The number of mitotic figures in the optic tectum of the chick increases from just over 20,000 on the fourth day of incubation (stages 25 and 26) to a peak of approximately 45,000 on the fifth and sixth days (stages 28 and 29). Thereafter, the number declines sharply to about 10,000 and 2,000 on the eighth and ninth days respectively and to as few as 100 and 18 on the eleventh and twelfth days. The dividing cells are not uniformly distributed throughout the tectum but show a clear rostral to caudal gradient such that at every stage the number of mitoses in the caudal half of the tectum exceeds that in its rostral half. In addition the number of mitotic figures in the ventrolateral half of the tectum exceeds that in its dorsomedial half between the fourth and seventh days of incubation, but this pattern is reversed during the last few days of mitotic activity.The process of cell proliferation in the optic tectum appears to be completely independent of the developing eye since there is no significant change in either the total number of mitotic figures in the tectum, or in their spatial distribution, after complete removal of the optic vesicle towards the end of the second day of incubation.Interest in mitotic activity in the developing nervous system has been focused principally upon two problems : the mechanism and timing of cellular division within the neural epithelium, and the role of peripheral factors in regulating cell proliferation in the neural tube and its derivatives. The extensive literature dealing with the first of these problems has recently been reviewed in detail by Watterson ('65) and need not be considered here, although it may be pointed out that there is still no generally accepted explanation for the inward movement of the dividing nuclei towards the luminal surface of the neural epithelium, or for the subsequent separation and outward migration of the daughter cells (see Martin, '67). Rather less attention has been paid to the significance of extrinsic factors in the control of mitotic activity in the nervous system. The findings in a number of studies have been quoted in support of the hypothesis that there is a peripheral regulative effect upon cell proliferation but, as Hamburger ('48) has stressed, much of this evidence is rather indirect and inconclusive. For example, in many experimental studies care has not been taken to distinguish clearly J. EXP. ZOOL., 169: 71-92. between the effects of peripheral changes upon mitotic activity per se, and later effects such as disturbances in neuroblast differentiation and growth, or even degenerative changes in differentiated cells. The evidence which is available from studies based upon actual mitotic counts (rather than total cell counts or volumetric measurements) is somewhat conflicting and at present it is by no means clear to what extent the apparent discrepancies reflect real differences between the central and peripheral nervous systems or between different groups of vertebrates.In one of the earliest quantitative studies of ...
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