Finspine morphogenesis in squalid and heterodontid sharks

Modern selachian finspines develop within a single, enlarged dermal papilla much earlier than any other papillary derived structure, and rapidly enlarge beyond the confines of the papilla. Within this, the spine primordium differentiates into two parts (mantle and trunk). Both components elongate basally and become mineralized by odontoblasts, but only the mantle has an enameloid layer. Mantle and trunk are reunited secondarily by dentine. The finspine vascular supply becomes blocked in adults, possibly to avoid loss of body fluids should the finspine (which is not shed) be damaged. Finspines probably have both defensive (perhaps mainly in juveniles) and locomotory (unctions. Odontocytes sometimes occur in the finspine base of Squalus, although the rest of the spine consists of acellular metadentine. The innermost dentine layers are centripetally deposited, resembling lamellar orthodentine of a tooth pulp. Over this is a continuous layer of centrifugally deposited lamellar dentine. Apically there is another centripetal layer (the mantle), which occurs only on certain parts of the spine. Enamelled tubercles can occur in Squalus, as a result of scleroblast disorganization, not as a result of secondary coalescence.