Final answer:
The descending limb of the loop of Henle's thick segment is lined with simple cuboidal epithelium, allowing water and ion exchange, while the thin segment's simple squamous epithelium permits water permeability through aquaporin channels. The ascending limb actively transports ions without water, maintaining a hypoosmotic filtrate.
Step-by-step explanation:
The descending limb of the nephron loop, also known as the loop of Henle, is vital for the concentration of urine. The thick descending limb is lined with simple cuboidal epithelium, which is similar to that found in the proximal convoluted tubule (PCT). This epithelium allows for the reabsorption of water and certain ions. The descending thin portion, however, is lined with simple squamous epithelium and is highly permeable to water due to the presence of aquaporin channel proteins. This leads to the movement of water into the surrounding interstitium, significantly concentrating the filtrate.
The ascending limb, by contrast, is impermeable to water in both its thin and thick segments. The thick ascending limb actively reabsorbs ions without water following, contributing to the generation of hypoosmotic filtrate and maintaining a hyperosmotic environment in the kidney medulla. This selective permeability at different portions of the nephron loop is crucial for the kidney's ability to concentrate urine and maintain fluid and electrolyte balance within the body.