Abstract
The actual three-dimensional microstructure of the mammalian liver has been reviewed by scanning electron microscopic (SEM) techniques.
Liver lobuli and acini easily revealed their microarchitecture showing interconnected liver plates, sinusoids and hepatocytes. Isolated liver cells appeared as polyhedrons with six or more facets covered by a large number of microvilli where fronting the sinusoidal wall, or relatively smooth where forming the biliary groove. In addition to microvilli, the biliary canalicular wall showed several pits and/or small holes probably related to secretory processes. Bile channels frequently bifurcated on the same side as the isolated liver laminae. Some bile canaliculi, showing lateral diverticula bordered by microvilli and arising from the cortical cytoplasm of the hepatocyte, were considered to be intracellular branches of bile canaliculi. Zones of minimal distance (0.1μm) were often observed between the spaces of Disse and the lumen of the bile canaliculus. These were interpreted as sites of the simple diffusion of substances from bile to blood and—actually representing loci minoris resisteutiae—they could be considered, pathologically, as possible pathways for bile regurgitation. Numerous blebs and laminar evaginations, probably related to phenomena of secretion and/or absorption, were present in the luminal wall of canaliculo-ductular junctions. Further bile ductules and ducts all possessed a variable number of unusual cilia of unknown roles (chemoreceptive and/or motile?).
Endothelial cells in sinusoids showed numerous fenestrations of different size. Small ones (0.1-0.2μm) were arranged in clusters, while others (-1μm) appeared often subdivided by slender strands of cytoplasm and doubled by laminar cellular extensions belonging to adjacent Kupffer cells or subjacent Ito cells located in the space of Disse. Kupffer cells closely resembled macrophages; their cell body showed numerous microvillous projections, blebs, lamellipodia and occasional filopodia. They were mainly located within the lumen of the sinusoids and often anchored to the endothelial wall with their long processes. In some cases, thin evaginations from Kupffer cells penetrated the endothelial gaps, enlarging them even further. As seen topographically, the subendothelial spaces of Disse appeared considerably larger than was generally evident in thin sections. This actually resulted in a continuous three-dimensional labyrinth of intercellular and pericapillary microlacunae in which hepatocytes were suspended. A few collagen fibers, fragments of basal lamina materials and fat storing cells were found in these spaces.
The fetal liver was composed of ramified cords and irregularly shaped hepatocytic laminae closely associated with wide vessels. The hepatoblasts in particular appeared as large cells uniformly covered with short microvilli. In some instances these were linearly distributed on the cellular surface and likely related to the formation of a biliary groove (morphogenetic event for biliary polarity of the cell).
In fetal sinusoids, transitional forms between Kupffer cells and endothelial cells were never found. Megakariocytes and other irregularly shaped cells were observed in the pericapillary spaces and in the sinusoidal lumen. They perhaps represented intermediary stages of the differentiation of Kupffer cells from monocytes and/or local macrophages.
