J.M. Villar

Ultrastructural Study of the Early Development of the Sheep Embryo

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.

Ultrastructural analysis of chick embryo blastoderms explanted in vitro in absence of endoderm. Differentiation capacity of mesoderm.

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.

Correlation between the Structural and Functional Differentiation of Chick Embryo Cardiomyocytes in Cellular Aggregates

In order to establish the possible relation of the structural characteristics acquired by cardiomyocytes and their functional differentiation (spontaneous contractions), a study of the myofibrillogenesis on chick embryo at Hamburger-Hamilton stage 5 was carried out using techniques of dissociation and cellular reaggregation.

Evidence of non-early ultrastructural regionalization in the neural epithelium of the chick by stereological methods

Cell populations of the anterior, medial and posterior regions of the neural epithelium of chick embryo were analyzed by ultrastructural morphometrical and stereological methods. No morphological subcellular differences appear to occur between the cells of these distinct areas, demonstrable by morphometrical and stereological conventional methods. The homogeneity of this cell population in each stage during neurulation is discussed in relation to the induction process.

A Morphological Study of Cultured Endodermal Cells of Chick Embryo: Characteristics of Adhesion, Spreading and Locomotion

A study is made of the morphological characteristics of the endodermic cells of the stage 5 chick embryo by means of in vitro cell culture techniques. The scanning electron microscope revealed that the endodermic cells in cultures were rounded, tended to be smooth and had few blebs and microvilli. Regarding cell projections typical of culture cells, such as filopodia, lamellipodia and pseudopodia, there was a noteworthy scarcity after 12 h growth, although greater cellular activity was observed at 24 h, characterized by the presence of filopodia and an ability of the cells to form clusters on the substratum. These facts show the morphological and adhesion movements of the endodermal cells studied to be related mainly with the presence of filopodia as the most abundant cell projections.

Stereological study of the early ultrastructural differentiation of chick embryo neuroepithelial cells during neurulation.

The neuroectodermal cells of chick embryos have been analyzed during neurulation by stereological and morphometrical ultrastructural methods in an attempt to describe their cytometric evolution. A profound change of cellular form coefficient was observed which is related to the typical process of columnarization of these cells. At stages 7 and 8, the nucleus appeared round in shape, probably due to a loss of pressure of the vitelline inclusions. In this sense, the volume density of these inclusions falls during this period. There was also a significant increase of the nuclear surface density, the significance of which is discussed on the basis of the nucleo-cytoplasmic interchanges and the differentiation process. At the same time, an increase in the number of mitochondria was observed, which is related to the neural folding process. Simultaneously, the amount of rough endoplasmic reticulum increases, presumably related to the remarkable changes of the embryonic extracellular matrix.

Bipolar Mesoderm Cells in vitro Examined by Scanning Electron Microscopy

Head mesoderm cells from chick embryo at stage 10 were cultured onto coverslips. The mesoderm cells with bipolar aspect were studied by means of scanning electron microscopy. Several features of the filopodia and lamellipodia of these cells, as number, length and width were quantified and the results ire discussed in view of the role proposed for these projections in the cellular locomotion. Moreover, the morphological characterization here carried out permits to have comparison with other cellular types of the same culture or between cultured cells of different stages of chick embryo development.

Quantitative ultrastructural changes of the endodermal cells in the early chick embryo analysed by stereological methods

The ultrastructure of endoderm cells of the area pellucida has been analysed in the chick embryo by stereological methods. These cells show a specific subcellular evolution which can be correlated with several aspects of morphogenetic behaviour. The cell form coefficient (CFc) changes notably from stage 5 (0.683) to stage 8 (0.446) accompanying the transformation of this layer into a squamous epithelium. An increase of the nuclear surface density is observed and is discussed in relation to the control of nucleocytoplasmic interchange. The mitochondrial volume and surface densities remain constant (3.12% of cellular volume and 0.727 mitochondria/mu(3) respectively). The endodermal cells possess higher levels of vitelline reserves (lipid bodies, 6.97% and yolk droplets, 8.90%) than other cellular types of the chick embryo. This fact is discussed with respect to the role of the endoderm in the phagocytosis of yolk. The RER length density shows an increase that could be related to some specific changes of the extracellular matrix during this period, but this fact remains to be demonstrated in relation to changes of Golgi membranes.

Stereological analysis of ultrastructural changes of surrounding tissues to neuroectoderm during chick embryo neurulation

The cytometric evolution of different subcellular components in the epiblast and the mesoderm of chick embryo during neurulation has been studied with stereological methods. The coefficient of cellular form (CFc) has specific values for each type, the epiblast having a mean CFc of 0.474, while the mesoderm, which has ellipsoidal cells, has a mean CFc of 0.643. The nucleus does not show any change of its coefficient of form although its surface density increases significantly. The proportion of mitochondria, present in the cells of each embryonic layer, remains constant during the 4 stages, being higher in the mesoderm cells (epiblast 3.6%; mesoderm 4.3%). The epiblast cells show a stable vitelline reserve, though the relative proportions of lipid bodies and yolk droplets vary: the volume density of yolk droplets increases from stage 5 (3.1%) to stage 8 (7.7%), while the lipid bodies diminish from 7 to 3.6% during this period. The mesoderm cells undergo a remarkable loss of vitelline volume during the same period. The rough endoplasmic reticulum of each cellular type has a remarkable length density increase, the significance of which is discussed in relation to production of extracellular matrix.