What structure forms cylindroids in the urine?

Cylindroids in urine are formed primarily in the collecting ducts. These structures are a type of cast that represents the presence of mucoproteins and cellular debris that have collected in the renal tubules. The collecting ducts are responsible for the final concentration of urine and play a key role in the excretion of certain substances. When urine passes through these ducts, conditions such as dehydration or other renal pathologies can lead to the aggregation of materials which ultimately forms cylindrical shapes due to the tubular structure of the ducts.

While the ascending loops of Henle, proximal tubules, and distal convoluted tubules are involved in various stages of filtrate processing and urine formation, they do not significantly contribute to the formation of cylindroids. The unique characteristics of the collecting ducts, including their cell types and the nature of the substances being concentrated during urine passage, make them the primary site for cylindroid formation.