Different populations of granules and their distribution in the hypothalamo-neurohypophysial tract of the rat under various experimental conditions. II. The median eminence.

In connection with previous studies (Krisch, 1974), a morphologic analysis of the median eminence of male, female, pregnant, and thirsting rats has been made, in order to evaluate: 1. whether the median eminence is an additional area where oxytocin and vasopressin may be released from neurosecretory axons in the zona interna; 2. whether it is possible to distinguish different populations of axons according to the mean diameters of their granules in the external zone of the median eminence. In the zona interna of the male and the female control animals, the oxytocin- and vasopressin-containing granules have already reached their final diameters, i.e. their size remains constant until they reach and are stored in the neurohypophysis. However, the relative proportions of these populations in the fibre layer of the median eminence indicate certain differences between male and female animals, concerning the turnover of oxytocin vasopressin. In pregnant, as well as in thirsting animals, there is an apparent decrease of the mean granule diameters in oxytocin- and vasopressin-containing axons. In particular in the latter group, ballooned axons in the fibre layer of the median eminence, containing disintegrating granules, tubular structures filled with electron dense material, and fine-flocculent material in the axoplasm, together with the morphometric results, support our concept that in case of highly increased peripheral demand the contents of the mature, large granules are released into the axoplasm of the fibre layer in the median eminence. In addition, axons containing granules smaller than 115 nm may be classified into four populations with mean diameters of 84 nm, 96 nm, 103 nm, and 100n. The same populations are found in the palisade zone of the median eminence as well as in the pericapillary zone, but in contrast to the zona interna there is a fifth population of axons containing very small granules with a mean diameter of about 70-75 nm. The relative proportions of these five populations differ between the zona palisadica and the pericapillary zone on the one hand and between the experimental groups on the other hand. Some morphological findings concerning the tanycytes are discussed with respect to previously published data.