Where Glia Come From

The microglia in your brain were not made by your brain. In mice, most of them descend from cells born in the yolk sac, a membrane outside the embryo, before the brain had a blood-brain barrier.

So “glia” is a job description, not a family. The word covers every non-neuronal cell that lives among neurons, and those cells come from at least three different places. The class on glial signaling opens with a figure of everything glia do and one question: “What functions are we missing?” This lesson asks an earlier question. Where does each kind of glial cell come from, and what tells a progenitor to make one?

The idea: one name, several birthplaces

Here is the cast. In the central nervous system (brain and spinal cord) there are astrocytes, star-shaped cells that touch blood vessels and synapses; oligodendrocytes, which wrap central axons in myelin, and their dividing oligodendrocyte precursors (OPCs); ependymal cells lining the ventricles; and microglia, the resident immune cells. In the peripheral nervous system there are Schwann cells along nerves and satellite glia around neuron cell bodies in ganglia.

For a long time these could not even be told apart. Cajal’s stains showed astrocytes well but left many small nuclei with no visible branches, which he grouped as a “third element”. Pío del Río-Hortega, using a silver carbonate stain he kept refining, split that group in two: microglia, described in 1919, and oligodendroglia, the “glia with few processes”, named in 1921. He saw many oligodendroglia lying between bundles of nerve fibres and, in a paper of 1922, proposed that they were the central counterpart of Schwann cells. But a stain shows shape and position. It cannot follow a cell’s descendants, so it could not settle origins.

Lineage methods did. Here is the answer they gave:

• Astrocytes, oligodendrocytes and ependymal cells come from the neuroepithelium of the neural tube, mostly through the radial glia that also made the neurons.
• Microglia come from the yolk sac.
• Schwann cells and satellite glia come from the neural crest, the cells that leave the edges of the closing neural tube (see the neural crest lesson).

Radial glia switch from neurons to glia

Radial glia are the stem cells of the developing cortex (see that lesson). In the mouse cortex they make neurons first, and most astrocytes appear late in gestation and after birth. The same progenitor pool changes its output. Think of a bakery that makes bread all morning and switches to pastry in the afternoon: same ovens, a different order sheet. What writes the new order?

Three signals do most of the work, and you have met two of them in Cell Signaling.

Notch and Hes. “Hes inhibits proneural genes.” Proneural genes push a cell to become a neuron (see proneural genes and lateral inhibition). Notch signaling turns on Hes repressors, which hold those genes down and keep the cell a progenitor (see the Notch lesson). Hes1 maintains neural stem cells: forced Hes1 expression in mice keeps stem cells going and prolongs the making of later-born neurons. Notch does not make astrocytes by itself. It keeps a pool of progenitors alive past the neurogenic phase and biases them toward glia.

Cytokines and JAK-STAT. CNTF, LIF and cardiotrophin-1 are members of the interleukin-6 family. They bind receptors that share the subunit gp130, which activates JAK kinases; JAK phosphorylates STAT3, which pairs up, enters the nucleus and switches on astrocyte genes (see the JAK-STAT lesson). In cultures of rodent cortical progenitors, CNTF drives the appearance of cells expressing GFAP, glial fibrillary acidic protein, the standard astrocyte marker. Early in neurogenesis the same cytokine does little, because the STAT3 binding site in the GFAP promoter is methylated and cannot be read. As development goes on, the site is demethylated and the cell becomes able to answer.

Where does the cytokine come from? One proposal, from work in mouse cortex, is the neurons themselves: newborn neurons make cardiotrophin-1, so the more neurons a region has made, the stronger its push toward glia. That is a neat feedback, timing glial onset by the work already done.

NFIA. In chick and mouse spinal cord, the transcription factor NFIA comes on in progenitors just as gliogenesis begins and is needed for it to start on time. In mouse cortical precursors, Notch signaling from newborn neurons has been shown to induce NFIA, which ties the three signals together.

Oligodendrocytes start in a ventral strip

Oligodendrocyte precursors are born from specific domains, and in the spinal cord the main one is ventral. Recall from the Shh and BMP lesson that Sonic hedgehog from the floor plate divides the ventral cord into progenitor domains. One of them, pMN, expresses the bHLH factor Olig2. In mice and chicks, pMN first makes motor neurons (see the motor neuron lesson) and then switches to making OPCs. Mice lacking both Olig1 and Olig2 have no spinal motor neurons and no spinal oligodendrocytes, which shows the shared origin as plainly as a knockout can.

The ventral strip is not the only source. In the mouse spinal cord, a later, smaller wave of OPCs comes from dorsal domains. In the mouse forebrain, Cre lineage tracing found three waves: from a ventral domain, then from the lateral and caudal ganglionic eminences, then from the cortex itself after birth. Much of the first ventral wave is lost after birth and replaced by the later ones. Once born, OPCs migrate widely and divide in response to PDGF-A, acting on the receptor tyrosine kinase PDGFRα (see receptor tyrosine kinases). How they then wrap axons belongs to the myelination lesson.

One cell, two futures

In 1983 Martin Raff, Robert Miller and Mark Noble cultured cells from the optic nerve of seven-day-old rats. Cells carrying the surface marker A2B5 became oligodendrocytes, carrying galactocerebroside (GC), in serum-free medium. In 10% fetal calf serum they became GFAP-positive cells with an unusual shape, called type-2 astrocytes. They named the progenitor O-2A. One caution: a clear counterpart of the type-2 astrocyte in the living animal has been hard to find, so treat it as a fate the progenitor can take in a dish.

A fate that follows the medium can mean two different things. Serum might instruct the cell, or it might only select which already-decided cells survive. In 1984 Raff, Brenda Williams and Miller switched media at different times. The choice could still be reversed after one to two days in serum, after which it became fixed. Counts of recovered cells were used to argue that the shift was not explained by death. The serum signal itself was not identified in 1984; later work showed that bone morphogenetic proteins in serum can push these cells toward the type-2 astrocyte fate, as can CNTF together with extracellular matrix. Separately, in 1988, Raff’s group and Noble’s group showed that PDGF made by astrocytes keeps O-2A progenitors dividing in culture.

Design problem

A single A2B5-positive cell from a P7 rat optic nerve gives an oligodendrocyte in serum-free medium and an astrocyte-like cell in serum. How would you decide whether serum instructs the fate of a cell or merely selects which pre-committed cells survive?

Microglia come from outside

Microglia are macrophages, and for decades it was argued whether they come from blood monocytes made in the bone marrow or from an earlier source. In 1999 Fabrice Alliot, Isabelle Godin and Bernard Pessac proposed, from where and when the cells appeared in mouse embryos, that microglia arise from the yolk sac.

In 2010 Florent Ginhoux, Miriam Merad and colleagues tested this with an inducible fate map in mice. A Runx1-driven Cre, activated by a single dose of tamoxifen at about embryonic day 7.5, permanently marked cells in the yolk sac and all their descendants. In adults, about a third of microglia carried the mark while very few blood cells did, and later pulses marked blood but few microglia. Embryos lacking a heartbeat (Ncx1 mutants) made yolk-sac macrophages but had no brain microglia, so the progenitors travel in through the early circulation. Microglia also need the receptor CSF1R: mice without it have few or none. Later work named the source more precisely as erythromyeloid precursors of the yolk sac.

One limit matters. In healthy adult mice, microglia renew themselves and blood monocytes do not replace them. After irradiation or some injuries, monocytes can enter the brain, which is why early bone marrow transplant experiments seemed to show replacement. Microglial roles in pruning synapses belong to the microglia and complement lesson.

Schwann cells come from the crest, in steps

In the peripheral nervous system, glia come from neural crest. Satellite glia, and the enteric glia of the gut wall, are crest derivatives. Schwann cells pass through stages that Kristján Jessen, Rhona Mirsky and colleagues defined in embryonic rat nerves: crest cell, then Schwann cell precursor, then immature Schwann cell, then myelinating or non-myelinating Schwann cell.

Each step has a different relationship to the axon. Schwann cell precursors die in culture unless neurons are present, and in a 1995 paper the group reported that neuregulin, a growth factor on axons read by ErbB2 and ErbB3, can keep them alive and drive their maturation into Schwann cells. Mouse genetics agreed: mice without ErbB3 lack Schwann cell precursors along their nerves. A conditional knockout went further. With a Cre line active in spinal roots around embryonic day 11 but in the sciatic nerve only near birth, deleting ErbB2 emptied the roots of most Schwann cells while sparing the sciatic nerve. The need for ErbB2 belongs to an early window in the life of the precursor pool. How glia later feed axons is in the metabolic partners lesson.