Mucus threads are similar to casts but lack:

Mucus threads and casts are both components that can be found in urine sediment, but they have distinct characteristics. The primary difference highlighted in this question is the edge definition. Mucus threads are generally more amorphous and lack the clear, defined edges associated with casts. Casts are formed in the renal tubules and have a solid, compact structure, often retaining the shape of the tubules from which they originated.

In contrast, mucus is secreted by the mucous membranes and appears in a more thread-like format without the distinct, structured boundaries. This lack of defined edges is indicative of mucus threads' more fluid and less organized form compared to casts.

Understanding this relationship helps in the differentiation between the various components found in urine, which is crucial for accurate urinalysis and diagnosis in veterinary practice. The other aspects, such as size variation, presence of crystals, and color variation, do not specifically define the key difference between mucus threads and casts, making them less relevant to this comparison.