Development of cartilage and bone tissues of the anterior part of the mandible in cichlid fish: a light and TEM study.

The present paper presents ultrastructural details of chondrogenesis of Meckel's cartilage and of ossification of its associated peri- and parachondral bones in a teleost fish, the cichlid Hemichromis bimaculatus. We have distinguished four stages during chondrogenesis, each of which is characterized by specific cellular and matrix features: blastema, primordium, differentiated cartilage and cartilage surrounded by perichondral bone. The blastema is characterized by prechondroblasts and the lack of cartilage matrix; the primordium by chondroblasts and the onset of secretion of matrix of fibrillar and granular nature; differentiated cartilage is characterized by chondrocytes and larger amounts of typical hyaline cartilage matrix. Once perichondral bone is laid down, the chondrocytes show degenerative features but not true hypertrophy. Differentiation of the cartilage cells is attended with cytoplasmic changes indicative of an increasing secretory activity. There is a regional calcification of the cartilage matrix by fusion of calcospherites. Chondrogenesis of the symphyseal area is continuous with that of the rami but starts slightly later. Formation of perichondral bone at the cartilage surface is attended with the deposition of a transitional zone apparently containing a mixture of the two matrices. The role of the perichondral cells is discussed and it is proposed that they may contribute to the formation of the two matrices. The transitional zone may then result either from a diffusion process or from the simultaneous deposition of elements of the two matrices. Growth of the cartilage is argued to be largely the result of matrix secretion, except in the symphyseal area where appositional growth probably occurs until the region is completely covered by perichondral bone. This paper provides a basis for further studies on the developmental interactions between cartilage, bone and teeth during mandibular development in cichlids.