MAMMALIAN REPRODUCTIVE CYCLE 



541 



apparatus whose final link to the pituitaiy 

 is neurohumoral by way of the hypophyseal 

 portal veins and whose activity precipitates 

 release of LH. (2) The apparatus includes 

 a hypothalamic center or centers whose ex- 

 citation depends on estrogen-progesterone 

 levels and afferent impulses of various 

 kinds. (3) The sensitivity of the center (s) 

 is influenced not only by the sex steroids, but 

 by other poorly understood factors that 

 vary from species to species and from time 

 to time in individuals, e.g., the diurnal 

 rhythm in rats. Here in bare outline is a 

 plausible hypothesis that may be generally 

 applied to the events immediately relating 

 to ovulation. 



Satisfactory hypotheses respecting other 

 phases of the cycle must await future de- 

 velopments. The extent and manner of in- 

 tervention of the nervous system in the fol- 

 licular and luteal phases remain unsettled. 

 Although the hypophyseal hormones con- 

 cerned in ovarian follicle development have 

 been characterized, their exact chemical de- 

 scrijition has not been accomplished. The 

 rate of their output at different stages of the 

 cycle is largely a matter of conjecture. 

 Structural changes that they jiroduce in the 

 ovary are well known, but in chemical terms 

 only the end products of ovarian activity are 

 well recognized, and these probably incom- 

 pletely. The fact that the estrogens, in turn, 

 have a regulating effect on follicle-stimulat- 

 ing activity of the hypophysis is known, but 

 the mechanisms by which this effect is ac- 

 complished are uncertain. The hypophyseal 

 hormones that maintain the luteal phase 

 are recognized with any certainty in only 

 three species and there is a wide difference 

 between rabbits, on the one hand, and rats 

 and mice, on the other. For mammals gener- 

 ally, the luteotrophic factors have not been 

 identified. Whether the hypothalamus is 

 actively concerned in maintenance of the 

 luteal i)hase in the majority of mammals is 

 unknown. The morphologic effects of luteo- 

 trojihic stimulation on corpora lutea are well 

 recognized, but here again the chemical 

 mechanisms leading to the end products are 

 obscure. The action of the corpus luteum 

 hormone in regulation of the cycle is par- 

 tially known, including the well established 

 fact that its continual jiresence in large 

 amount will suiijiress the estrogenic and 



ovulatory phases. Yet, one cannot say 

 whether this effect is accomplished by direct 

 action on the hypophysis or by indirect ac- 

 tion through the central nervous system. 

 Nor can one state how the hypophysis- 

 gonad equilibrium of the luteal phase is in- 

 terrupted in the absence of a conceptus. 

 With respect to the ovulation mechanism 

 itself, the hypothesis outlined above requires 

 verification in additional species. Assuming 

 its validity, many details remain to be stud- 

 ied, e.g., the neural pathways and nuclei 

 involved, identification of neurochemical 

 activators of the pars distalis and their 

 sources and loci of action, the precise nature 

 of mechanisms whereby the gonadal steroids 

 excite or suppress, the cellular mechanisms 

 by which ovulating hormone is released into 

 circulation by the hypoi^hysis, and the cyto- 

 chemical effects within the ovary. All too 

 evidently an encompassing theory of the 

 female reproductive cycle is far from reali- 

 zation. 



IX. References 



Allex. E., Daxforth, C. H., .and Doisy, E. A., 

 Eds. 1939. Sex and Internal Secretions, 2nd 

 ed. Baltimore: The Williams & Wilkins Com- 

 pany. 



Allen, E. 1923. Racial and familial cyclic in- 

 heritance and other evidence from the mouse 

 concerning the cause of estrous phenomena. 

 Am. J. Anat., 32, 293-304. 



Allen, E. 1927. The menstrual cycle of the mon- 

 key, Macacus rhesus: observations on normal 

 animals, the effects of removal of the ovaries 

 and the effects of injection of ovarian and pla- 

 cental extracts into the spayed animals. Contr. 

 Embrvol., Carnegie Inst. Washington. 19, 

 1-14. 



Allen, E., Pratt, J. P., Newell, Q. U.. and Bland, 

 L. J. 1930. Human ova from large follicles: 

 including a search for maturation divisions and 

 observations of atresia. Am. J. Anat., 46, 1-53. 



Alloiteau, J. J. 1952. La lordose reflexe par ex- 

 citation du col uterin chez la rate. Son con- 

 trole endocrinien. Compt. rend. Soc. biol., 146, 

 237-238. 



Alloiteau, J. J. 1954. Effets de doses minimes de 

 progesterone sur les fonctions gonadotropes de 

 la Ratte en oestrus permanent hypothala- 

 mique. Compt. rend. Soc. biol., 148, 875-877. 



Alloiteau, J. J., .\nd Courvoisier, B. 1953. Effets 

 de I'excitation du col uterin chez la rate apres 

 lesions hypothalamique. Compt. rend. Soc. 

 biol., 147, 1040-1042. 



Arm.strong, D. T., and Hansel, W. 1958. Altera- 

 tion of the bovine estrous cvcle with oxvtocin. 

 Fed. Proc, 17, 6. 



Arox, C, and Aron-Brunetiere, R. 1953, Effets 



