Ultrastructure of hypothalamic paraventricular neurons.

The hypothalamic paraventricular nucleus (PVN) plays a pivotal role in the regulation of endocrine processes and the modulation of autonomic functions. The multi-channel outputs of the nucleus are directed toward the anterior and posterior lobes of the pituitary, autonomic centers, and limbic structures. Counterbalancing the wide spectrum of efferent projections, the nucleus receives humoral signals from endocrine glands and neuronal afferents from several loci of the brain. The multiple functions of the nucleus are executed by neurons that are organized in distinct subnuclei and display complex chemotypes. The review demonstrates and discusses the organization, architecture, chemical composition, plasticity, and pathology of paraventricular neurons of the rat hypothalamus from the perspective of ultrastructural analysis. Electron microscopy--by its high resolution--offers a powerful tool that is suitable for revealing the structural background of physiological processes that modulate and govern the operation of paraventricular neurons.

Quantitative fine structure of capillaries in subregions of the rat subfornical organ.

The differentiated cytology across subregions of the rat subfornical organ (SFO) prompted our hypothesis that ultrastructural features of capillary endothelial cells would vary topographically and quantitatively within this small nucleus. We used electron microscopic and computer-based morphometric methods to assess fine structural dimensions of the capillary endothelium in four distinct subregions of the SFO from Long-Evans and homozygous Brattleboro rats. Three types of capillary were present. Type III capillaries (resembling those of endocrine glands) had an average wall thickness of 0.17 microns, 54% thinner than those of Type I and II capillaries. Pericapillary spaces around Type III capillaries measured 56 microns2, 100% larger than for Type I vessels (resembling those of skeletal muscle). Only Type III capillaries contained fenestrations (9 per microns2 of endothelial cell) and were the predominant type of capillary in central and caudal subregions of the SFO. Type I capillaries, prevalent in the transitional subregion between the central and rostral parts of the SFO, had 10 cytoplasmic vesicles per micron2 of endothelial cell area, a number not different from that of Type III capillaries but 3x the frequency found in Type II vessels. Type II capillaries (those typical of "blood-brain barrier" endothelium) had low vesicular density (3 per microns2), no fenestrations, and no pericapillary spaces. Luminal diameters and the densities of mitochondria and intercellular junctions were not different among capillary types or subregions in the SFO. Furthermore, there were no morphometric differences for any capillary dimensions between Long-Evans and Brattleboro rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Topography and morphometry of capillaries in the rat subfornical organ.

A comprehensive stereological analysis was performed to define capillary dimensions in individual subregions of the subfornical organ in Long-Evans, homozygous Brattleboro, and Sprague-Dawley rats. Capillary density, volume fraction, length, surface area, and diameter were assessed in four regions in the sagittal plane (rostral, "transitional," central, and caudal) and two zones in the coronal plane (dorsal and ventromedial). The ventromedial zones in the central and caudal regions correspond to areas of dense perikarya and neuropil containing neural afferent inputs to the subfornical organ (e.g., putative fiber terminals for angiotensin II), whereas the dorsal zones of these regions are apparently the predominant sites of perikarya having efferent projections directed outside of the organ. The morphometric analysis revealed heterogeneous capillary density across subregions of the subfornical organ (range of 132 to 931 capillaries/mm2 in the three rat groups). Capillaries in the ventromedial zones of the central and caudal regions had significantly greater density, volume fraction, and surface area, but smaller diameters, than those in the adjacent dorsal zones and more rostral regions. Across all subregions within the dorsal zone, there was generally a consistent morphometric pattern in the three rat groups. No differences in capillary dimensions in any part of the subfornical organ were found between the Long-Evans and Brattleboro rats. A qualitative electron microscopic investigation of endothelial cells in each subregion of the subfornical organ in Long-Evans rats revealed at least three types of capillary oriented according to region: in the rostral region were capillaries having no endothelial fenestrations or pericapillary spaces, and few vesicles, in the "transitional" region between the rostral and central regions, capillaries having no endothelial fenestrations, substantial numbers of vesicles, and narrow but perceptible pericapillary spaces were found, and in the central and caudal regions, capillaries having abundant endothelial fenestrations and vesicles, expansive pericapillary labyrinths, and relatively thin walls were present. These findings from light microscopic morphometry and electron microscopy in rats indicate a heterogeneity of capillary organization that shows topographical correspondence to the cytology and putative functions of the subfornical organ.

Immunocytochemical studies of the peptidergic innervation of the thyroid gland in the Brattleboro rat.

An indirect immunofluorescence technique was used to study the peptidergic innervation of the thyroid gland in homozygous Brattleboro rats (DI) and normal Long-Evans rats (LE). The primary goal of this study was to determine whether the previously demonstrated decrease in thyroid responsiveness to TSH in DI might be due to an abnormality in the innervation of the thyroid. Thyroids from both types of rats were found to contain nerve fibers containing immunoreactivity for vasoactive intestinal peptide (VIP), substance P (SP), neuropeptide Y (NPY), and peptide HI (PHI). All four types of fibers were found in close association with both follicle cells and blood vessels. Well developed networks of fibers surrounding blood vessels were particularly apparent in the case of NPY. The density of fibers associated with follicle cells in DI was at least as great as that in LE in regard to SP, NPY, and PHI. Fibers containing VIP were found in greater abundance in DI than in LE. Additional studies revealed no evidence of thyroid fibers containing either somatostatin or neurophysin, which was used as a marker for vasopressin. We conclude that the reduced responsiveness of the thyroid in DI is not due to an inadequate supply of any of the neuropeptides included in this study. Since VIP is known to enhance thyroid secretion, we suggest that the apparent proliferation of VIP-containing fibers in DI may be a reflection of a neural mechanism attempting to compensate for a thyroid gland deficiency analogous to the humoral mechanism by which TSH secretion increases in response to thyroid deficiency.