The acrosome of eutherian mammals.

The acrosome is not just a bag of enzymes, most of which, if not all, are singly non-essential for sperm-oocyte interaction. The Golgi-derived acrosomal cap reveals some extraordinary development and structure particularities. The acrosome of eutherian spermatozoa basically consists of two parts, the anterior and equatorial segments; the present review is devoted to the former, the initial actor in fertilization. Its occasional fanciful morphological changes during epididymal maturation are analyzed, together with its heterogeneous contents: enzymes, zona binding proteins, structural proteins (matrix) and yet to be chemically characterized crystalloids. The plasma and acrosomal membranes present stabilized ordered domains, whereas glycoprotein-free areas appear during capacitation and before fusion. Exocytosis, induced by the cumulus oophorus and/or the zona pellucida, may generally start proximally and progress anteriorly, resulting in the detachment of a hybrid membrane shroud, whose entity is probably maintained by the bound matrix. Immediately released soluble enzymes must be active during the first interactions of the gametes, whereas other lysins, bound to the matrix or stored as proenzymes, are only progressively released. Zona binding is probably achieved via the shroud and/or the IAM (depending on species). Penetration along an incurved slit through the stratified zona is allowed by the rigid and denuded head tip and flagellar hyperactivity, and assisted by the local proteolytic activity of proteasomes bound to the IAM, the unique essential zona lysin system.

Planar polarity of the extraembryonic epithelia in the preimplantation porcine conceptus.

The present review deals with the trophoblast structure during the free intrauterine life of the pig blastocyst. The term trophoblast is used here to describe the association of the first extraembryonic cell layers, the trophectoderm and the primitive endoderm that are polarized epithelia, a fact established by ultrastructural and immunocytochemical data. The aim of this synthesis is to gather the relative works dispersed in the litterature and to explain the implication of the planar polarity of these cell layers on their developmental fate and roles. These epithelia are intricately dependent on each other for the maintenance of their differentiated state and continuity. The modalities of their spectacular expansion can be explained in part by biomechanical concepts.

Nucleoskeleton of early bovine embryos and differentiated somatic cells: an ultrastructural and immunocytochemical comparison.

The nucleoskeleton is a complex structure involved in structural and functional organisation of the genome in eukaryotic cells. As little information on the nucleoskeleton is available from early embryonic development stages, we describe here the morphology and composition of the nucleoskeleton in cleaving bovine embryos (stages 1-16 cells). Ultrastructural observations were performed using thick resinless sections after chromatin removal by nucleases. The localisation of nucleoskeleton-related lamins A and C, NuMA, SRm160 and hnRNP H was tested by immunofluorescence. The characteristic structures of the nucleoskeleton (nuclear lamina, core filaments and the 'diffuse' nucleoskeleton) were present throughout all embryonic stages studied, although less discernable during the 1-cell stage. Lamins A and C as well as the NuMA protein were observed in embryo nuclei from the 1-cell stage; a diffuse hnRNP H and speckled SRm160 immunofluorescence appeared from the 4-cell stage. During the 8- to 16-cell stages (major transcriptional activation), the immunofluorescence patterns were identical with those of differentiated cells (fibroblasts). The temporal pattern of immunolabelling confirmed that transcription and splicing compartmentalisation was established progressively during cleavage, and that some of the proteins tested can be used as markers in studies on nuclei reprogramming after transfer into enucleated oocytes.