Day 14: The Skeleton

Development of Bone

Membrane bones:

There are two main types of bones: membrane bones and cartilage bones. Membranous bone is mostly found in the flat bones of the face and skull. During the first few hours of development, groups of mesodermal cells, called mesenchyme, migrate from the body of the embryo to the head region. There, the mesenchymal cells form a mesh into which bone can be laid. Some of the mesenchymal cells then differentiate into osteoblasts and begin to fill the mesh with bone. Once surrounded by bone, the osteoblasts mature into osteocytes which are responsible for nourishing and maintaining the bone. As the skull grows, it is constantly being remodeled by osteoclasts which remove bone and osteoblasts which form new bone to shape the skull.

The process of laying down bone into a pre-existing framework whether the framework it is mesenchyme or cartilage - is called ossification. Centers of ossification appear in individual embryonic bones at differing times depending on the animal. This photograph shows the skull of a 40-day old cat fetus. Calcium is stained red to show the areas of ossification. In most bones ossification begins at the center of a bone and spreads toward its outer edges. This pattern allows the joints or sutures - outer boundaries of the bone where one bone articulates with another - to adapt as the bones and soft tissue organs grow and continually change shape.

In chick embryos, the portion of the skull composed of membrane bone begins to ossify between days 9 and 14.

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