The fine structure of the medullary vessels

EFFERENT ARTERIOLES - The efferent arterioles of the juxtamedullary glomeruli are long and have a relatively thick wall. They have not been studied systematically in the human kidney and the following account is based upon their structure in the rat as described by Moffat (1967) and later by other workers (see Moffat 1975 for references).

The wall of the juxtamedullary efferent is thicker than that of the cortical efferent since it has a well-developed muscular media. The endothelium is relatively thick and non-fenestrated and it is surrounded by two or three layers of smooth muscle cells. Outside the media there are bundles of non-myelinated nerve fibres, a few fibroblasts and bundles of collagen fibres which separate the arterioles from the surrounding renal tubules.

The sphincter-like arrangement of smooth muscle around these arterioles could control the medullary blood flow and in injected animal preparations there often appears to be a narrow constriction at this site (Fourman & Moffat 1971). The muscle could also vary the glomerular filtration rate by altering the postglomerular resistance but direct evidence for either of these functions is lacking at present.

VASA RECTA

Immediately after the division of the efferent arteriole in the subcortical zone, the descending vasa recta are grouped together and each is surrounded by a layer of smooth muscle cells, usually only one cell thick. This, in turn, is surrounded by bundles of non-myelinated nerves. The vessels showing this structure are found in the deepest layers of the subcortex and the most superficial layers of the outer medulla.

Throughout the rest of the outer medulla, the descending vasa recta are closely intermingled with the ascending vasa recta to form the central part of the vascular bundles (see below) and here" the structure of their wall changes. The endothelium of the descending vasa recta is similar to that of the efferent arteriole (Fig. 1.11), consisting of a relatively thick, non-fenestrated endothelium resting on a basal lamina.

This is in marked contrast to the endothelium of the ascending vasa recta which resembles that of the peritubular capillaries. It is extremely attenuated except in the region of the nucleus and it is extensively fenestratcd.

The fenestrations, like those in the peritubular capillaries of the cortex, are regular in shape, about 70 nm in diameter, and are closed by a thin membrane. The membrane does not, however, appear to impede the passage of large molecules through the fenestrations, at least in experimental animals (Moffat 1969).

Fig. 1.11: Part of a vascular bundle from a rat kidney, to show a perivascular cell separating ascending and descending and descending vasa recta. Note the zone of filaments on the side facing the descending vessel and the pinocytotic vesicles facing, the ascending vessel. From Moffat (1967), by permission of the editor and Academic Press Inc.

PERIVASCULAR CELLS

In many places the ascending and descending vasa recta in the vascular bundles are separated only by their basal laminae and a few collagen fibres, but elsewhere the descending vasa recta are surrounded by a single, and sometimes incomplete, layer of very characteristic perivascular cells. These are thin cells which are wrapped around the descending vasa, sometimes completely encircling them.

On the side of the cell which faces the thick endothelium of the descending vessel there is a zone which is devoid of mitochondria, ribosomes and other organelles but is packed with fine parallel filaments that resemble the myofilaments of smooth muscle cells (Fig. 1.11). On the opposite side of the cell, which is closely related to the fenestrated endothelium of the ascending vessel, there are numerous pinocytotic vesicles.

These relationships are invariable; similar cells have been identified in the outer medulla of the human kidney (Fig. 1.12). The function of the perivascular cells is unknown at present but it seems possible that the filaments are contractile and can vary the diameter of the descending vasa recta along the whole length of their course in the outer medulla.

Fig. 1.12: Perivascular cell in the human kidney. The cell contains a very distinct band of filaments on the descending vessel side, and pinocytotic vesicles  on the ascending vessel side. The interstitial tissue contains collagen fibres.

It is tempting to suppose that the pinocytotic vesicles indicate the sampling of the blood leaving the medulla via the ascending vasa recta, since these vessels are freely permeable, but attempts to ‘fecd’ the cells by introducing electron-opaque material into the blood-stream and thence into the interstitial tissue have so far been unsuccessful.

MEDULLARY INTERSTITIAL CELLS

As the descending vasa recta approach the inner medulla, the character of the perivascular cells changes (Fig. 1.14). They become wider and less exclusively associated with the descending vasa recta, lose their regular arrangement of filaments and pinocytotic vesicles and begin to accumulate large lipid droplets in their cytoplasm.

Finally, in the inner medulla itself, the cells take on the characteristics of the medullary interstitial cells (Fig. 1.13) which have been described by a number of authors (see Bulger & Nagle 1973 for references). These cells are mostly arranged at right angles to the longitudinally running vessels and tubules of the medulla so that they give the appearance of the rungs of a ladder.

They are, however, fewer in number and less regularly arranged in the human kidney than in many animals. The electron microscope shows these cells to be irregular in shape with numerous processes, They contain densely staining lipid droplets and a great deal of granular endoplasmic reticulum, the cisternae of which may contain flocculent material.

There is also a rather prominent Golgi apparatus so that the general impression given is that of a secreting cell. Many authors have described other peculiar features such as greatly dilated cisternae of endoplasmic reticulum, an enlarged perinuclear cistern and gaps in the cell membrane through which the lipid droplets can escape.

In addition, many of the illustrations in the literature show the stigmata of poor fixation such as ‘exploded’ mitochondria, empty cytoplasmic vesicles and a general washed-out appearance. All these effects are probably the result of trying to fix a tissue whose osmolality may be 2,000 mOsm in a fixative with an osmolality between 300 and 400 mOsm.

Fig. 1.13: A characteristic medullary interstitial cell from the rat kidney. It contains much granular endoplasmic reticulum and irregular lipid droplets. The copious interstitial tissue contains parts of numerous other interstitial cells. The capillary (cap) has a fenestrated endothelium cut tangentially at the arrow.

The same difficulty is probably the cause of the differing appearances of the interstitial cells in diuresis and antidiuresis which have been reported by a number of authors. Bohman (1974) has fixed the medulla of the rat kidney in special fixatives whose osmolality was tailored to that of the part of the medulla from which the specimen was taken and found that he could obtain good fixation of all levels of the medulla.

The function of these interstitial cells remains uncertain but it seems highly likely that they are concerned with the synthesis and/or storage of the prostaglandins. Both histochemical and analytical investigations have identified prostaglandins or their precursors in the medulla and Muirhead et al. (1972) have succeeded in culturing interstitial cells and have found that the cultures were able to synthesize all three of the medullary prostaglandins (PGA2, PGE2 and PGF2 alfa).

More recently Bohman and Larsson (1975) have analysed membrane fractions from the rabbit renal medulla and found that prostaglandin synthetase was most concentrated in the fraction that was (probably) derived from“ the endOplasmic reticulum of the interstitial cells. The prostaglandins are of great interest in any consideration of the vascular system since they have important effects on the intrarenal circulation which will be discussed in the next chapter.

Fig. 1.14: Diagram to show the cells associated with the medullary vessels. From Moffat (1967), by courtesy of the editor and Academic Press Inc.

CAPILLARIES

The medullary capillaries have a thin, fenestrated endothelium (Fig. 1.13). In the inner medulla, where the vasa recta are not arranged in organized bundles, it is therefore difficult to distinguish between ascending vasa recta and capillaries since both have a similar type of endothelium.

Read the full article: The Anatomy of the Renal Circulation