Spine skeleton morphogenesis during regeneration in clypeasteroid and camarodont sea urchins
The process of skeleton morphogenesis is described for broken and totally removed spines in clypeasteroid (hollow spine) and camarodont (solid spine) sea urchins. Spine regeneration after total spine removal is completed in 40–45 days in clypeasteroids and in 60–70 days in camarodont sea urchins. Along with common stages of formation of longitudinal ribs in both hollow and solid spines, fundamental differences were found between the initial stages of reparative growth of the spine shaft. The spine shaft is formed from a single median process in clypeasteroids and from many simultaneously growing processes in camarodont sea urchins. Reparative morphogenesis of totally removed and partly broken spines in clypeasteroid sea urchins and totally removed spines in camarodont sea urchins leads to the formation of a skeletal structure identical to the intact spine. However, during the regeneration of broken camarodont spines, lateral growth is markedly retarded. As a result, the regenerated part of the spine shaft has a smaller diameter when the initial spine length is achieved. A hypothesis is proposed on a paedomorphic origin of spines in the clypeasteroid sea urchins on the basis of the juvenile stage of definitive spines in the camarodont sea urchins.
Zur Morphologie und Regeneration der Hohlstacheln vonDiadema antillarum Philippi undEchinothrix diadema (L.) (Echinoidea, Diadematidae)
SummaryThe structure and regeneration of the primary and secondary spines ofDiadema antillarum andEchinothrix diadema were studied and compared with the light and scanning electron microscopes. To evaluate the effect of temperature on the rate of regeneration, spines ofD. antillarum were regenerated at different temperatures under careful control of other environmental factors.The opinion of Borig (1933) that the regeneration of hollow spines begins analogously to that of solid spines on the inner zone of the fractured surface of the spine shaft by addition of loose meshwork, was refuted. The new skeleton first appears as minute, conical “micro-spines” on the margin of the perforated tube. The same course of regeneration was observed inEchinothrix spines, which possess an inner meshwork. This indicates that the inner meshwork of the spines of the Diadematidae grows into the cavity of the spine secondarily during the course of spine growth and that the central cavity does not arise by resorption of a primordial axial meshwork. Thus it is understandable that old spines possess more inner meshwork than younger ones.The hypothesis of Durham and Melville (1957) is discussed. The present morphological findings do not favour the contention that the Diadematidae are derivable from cidaroid ancestors with solid spines.ZusammenfassungDie Primär- und Sekundärstacheln vonDiadema antillarum undEchinothrix diadema sowie die Regeneration dieser Stacheln wurden mittels licht- und rasterelektronenmikroskopischer Methoden vergleichend untersucht. Darüber hinaus wurden experimentelle Untersuchungen über den Einfluß der Temperatur auf die Stärke der Stachelregeneration durchgeführt.Die Behauptung (Borig, 1933), daß die Regeneration der Diadematidenstacheln damit beginnt, daß der axiale Hohlraum auf Höhe der Bruchfläche mit Maschenwerk verschlossen wird und die Regeneration — analog zu den Vollstacheln — auf diesem zentralen Maschenwerk einsetzt, wurde widerlegt. Die Regeneration beginnt stets auf Höhe des Siebzylinders; das gilt sogar für solche Diadematidenstacheln, die ein inneres Maschenwerk besitzen. Dieser Befund ist ein Hinweis darauf, daß das innere Maschenwerk der Diadematidenstacheln eine sekundäre Bildung und dem Mark der Vollstacheln nicht homolog ist. Der axiale Hohlraum entsteht nicht sekundär durch Resorption eines primär vorhandenen axialen Kalkgerüstes. In diesem Sinne läßt sich auch der Umstand deuten, daß alte Stacheln mehr inneres Maschenwerk besitzen als junge Stacheln.Die von Durham u. Melville (1957) aufgestellte Hypothese, daß alle rezenten Seeigel von cidaroiden Ahnen herzuleiten sind, wird diskutiert. Die morphologischen Befunde sprechen nicht dafür, daß die Diadematiden mit ihren Hohlstacheln von cidaroiden Ahnen, die Vollstacheln besaßen, abgeleitet werden können.
Origin of calcified tissue in regenerating spines of the sea urchin, Strongylocentrotus purpuratus (Stimpson): a quantitative radioautographic study with tritiated thymidine.
Cellular proliferation in the calcified dermis during regeneration of fractured spines of the sea urchin, Strongylocentrotus purpuratus (Stimpson), was studied quantitatively in radioautograms of tissues labeled with tritiated thymidine in vivo or in vitro to establish the origin of regenerated calcified tissue. Non-fractured, whole spines were included for comparison. In short term experiments, incorporation of tritiated thymidine occurred throughout the length of regenerating and non-fractured spines, with a peak in the number and proportion of labeled cells in the shaft occurring near the milled ring, declining to low levels in the the base and tip. A localized apical growth zone was absent in these spines. In pulse-labeling experiments, there was a marked distal shift in the distribution of labeled cells during regeneration of fractured spines and in whole spines presumably due to migration. Significantly, labeled cells appeared in increasing numbers throughout the regenerate during growth. Few migratory coelomocytes within the calcified dermis were observed to incorporate detectable quantities of tritiated thymidine. The results of this study are consistent with the hypothesis that regenerated connective tissue of the calcified dermis originates by distal migration of cells, primarily from proximal regions of relatively high mitotic activity within the shaft near the level of the milled ring.