# Dynamics of postnatal bone development and epiphyseal synostosis in the caprine autopod

## TL;DR

This study explores how bones in goats develop after birth, showing how movement affects bone structure and fusion in the lower forelimbs.

## Contribution

The study identifies a novel postnatal synostosis event in the caprine metacarpal epiphysis linked to ambulatory loading.

## Key findings

- P1 cortical bone densified rapidly after birth, with continual increases in cortical thickness through adulthood.
- Metacarpal epiphyses undergo postnatal synostosis through fibrocartilaginous endochondral ossification prior to growth plate closure.

## Abstract

Bones develop to structurally balance strength and mobility. Bone developmental dynamics are influenced by whether an animal is ambulatory at birth. Precocial species, which are ambulatory at birth, develop advanced skeletal maturity in utero and experience postnatal development under mechanical loading. Here, we characterized postnatal bone development in the lower forelimbs of precocial goats using microcomputed tomography and histology. Our analysis focused on the two phalanges 1 (P1) bones and the partially fused metacarpal bone of the goat autopod from birth through adulthood.

P1 cortical bone densified rapidly after birth, but cortical thickness increased continually through adulthood. Upon normalization by body mass, the P1 normalized polar moment of inertia was constant over time, suggestive of changes correlating with ambulatory loading. P1 trabecular bone increased in trabecular number and thickness until sexual maturity (12 months), while metacarpal trabeculae grew primarily through trabecular thickening. Unlike prenatal synostosis (i.e., bone fusion) of the metacarpal diaphysis, synostosis of the epiphyses occurred postnatally, prior to growth plate closure, through a unique fibrocartilaginous endochondral ossification.

These findings implicate ambulatory loading in postnatal bone development of precocial goats and identify a novel postnatal synostosis event in the caprine metacarpal epiphysis.

Continuous ambulatory loading on the caprine autopod caused continual increases in development of the phalanges 1 (P1) and metacarpal bones.P1 polar moment of inertia normalized by body mass was constant over time, suggesting P1 bones undergo structural load adaptation.P1 trabecular increased in trabecular number and thickness until skeletal maturity, while metacarpal trabeculae grew primarily through trabecular thickening.Metacarpal bones underwent diaphyseal synostosis (i.e., bone fusion) in utero, but synostosis of the epiphyses occurred postnatally, prior to growth plate closure, through a unique fibrocartilaginous endochondral ossification.

Continuous ambulatory loading on the caprine autopod caused continual increases in development of the phalanges 1 (P1) and metacarpal bones.

P1 polar moment of inertia normalized by body mass was constant over time, suggesting P1 bones undergo structural load adaptation.

P1 trabecular increased in trabecular number and thickness until skeletal maturity, while metacarpal trabeculae grew primarily through trabecular thickening.

Metacarpal bones underwent diaphyseal synostosis (i.e., bone fusion) in utero, but synostosis of the epiphyses occurred postnatally, prior to growth plate closure, through a unique fibrocartilaginous endochondral ossification.