Wandering epithelial cells in the rabbit embryo milk line: A preliminary scanning electron microscope study

In order to confirm the previous results obtained during experimental studies on mammary gland organogenesis and elucidate by what means epidermal cells are clustered together into a protruding ridge, the rabbit embryo milk line was examined under the scanning electron microscope. The discovery of wandering epithelial cells strengthens the current hypothesis that cell migration is a fundamental process during milk line morphogenesis.     

An electron microscope study of smooth muscles in pregnant uterus of the rabbit

In the smooth muscle of the rabbit uterus one type of myofilament can be found; this is about 50 A thick. In the sarcoplasmic reticulum there are smooth walled vesicles and many vesicles with particles attached. It is suggested that these particles may correspond to the ribonucleoprotein-containing particles of Palade and Siekevitz. The diameter of the particles is 250 A. There are also unattached particles some of which may be glycogen. The membranes of the muscle cells and the connective tissue lying between them are described.     

An electron microscope study of myofibril formation in embryonic rabbit skeletal muscle

Trunk and limb muscles from fetal and newborn rabbits were investigated by means of light and electron microscopes. At 14 days gestation, the presumptive myoblasts migrate away from the myotome to form the anlage of the muscle of the trunk and limb. Among the population of undifferentiated cells, the myoblasts were recognized due to the presence of actin and myosin filaments. The aggregates of thin and thick filaments appear at the periphery of the cells. There is a great variety of filament assembly. The presence of Z band material appears to be essential for sarcomere formation. At 14 days of gestation the myotubes are more numerous in the limb than in the trunk. The presence of unmaturated fibrils with absence of the M line in the sarcomeres was observed. By day 18 of gestation the myotubes are wider and aggregate to form small bundles. The myofibrils were more numerous and the vesicles of the SR precursor, partly incrustated with ribosomes were dispersed among them. At day 22 of gestation the myotubes are thicker because of the myofibrils which are far more numberous. The sarcomeres were more fully developed, with the M line present. At day 28 of gestation and 3 days after delivery the already developed myofibers were present with a well organized SR system and fully developed sarcomeres.     

Synaptic junction development in the spinal cord of an amphibian embryo: An electron microscope study

Summary An electron microscopical study has been made of the cervical spinal cord of Xenopus laevis embryos, from the time that the neural tube closes until the larvae were hatched and could swim. Sections of the whole cord were searched for intercellular junctions during this period. Two nonsynaptic types were found, the first were widely distributed puncta adherentia, the second were rare and similar to gap junctions. Membrane specializations with synaptic vesicles were first found when the neural folds had fused; membrane-vesicle clusters which looked like the presynaptic half of a synaptic junction were present, together with synaptic junctions lacking any postsynaptic membrane thickening or cytoplasm density. About four hours later, mature synaptic junctions with full thickening of the postsynaptic membrane, dense cytoplasm and striated or dense material in the synaptic cleft were present. Presynaptic mitochondria, dense-cored and flattened vesicles, fibre to fibre and fibre to cell body synapses were present from the first, as were synapses onto very fine dendrites which might be filopodia from dendritic growth cones. Synaptogenesis may start with the accumulation of vesicles in dense cytoplasm near a thickened cell membrane; the postsynaptic element becomes associated with this membrane-vesicle cluster and matures by increasing cleft and cytoplasmic density, and by membrane thickening.     

The vasculature of the rat embryo: an electron microscope study

The ultrastructure of portions of the arterial and venous systems of the 11.5 day old Wistar rat embryos has been studied by scanning and transmission electron microscopy. The vessels at this stage of development are in the form of capillaries, and the arterial and venous types can be distinguished by the morphology of the endothelial cells by SEM. The endothelial cells of the arterial vessels gave prominent nuclear bulges and numerous microvilli apart from their spindle shape, whilst those of the veins appear flattened, are polygonal in shape, and have few microvilli. Transmission electron microscopy shows that the endothelial cells of the arteries and veins are identical in structure. The ultrastructure of these cells resembles that of endothelial cells at later stages of development including the adult type in that mature forms of cytoplasmic organelles are obtained. In studies on the intercellular junctions and fenestrations with lanthanum nitrate, the impression is formed that the vessels at this stage are impermeable to small molecular size particles, compared with adult capillaries. This suggests that cytoplasmic vesicles must play a major role in the transport of macromolecules in the 11.5 day embryonic vessels.     

An electron microscope study of the perfusion-fixed spleen

Summary By preserving the intercellular or plasmatic space, perfusion fixation provides a particularly clear picture of the histological and cytological constitution of the spleen. An analysis of documents obtained with this technique has contributed new information concerning the organization of the intermediate circulation in that organ, and on the topographical relationships and ultrastructure of the various types of cell which are usually grouped under the common denomination of reticulo-endothelial system.Currently available physiological data concerning the flow of blood through the spleen, together with the observed scarcity of sinuses, and of interstices in the latters' walls, to which must be added the narrowness of these interstices, incline us to question the existence of a circulatory system involving the passage of blood from the arterioles into the pulp cords of Billroth, and thence into the sinuses. Thus we are led to conceive of a system whereby about 97% of the blood entering the spleen would flow directly into the sinuses. Phagocytosis of worn blood cells would be carried out by a quantitatively less important side-stream, running parallel to the main stream, but nevertheless sufficiently fast to ensure that every blood cell should be shunted through the spaces of Billroth at least once every 24 hours.The unitary concept of the RES, as it appears from the bulk of existing literature, is based upon the postulated, but never proven, existence of a single cell type capable of developing distinct morphological characteristics, and of carrying out distinct functions, namely phagocytosis and the laying down of reticulin, depending on its situation in the tissue. An ultrastructural study of the various cells encountered in sections of spleen, and a comparison with their counterparts in other organs, seem to rule out this unitarian view.

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Résumé La fixation par perfusion, en conservant l'espace intercellulaire ou plasmatique, donne une image de l'organisation structurale et cytologique de la rate particulièrement claire. L'analyse des documents aínsí obtenus fournit des données nouvelles sur l'organisation de la circulation intermédiaire et sur la position et l'aspect ultrastructural des cellules habituellement réunies sous le nom de système réticulo-endothélial. Les données de physiologie sur le débit splénique, le nombre de sinus observables et le relativement petit nombre et l'étroitesse des fentes sinusiennes font douter de l'existence d'un système circulatoire faisant passer le sang des artérioles dans les cordons de Billroth et de là dans les sinus. Ceci fait envisager un schéma de circulation faisant passer environ 97% du sang directement à travers le sinus. La phagocytose des éléments à détruire est assurée par une circulation parallèle peu importante, mais suffisante pour que chaque élément sanguin ait chaque jour la possibilité de passer par les cordons de Billroth.La théorie du SRE est basée sur l'existence d'une cellule unique, différant seulement par sa position et assurant la macrophagie et la formation de la réticuline. L'étude de ces différents types de cellules au niveau de la rate et leur comparaison au niveau d'autres organes permet d'assurer qu'aucun argument de morphologie ultrastructurale ne peut l'étayer.

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This work was supported by grant No 4237 of the Swiss National Foundation for Scientific Research.

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We wish to thank Mrs. Sidler-Ansermet for her invaluable technical assistance.     

An electron microscope study of lampbrush chromosomes

Lampbrush chromosomes were isolated from germinal vesicles of oocytes from Necturus maculatus, Triturus viridescens, Pseudotriton montanus and Rana pipiens. After treatment of isolated nuclei with 10 per cent sucrose, chromosomes free of nuclear sap are obtained for examination in either the light microscope or in the electron microscope. For electron microscopy the chromosomes were prepared either by Anderson's critical-point procedure or were embedded in methacrylate and sectioned. The evidence presented in favor of the view that the loops, axis, and the chromomeres of lampbrush chromosomes are formed by two chromonemata is based on the following observations: 1. Treatment of isolated chromosomes with 0.002 M KCN loosens the structure of the loops, and a more or less coiled organization is then observed in most of them with the light microscope. At the electron microscope level, each loop consists of a bundle of microfibrils. The latter are 500 A in diameter, and their complex arrangement within the loops is best studied in stereoscopic preparations. 2. Treatment of chromosomes with 0.002 M KCN also unravels the "chromomeric" regions of the axis. A fibrillar organization then becomes visible in the light microscope. In the electron microscope, wide strands are seen within some chromomeres; their diameter corresponds closely to that of the chromonemata forming the loops associated with the same chromomeres. In thin transverse sections of isolated chromosomes, no special structure is visible in the axial region except random profiles of fibrils similar to those seen in the loops of the same preparations. 3. Two strands sometimes connect adjacent chromomeres. Where gaps exist along the axis, after stretching of the chromosomes, a loop occasionally straddles the break and returns to a chromomere on each side.     

Ultrastructural study of polyspermy during early embryo development in pigs, observed by scanning electron microscope and transmission electron microscope

Polyspermy is generally considered a pathological phenomenon in mammals. Incidence of polyspermy in porcine eggs in vivo is extremely high (30-40%) compared with other species, and polyspermy rate in the in vitro fertilized eggs in pigs can reach 65%. It is still unknown whether polyspermy to a certain degree is a physiological condition in pigs, and whether porcine eggs have any capability with which to remove the accessory sperm in the cytoplasm. The objectives in the present study are to observe the ultrastructural changes of accessory sperm during early embryonic development in pigs. A total of 58 normal, early embryos at one-, two, three-, and four-cell and morular stages were collected from gilts and were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The surface ultrastructure showed that sperm fusion with the zona pellucida was a continuous process during one-, two-, three-, and four-cell and morular stages, as observed by the SEM. Accessory sperm were present in the cytoplasm of cleaved embryos. The sperm heads in the cytoplasm of cleaved embryos did not decondense. TEM revealed the presence of a condensed sperm head within a lysosome (or phagolysosome) in a three-cell embryo. These observations suggest that polyspermy may be a physiological condition in pigs and that early embryos may develop to term if accessory sperm do not interrupt the embryo genome. Furthermore, lysosome activity could be another physiological mechanism for removing accessory sperm in the cytoplasm of fertilized eggs and cleaved embryos after fertilization in pigs.     

An electron microscope study of the development of the blastula of the sea urchin embryo and its radial polarity

An attempt has been made to correlate the changes in structure of the developing blastula, as revealed by electron microscopy, with events at both the molecular and cellular level.The outer surface of the early blastula is a well defined complex, similar to that seen in the fertilised egg, and comprises the highly convoluted plasma membrane which is firmly attached to a fibrous diffuse hyaline layer. The internal surfaces of the cells, where they are in contact with one another and where they line the blastocoel, are by contrast smooth and of simple structure. During development the hyaline layer condenses into two well defined layers, particularly evident after hatching; desmosomes develop between the outer edges of the cells, and a fibrous secretion occurs at the inner surface of the cells and forms a coating to the inner surface. These structural changes have been correlated with the histochemical studies of other workers which indicate the secretion of acid mucopolysaccharide and protein from the cells.The nature of the cell contacts is discussed in relation to observations on cellular activities during blastula formation; the strong attachment of the cells to the hyaline layer, and the intracellular attachment via desmosomes, are stressed as morphogenetic factors.The cells of the hatched blastula show marked radial polarity, as regards structure of the cell surfaces, staining of polysaccharides, and distribution of the internal contents of the cell. Radial polarity is also revealed by grafting experiments. It is suggested that all these polarisations may be referred back to an initial polarisation due to the original external surface complex which appears at fertilisation and the new internal surfaces formed at cleavage. The difference between these can provide the basic polarity which becomes progressively enhanced by symmetric secretion of material from the cells. The system, so conceived, is automatically self-complexing, and its complicated differentiated properties may be generated by only a few cellular activities.     

An electron microscope study of the regenerating nerve fibers

Summary Guinea pig sciatic nerves were severed in order to obtain a regenerative process of the nerve fibers. The animals were killed at different periods of time after the severing (24 hours, 6 days and 12 days) and the specimens obtained were prepared for electronmicroscopic study.The nerve fiber growing extremities (growing cones) were specially studied. The growing cones showed the following components: a) microvesicles; b) mitochondria; c) multivesicular bodies.The microvesicles are hollow elements of about 200 to 700 Å. They constitute the main component of the growing cone. The mitochondria were seen as elongated bodies of 80 m. They were seen in many cases changing to round dense bodies which appear to break-up in irregularly-shaped fragments.The multivesicular bodies were found present in most of the growing cones. Protoneurofibrils do not exist in the growing cone but a close relationship between microvesicles and protoneurofibrils was found in the segments next to the growing cone.The above-mentioned components were found in all growing cones, disregarding the time elapsed after the nerve severing.Director of the Institute.Assistant of the Department of Neurohistology and Experimental Histology.Head of the Department of Cell Ultrastructure.     

An electron microscope study of the yeast Pityrosporum ovale

Summary Cells of Pityrosporum ovale were prepared for electron microscopy by different methods of fixation and embedding, all of them causing some degree of damage to the cells. Apart from the usual organelles seen in other yeast cells, a body was found which showed an electron-dense outer layer and an electron-light centre when stained with permanganate. The cell wall showed layers of different electron-density. Buds were formed at one pole only, leaving a collar on the mother cell.     

An electron microscope study of intrusions into alarm substance cells of the channel catfish

Thin and ultrathin sections of the epidermis of two channel catfish Ictalurus punctatus were studied in light and electron microscopes, respectively, to learn more about intrusions of entire other cell types into alarm substance cells, first noted in 1981. Several degrees of intrusion and several stages in the process of total intrusion are described: microvillus-like projections, pseudopod-like projections, telophase-stage projections, total cell intrusion. In addition, several different cell types intrude: lymphocyte-like cells, general epidermal cells, virus-infected cells and other alarm substance cells. These findings indicate that the alarm substance cell is extraordinarily subject to invasion by other cell types. They suggest that its plasma membrane may have been modified so as to be somewhat less effective in preserving cell integrity (and thus more easily release its alarm pheromone on injury) than is the case with most other cell types or that the alarm substance acts as an attractant for other epidermal cells.