Final answer:
The myelin sheath is primarily composed of lipids, which include the phospholipids of the glial cell membrane, and it is produced by oligodendrocytes in the CNS and Schwann cells in the PNS to insulate axons and facilitate electrical transmission. Option D.
Step-by-step explanation:
The myelin sheath is composed mostly of lipids. Specifically, myelin is a lipid-rich sheath that surrounds the axon and facilitates the transmission of electrical signals along the axon. Myelin is produced by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS).
Although myelin includes some proteins that are integral to the function of the sheath, it is the lipids—predominantly phospholipids—that make up the bulk of its composition. These lipid layers act as a very good electrical insulator, similar to the plastic or rubber that encases an electrical cord.
Therefore, the correct answer to the composition of the myelin sheath is D. Lipids.