Granule formation in rat mast cells. Study with serial ultrathin sections.

The process of granule formation in mast cells was investigated with young mast cells in the milky spot of adult rats, seven days after an intraperitoneal injection of distilled water. Serial ultrathin sections were made and examined with an electron microscope to reveal the following results. The early step of granule formation is observed as a complex which consists of an irregularly shaped narrow vestibular space and several primary aggregate granules which are bowl-like in form, attached to the vestibular space and contain a few subgranules. The vestibular space has microvilli. The precursor materials seem to be secreted from the microvilli and transported through the vestibular space into the primary aggregate granules, being coagulated to become subgranules. As the subgranules increase in number, the primary aggregate granules become aggregate granules which grow further to their maximum size. With further addition of the materials, the aggregate granules become, through a step of ropy cord granule, mature dense homogeneous granules. The complex seems to remain as a unit after the maturation of granules and to react as a whole to any stimulus.