In intramembranous ossification, bone is formed by differentiation of mesenchymal cells into osteoblasts.
The first evidence of intramembranous ossification is seen around the eighth week of gestation in humans. Some of the pale-staining, elongated mesenchymal cells within the mesenchyme migrate and aggregate in specific areas, the sites where bone is destined to form. This condensation of cells within the mesenchymal tissue initiates the process of intramembranous ossification (Fig. 8.16 and Plate 15, page 252). Mesenchymal cells then differentiate into osteoprogenitor cells expressing Cbfa1 transcription factor. This transcription factor is essential for osteoblast differentiation and expression of genes necessary for both intramembranous and endochondral ossification. As the process continues, the newly organized tissue at the presumptive bone site becomes more vascularized, and the aggregated mesenchymal cells become larger and rounded. The cytoplasm of osteoprogenitor cells changes from eosinophilic to basophilic, and a clear Golgi area becomes evident. These cytologic changes result in the differentiated osteoblast, which then secretes the collagens (mainly type I collagen molecules), bone sialoproteins, osteocalcin, and other components of the bone matrix (osteoid). The osteoblasts within the bone matrix become increasingly separated from one another as the matrix is produced, but they remain attached by thin cytoplasmic processes. Because of the abundant collagen content, the bone matrix appears denser than the surrounding mesenchyme, in which the intercellular spaces reveal only delicate connective tissue fibers.
 FOLDER 8.3 Clinical Correlation: Nutritional Factors in Bone Formation
Both nutritional and hormonal factors affect the degree of bone mineralization. Calcium deficiency during growth causes rickets, a condition in which the bone matrix does not calcify normally. Rickets may be caused by insufficient amounts of dietary calcium or insufficient vitamin D (a steroid prohormone), which is needed for absorption of calcium by the intestines. An X-ray of the child with ad-vanced rickets presents classic radiological symptoms: bowed lower limbs (outward curve of long bones of the leg and thighs) and a deformed chest and skull (often having a distinctive “square” appearance). If rickets is not treated while children are still growing, then skeletal deformities and short stature may be permanent. In adults, the same nutritional or vitamin deficiency leads to osteomalacia. Although rickets and osteomalacia are no longer major health problems in populations where nutrition is adequate, it is among the most frequent childhood diseases in many developing countries. In addition to its influence on intestinal absorption of calcium, vitamin D is also needed for normal calcification. Other vitamins known to affect bone are vitamins A and C. Vitamin A deficiency suppresses endochondral growth of bone; vitamin A excess leads to fragility and subsequent frac-tures of long bones. Vitamin C is essential for synthesis of collagen, and its deficiency leads to scurvy. The matrix pro-duced in scurvy is not calcifiable. Another form of insufficient bone mineralization often seen in postmenopausal women is the condition known as osteoporosis (see Folder 8.2).