Stuart Young

Effects on sheep of the milkweeds Asclepias eriocarpa and A. labriformis and of cardiac glycoside-containing derivative material.

Abstract J. M. Benson , J. N. Seiber , C. V. Bagley , R. F. Keeler , A. E. Johnson and S. Young . Effects on sheep of the milkweeds Asclepias eriocarpa and A. labriformis and of cardiac glycoside-containing derivative material. Toxicon17, 155–165, 1979.—Some milkweeds native to the western United States are extremely toxic to range animals. As little as 0·05 and 0·25% of an animals weight of Asclepias labriformis and Asclepias eriocarpa, respectively, may be lethal ( Kingsbury , 1964). Cardiac glycosides (cardenolides) in these species have been implicated as toxic principles. In experiments conducted to verify this toxicity, four preparations were tested on sheep: dried, ground A. labriformis and A. eriocarpa plants, a crude ethanolic A. eriocarpa extract, and a partially purified extract from which pigments and fats had been removed. In addition, labriformin (a purified cardenolide present in A. labriformis and A. eriocarpa), and digitoxin (a clinically important cardenolide) were administered. Toxic symptoms and gross and histopathological lesions were qualitatively similar regardless of whether plants, extracts, labriformin or digitoxin were administered. Results of these experiments strongly suggest that cardenolides found in A. labriformis and A. eriocarpa account for the effects of these plants on sheep.

The external limiting membrane in developing normal and dysplastic canine retina

Development of the external limiting membrane of the retina in English springer spaniel fetuses and neonates with incipient stages of retinal dysplasia and in normal mongrel fetuses and neonates was examined using transmission electron microscopy and the freeze-fracture technique. The external limiting membrane of the retina of normal canine fetuses was composed of zonulae adherentes in which there were focal areas of apparent apposition between adjacent ventricular cell membranes. Freeze-fracture examination revealed gap junctions within the external limiting membrane that were composed of 8-10 nm P-face particles in aggregates of variable size and shape. These junctions corresponded to areas of focal membrane apposition. Tight junction strands were also identified in deep E-face grooves which were located perpendicular to the external limiting membrane. At day 46 of gestation in incipiently dysplastic English springer spaniel fetal eyes, there was a marked decrease in the size and area occupied by gap junctions within the external limiting membrane as compared with retinas of mongrel control fetuses at the same age and of other age groups. This apparent loss of gap junctions, coincident with the incipient onset of histologically detectable dysplastic events in the sensory retina, may contribute to the morphogenesis of the defect.

Cilia in the fetal and neonatal canine retina

Cilia in the canine retina were examined at 40, 46 and 50 days of gestation and at birth by scanning electron microscopy, transmission electron microscopy, and by the freeze-fracture technique. Cilia were similar in all age groups examined. Scanning electron micrographs showed them to be smooth-surfaced conical to tubular extensions arising from putative photoreceptor inner segments. Cilia when freeze-fractured contained variable numbers of circumferential rows of 10 nm P-face particles: these constitute the ciliary necklace. Transmission electron micrographs showed the ciliary membrane to contain electron-dense beads which corresponded to the ciliary necklace seen in freeze-fracture replicas. The ciliary necklace identified in the developing canine retina was similar to those found in other types of motile and sensory cilia.

Retinal vessls of canine eyes at different ages—A qualitative and quantitative study

Diameter and number of cells per unit length of vessel wall and endothelial cell frequency were determined in capillaries of trypsin-digested Foxhound retinas from different age groups. Capillary diameters increased and number of cells per unit length of capillary wall decreased with age and distance from the optic disc. Endothelial cell frequency was constant at approximately 79% of the total cells in capillary walls in all areas measured. Peripheral cystoid degeneration and peripheral annular and focal degeneration were found in aged dog retinas. Sclerosis of retinal arterioles was observed ophthalmoscopically, histologically, and in trypsin-digested retinas from aged dogs. The significance of this change in relation to the peripheral retinal degeneration is undetermined. It is proposed that thickening of basement membranes observed in peripheral capillaries of retinas causes chronic, low-level hypoxia leading to peripheral retinal degeneration in aged retinas.