Chondrogenesis in chick embryo somites in vitro.

During the embryonic development of vertebral cartilages, cells from the somite mesoderm differentiate into chondrocytes around the spinal cord and notochord. Grafting experiments in amphibian and chick embryos have indicated that the spinal cord and notochord have some influence on this differentiation of somite cells to cartilage (Holtzer & Detwiler, 1953; Watterson, Fowler & Fowler, 1954). Further analysis in vitro has established that, under specified culture conditions, cartilage formation from somites is, in fact, dependent on the presence of either spinal cord and/or notochord or their extracts (Grobstein & Parker, 1954; Grobstein & Holtzer, 1955; Strudel, 1962, 1963; Lash, 1963). For example, Lash (1963) showed that in his system on a liquid medium stage 16 (Hamburger & Hamilton, 1951, stages) somites alone formed no cartilage, but that cartilage did develop when notochord or spinal cord was cultured with the somites. This suggested that the spinal cord and notochord were 'inducing' somite mesoderm cells to form chondrocytes. However, more recent observations showed that in some instances cartilage production by somites in culture is independent of 'inducing' tissues, and this independence varies with the conditions. Lash (1964) noted that a percentage of 56 h chick somite explants became able to differentiate without inducer when the liquid medium of the culture system was solidified with agar. Strudel (1963) was able to reduce the minimum age for ' autodifferentiation' to an equivalent of stage 12 (17 somites stage) by enclosing the explant in vitelline membrane. Holtzer's (1964) analysis of the effect of environmental factors on the somites showed that, while the minimum age for differentiation independent of inducers was stage 18 when plasma clots were used, stage 16 gave positives on agar medium, and even stage 14 material could differentiate when the size of the explant was increased from 10 to 30 somites. These findings raise questions concerning the state of the somites on explantation and the role of the interaction between spinal cord and notochord, and the somites in combined cultures. Are the environmental factors which seem to favour cartilage formation in 1 Authors' address: Chester Beatty Research Institute, Institute of Cancer Research, Royal Cancer Hospital, London, S.W.3.