Dendrite Biology and Ultrastructure

Introduction

Dendrites are the primary input-receiving compartments of neurons. They extend from the soma as tapering, branching processes that collectively form the dendritic arbor — the antenna system through which a neuron samples its synaptic environment. In electron microscopy, dendrites present a distinctive set of ultrastructural features that distinguish them from axons and glia. This script provides annotators and instructors with a comprehensive guide to dendritic morphology, spine classification, and the organelle signatures that define the dendritic compartment.

1. Overview of Dendritic Function

Dendrites receive synaptic input from presynaptic terminals, integrate excitatory and inhibitory signals through passive cable properties and active conductances, and transmit the resulting electrical signals toward the soma. Unlike axons, dendrites:

Taper distally: Their caliber decreases progressively from the soma toward distal tips.
Branch extensively: A single pyramidal neuron may have 30-40 branch points in its basal dendrites alone.
Contain ribosomes: Dendrites support local protein synthesis, a feature absent from mature vertebrate axons under normal conditions.
Have mixed microtubule polarity: Both plus-end-out and minus-end-out microtubules coexist (Baas et al., 1988), unlike the uniform plus-end-out polarity of axonal microtubules.

2. Proximal vs. Distal Morphology

The character of a dendrite changes dramatically from its base to its tips.

Proximal Dendrites (within ~50 micrometers of the soma)

Caliber: 5-10 micrometers in diameter for large pyramidal neurons.
Organelle content: Rich in rough ER (continuous with somatic Nissl substance), multiple Golgi outposts, abundant mitochondria, and dense microtubule arrays.
Ribosomes: Polyribosomes are plentiful both on ER membranes and free in the cytoplasm.
EM appearance: The cytoplasm appears relatively dark and granular due to the density of ribosomes and organelles. Microtubules run longitudinally in loose parallel bundles.

Distal Dendrites (terminal branches)

Caliber: Often less than 1 micrometer, sometimes as thin as 0.3 micrometers.
Organelle content: Rough ER is reduced to scattered polyribosomes. Smooth ER tubules persist. Mitochondria are present but fewer and smaller.
Spines: Dendritic spines are most abundant on mid-to-distal dendrite segments of spiny neurons (pyramidal cells, medium spiny neurons).
EM appearance: The cytoplasm is lighter, with fewer ribosomes. Microtubules are still present but in smaller numbers. The process may be difficult to distinguish from thin axons without careful examination of organelle content.

The Organelle Gradient

This proximal-to-distal gradient of decreasing organelle density is a key concept for annotators. When tracing a process away from a soma and it gradually loses rough ER, becomes thinner, and develops spines, you can be confident you are following a dendrite.

3. Dendritic Spines in Detail

Dendritic spines are small protrusions from the dendritic shaft that serve as the postsynaptic elements for most excitatory synapses in the mammalian brain. They are among the most intensively studied structures in neuroscience and are critical for annotators to identify correctly.

3.1 Thin Spines

Morphology: Long, narrow neck (often 0.1-0.2 micrometers wide, up to 2 micrometers long) topped by a small bulbous head.
Head volume: Approximately 0.01-0.1 cubic micrometers.
Frequency: The most common spine type in adult cortex, often comprising 50-65% of all spines (Harris et al., 1992).
Functional significance: Often called “learning spines” because they are thought to be dynamic structures that can enlarge (becoming mushroom spines) during synaptic potentiation or retract during depression.
EM identification: Look for a narrow stalk connecting to the dendrite shaft, with a small terminal swelling containing a PSD.

3.2 Mushroom Spines

Morphology: Short, wide neck supporting a large, bulbous head.
Head volume: Greater than 0.6 cubic micrometers in many classification schemes; some mushroom spines exceed 1 cubic micrometer.
Frequency: Approximately 25-35% of spines in adult cortex.
Functional significance: Called “memory spines” because they are stable over time and associated with strong, potentiated synapses. The large head accommodates a larger PSD with more AMPA receptors.
EM identification: The large head is conspicuous and often contains a spine apparatus. The PSD is prominent and easy to identify.

3.3 Stubby Spines

Morphology: No clear neck; the head appears to sit directly on the dendritic shaft.
Head volume: Variable, typically intermediate.
Frequency: More common in developing tissue and in proximal dendritic segments. In mature cortex, they are relatively rare (5-10%).
Functional significance: May represent a transitional form. Some authors argue that many “stubby” spines are artifacts of fixation or section angle.
EM identification: A PSD-bearing protrusion with little or no constriction at its base.

3.4 Branched and Complex Spines

Morphology: A single spine stalk branches to produce two or more heads, each potentially bearing its own synapse.
Frequency: Rare (less than 5%), but more common in some brain regions (e.g., CA1 stratum radiatum) and after learning paradigms.
Significance: Multiple synaptic contacts on one spine suggest complex local computation. They may represent recently split synapses or multi-innervated structures.

3.5 The Spine Apparatus

The spine apparatus is a smooth ER derivative found within the necks and heads of a subset of dendritic spines, particularly mushroom spines. In EM:

Appearance: Stacked, flattened cisternae of smooth membrane separated by electron-dense plates (containing the protein synaptopodin).
Size: Typically 2-4 cisternae stacked together, spanning 100-300 nm.
Function: Serves as a local calcium store and is implicated in synaptic plasticity. Knockout of synaptopodin eliminates spine apparatuses and impairs long-term potentiation (Bhatt, Bhatt & Bhatt; Bhatt DH et al., 2009).
Not all spines have one: Spine apparatuses are found in roughly 10-30% of spines, predominantly in larger mushroom-type spines (Spacek & Harris, 1997).

4. The Postsynaptic Density (PSD)

The PSD is the defining ultrastructural feature of excitatory postsynaptic sites. In EM it appears as an electron-dense band on the cytoplasmic face of the postsynaptic membrane.

Dimensions: 20-500 nm in diameter (en face), 30-50 nm thick for Type I (asymmetric) synapses.
Composition: A dense meshwork of scaffolding proteins (PSD-95, Homer, Shank) that anchor glutamate receptors (AMPA and NMDA subtypes), adhesion molecules, and signaling enzymes.
PSD size correlates with synapse strength: Larger PSDs contain more AMPA receptors and correlate with larger spine heads and higher synaptic efficacy (Harris & Weinberg, 2012).
EM appearance: A dark band closely apposed to the postsynaptic membrane, usually thicker than any corresponding presynaptic density. In en face reconstructions from serial sections, the PSD appears as a disc or irregular patch.

5. Microtubule Organization in Dendrites

Microtubules in dendrites have a characteristic mixed polarity arrangement (Baas et al., 1988):

Plus-end-out microtubules: Similar to axonal microtubules, oriented with growing ends pointing distally.
Minus-end-out microtubules: Unique to dendrites in vertebrate neurons, oriented with growing ends pointing toward the soma.
Proportion: Roughly 50% plus-end-out in proximal dendrites, shifting to a higher proportion of plus-end-out in distal segments.
Functional significance: Mixed polarity enables bidirectional transport by kinesin (plus-end-directed) and dynein (minus-end-directed) motors without requiring both motor types for retrograde transport.
Contrast with axons: Vertebrate axons have uniformly plus-end-out microtubules. This polarity difference is one of the fundamental molecular distinctions between the two compartment types.

In EM, microtubules appear as hollow cylinders approximately 25 nm in outer diameter. They are visible in longitudinal section as parallel lines and in cross-section as small circles. Annotators cannot determine polarity from standard EM images, but the distinction is important for understanding why dendrites and axons have different organelle distributions.

6. Ribosomes in Dendrites: Local Protein Synthesis

A landmark discovery by Steward and Levy (1982) demonstrated that polyribosomes are selectively positioned beneath dendritic spine synapses. This finding revolutionized the understanding of synaptic plasticity.

Polyribosome clusters: Groups of 5-10 ribosomes arranged in rosettes or spirals, often found at the base of dendritic spines and within the dendritic shaft.
mRNA localization: Specific mRNAs (CaMKII-alpha, Arc/Arg3.1, MAP2) are transported into dendrites and translated locally in response to synaptic activity.
Functional significance: Local translation allows individual synapses to modify their protein composition independently, supporting synapse-specific plasticity.
EM identification: Ribosomes appear as electron-dense particles approximately 20-25 nm in diameter. Polyribosomes are visible as clusters. Their presence in a process is strong evidence for dendritic (not axonal) identity.
Rough ER in dendrites: In proximal dendrites, polyribosomes are often attached to ER membranes (rough ER). In distal dendrites, free polyribosomes predominate.

7. Mitochondria in Dendrites

Dendritic mitochondria have distinctive features compared to axonal mitochondria:

Size: Generally larger and more branched than axonal mitochondria. Dendritic mitochondria can be 2-6 micrometers long.
Distribution: Found throughout the dendritic shaft and at branch points. They cluster near active spine synapses, where energy demand for ion pumping and protein synthesis is high.
Cristae: Well-developed lamellar cristae, indicating high metabolic capacity.
Contrast with axons: Axonal mitochondria tend to be smaller (0.5-2 micrometers), more uniform in size, and more elongated.
Annotation cue: If a process contains large, branched mitochondria alongside ribosomes, it is very likely a dendrite.

8. Smooth Endoplasmic Reticulum in Dendrites

The smooth ER forms a continuous tubular network extending throughout dendrites:

Dendritic shaft: SER runs as a tubular network parallel to microtubules, sometimes forming a continuous lumen extending from the soma to distal tips.
Spine entry: SER tubules enter dendritic spines, sometimes forming the spine apparatus (see Section 3.5).
Calcium signaling: The SER serves as the primary intracellular calcium store. IP3 receptors and ryanodine receptors on the SER membrane mediate calcium release during synaptic signaling.
EM appearance: Smooth-walled tubular profiles, 30-50 nm in diameter, without ribosomes on their surface. In cross-section they appear as small circular profiles.

9. Worked Example: Identifying a Spine Synapse

Scenario: In a cortical EM volume, you see a small protrusion extending from a larger process, with a darkened presynaptic terminal apposed to it.

Step-by-step identification:

Identify the dendrite: The larger parent process (approximately 1.5 micrometers diameter) contains microtubules, a few mitochondria, and scattered polyribosomes. This confirms it as a dendrite.
Identify the spine: A narrow neck (approximately 0.15 micrometers) extends from the dendrite shaft, widening into a small head (approximately 0.4 micrometers across).
Find the PSD: On the head of the spine, a thick electron-dense band (approximately 200 nm long, 40 nm thick) is visible on the cytoplasmic face of the membrane.
Check the presynaptic side: Apposed to the PSD, a terminal containing clustered round vesicles (approximately 40 nm diameter) is present. The presynaptic membrane shows active zone densification.
Classify the synapse: Thick PSD + round vesicles + wide cleft = asymmetric (Type I) excitatory synapse on a spine.
Classify the spine: The moderate head size and clearly defined neck suggest a thin-to-mushroom transitional morphology.
Check adjacent sections: Verify the spine connection to the parent dendrite in 2-3 neighboring sections to confirm it is not an isolated profile.

10. Worked Example: Distinguishing a Thin Dendrite from an Axon

Scenario: You encounter a small-caliber process (approximately 0.5 micrometers) running through the neuropil. Is it a thin dendrite or an unmyelinated axon?

Feature	Thin Dendrite	Unmyelinated Axon
Caliber	Gradually tapering, may vary	Uniform caliber along length
Ribosomes	Scattered polyribosomes present	Absent (no local translation)
Microtubule polarity	Mixed (cannot see directly in EM)	Uniform plus-end-out
Microtubule spacing	Loosely spaced, irregular	More regular spacing
Rough ER	May have sparse RER profiles	Absent
Smooth ER	Tubular SER network present	Single SER tubule or absent
Mitochondria	Intermediate size, well-developed cristae	Smaller, more elongated
Spines	May bear spines (if spiny neuron)	Never bears spines
Synaptic contacts	Receives synapses (postsynaptic)	Makes synapses (presynaptic)

Decision process:

Look for ribosomes or rough ER. If present, the process is a dendrite. This is the single most reliable cue.
Look for spines or PSDs on the process. Postsynaptic specializations indicate a dendrite.
Check for vesicle clusters within the process. Synaptic vesicles indicate an axon terminal.
Examine caliber changes. Tapering suggests a dendrite.
Consider context. Trace the process toward a soma if possible.

11. Common Misconceptions

Misconception	Reality
“All dendrites have spines.”	Only certain neuron types are spiny (pyramidal cells, medium spiny neurons). Many interneuron subtypes have smooth (aspiny) dendrites that receive synapses directly on the shaft.
“Spine size is fixed.”	Spines are highly dynamic structures that change size and shape over minutes to hours in response to activity. Long-term potentiation enlarges spines; depression shrinks them (Bourne & Harris, 2008).
“Thin spines are immature.”	Thin spines are found abundantly in adult tissue. They may represent learning substrates, not just developmental precursors.
“Dendrites do not conduct action potentials.”	Many dendrites support backpropagating action potentials and dendritic spikes (calcium or sodium), though these are not visible in EM.
“Ribosomes are always on rough ER.”	Free polyribosomes (not attached to ER membranes) are abundant in dendrites and are the primary site of local dendritic translation.
“The PSD is a membrane structure.”	The PSD is a cytoplasmic protein meshwork on the intracellular face of the postsynaptic membrane, not a membrane itself.

References

Harris KM, Jensen FE, Bhatt DH (1992) “Three-dimensional structure of dendritic spines and synapses in rat hippocampus (CA1) at postnatal day 15 and adult ages.” Journal of Neuroscience 12:2685-2705.
Bourne JN, Harris KM (2008) “Balancing structure and function at hippocampal dendritic spines.” Annual Review of Neuroscience 31:47-67.
Harris KM, Weinberg RJ (2012) “Ultrastructure of synapses in the mammalian brain.” Cold Spring Harbor Perspectives in Biology 4:a005587.
Fiala JC, Harris KM (1999) “Dendrite structure.” In: Dendrites (Stuart G, Spruston N, Hausser M, eds), pp 1-34. Oxford University Press.
Baas PW, Deitch JS, Black MM, Bhatt GA (1988) “Polarity orientation of microtubules in hippocampal neurons: uniformity in the axon and nonuniformity in the dendrite.” Proceedings of the National Academy of Sciences 85:8335-8339.
Steward O, Levy WB (1982) “Preferential localization of polyribosomes under the base of dendritic spines in granule cells of the dentate gyrus.” Journal of Neuroscience 2:284-291.
Peters A, Palay SL, Webster HdeF (1991) The Fine Structure of the Nervous System, 3rd edition. Oxford University Press.
Spacek J, Harris KM (1997) “Three-dimensional organization of smooth endoplasmic reticulum in hippocampal CA1 dendrites and dendritic spines of the immature and mature rat.” Journal of Neuroscience 17:190-203.

This document is part of the NeuroTrailblazers Content Library. It is intended as an instructor reference and annotator training script. Last updated: 2026.