MICROTUBULES IN THE FORMATION AND DEVELOPMENT OF THE PRIMARY MESENCHYME IN ARBACIA PUNCTULATA

Abstract

Prior to gastrulation, the microtubules in the presumptive primary mesenchyme cells appear to diverge from points (satellites) in close association with the basal body of the cilium; from here most of the microtubules extend basally down the lateral margins of the cell. As these cells begin their migration into the blastocoel, they lose their cilia and adopt a spherical form. At the center of these newly formed mesenchyme cells is a centriole on which the microtubules directly converge and from which they radiate in all directions. Later these same cells develop slender pseudopodia containing large numbers of microtubules; the pseudopodia come into contact and fuse to form a "cable" of cytoplasm. Microtubules are now distributed parallel to the long axis of the cable and parallel to the stalks which connect the cell bodies of the mesenchyme cells to the cable. Microtubules are no longer connected to the centrioles in the cell bodies. On the basis of these observations we suggest that microtubules are a morphological expression of a framework which opeartes to shape cells. Since at each stage in the developmental sequence microtubules appear to originate (or insert) on different sites in the cytoplasm, the possibility is discussed that these sites may ultimately control the distribution of the microtubules and thus the developmental sequence of form changes.

Transformation mechanism of amorphous calcium carbonate into calcite in the sea urchin larval spicule

“…The spicules are formed inside a syncytium produced by specialized cells (13). The first deposit is a single rhombohedralshaped calcite crystal.…”

Seawi—a sea urchin piwi/argonaute family member is a component of MT-RNP complexes

“…By electron microscopy, the MT-RNP complexes consisted of microtubule arrays studded with ribosomes that appeared to be connected via a salt-extractable, protease-sensitive stalk (Hamill et al 1994). Similar studding of ribosomes on MTs has been reported to occur in vivo along the MT arrays of the mitotic spindle (Hirokawa et al 1985;Suprenant et al 1989) and ciliary platform rootlets of epithelial cells at the primary mesenchyme blastula stage of sea urchin embryogenesis (Gibbins et al 1969).…”

Transient precursor amorphous phases in biomineralization.In the footsteps of Heinz A. Lowenstam

“…Within the syncytium, mineral deposition occurs inside a membrane-delineated vacuole [39,41]. The size and the shape of the syncytium constantly increase and change during growth of the spicule [35,42].…”