ELECTRON MICROSCOPIC OBSERVATION ON THE HYPOTHALAMOHYPOPHYSEAL SYSTEM IN RATS:IV. ULTRAFINE STRUCTURE OF THE DEVELOPING MEDIAN EMINENCE

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Ultrafine structure of a developing median eminence was examined with the electron microscope in 20-day-old fetuses, 1-day-, 3-day-, 6-day-, 15-day-and 30-day-old female rats. In 20-day-old fetuses and 1-day-old rats, zonation of the neuronal layer is not distinguishable. Such a structural uniformity of the neuronal layer is mainly due to the lack of the myelinated nerve fibers as well as the lack of large electron dense granules (over 180 mμ in diameter), both of which characterize the internal layer of the adult rat. In these stages, the subependymal neuronal layer contains innumerable unmyelinated nerve fibers. The electron dense granules and electron lucent vesicles contained in these nerve fibers vary from 20 to 180 mμ in diameter, with the largest population at 80-100 mμ and 40-60mμ respectively. In the peripheral region, the structural aspect is largely similar but granules and vesicles are larger in number, especially in the axon endings abutting on the portal vessel. On the 3rd day of birth, the neuronal layer increases in width. The electron dense granule shows a slight increase in number. The myelinated nerve fiber has occured in the subependymal layer of a 6-day-old rat. On the 15th day of birth the zonation of the internal and the external layer is distinct. The unmyelinated nerve fibers containing large granules (over 180 mμ in diameter) are not infrequently encountered in the internal layer but not at all in the external layer. In the internal layer, the electron dense granules expand their size range to as large as 280mμ. In the external layer, granules and vesicles show remarkable increase in number. The peak in the size distribution histogram of granules shifts to the larger diameter (100-120mμ) but the size range remains unchanged (60-160mμ). On the 30th day of birth, the difference between internal and external layer becomes more prominent. In the internal layer, myelinated nerve fibers containing many electron dense granules are often observed. Unmyelinated nerve fibers containing large granules (over 180mμin diameter) increase in number. The pattern of the size distribution histogram is much the same as that of a 15-day-old rat. In the external layer, granules and vesicles show a striking increase in number.
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ELECTRON MICROSCOPIC OBSERVATION ON THE HYPOTHALAMOHYPOPHYSEAL SYSTEM IN RATS:IV. ULTRAFINE STRUCTURE OF THE DEVELOPING MEDIAN EMINENCE

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