Electron microscopic studies of the assembly, intracellular transport, and secretion of chylomicrons by rat intestine.

A detailed ultrastructural investigation of the assembly, intracellular transport, and secretion of chylomicrons by rat proximal jejunal intestinal cells was performed in rats fed corn oil. Following fat feeding the smooth endoplasmic reticulum of the absorptive cells becomes laden with triglyceride droplets which are transported through channels of the endoplasmic reticulum to the Golgi apparatus. The Golgi zones become extremely prominent due to the accumulation of osmiophilic droplets, similar in size and configuration to chylomicrons, within proliferated Golgi vesicles. Golgi-derived secretory vesicles, containing nascent chylomicrons, migrate towards the lateral cell membrane. The secretory vesicle membranes fuse with the lateral plasmalemma and nascent chylomicrons are then discharged into the intercellular spaces. Alterations of specific domains of the secretory vesicles were prominent, appearing as coated pits. Coated pits were apparent in the lateral plasmalemma in areas of active chylomicron exocytosis suggesting their derivation from secretory vesicle-membrane fusion. Chylomicrons, within the intercellular spaces, pass through the basement membrane that lines the basal surfaces of the epithelial cells, traverse the cellular elements of the lamina propria, and finally gain access to the lymphatics entering these channels through gaps between adjacent endothelial cells. These observations indicate that nascent chylomicrons accumulate within Golgi vesicles as a pre-requisite to secretion and that secretion occurs by exocytosis resulting in the release of nascent chylomicrons from secretory vesicles.