Two theories for the evolution of the articular cavity have long been disputed: the first is that the articular cavity originates from the liquefaction of tissue, and the second is that it is a result of dehiscence of tissue. We investigated the development of the articular cavity using chick embryos.
The study included 24 embryos in all, of which one each was incubated every 6hours from a 6-day,0-hour embryo to an 11-day,18-hour embryo. Their heads were embedded in paraffin and sectioned serially at 10μm nearly parallel to the upper surface of the lower beak.
In the 6-day,0-hour embryo, the anlage of the quadrate-mandible articulation had already formed. In the 6-day,18-hour embryo, prominent changes were recognized in the cluster of mesenchymal cells which lay between the two cartilages composing the articulation; in this region, a primitive articular cavity, i. e. an apparent tendency for two cells to separate, was recognized in several places, and moreover, the cells facing this primitive articular cavity formed an ellipse with the long axis orientated perpendicularly to the line linking the two cartilages. The nuclei of these cells were large and the cells had little cytoplasm. These primitive articular cavities enlarged and fused as time passed, and the cells gradually became long and slender and their shape changed from an ellipse to a spindle. In the 8day,18-hour embryo, a cell layer stripped off from the mesenchymal cells was seen in the articular aperture. In the 10-day,0-hour embryo, one part of it separated from the mesenchymal cells, and clusters of cells which became isolated in this articular aperture were recognized. At this stage, no liquefaction of tissue could be recognized.
Thus, based on our findings, no liquefaction of tissue was seen in the process of articular cavity formation, and it was assumed that the articular cavity originated by the dehiscence of tissue.