Transverse sections through left branchial arch skeletons showing metamorphic changes in cartilage histology, shape and cell features.

Dorsal is up, medial is to the right. (a) Alcian-blue stained second and third partitions at a late larval stage (NF 58) showing ceratobranchial bases (cb2 and 3), the intervening gill slit (gs), and the vertical rods (r) that partition the space into flow chambers. The rods have anterior and posterior extensions that support filter surfaces comprised of ornate processes (op). Dorsal folds of pharyngeal epithelium (*) hang down into each flow chamber to force water to flow through the filter surfaces (arrows), into the spaces between ornate processes and rods, and out the gill slit. (b) a close-up of the rectangular region outlined in a, showing a rod, ornate processes and blood vessels (bv) within the rod. (c) brown E-cadherin staining of the squamous epithelium that lines rods and ornate processes at NF 59, and captures food particles. The columnar, mucous secreting epithelium (mse) that caps the inner walls and partitions transports the food particles to the esophagus. (d and e) progressive collapse of the dorsal portions of the partitions in early metamorphosis (NF 61 and 62), and the peripheral matrix in ceratobranchial bases ceasing to stain blue for chondroitin sulphate. (f) BrdU labeling of epithelial cells, chondrocytes, and perichondral cells in ventral portions at a late larval stage (NF 59). (g) BrdU-labeled chondrocytes at a mid-metamorphic stage (NF 62). (h and close up i) TUNEL labeling of epithelial cells, but not chondrocytes at an early metamorphic stage (NF 60). (j) caspase-3 labeling of epithelial cells, and less frequently and more faintly of chondrocytes at NF 62. Scale bars are 1 mm for a, 0.5 mm for b-e and h, and 0.2 mm for f-g and i-j.