Schwann cell proliferation in the sciatic nerve of hypothyroid chick embryos studied by autoradiography and image analysis.

The proliferation of Schwann cells in the sciatic nerve of chick was studied from day 11 to day 27 of development in control and thyroid-deficient embryos. Hypothyroidism was induced by tetramethylthiourea injection on days 8 and 19 of incubation. The parameters of the cell cycle were determined using autoradiographs (tritiated thymidine) and by image analysis of Feulgen-stained nuclear smears. The duration of the cell cycle was lengthened and the growth fraction was reduced in hypothyroid animals, at 11 and 15 days of incubation. At later stages (days 21 and 27), these parameters were not significantly different from the controls as if the sensitivity of Schwann cells to thyroid hormones was scheduled to occur during a limited period of development. The total number of axons was the same in control and hypothyroid animals suggesting that the slowing down of Schwann cell proliferation is not a consequence of neuronal cell death. The consequence of that slowing down is a delay in the isolation of promyelin axons and a reduction in the proportion of myelinated axons at all the stages studied.