ELECTRON M l( ;|{()S( OPV 



sists of the glomerulus, a small spherical body beginning of the tubular duct, the capsule of 



in which the urine is formed by filtration Bowman (Fig. 1), An afferent arteriole leads 



from the blood capillaries; and the attached the blood into the capillaries and an efferent 



elongated tubule, which conducts the urine arteriole drains the vascular bed, directing 



towards the renal pelvis by a simultaneous most of the blood towards the capillary net- 



reabsorption of fluid and substances. The re- work which winds around the tubule. The 



absorbed units are returned to the blood wall of the afferent arteriole contains a num- 



stream b}' a matter of diffusion into abun- ber of highly granulated cells, the so called 



dant capillaries which wind around the tu- juxtaglomerular apparatus. These cells have 



bule. By this mechanism the body is pre- been suggested to play a certain role in the 



vented from a loss of large amounts of fluid production of the hormone renin which seems 



and important substances. to have a constringent effect upon the wall 



While the glomerulus offers a fairly simple of the arterioles. The fine structure of the 



structural background for the filtration granules in the juxtaglomerular cells is 



process, the tubule is more complicated from reminiscent of that which characterizes the 



a structural point of view. The explanation granules of the endocrine glands elsewhere 



for this is to be found in the manner in which in the body. When the afferent arteriole has 



reabsorption occurs. The tubule is divided entered the capsule of Bowman, it divides 



into several structurally different segments into several main branches, each of which in 



and each segment seems to be responsible for turn gives rise to a number of interconnected 



the reabsorption of only certain substances, smaller capillaries. The blood leaves the 



The segmentation of the nephron can capillary bed of the glomerulus through the 



easily be revealed in the light microscope, efferent arteriole w^hich is formed by a con- 



The first portion of the tubule following the fluence of several main capillary branches, 



glomerulus is called 'proximal convolution. No granulated cells can be detected in the 



The next part is the loop of Henle, consisting wall of the afferent arteriole, 

 of the straight descending loop, the thin seg- The capsule of Bowman is lined by squa- 



ment, and the thick (straight) ascending mous epithelial cells which are continuous 



limb. The latter connects with the distal con- %vith the columnar epithelial cells of the 



volution in the neighborhood of the glomeru- proximal convolution. The squamous cells 



lus, followed by the short cortical collecting form the parietal layer of Bowman's capsule, 



duct. This in turn leads into the collecting whereas the cells which cover the capillaries 



tubule and conducts the urine all the way to and actually represent the indented portion 



the renal pelvis. of Bowman's capsule form the visceral layer 



In low magnification electron micrographs, of Bowman's capsule. The space between the 



the various segments of the nephron are parietal and visceral layers is called the space 



easily compared and identified with those of Bowman's capsule. This space is continu- 



seen in the light microscope. But it is easier ous with the lumen of the tubule and it is 



to note in what respect they differ struc- into this space the urine is filtered. The en- 



turally. The complexity of structures seen tire Bowman's capsule is surrounded by a 



in high magnification electron micrographs basement membrane which, at the vascular 



more than well stresses the physiological pole (where the arterioles enter and leave the 



differences recorded in the function of dif- capsule) is continuous with the basement 



ferent parts of the nephron. membranes of the arterioles and at the uri- 



Glomerulus. The glomerulus is essen- nary pole (beginning of the tubule) is con- 



tially a richly branched and interconnected tinuous with the basement membrane of the 



capillary network w^hich is indented at the tubule. 



164 



