Macrophage is CNS
**Core Concept**
Microglia are the resident macrophage-like cells in the central nervous system (CNS). They play a crucial role in the maintenance of CNS homeostasis, participating in the surveillance of the brain environment, and responding to pathogens and injury.
**Why the Correct Answer is Right**
Microglia are derived from the yolk sac and migrate to the CNS during embryonic development. They are the primary immune cells in the brain, involved in the phagocytosis of apoptotic cells, debris, and pathogens. Microglia express receptors that allow them to detect changes in the CNS environment, such as the release of chemokines and cytokines. Upon activation, microglia undergo morphological changes, increasing their surface area and releasing various factors that modulate immune responses.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the question specifically asks about macrophages in the CNS, not other types of immune cells.
**Option B:** This option is incorrect because while astrocytes do play a role in the CNS, they are not macrophages.
**Option C:** This option is incorrect because oligodendrocytes are responsible for myelination in the CNS, not immune surveillance.
**Clinical Pearl / High-Yield Fact**
Microglia are the only immune cells in the CNS that are capable of movement, allowing them to survey the brain environment and respond to changes.
**Correct Answer:** D. Microglia.