New aspects of the morphology of circulating megakaryocytes in rabbits.

Circulating cytoplasmatic megakaryocytes, isolated from central venous blood of rabbits by a saponinhaemolysis leuco-concentration technique, were found to have two morphological appearances: a globular and an oblong form, referring to the shape of the cells as a whole as well as to the shape of the nuclei. The nucleus was either centrally or peripherally located. Of the megakaryocytes observed, 40% were cytoplasmatic and 60% naked nuclei. Regressive changes in the residual nucleus was observed, only 1.5% showing karyorrhexis or karyolysis. Megakaryoblasts and promegakaryocytes were never encountered. The average megakarycyte content per ml central venous blood was 3.8 (range: 0-10). A not previously described cytoplasmatic-ligating process, in which the nucleus seemed to move out of the cytoplasm, forming a naked nucleus and leaving the total cytoplasm intact, was observed. We call this phenomenon "nucleus escape". The function and fate of the intravascular megakaryocytes are briefly discussed.