Formation of Bone

Formation of Bone

Bones are formed by two mechanisms: intramembranous ossification (bones of the skull, part of the mandible and clavicle) or endochondral ossification.

Intramembranous Ossification

Intramembranous ossification occurs within a membranous, condensed plate of mesenchymal cells. At the initial site of ossification (ossification centre) mesenchymal cells (osteoprogenitor cells) differentiate into osteoblasts. The osteoblasts begin to deposit the organic bone matrix, the osteoid. The matrix separates osteoblasts, which, from now on, are located in lacunae within the matrix. The collagen fibres of the osteoid form a woven network without a preferred orientation, and lamellae are not present at this stage. Because of the lack of a preferred orientation of the collagen fibres in the matrix, this type of bone is also called woven bone. The osteoid calcifies leading to the formation of primitive trabecular bone.

Further deposition and calcification of osteoid at sites where compact bone is needed leads to the formation of primitive compact bone.

Note the distinction between macroscopic and microscopic appearance when the bone is named. We again have the two macroscopically different forms of bone - trabecular bone and compact bone - but their early developmental ("primitive") forms consist of woven bone.

Through subsequent reorganisation the primitive compact and trabecular bone is converted into mature compact and trabecular bone. During reorganisation and growth, woven bone will, in time, be replaced by lamellar bone.

Intramembranous ossification does not require the existence of a cartilage bone model.

Suitable Slides

sections of the developing mandible (or some other bones of the skull) or clavicle - H&E, van Gieson

Sections prepared to show endochondral ossification (see below) may be an alternative if no specifically prepared slides of intramembranous ossification are available. The periosteal collar, i.e. the manchette of bone forming around the diaphysis of the cartilage model of the bone, is formed by a mechanism similar to intramembranous ossification and results in the deposition of woven bone.

Mandible, intramembranous ossification - H&E

The developing bone will in sections usually be associated with a number of other tissues which develop in close association with it. In case of the mandible, there can be developing teeth, the tongue, skin and salivary glands.
The first job - best done at low magnification - is to find the developing bone. It should look like a coarse meshwork (trabecular bone) of pink tissue surrounding patches of much lighter or unstained tissue. Lamellae are not visible (woven bone) and the lacunae are larger than lacunae in mature bone. Ossification centres appear as areas of a gradual transition from connective tissue to bone. Light, pinkish bone matrix is found between the osteoblasts.

Depending on the state of development of the bone, it is occasionally possible to find bone trabeculae which are lined by a layer of osteoblasts. These osteblasts are depositing the first lamellae on the already existing trabeculae. The trabeculae will therefore have a core of woven bone, which is surrounded by lamellar bone. Compare the shapes, sizes and frequencies of lacunae in lamellar and woven bone if both types of bone are present.