Trafficking and metabolism of sphingolipids were examined in undifferentiated (G+) and differentiated (G+ reversed) HT29 human colon adenocarcinoma cell lines. Metabolic experiments employing a fluorescently labeled sphingolipid precursor, 6-[N-(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]hexanoylceramide++ + (C6-NBD-ceramide) revealed that both qualitative and quantitative differences exist in sphingolipid synthesis between the 2 cell lines. One of the C6-NBD-sphingolipids synthesized in G+ cells is not found in the G+ reversed cells. Furthermore, the ratio of the 2 main products, C6-NBD-glucosylceramide and C6-NBD-sphingomyelin, differs: in G+ cells glucosylceramide is by far the main product, whereas G+ reversed cells synthesize C6-NBD-sphingomyelin in slight excess. Once established, these ratios of sphingolipids are quickly restored metabolically when distortion of the ratio is caused by experimental manipulation. This indicates that they represent a true metabolic equilibrium situation of the 2 sphingolipids in these cells, while the distinct ratios are mainly determined by the NBD-lipid pool in the plasma membrane. Preferential synthesis and transfer of glucosylceramide from its site of synthesis to the cell surface do not occur when the plasma membrane pool of glucosylceramide is selectively removed. This suggests that instantaneous replenishment via specific signalling is probably not involved as a mechanism in re-establishing perturbed lipid pools. In conjunction with observations on distinct lipid trafficking pathways of glucosylceramide in G+ and G+ reversed cells, the present metabolic studies emphasize a relation between the expression of this glycolipid and the state of differentiation of HT29 cells.

Tanycytes with foot processes contacting capillary basal membranes were identified in the rat medial habenular nucleus. They constitute a relatively small but constant population of cells among the conventional ependymal cells. In contrast to tanycytes in most circumventricular organs, habenular tanycytes possess cilia. Superimposed upon the cells are nerve fibers belonging to the serotonergic supraependymal axon plexus. Their ultrastructure differs in many respects from that of hypothalamic tanycytes.

The surface of the recessus infundibularis of the third ventricle has been studied with the scanning and transmission technique in normal and experimental material. Surface specializations such as microvilli, craters and areas of discontinuous lining are described. Supraependymal cells and fibres have been found; some of these cells form wide-meshed networks. The supraependymal fibres may be regular or varicose; the former seem to perforate the ependyma. With the transmission electron microscope the supraependymal cells are divided into three categories; nerve cells, lymphocytes and "dense cells". Two fibre populations are distinguished: thin profiles (nerve fibres) and thick profiles (nerve terminals). Axosomatic and axoaxonic synapses are described. Synapses between supraependymal fibres and ependyma cells have also been found.

In the medial habenular nucleus of the rat, ependymal and endothelial membrane specializations were studied with TEM and freeze-fracturing. They comprise ependymal adherent junctions - not manifest in freeze-fracture replicas-, gap junctions, and membrane-associated orthogonal particle complexes ("assemblies") - not identifiable in thin-sectioned material. Ependymal tight junctions being absent, no brain-liquor barrier exists. The capillary endothelium is provided with tight junctions only. Intraventricularly injected HRP was transported in large amounts through the ependyma, mainly through the intercellular spaces and additionally by way of massive pinocytosis through the cytoplasm of particular ependymal cells only, and finally through the parenchymal intercellular compartments towards habenular capillaries. Following intravenous injection of HRP, considerable transport of the enzyme took place by means of transendothelial pinocytosis, followed by some pinocytotic transport through diverse parenchymal elements and markedly profuse incorporation and lysis within pericytes. The habenular blood-brain barrier appeared to be considerably leaky with respect to HRP.

The fine structure of supraependymal cell clusters of the median eminence was studied with TEM. The cluster cells were identified on the basis of ultrastructure and histochemical determination of glial fibrillar acidic protein (GFA). The phagocytic properties were also studied by means of intraventricular injections of HRP. Neurons, neuroglia cells and degenerating ependyma- and glial cells were found. The extrusion of degenerating infundibular elements into the ventricle is a constant phenomenon but its precise localization and intensity are variable. The close proximity of the clusters to capillary loops is stressed. Because of the broken ependyma at the neck of the cluster, the permeability of the infundibular lining for HRP is increased. Clusters may be seen as sites lacking a brain--CSF barrier.