Developmental origin of the frontoparietal bone in Bombina variegata (Anura: Discoglossidae).

Abstract

Development of the frontoparietal bone in the European yellow-bellied toad, Bombina variegata, was followed on the basis of histological analysis of transverse serial sections through the larval skulls to recognize early stages of ossification represented by osteoid (uncalcified bone matrix) and on cleared and stained specimens to investigate more advanced stages. Ossification of the frontal begins as three tiny areas of osteoid (F(1), F(2), F(3)) adjoining the dorsal surface of the orbital cartilage, which are separated by areas without osteoid. F(3) is the largest (most advanced). Prior to calcification, F(3) extends to fuse with F(2) and then with F(1), but it does not expand over the prootic fissure posteriorly. As calcification begins the strip of bone is joined posteromedially by F(4). Only then does a single ossification center corresponding to the parietal arise on the anterodorsal surface of the otic capsule. This ossification sequence corresponds to those observed in the Actinopterygii and in caudate amphibians.