Electron microscope study on the innervation of guinea pig liver--proposal of sensory nerve terminals in the hepatic parenchyme.

Innervation of guinea pig livers was investigated by electron microscopy. Thinner unmyelinated autonomic nerves in the portal tract were found to contain many synaptic vesicles in their varicosities: small granular, agranular vesicles and a few large dense-cored vesicles, in addition to a few small mitochondria with sparse cristae, glycogen particles and occasional lysosomes. These adrenergic varicosities showed morphological signs suggesting emiocytotic release of transmitter substance through their naked surface into the portal space; they showed no synaptic contact with cells in the portal space. A cross section of a possible sensory nerve composed of several large axons filled with small mitochondria was identified in the portal space. Occasional large mitochondria-rich axon profiles, probably sensory in nature, were also present in the autonomic nerve. They seemed to be distributed in the hepatic parenchyme making large sensory terminals. In the Disse's spaces thin autonomic nerves composed of varicose axons were observed. Here also, the varicosities contained synaptic vesicles with adrenergic characteristics. Some of them freely terminated in the Disse's space suggesting the release of transmitter substance into the space, but the majority of them made synaptic contacts with hepatocytes. Ito cells received relatively numerous synaptic contacts from the adrenergic terminals. The sinusoidal endothelial lining received no synaptic contacts but only nerve approaches over a wide distance ("synapses per distance"). The Kupffer cell infrequently sent a cytoplasmic process into the Disse's space to form a synaptic contact with an axon varicosity. Presumed sensory terminals were characterized by their extraordinarily large sizes and by their contents of abundant small mitochondria with sparse cristae, glycogen particles and scanty small granular and agranular vesicles. These terminals closely contacted the Ito cell and/or hepatocyte. In guinea pig liver, only adrenergic varicosities or terminals were identified in addition to probable sensory terminals.