Ultrastructure of primary mesenchyme cell ingression in the sea urchin Lytechinus pictus

Abstract

The ultrastructural changes associated with primary mesenchyme cell formation in the sea UrchinLytechinuspictus were studied by transmission (TEM) and scanning electron microscopy (SEM). Early during ingression, one can recognize about 30 presumptive primary mesenchyme cells, which form a ring around eight central cells, located at the apex of the vegetal pole. The first steps involve the loss of the basal lamina in the vegetal plate region and the loss of cilia from the 30 presumptive primary mesenchyme cells. These cells then elongate into bottle-shaped cells which have a bulbous, appendage-free basal end that protrudes into the blastocoel, and a thin neck region that forms numerous apical cell processes. Longitudinally oriented microtubules are observed in the peripheral cytoplasm in the neck region of the bottle-shaped cells and in the adjacent cytoplasm of closely adherent neighboring non-mesenchymal blastomeres. After the disappearance of apical junctions, the neck regions of the bottle-shaped cells shorten, resulting in the transposition of the cells into the blastocoel as rounded primary mesenchyme. Mesenchyme cell displacement results in the temporary formation of fenestrae in the blastular wall, which are filled in by apical cell processes that extend from neighboring blastomeres. These morphological changes have several features in common with other examples of cells which exit from epithelia and are consistent with a morphological sequence involving the relaxation of the basal cytoplasm of the presumptive primary mesenchyme cells, followed by their active elongation and lateral compression by adjacent cells. PRIMARY MESENCHYME CELL INGRESSION IN SEA URCHIN

Serotonin stimulates [Ca2+]i elevation in ciliary ectodermal cells of echinoplutei through a serotonin receptor cell network in the blastocoel

“…PKCrelated break down of inositol phospholipids occurs in association with desmosomes (Kitajima et al, 1992), the intercellular junction also found in sea urchin ectoderm (Katow and Solursh, 1980). Thus, desmosomes could be involved in the present intercellular propagation of [Ca 2+ ] i elevation in the ciliary epithelium.…”

“…However, since ectodermal cells are less than 1/10 of the diameter of an oocyte [about 110·m and often the subject of [Ca 2+ ] i wave propagation studies (e.g. Kyozuka et al, 1998) from the previous cell to the next through gap junctions (Braet et al, 2003) also seems to be an unlikely mechanism in sea urchin larvae, because gap junctions were not found in the ectoderm (Katow and Solursh, 1980), and connexin-like proteins have not been found in the Sea Urchin Genome Resources to date (Sea Urchin Genome Sequencing Consortium, 2006).…”

Mechanisms of the epithelial-to-mesenchymal transition in sea urchin embryos

“…2). [6][7][8][9][10] The major model species have consisted of Hemicentrotus pulcherrimus in Japan, Strongylocentrotus purpuratus and Lytechinus variegatus in North America, and Paracentrotus lividus in Europe.…”

“…Two of 3 NMC genes, Hp-ecpn 6 and Hp-gad, 5 are expressed in the ectoderm as early as the 15-hpf swimming blastula stage until at least the 2-arm pluteus stage in H. pulcherrimus. The Hp-ECPN expressing cells (ECPN cells) plays an essential role in photoresponsive larval swimming.…”

The Snail repressor is required for PMC ingression in the sea urchin embryo

“…These nascent PMCs ingress through the basal lamina and migrate into the blastocoele (for details, see Katow and Solursh, 1980) (reviewed by Solursh, 1986). During EMT, the PMCs downregulate cell-cell adhesion (Fink and McClay, 1985), increase motility, change cell shape, and finally become mesenchymal cells with a migratory behavior.…”