Gene Networks Database
Lytechinus variegatus Genes in Development: Primary mesenchyme-specific genes
the ectoderm; as a consequence, the arrangement of PMCs
in the blastocoel is altered and the number of spicule rudiments
is increased (Hardin et al., 1992; Armstrong et al., 1993).
When embryos were raised continuously from fertilization in 0.05 mM NiCl2, both the patterning of the PMC ring and the expression of LvSM30 were altered. In most NiCl2-treated embryos, LvSM30 expression was relatively uniform throughout the PMC ring. In some embryos, multiple, discrete clusters of cells expressing high levels of LvSM30 were visible, with lower transcript levels exhibited by cells connecting these clusters. This pattern of expression was more common in embryos cultured in higher concentrations of nickel (e.g. 0.2 mM).
To determine whether the aberrant LvSM30 expression pattern was due to an effect of nickel on the ectoderm or the PMCs, we examined LvSM30 expression after transplanting nickel-treated PMCs into untreated, PMC-depleted host embryos. In such embryos, nickel-treated PMCs formed a ring and exhibited a pattern of LvSM30 expression similar to that observed in control embryos. In 6/7 cases, the PMCs formed a ring in which two clusters and either the ventral and/or the dorsal chain were distinguishable. PMCs in the two ventrolateral clusters exhibited the highest expression of LvSM30. The dorsal chain, when present, exhibited faint to strong staining, whereas the ventral chain showed consistently low expression. As a control, a sample of nickel-treated donor embryos was allowed to continue development in IO in parallel with the experimental embryos. These donor embryos were ventralized and showed an aberrant pattern of LvSM30 expression. These results indicate that nickel causes a misregulation of LvSM30 expression by altering the ectoderm (Guss et al., 1997).
Transplantation of PMCs into recipient embryos of different
developmental stages has been used to provide evidence that
aspects of PMC behavior are regulated by extrinsic cues
(Ettensohn and McClay, 1986). Here the PMCs from
mesenchyme blastulae were transferred into younger embryos (pre-hatching
blastulae) and LvSM30 expression was examined after 5- 5,5 hours,
when sibling donor embryos had reached the mid-late gastrula
stage and were expressing high levels of LvSM30 mRNA. As
a control for the transplantation procedure, PMCs were
removed from mesenchyme blastulae and microinjected into
recipient embryos of the same developmental stage from which
all endogenous PMCs had been removed (isochronic controls).
In the combined results of three series of heterochronic transplants, in 23 of 40 experimental embryos, the PMCs showed no significant signal above background The remaining embryos contained 1-2 cells exhibiting signal above background (6/40) or more than two cells above background (11/40).
In contrast to the low levels of LvSM30 expression observed following heterochronic transplantations, 26/27 isochronic control embryos exhibited more than two positive cells, although signal intensity ranged from just above background to very strong. In 17 of those embryos, a ring was distinguishable and the PMCs exhibited some degree of spatial regulation of LvSM30 expression, e.g. the highest degree of signal was localized to cells in at least one cluster. In the remaining cases, the presence of a ring could not be confirmed because of the orientation of the embryos (five cases), or the ring was incompletely formed and/or there were no differences in levels of staining in cells in different regions of the ring (four cases). Thus, the delay in LvSM30 expression following heterochronic PMC transplantation cannot be attributed to the effects of microsurgery alone, since almost all of the isochronic controls showed detectable levels of LvSM30 expression after the same period of time (Guss et al., 1997).