The morphological events during the self-reassembly of the sea urchin embryo in culture Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/f1881p74j

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  • The gastrula of the sea urchin Strongylocentrotus purpuratus was dissociated into single cells. In culture these cells appeared apolar, but reassembled into quasi-normal embryos composed of structures with polarized cells. In this study emphasis was placed on those morphological events coincident with the reestablishment of a polar orientation within the epithelial cells. Using both time-lapse dark-field-phase microscopy and electron microscopy, the major morphological events followed in detail include: (a) the migration of lysosome-like vesicles to specific margins within the epithelial cells; (b) the migration of the epithelial cells during the re-construction of the epithelium: and (c) the reformation of the cell junctions between the epithelial cells. Evidence indicates that: (1) The orientation resulting from the migration of the lysosome-like vesicles is transient and may not determine any lasting polarity of the epithelial cell. The sequential migration of the lysosome-like vesicles first to the epithelial cell to cell interface, then to the basal margin and finally to the apical margin of the epithelial cell was coincident with and apparently in response to the presence of cell debris often trapped at these sites. The Lysosome-like vesicles formed tight clusters along the epithelial cellular margins where phagocytosis of cell debris and resorption of specialized structures occurred. In contrast, the mesenchyme cell had smaller numbers of phagosomes and lysosome-like vesicles and was observed to participate to a more limited degree in phagocytosis and resorption. (2) The orientation resulting from the formation of a leading edge during the migration of the epithelial cell is transient. The sequence of cytological events associated with epithelial cell migration included the formation of: (a) hyaloplasmic blebs along the cell free margin; (b) filopodia and microvilli; and (c) lamellae with a number of lamellipodia at the leading edge. In contrast, the mesenchyme cell formed filopodia on a smaller scale and was observed to participate in migration only to a limited degree. Coincident with the cessation of epithelial cell migration was the enlargement of the lamella between the epithelial cells into a hyaloplasmic sheet which forms the blastocoel wall. Embryonic cells from reassembling aggregates were cultured as monolayers on glass treated with an adhesion enhancing factor derived from the incubation medium. Cords of cells, composed of both epithelial and mesenchyme cells migrate away from the aggregate and leave in their wake clusters of cells. Epithelial cells were found at the leading margins of these cell cords and were observed forming cytoplasmic blebs, filopodia and hyaloplasmic leading edges during the migration of the cell cord. No apparent coordinate orientation was detectable within the epithelial cells of the migrating cell cord or stationary cell cluster. A hypothesis is presented suggesting a role for resorption during epithelial cell migration. (3) Cell contacts formed during reaggregation become more adhesive as development proceeds. The sequence of fine structural events associated with cell contact formation include the formation of (a) a non--junctional cell apposition, which was characterized by parallel apposed membranes; (b) an incipient continuous junction, which was characterized by parallel apposed membranes; and (c) a continuous junction, which was characterized by the presence of an electron dense region both superjacent and subjacent to the plasma membrane at the site of the junction. The cell appositions between premigratory cells were easier to disrupt mechanically than the later forming continuous junctions. The forming continuous junction was first focal in extent and later zonal in girth around the apical margin of the epithelial cell. Concurrent with the formation of the strongly adhesive continuous junction by the lamella was a cessation of epithelial cell migration, followed by an expression of an apical-basal polarity within the epithelial cell. A hypothesis is presented suggesting a role for cell contacts during the coordinated orientation of the epithelia during the formation of the blastocoel wall.
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