Age-dependent development and microarchitecture of the osteochondral unit of the humeral head in harbour porpoises (Phocoena phocoena)

The body morphology of harbour porpoises (Phocoena phocoena), like that of other fully aquatic mammals, differs greatly from terrestrial species due to adaptations to aquatic life. Thus, the developmental processes of the osteochondral unit (OCU) and its postnatal adaptation in harbour porpoises remain poorly understood. Here, we compared the humeral head microarchitecture in neonate, juvenile and adult harbour porpoises to better understand the relationship between environmental loading and the organisation of the OCU within diarthrodial joints. In neonates, the superficial layer of hyaline cartilage showed parallel collagen fibres, while the remaining cartilage exhibited an oblique orientation. Neither a calcified cartilage layer (CCL) nor a subchondral bone plate (SBP) was present. In juveniles the early formation of distinct zones in the articular cartilage, initial cartilage calcification and SBP formation were observed. Adults exhibited a layered hyaline cartilage structure within an arcade-like collagen arrangement, similar to terrestrial mammals. The formation of the CCL and SBP was age-dependent, as in terrestrial species, but appeared to occur later in the maturation process towards adults. These findings provide new insight into how aquatic environments shape age-related remodeling and structural adaptation of the OCU in cetacean diarthrodial joints.