Surface morphology of the subfornical organ in the rabbit's brain

Scanning electron microscopy of central nervous system cerebrospinal-fluid-contacting surfaces: a bibliography (1963-1995)

This bibliography is compiled to assist in locating papers related to the application of scanning electron microscopy (SEM) to cerebrospinal-fluid-contacting surfaces in vertebrates. The use of SEM by neuroscientists has continued apace since the publication of the first bibliography in 1980. SEM studies now include more than 50 species of vertebrates and range from cyclostomes to humans; they encompass development from embryo to senescence and concern both normal and pathologic morphology. Although remarkable strides have been made toward a greater understanding of many aspects of the structure and function of cerebrospinal-fluid-contacting surfaces, many significant problems await the judicious application of scanning electron microscopy.

Expression of basic fibroblast growth factor and its receptor in the rat subfornical organ

Basic fibroblast growth factor (FGF), its mRNA and the mRNA that encodes for its receptor have all been localized in the rat subfornical organ (SFO). Basic FGF is widely distributed throughout the SFO; it is present in neurons, in the vascular basement membrane of lateral blood vessels (but not those within the SFO) and in ependymal cells surrounding the SFO. Results of in situ hybridization show that the expression of basic FGF mRNA is detected throughout the organ. Similarly, the expression of flg, the gene for the putative basic FGF receptor, can also be detected in the SFO. The results all support the possibility that this growth factor may modulate the known physiological functions of the SFO.

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.

The subfornical organ as a model of neurohumoral integration

Through its specialized morphology, sensitivity to angiotensin II, and strategic anatomical connections, the subfornical organ is intimately involved in the translation of peripheral thirst to behavioral and physiological actions for establishing body fluid homeostasis. Evidence identifying the subfornical organ as a "neuroendocrine transducer" is summarized in categories based on its structure-function relationships with systemic blood, cerebrospinal fluid, and its afferent and efferent neural connections.

The subfornical organ: biochemical and neuroendocrine comparisons with the hypothalamo-neurohypophysial system

Biochemical, cytochemical and immunological methods were used to compare the metabolic and neuroendocrine properties of the subfornical organ (SFO) with the hypothalamo-neurohypophysial system (HNS) in the rat. The SFO resembles the HNS in that both have (a) increased label incorporation into RNA during dehydration; (b) an intense reaction for glucose-6-phosphate dehydrogenase; (c) NADPH-diaphorase and the Type I pathway for hydrogen utilization from NADPH, presumably as part of the mixed-function oxidase system for the metabolism of endogenous substrates and xenobiotics; (d) immunoreactive vasopressin and oxytocin. Gel filtration of extracts of the SFO area using Sephadex G-25 chromatography resulted in immunoreactive peaks for both AVP and OT which were similar to synthetic hormones. One other fraction in the SFO extract, containing a substance(s) of higher molecular weight than AVP, was detected using the antiserum for AVP. The concentration of immunoreactive AVP in the SFO area was increased after colchicine, decreased by hypophysectomy, and unaltered by: (a) infusion (4.6 pg/min for 3 hr) or injection (1 or 6 ng) of AVP into the lateral cerebroventricle; (b) dehydration; (c) renin administered intracerebroventricularly; (d) pinealectomy; or (e) hypertension in the spontaneously hypertensive rat. In conclusion, cells in the SFO have specialized metabolic and neuroendocrine properties similar to the HNS. It can be inferred from these biochemical specializations that the SFO has metabolic and secretory activities.

Surface morphology of the subfornical organ: effects of low and high sodium chloride diet

Ependymal cells found in the subfornical organ of the rat were counted. Cells covered by small microvilli, small protrusions and smooth cells were frequently found. Also present were cells with long or short cilia, cels with large protrusions and supraependymal cells. High and low sodium diets reduced the number of cells with large protrusions. Microvilli-covered cells increased after a low sodium diet.

Ultrastructure of the subfornical organ of the chicken (Gallus domesticus)

The SFO of the chicken is divided in half by a large central blood sinus; ventrally it is covered by a thin layer of ependyma (including tanycytes, dendrites, and axons) which connects the two lateral halves and protrudes as a midsagittal crest into the lumen of the third ventricle. The ependyma consists predominantly of tanycytes with long basal processes which terminate upon perivascular spaces. These cells have an extensive Golgi apparatus and abundant lysosomes; their cellular apices containing polyribosomes and a few vesicles frequently protrude into the ventricle. In addition to astrocytes, oligodendrocytes, and microglial cells, there is another glial cell population that is distinguished by the presence of parallel stacks or spherical to ovoid conglomerates of rough ER and their unique location, i.e., limited to areas ventral and ventral-lateral to the large blood sinus. Two types of neurons are present: neurons in which there is a paucity of granulated vesicles and occasional vacuoles in both the cytoplasm and nuclei, the second type of neuron elaborates many granulated vesicles. Numerous puncta adhaerentia are observed between adjacent neuronal perikarya and between glial processes and neuronal perikarya. Diverse axon types are found within the chicken SFO. Axo-dendritic and axo-somatic axon terminals and presynaptic axon dilations contain assorted combinations of electron-lucent and granulated vesicles of different maximal diameters. Based on the morphology of these axons, cholinergic, peptidergic, and serotoninergic fibers are described. There are two additional groups of axons whose classification awaits further investigation. The chicken SFO differs from the mammalian SFO in several respects: it possesses an ependyma with secretory and/or absorptive tanycytes predominating; it is divided midsagittally by a central blood sinus; its lateral and dorsal limits are nebulous; a previously undescribed peculiar type of glial cell is found in a limited portion of the organ; supraependymal neurons are lacking.

Regional differences in the morphology of the rat subfornical organ

Based upon scanning and transmission electron microscopy 3 regions are distinguished in the rat subfornical organ. The rostral region is dominated by nerve fibers interspersed with relatively few neurons and glial cells. Squamous to low cuboidal ependymal cells with flat ventricular surfaces bearing a few short microvilli line the center of this region; laterally, ciliated cuboidal ependymal cells predominate. The central region occupies the largest area of the organ and contains most of the neuronal perikarya and glial cells. Many perikarya and neuronal processes are located immediately underneath the ependymal surface. A dense capillary network with wide pericapillary spaces permeates the tissue. In the rostral two-thirds of this region the ependymal cells are either squamous or cuboidal, sometimes with slightly bulging ventricular surfaces bearing longer microvilli. Here supraependymal neurons are particularly numerous. The caudal one-third of the central region is characterized by squamous, cuboidal and columnar ependymal cells whose hemispherical ventricular surfaces are studded with long microvilli and occasional cilia and vesicular protrusions. The caudal region, like the rostral region, is dominated by nerve fibers between which neuronal perikarya and glial cells are present. At this level the choroid plexus is attached to the SFO through highly vascularized pial connective tissue. It is the major point of penetration of the SFO's capillary plexus. The possible significance of these observations and their importance in experimental interventions are discussed.

Effects of subfornical organ extracts of salt-water balance in the rat

The subfornical organ (SFO) is regarded as a neurosecretory structure but no information is available on the nature or biological effects of the secretory products(s). Supernatants of water homogenates of rat SFO were lyophilized and reconsittuted in artificial cerebrospinal fluid (CSF). Intracerebroventricular (IVT), but not subcutaneous, administration of this material to rats produced diuresis, natriuresis and kaliuresis in the following 8 h daylight period. During the overnight cycle, consummatory behavior and excretion of sodium and potassium were reduced. Similar responses were obtained after IVT administration of cerebellar cortex (CB) or large amounts of plasma. SFO, CB and cerebral cortex (CC) were incubated in potassium-enriched CSF to enhance release of secretory products. Urine volume was increased 8 h after IVT injection of SFO media; in the overnight cycle, food consumption, absolute urinary sodium and potassium, and [Na+-a1 were reduced. These effects were not produced by IVT injection of CC or CB media, or equal amounts of plasma proteins. Additional experiments demonstrated that choroid plexi and SFO effects were similar and that the active SFO material was dialyzable and thermal stable. These data suggest that SFO contains a water-soluble substance which is released into a posassium-enriched medium. The material is heat stable, has a relatively low molecular weight, and alters salt-water balance after injection into ventricular cerebrospinal fluid.

Pathogenesis of aqueductal occlusion in congenital murine hydrocephalus

In a study of congenital hydrocephalus in the murine mutant (hy-3/hy-3), the authors found that aqueductal stenosis develops during the progression of hydrocephalus. In Stage 1 hydrocephalus (ventricular dilation and open aqueduct), a block in the subarachnoid space over the cerebral convexities causes the lateral and third ventricles to enlarge. The ependyma becomes stretched and a collection of edematous fluid forms in the subependymal layer. In Stage 2 hydrocephalus (edema in white matter around lateral ventricles and compression of quadrigeminal plate), edema develops peripheral to ependyma in the aqueduct and compresses the lateral surfaces of the aqueductal wall to obstruct the lumen. While periaqueductal edema is spreading, the forces of the expanding midline structures and the cystic occiital horns alter the relationship of brain structures. There is no proliferation of glia, but, rather, a "simple stenosis" which results from a combination of ventricular dilation, cerebral edema, brain shift, brain-stem compression, and brain-stem edema. In this study, normal ependymal specializations were observed that indicate a more active functional role for aqueductal ependyma than previously recognized.

Surface fine structure of supraependymal elements and ependyma of hypothalamic third ventricle of mouse

The ependymal surface of the hypothalamic third ventricle of the adult female and male mouse was examined with the scanning electron microscope. The material was dried with the critical point method. Supraependymal cells, exhibiting a variety of forms, lie among the cilia of the rostrodorsal two-thirds of the ventricle. The cell bodies are usually between 8 and 10 mu in diameter and have dividing pseudopodial processes which may be broad or narrow, flat or stout, smooth or varicosed. The textured surfaces of the cells seen in this material appear to have no cilia, microvilli or membraneous folds though they may have small bleb-like protrusions. Some cells and adjacent cilia bear clusters of friable-looking material that may be secretory. In several female mice, mounds of debris, that could be degenerating cells or secretory material, were seen in intimate association with clusters of cells. The cells' processes extend over and through the surface of the mound. While some supraependymal cells may be neurons or glia, it is suggested that many of the cells seen may be morphological variants of one cell line, mononuclear phagocytes. Sausage-shaped structures at the end of long cilia-like stalks are described and their possible identity discussed.