Beginning Of Diameter Deviation Research Articles

Variations in follicle-diameter deviation and a growth spurt in the dominant follicle at deviation in Bos taurus heifers

Follicle diameter deviation during a follicular wave in cattle is preceded by a common-growth phase and begins with continued growth rate of the future dominant follicle (DF or F1) and reduced growth rate of the future largest subordinate follicle (SF or F2). Follicle diameters were analyzed daily for wave 1 (n=100) and wave 2 (n=91). The beginning of expected deviation was designated day 0 and was defined as the day the largest follicle was closest to 8.5mm. Conventional classification of deviation (F2≥7.0mm at expected deviation) was more frequent (P<0.0001) in wave 1 (59%) than in wave 2 (35%); deviation began abruptly. An F2-undersized classification (F2<7.0mm at expected deviation) was greater (P<0.0001) for wave 2 (40%) than for wave 1 (15%); separation of diameters of F1 and F2 occurred gradually during the last 2days of the common-growth phase. The growth rate of F1 in conventional deviations of wave 1 was greater (P<0.002) on days 0 to 1 (1.8±0.1mm/day) than on days −1 to 0 (1.5±0.1mm/day). Several other novel growth spurts of F1 were observed in association with deviation including during switching of diameter rank between F1 and F2 in each of waves 1 and 2. Hypotheses were supported that (1) conventional deviation is more common during wave 1 whereas F2-undersized deviation is more common during wave 2 and (2) a transient growth spurt occurs in F1 of wave 1 at the beginning of diameter deviation.

Changes in follicular blood flow and nitric oxide levels in follicular fluid during follicular deviation in cows

Two experiments were conducted to test the hypothesis that there are dynamic changes in follicular blood flow during follicular deviation and that nitric oxide (NO) in follicular fluid (FF) plays a role in regulation of follicular blood flow. In Experiment I, follicular blood flow of the two largest follicles was monitored by using Power Doppler ultrasonography during follicular deviation in sixteen follicular waves during eight estrous cycles in eight cows. Blood flow did not differ (P>0.05) between the dominant follicle (DF) and the largest subordinate follicle (SF) until the beginning of the deviation of the follicular size, but was higher (P<0.05) in DF than in the largest SF one and two days after the beginning of diameter deviation in ovulatory (n=5) and atretic (n=11) waves; respectively. In Experiment II, FF was aspirated from DF and the largest SF on the day of diameter deviation (DF, n=6; SF, n=6) and two days later (DF, n=12; SF, n=9). Nitric oxide did not differ (P>0.05) between DF and the largest SF on the day of diameter deviation but, one or two days after observed diameter deviation NO concentrations were lower (P<0.01) in DF compared to the largest SF. On the day of diameter deviation and two days later E2 levels in FF were higher (P<0.01) in DF than in the largest SF. P4 concentrations in FF were higher (P<0.05) in DF than in the largest SF on the day of diameter deviation, but did not (P>0.05) differ two days later. E2/P4 ratio in FF was the same (P>0.05) in DF and the largest SF on the day of diameter deviation, but was higher (P<0.01) in DF than in the largest SF one or two days later. In conclusion, area of follicular blood flow of DF and the largest SF increased in parallel with follicular size during follicular deviation. Furthermore, there were relationships between changes in follicular blood flow, NO concentrations and E2/P4 ratio in FF following the beginning of diameter deviation in cattle.

Seasonal effects on the response of ovarian follicles to IGF1 in mares

The response of follicles to IGF1 was compared between the transition into the ovulatory season (transitional period) and the ovulatory season (ovulatory period) in eight mares using a cross-over experimental design within periods. Granulosa cells were collected from follicles 15-24 or 25-34 mm and expression of IGF1R, IGF2R, FSHR, LHCGR and PAPPA was determined by qPCR. In addition, 10 mg IGF1 or vehicle were injected into the largest follicle (transitional period) or the second largest follicle (ovulatory period) of a follicular wave before the beginning of diameter deviation between the two largest follicles (mean diameters at injection 19.2 and 20.0 mm during transitional and ovulatory periods respectively). Follicular fluid was collected 24 h after injection for determination of free IGF1, IGFBP, inhibin A and oestradiol levels. Granulosa cells from follicles 25-34 mm, but not follicles 15-24 mm, expressed higher levels of IGF1R (P=0.01), FSHR (P<0.007) and LHCGR (P=0.09) during the ovulatory period than during the transitional period, whereas IGF2R expression was higher in transitional than ovulatory follicles (P=0.06). Follicular IGFBP2 levels were not different (P>0.1) between periods and treatments, whereas IGFBP5 levels were higher (P<0.05) during the ovulatory period. Finally, IGF1 injection before the beginning of deviation induced an approximately twofold increase (P=0.01) in follicular inhibin A levels during each period and did not affect oestradiol (P>0.1). These results suggest that, as during ovulatory waves, equine follicles during transitional waves are responsive to IGF1 before the beginning of deviation and that, therefore, inadequate IGF1 responsiveness before deviation may not underlie the deficient development of dominant follicles during transition.

Mechanism of follicle deviation in monovular farm species

Diameter deviation is a distinctive change in growth rates among the follicles of a wave, heralding the formation of a dominant follicle and subordinate follicles. When the follicles are about 5 mm in cattle and 13 mm in horses, the wave-stimulating FSH surge reaches peak concentrations. Follicle and FSH manipulation studies in both species have shown that the declining portion of the surge before the beginning of deviation is a function of multiple growing follicles that require the decreasing FSH. During this time, all follicles of the wave have the potential for future dominance. Deviation begins when the two largest follicles on average are 8.5 and 7.7 mm in cattle and 22.5 and 19.0 mm in horses or about 3 days after the FSH peak in both species. The FSH/follicle relationship is close so that a change in one event soon causes a detectable change in the other. Thus, the difference in diameter between the two largest follicles at the beginning of deviation is compatible with rapid establishment of the destiny of the two follicles before the second-largest follicle can also show dominance. The deviation mechanism is initiated when FSH concentrations are low and the most advanced follicle reaches a specific developmental stage. In cattle, the future dominant follicle develops greater LH-receptor expression than the other follicles about 8 h before the beginning of diameter deviation. Estradiol and free IGF-1 begin to establish higher concentrations in the future dominant follicle than in other follicles and activin-A is transiently elevated in both follicles a few hours before the beginning of diameter deviation. In horses, estradiol, free IGF-1, activin-A, and inhibin-A begin to increase differentially in the future dominant follicle about 1 day before deviation. These changes underlie a greater responsiveness to LH and FSH by the developing dominant follicle than for other follicles, thereby accounting for deviation. Results of in vitro studies, although frequently done in other species, support this conclusion.

Changes in concentrations of follicular fluid factors during follicle selection in mares.

The temporal relationships in the changes in concentrations of follicular fluid factors during follicle selection were characterized in mares. All follicles > or =5 mm were ablated 10 days after ovulation, followed by follicular fluid collection from the three largest follicles (F1, F2, and F3) when F1 of the new wave reached a diameter of 8.0-11.9, 12.0-15.9, 16.0-19.9, 20.0-23.9, 24.0-27.9, or 28.0-31.9 mm (n = 4-8 mares/range). Diameter deviation between F1 and F2 began during the 20.0- to 23.9-mm range, as indicated by a greater difference in diameter between the two follicles at the 24.0- to 27.9-mm range than at the 20.0- to 23.9-mm range. Androstenedione concentrations increased in F1, F2, and F3 between the 16.0- to 19.9- and 20.0- to 23.9-mm ranges. In contrast, estradiol, free insulin-like growth factor (IGF)-1, activin-A, and inhibin-A concentrations increased only in F1 beginning at the 16.0- to 19.9-mm range. As a result, the concentrations of all four factors were higher in F1 than in F2 and F3 at all the later ranges, including the 20.0- to 23.9-mm range (beginning of diameter deviation). Concentrations of progesterone differentially increased in F1, concentrations of androstenedione and IGF-binding protein (IGFBP)-2 increased only in F2 and F3, and concentrations of inhibin-B differentially decreased in F2 and F3 simultaneous with the beginning of deviation. Concentrations of FSH, LH, pro-alphaC inhibin, and total inhibin did not change differentially among follicles. Results indicated that, on a temporal basis, estradiol, free IGF-1, activin-A, and inhibin-A may have played a role in the initiation of follicle deviation. In addition, these four factors as well as progesterone, androstenedione, IGFBP-2, and inhibin-B may have been involved in the subsequent differential development of the follicles.

Follicle selection in cattle: role of luteinizing hormone.

The circulating concentrations of LH were reduced by administration of 50 mg of progesterone every 8 h for 72 h, beginning when the largest follicle was 6.0 mm (experiment 1; n = 10). Progesterone treatment prevented the transient increase in LH that accompanied deviation (partitioning into dominant and subordinate categories) in control heifers (n = 10). The reduced LH concentrations were associated with reduced growth of the largest follicle, beginning a mean of 31 h after deviation, but did not alter the time of deviation or the growth and regression of the second-largest follicle. In experiment 2, 0 mg (controls) or 50 mg of progesterone was given every 8 h for three injections, beginning when the largest follicle was 7.0 mm (predeviation group) or 9.0 mm (postdeviation group; n = 8 for each of the four groups). Blood samples from the jugular vein and follicular-fluid samples from the two largest follicles were taken 8 h after the last treatment when the largest follicle was a mean of 8.7 mm in the predeviation group and 10.8 mm in the postdeviation group. In the controls, follicular-fluid concentrations of estradiol and free insulin-like growth factor (IGF)-1 in the largest follicle and IGF binding protein (IGFBP)-2 in the second-largest follicle were higher (P: < 0.05) in the postdeviation group than in the predeviation group. Progesterone treatment lowered (P: < 0.006) the circulating LH concentrations to a similar extent in both groups. In the predeviation group, progesterone treatment did not have a significant effect on any of the characteristics of the largest follicle. In the postdeviation group, the largest follicle of the progesterone-treated heifers had significant reductions in diameter and in follicular-fluid concentrations of estradiol and free IGF-1. Follicular-fluid concentrations of immunoreactive inhibin were not different for any of the comparisons. The results supported the hypothesis that LH has a positive effect on diameter of the largest follicle but not until after the beginning of diameter deviation. In addition, the results indicated that LH is involved in the production of estradiol by the largest follicle and that free IGF-1 concentrations increase in the largest follicle during deviation.

Selection of the dominant follicle in cattle and horses

The nature of selection of the dominant follicle is reviewed by comparing research results between cattle and horses. In both species, emergence of a follicular wave is stimulated by an FSH surge. The surge reaches a peak by the time the follicles attain 4 mm in diameter in cattle and 13 mm in mares. In cattle, all of the growing follicles ≥5 mm contribute to the decline in FSH concentrations. However, the declining FSH concentrations are still needed by the growing follicles. Several days after the peak of the FSH surge and emergence of the wave, the two largest follicles reach means of 8.5 and 7.7 mm in cattle and 22 and 19 mm in horses. At this approximate time, the follicles begin to undergo deviation in follicle diameters, which is characterized by continued growth of the largest follicle to become the dominant follicle and reduced or terminated growth of the remaining follicles to become subordinate follicles. In both species, on average, the future dominant follicle emerges before the future largest subordinate follicle, and the two follicles grow in parallel until deviation. The difference in diameter between the two largest follicles at the beginning of deviation is equivalent in growth to approximately 8 h in cattle and 24 h in mares. Apparently, this is adequate time for the largest follicle to establish the deviation process before the second-largest follicle reaches a similar diameter. During this time, the largest follicle plays the primary role in further suppressing circulating FSH concentrations to below the requirements of the smaller follicles, which causes their regression. The follicle-produced FSH suppressants appear to be estradiol and inhibin. In addition to enhancing its FSH-suppressing ability, the largest follicle also develops the ability to utilize the reduced concentrations of FSH for its continued growth. It is therefore postulated that the essence of selection of a dominant follicle in these two species is a close two-way functional coupling between changing FSH concentrations and follicle growth and development. Elevated concentrations of circulating LH encompass deviation in both species and may play a role in continued growth of the largest follicle. It is not known if LH begins to be utilized by the largest follicle before, at, or after the beginning of diameter deviation. However, results of studies in mares suggested that LH does not influence growth of the dominant follicle until after the beginning of deviation.

Echotextural changes in the follicular wall during follicle deviation in mares

The echotextural changes occurring in the follicular wall in association with deviation in diameters were studied in 8 pony mares. Echotextural changes could be useful as a reference point for future studies of the follicle-selection phenomenon. Follicles were examined daily by transrectal ultrasound from 3 d before to 3 d after the beginning of deviation (Day 0). The following echotextural end points were recorded based on a scoring or percentage system: 1) thickness of granulosa, 2) echogenicity of granulosa, 3) prominence of an anechoic layer located beneath the granulosa, and 4) extent of involvement of the circumference of the follicle by the anechoic layer. Significant main effects indicated increasing values over Days −3 to 3 and a higher mean value for the dominant-follicle status for each end point. The interaction approached significance for thickness of granulosa and was significant for each of the other 3 end points. The interaction seemed due primarily to lower values in the subordinate follicle on Days 2 and 3. The first day of an overall echotextural difference between the 2 largest follicles ( 15.9 ± 0.5 d after ovulation) or between days within the largest follicle (16.1 ± 0.5 d) was about 1 d earlier (P<0.05) than the beginning of diameter deviation (17.4 ± 0.8). The effects of follicle diameter versus future follicle status were compared for Days −3 to 0 by grouping follicles for each status into 2 diameter categories (16.0 to 18.9 mm and 19.0 to 22.5 mm). Increasing echotextural values for the granulosa (thickness and echogenicity) were more attributable to follicle growth, whereas the increasing values for the anechoic layer were more attributable to status as a future dominant follicle. Results indicated that development of an anechoic layer beneath the granulosa distinguished the future dominant follicle from the largest future subordinate follicle before the beginning of diameter deviation.