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Intraovarian effect of the corpus luteum on ovarian follicles during early pregnancy in heifers
Animal Reproduction Science, 15 (1987) 53-60
53
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Intraovarian Effect of the Corpus Luteum on Ovarian Follicles during Early Pregnancy in Heifers R.A. PIERSON and O.J. GINTHER
Department of Veterinary Science, University of Wisconsin-Madison, 1655Linden Drive, Madison, WI53706 (U.S.A.) (Accepted 18 May 1987)
ABSTRACT Pierson, R.A. and Ginther, O.J., 1987. Intraovarian effect of the corpus luteum on ovarian follicles during early pregnancy in heifers. Anita. Reprod. Sci., 15: 53-60. Ovarian follicles/> 2 mm were studied in 14 Holstein heifers by daily ultrasound examinations from Day 0 to Day 60 of pregnancy. Data were partitioned by ovary bearing the corpus luteum versus the contralateral ovary. There were more (P < 0.04 ) follicles 2-3 mm and fewer (P < 0.05) follicles 7-10 ram, 11-13 ram, and > 13 mm in the ovary bearing the corpus luteum. The interaction of day of pregnancy and location of the corpus luteum was significant (P<0.05) for the numbers of follicles 7- 10 mm and 11- 13 mm and tended to be significant (P < 0.1 ) for the number of follicles > 13 mm. The differences in the numbers of follicles 7-10 mm, 11-13 mm, and > 13 mm between the ovary bearing the corpus luteum and the contralateral ovary became manifest after approximately Day 22. Diameters of the largest and second largest follicles were greater (P<0.02 and P<0.05, respectively) in the ovary contralateral to the corpus luteum. The interaction of day of pregnancy and luteal location was significant for diameter of the largest follicle (P<0.02), but not for diameter of the second largest follicle. The presence of a corpus luteum appeared to exert a local control over the growth of follicles. The corpus luteum apparently was a negative influence for the growth of follicles larger than approximately 7 mm after Day 21 or 22 of pregnancy.
INTRODUCTION
Studies of follicular population dynamics and differences in the levels of follicular activity in the ovaries of cattle due to the influence of right or left side and presence of a corpus luteum have been evaluated in slaughterhouse studies (England et al., 1973; Rexroad and Casida, 1975; Matton et al., 1981 ) and more recently by diagnostic ultrasonography (Pierson and Ginther, 1984a, 1986, 1987a,b). In a previous study utilizing ultrasonography (Pierson and Ginther, 1987b), more follicles 2-3 ram, > 13 mm, and >i 2 mm were observed 0378-4320/87/$03.50
© 1987 Elsevier Science Publishers B.V.
54 in the ovary bearing the corpus luteum than in the contralateral ovary during the estrous cycle. These observations supported the hypothesis that the corpus luteum has a positive intraovarian effect. It has been demonstrated that the corpus luteum plays a positive role in local control of the development of follicles during the estrous cycle in sheep (Dufour et al., 1971, 1972). More recently, it was proposed that both the corpus luteum and the largest follicle are involved in the control of development of other follicles during the estrous cycle in cattle ( Staigmiller and England, 1982). During pregnancy, the role of the corpus luteum on follicular populations in sows, ewes, and cattle has been investigated in a slaughterhouse study in which stages of pregnancy were estimated by embryonic and fetal characteristics ( Rexroad and Casida, 1975). In the sow, more follicles were present throughout pregnancy in the ovary with the greater number of corpora lutea. In the ewe, more follicles were observed in the ovary bearing the corpus luteum, although the ovary with the corpus luteum had a lesser volume in small follicles at Day 7 of pregnancy and a greater volume in small follicles at Day 35. In cattle estimated to be from Days 0 to 20 of pregnancy, similar numbers of follicles > 4 m m were observed in the ovary bearing the corpus luteum and the contralateral ovary; however, there were more follicles > 4 mm in the ovary contralateral to the corpus luteum in cattle estimated to be from Days 21 to 70 and Days 71 to 167 of pregnancy. T h a t is, a negative effect of the corpus luteum on the number of follicles was observed during mid-pregnancy. This association was thought to be due to a local influence of the corpus luteum, perhaps through locally high levels of progesterone, although the mechanism has not been elucidated. The purpose of the present study was to test the hypothesis that the corpus luteum has an intraovarian effect on the number of follicles in various diameter categories during the first 60 days of pregnancy in heifers. MATERIALSAND METHODS Nulliparous Holstein heifers between 1.5 and 2.0 years of age and weighing 330-450 kg were used. The data set analysed herein is the same as that used for a portion of a previous study that compared pregnant and nonpregnant heifers (Pierson and Ginther, 1986); detailed analyses were made of the day effects for the combined data of both ovaries regardless of side or location Of the corpus luteum status during Days 0 to 60. The reproductive status (pregnant or nonpregnant) of the heifers was determined as described (Pierson and Ginther, 1984b; Curran et al., 1986a,b). Fourteen heifers maintained an embryo until at least Day 60. Four additional heifers lost the conceptus prior to the detection of an embryo proper and were not considered in this study. For all heifers, the embryonic vesicle and subsequent embryo proper were located in the uterine horn ipsilateral to the corpus luteum. Twelve of the 14 heifers
55 (86%) ovulated from the right ovary; thus the influence of right versus left ovary regardless of the presence of a corpus luteum could not be evaluated. A real-time B-mode diagnostic ultrasound instrument (Equisonics 210, Equisonics Inc., Roselle, IL) equipped with a linear-array, 5 MHz transducer designed for intrarectal placement was used for the examinations. Ultrasound examinations were done and follicles >/2 m m were counted and measured as described. The reliability of diagnostic ultrasonography for measuring and counting ovarian follicles in heifers has been substantiated (Pierson and Ginther, 1984a; 1987c ). The day of ovulation (Day 0) was determined by the disappearance of a large ( > 11 m m ) follicle and confirmed by the subsequent formation of a corpus luteum ( Pierson and Ginther, 1984a). The heifers were examined daily from at least 3 days prior to ovulation until Day 60 of pregnancy. Follicular patterns were studied by grouping the follicles into diameter ranges of 2-3 mm, 4-6 mm, 7-10 mm, 11-13 m m and > 13 m m as well as the total number of follicles >/2 mm. Diameters of the largest follicle and second largest follicle were also determined. The examinations were made without knowledge of the number of days post ovulation or of the previous day's result. The reproductive tract was not directly manipulated before or during the ultrasound examination. Examination of the uterus for the presence of a conceptus was done after the ovarian examinations were made. Statistical analyses used the Statistical Analysis System ( SAS ) general linear models univariate analyses for repeated measures (SAS Institute, 1985). Analyses were performed using a model statement written to determine the main effect of presence of the corpus luteum from Days 0 to 60. The main effect was tested using an F test with degrees of freedom modified by the Greenhouse-Geyser epsilon (SAS Institute, 1985). The interactions between day and location of corpus luteum also were examined. Days of significant difference due to location of the corpus luteum were determined using univariate t tests for each day (Snedecor and Cochran, 1980). RESULTS The mean diameters of the largest and second largest follicles are depicted for Days 0 to 60 (Fig. 1 ). There was a main effect of presence of the corpus luteum for diameter of the largest follicle. Averaged over all days, the diameter of the largest follicle was greater ( P < 0.02) in the ovary contralateral to the corpus luteum than in the ovary bearing the corpus luteum. The interaction of day of pregnancy and luteal location was also significant ( P < 0.002). The mean diameter of the largest follicle was significantly greater in the non-corpus luteum-bearing ovary on Days 22 to 33 and on Days 46 to 60 ( P < 0.01 ) but not on the other days. The diameter of the second largest follicle was greater (P < 0.05) in the ovary contralateral to the corpus luteum. There was not a
56
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Fig. I.Mean diameter ( _+S E M ) of the largestand second largestfolliclesin the ovary bearing the corpus luteum and the contralateralovary for Days 0 to 60 (n = 14). The main effectof the corpus luteum was significant (P < 0.05) for both endpoints. The interaction of day and location of the corpus luteum was significant (P < 0.005) for diameter of the largest follicle but not for the diameter of the second largest follicle.
significant interaction of day of pregnancy and luteal status for diameter of the second largest follicle. Data for the numbers of follicles 2-3 mm, 4-6 mm, 7-10 mm, 11-13 mm, > 13 mm, and total number >t 2 mm are depicted from Day 0 to 60 (Fig. 2). There were main effects of luteal status for the number of follicles 2-3 mm, 7-10 mm, 11-13 mm, and > 13 ram. There were more 2-3 mm follicles ( P < 0.04) and fewer follicles 7-10 mm ( P < 0.05), 11-13 mm ( P < 0.007), and > 13 mm ( P < 0 . 0 2 ) in the ovary bearing the corpus luteum. The interaction of day of pregnancy and location of the corpus luteum was significant ( P < 0.05) for the number of follicles 7-10 mm and 11-13 mm and there was a tendency ( P < 0.1 ) towards an interaction for the number of follicles > 13 mm. The day of first significant difference between the ovary bearing the corpus luteum and the contralateral ovary was Day 23 for the number of follicles 7-10 mm, Day 22 for the number of follicles 11-13 mm, and Day 30 for the number of follicles > 13 ram. Fig. 2. Mean numbers of follicles ( + SEM) for the ovary bearing the corpus luteum and the contralateral ovary in the 2-3 ram, 4-6 ram, 7-10 ram, 11-13 ram, > 13 mm and >/2 mm diameter categories for Days 0 to 60 ( n = 14). The main effect of luteal status was significant for the number of follicles 2-3 mm (P<0.04), 7-10 mm (P<0.05), 11-13 mm (P<0.007), and >13 mm (P < 0.02 ). The interaction of day and location of the corpus luteum was significant (P < 0.02) for the number of follicles 7-10 mm and 11-13 ram.
57
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58 DISCUSSION Data for the various diameter categories and diameters of the largest and second largest follicles for the days of pregnancy corresponding to approximately the length of an interovulatory interval (21 days) in the present study were consistent with previously reported follicular numbers in the ovary bearing the corpus luteum and the contralateral ovary during the estrous cycle (Pierson and Ginther, 1987b). The numbers of follicles also appeared to be consistent with the slaughterhouse study of the follicular population in cattle estimated to be from Days 0 to 20 and Days 21 to 70 of pregnancy as determined by embryonic and fetal characteristics (Rexroad and Casida, 1975 ). The mean profile of diameter of the largest follicle in the ovary bearing the corpus luteum was characterized by an increase in mean diameter to approximately 10 m m from Day 0 to Day 12 or 13 and subsequent decrease until Day 26. Another increase in mean diameter, to approximately 10 mm, occurred from approximately Day 29 to 40. There was a subsequent decrease in mean diameter until Day 60. Mean diameter of the largest follicle in the ovary contralateral to the corpus luteum was reciprocally related to the mean diameter of the largest follicle in the ovary bearing the corpus luteum from approximately Days 20 to 60. The mean diameter of the second largest follicle remained greater in the ovary which did not contain the corpus luteum from Day 20 to Day 60. There were more follicles 2-3 m m and fewer follicles 7-10 mm, 11-13 mm, and > 13 m m in the ovary bearing the corpus luteum. The greater number of 2-3 mm follicles was consistent with a previous study utilizing diagnostic ultrasonography to study the effects of the corpus luteum during the intraovulatory interval. There were, however, no significant differences for the numbers of follicles 7-10 mm, 11-13 mm, or > 13 m m between the ovary bearing the corpus luteum and the contralateral ovary during the interovulatory interval. The patterns exhibited by the mean profiles of data in the various diameter categories during early pregnancy in the present study were similar to the mean profiles of data partitioned by corpus luteum-bearing ovary and non-corpus luteum-bearing ovary in comparable categories for the length of time associated with the interovulatory interval (21 days; Pierson and Ginther, 1987b). The differences in the numbers of follicles 2-3 mm, 7-10 mm, 11-13 mm, and > 13 m m between the ovary bearing the corpus luteum and the contralateral ovary in the present study became manifest after approximately Day 22, that is, after the time nonpregnant heifers would be expected to return to estrus and ovulate. The number of follicles 2-3 mm, > 13 mm, and >/2 m m were similar during the interovulatory interval (Pierson and Ginther, 1987b) and the first 21 days of pregnancy in the present study. During the interovulatory interval, however, the number of follicles 7-10 mm and 11-13 m m were not different between the ovary bearing the corpus luteum and the contralateral
59 ovary. Perhaps two separate mechanisms influence the follicular population. A positive intraovarian effect of the corpus luteum on the follicular population may be exerted during the interovulatory interval and during the first 21 days of pregnancy whereas, after approximately Day 25, the conceptus or products of the conceptus may have a unilateral, negative influence. It was unfortunate that the influence of right and left side on the numbers of follicles in the various diameter categories could not be evaluated in the present study. The corpus luteum was in the right ovary for 12 of the 14 heifers. However, previous studies have shown a greater level of follicular activity and more frequent ovulation in the right versus the left ovary ( Casida et al., 1935; Rajakoski, 1960; Pierson and Ginther, 1987b). Nevertheless, the follicular activity and diameter of the largest and second largest follicle in the ovary bearing the corpus luteum were depressed; therefore, the unilateral effect observed in the present study was more likely related to the location of the corpus luteum than to an influence of side. In addition, diameters of the largest and second largest follicles were greater in the non-corpus luteum-bearing ovary during Days 0 to 60 of pregnancy but were not significantly different during the interovulatory interval ( Pierson and Ginther, 1987b). The interaction of day and location of the corpus luteum for the number of follicles 7-10 m m and 11-13 mm appeared to be due primarily to a greater difference between ovaries after Day 20 than before, due to decreased numbers of follicles in the ovary containing the corpus luteum. The interaction for the 11-13 mm category seemed due to differences similar to those for the 7-10 m m category, except for Days 36 to 44. The patterns of the mean profiles for follicles 11-13 m m and > 13 m m in the ovary bearing the corpus luteum versus the contralateral ovary were reflected in the profiles representing diameter of the largest follicle. Examination of the data from the individual heifers indicated that the increases and decreases in the mean profiles of diameter of the largest follicle and the numbers of follicles 7-10 mm, 11-13 mm, and > 13 m m were due to the growth and regression of one follicle in each heifer from approximately Days 30 to 50; however, this hypothesis was not critically tested. In conclusion, differences due to the presence of the corpus luteum observed in the population of follicles >i 2 m m in pregnant heifers occurred in the numbers of follicles 2-3 mm, 7-10 mm, 11-13 mm, and > 13 mm and the diameter of the largest follicle. There were more follicles < 4 m m in the ovary bearing the corpus luteum and more follicles > 7 m m in the contralateral ovary. Thus, the presence of a corpus luteum appears to exert a local control over the growth of follicles. The corpus luteum apparently is a negative influence for the growth of follicles larger than approximately 7 mm after Day 21 or 22 of pregnancy. ACKNOWLEDGEMENTS Supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison and USDA Grant 84-CRSR-2-2451. The authors wish to
60
thank Dr. Murray Clayton, Department of Statistics, and Peter Crump, Department of Computing and Biometry, for assistance in their respective specialities. Appreciation is expressed also to Marcia Campbell, Peggy Hoge, Susan Krebs, Lisa Kulick, and Diane Stuber for technical assistance.
REFERENCES
Casida, L.E., Chapman, A.B. and Rupel, I.W., 1935. Ovarian development in calves. J. Agric. Res., 50: 953-960. Curran, S., Pierson, R.A. and Ginther, O.J., 1986a. Ultrasonographic appearance of the bovine conceptus on days 10 through 20. J. Am. Vet. Med. Assoc., 189: 1289-1294. Curran, S., Pierson, R.A. and Ginther, O.J., 1986b. Ultrasonographic appearance of the bovine conceptus on days 20 through 60. J. Am. Vet. Med. Assoc., 189: 1295-1303. Dufour, J., Ginther, O.J. and Casida, L.E., 1971. Corpus luteum action on ovarian follicular development after destruction of macroscopically visible follicles in ewes. Proc. Soc. Exp. Biol. Med., 138: 475-478. Dufour, J., Ginther, O.J. and Casida, L.E., 1972. Intraovarian relationship between corpora lutea and ovarian follicles in ewes. Am. J. Vet. Res., 33: 1445-1447. England, B.G., Karavolas, H.J., Hauser, E.R. and Casida, L.E., 1973. Ovarian follicular estrogens in Angus heifers. J. Anita. Sci., 37: 1176-1179. Matton, P., Adelakoun, V., Couture, Y. and Dufour, J.J., 1981. Growth and replacement of the bovine ovarian follicles during the estrous cycle. J. Anim. Sci., 52: 813-820. Pierson, R.A. and Ginther, O.J., 1984a. Ultrasonography of the bovine ovary. Theriogenology, 21: 495-504. Pierson, R.A. and Ginther, O.J., 1984b. Ultrasonography for detection of preguancy and study of early embryonic development in the bovine. Theriogenology, 21: 225-233. Pierson, R.A. and Ginther, O.J., 1986. Ovarian follicular populations during early pregnancy in heifers. Theriogenology, 26: 649-659. Pierson, R.A. and Ginther, O.J., 1987a. Follicular populations during the estrous cycle in heifers. I. Influence of day. Anim. Reprod. Sci., 14: 165-176. Pierson, R.A. and Ginther, O.J., 1987b. Follicular populations during the estrous cycle in heifers. II. Influence of right and left sides and intraovarian effect of the corpus luteum. Anita. Reprod. Sci., 14: 177-186. Pierson, R.A. and Ginther, O.J., 1987c. Reliability of diagnostic ultrasonography for identification and measurement of follicles and detecting the corpus luteum in heifers. Theriogenology (accepted). Rajakoski, E., 1960. The ovarian follicular system in sexually mature heifers with special reference to seasonal, cyclical and left-right variations. Acta Endocrinol., Suppl., 52: 1-68. Rexroad, C.E. and Casida, L.E., 1975. Ovarian follicular development in cows, sows, and ewes in different stages of pregnancy as affected by number of corpora lutea in the same ovary. J. Anita. Sci., 41: 1090-1097. SAS Institute, 1985. SAS User's Guide: Statistics, Version 5 Edition. SAS Institute, Cary, NC, 956 pp. Snedecor, G.W. and Cochran, W.G., 1980. Statistical Methods, 7th Edition. Iowa State University Press, Ames, IA, 507 pp. Staigmiller, R.B. and England, B.G., 1982. Folliculogenesis in the bovine. Theriogenology, 17: 43-52.
53
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Intraovarian Effect of the Corpus Luteum on Ovarian Follicles during Early Pregnancy in Heifers R.A. PIERSON and O.J. GINTHER
Department of Veterinary Science, University of Wisconsin-Madison, 1655Linden Drive, Madison, WI53706 (U.S.A.) (Accepted 18 May 1987)
ABSTRACT Pierson, R.A. and Ginther, O.J., 1987. Intraovarian effect of the corpus luteum on ovarian follicles during early pregnancy in heifers. Anita. Reprod. Sci., 15: 53-60. Ovarian follicles/> 2 mm were studied in 14 Holstein heifers by daily ultrasound examinations from Day 0 to Day 60 of pregnancy. Data were partitioned by ovary bearing the corpus luteum versus the contralateral ovary. There were more (P < 0.04 ) follicles 2-3 mm and fewer (P < 0.05) follicles 7-10 ram, 11-13 ram, and > 13 mm in the ovary bearing the corpus luteum. The interaction of day of pregnancy and location of the corpus luteum was significant (P<0.05) for the numbers of follicles 7- 10 mm and 11- 13 mm and tended to be significant (P < 0.1 ) for the number of follicles > 13 mm. The differences in the numbers of follicles 7-10 mm, 11-13 mm, and > 13 mm between the ovary bearing the corpus luteum and the contralateral ovary became manifest after approximately Day 22. Diameters of the largest and second largest follicles were greater (P<0.02 and P<0.05, respectively) in the ovary contralateral to the corpus luteum. The interaction of day of pregnancy and luteal location was significant for diameter of the largest follicle (P<0.02), but not for diameter of the second largest follicle. The presence of a corpus luteum appeared to exert a local control over the growth of follicles. The corpus luteum apparently was a negative influence for the growth of follicles larger than approximately 7 mm after Day 21 or 22 of pregnancy.
INTRODUCTION
Studies of follicular population dynamics and differences in the levels of follicular activity in the ovaries of cattle due to the influence of right or left side and presence of a corpus luteum have been evaluated in slaughterhouse studies (England et al., 1973; Rexroad and Casida, 1975; Matton et al., 1981 ) and more recently by diagnostic ultrasonography (Pierson and Ginther, 1984a, 1986, 1987a,b). In a previous study utilizing ultrasonography (Pierson and Ginther, 1987b), more follicles 2-3 ram, > 13 mm, and >i 2 mm were observed 0378-4320/87/$03.50
© 1987 Elsevier Science Publishers B.V.
54 in the ovary bearing the corpus luteum than in the contralateral ovary during the estrous cycle. These observations supported the hypothesis that the corpus luteum has a positive intraovarian effect. It has been demonstrated that the corpus luteum plays a positive role in local control of the development of follicles during the estrous cycle in sheep (Dufour et al., 1971, 1972). More recently, it was proposed that both the corpus luteum and the largest follicle are involved in the control of development of other follicles during the estrous cycle in cattle ( Staigmiller and England, 1982). During pregnancy, the role of the corpus luteum on follicular populations in sows, ewes, and cattle has been investigated in a slaughterhouse study in which stages of pregnancy were estimated by embryonic and fetal characteristics ( Rexroad and Casida, 1975). In the sow, more follicles were present throughout pregnancy in the ovary with the greater number of corpora lutea. In the ewe, more follicles were observed in the ovary bearing the corpus luteum, although the ovary with the corpus luteum had a lesser volume in small follicles at Day 7 of pregnancy and a greater volume in small follicles at Day 35. In cattle estimated to be from Days 0 to 20 of pregnancy, similar numbers of follicles > 4 m m were observed in the ovary bearing the corpus luteum and the contralateral ovary; however, there were more follicles > 4 mm in the ovary contralateral to the corpus luteum in cattle estimated to be from Days 21 to 70 and Days 71 to 167 of pregnancy. T h a t is, a negative effect of the corpus luteum on the number of follicles was observed during mid-pregnancy. This association was thought to be due to a local influence of the corpus luteum, perhaps through locally high levels of progesterone, although the mechanism has not been elucidated. The purpose of the present study was to test the hypothesis that the corpus luteum has an intraovarian effect on the number of follicles in various diameter categories during the first 60 days of pregnancy in heifers. MATERIALSAND METHODS Nulliparous Holstein heifers between 1.5 and 2.0 years of age and weighing 330-450 kg were used. The data set analysed herein is the same as that used for a portion of a previous study that compared pregnant and nonpregnant heifers (Pierson and Ginther, 1986); detailed analyses were made of the day effects for the combined data of both ovaries regardless of side or location Of the corpus luteum status during Days 0 to 60. The reproductive status (pregnant or nonpregnant) of the heifers was determined as described (Pierson and Ginther, 1984b; Curran et al., 1986a,b). Fourteen heifers maintained an embryo until at least Day 60. Four additional heifers lost the conceptus prior to the detection of an embryo proper and were not considered in this study. For all heifers, the embryonic vesicle and subsequent embryo proper were located in the uterine horn ipsilateral to the corpus luteum. Twelve of the 14 heifers
55 (86%) ovulated from the right ovary; thus the influence of right versus left ovary regardless of the presence of a corpus luteum could not be evaluated. A real-time B-mode diagnostic ultrasound instrument (Equisonics 210, Equisonics Inc., Roselle, IL) equipped with a linear-array, 5 MHz transducer designed for intrarectal placement was used for the examinations. Ultrasound examinations were done and follicles >/2 m m were counted and measured as described. The reliability of diagnostic ultrasonography for measuring and counting ovarian follicles in heifers has been substantiated (Pierson and Ginther, 1984a; 1987c ). The day of ovulation (Day 0) was determined by the disappearance of a large ( > 11 m m ) follicle and confirmed by the subsequent formation of a corpus luteum ( Pierson and Ginther, 1984a). The heifers were examined daily from at least 3 days prior to ovulation until Day 60 of pregnancy. Follicular patterns were studied by grouping the follicles into diameter ranges of 2-3 mm, 4-6 mm, 7-10 mm, 11-13 m m and > 13 m m as well as the total number of follicles >/2 mm. Diameters of the largest follicle and second largest follicle were also determined. The examinations were made without knowledge of the number of days post ovulation or of the previous day's result. The reproductive tract was not directly manipulated before or during the ultrasound examination. Examination of the uterus for the presence of a conceptus was done after the ovarian examinations were made. Statistical analyses used the Statistical Analysis System ( SAS ) general linear models univariate analyses for repeated measures (SAS Institute, 1985). Analyses were performed using a model statement written to determine the main effect of presence of the corpus luteum from Days 0 to 60. The main effect was tested using an F test with degrees of freedom modified by the Greenhouse-Geyser epsilon (SAS Institute, 1985). The interactions between day and location of corpus luteum also were examined. Days of significant difference due to location of the corpus luteum were determined using univariate t tests for each day (Snedecor and Cochran, 1980). RESULTS The mean diameters of the largest and second largest follicles are depicted for Days 0 to 60 (Fig. 1 ). There was a main effect of presence of the corpus luteum for diameter of the largest follicle. Averaged over all days, the diameter of the largest follicle was greater ( P < 0.02) in the ovary contralateral to the corpus luteum than in the ovary bearing the corpus luteum. The interaction of day of pregnancy and luteal location was also significant ( P < 0.002). The mean diameter of the largest follicle was significantly greater in the non-corpus luteum-bearing ovary on Days 22 to 33 and on Days 46 to 60 ( P < 0.01 ) but not on the other days. The diameter of the second largest follicle was greater (P < 0.05) in the ovary contralateral to the corpus luteum. There was not a
56
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----
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Fig. I.Mean diameter ( _+S E M ) of the largestand second largestfolliclesin the ovary bearing the corpus luteum and the contralateralovary for Days 0 to 60 (n = 14). The main effectof the corpus luteum was significant (P < 0.05) for both endpoints. The interaction of day and location of the corpus luteum was significant (P < 0.005) for diameter of the largest follicle but not for the diameter of the second largest follicle.
significant interaction of day of pregnancy and luteal status for diameter of the second largest follicle. Data for the numbers of follicles 2-3 mm, 4-6 mm, 7-10 mm, 11-13 mm, > 13 mm, and total number >t 2 mm are depicted from Day 0 to 60 (Fig. 2). There were main effects of luteal status for the number of follicles 2-3 mm, 7-10 mm, 11-13 mm, and > 13 ram. There were more 2-3 mm follicles ( P < 0.04) and fewer follicles 7-10 mm ( P < 0.05), 11-13 mm ( P < 0.007), and > 13 mm ( P < 0 . 0 2 ) in the ovary bearing the corpus luteum. The interaction of day of pregnancy and location of the corpus luteum was significant ( P < 0.05) for the number of follicles 7-10 mm and 11-13 mm and there was a tendency ( P < 0.1 ) towards an interaction for the number of follicles > 13 mm. The day of first significant difference between the ovary bearing the corpus luteum and the contralateral ovary was Day 23 for the number of follicles 7-10 mm, Day 22 for the number of follicles 11-13 mm, and Day 30 for the number of follicles > 13 ram. Fig. 2. Mean numbers of follicles ( + SEM) for the ovary bearing the corpus luteum and the contralateral ovary in the 2-3 ram, 4-6 ram, 7-10 ram, 11-13 ram, > 13 mm and >/2 mm diameter categories for Days 0 to 60 ( n = 14). The main effect of luteal status was significant for the number of follicles 2-3 mm (P<0.04), 7-10 mm (P<0.05), 11-13 mm (P<0.007), and >13 mm (P < 0.02 ). The interaction of day and location of the corpus luteum was significant (P < 0.02) for the number of follicles 7-10 mm and 11-13 ram.
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58 DISCUSSION Data for the various diameter categories and diameters of the largest and second largest follicles for the days of pregnancy corresponding to approximately the length of an interovulatory interval (21 days) in the present study were consistent with previously reported follicular numbers in the ovary bearing the corpus luteum and the contralateral ovary during the estrous cycle (Pierson and Ginther, 1987b). The numbers of follicles also appeared to be consistent with the slaughterhouse study of the follicular population in cattle estimated to be from Days 0 to 20 and Days 21 to 70 of pregnancy as determined by embryonic and fetal characteristics (Rexroad and Casida, 1975 ). The mean profile of diameter of the largest follicle in the ovary bearing the corpus luteum was characterized by an increase in mean diameter to approximately 10 m m from Day 0 to Day 12 or 13 and subsequent decrease until Day 26. Another increase in mean diameter, to approximately 10 mm, occurred from approximately Day 29 to 40. There was a subsequent decrease in mean diameter until Day 60. Mean diameter of the largest follicle in the ovary contralateral to the corpus luteum was reciprocally related to the mean diameter of the largest follicle in the ovary bearing the corpus luteum from approximately Days 20 to 60. The mean diameter of the second largest follicle remained greater in the ovary which did not contain the corpus luteum from Day 20 to Day 60. There were more follicles 2-3 m m and fewer follicles 7-10 mm, 11-13 mm, and > 13 m m in the ovary bearing the corpus luteum. The greater number of 2-3 mm follicles was consistent with a previous study utilizing diagnostic ultrasonography to study the effects of the corpus luteum during the intraovulatory interval. There were, however, no significant differences for the numbers of follicles 7-10 mm, 11-13 mm, or > 13 m m between the ovary bearing the corpus luteum and the contralateral ovary during the interovulatory interval. The patterns exhibited by the mean profiles of data in the various diameter categories during early pregnancy in the present study were similar to the mean profiles of data partitioned by corpus luteum-bearing ovary and non-corpus luteum-bearing ovary in comparable categories for the length of time associated with the interovulatory interval (21 days; Pierson and Ginther, 1987b). The differences in the numbers of follicles 2-3 mm, 7-10 mm, 11-13 mm, and > 13 m m between the ovary bearing the corpus luteum and the contralateral ovary in the present study became manifest after approximately Day 22, that is, after the time nonpregnant heifers would be expected to return to estrus and ovulate. The number of follicles 2-3 mm, > 13 mm, and >/2 m m were similar during the interovulatory interval (Pierson and Ginther, 1987b) and the first 21 days of pregnancy in the present study. During the interovulatory interval, however, the number of follicles 7-10 mm and 11-13 m m were not different between the ovary bearing the corpus luteum and the contralateral
59 ovary. Perhaps two separate mechanisms influence the follicular population. A positive intraovarian effect of the corpus luteum on the follicular population may be exerted during the interovulatory interval and during the first 21 days of pregnancy whereas, after approximately Day 25, the conceptus or products of the conceptus may have a unilateral, negative influence. It was unfortunate that the influence of right and left side on the numbers of follicles in the various diameter categories could not be evaluated in the present study. The corpus luteum was in the right ovary for 12 of the 14 heifers. However, previous studies have shown a greater level of follicular activity and more frequent ovulation in the right versus the left ovary ( Casida et al., 1935; Rajakoski, 1960; Pierson and Ginther, 1987b). Nevertheless, the follicular activity and diameter of the largest and second largest follicle in the ovary bearing the corpus luteum were depressed; therefore, the unilateral effect observed in the present study was more likely related to the location of the corpus luteum than to an influence of side. In addition, diameters of the largest and second largest follicles were greater in the non-corpus luteum-bearing ovary during Days 0 to 60 of pregnancy but were not significantly different during the interovulatory interval ( Pierson and Ginther, 1987b). The interaction of day and location of the corpus luteum for the number of follicles 7-10 m m and 11-13 mm appeared to be due primarily to a greater difference between ovaries after Day 20 than before, due to decreased numbers of follicles in the ovary containing the corpus luteum. The interaction for the 11-13 mm category seemed due to differences similar to those for the 7-10 m m category, except for Days 36 to 44. The patterns of the mean profiles for follicles 11-13 m m and > 13 m m in the ovary bearing the corpus luteum versus the contralateral ovary were reflected in the profiles representing diameter of the largest follicle. Examination of the data from the individual heifers indicated that the increases and decreases in the mean profiles of diameter of the largest follicle and the numbers of follicles 7-10 mm, 11-13 mm, and > 13 m m were due to the growth and regression of one follicle in each heifer from approximately Days 30 to 50; however, this hypothesis was not critically tested. In conclusion, differences due to the presence of the corpus luteum observed in the population of follicles >i 2 m m in pregnant heifers occurred in the numbers of follicles 2-3 mm, 7-10 mm, 11-13 mm, and > 13 mm and the diameter of the largest follicle. There were more follicles < 4 m m in the ovary bearing the corpus luteum and more follicles > 7 m m in the contralateral ovary. Thus, the presence of a corpus luteum appears to exert a local control over the growth of follicles. The corpus luteum apparently is a negative influence for the growth of follicles larger than approximately 7 mm after Day 21 or 22 of pregnancy. ACKNOWLEDGEMENTS Supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison and USDA Grant 84-CRSR-2-2451. The authors wish to
60
thank Dr. Murray Clayton, Department of Statistics, and Peter Crump, Department of Computing and Biometry, for assistance in their respective specialities. Appreciation is expressed also to Marcia Campbell, Peggy Hoge, Susan Krebs, Lisa Kulick, and Diane Stuber for technical assistance.
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