Which cells produce the myelin sheath? What is the myelin sheath made of?

Explanation

### Step-by-Step Solution:

1. **Identify the Cells Producing Myelin Sheath**:
- In the **Central Nervous System (CNS)**, the myelin sheath is produced by **Oligodendrocytes**. These cells create the myelin sheath by the apposition (close placement) of their membranes around the axons.
- In the **Peripheral Nervous System (PNS)**, the myelin sheath is produced by **Schwann Cells**. These cells secrete myelin on the surface of the axons.

2. **Understanding the Structure of Myelin Sheath**:
- The myelin sheath is primarily composed of **lipids**, which are fatty substances that provide insulation.
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