Mechanisms of branchial cartilage growth in Astatotilapia elegans (Teleostei: Cichlidae).

The growth of the cartilaginous infrapharyngobranchial II (part of the upper pharyngeal jaws) in the cichlid fish Astatotilapia elegans was investigated in four successive early postembryonic stages. Total cell number, individual cell volumes, total cell, total matrix and total cartilage volume were accurately measured or calculated. The share of cell and matrix volume increase in the overall growth was calculated for each of the transitions. Matrix volume increase accounted for almost 60% of the overall rudiment increase. Non-hypertrophic cells and the matrix produced by them together accounted for about three quarters of the total cartilage volume increase. Zones with particular characteristics (cell orientation, cell shape, degenerative features, etc.) were distinguished in each of the four stages separately. The sequence of increasing mean cell volume between zones was interpreted as reflecting growth directions within the cartilage. Apposition probably largely contributed to the observed cell volume increase. Mean cell volume and mean matrix volume per cell showed a highly significant positive relation. Cell density showed a highly significant negative relation to both mean cell volume and mean matrix volume. Our results indicate that the area of hypertrophy and cartilage resorption may be established quite early in the rudiment before overt manifestation of these processes.