Ontogenetic changes of geometrical and mechanical characteristics of the avian femur: a comparison between precocial and altricial birds

Effects of Development on Bone Mineral Density and Mechanical Properties in the Aquatic Frog, Xenopus Laevis, and a Terrestrial Frog, Lithobates Catesbianus

The limb bones of vertebrates have a critical role in supporting the weight of the body and transmitting forces that power locomotion. The loads that limb bones experience can vary in association with a range of factors, including locomotor environment or developmental stage. Limbed vertebrates that are habitually found in environments with low locomotor loads (e.g., water) might be predicted to also exhibit limb bones with less elevated mechanical properties, such as yield stiffness and yield stress. Frogs provide a distinctive case, in which these ideas can be tested as they experience changes in both locomotor style and habitat as they develop. However, while many frog taxa shift from aquatic to terrestrial habitats as they metamorphose, some lineages, such as pipids, maintain an aquatic lifestyle even after metamorphosis, providing a comparative framework for the effects of habitat shifts on developing limbs in vertebrates. This study compares the material composition and mechanical properties of the femur between frog species that are aquatic specialists (Xenopus laevis) vs generalists that spend considerable time both on land and in water (Lithobates catesbeianus) as they transition from metamorphic tadpoles to fully grown adults. MicroCT scanning was used to determine changes in bone density related to developmental stage and hindlimb use during swimming. Microindentation was then used to collect hardness values from the cortical bone of each femur, which was used to evaluate bone material properties. We found that aquatic frogs had less overall bone mineral density (BMD) than terrestrial frogs and that BMD was more elevated in the cortical region of the diaphysis than trabeculae and distal and proximal epiphyses. Despite its less elevated BMD, bone mechanical properties were not significantly different in aquatic specialist X. laevis than in more terrestrial L. catesbeianus. Our results suggest that the limb bones of aquatic frogs may experience compensatory effects through development to offset their lower BMD. Furthermore, changes in bone density and material properties across development may help to explain some of the differences in locomotor performance found between aquatic and terrestrial metamorphic frogs, providing insight into how environmental factors might correlate with bone ossification.

Developmental factors influencing bone strength in precocial mammals: An infant pig model

Most vertebrates are precocial in locomotion, able to walk and run soon after birth. Precociality requires a bony skeleton of sufficient strength to resist mechanical loading during early locomotor efforts. The aim of this study was to use an animal model-the preterm infant pig-to investigate some of the proximate factors that might determine variation in bone strength in precocial animals. Based on the prior literature, we tested the null predictions that skeletal integrity would be significantly compromised by truncated gestation (i.e., preterm birth) and reduced body mass at birth. We generated a suite of both morphometric measures (tissue mineral density and cross-sectional geometry) and performance-related metrics (ability to resist loading, deformation, and fracture during three-point bending tests) of the appendicular skeleton of preterm and full-term infant pigs. Results showed that very few measures in our ontogenetic infant pig sample significantly varied with either gestation length or birth mass. Overall, our results contribute to a growing body of literature demonstrating the early functional capacity of the precocial infant musculoskeletal system and suggest that bone strength in perinatal precocial mammals may be robust to the factors shown to compromise skeletal integrity in more altricial taxa.

Developmental Transcriptome Profiling of the Tibial Reveals the Underlying Molecular Basis for Why Newly Hatched Quails Can Walk While Newly Hatched Pigeons Cannot

Birds can be classified into altricial and precocial species. The hatchlings of altricial birds cannot stand, whereas precocial birds can walk and run soon after hatching. It might be owing to the development of the hindlimb bones in the embryo stage, but the molecular regulatory basis underlying the divergence is unclear. To address this issue, we chose the altricial pigeon and the precocial Japanese quail as model animals. The data of tibia weight rate, embryonic skeletal staining, and tibia tissues paraffin section during the embryonic stage showed that the Japanese quail and pigeon have similar skeletal development patterns, but the former had a faster calcification rate. We utilized the comparative transcriptome approach to screen the genes and pathways related to this heterochronism. We separately analyzed the gene expression of tibia tissues of quail and pigeon at two consecutive time points from an inability to stand to be able to stand. There were 2910 differentially expressed genes (DEGs) of quail, and 1635 DEGs of pigeon, respectively. A total of 409 DEGs in common in the quail and pigeon. On the other hand, we compared the gene expression profiles of pigeons and quails at four time points, and screened out eight pairs of expression profiles with similar expression trends but delayed expression in pigeons. By screening the common genes in each pair of expression profiles, we obtained a gene set consisting of 152 genes. A total of 79 genes were shared by the 409 DEGs and the 152 genes. Gene Ontology analysis of these common genes showed that 21 genes including the COL gene family (COL11A1, COL9A3, COL9A1), IHH, MSX2, SFRP1, ATP6V1B1, SRGN, CTHRC1, NOG, and GDF5 involved in the process of endochondral ossification. These genes were the candidate genes for the difference of tibial development between pigeon and quail. This is the first known study on the embryo skeletal staining in pigeon. It provides some new insights for studying skeletal development mechanisms and locomotor ability of altricial and precocial bird species.