An electron microscopic study on the structural development of the neural lobe in the human fetus.

Structural development of the neural lobe of the hypophysis was studied by light and electron microscopy in five human fetuses ranging from 7.5 to 19 weeks of ovulation age. Clusters of small clear vesicles were found occasionally in axon profiles in a fetus of 7.5 weeks. Granular vesicles in axon profiles first accumulated in a fetus of 8.5 weeks. Concurrently, fenestration of capillary walls and expansion of perivascular spaces were observed. After 8.5 weeks, three types of vesicle-containing axon profiles were found, which contained predominantly granular vesicles (GP), small clear vesicles (SP), and a mixed population of both granular and small clear vesicles (MX), respectively. Their numbers increased significantly during two periods, 7.5-8.5 weeks and 15.5-19 weeks. Ratios of GP, SP, and MX to the total number of the three types of axon were almost the same in all the fetuses studied, i.e., about 40%, 40%, and 20%, respectively. Developmental changes in the diameter of granular vesicles were examined in GP, SP, and MX of each fetus. Apart from a few exceptions, the mean diameter of granular vesicles was larger in GP than in MX, which was in turn larger than in SP. The diameters increased with development and those in GP reached almost the same value as in the adult by 19 weeks.