Meckel's cartilage in Xenopus laevis during metamorphosis : a study of the morphological and cellular changes taking place in Meckel's cartilage during metamorphosis in Xenopus laevis (Daudin)
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This thesis presents an analysis of the normal pattern of changes which occur in Meckel's cartilage in Xenopus laevis during metamorphosis. The investigation has concentrated on morphological as well as microscopic and ultrastructural changes which take place. A method for the preparation of the very fragile premetamorphic cartilage for electron microscopy has been developed. Some of the changes occurring in the cartilage were quantified using stereological techniques. Appearance During metamorphosis Meckel's cartilage changes from a shallowly curved bar to a deeper "U" shape. During this alteration, the cartilage changes from a structure with little intercellular matrix to a tissue in which both matrix and cellular components have increased considerably. Cell population During the early part of metamorphic climax there is a slow increase in the number of cells. A dramatic increase in the cell population occurs in the middle of the period, followed by a levelling off in the rate of cell divisions towards the end. Metamorphic climax has therefore been described in these three phases in this study. These changes have been quantified by carrying out counts on the whole cartilage and methods of analysing such counts have been discussed. Matrix production The increase in the amount of matrix present is considerable, and largely accounts for the different appearances before and after metamorphic climax. This increase occurs in the later phase of climax, after the phase of maximal cell division has taken place. This change has been quantified using a point counting technique, and the analysis of data from such techniques has been described. Sites of cell division The areas where most cell divisions are occurring during metamorphic climax appear to be mainly in the immediate subperichondrial zones of the tissue. This has been investigated by examining sections for mitotic figures and by treating tadpoles during the period of maximal cell division with colchicine. A suitable method of using the drug for stathmokinesis during metamorphic climax has been described. In addition to this investigation of normal metamorphic changes, a preliminary study of the effect on Meckel's cartilage of exposing tadpoles to thyroid hormone at different times during metamorphic climax has also been undertaken, indicating that the hormone appears to act at a specific time during climax. At attempt has been made to correlate the changes of shape and internal architecture of the cartilage with the changing feeding habit of the animals.