An ultrastructural study of the different stages of pre-implantation in sheep was carried out, analysing the changes brought about mainly in the morula and blastocyst stages. The analysis of the embryos obtained showed a series of common characteristics in all stages, most noticeable being the presence of a high number of vesicles distributed in a uniform way in the cytoplasm, and also the presence of numerous electron-dense mitochondria in many varied forms. The most important ultrastructural modifications took place at the 16-cell stage and affected, principally, the nucleus, which presented numerous condensations of chromatin distributed along the nucleoplasm. The nucleoli adopted a reticular morphology, abandoning the compact aspects presented in the previous stage. These changes might be involved in the synthesis of embryonic RNA, and, accordingly, in the activation of the genome of this species. These data indicate that this stage is critical to the embryonic development and might be related to the blockage produced in the development of cultivated sheep embryos at the point of transition from 8 to 16 cells. Nevertheless, it should be pointed out that the first signs of modifications in the aspect of the nucleus are observed at the four-cell stage, being characterized by the appearance of vacuolated areas in the nucleolus, indicating the first signs of embryonic nucleic activity, which would anticipate the main change in the control of the protein synthesis.

A study of the structural characteristics of stage 5 chick embryo blastoderms, cultured in vitro, was carried out after previous elimination of endoderm to analyse the differentiation capacity of mesoderm. Our results show that only the precardiac mesoderm is determined at this early stage and that it is able to differentiate cardiac tissue in the absence of the endoderm. The ultrastructural characteristics of this tissue are described. However, the degree of differentiation of cardiomyoblasts does not reach the expected level of organization and myofibrillar complexity.

SummaryProduction of embryos that are free of tough outer coats facilitates studies that are not possible with embryos surrounded by impenetrable envelopes. This report describes a new procedure for preventing formation of fertilisation membranes in the sea urchin (Lytechinus pictus) model. This procedure involves treating unfertilised eggs with the enzyme alpha-amylase, which cleaves alpha-1,4 glucosidic bonds in the vitelline layer. A major advantage of this method is that it is very well defined and completely controllable with alpha-amylase inhibitor. The results suggest that intact alpha-1,4 glucosidic bonds are essential for vitelline layer integrity required for formation of the fertilisation membrane. Eggs treated with alpha-amylase possessed the same surface lectin receptors as untreated eggs and, as shown by light and transmission electron microscopy, produced healthy, cleaving embryos that were free of fertilisation envelopes.

We observed a higher staining of WGA, DBA, and SBA lectins in the intercellular contact zone with respect to the free cell surface of blastomeres during cleavage. This indicates that the N-acetyl galactosamine and N-acetyl glucosamine residues may be involved in sheep morula compaction. In contrast, the trophoblast cell displays an increase of staining of some lectins previously identified during cleavage (LcH, WGA, SBA, MPA, and PNA) on the free membrane, and a lack of sugar residues in the intercellular surface. This polarization of the trophoblast cell surface is not observed in the inner cell mass and could provide a mechanism for differentiation within the blastocyst. Intracytoplasmic vesicles show a cytochemical identity with lysosomes in the blastocyst (abundant GlcNAc and Man/Glc residues) that may reflect a functional relationship between both organelles in an intracellular cycle. The zona pellucida presents abundant GalNAc, GlcNAc, and Gal residues during preimplantation ovine development.

SummaryStudy of the Termination of Cervical Nerves Innervating the Rhomboideus, Serratus Ventralis and Trapezius Muscles Part II: Equus and RuminantiaA systematic anatomic study of the origin, course and termination of cervical nerves innervating the Rhomboideus, Serratus ventralis and Trapezius muscles was carried out in the cow, the horse and the sheep. The organization of this innervation is discussed in regard to mesodermal origin of these muscles and the morphogenetic processes demonstrated through innervation.