TIME OF ORIGIN AND PATTERN OF SURVIVAL OF NEURONS IN ISTHMO-OPTIC NUCLEUS OF CHICK

The time of origin of the cells in the isthmo-optic nucleus (ION; the nucleus of origin of centrifugal fibers to the avian retina) was determined in the chick by a variant of the cumulative labeling method, using 3H-thymidine autoradiography. All the neurons of the ION were generated (i.e., pass through their last phase of DNA synthesis) over a 50-h period between the latter part of the 5th and the 7th days of incubation (stages 28-31 of the Hamburger and Hamilton series) but the cells came to be assembled within the nucleus along a distinct temporo-spatial gradient. The earliest-formed cells occupied the ventrolateral part of the nucleus, while the last neurons to be generated came to lie along its dorsomedial margin. When the nucleus was numerically complete, around the 11th day of incubation, it contained about 22,000 neurons, but between the 13th and 17th days (stages 39-43) this number was reduced by nearly 60% to about 9500 cells. Following the radical extirpation of 1 optic cup or the circumscribed removal of the neural retina early on the 3rd day of incubation, the cell loss in the contralateral ION was greatly accentuated, so that by the 18th day of incubation no cells remained in the nucleus. Serial counts of the numbers of cells in the nucleus on the side of the eye (or retinal) removals showed that it, too, underwent additional cell degeneration, so that by the end of the phase of naturally occurring neuronal loss, only about half the normal number of cells persist in these experimental animals.