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Hodgson E. Modern toxicology [2004].pdf
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346 REPRODUCTIVE SYSTEM

20.4FEMALE REPRODUCTIVE PHYSIOLOGY

As described previously for the male, the female hormonal signaling is composed of four primary levels: CNS, hypothalamus, anterior pituitary, and gonads. The gonado- tropin-releasinghormones of the hypothalamus stimulates the anterior pituitary to release LH and FSH. Subsequently LH and FSHS stimulate the release of estrogen and progesterone from the ovaries (Figure 20.3). Estrogen is secreted in the growing follicle and has effects on the uterus. The oocytes are formed before birth, then develop into the primary oocytes after meiosis. At the time of puberty, the release of gonadotropin stimulates the oocytes to develop into graafian follicles (Figure 20.3).

Ovarian Cycles. Estrus is the period when the female mammal is most receptive to the male (coincides with high levels of circulating estrogen). Rodents are considered to be polyestrous and have a succession of estrus cycles. Cats are seasonally (spring, early fall) polyestrus, while dogs are monestrous. Humans and higher primates cycle at monthly intervals. Although most mammals ovulate spontaneously, some mammals (cats and minks) undergo provoked or induced ovulation (i.e., stimulated by mating). The estrus cycle and the resulting differences in circulating hormone concentrations at different stages of the cycle are depicted in Figure 20.4. The changes in circulating hormone and the stage of follicle development during an adverse toxicant insult results in a variety of toxicological manifestations.

Germinal

Primary

Secondary Follicle

 

(Antrum forming)

Follicles

 

Epithelium

Oocyte

 

 

Stratum

 

 

Granulosum

Mesovarium

Time

Corpus

 

 

Cumulus

Luteum

 

 

 

Maturing

Oophorus

 

 

Theca

 

 

Theca

Follicle

Zona

Externa

Interna

 

Pellucida

 

 

Figure 20.3 The arrow follows the ovarian follicles (time course) from their maturation from primary follicles to the corpus luteum. (Adapted from Web site of Dr. Steven Scadding and Dr. Sandra K. Ackerley, http://www.uoguelph.ca/zoology/devobio/210labs/ovary4.html.)