Investigations on myelinogenesis in vitro: a study of the critical period at which thyroid hormone exerts its maximum regulatory effect on the developmental expression of two myelin associated markers in cultured brain cells from embryonic mice.

Cultures of cells dissociated from embryonic mouse brain were used to assess the period in which thyroid hormone exerts its maximum influence on the regulation of the expression of two myelin associated metabolites, sulfolipids and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP-ase). Cultures were grown for a specified number of days on a medium containing normal calf serum and then a portion were switched to a medium containing hypothyroid calf serum for 2 days. One half of these cultures were then supplemented with 50 nM triiodothyronine and growth was continued in all cultures for 3 more days. The cells were then assayed for CNP-ase activity and for their ability to incorporate 35SO4 into sulfolipids. Studies with both myelin markers showed that in the earlier culture ages of 5, 8, and 11 days, thyroid hormone was able to fully restore the activities when added to cultures grown on hypothyroid calf-serum. In contrast, in the intermediate age range (15, 19, and 22 days) the restoration was partial, while in the higher ages, there was practically negligible restoration with T3. Since the culture system eliminates the possibility of a blood brain barrier and drastically decreases the complicity of other hormones, the lack of a myelinogenic response to thyroid hormone after a certain age must be attributed to the loss of sensitivity of the oligodendroglia to T3 possibly through genetic programming.