Medulla

Proximal straight tubules (thick descending limbs of the loop of Henle) are lined by a tall, simple cuboidal epithelium with a prominent brush border.  Many of the proximal tubules seen here have precipitate filling the lumens and show considerable vacuolation in their cytoplasm.

The thin limbs of the loop of Henle are lined by a simple squamous epithelium.  Usually this epithelium, although squamous, is thicker than that seen in a capillary.

Thick ascending limbs of the loop of Henle (distal straight tubules) have a lower epithelium than do proximal tubules and lack a brush border.  Additionally, the nuclei are spaced irregularly around the lumen, into which they frequently bulge.

Collecting ducts are identified by a tall, eosinophilic, simple columnar epithelium.  Lateral cell boundaries are distinct in a collecting duct, and a brush border is absent.

Vasa recta arise from efferent arterioles of renal corpuscles located near the cortico-medullary junction.  These capillaries accompany the tubules through the medulla.