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The fine structure of the normal rat aorta is described. The presence of a subendothelial layer, the oblique orientation of the smooth muscle cells with respect to the aortic axis, and the occurrence of desmosomes between these cells and adjacent elastic laminae, are emphasized. Lead-stained collagen presented a characteristic signetMng appearance on cross-section. The rats examined were the pair-fed controls for the lathyritic series described in a separate communication.Electron microscope studies on the normal aorta and large arteries of mammals are scant. Buck (1) described the endothelium of the aorta, femoral and splenic arteries of rats and rabbits, and Parker (2) reported the normal architecture of rabbit coronary arteries. The only papers that could be found on the whole aortic wall were by Berrian (3) on the rat, and a short abstract on the kitten aorta by Pease (4). Although the present study is in general agreement with these, it differs with Berrian on several points.I t was considered of interest, therefore, to investigate the fine structure of the normal rat aorta using the newer embedding and staining techniques. In addition, this investigation serves as a basis for the lathyritic and enzyme studies reported in separate communications. Materials and MethodsFive albino rats of Sprague-Dawley strain were weaned at 21 days and then fed Purina fox chow. Each was pair-fed with a litter mate given a sweet pea diet, and thus constituted the controls for the lathyritic animals reported in a separate paper (5). The intake averaged 9 to 11 gin. daily and all animals remained apparently healthy though hungry. They were sacrificed when 37, 38, 41, 54, and 60 days old. After anaesthetizing the animal with ether, the thoracic cage was opened and both ascending and descending aortae were removed as quickly as possible. (The actual arch was carefully avoided owing to the multidirectional elastic laminae at the origin of the great vessels (6)). Unwanted fat and connective tissue were removed and 2 mm. lengths of the ascending aorta were placed in ice-cold 2 per cent O~O4 buffered to pH 7.3 with veronal-acetate and containing sucrose as described by Caulfield (7). Short lengths of the descending thoracic aorta were put in 10 per cent formalin for light microscopy. After fixation for 1 to la/} hours at 0-5°C. and dehydration at room temperature, duplicate specimens for electron microscope examination were embedded in butyl methacrylate and araldite. The best results with methacrylate were obtained by polymerizing at 60°C. for 2 days. Difficulty was experienced in obtaining undistorted sections of tissue embedded in araldite, and this was partly due to the nature of the aortic wall which consists of alternating layers of elastin and muscle cells, each layer presenting a different cutting property. The commonest artifacts were washboarding and macroscopic disintegration of the specimen in the microtome collecting trough. Since methaerylate did not appear to produce such an accurate anatomical picture as arald...

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The kinetics of collagen reprecipitation from solutions of salt-extracted calf dermis in the presence of small amounts of mucopolysaccharidc and nucleic acids (0.005 per cent in the final reaction mixture) has been reported by . The present paper is a parallel study using the same materials, and describes the electron microscopic (EM) morphology of the collagen precipitates replicated after 24 hours at room temperature. Satisfactory, uncontaminated EM preparations were obtained which showed that all the deposits were fibrous and bore the 640 A cross-banding characteristic of collagen except some narrow, background fibrils 200 to 1000 A wide precipitated in the presence of heparin. These exhibited fine striations about 220 A apart. Chondroitin sulfate greatly increased the rate of precipitation to give a deposit of low optical density consisting of narrow, rigid, discrete fibrils resembling fresh dermis. In contrast, heparin prevented macroscopic gelation, delayed precipitation, and only produced a scanty deposit of abnormal, short, wide, striated tactoids and compound fibers of varying length. The control preparations and the deposits formed in the presence of hyaluronic acid were intermediate between these two extremes. Delayed precipitation was associated with a coarser deposit and aggregation of the fibrils. A duplicate series of deposits precipitated in the presence of RNA and DNA, together with their controls, were examined after 1~, 1, 1}~, 3, 9, and 24 hours. One set employed an acetic extract of whole calf dermis and the other salt-extracted dermis. The presence of 0.005 per cent DNA in the reaction mixture markedly delayed collagen precipitation with the slow formation of abnormal, short, wide tactoids and compound fibers. RNA also interfered with the quantity and quality of the deposits which contained far less collagen resembling unfixed, normal, adult human dermis, than the controls at the corresponding time intervals. Comparison of the experiments employing whole calf dermis with those employing the salt-extracted material demonstrated that at every time interval in all the experiments the deposits were retarded when salt-extracted dermis was used. This suggests that the saltsoluble components of the dermis play a part in fiber formation.Previous work in this series of experiments (1-3) described the kinetics of collagen precipitation from salt-extracted calf dermis based on a systematic study of the effect of experimental conditions on both the rate of fibril formation and the morphology as seen under the electron microscope. Samples of the identical solutions used for the rate studies revealed that factors which speeded up the precipitation of neutral salt collagen (increased temperature; decreased ionic strength)