Extensive soma‐soma plate‐like contact sites (ephapses) connect suprachiasmatic nucleus neurons

The hypothalamic suprachiasmatic nucleus (SCN) is the central pacemaker for mammalian circadian rhythms. As such, this ensemble of cell‐autonomous neuronal oscillators with divergent periods must maintain coordinated oscillations. To investigate ultrastructural features enabling such synchronization, 805 coronal ultrathin sections of mouse SCN tissue were imaged with electron microscopy and aligned into a volumetric stack, from which selected neurons within the SCN core were reconstructed in silico. We found that clustered SCN core neurons were physically connected to each other via multiple large soma‐to‐soma plate‐like contacts. In some cases, a sliver of a glial process was interleaved. These contacts were large, covering on average ∼21% of apposing neuronal somata. It is possible that contacts may be the electrophysiological substrate for synchronization between SCN neurons. Such plate‐like contacts may explain why the synchronization of SCN neurons is maintained even when chemical synaptic transmission or electrical synaptic transmission via gap junctions is blocked. Such ephaptic contact‐mediated synchronization among nearby neurons may therefore contribute to the wave‐like oscillations of circadian core clock genes and calcium signals observed in the SCN. Significance Three‐dimensional reconstruction of SCN tissue via serial electron microscopy revealed a novel structural feature of SCN neurons that may account for interneuronal synchronization that persists even when the predominant mechanisms of neuronal communication are blocked. We found that SCN core neurons are connected by multiple soma–soma contact specializations, ultrastructural elements that could enable synchronization of tightly packed neurons organized in clustered networks. This extensive network of plate‐like soma–soma contacts among clustered SCN neurons may provide insight into how ∼20,000 autonomous neuronal oscillators with a broad range of intrinsic periods remain synchronized in the absence of ordinary communication modalities, thereby conferring the resilience required for the SCN to function as the mammalian circadian pacemaker. The suprachiasmatic nucleus comprises cell‐autonomous neuronal oscillators with divergent intrinsic circadian periods that maintain coordinated oscillations even when chemical synapses or gap junctions are blocked. To investigate ultrastructural features enabling such synchronization, a three‐dimensional reconstruction of SCN core neurons was performe