PubMed Health⌕ Search

Biomedical subjects

P Schook

Publications and source records attributed to P Schook.

10 recordsLinked to original sources
PubMed1980

The effective osmotic pressure of the fixative for transmission and scanning electron microscopy.

In previous studies concerning the morphogenetic movements in the invaginating lens placode of the chick eye, a discrepancy between the TEM and SEM images was observed. With a fixation procedure giving good results in the TEM, certain changes seemed to have occurred in the SEM specimens (opening of apical cell blebs, bubble-like swelling of microvilli). These changes were suspected of being artefacts arisen during processing. Comparing SEM and TEM images of the invaginating lens placode and the surrounding surface applying two different osmolalities of the glutaraldehyde/formaldehyde fixative vehicle, and comparing SEM as well as TEM images with LM images of living embryos using differential interference contrast, yielded to following information. Recent reports in literature that ideal fixation techniques for SEM, certainly those preserving delicate surface structures as apical blebs and microvilli, can differ from those suitable for TEM, were confirmed. Apparently, the cell blebs which can be demonstrated in vivo to be closed, burst during SEM processing as a consequence of a relative hypo-osmolality of the fixative vehicle (a too low effective osmotic pressure). The TEM images appeared to suffer much less from changes in the osmolality of the fixative vehicle. It is suggested, therefore, that the artefacts are brought about by some step specific to the processing for the SEM (E.g. critical point drying).

Animals↗
PubMed1980

Morphogenetic movements during the early development of the chick eye. An ultrastructural and spatial study. C. Obliteration of the lens stalk lumen and separation of the lens vesicle from the surface ectoderm.

The obliteration of the lens stalk lumen and the separation of the lens vesic from the surface ectoderm were studied by means of light microscop transmission electron microscopy, scanning electron microscopy and three-dimensional reconstruction. Our observations suggest that the obliteration of the lens stalk lumen is effected by means of cellular protrusions which grown from the apical cell surface, intertwine, meet, form interconnecting junctions and finally fuse. Junctions, and also microfilaments, which probably contract, seem to be temporary structures which assist in drawing the cell projections together and forming a firm tissue complex. Separation of the lens vesicle from the surface ectoderm takes place when the obliteration of the lens stalk lumen is completed. Cell death in a circumscribed area of the lens stalk and absence of the basal lamina of the lens stalk accompany the separation of the vesicle. Necrotic cells are phagocytosed by neighbouring epithelial cells or are expelled into the lens stalk lumen or into the interspace between lens vesicle and surface ectoderm.

Animals↗
PubMed1980

Morphogenetic movements during the early development of the chick eye. A light microscopic and spatial reconstructive study.

Morphogenetic movements were studied with light microscopic and spatial reconstructive techniques, using the embryonic chick eye as a model system. From the literature at our disposal we learned that the process of formation of lens vesicle and optic cup, during which process invagination, fusion, obliteration and separation are very important features, represents a finely balanced interplay between mitosis, cell differentiation, cell transformation, cell death and perhaps cell migration. Literature also revealed contradictory observations, controversial interpretations and many unsolved questions, which justified a systematic approach. For our analysis of eye development during the third day of incubation, a classification was developed which consisted of five phases and was based on morphological criteria. Invagination of the lens placode and invagination of the optic vesicle are undoubtedly two different processes. As to lens placode invagination, current thinking has been focused on apical cell contraction, cell proliferation and tissue interaction as possible underlying mechanisms. However, these theories can not explain the asymmetry of the invaginating lens placode. We found visible changes in the surface of the lens pit which might have to do with bringing about its asymmetric shape. In our opinion, localised cell death contributes as much to transformation of the optic vesicle into a double layered optic cup, as mitotic zones. We could not prove that the two optic cup layers really fuse at the end of the third day of incubation. The separation of the lens vesicle from the surface ectoderm seems to be a twofold process. The lens stalk lumen obliterates, and a process in which phagocytosis plays probably a role, separates the lens vesicle from the surface ectoderm at the same time. For further study of all these morphogenetic movements light microscopic and ultrastructural (TEM and SEM) description is needed, using three-dimensional models to deepen our understanding of these transformations in space and in time. Such a study will be the subject of future papers.

Animals↗
PubMed1980

Morphogenetic movements during the early development of the chick eye. An ultrastructural and spatial reconstructive study. A. Invagination of the lens placode.

An electron microscopic study of the invagination of the chick lens placode led to the following observations: In addition to the cyclic movement of the dividing lens placode cells there is a continuous apical extrusion of delaminated lumps of cytoplasm and membranous material, which has been collected in vesicles. Beneath the level of the apical terminal network (junctional complexes and microfilaments) new intercellular junctions are formed. During the invagination the average length of the cells and consequently the thickness of the lens placode remains constant. By combining data from literature as well as from our own observations, we arrived at the following conception: The lens placode cells are in continual movement, and shift their external boundaries as a result of apical shrinkage by extrusion of cellular material and a shift of the apical terminal network on one side and presumably by an outgrowth of cells in the basal direction on the other. This may be the explanation to the change in shape of the entire ectodermal placode, described as invagination. A gradient in this process could account for the final asymmetry in the formation of the lens pit.

Animals↗
PubMed1980

Morphogenetic movements during the early development of the chick eye. An ultrastructural and spatial reconstructive study. B. Invagination of the optic vesicle and fusion of its walls.

Cell death as occurring in the developing optic cup of two to three-day-old chick embryos was investigated with the transmission electron microscope. Two circumscribed areas of cell death were found: The first appears during phase 1 (18 somites, stage 13-) in the middle of the invaginating optic vesicle wall; the second appears during phase 3 (24-28 somites, stages 15 and 16) in the optic stalk where it is connected with the diencephalon. Ultrastructural observations of dead cells with clumped nuclei are phagocytosed by neighbouring neuroepithelial cells. No invasion of macrophages from the mesenchyme was observed. Necrotic cells were occasionally found in the optocoel. After the invagination of the optic vesicle has been completed (at the end of the third day incubation) the layers of the optic cup meet, but there is no real cytoplasmic fusion. Some intercellular junctions are suggested to be precursors of gap junctions occurring in later stages. The role of changing cellular connections between lateral cell surfaces during invagination of the optic vesicle is discussed. Such a process could be responsible for the lengthening of the non-invaginating optic vesicle wall, i.e. the outer layer of the optic cup.

Animals↗
PubMed1980

A spatial analysis of the localization of cell division and cell death in relationship with the morphogenesis of the chick optic cup.

In an attempt to analyze the possible mechanisms underlying the morphogenesis of the optic cup and the optic fissure, two features of generally accepted morphogenetic importance, mitosis and cell death, were studied in their spatial distribution in the chick. The mitotic figures and necrotic remnants visible in serial sections as stained with Feulgen/naphthol yellow-S, were marked on photomicrographs and traced on glass-plates. By piling up the plates, glass reconstructions of five successive developmental phases in the course of the third day of incubation were obtained. Mitotic figures in the walls of the optic vesicle and cup appeared to be distributed at random in all phases. Four areas of cell death, in the lateral wall of the optic vesicle, in the ventral part of the proximal optic stalk, in the uppermost part of the optic cup rim and in the outer layer of the optic cup, were found lying in a frontal plane somewhat rostral from the center of invagination. Evidence for the role of local differences in cell division and localized cell death during the morphogenesis of the optic cup, was not found. Current theories in literature and our observations were discussed. We concluded that for the present, current or conceivable theories can only be teleological explanations of circumstantial evidence. A topographical relationship between cell death and the outgrowth of the optic nerve fibers probably does exist, however. This relationship is born out by studies of congenital anomalies. Possible avenues for further investigation are suggested.

Animals↗
PubMed1978

A review of data on cell actions and cell interaction during the morphogenesis of the embryonic eye.

The early morphogenesis of the chick embryo seems to be an excellent model-system for study of morphogenetic movements, usually summarized in general terms like invagination, fusion and separation. This article reviews literature data about the early morphogenesis of the eye. The formation of lens vesicle and optic cup, during which invagination, fusion and separation are very important, were seen to represent a finely balanced interplay between cell mitosis, cell differentiation, cell death, cell transformation and cell migration. Differing observations and contradictory interpretations justify a systematic approach to to these embryological questions. We will attempt this in our next study. Apart from microscopic and submicroscopic description of closely succeeding developmental stages, three-dimensional reconstruction techniques will be indispensable to understand the spatial arrangement of the structures concerned.

Animals↗