Prenatal developmental process of human temporomandibular joint.

The prenatal development of 20 human TMJs from the fourteenth gestational week to full term was studied and the following observations were made. Radiologic findings: at the sixteenth gestational week, the outline of the TMJ as radioopaque structures appeared. Curvature of the articular eminence of the temporal components was observed at gestational week 33, and convexity of the condylar head of the condylar process was observed at the thirty-first week. Histologic findings: at the fifteenth gestational week, thin intramembranous ossification of the temporal components and endochondral ossification of the condylar process were observed, and secondary cartilage occupied the whole condylar process. The articular disk was distinguishable and was composed of fine collagen fibers. At the seventeenth gestational week, endochondral ossification of the condylar process appeared, the formation of joint cavities was fairly completed, and synovial tissues were easily observed. From gestational week 17 through 21, endochondral ossification of the condylar process developed rapidly and the layer structures, which consisted of a fibrous covering layer, transitional cell layer, hypertrophic layer, and erosive layer, were observed. At the twenty-first gestational week, Meckel's cartilage disappeared and hematopoietic foci appeared in the temporal components and the condylar process. Week 21 seemed to be the greatest turning point of the prenatal development of the human TMJ. Secondary cartilage of the condylar process diminished gradually throughout fetal life, but was vestigial at full term.(ABSTRACT TRUNCATED AT 250 WORDS)