Origin of the hypoblast in Salmo . I. Does the blastodisc edge turn inward?

1966

Self Cite

Vital dye carriers implanted under the blastodisc and chalk markers injected among the blastomeres provide a new interpretation of the origin of the hypoblast in teleost embryos. This first inner layer of cells is formed along the sides of the embryonic area and around the germ ring, not by invagination from the surface, or by wheeling in of deeper cells, or by delamination at the blastodisc rim, but by the migration outward of deep central cells. The layering is interpreted as the effect of a shear between superficial cells in pure epibolic motion and deeper cells which are at the same time undergoing convergent movement toward the axis. The whole blastodisc could be stained to controllable depths by suitable exposure to vital dye solutions. Subsequent location of the colored cells indicates that at the late "pregastrula" stage the prospective CNS cells overlap those which will become the first 10 pairs of somites, and all the prospective somite material overlaps prospective notochord and endoderni. A teleost pregastrular fate map thus cannot be a surface map as in amphibia. The rim of the teleost blastodisc and the amphibian blastopore carry out totally different functions.

Origin of the hypoblast in Salmo. II. Outward movement of deep central cells

1966

Self Cite

The morphogenetic movements at the lower surface of the blastodisc in salmonid embryos

Dodes

1968

Self Cite

Patterns of hitherto undescribed inorphogenctic movements have been found in Salmo gairdneri and Salvelinus fontinalis by removing samples of blastodiscs from their yolk spheres at intervals of several hours and inspecting their lower surfaces. Before onset of epiboly, the former species usually produces no pregastrular subgerminal cavity, but the latter usually produces one or several, with wide variation. In both species the inner blastomeres consolidate from an earlier dispersed and migratory condition, forming a coherent flattened sheet 6 to 10 cells thick before active epiboly begins.As the first evidence of polarity in the salmonid embryo, one sector of the blastodisc becomes condensed and slightly thickened. Then a broad central area of the disc rapidly thins out as a result of two types of movement: (1) large numbers of cells disengage from the under surface of the condensed blastodisc, and they migrate away from the center as individuals and small groups; ( 2 ) the rest of the internal cells engage in a similar centrifugal movement toward the germ ring and the embryonic shield, but as a coherent sheet, sliding just within the cellular envelope.Some, perhaps all, of the disengaged cells of the first group are slowly gathered into the endoderin sheet. The coherent cells of the second group accumulate in germ ring and embryonic shield, where they become split temporarily into upper and lower sheets, the former adhering to the external layer, the cellular envelope, with which it forms the epiblast, and the latter associating with the disengaged cells as the hypoblas t.The first internal sign of axis formation is a hitherto undescribed nubbin of cells attaching to the yolk syncytium at one sector on the inside of the blastodisc rim. The nubbin cells take no further part in the epibolic spread of the disc. It is suggested that they form an anchor for the later convergence movements from right and left, and that the whole pattern of movements from then on may be controlled by a gradient of increasing cell adhesiveness that spreads from them as a center. These nubbin cells have never been at the surface of the blastodisc. They neither invaginate nor migrate forward. They may represent the prechordal plate.

History of the hypoblast in Salmo

1968

Spots printed on the blastodisc o f Salmo gairdneri with Nile blue sulfate prior to and during the formation of the germ ring and embryonic shield show during the morphogenetic movements that not many of the germ ring cells are derived from the center of the blastodisc. Evidence suggests that only a few of the loose round cells of the deep part of the ring and of the axial nubbin are produced by local dropout of cells at the margin of the blastodisc, but that most of them arrive there by outward migration from the outer two-thirds of the disc. The embryonic shield is produced by a convergence of internal cells that starts early in the fifth day at 10" C, augmented by a massive descent of consolidated internal cells from the central one-third of the blastodisc which then form the forebrain and optic vesicles. The cellular envelope is not involved in an invagination, nor does i t take part in the general convergence movements. Chalk grains incorporated into the lower surface of the germ ring and the embryonic shield reveal the destination of the cells which carry them along. The hypoblast of the ring continues to converge toward the shield until the closure of the yolk plug on the ninth day. The epiblast of the ring meanwhile forms (the cell layers of the yolk sac. Head mesoderm, pharynx endoderm and notochord materials are assembled directly into the embryonic shield while the germ ring is forming. Additional evidence is presented thait these parts are not formed by invagination. Materials of the hypoblast originally placed as far as 90" out on the ring from the early shield reach the trunk endoderm, and materials from the hypoblast of the entire ring are passed along toward and into the somite and lateral plate strips. Their oblique paths of migration, combining both epiboly and convergence, are at marked angles to the simple epibolic paths of the epiblast cells, and it is suggested that the split between epiblast and hypoblast in the germ ring is a result of this shearing effect. This paper pursues questions which arose out of the observations of Ballard and Dodes ('68) about the origin and fate of the hypoblast components during the morphogenetic movements of the salmonid embryo. In Salmo gairdneri the hypoblast separates from the more superficial cells at the perimeter of the expanding blastodisc during the sixth day of development at 10" C. It can be identified first near the embryonic shield, and the cleft that defines it quickly spreads around the germ ring. Experimental evidence has been submitted (Ballard, '66a, b,c) for the view that this hypoblast is not invaginated from the surface, and that it is not formed by any similar movement of wheeling-in, such as one might imagine being executed by internaI cells that lie just under the cellular envelope at the perimeter. Instead, the evidence so far suggests that more centrally located deep cells move toward the periphery to take part in the nubbin, the embryonic shield and the hypoblast.The newly described detail and the precise time schedule now av...