The dynamic structure of rabbit blastocyst coverings. II. Ultrastructural evidence for a role of the trophoblast in neozona formation

Why choose the rabbit to work in reproductive technology?

Rabbits have played a significant role in both livestock production and the advancement of reproductive scientific research. Their unique biological traits, including induced ovulation and a reproductive process that closely mirrors that of humans, have been pivotal in their use as a model. Moreover, their body size is perfectly aligned with the 3Rs principles: Replacement, Reduction, and Refinement. Consequently, techniques for gamete collection and embryo recovery, followed by their use in artificial insemination or embryo transfer, are characterized by being minimally invasive. However, refining in vitro fertilization and embryo culture techniques continues to present challenges. The incorporation of cutting-edge genomic editing tools, such as CRISPR/Cas9, has reestablished rabbits as essential models in genetic and biomedical research, driving scientific progress. This review aims to describe the most effective reproductive biotechnologies for both male and female rabbits and how these methodologies are in line with the 3Rs principles-Replacement, Reduction, and Refinement-highlighting their significance in conducting ethical research.

Rise of the rabbit model for implantation research. A commemoration of the scientific work of Bent G. Boving (1920-2019)

Bent G. Boving, a remarkable pioneer of research on mechanistic aspects of embryo implantation and specifically of using the rabbit model for this purpose, passed away peacefully on 15 November 2019, not long before he would have been able to celebrate his 100th birthday on 23 February 2020. His work has been very stimulating for other researchers, in part possibly because it elicited controversial discussions. This article attempts to give an overview of his scientific contributions, which still harbor some treasures, including potentially stimulating ideas for future research on implantation.

Exposure to high ambient temperatures alters embryology in rabbits

High ambient temperatures are a determining factor in the deterioration of embryo quality and survival in mammals. The aim of this study was to evaluate the effect of heat stress on embryo development, embryonic size and size of the embryonic coats in rabbits. A total of 310 embryos from 33 females in thermal comfort zone and 264 embryos of 28 females in heat stress conditions were used in the experiment. The traits studied were ovulation rate, percentage of total embryos, percentage of normal embryos, embryo area, zona pellucida thickness and mucin coat thickness. Traits were measured at 24 and 48 h post-coitum (hpc); mucin coat thickness was only measured at 48 hpc. The embryos were classified as zygotes or two-cell embryos at 24 hpc, and 16-cells or early morulae at 48 hpc. The ovulation rate was one oocyte lower in heat stress conditions than in thermal comfort. Percentage of normal embryos was lower in heat stress conditions at 24 hpc (17.2%) and 48 hpc (13.2%). No differences in percentage of zygotes or two-cell embryos were found at 24 hpc. The embryo development and area was affected by heat stress at 48 hpc (10% higher percentage of 16-cells and 883 μm² smaller, respectively). Zona pellucida was thicker under thermal stress at 24 hpc (1.2 μm) and 48 hpc (1.5 μm). No differences in mucin coat thickness were found. In conclusion, heat stress appears to alter embryology in rabbits.

Detection of glycosylated proteins in rabbit oviductal isthmus and uterine endometrium during early embryo development

In this study, lectin histochemistry was performed on paraffin sections to compare carbohydrate expression of oviductal isthmus and uterine endometrium in rabbits during early embryo development. Rabbit embryos are surrounded not only by the zona pellucida but also by tubal secretion-derived mucinous coat material, the mucin coat. Twenty sexually mature females were euthanized at 0 (pre-ovulatory group) and 24, 72 and 96 h after insemination (pseudopregnancy group). The following lectin-binding agents were used: Arachis hypogaea, Peanut (PNA) to label galactosyl (β-1,3)N- acetyl-galactosamine, Dolichos biflorus Agglutinin (DBA) to label galactosyl (β-1,4)N- acetyl-galactosamine, Lens curinaris (LCA) to label α--mannose, α-d-glucose and Pisum sativum agglutinin (PSA) to label α-d-mannose, α-d-glucose. Blood was collected by cardiac puncture, and direct enzyme immunoassay technique was used to measure progesterone concentration. A significant increase in total plasma progesterone concentrations was detected at 96 h post-ovulation when compared with 0, 24 and 72 h post-ovulation (2.9 ± 0.5 vs 0.5 ± 0.15, 1.6 ± 0.5 and 1.5 ± 0.4 ng/ml, at 96 h vs 0, 24 and 72 h post-ovulation, respectively). No differences between pre-ovulatory and pseudopregnant females were observed for glycoprotein localization in isthmus. In contrast, in the endometrium, differences in the glycoprotein detection between pre-ovulatory and pseudopregnant stages were detected. PNA to label galactosyl (β-1,3)N- acetyl-galactosamine was not detected at the pre-ovulatory stage, but its presence was detected at 24 h after ovulation. Both PSA and LCA to label α-d-mannose, α-d-glucose were only detected at 72 h after ovulation. DBA detection was similar for all stages of the reproductive cycle. Therefore, N-acetyl-galactosamine secreted from isthmus could be involved in the formation of the embryonic mucin coat. d-galactose (PNA), d-glucose and d-mannose (PSA and LCA) might be crucial for the implantation period.

Polarity in the rabbit embryo

The main aim of the gastrulation process is commonly regarded to be the generation of the definitive germ layers known as mesoderm, endoderm and ectoderm. Here we discuss how the topography of gene expression, cellular migration and proliferative activity in the preliminary germ layers (hypoblast and epiblast) of the rabbit embryo reveal the sequence of events that establishes the three major body axes. We present a testable model in which a combination of cellular movement in the hypoblast with a morphogen gradient created by the (extraembryonic) trophoblast creates morphological polarity in the embryo and, hence, the co-ordinates for germ layer formation.

Horse conceptuses secrete insulin-like growth factor-binding protein 3

Insulin-like growth factor-I (IGF-I) promotes early embryonic development in several species. In the rabbit, IGF-I binds to the embryonic coats from Day 3 of development onward by a 38-kDa protein that is probably insulin-like growth factor-binding protein 3 (IGFBP3). In the present study, ligand, Western, and Northern blot analyses were used to demonstrate the presence of IGF-I-binding activity, several immunoreactive IGFBP3 proteins, and IGFBP3 mRNA in horse conceptuses with particularly large amounts of immunoreactive IGFBP3 in the conceptus capsule. In addition, immunoprecipitation of radiolabeled proteins showed that cultured horse conceptuses secreted IGFBP3 into the culture medium. Endometrial samples from mares also contained IGFBP3 mRNA and protein; but there was no evidence of secretion of IGFBP3 into the uterine lumen by ligand blot analysis, and there was evidence of only very small amounts by Western blot analysis. These results indicate that the horse conceptus secretes significant quantities of IGFBP3 toward the conceptus capsule from as early as Day 10 after ovulation. Thus, most of the IGFBP3 contained within the capsule, which binds IGF-I to this special extracellular matrix of the preimplantation horse conceptus, is likely to be embryonic in origin. IGFBP3 in the horse conceptus capsule may enhance or modulate the action of IGFs on the developing conceptus.

Binding of IGF-I to preimplantation rabbit embryos and their coats

It is known that insulin-like growth-factor I (IGF-I) promotes early embryonic development from the morula to the blastocyst stage in rabbits (28). Therefore we used autoradiography to investigate whether IGF-I binds to preimplantation embryos and its coats. From Day 3 after mating onwards, a clear binding of IGF-I to the embryos was observed. There was no difference in binding to the embryoblast or trophoblast cells. Using ligand blot, several IGF-binding proteins (IGFBP; 31 kDa, 33 kDa, 36 kDa, three overlapping bands at 40 to 55 kDa) were obvious in the embryoblast and trophoblast. A 120 to 130 kDa protein was observed exclusively in the embryoblast. Significant binding of (125)I IGF-I to the coats of embryos older than 3 d was detected, and IGF-I was bound via a 38 kDa protein, as detected by ligand blot. To investigate the origin of this protein, the patterns of IGFBP were determined in the oviductal and uterine fluids of pregnant animals (Days 0 to 6). The following binding proteins were observed regularly in the oviductal and uterine flushings: 28 kDa, 32 kDa and 3 overlapping bands in the area of 40 to 55 kDa. In the oviduct the main IGF binding protein was the 32 kDa band (38.7% to 45.9%), while in the uterus it was the 3 overlapping bands at 40 to 55 kDa (42.5% to 24.1%). Because IGF-I is produced in the oviduct and uterus (27), IGFBPs are found in oviductal and uterine fluids, IGF-I is stored in the coats, IGF-I binds to preimplantation embryos and IGF-I promotes early embryonic development (28), the IGF system seams to have a function in the maternal-embryonic interaction.

Polar trophoblast (Rauber's layer) of the rabbit blastocyst

The germinal area of the rabbit blastocyst between 108 h postcoitum (pc) and 168 h pc has been examined by scanning electron and transmission electron microscopy. At 108 h and 120 h pc the polar trophoblast (Rauber's layer) is an intact epithelium overlying the epiblast of the inner cell mass. By 132 h pc the polar trophoblast cells begin to separate at multiple foci, exposing the underlying epiblast. Most of the polar trophoblast cells have become individually separated at 144 h pc. The villous, electron-dense polar trophoblast cells can be easily distinguished from the cells of the epiblast, which have smooth apical surfaces. By 162 h pc the polar trophoblast cells have disappeared from the germinal area. Before the polar trophoblast breaks up, the underlying epiblast cells are only loosely attached to one another. Concurrent with the disintegration of the trophoblast epithelium, the epiblast cells change in shape so that their lateral borders become closely apposed, and junctions develop to form a new epithelium. The epiblast becomes contiguous with the mural trophoblast, and thus the blastocyst does not lose its turgidity as the permeability seal is maintained. There are two classical theories on the fate of the polar trophoblast: the cells die, or they become incorporated into the epiblast as living cells. In newly exposed epiblast the presence of very large phagosomes, which are not found when the polar trophoblast is still intact, favors the first hypothesis and indicates that in the rabbit the epiblast is involved in the phagocytosis of the polar trophoblast.

Implantation, development of the fetal membranes, and placentation in the captive black mastiff bat, Molossus ater

Uterine events during pregnancy were examined histologically in laboratory-bred black mastiff bats (Molossus ater) as part of an effort to develop this species as a model for studies of the factors controlling trophoblastic growth. Embryos entered the uterus at the morula stage and in most cases shed their zonae pellucidae reasonably intact, apparently as blastocyst expansion occurred. Implantation was superficial and observed to occur only in the right uterine horn. During implantation to the endometrium by both blastocyst expansion and closure of the uterine lumen. A decidual reaction was evident at an early stage of uterine epithelial displacement and spread rapidly through the endometrium. Initial trophoblastic proliferation occurred along the uterine lumen and into the glands, while its invasion of the endometrial stroma was delayed. Although one or several primordial cavities have been observed to develop within the epiblast during implantation, these subsequently opened to a trophoepiblastic cavity, and the definitive amnion was formed by folding. A choriovitelline placenta was present briefly at thesomite stage, but disappeared as the exocoelom enlarged and the yolk sac collapsed. The latter persisted through pregnancy, however, as a glandular-appearing body. As the yolk sac retracted from the chorion, it was replaced by allantoic mesoderm, creating a diffuse labyrinthine endotheliodichorial placenta. This was prominent during mid-gestation, but was supplanted by the discoidal hemochorial placenta as the major site of feto-maternal exchange during late pregnancy.