## Core Concept
Gitter cells are a type of cell that plays a significant role in the pathological process of certain neurodegenerative diseases. They are characterized by their involvement in the removal of debris and are often seen in conditions associated with neuronal damage or death. The core concept here revolves around the understanding of gitter cells' origin and function within the central nervous system (CNS).

## Why the Correct Answer is Right
Gitter cells are essentially microglial cells that have transformed in response to pathological conditions within the CNS, such as Alzheimer's disease, cerebral infarction, or other neurodegenerative disorders. They are involved in the phagocytosis of cellular debris, including dead neurons and myelin. This process is crucial for clearing the extracellular space of harmful substances that could otherwise exacerbate tissue damage. The correct answer identifies gitter cells as a type of microglia, which is accurate given their origin from microglial cells.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because gitter cells are not primarily associated with being a type of astrocyte. While astrocytes are a type of glial cell in the brain and play critical roles in brain function and response to injury, gitter cells specifically originate from microglia.
- **Option B:** This option is incorrect as it does not accurately describe gitter cells. The description might fit other types of cells or pathological features but does not specifically relate to the definition or function of gitter cells.
- **Option C:** Without the specific details of this option, it's understood that any description not aligning with gitter cells being a form of microglia or accurately describing their function would be incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that gitter cells are often seen in areas of brain injury or disease and are a hallmark of certain pathological conditions. Their presence indicates an active process of cleaning up cellular debris, which can be a response to various types of neuronal injury or disease.

## Correct Answer: D. Microglia.