Histological and Immunohistochemical Analyses on the Formation of the Split Plane in Developing Autotomous Tail Vertebrae of the Lizard <i>Anolis lineatopus</i>

The present histological, autoradiographic, and immunohistochemical study describes the maturation of caudal vertebrae and the origin of autotomous planes in the embryo of the lizard Anolis lineatopus. Autotomous vertebrae associated with connective planes for the detachment of the tail ensure higher survival chances for lizards. The study on late embryonic and pre-hatching stages shows that caudal vertebrae are largely colonized by hemopoietic cells and are not completely surrounded by compact bone laminae in some small areas. The periosteum surrounding the vertebral centrum and neural arch is not continuous, so that the spinal cord contacts through the meninges the veins inside the bone marrow of vertebral bodies. The formation of an alamellar bone outlining completely the vertebrae, as observed in adult lizards, is likely reached sometime after hatching. Discontinuities in the thin alamellar bone outlining the vertebrae are observed in the split or autotomous plane and in the foramen of spinal nerves. The chemical signal for the localization of the split is unknown. Tritiated thymidine autoradiography and 5BrdU-immunolabeling indicate that some periosteum osteoblasts surrounding the centrum and neural arches are proliferating in these late developmental stages, in addition to sclerotomic cells invading the inter-centrum. The remodeling process also includes the formation of a split plane due to the penetration of osteoblasts into the initial alamellar bone and the separation of the borders of the alamellar bone where proliferating osteoblasts are present. These cells also contain matrix metalloproteinaes and alkaline phosphatase, indicating that they are involved in remodeling the bones of caudal vertebrae during vertebral development, but likely also after hatching. Vertebrae therefore contain a large mass of bone marrow which stem cells can be liberated on the tail stump after tail autotomy, providing hematogenous cells that contribute to the initial formation of the blastema together mesenchymal cells and fibroblasts.