The fine structure of the inner limiting membrane of the rat retina as revealed by ruthenium red staining

Isolation, characterization, and substrate properties of the external limiting membrane from the avian embryonic optic tectum

The external limiting membrane of the avian embryonic optic tectum is isolated by mechanically separating the neuronal mesencephalon from the overlying mesenchymal tissue. The preparation consists of a basal lamina which is covered on its neural side by endfeet of neuroepithelial cells and has attached to it on its meningeal side a collageneous stroma, containing blood vessels. The external limiting membrane can be flat-mounted on a piece of nitrocellulose filter as mechanical support. It covers an area between 0.3 and 1 cm2, depending on the age of the donor embryo. The endfeet can be removed together with all cellular components of the meninges by treatment with 2% Triton-X-100 or with distilled water. The basal lamina itself is approximately 80 nm thick and consists of two laminae rarae and a central lamina densa. Immunohistochemical staining reveals that the basal lamina in the embryo, after isolation and after detergent extraction of the isolated preparation, contains type IV collagen, nidogen, laminin, and low density heparan sulfate proteoglycan as do other basement membranes. Antibodies against the neural cell adhesion molecule (N-CAM), chondroitin sulfate proteoglycan, and fibronectin fail to stain the external limiting membrane, but these proteins were clearly identified in the blood vessel-containing meninges or in the optic tectum. The flat-mounted external limiting membrane preparation was used as substrate to culture several different neural tissues of central and peripheral origin. Explants of neural crest cells, dorsal root ganglia, and sympathetic ganglia can be cultured on the external limiting membrane. All explants grow well on the basal lamina preparations whether the endfeet are attached or detergent-extracted prior to explantation; however, neurite outgrowth from sympathetic ganglia is reduced in the presence of the endfeet. Although the endfoot-lined external limiting membrane represents at least part of the immediate environment encountered by retinal axons as they invade the optic tectum and despite its excellent properties as a substrate for retinal axons in vitro, cues guiding the orientation of axons were not detected in the flat-mounted preparation.

Ultrastructural studies of bovine retinal microvascular basement membranes with the cationic dye ruthenium red

Our recent observation that the basement membranes of brain microvessels do not stain with the cationic dye ruthenium red has raised the question of whether the basement membranes of this and other vascular beds functioning as barriers between blood and neural tissues are deficient in the polyanionic macromolecules, such as glycosaminoglycans, which are responsible for the ruthenium red staining of other vascular basement membranes. We therefore attempted to produce staining in the only barrier-type microvascular basement membrane known to contain heparan sulfate. Bovine retinas were fixed by immersion in aldehyde fixatives containing ruthenium red, buffered with either 10 mM or 100 mM sodium cacodylate. We found discrete, electron-dense deposits of ruthenium red in vascular basement membranes, quite similar to those seen in vascular basement membranes of nonneural tissues after exposure to ruthenium red. These deposits were more distinct and more frequent in tissue exposed to ruthenium red-aldehyde solutions buffered with 10 mM cacodylate. They were not seen if ruthenium red was omitted from the fixative. The results demonstrate that anionic macromolecules in basement membranes of barrier-type microvessels can be stained with cationic dyes, and suggest that the failure of brain microvessels to stain with ruthenium red may be the result of a relative or total lack of polyanion in this basement membrane, or of other unique properties.