Growth of the calcareous skeleton during regeneration of spines of the sea urchin, strongylocentrotus purpuratus (stimpson): A light and scanning electron microscopic study

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Abstract
Growth of the skeleton of regenerating spines of the sea urchin, Strongylocentrotus purpuratus, was studied with the light and scanning electron microscopes during the formation of a growth ring or cycle.Growth was initiated about three days after fracture and was linear between 5 and about 40 days after fracture, with a mean rate of 0.16 mm/day. There‐after, a decline in growth rate was observed, being attributed to abrasion.The new skeleton first appeared as minute, conical „micro‐spines”︁ on the fractured surface of the spine shaft initiating regeneration of the inner zone of meshwork. Subsequent growth of micro‐spines of both the developing inner zone of meshwork, and an outer zone of radiating wedges, formed a conical fenestrated skeleton on the fractured surface of the shaft. Further deposition of micro‐spines along the shaft, initially at the level of fracture, formed meshwork which gradually became solidified externally resulting in a new cycle about 60 days after fracture. In contrast, a new cycle was initiated at the milled ring in non‐fractured spines during total regeneration on bare tubercles, demonstrating that growth of spines also takes place in the absence of fracture.Experiments conducted in vitro demonstrate that spine regeneration is not a polar phenomenon.

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