Morphology and function of capillary networks in subregions of the rat tuber cinereum.

The differentiated cytology, cytochemistry, and functions within subdivisions of the tuber cinereum prompted this morphometric and physiological investigation of capillaries in the medium eminence and arcuate nucleus of albino rats. Morphometric studies established that the external zone of the median eminence had 3-5 times the number and surface area of true and sinusoidal capillaries than the internal or subependymal median eminence zones, or either of two subdivisions examined in the arcuate nucleus. Type-I true capillaries, around which Virchow-Robin spaces comprise 1% of arcuate tissue area, were situated proximally to the median eminence border. This finding is consistent with a premise that confluent pericapillary spaces enable infiltration of arcuate neurons by factors from capillary blood from the median eminence or Virchow-Robin spaces. Physiologically, the rate of penetration across the median eminence capillaries by blood-borne [14C]alpha-amino-isobutyric acid (a neutral amino acid used as a capillary permeability tracer) was 142 times greater than for capillaries in the distal arcuate nucleus within 12 s of tracer administration. A new finding was that the proximal arcuate nucleus had a permeability x surface area product of 69 microliters g-1 min-1, 34 times greater than that in more distal aspects of the tuber where blood-brain barrier properties exist. We also found that the microcirculatory transit time of a plasma space marker, [14C]sucrose, was considerably longer (1.2 s) in the median eminence and proximal arcuate nucleus than in the distal arcuate or ventromedial nucleus (0.4 s). By virtue of its high capillary permeability and extensive blood-tissue surface area, including the wide Virchow-Robin spaces, the median eminence external zone could be a gateway for flooding other tuberal compartments with blood-borne factors. This effect may be compounded by capillary bed specializations in the proximal arcuate nucleus where Type-I true capillaries, Type-III sinusoids, and pericapillary spaces are confluent with those in the median eminence. The results indicate that the proximal arcuate parenchyma could be exposed to circulating neuroactive substances on a moment-to-moment basis.

[Ultrastructure of the proximal neurosecretory contact region in a sturgeon]. / Ul'trastruktura proksimal'noi neirosekretornoi kontaktnoi oblasti u osetra

The proximal neurosecretory contact region (PCR), a homologue of the tetrapoda median eminence occupies the rostral and chiefly the ventral surfaces of the hypothalamic tuber cinereum. Three types of granule-containing neurosecretory fibres and their terminals have been found in the PCR with the electron microscope. Peptidergic, A1 and A2 type fibres contain granules of 120--300 nm and 100--170 nm in diameter, correspondingly, and monoaminergic B fibres have granules of 80--100 nm in diameter. Neurosecretory terminals and tanycyte vascular "endfeet" make contacts with a 70 nm thick outer basement membrane of the primary portal capillaries. Some thin horizontally oriented tanycyte branches form a multiserial layer bordering the external zone of the PCR from the preoptico-hypophyseal tract. Few neuroglial cells with pale cytoplasm and numerous lysosomes and lipofuscin granules are seen there. It is hypothesized that, like in other vertebrates, in the PCR of Acipenseridae both peptide and monoamine hypophysiotropic neurohormones may be discharged in the portal circulation to affect the functional activity of glandular cells of the pars distalis.

Scanning electron microscopy of the wall of the third ventricle of the brain of Rana temporia. II. Electron microscopy of the ventricular surface of the pars ventralis of the tuber cinereum.

Scanning electrom microscopy of the ventricular surface of the pars ventralis of the tuber cinereum of the frog Rana temporaria shows that it can be divided in (1) a dorsolateral area, characterized by the presence of very numerous large, solid, bulbous protrusions (2) a medial area, where the large bulbous structures are completely absent and which is diffusely covered with very numerous cilia, (3) a transiotional area located between the two other regions. The large bulbs, shown by scanning electron microscopy, correspond with the end-bulbs of thick ventricular dendrites of nerve cells revealed by transmission electron microscopy. At least many of these intra-ventricular bulbs are dendritic endings of peptidergic neurosecretory neurons, which have been tentatively identified as adenohypophysiotropic neuro-hormone producing neurons (Derickx et al., 1972, 1973a, 1973b). The structural features of the dendritic endings plead in favour of a possible receptive role.