Polyovular Follicles and Multinucleate Ova in the Ovaries of Young Mice HARRY A. KENT, JR. Zoology Department, University o f Georgia, Athens, Georgia' Polyovular follicles and multinucleate and the arithmetic means calculated for ova have been observed in the immature each age group, the information is as premembers of numerous species (Hartman, sented in figures 1 and 2. '26; Lane, '38; Davis and Hall, '50 and DISCUSSION Dawson, '51 ). Most of the research studies The Swiss mouse reaches maturity, as and conclusions drawn from studies in this area have dealt with the possibility of a based on successful litter production, by sequential relationship between the two the 7th week of age. Follicle counts, as types of juvenile ovarian structures. Kent presented in table 1, indicate an early ('59) has presented evidence of low estro- production of corpora lutea at approxigen level as a primary cause for the pres- mately the 6th week with a steady increase ence of polyovular follicles and multinu- in numbers through the last analysis pecleate ova in the immature hamster. As a riod, the 10th week. On the basis of proresult of Kent's study it has become ap- portion of total follicles, the level of corparent that a survey of the common labora- pora lutea reaches a peak by the 7th week tory animals, with regard to establishing (fig. 1) . These figures may be presumed to the levels and types of ovarian structures indicate establishment of a static level of at various ages, should be undertaken so follicle maturation by the 7th week. Production of primary follicles, as preas to complete the picture of immature patterns and to provide a more substantial sented in table 1, presents a picture of inbackground for further experimental stud- creasing numbers throughout the analytiies. The present work initiates the project cal period, while secondary follicles reach with a timed age survey of the Swiss a maximum at approximately the 6th week. On the basis of proportions of primouse. mary and secondary follicles (fig. l ) , an MATERIALS AND METHODS anomalous situation is established in which Forty female Swiss mice, of the strain there is an apparent interacting cyclic bemaintained at the Worcester Foundation havior of the primary and secondary for Experimental Biology, were used. Five follicle levels. The reverse curves (fig. 1) are due to animals were sacrificed at each week of age, from the third to the 10th week. Ovar- an early increase in proportion of secondies were removed, fixed in Carnoy's fluid, ary follicles which reaches a plateau by imbedded in paraffin, sectioned serially at the 6th to 7th week, followed by a contin8 and stained with hematoxylin and ued increase in proportion of primary foleosin. All follicles were counted and class- licles continuing through the 9th week at ified following criteria established by Kent which time the secondary follicles regain their earlier proportionate value. ('59). The three curves of figure 1 are of interRESULTS AND OBSERVATIONS est primarily as a key to the onset of maThe results of the study are presented in turation processes in the ovary. On the table 1. The count data are further ex- basis of corpora lutea production, luteinizpressed in terms of per cent in order to ing hormone (LH) becomes available in provide statements of the proportions of This work was conducted at the Worcester each type of structure per total number of Foundation for Experimental Biology under the follicles. When this is done for each ovary auspices of the American Physiological Society. 52 1 HARRY A. KENT, JR. quantity somewhere during the 5th to 6th week, while the increasing numbers of secondary follicles would indicate perhaps an earlier date for the onset of LH production. Increasing numbers of primary follicles throughout the analysis period suggest an increasing estrogen level in the ovary (Payne, '56) with a preceding elevation in amount of available follicle stimulating hormone (FSH). Numerous investigators have approached the problem of immature mammalian ovarian structures with the attitude that the multinucleate condition precedes the polyovulate. Such an assumption appears to be substantiated by the present study (table 1 and fig. 2) although the actual number of multinucleate ova is inadequate to serve as precursors to the more extensive polyovular follicles. Atretic follicles were not included in the counts of this study, since they represent anomalous structures not directly correlated with the production of fertilizable ova. It is conceivable that atretic follicles may provide an explanation of the above statements in that only a certain proportion of multinucleate ova may survive to undergo further development. As an indication of possible approaches to a solution of causative factors in the development of multinucleate and polyovulate conditions, certain tentative conclusions may be based on the present study. It is apparent that FSH is dominant during the third to 4th week with increasing levels of LH after that time (Fevold, '41). The increasing numbers of primary and secondary follicles may be assumed to indicate an accordant increase in estrogen level in the ovary (Hisaw, '47). It must be doubted that progesterone is available in any quantity before the 6th week. On the basis of these assumptions it would appear that a low level of FSH permits development of multinucleate structures, and that a subsequent rise in FSH level limits production of the same structures. Increasing levels of LH presage development of polyovulate follicles and attainment of the adult FSH/LH balance limits further development of either multinucleate ova or polyovular follicles These statements are, of course, only suppositional, indicating what course fur- 523 OVARIAN ABNORMALITIES IN THE MOUSE 100 ' 90. 80 70. 60 t- z W 50. a w a 4 0- SECONDARY FOLLICLES 30. 2 0. 10CORPORA LUTEA +-*+ -+ -, -t 21 28 35 42 49 56 63 70 AGE IN DAYS Fig. 1 Relative number of monovular and uninucleate primary and secondary follicles and corpora lutea in the ovaries of mice. One standard error is indicated. ther studies may take. The purpose of this work is to establish the normal condition of the immature mouse ovary with respect primarily to multinucleate ova and POlYovular follicles. SUMMARY The number of monovular, uninucleate, multiovular and multinucleate follicles in Swiss mice from the third to 10th week of age were counted. Polynuclear ova were most numerous in 4-week-old mice. Poly- ovular follicles reached a peak at 6 weeks and decreased in frequency from that point on. Secondary follicles reached a maximum value at the 7th week and held a level while primary follicles continued to increase throughout the duration of the study. The data are explained on the basis that estrogen level affects the incidence of polyovular and polynuclear follicles and that both estrogen production and primary and secondary follicle development may be correlated with FSH and LH levels. 524 HARRY A. KENT, JR. PO LY OVU L A R F 0 LLIC L E S 1. I- z BIOVATE A W u II: W CL 21 I L A'+YATE / + 21 /*-- 12. 28 42 35 I *'-+-*-+ 49 $6 63 70 POLYNUCLEAR OVA 10. 1 AGE IN DAYS Fig. 2 Relative number of multinucleate ova and polyovular follicles in the ovaries of mice. One standard error i s indicated. LITERATURE CITED Adams, C. E. 1953 Some aspects of ovulation, recovery and transplantation of ova i n the immature rabbit. Ciba Found. Symp. On Mammalian Germ Cells, up. 198-211. Davis, D. E. 1950 Polyovuly and anovular follicles in the wild Norway Rat. Anat. Rec., 107: 187-192. Dawson, A. B. 1951 Histogenetic interrelationships of oocytes and follicle cells. A possible explanation of the mode of origin of certain polyovular follicles in the immature rat. Ibid., 110: 181-197. Fevold, H. L. 1941 Synergism of the follicle stimulating and luteinizing hormones in pro- ducing estrogen secretion. Endocrinology, 28: 33-36. Hartmann, G . G. 1926 Polynuclear ova and polyovular follicles i n the opossum and other mammals, with special reference to the problem of fecundity. Am. J. Anat., 37: 1-51. Hisaw, F. L. 1947 Development of the graafian follicle and ovulation. Physiol. Rev., 27: 95119. Kent, H. A., Jr. 1959 Reduction of polyovular follicles and polynuclear ova by estradiol monobenzoate. Anat. Rec., 134: 455462. Lane, C. E. 1938 Aberrant follicles in the immature rat. Ibid., 71: 243-247.