Effect of the brachypod mutation on early stages of chondrogenesis in mouse embryonic hind limbs: An ultrastructural analysis

This paper is an ultrastructural investigation of the early chondrogenic events which occur in the tibia and fibula blastemata of normal and brachypod embryos during 12‐; and 13‐days of development. Transverse sections taken within the central core of the 12‐day normal fibular blastema revealed a characteristic morphology consisting of elongated cells with long cytoplasmic filopodia separated by matrix which contained granules and fibers. In 12‐day brachypod embryos the cells within the fibular blastema portrayed an atypical morphology. The cells were more closely associated, exhibited small spiky microvilli rather than long cytoplasmic filopodia, had more extensive intercellular contacts and were deficient in matrix. The brachypod tibial blastema in embryos of the same age displayed a quasi normal appearance. More matrix was present, filopodia were apparent but shorter than normal and numerous sections of filopodial processes were observed in the intercellular spaces. Fibular regions of 13‐day normal limbs exhibited spacing and morphology similar to 12‐day limbs, but had more matrix material. Cells in the fibular region of 13‐day brachypod limbs presented a normal morphology and arrangement, but matrix granules were still scarce and aggregation of collagen was abnormal. An explanation of how these differences may contribute to the expression of the brachypod phenotype is offered.