Sections of mouse renal glomerulus fixed by perfusion with buffered osmium tetroxide solution have been studied with the electron microscope.

Four components are recognized in the mouse glomerulus: epithelium, basement membrane, endothelium, and intercapillary cell. The three cellular components all display in their cytoplasm mitochondria, Golgi bodies, endoplasmic reticulum, and uncharacterized vesicles.

The concepts of Hall, of Pease, and of Rhodin regarding the glomerular filtration surface are confirmed.

The epithelial cells are characterized by intricate, branching, interdigitating ridge-like processes or pedicels, the summits of which press against the urinary surface of the basement membrane, covering the glomerular capillary tuft almost completely except for narrow spaces about 200 to 300 A wide between the processes. These spaces are termed the epithelial filtration slits, and are bridged by a very delicate gossamer-like membrane about 30 A thick,—the filtration slit membrane.

The basement membrane is interposed everywhere between epithelial processes and endothelium, and between epithelial and intercapillary cells. The basement membrane of the filtration surface of the glomerular capillary has smooth surfaces and is about 800 A thick. It consists of three layers—a thick central lamina densa, appearing to have a very delicate felt-like structure, flanked on each side by thinner lamina rara externa and lamina rara interna. This membrane continues to the intercapillary space and makes a complicated sponge-work of varying thickness in which the intercapillary cells are enmeshed.

The endothelial cells are of moderate thickness in the nuclear region, but send out thin sheet-like extensions over the filtration surface. These extensions are about 300 to 400 A thick and are characterized by numerous round endothelial filtration pores about 500 to 1000 A in diameter.

The intercapillary tissue or mesangium is composed of the network of the basement membrane and the intercapillary cells.

The intercapillary cells, with characteristic fine fibrillar cytoplasm, make contact with epithelial and endothelial cells and are enmeshed within the network of the basement membrane. Rounded processes of intercapillary cells penetrate into the endothelial cell through the basement membrane, and may even perforate entirely through the endothelium. Such processes are called (after Zimmermann) the intracapillary colliculi.

In the endothelial cytoplasm close to the intracapillary colliculi are many dense endothelial juxtacollicular vesicles and caveolae.

The cell boundaries of the endothelial cell resemble terminal bars.

Some physiological speculations relating to glomerular structure are advanced.

The description of Zimmermann (54), based on a light microscope study, is confirmed in many respects.

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