Sea urchin embryos have served as a model system for the investigation
of many concepts in developmental biology. Their gastrulation consists of
two stages; primary invagination, where part of the epithelium invaginates
into the blastocoel, and secondary invagination, where the archenteron
elongates to completely traverse the blastocoel. Primary invagination
involves proliferation of cells within the vegetal plate during primary
invagination, but until recently, it was assumed that the larval
gastrointestinal (GI) tract developed without further involution of
epithelial cells. To investigate rigorously the contribution of epithelial
cell involution during archenteron and GI tract development in the sea
urchin, Lytechinus variegatus, we developed a new method for cell
tracking based on two-photon excited photorelease of caged fluorophores.
Single-cell embryos were injected with caged dye and two-photon excitation
uncaging was employed to mark small groups of cells throughout
gastrulation. Two-photon excitation allowed for noninvasive,
three-dimensionally resolved uncaging inside living cells with minimal
biological damage. Cellular involution into the archenteron was observed
throughout primary and secondary invagination, and the larval intestine
was formed by further involution of cells following secondary
invagination, which is inconsistent with the traditional model of sea
urchin gastrulation. Further, as two-photon excitation microscopy becomes
accessible to many researchers, the novel techniques described here will
be broadly applicable to development of other invertebrate and vertebrate
embryos.