Fate of the nucleus of the marrow erythroblast.

Nucleated red cells lose their nuclei during passage through the endothelium of marrow sinuses. The passage occurs through cytoplasmic pores which are not gaps at the junction of two endothelial cells but perforations within the endothelium. Enucleation occurs because the pores are of relatively fixed size. Whereas the cytoplasm is flexible and squeezes through the pore, the nucleus is rigid and cannot conform to the pore size. It is, thus, caught, and the red cell becomes enucleated.