**Core Concept**
Glial cells, also known as neuroglia, are non-neuronal cells in the central and peripheral nervous systems. They provide support, insulation, and protection to neurons, and play a critical role in the maintenance of the nervous system environment. Glial cells are classified into two main categories: macroglial cells and microglial cells. Macroglial cells are further divided into oligodendrocytes, astrocytes, and ependymal cells, which are all of ectodermal origin.

**Why the Correct Answer is Right**
Microglial cells are distinct from other glial cells in that they are of mesodermal origin. They are derived from the yolk sac, a structure that is itself of mesodermal origin. As such, microglial cells are the only glial cells that are not of ectodermal origin. This is a critical distinction, as it affects the development, function, and behavior of microglial cells in the nervous system. Microglial cells are involved in immune surveillance, phagocytosis of debris, and the modulation of neuroinflammatory responses.

**Why Each Wrong Option is Incorrect**
**Option A:** Macroglial cells, including oligodendrocytes, astrocytes, and ependymal cells, are all of ectodermal origin, making this option incorrect.
**Option C:** Oligodendrocytes are a type of macroglial cell that are responsible for myelinating axons in the central nervous system. They are of ectodermal origin, making this option incorrect.
**Option D:** Ependymal cells line the ventricles and central canal of the spinal cord and are involved in the production of cerebrospinal fluid. They are also of ectodermal origin, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that microglial cells are the only glial cells of mesodermal origin, which sets them apart from other glial cells in terms of their development, function, and behavior. This distinction is critical in understanding the role of microglial cells in neuroinflammatory responses and their potential involvement in neurological disorders.

✓ Correct Answer: B. Microglial cells