Fig 9. Frontal sections of the distal region of left lower jaws showing metamorphic changes in histology, shape and cell features.

Right is to the front of the head and up is to the lateral side. (a) a late larval specimen (NF 58) showing BrdU labeling in transversely aligned cell clusters inside the cartilage, and the jaw joint with the palatoquadrate (arrow), which lies in the plane of the long axis of the cartilage at larval stages. (b) a resin embedded, H&E stained section at NF 59 (the smaller cartilage is the palatoquadrate) showing matrix and clusters of round chondrocytes near the distal end (*) and a layer of flattened cells outside the distal end (arrow). (c-f) a series from NF 61 to 64+ specimens showing the emergence of rows of small chondrocytes aligned with the distal end (arrow in c). BrdU labeling (c, e and f) remains low in perichondral cells, the newly emerging chondrocytes, and the larger, more proximally situated chondrocytes of the original larval cartilage. (g) a NF 67 specimen showing light Alcian blue staining of matrix around the outermost rows of new chondrocytes, strong staining around the larger, more proximally positioned chondrocytes, and moderate staining around the innermost row of new chondrocytes (arrow), which are larger and more circular than in outer rows. (h) the head of a humerus from the same specimen as g showing a similar pattern of chondrocytes and matrix staining. Scale bar is 0.2 mm.