Myelin

• Ependymal cell—a cell that creates CSF (cerebrospinal fluid)

The astrocyte is star-shaped and have many processes coming from their cell body (which are

not dendrites or axons). They interact with blood vessels, connective tissue, and neurons. They

support by maintaining the concentration of chemicals in the extracellular space, by removing

neurotransmitters, by contributing to the blood-brain barrier, and by reacting to tissue damage.

The blood-brain barrier or BBB is what separates the CNS from the rest of the body. The BBB

will allow nutrients in the CNS but other molecules will not be allowed into the brain. The brain

has a special blood supply with little ability for molecules to pass through passively. Because of

this, amino acids and glucose must be pumped into the CNS through active transport. Gases

and ions can also get through the CNS. WBCs and larger molecules cannot get into the brain or

spinal cord. Pharmaceuticals often have a limited ability to pass through the BBB.

Oligodendrocytes have few branches (and thus the name). Each oligodendrocyte reaches out

and surrounds an axon in order to insulate it with myelin. One such cell will make myelin for

multiple axon segments or for different axons.

Microglia are probably white blood cell types called macrophages that become microglia as part of early embryonic development. They encounter diseased or damaged cells and ingest/digest

these cells or possibly pathogens. Because of their job, microglia are also called CNS-resident

macrophages.

An ependymal cell is a glial cell that filters the blood and turns it into cerebrospinal fluid or CSF,

which is the fluid that circulates throughout the central nervous system. These cells line the

ventricles in the CNS, which are embryological remnants of the neural tube. There is a

specialized structure in the ventricles called the choroid plexus in which ependymal cells come

into contact with blood vessels, where blood can be filtered. This makes the ependymal cells a

part of the blood brain barrier. The ependymal cells are similar to epithelial cells in the CNS.

MYELIN

The insulation for axons or “myelin” is made by the oligodendrocytes in the CNS and by

Schwann cells in the PNS. Myelin is a lipid-rich substance that surrounds the axon to make a