow until adult appearance and secretory properties areattained between PND 40 and 50. 84 Thus, proliferation and differentiation of the seminal vesiclesparallels the postnatal increase in testosterone levels.3 Neuroendocrine Control of the Reproductive SystemIn mammals, the pituitary and gonads are capable of supporting gametogenesis prior to puberty;however, events in the brain are required to change the hypothalamic-pituitary-gonadal (HPG) axisand trigger maturational changes.3.1 HumanIn humans, much of the process controlling testicular androgen production is present at the timeof birth. In the fetal testis, production of testosterone and antimüllerian hormone begins at the endof the first trimester of pregnancy. 18 Sertoli cells, triggered through an unknown mechanismmediated by the Y chromosome, differentiate and produce antimüllerian hormone. Subsequently,Leydig cells differentiate at approximately 7-8 weeks of gestation and begin producing androgens 96that ultimately come under the control of the placental gonadotrophin, human chorionic gonadotrophin(hCG). 97 Pituitary gonadotrophin synthesis begins around week 12 of gestation with initiallyhigh levels that decline towards the end of gestation, the likely period for the onset of negativefeedback regulation. 98 Thus, the hypothalamic-pituitary-gonadal axis is fully functional in the fetusand neonate. Neonatal exposure of the hypothalamus to androgen is need for sexual differentiationof the LH release mechanism, allowing LH secretion to be modified by either androgen or estrogen. 16Thus, transient elevations of FSH, LH and testosterone have been noted in boys during the first 6months of life, 7 then pulsatile secretion of gonadotrophin declines, reaching its lowest point at 6years of age. 99 Thereafter, pulsatile gonadotrophin secretion begins to increase.© 2006 by Taylor & Francis Group, LLC