An exact miniature model of the skeleton develops in the fetus from hyaline cartilage. The cartilage model grows within condensations of embryonic mesenchyme, a tissue that is derived from the sclerotome of the somites. Once the cartilage skeleton is formed it is replaced by immature bone in a process called endochondral ossification. This process of ossifying the cartilage model occurs throughout a new bone except for a thin plate close to each epiphysis (the end of a bone). It is here that our bones retain the ability to grow in length. The cartilage cells in this ‘epiphyseal growth plate’ constantly divide, grow, die and are replaced by bone in a finely balanced and co-ordinated manner that allows our bones to elongate. This process continues until puberty hits. At puberty, the rate of cartilage growth in these plates slows but the rate of ossification remains the same. This shift in balance causes the plates to fuse. As a result, our bones can no longer grow in length and we achieve our adult height.
(a developing fetal arm, rat, Mallory-Azan stained)