Unusual Development of Basement Membrane about Small Blood Vessels

Basement m e m b r a n e is usually found where connective tissue comes into contac t wi th epithelial s tructures (1). I t occurs on the outer surface of the endothe l ium of small blood vessels, t hough it is flimsy and deficient about sinusoidal vessels of the spleen, liver, bone marrow, and frog hear t (2-6). Consisting of a feltwork of fine filaments embedded in a homogeneous ma t r ix (4, 5), it comprises three layers: viz., the l amina ra ra interna, l amina densa, and l amina ra ra externa (5, 7-9). U n d e r normal or physiological conditions, endothel ia l basement membranes are known to vary in thickness with age (9-13), species (9, 11, 13-17), and the site and na tu re of the vessel (2, 3, 9-20). Pathological th ickening of the glomerular basement membranes occurs in bo th spontaneous and experimental ly induced lesions (9, 1 l, 21-23). However , in the present repor t an exceedingly thick basement membrane , wi thout any recognizable pathological cause, was found in small vessels of the frog. In addit ion, the uhras t ruc tu ra l organizat ion of the basement m e m b r a n e was qui te unusua l in tha t it was related not only to endothe l ium bu t also to pericytes and advent i t ia l cells.