Ultrastructural Analysis of Murine Megakaryocyte Maturation In Vitro :

Two morphologically distinct types of murine megakaryocyte (MK) colonies are present after three to seven days in soft agar culture: (a) big-cell” colonies composed of ten to 30 large, mature-appearing megakaryocytes and (b) “heterogeneous” colonies consisting of approximately 1 00 or more cells at various stages of differentiation. Cytochemical and immunocytochemical techniques were used to study MK maturation in colonies as well as normal mouse bone marrow. Acetylcholinesterase (AChE). a specific marker for murine platelets and MK. was found in the perinuclear cisterns. endoplasmic reticulum, and occasionally. Golgi cisternae of MK in three-day big-cell colonies and immature bone marrow MK. MK in seven-day big-cell colonies and mature bone marrow MK showed additional reaction sites in the demarcation membrane system and occasional granules. In seven-day heterogeneous colonies. small cells resembled immature bone marrow MK with respect to AChE localization. whereas large cells corre-