In the avian embryo ectomesenchyme cells, derived from the mesencephalic level of the cranial neural crest, migrate into the presumptive maxillary region and subsequently differentiateinto the membrane bones and associated secondary cartilage of the upper jaw skeleton. The cartilage arises secondarily within the periosteum at points of articulation between membrane bones and provides an embryonic articulating surface. The stimulus for the differentiation of secondary cartilage is believed to be intermittent pressure and shear created at the developing embryonic movement. The development of one such system - the quadratojugal, has been analysed using organ and explant culture techniques and studied with particular reference to the differentiation of periosteal cells into secondary cartilage. A number of conclusions were reached.

(1) Normally only cells at discrete loci express a chondrogenic potential ,in vivo: the periosteal cells at these sites of future articulation become committed to chondrogenesis during stage 35, more than 24 h before cartilage is identifiable ,in vivo.

(2) However, cells with a ‘latent’ chondrogenic potential are widespread in membrane bone periosteum and occur over most, if not all, of the surface area of the bone. This potential is expressed in the ‘permissive’ environment created by submersion of the tissue in explant culture or in submerged organ culture.

(3) This chondrogenic potential exists long before the time at which commitment of cartilage-forming cells occurs and even presumptive maxillary ectomesenchyme at stage 29 has a limited ability to form cartilage ,in vitro.

It is suggested that spatial position is a principal factor controlling the differentiation of secondary cartilage. Ectomesenchyme cells with the potential to form secondary cartilage are widespread but it is only those cells whose migration from the neural crest positions them and their progeny at the site of a presumptive joint which subsequently express this potential. This epigenetic interpretation is discussed in the general context of development mechanisms underlying the spatial and temporal patterns in which neural crest-derived cells differentiate to produce bone and cartilage during the formation of the head skeleton.

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