Ultrastructure of Reichert ' s Membrane , a Multilayered Basement Membrane in the Parietal Wall of the Rat Yolk Sac

The ultrastructure of Reichert's membrane, a thick basement membrane in the parietal wall of the yolk sac, has been examined in 13-14-d pregnant rats. This membrane is composed of more or less distinct parallel layers, each one of which resembles a common basement membrane. After routine fixation in glutaraldehyde followed by osmium tetroxide, the layers appear to be mainly composed of 3-8-nm thick cords arranged in a threedimensional network. Loosely scattered among the cords are unbranched, straight tubular structures with a diameter of 7-10 nm, which mainly run parallel to the surface and to one another; they are referred to as basotubules. Permanganate fixation emphasizes the presence of a thick feltwork of irregular material around basotubules. Finally, minute dot-like structures measuring 3.5 nm and referred to as double pegs are present within the meshes of the cord network. Reichert's membranes have been treated for 2-48 h at 25°C with plasmin, a proteolytic enzyme known to rapidly digest laminin and fibronectin. After a 2-h treatment, most of the substance of the cords is digested away leaving a three-dimensional network of 1.5-2.0-nm thick filaments. The interpretation is that the cords are formed of a plasmin-resistant core filament and a plasmin-extractable sheath. When plasmin treatment is prolonged for 15 h or longer, the filaments are dissociated and disappear, while basotubules are maintained. Plasmin digestion also reveals that basotubules are composed of two parts: a ribbon-like helical wrapping and tubule proper. Further changes in the tubule under plasmin influence are interpreted as a dissociation into pentagonal units suggestive of the presence of the amyloid P component. After 48 h of plasmin treatment, basotubules are further disaggregated and dispersed, leaving only linearly arranged double pegs. Reichert's membranes with or without a 2-hr plasmin treatment have been immunostained by exposure to antibodies against either laminin or type IV collagen with the help of peroxidase markers. The results indicate that the sheath of the cords contains laminin antigenicity, while the core filament contains type IV collagen antigenicity. It is proposed that Reichert's membrane consists mainly of a three-dimensional network of cords composed of a type IV collagen filament enclosed within a laminin-containing sheath. Also present are basotubules--which may contain the amyloid P component--and double pegs whose nature is unknown. Basement membrane is a thin matrix that separates connective tissue from epithelial, endothelial, fat, and muscle cells, and also from the nervous system. The electron microscope shows that basement membranes commonly are composed of three layers. The two main ones are the 10-50-nm thick lamina lucida (also called tara) adjacent to the cells and, next to it, the 20-300-nm thick lamina densa (also called basal lamina), while a third, poorly defined layer, the lamina or 1524 pars fibroreticularis, makes up the transition with connective tissue 1 (1, 2). Basement membranes are said to be composed The layers of the basement membrane are described in the terms recently recommended by the International Anatomical Nomenclature Committee as reported in the "Nomina Histologica" section of Nomina A natomica (1983, 5th Edition, Williams and Wilkins, Baltimore, MD), that is lamina tucida, lamina densa, and lamina (pars) ]HE JOURNAL OF CELL BIOLOGY VOLUME 97 NOVEMBER 1983 1524-1537 © The Rockefeller University Press • 0021-9525/83111/1524/14 $1.00 on July 3, 2017 jcb.rress. org D ow nladed fom