Bone histology

Bone tissue formation

Please take into account that unlike most organ systems that complete organogenesis during the antenatal period, skeletal development is spread out over the gestational period and continues into extra-uterine life. Bone is derived from three embryonic sources. The neurocranium and the viscerocranium originate from derivatives of the neural crest cells as well as paraxial mesoderm. The paraxial mesoderm also contributes to the formation of the axial skeleton, while the appendicular skeleton originates from the lateral plate mesoderm.

The so-called flat bones of the body such as calvaria, mandible, maxilla, etc. and long bones such as those of the limbs, are formed by two different processes. The former originates by way of intramembranous ossification, while the latter undergoes endochondral ossification. The initiation of either process depends on the differentiation of the preceding mesenchymal cell line. If the mesenchymal cells differentiate into chondrocytes, then endochondral ossification will occur. However, should the mesenchymal cells differentiate into osteoblasts intramembranous ossification would ensue. 

Irrespective of the pathway taken, ossification begins around the 6th or 7th gestational week and persists well into extra-uterine life; the clavicle can take up to 20 – 21 years for complete fusion to occur, and 26 years for the epiphyseal scar to disappear. Extra-uterine bone development has been classified into 5 stages.

• In stage 1, the epiphysis is not yet ossified.
• Once ossification becomes apparent in the epiphyses, then the bone is in stage 2 of development.
• At the point where the epiphyses and diaphysis begin to fuse, then the bone has entered stage 3.
• Stage 4 represents complete fusion of the epiphyses and diaphysis, leaving behind an epiphyseal scar at the site of the epiphyseal growth plate.
• The final stage is characterized by disappearance of the epiphyseal scar.

Endochondral ossification relies on an analgen in the form of hyaline cartilage laid down during embryogenesis. Initially, a hyaline cartilaginous framework is laid down as a template for osteogenesis. It is encased by a perichondrial layer that comprises of a condensed vascular mesenchyme. The model grows by both interstitial (replication of chondrocytes and secretion of new matrix) and appositional (absorption of old cartilage and deposition of new matrix) methods. The chondrocytes (cartilaginous cells) in the mid shaft of the cartilaginous template (diaphysis) begin to replicate and hypertrophy. An increase in the number of vacuoles can be observed in the cytoplasm in this phase. Subsequently, the matrix is compressed, forming thin fenestrated septae. The cartilage model subsequently calcifies, resulting in decreased diffusion of nutrients to the cells. They eventually degenerate, die, and calcify; leaving confluent lacunae in their absence. 

As the cartilage calcifies, the inner layer of the perichondrium begin to express osteogenic (i.e. bone forming) properties; and thus become osteoblasts. Osteoblasts are responsible for production of bone matrix; they eventually produce a bony collar around the diaphysis called the periosteal collar. The connective tissue superficial to the periosteal collar is subsequently referred to as the periosteum. The visceral periosteum contains mesenchyme cells that evolve into osteoprogenitor cells. This cell line replicates and further differentiates into osteoblasts. These cells travel with osteogenic buds, which are terminal capillary sprouts.

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The osteoclasts break down previously formed bone and as a result, facilitate the breakdown of calcified cartilage to allow invasion of osteoblasts (that will lay down new bone matrix) and osteogenic buds (to establish nutrient supply to the developing bone). The inner surface of bone (i.e. endosteum) that lines all bony cavitation is also covered by a single layer of osteoprogenitor cells that provides a supply of stem cells for future differentiation.

The bony development occurring in the diaphysis is referred to as the primary ossification centre. However, as the bones continue to elongate and increase in diameter, a secondary ossification centre develops in the epiphyses (distal articular ends) of the long bones. The secondary ossification centre is also invaded by a vascular supply and mesenchymal derivatives similar to those present in the primary ossification centre.

At both the primary and secondary ossification centres, cartilage is replaced by bone. However, there is a region where cartilage is preserved, known as the epiphyseal growth plate. The bone continues to grow by appositional and interstitial mechanisms at this region until the ideal length is achieved. Fusion of the epiphyses with the diaphysis marks the cessation of bone growth. At this point, the only remnant of hyaline cartilage is found at the articulating surfaces of the bone.

Unlike endochondral ossification, the intramembranous ossification pathway does not require a cartilaginous scaffold. Instead, the bone is formed within primitive mesenchymal layers that have rich blood supplies. The stem cells within the mesenchyme differentiate into osteoprogenitor cells that replicate adjacent to capillary beds. The end result is scattered layers of osteoblasts producing bone matrix. Consequently, there are multiple ossification centres observed in intramembranous ossification. The osteoblasts are described as being polarized cells, owing to the fact that osteoid secretion occurs at the surface furthest away from the blood supply. The ossification centres subsequently anastomose, leaving a woven pattern of trabeculae, referred to as primary spongiosa or spongy bone.

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