**Core Concept**
Pilocytic astrocytomas are a type of slow-growing brain tumor characterized by distinct histological features. These features include specific cellular and vascular changes that aid in diagnosis.

**Why the Correct Answer is Right**
Pilocytic astrocytomas typically exhibit an increased number of blood vessels, which is a hallmark of this tumor type. This is due to the high vascularity and the presence of microvascular proliferation, making option D incorrect. The other characteristic features of pilocytic astrocytoma include the presence of **Rosenthal fibers**, which are eosinophilic, cigar-shaped structures formed by the accumulation of intermediate filaments, and **eosinophilic granular bodies**, which are collections of degenerated astrocytic processes. Additionally, **bipolar cells** are a common feature, often showing a characteristic "fried egg" appearance due to the presence of a large, pale nucleus surrounded by a clear halo.

**Why Each Wrong Option is Incorrect**
**Option A:** Bipolar cells are indeed a characteristic feature of pilocytic astrocytoma, often showing a "fried egg" appearance due to the presence of a large, pale nucleus surrounded by a clear halo. Therefore, option A is correct in the context of pilocytic astrocytoma.
**Option B:** Rosenthal fibers are a hallmark of pilocytic astrocytoma and are formed by the accumulation of intermediate filaments. They are a key feature in the diagnosis of this tumor type, making option B incorrect.
**Option C:** Eosinophilic granular bodies are collections of degenerated astrocytic processes and are a characteristic feature of pilocytic astrocytoma. They are often found in the vicinity of Rosenthal fibers and are an important diagnostic feature, making option C incorrect.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize the characteristic histological features of pilocytic astrocytoma, including Rosenthal fibers, eosinophilic granular bodies, and bipolar cells, to distinguish it from other glioma types. A high index of suspicion and awareness of these features are crucial in making an accurate diagnosis.

**✓ Correct Answer: D. Decrease in the number of blood vessels**