Walk one merge (unit §1): a fine astrocytic process fused onto a dendrite. The neuron gains a branch that does not exist. Because astrocytic processes ensheathe synapses, the merged path runs directly past many of them, and those detections get attributed to the neuron — inputs from cells it never contacted. The false inputs are spatially local, so they inflate local clustering and short-range connectivity. Instructor script: "Again the bias points toward an interesting result. That is why glia–neuron merges rank above many neuron–neuron splits in the queue, even though the split is more visually obvious."
Set the stake before anyone starts looking: a glia–neuron merge does not merely add a branch — it drags a neuron's arbor past synapses it never contacted, so the false-input yield per micrometre of merged path is unusually high. Each glial class has one near-diagnostic feature. Learn those three first: astrocyte — glycogen granules; oligodendrocyte — the darkest nucleus in the field; microglia — a dense elongated nucleus plus lysosomal content.
Read cross-sectional shape before anything else. An astrocytic process takes whatever contour is left over after the neurites pack; a neurite holds a tube shape of its own. That cue survives weak staining better than glycogen granules do, which makes it the one to reach for where everything else fails. Other astrocyte cues from unit §2: pale "watery" cytoplasm with few organelles; glycogen granules 20–30 nm, very dark, clustered (neurons do not contain them); perisynaptic processes under 100 nm thick; endfeet flattened against vessels; no vesicles, no PSDs, few microtubules.
Instructor script: "Trace the outline with your finger. Does it hold its own shape, or does it fill the gaps the neurites left?" Use the unit's check-yourself: a pale, irregular profile between three neurites, no glycogen visible. Probably astrocytic — medium confidence at best. Absence of glycogen is weak evidence because granules are clustered and sparse. Better evidence, in order: space-filling behaviour over several sections; no vesicles or PSD on it; continuity to an endfoot; microtubules at higher magnification.
Walk the unit's worked example here: a pale profile a few hundred nanometres thick, pressed flat against a capillary, currently assigned to a nearby dendrite. Rule out synaptic participation first — even when the vascular context is shouting the answer. Glycogen absent: weak evidence, noted not spent. Over 5–10 sections the contour spreads to track the vessel wall: space-filling. Context: a flattened expansion covering a vessel is an endfoot. Dismiss microglia for cause — the parent process is pale and organelle-poor, not inclusion-rich. Call: astrocytic endfoot, high confidence; the dendrite assignment is a glia–neuron merge and goes above conspicuous splits in the queue.
Look for the pairing that carries the call: a dark, often elongated or bean-shaped nucleus with heterochromatin clumped against the envelope, plus lysosomal and phagosomal content in dense cytoplasm with long, narrow ER cisternae. Microglia are the least stereotyped of the three classes because morphology tracks activation state — which is why nuclear evidence outranks process shape, and why a microglia call needs a nucleus or a lysosome-rich parent process, never a fragment.
Instructor script: "If you remember one thing about oligodendrocytes, it is the nucleus: distinctly darker than neuronal or astrocytic nuclei, usually identifiable at a glance." Then spend the time on the OPC: the correct action for a "not quite" cell is to flag rather than force. A flagged OPC candidate is a useful annotation; a forced wrong class is not.
The panel highlights every myelinated axon the H01 myelin mask finds in one grey-matter field — a small minority of profiles in cortex. Instructor cue: ask how myelin context changes proofreading priority. Context cues from the protocol: the inner or outer tongue of a myelin sheath, or a paranodal loop, is a high-confidence oligodendrocyte call.
The unit's identification protocol, which feeds step 1: (1) soma? read the nucleus first — very dark and round -> oligodendrocyte; dark, elongated, peripheral heterochromatin plus lysosomes -> microglia; pale, irregular, glycogen -> astrocyte; pale but "not quite" -> OPC candidate, flag. (2) Synaptic participation -> neurite, go to Unit 06. (3) Glycogen -> astrocyte. (4) Cross-sectional character over 5–10 sections. (5) Context: endfoot, myelin tongue, lysosome-rich parent. (6) Unresolved -> uncertain, with the missing cue named. Context is the strongest cue in the protocol, but it is only safe after the exclusion steps have run.