New data on the structure and the chondrocyte populations of the haemal cartilage of abdominal vertebrae in the adult carp Cyprinus carpio (Ostariophysii, Cyprinidae)
The bulk of cartilage retained in the haemal arch of the abdominal vertebrae of adult carps was examined using scanning and transmission electron microscopy. A special attention was devoted to the mineralization of the cartilage. The mineralization is spheritic but the processes differ in the proximal and distal parts. The proximal hyaline cartilage contacts the lamellar bone of the centrum without an intermediate chondroid bone; it mineralizes although chondrocytes bearing a hypertrophic phenotype were not identified in this area. The distal part of the cartilage contains hypertrophic chondrocytes associated with the mineralization of the cartilage matrix. Mineralized globules fuse to form longitudinal septa that are replaced by trabecular bone after their resorption by multinucleated chondroclasts. The distal part, resembling a mammalian epiphyseal growth plate, was coined the “haemal growth plate”. Two types of chondrocytes are identified in the distal part by transmission electron microscopy. Apart from typical chondrocytes called “light chondrocytes”, a population of “dark chondrocytes” is present in all stages of chondrocyte differentiation. The dark chondrocytes are electron dense and they are characterised by an abundant rough endoplasmic reticulum and by Golgi complexes. Their nuclei show irregular outlines and they contain a very-condensed chromatin. The compacted chromatin is scattered within the nuclei. Some dark chondrocytes show signs of shrinking and self-destruction through autophagic vacuoles and blebbing with disruption of the cellular membrane. Such chondrocytes, considered as cells exhibiting chondroptosis, a variety of programmed cell death, have only been described so far in the growth plate of amniotes. In this study, dark chondrocytes following a chondroptotic pathway are described for the first time in the cartilage of a non-amniote vertebrate. The present study raises the possibility that specific microenvironmental conditions in the abdominal vertebrae of the adult carp prompt differentiation of dark cells.