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A histological analysis of gill skeletogenesis in silver carp: Endochondral ossification, cartilage and bone
1Department of Histology, Faculty of Veterinary Medicine, South Valley University, Qena, Egypt.
Abstract:
This study provides a detailed histological analysis of the gill arch skeleton in the silver carp (Hypophthalmichthys molitrix). The gill rakers contain discrete cartilaginous units composed of a basophilic matrix with chondrocytes, including hypertrophic subtypes, separated by fibroblast-rich connective tissue. The filament cores are supported by specialized hyaline cartilage with two distinct zones: a highly cellular central core and a matrix-rich peripheral region. Active appositional growth at the filament tips is indicated by a high density of peripheral chondrocytes. A spectrum of cartilage subtypes-including fibro-hyaline, matrix-rich, and highly cellular hyaline was identified. Ossification analysis revealed two distinct pathways: intramembranous ossification formed the opercle bone, while the ceratobranchial developed via endochondral ossification, progressing through proliferative and hypertrophic zones, perichondral bone collar formation, and eventual cartilage replacement by bone. Within the ceratobranchial, two bone types coexist: cellular bone with osteocytes in lacunae, and low-density (acellular) bone with sparse osteocytes, located in metaphyseal regions. These findings illustrate the gill arch as a sophisticated composite structure, where diverse cartilage specializations and dual ossification pathways are integrated to meet the biomechanical demands of respiration.
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