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Bos taurus indicusfemales provide a greater in vivooocyte recovery (2-4 times more) in comparision with Bos taurus taurus. This aspect has strongly contributed for the success of the embryo industry in Brazil. Little information is available to explain this difference. Our objective was to test the hypothesis that the difference in oocyte yield is a result of higher numbers of preantral follicles in the ovaries of B. indicusfemales. Ovaries (n= 44) of fetuses aging from 180 to 240 days and of heifers from 20 and 24 months were collected from Bos taurus indicus(Nelore) and Bos taurus taurus(Aberdeen Angus) females at a slaughterhouse. Fetal age was estimated from the crown-rump length (Evans HE and Sack WO 1973 Anat. Hist. Embryol. 2, 11-45). Ovaries from Nelore fetuses (n= 10) and heifers (n= 12) and Aberdeen Angus fetuses (n= 10) and heifers (n= 12) were cut longitudinally into two halves, fixed in Bouin''s solution, and processed for histological evaluation. The ovarian halves were dehydrated in alcohol, cleared with xylene, embbebed in paraffin, and serially sectioned at 7 μm. Every 120th section was mounted and stained with periodic acid Schiff and hematoxylin. The number of preantral follicles was estimated through the counting of follicles in each section using the nucleus of the oocyte as a marker and a correction factor (Gougeon A and Chainy GBN 1987 J. Reprod. Fertil. 81, 433-442). Only 1 ovary per female was analyzed. Preantral follicles were classified according to the developmental stage as primordial (1 layer of flattened granulosa cells surrounding the oocyte), primary (1 layer of cuboidal granulosa cells), or secondary (2 or more layers of cuboidal granulosa cells), and as normal or degenerated according to their morphological appearance. The number of preantral follicles was not normally distributed and was compared using the Mann-Whitney test. There was no difference (P> 0.05) between the average number of preantral follicles in the ovaries of Bos indicusand Bos taurusfemales. The average number of preantral follicles per female was 143 929 ± 253 (mean ± SD) and 285 155 ± 570 for Bos taurusand Bos indicusfetuses, respectively, and 76 851 ± 280 and 109 673 ± 293 for Bos taurus andBos indicus heifers (P> 0.05), respectively. A large variation in numbers of preantral follicles was observed among individuals within the same category and between breeds. Our results suggest that there might be differences in mechanisms controlling follicle development after the preantral stage accounting for the greater oocyte yield from Bos indicusfemales. This work was supported by grants from the Universidad Nacional del Litoral (CAI+D program), Universidad Nacional de Lomas de Zamora, the Argentine National Agency for the Promotion of Science and Technology (ANPCyT), and the Argentine National Council for Science and Technology (CONICET). [ABSTRACT FROM AUTHOR]

In recent years, Brazil has become the leading country in the world for the number of embryos produced in vitro(Thibier M 2009 IETS Embryo Transfer Newsletter 22, 12-19). This is partly due to the large numbers of Bos indicusanimals in Brazil, making up about 80% of the total cattle. The mean oocyte production per ultrasound-guided follicular aspiration from Bos indicusis higher than those for European breeds (Pontes JHF et al.2009 Theriogenology 71, 690-697). In the present study, we analyzed 5407 ovum pick ups (OPU) and compared the average production of total (n= 90,086) and viable (n= 64,826) oocytes and the number of embryos produced in vitrofrom Gir (Bos taurus indicus), Holstein (Bos taurus taurus), 1/4 Holstein × 3/4 Gir, and 1/2 Holstein-Gir crossbreed cows. To obtain oocytes, OPU was repeated from 4 to 7 times (mean = 5.7 ± 2.4) in each donor cow aged from 3 to 7 years (mean = 5.0 ± 2.3) during a 12-mo period. COCs (n= 90,086) obtained were classified according to the presence of cumulus cells and the oocyte cytoplasm aspect (homogeneous or heterogeneous/fragmented). The viable oocytes (n= 64,826) were in vitromatured for 24 h at 38.8°C in an atmosphere of 5% CO2in air. Since this was a commercial programm, frozen sexed semen (2 × 106mL-1) from Gir (n= 8) or Holstein (n= 7) sires previously tested for high efficiency was used for IVF. Fertilization was carried out (18-20 h) and the presumed embryos were cultured for 7 days in the same conditions as were used for IVM. Data were analyzed by ANOVA. On average, 16.7 ± 6.2 oocytes were obtained per OPU/IVF procedure and 71.96% were considered viable. The mean numbers of total oocytes per OPU/IVF procedure were 17.1 ± 4.4 for Gir cows (n= 617), 11.4 ± 3.9 for Holstein cows (n= 180), 20.4 ± 5.8 for 1/4 Holstein × 3/4 Gir (n= 44), and 31.4 ± 5.6 for 1/2 Holstein-Gir crossbreed females (n= 37, P< 0.01). The mean numbers of viable oocytes per OPU/IVF procedure were 12.1 ± 3.8 for Gir cows, 8.0 ± 2.6 for Holstein cows, 16.8, ± 5.0 for 1/4 Holstein × 3/4 Gir, and 24.3 ± 4.7 for 1/2 Holstein-Gir crossbreed females (P< 0.01). The average number of embryos produced by OPU/IVF were 3.2 (n= 12,243/3378) for Gir cows, 2.2 (n= 2426/1138) for Holstein cows, 3.9 (n= 1033/267) for 1/4 Holstein × 3/4 Gir, and 5.5 (n= 1222/224) for 1/2 Holstein-Gir. The average number of embryos produced per IVF session from 1/2 taurus × indicusdonor cows was greater (P< 0.01) than from Bos indicuscows. The number of recoverable and viable oocytes and the number of embryos produced in vitrofrom Bos indicusdonors were higher than from Bos taurusfemales. Therefore, the highest oocyte yield and the greatest embryo production were obtained from 1/2 taurus × indicusfemales. This work was supported by In Vitro Brasil. [ABSTRACT FROM AUTHOR]