No abstract

Cellular polarization during preimplantation development of the embryo is believed to be a crucial event in the transition of a zygote to a blastocyst stage embryo with morphologically and functionally differentiated cell types. While extensive studies have been done on polarity development in mouse embryos, little information is available in other species, particularly in cattle. The objective of this study was to examine the initiation of polarity by microvilli distribution on blastomeres of cattle and rabbit embryos using scanning electron microscopy (SEM). Bovine embryos were obtained by in vitro fertilization of in vitro-matured follicular oocytes. Rabbit embryos of various stages were collected from superovulated rabbits. Intact embryos and isolated blastomeres were examined in both species. Blastomeres from 1- to 8-cell embryos in both cattle and rabbits showed no polarity. The onset of transitional polarization of microvillous distribution occurred in some blastomeres of cattle embryos at the 9- to 15-cell stage; but typical, distinct polarity was not manifested until after the 16-cell stage with approximately 40% polar cells per embryo. In the rabbit blastomere polarity occurred one cell cycle later, with 46% polar cells per embryo after the 32-cell stage. The difference in cell numbers at the time polarity is evident is probably related to the different cell stages for embryo compaction and blastocyst formation in the two species.

Abstract. Although the number of dairy farms is decreasing, that of large farms is increasing in Japan. Milk production in Japanese dairy cows has increased from 62 kg/ year to 88 kg/ year over the last 2 decades. However, Japanese dairy cows are experiencing a sustained decline in reproductive performance, calving intervals, and days open; further, the number of inseminations required for conception have increased, and the conception rate has decreased. In order to improve fertility in high milk-producing dairy cows, it is necessary to evaluate their reproductive characteristics. In this study, the postpartum body condition score (BCS) was remarkably low, and the functional recovery of reproduction was consequently delayed. Moreover, the results indicate that the estrus duration varies among individual cows. However, it is possible to improve the conception rate by inseminating cows 8-12 h after the onset of estrus. Reproductive management systems suitable for the current dairy farming system with large herd sizes are required. Key words: Artificial insemination, Body condition score, Conception rate, Dairy cows, Reproduction (J. Reprod. Dev. 56: S61-S65, 2010) lthough milk production per dairy cow has dramatically increased over the last 2 decades, the reproductive performance of dairy cows has declined worldwide [1][2][3][4][5][6]. The cause of this low reproductive performance may vary considerably across countries and is probably multifactorial. The factors that contribute to the decrease in the reproductive performance of dairy cows are increase in milk production per cow, concrete flooring, confined housing, and increase in herd size.According to the latest livestock statistics reported by the Ministry of Agriculture, Forestry and Fisheries, in July 7, 2009 (http:// www.maff.go.jp/toukei/sokuhou/data/siyou_doukou09/ siyou_doukou09.pdf), the total number of dairy farms in Japan is 22,300. This number is decreasing; however, the number of farms with more than 100 cows (aged above 24 months) is increasing, and this number is 1,860 (8.3%). The number of dairy farms with more than 300 cows is 173. This trend in the increase in herd size in Japan is the same as that in other countries such as the USA [2].According to data presented by the Livestock Improvement Association of Japan (http://liaj.lin.gr.jp/japanese/kentei/ kentei.html), over the last 2 decades, milk production (305 d) in Japanese dairy cows has increased by approximately 62 kg/year in Hokkaido and 88 kg/year all over Japan, except in Hokkaido (Fig. 1). This trend in the increase in milk production in Japan is the same as that in other countries [2,4]. However, a sustained decline in reproductive performance has been observed in Japanese dairy herds (Table 1) The Hokkaido Artificial Insemination Technician Association Inc. conducts annual surveys and reports the conception rates at the first artificial insemination (AI) in dairy cattle in the Hokkaido district. The conception rate in dairy cows was found to decrease from 53.4% in 1989 to 41.2% ...

Development of cellular polarity is an important event during early mammalian embryo development and differentiation. Blastomeres of hamster embryos at various stages were examined by scanning electron microscopy (SEM) and immunocytochemical staining. SEM observations revealed that 1- to 7-cell-stage embryos showed a uniform distribution of microvilli throughout the cell surface. Microvillous polarization was initially noted in the blastomeres (10-35%) of 8-cell-stage embryos. The polarized microvilli were observed mostly in the basal region of cell-cell contact and occasionally at the apical, outward-facing surface of the blastomere. Fluorescein-isothiocyanate-conjugated concanavalin A failed to reveal any polarity in the blastomeres regardless of the stages of the embryos. Actin staining showed that microfilaments were present beneath the cell surface, and in addition, areas of cell contact were more heavily stained, indicating a thick microfilament domain. Microtubules were located throughout the cytoplasm and were heavily concentrated near the nucleus during interphase, although they became redistributed in the region of the mitotic spindle during karyokinesis. The position of nucleus changed from the cell center to the apical, outward-facing surface of the cell, and it distanced itself from the basal microvillous pole. It is suggested that the changes in the cell surface and nuclear position are the first manifestations of cell polarity in peri-compacted hamster embryos, which appear as early as the 8-cell stage; furthermore, the outward migration of the nuclei may parallel the redistribution of microtubules in the cytoplasm.

Treatments with GnRH and PGF2α for synchronization of ovulation has resulted in acceptable pregnancy rates after fixed-time artificial insemination in dairy cows without estrus detection. The objective of the present study was to evaluate the practicability of ovulation synchronization (Ovsynch, Pursley JR et al. 1995 Theriogenology 44, 915–923) in dairy cattle using GnRH and PGF2α for the embryo transfer recipients. Dairy cattle (cows; n=100, heifers; n=88) were randomly allocated to one of two groups. The control group (cows; n=45, heifers; n=37) was composed of cows in natural estrus. The ovulation synchronization group (cows; n=55, heifers; n=51) was treated with an intramuscular injection of 100μg of GnRH at a random stage of the estrous cycle. Seven days later, the cattle received PGF2α (Cows; 25–30mg) or PGF2α analog (Heifers; 0.5mg) in order to regress the corpora lutea (CL). Forty-eight hours later, cows and heifers received a second injection of 100μg GnRH. Embryo transfer was carried out 7 days after the second injection of GnRH in the ovsynch group and 7 days after estrus in the control group. The cattle judged to have CL 17mm were classified as acceptable recipients. The size of the follicles and the CL were determined to be of estrus stage and embryo transfer by means of ultrasonography. The mean numbers of follicles and CL were analyzed by ANOVA, while pregnancy rates were analyzed by chi-square test. The results are presented in the Table. The proportion of cows and heifers determined to be acceptable embryo transfers was not different between the control group and the ovsynch group. There were no differences in the proportion of acceptable embryo transfers between the control group and the ovsynch group. Follicle diameter at the time of estrus in the control group (cows; 20.7±0.7mm, heifers; 16.8±0.5mm) were significantly larger than that of the ovsynch group (cows; 18.0±1.0mm, heifers; 14.7±0.2mm) (P<0.05). Although CL diameter at the time of embryo transfer in heifers showed no differences between the control group and the ovsynch group (25.0±1.0mm v. 22.8±1.5mm), The CL diameter of the control cow group was larger than that of the ovsynch group (29.8±0.7mm v. 26.1±1.0mm, P<0.05). However, no differences in pregnancy rate were seen between the control group and the ovsynch group. These results suggest that ovsynch can be effectively applied in an embryo transfer program for cattle. Table 1 Proportion of acceptable embryo transfer recipients and pregnancy rate in dairy cattle in the control ovsynch groups