The insertion of mesenchyme cells into the ectoderm during differentiation in Sea urchin embryos

Mechanical modulation of osteochondroprogenitor cell fate

Mesenchymal cells are natural tissue builders. They exhibit an extraordinary capacity to metamorphize into differentiated cells, using extrinsic spatial and temporal inputs and intrinsic algorithms, as well as to build and adapt their own habitat. In addition to providing a habitat for osteoprogenitor cells, tissues of the skeletal system provide mechanical support and protection for the multiple organs of vertebrate organisms. This review examines the role of mechanics on determination of cell fate during pre-, peri- and postnatal development of the skeleton as well as during tissue genesis and repair in postnatal life. The role of cell mechanics is examined and brought into context of intrinsic cues during mesenchymal condensation. Remarkable new insights regarding structure function relationships in mesenchymal stem cells, and their influence on determination of cell fate are integrated in the context of de novo tissue generation and postnatal repair. Key differences in the formation of osteogenic and chondrogenic condensations are discussed in relation to direct intramembranous and indirect endochondral ossification. New approaches are discussed to elucidate and exploit extrinsic cues to generate tissues in the laboratory and in the clinic.

A putative role for carbohydrates in sea urchin gastrulation

Many studies have examined the effects of lectins on embryonic development. Recently, it has been shown that lectins actually enter the blastocoel of sea urchin embryos without microinjection and bind to specific cell types. The present study was performed to examine the effects of lectins on sea urchin gastrulation. Strongylocentrotus purpuratus sea urchin embryos were incubated with several lectins at concentrations from 0.01 microgram/ml to 100 micrograms/ml at 15-28 h in the presence or absence of the preferential binding sugars. The most interesting findings were that the mannose specific lectins Lens culinaris agglutinin (LcH) which binds to secondary mesenchyme cells involved in archenteron anchoring and Pisum sativum (PSA) caused exogastrulation. Wheat germ agglutinin (WGA) which binds to primary mesenchyme cells involved in skeletogenesis caused defective skeletogenesis. Our findings suggest that D-mannose-like residues (LcH and PSA specific sugar) may function in archenteron development and anchoring, while N-acetyl-D-glucosamine-like groups (WGA specific sugar) may contribute to control of primary mesenchyme positioning and function. Specific carbohydrate-containing receptors may, therefore, be of importance in specific gastrulation events.

Primary mesenchyme cell migration in the sea urchin embryo: distribution of directional cues

The directional migration of the primary mesenchyme cells (PMCs) of the sea urchin embryo is a critical step in the process of gastrulation. Although interactions between the migrating cells and the blastocoel environment are necessary for guiding the PMCs to their subequatorial target site, the nature of these interactions and the localization of guidance cues involved in directing the cells are not yet known. Previous studies have suggested that PMC migration is the result of random exploration and selective trapping at the target site by a pattern of adhesiveness in the ectoderm or basal lamina. To better characterize the distribution of guidance cues in the blastocoel we used a combination of time-lapse microscopy, microsurgery, and fluorescence photoablation to study the behavior of the migrating cells. By using fluorescence time-lapse microscopy, and a two-dimensional random-walk analysis of cell trajectories, we demonstrated that fluorescently labeled PMCs injected near the animal pole move in a directed fashion over a relatively long distance to reach the target site. This suggests that guidance cues are distributed globally throughout the embryo and are not restricted to the immediate ring area. To further test this hypothesis we investigated the migratory behavior of PMCs that were prevented from interacting directly with the target site. First, we examined the behavior of PMCs injected into animal embryo fragments lacking the target site. We found that PMCs move to the vegetal-most area of such embryo fragments, regardless of their size. Second, we studied the effects of photoablating a stripe of ectoderm between PMCs injected at the animal pole region (APR) and the target site. PMCs were found to accumulate along the ablated stripe and were unable to cross it for up to 6 hr after ablation. We also examined the migratory behavior of endogenous PMCs in embryos treated with lithium, a vegetalizing agent which shifts the position of the PMC ring toward the animal pole. We found that PMCs accumulated along an ablated stripe of ectoderm positioned below the shifted target site, suggesting that endogenous PMCs follow a set of directional cues to the target site which may be similar to those used by PMCs injected into the APR. As a whole, these results suggest that migrating PMCs follow a set of directional cues that are widely distributed throughout the blastocoel and that may be arranged in a gradient.