Whole-mount in situ immunofluorescence of early rabbit embryos

BMP signals and the transcriptional repressor BLIMP1 during germline segregation in the mammalian embryo

Molecular factors and tissue compartments involved in the foundation of the mammalian germline have been mainly described in the mouse so far. To find mechanisms applicable to mammals in general, we analyzed temporal and spatial expression patterns of the transcriptional repressor BLIMP1 (also known as PRDM1) and the signaling molecules BMP2 and BMP4 in perigastrulation and early neurulation embryos of the rabbit using whole-mount in situ hybridization and high-resolution light microscopy. Both BMP2 and BMP4 are expressed in annular domains at the boundary of the embryonic disc, which—in contrast to the situation in the mouse—partly belong to intraembryonic tissues. While BMP2 expression begins at (pregastrulation) stage 1 in the hypoblast, BMP4 expression commences—distinctly delayed compared to the mouse—diffusely at (pregastrulation) stage 2; from stage 3 onwards, BMP4 is expressed peripherally in hypoblast and epiblast and in the mesoderm at the posterior pole of the embryonic disc. BLIMP1 expression begins throughout the hypoblast at stage 1 and emerges in single primordial germ cell (PGC) precursors in the posterior epiblast at stage 2 and then in single mesoderm cells at positions identical to those identified by PGC-specific antibodies. These expression patterns suggest that function and chronology of factors involved in germline segregation are similar in mouse and rabbit, but higher temporal and spatial resolution offered by the rabbit demonstrates a variable role of bone morphogenetic proteins and makes “blimping” a candidate case for lateral inhibition without the need for an allantoic germ cell niche.

Mounting, embedding, and sectioning of early rabbit embryos

This protocol describes various methods for visualizing cellular labeling in early rabbit embryos at high resolution using stereomicroscopy, compound light microscopy, or electron microscopy. Embedding media are chosen according to the level of resolution needed: For low-magnification stereomicroscopy, optimal spreading of large, flat, thin embryonic discs is achieved by mounting embryos in Mowiol on regular microscope slides. Gelatin/BSA embedding is useful for quickly cutting a complete series of thick (e.g., 30-μm) sections in defined planes from large (e.g., somite stage) paraformaldehyde-fixed embryos prelabeled by immunofluorescence or in situ hybridization. Higher cellular and subcellular resolution of such specimens usually requires embedding in Technovit, a hydrophilic methacrylate-based resin suited for light microscopic analysis of 5-μm serial sections. Araldite is a translucent, hydrophobic epoxy-based resin used for serial semi-thin (1-μm) and ultrathin (80-nm) sections amenable to consecutive (and correlative) light and electron microscopic analysis. Specimens to be embedded in Araldite must be much harder than those embedded in hydrophilic resins, requiring at least 1% glutaraldehyde in the primary fixative. Unfortunately, strong cross-linking by glutaraldehyde prevents the use of antibodies/nucleotide probes post-fixation; glutaraldehyde autofluorescence also thwarts the use fluorescent stains. Finally, semi-thin cryosections can be cut from unfixed or paraformaldehyde-fixed specimens embedded in “tissue freezing medium,” offering the opportunity for highly sensitive light microscopic analysis of antigen distribution. Here, subcellular structures can be identified up to the level of individual mitochondria; greater resolution can be obtained by correlating such samples with similar specimens analyzed using other techniques.

Staining and imaging of live rabbit embryos

This protocol describes methods for supravital labeling of cellular proliferation and movement in rabbit embryos as well as for imaging of live cell movements. The techniques use localized labeling of epiblast cells with DiI by injection or by using comprehensive nuclear staining of all cell layers before and during early gastrulation. Because rabbit embryos implant relatively late during embryonic development, gastrulation-stage embryos can be isolated by flushing them from the uterus. Moreover, the embryonic disc is flat and translucent, providing a direct uncompromised overview of gastrulation while the whole blastocyst is protected by the zona pellucida. These properties allow examination of whole blastocysts either under the impact of changing environmental conditions (e.g., medium additives, oxygen concentrations) or following mechanical manipulation of the embryonic disc using a needle penetrating the zona pellucida. Further manipulation (e.g., transplantation) can be performed after removing the embryo from the zona and explantation of the embryonic disc. These techniques allow embryonic growth centers to be identified and single-cell movements to be recorded.

Rabbit mating and embryo isolation

This protocol describes how to generate and collect rabbit embryos of defined developmental stages for experiments such as live embryo micromanipulation and culture or whole-mount in situ hybridization and immunofluorescence. Most values and measurements given in the procedure are optimally suited for the New Zealand White strain (the most commonly used strain because of its relatively large size and therefore large blood volumes for polyclonal antiserum collection). Successful embryo generation and isolation depend on a well-equipped animal care facility and dedicated veterinary care.

The rabbit (Oryctolagus cuniculus): a model for mammalian reproduction and early embryology

The rabbit is a valuable animal model for a variety of biomedical research areas including in vitro fertilization, early embryology and organogenesis, neurophysiology, ophthalmology, and cardiovascular research. The rabbit is also used as a model for toxicology studies and analyses of drug effects on embryo and fetal development, as well as for research involving the immune system (e.g., host/pathogen interactions, asthma, arthritis, systemic lupus erythematosus). The rabbit’s most prominent use as a laboratory animal is antibody production; most biomedical research institutions thus provide the infrastructure and veterinary expertise for keeping rabbits. This article discusses current advances in applying modern life science research methods to basic and applied embryological research using the rabbit as a model for early intrauterine development in mammals, including state-of-the-art genomic tools and the sequencing data available (and planned) for the rabbit.