Osteohistology of the Antarctic penguin Pygoscelis adeliae (Aves, Sphenisciformes): definitive evidence of medullary bone

Osteohistological studies allow us to obtain valuable information on different aspects related to the bone microstructure, physiology and ecology of organisms. Although the anatomy and morphology of penguin bones are well known, studies in osteohistology are still insufficient. In order to analyze the osteohistological variations between male and female adults of Pygoscelis adeliae (Aves, Spheniscidae), histological sections were prepared from various bones including appendicular (humerus, radius, ulna, carpometacarpus, femur, tibiotarsus, tarsometatarsus) and axial (vertebral and sternal ribs) elements. The results indicate that all sections showed compact tissue with reduced or absent medullary cavities and high bone density due to internal tissue compaction. Histologically, we identified three distinct tissue regions based on their bone matrix, vascular channels organization and compactions degree. Our results indicate osteohistological variations between sexes throughout the whole skeleton, including the first definitive evidence of medullary bone in the Sphenisciformes females. While the male specimen exhibited a medullary cavity characterized by an inner circumferential layer, the female displayed a medullary region without an inner circumferential layer, lower bone compaction, presence of medullary bone in all the sections, and greater development of intertrabecular spaces. These results are consistent with previous reports of medullary bone in females from other birds and provided an auxiliary criterion for sex differentiation.