What histological feature is characteristic of a well-differentiated tumor?

A well-differentiated tumor is characterized by its ability to maintain some of the structural characteristics of the tissue from which it arises. This means that, despite being neoplastic, the tumor cells resemble normal cells in appearance and function, preserving the architecture of the tissue. This preservation often indicates that the tumor is less aggressive and more likely to have a better prognosis compared to poorly differentiated tumors, which exhibit a loss of structural organization and a more abnormal appearance.

In terms of histological analysis, well-differentiated tumors will show features such as organized tissue structures, recognizable cell types, and maintained features like gland formation or the pattern of the tissue. This contrasts with high-grade tumors, where significant alterations to cellular architecture typically occur, leading to a lack of resemblance to the tissue of origin.

High-grade nuclear pleomorphism, necrosis, and increased mitotic activity are indicative of more aggressive cancer types and correlate with poorer differentiation. The presence of such features suggests a greater likelihood of rapid growth, invasiveness, and potential metastasis, which detracts from the "well-differentiated" classification. Thus, the correct answer reflects a key aspect of tumor pathology that is crucial for understanding tumor behavior and prognosis.