Vegetal plate specification was assessed in S. purpuratus embryos after micromere deletions at the 4th, 5th and 6th cleavages, by assaying expression of the early vegetal plate marker Endo 16, using whole-mount in situ hybridization. After 4th cleavage micromere deletions, the embryos typically displayed weak Endo16 expression in relatively few cells of the lineages that normally constitute the vegetal plate, while after 5th and 6th cleavage micromere deletions the embryos exhibited strong Endo16 expression in larger fractions of cells belonging to those lineages. When all four micromeres were deleted, the embryos were severely delayed in initiating gastrulation and sometimes failed to complete gastrulation. However, if only one micromere was allowed to remain in situ throughout development, the embryos exhibited strong Endo16 expression and gastrulation occurred normally, on schedule with controls. Additional measurements showed that these microsurgical manipulations do not alter cleavage rates or generally disrupt embryo organization. These results constitute direct evidence that the micromeres provide signals required by the macromere lineages for initiation of vegetal plate specification. The specification of the vegetal plate is completed in a normal manner only if micromere signaling is allowed to continue at least to the 6th cleavage stage.

Cameron
R. A.
,
Hough-Evans
B. R.
,
Britten
R. J.
,
Davidson
E. H.
(
1987
)
Lineage and fate of each blastomere of the eight-cell sea urchin embryo.
Genes Dev
1
,
75
84
Cameron
R. A.
,
Fraser
S. E.
,
Britten
R. J.
,
Davidson
E. H.
(
1991
)
Macromere cell fates during sea urchin development.
Development
113
,
1085
1091
Davidson
E. H.
(
1989
)
Lineage-specific gene expression and the regulative capacities of the sea urchin embryo: A proposed mechanism.
Development
105
,
421
445
Davidson
E. H.
(
1990
)
How embryos work: A comparative view of diverse modes of cell fate specification.
Development
108
,
364
389
Davidson
E. H.
(
1991
)
Spatial mechanisms of gene regulation in metazoan embryos.
Development
113
,
1
26
Fink
R. D.
,
McClay
D. R.
(
1985
)
Three cell recognition changes accompany the ingression of sea urchin primary mesenchyme cells.
Dev. Biol
107
,
66
74
Hörstadius
S.
(
1935
)
Uber die Determination im Verlaufe der Eiachse bei Seeigeln.
Pubb. Staz. Zool. Napoli
14
,
251
479
Hörstadius
S.
(
1939
)
The mechanics of sea urchin development studied by operative methods.
Biol. Rev. Cambridge Phil. Soc
14
,
132
179
McClay
D. R.
,
Armstrong
N. A.
,
Hardin
J.
(
1992
)
Pattern formation during gastrulation in the sea urchin embryo.
Development
1992
,
33
41
Nocente-McGrath
C.
,
Brenner
C. A.
,
Ernst
S. G.
(
1989
)
Endo16, a lineage-specific protein of the sea urchin embryo, is first expressed just prior to gastrulation.
Dev. Biol
136
,
264
272
Ransick
A.
,
Davidson
E. H.
(
1993
)
A complete second gut induced by transplanted micromeres in the sea urchin embryo.
Science
259
,
1134
1138
Ransick
A.
,
Ernst
S.
,
Britten
R. J.
,
Davidson
E. H.
(
1993
)
Whole mount insitu hybridization shows Endo16 to be a marker for the vegetal plate territory in sea urchin embryos.
Mech. Dev
42
,
117
124
Soltysik-Española
M.
,
Klinzing
D. C.
,
Pfarr
K.
,
Burke
R. D.
,
Ernst
S. G.
(
1994
)
Endo16, a large multidomain protein found on the surface and ECM of endodermal cells during sea urchin gastrulation, binds calcium.
Dev. Biol
165
,
73
85
This content is only available via PDF.