Development of cellular polarity and tight junctions in parotid acinar cells of postnatal rat

The morphogenesis of acino-tubular structures and cytodifferentiation of acinar cells in developing rat parotid glands from the day of birth to the 7th day after birth were studied by conventional ultrathin-section electron microscopy in conjunction with freeze replica and space tracer methods. An ultrathin-section study indicated that the acinar cells developed sequentially in the order of the following three stages: (1) the stage of undifferentiated cell immediately after birth, in which the presumptive acinar cells showed very scanty cell cytoplasm, poorly developed organellae, and no distinctive cellular polarity; (2) the stage about the 3rd day after birth, in which cells were arranged into a single layer, resulting in the establishment of three recognizable domains in the plasma membranes, and developing cellular organellae started to distribute with distinctive polarity; and (3) the stage of the 5th day after birth and thereafter, in which secretory granules were formed, indicating the beginning of exocrine functions. Freeze replica and a tracer study demonstrated that the formation of a sealing strand of tight junctional belt took place in correspondence to the establishment of cellular polarity. These results indicated that the development of cellular polarity, plasma membrane domains, tight junctions, and acino-tubular structure were closely interrelated to each other, and preceded the onset of secretory functions.