OSTEOGENESIS

Bone formation (osteogenesis or ossification)

Bone develops by replacement of a preexisting connective tissue. The two processes of bone formation or osteogenesis observed in the embryo are: (1) intramembranous bone formation, in which bone tissue is laid down directly in primitive connective tissue or mesenchyme (Figures 5-1 and 5-2), and (2) endochondral bone formation, in which bone tissue replaces a preexisting hyaline cartilage, the template or anlage of the future bone (Figures 5-3 to 5-5).

Figure 5-1 Intramembranous ossification

Figure 5-2 Intramembranous ossification

Figure 5-3 Endochondral ossification: Primary ossification center

Figure 5-4 Endochondral ossification: Secondary ossification centers

Figure 5-5 Endochondral ossification: Four major zones

The mechanism of bone matrix deposition during intramembranous and endochondral ossification is essentially the same: A primary trabecular network or primary spongiosa is first laid down and then transformed into mature bone. But there is a difference: In endochondral ossification, cartilage is replaced by bone matrix.

Intramembranous bone formation

Membrane bones such as the flat bones of the skull develop by intramembranous ossification. Intramembranous ossification occurs in the following sequence (see Figure 5-1):

Osteocytes remain connected to each other by cytoplasmic processes enclosed within canaliculi, and new osteoblasts are generated from osteoprogenitor cells adjacent to the blood vessels.

The final developmental events include:

At birth, bone development is not complete, and the bones of the skull are separated by spaces (fontanelles) housing osteogenic tissue. The bones of a young child contain both woven and lamellar bony matrix.

Endochondral ossification

Endochondral ossification is the process by which skeletal cartilage templates are replaced by bone. As you may recall, intramembranous ossification is the process by which a skeletal mesenchymal template is replaced by bone without passing through the cartilage stage. Bones of the extremities, vertebral column, and pelvis derive from a hyaline cartilage template.

As in intramembranous ossification, a primary ossification center is formed during endochondral ossification (Figure 5-3). Unlike intramembranous ossification, this center of ossification derives from proliferated chondrocytes that have deposited an extracellular matrix containing type II collagen.

Shortly thereafter, chondrocytes in the central region of the cartilage undergo maturation to hypertrophy and synthesize a matrix containing type X collagen, a marker for hypertrophic chondrocytes. Angiogenic factors secreted by hypertrophic chondrocytes (vascular endothelial cell growth factor [VEGF]) induce the formation of blood vessels from the perichondrium. Osteoprogenitor and hematopoietic cells arrive with the newly formed blood vessels.

These events result in the formation of the primary ossification center. Hypertrophic chondrocytes undergo apoptosis as calcification of the matrix in the middle of the shaft of the cartilage template takes place.

At the same time, the inner perichondrial cells exhibit their osteogenic potential, and a thin periosteal collar of bone is formed around the midpoint of the shaft, the diaphysis. Consequently, the primary ossification center ends up located inside a cylinder of bone. The periosteal collar formed under the periosteum by intramembranous ossification consists of woven bone. As we will discuss later on, the periosteal collar is converted into compact bone.

The following sequence of events defines the next steps of endochondral ossification (Figure 5-4):

The growth in length of the long bones depends on the interstitial growth of the hyaline cartilage while the center of the cartilage is being replaced by bone at the equidistant zones of ossification.

Secondary centers of ossification and the epiphyseal growth plate

Up to this point, we have analyzed the development of primary centers of ossification in the diaphysis of long bones that occurs by the third month of fetal life.

After birth, secondary centers of ossification develop in the epiphyses (see Figure 5-4). As in the diaphysis, the space occupied by hypertrophic chondrocytes is invaded by blood vessels and osteoprogenitor cells from the perichondrium. Most of the hyaline cartilage of the epiphyses is replaced by the spongy bone, except for the articular cartilage and a thin disk, the epiphyseal growth plate, located between the epiphyses and the diaphysis. The epiphyseal growth plate is responsible for subsequent growth in length of the bone.

Clinical significance: The epiphyseal growth plate and dwarfism

Indian hedgehog (Ihh), a member of the hedgehog family of proteins secreted by chondrocytes, regulates chondrocyte proliferation of the growth plate in a paracrine fashion and delays chondrocyte hypertrophy (see Figure 5-9). Ihh also regulates bone formation in the perichondrial collar. A lack of expression of Ihh protein in mutant mice results in dwarfism and lack of endochondral ossification. Essentially, Ihh maintains the pool of proliferating chondrocytes in the growth plate by delaying their hypertrophy. In addition, Ihh stimulates the expression of parathyroid hormone-related peptide (PTH-RP) in perichondrial chondrocytes adjacent to the articular surface. A feedback loop between Ihh and PTH-RP regulates the balance between proliferating and hypertrophic chondrocytes.

Figure 5-6 Endochondral ossification: Zones of proliferation, hypertrophy, and vascular invasion

Figure 5-7 Endochondral ossification: Zones of proliferation and hypertrophy

Figure 5-8 Endochondral ossification: Zones of hypertrophy and vascular invasion

Figure 5-9 Growth plates and bone growth in length

At the end of the growing period, the epiphyseal growth plate is gradually eliminated by a continuum established between the diaphysis and the epiphyses. No further growth in length of the bone is possible once the epiphyseal growth plate disappears at puberty.

Zones of endochondral ossification

As we have seen, the deposition of bone in the center of the diaphysis is preceded by an erosion process in the hyaline cartilage template (see Figure 5-4). This center of erosion, defined as the primary ossification center, extends in both directions of the template, in parallel with the formation of a bony collar.

The bony collar provides strength to the midsection of the diaphysis or shaft as the cartilage is weakened by the gradual removal of the cartilage before its replacement by bone.

The continuing process of cartilage erosion and bone deposition can be visualized histologically (Figure 5-5). Four major zones can be distinguished, starting at the end of the cartilage and approaching the zone of erosion: