Correlation between cell density, membrane fluidity, and the availability of transferrin receptors in friend erythroleukemic cells

Undifferentiated Friend erythroleukemic cells (FL cells) acquire membrane microviscosity ( \(\bar \eta \) ), in accord with the culture cell density. At low cell density \(\bar \eta (21^\circ ) \sim 2.8\) poise, whereas at confluency it increases to \(\bar \eta (21^\circ ) \sim 5.3\) poise. Concomitantly, the total number of available transferrin receptors per cell decreases by about 80% upon increase in cell density. Modulation of membrane microviscosity, by artificial alteration of the membrane cholesterol level, mediates similar modulations of the availability of the transferrin receptors. The correlation between the availability of the transferrin receptors and the membrane lipid fluidity may take part in the overt decrease in iron uptake by erythroid cells along the erythropoiesis pathway.