Whereas the radiologic features of achondroplasia-the most common type of skeletal dysplasia in adults-are clearly defined, there is still some debate about the severity and the type of histologic changes. Earlier descriptions reported severe disturbance of enchondral ossification to be typical of achondroplasia: They are, however, misleading by dealing mostly with cases of lethal neonatal dwarfism (e.g. thanatophoric dysplasia). Newer findings confirmed that only minor, quantitative lesions are typical of heterozygous achondroplasia. But even in recent years some observers noted more severe changes of enchondral ossification. An extensive histologic skeletal survey in a newborn achondroplastic male revealed remarkable findings: It clearly showed that in addition to a generalized, but moderate narrowing of the zones of enchondral ossification focal severe changes were present in various epiphyseal plates including clusterlike arrangement of enlarged chondrocytes, vacuolization, premature calcification and important fibrosis of cartilagineous matrix with membranous ossification. Our findings thus enable us to reconcile the seemingly divergent statements made before.
In a more recent article in Hum Mol Genet. 2012 Jun: ” An activating FGFR3 mutation affects trabecular bone formation via a paracrine mechanism during growth” by Mugniery E, Dacquin R, Marty C, Benoist-Lasselin C, de Vernejoul MC, Jurdic P, Munnich A, Geoffroy V, Legeai-Mallet L. In the abstract you can read:
“The fibroblast growth factor receptor 3 (FGFR3) plays a critical role in the regulation of endochondral ossification. Fgfr3 gain-of-function mutations cause achondroplasia, the most common form of dwarfism, and a spectrum of chondrodysplasias. Despite a significant number of studies on the role of FGFR3 in cartilage, to date, none has investigated the influence of Fgfr3-mediated effects of the growth plate on bone formation. We studied three mouse models, each expressing Fgfr3 mutation either ubiquitously (CMV-Fgfr3(Y367C/+)), in chondrocytes (Col II-Fgfr3(Y367C/+)) or in mature osteoblasts (Col I-Fgfr3(Y367C/+)). Interestingly, we demonstrated that dwarfism with a significant defect in bone formation during growth was only observed in mouse models expressing mutant Fgfr3 in the cartilage. We observed a dramatic reduction in cartilage matrix mineralization and a strong defect of primary spongiosa. Anomalies of primary spongiosa were associated with an increase in osteoclast recruitment and a defect of osteoblasts at the mineralization front. A significant decrease in bone volume, trabecular thickness and number was also observed in the trabecular bone. Interestingly, no anomalies in proliferation and differentiation of primary osteoblasts from CMV-Fgfr3(Y367C/+) mice were observed. Based on these data, we excluded a potential function of Fgfr3 directly on osteoblasts at 3 weeks of age and we obtained evidence that the disorganization of the growth plate is responsible for the anomalies of the trabecular bone during bone formation. Herein, we propose that impaired FGFR3 signaling pathways may affect trabecular bone formation via a paracrine mechanism during growth. These results redefine our understanding of endochondralossification in FGFR3-related chondrodysplasias”