Histological study of the developing pterygoid process of the fetal mouse sphenoid

The pterygoid process undergoes ossification of both the cartilage and membrane. However, few studies have attempted to explore the sequential development of the pterygoid process. Using histological examination, we performed morphological observations of the pterygoid process and surrounding tissue. ICR mice at embryonic days 13.5–18.0 and postnatal day 0 were used for morphological observations of the pterygoid process. By embryonic day 14.5, a mesenchymal cell condensation forming the anlage of the future medial pterygoid process differentiated into osteoid-like tissue and cartilage. At embryonic days 15.5–16.5, cartilage cells were clearly evident in the medial pterygoid process. In the medial pterygoid process, a bone collar was evident and calcified bone tissue surrounded the cartilage. At this point, a mesenchymal cell condensation formed the anlage of the pterygoid hamulus. At embryonic days 17.0–18.0, the cartilages were located along the lower and posterior border of the medial pterygoid process. A metachromatically stained matrix first became detectable around cells located in the pterygoid hamulus. On the other hand, at embryonic day 13.5, a metachromatically stained matrix was already evident in the space between the flattened cells in the lateral pterygoid process. At embryonic day 17.0, a hypertrophic cell zone had clearly formed in the diaphysis. On the basis of our present investigation, the lateral pterygoid process can be classified as primary cartilage, whereas the medial pterygoid process can be classified as secondary cartilage. Furthermore, it was found that the pterygoid hamulus is formed latest in the medial pterygoid process.