Bone marrowstromal cells are requisite for the proliferation of hematopoietic cells and com-municate with each other via gap junctions. Marrowstromal cells expressed connexin 43, but not connexin 32. Hl/A, a murine marrowstromal cell line, underwent adipocyte differentiation at confluence, and expressed the 3.0 kilobase mRNAspecies of connexin 43 before differentiation. Hl/A cells studied with the dye-transfer method showedgap-junctional communication with adjacent cells but lost this communication during differentiation. Theconnexin 43 transcripts in Hl/A cells were down-regulated before the expression of glycero-monophosphate dehydrogenase wasinduced. Loss of gap-junctional communication wasregulated at the mRNA level of connexin 43. Connexin 43 expression was down-regulated at the transcriptional level.

Gapjunctions between cells are present in manytissues and in a wide range of species. Gapjunctions are composedof protein channels that allow molecules with molecular weights of less than 1500 to pass directly from the inside of one cell to that of another (16, 17, 21). Cells connected by gap junctions share manyof their small molecules, hence gap junctions are clearly important in coordinating the activity of these cells and expressing a cell's individuality. CDNAfor mammalian gap-junction proteins has been cloned, and it appears