Formation, structure, and function of extra‐skeletal bones in mammals

Abstract

This review describes the formation, structure, and function of bony compartments in antlers, horns, ossicones, osteoderm and the os penis/os clitoris (collectively referred to herein as AHOOO structures) in extant mammals. AHOOOs are extra‐skeletal bones that originate from subcutaneous (dermal) tissues in a wide variety of mammals, and this review elaborates on the co‐development of the bone and skin in these structures. During foetal stages, primordial cells for the bony compartments arise in subcutaneous tissues. The epithelial–mesenchymal transition is assumed to play a key role in the differentiation of bone, cartilage, skin and other tissues in AHOOO structures. AHOOO ossification takes place after skeletal bone formation, and may depend on sexual maturity. Skin keratinization occurs in tandem with ossification and may be under the control of androgens. Both endochondral and intramembranous ossification participate in bony compartment formation. There is variation in gradients of density in different AHOOO structures. These gradients, which vary according to function and species, primarily reduce mechanical stress. Anchorage of AHOOOs to their surrounding tissues fortifies these structures and is accomplished by bone–bone fusion and Sharpey fibres. The presence of the integument is essential for the protection and function of the bony compartments. Three major functions can be attributed to AHOOOs: mechanical, visual, and thermoregulatory. This review provides the first extensive comparative description of the skeletal and integumentary systems of AHOOOs in a variety of mammals.

Unexpected cranial sexual dimorphism in the tragulid Dorcatherium naui based on material from the middle to late Miocene localities of Eppelsheim and Hammerschmiede (Germany)

“…The bulges show a rough surface, while the rest of the skull roof has a smooth surface texture ( Fig 9 ) as in some types of headgear known from Pecora (e.g. [ 53 , 58 , 59 ]). Moreover, the results of the μCT-analysis of the male D .…”

Osteohistology of the hyperelongate hemispinous processes of Amargasaurus cazaui (Dinosauria: Sauropoda): Implications for soft tissue reconstruction and functional significance

“…The absence of a keratinized horn sheath covering the hemispinous processes casts doubt towards the interpretation of the hyperelongated hemispinous processes acting as a defensive structure against predation attacks (Apesteguía et al, 2019; Gallina et al, 2019; Salgado & Bonaparte, 1991). Furthermore, the hemispinous processes lack internal sinus or abundant cancellous bone typical from bovid horn cores, which increases the mechanical resistance and act as shock absorbers in these structures (Farke, 2008; Nasoori, 2020). Additionally, as noted above, a pronounced ‘lip’ or bony overgrowth probably acting as a reinforcement of this critical section, is also absent.…”

Hibernating bear serum hinders osteoclastogenesis in-vitro

“…Such differences might be due to species-specific osteogenic alterations, which have also been reported in other mammals. For instance, a transient physiological osteoporosis due to a rapid antler osteogenesis may occur in cervids, which is a distinct feature in comparison with other artiodactyls [ 67 ]. Hence, comprehensive and comparative studies on mammalian physiological bone preserving/ or osteoporotic mechanisms are needed to provide insights into mammalian bone biology.…”