Three of resident cells of the hepatic sinusoidal wall and in connection with these, the hepatic parenchyma were electron microscopically observed in four species of birds.
Avian hepatocytes were characterized by densely packed, abundant large mitochondria probably in accordance with mandatory postulation of high energy for flight. Supra- and paranuclear location of Golgi complexes in avian hepatocytes was characteristic, and this and other cytological signs were suggestive both of emiocytotic discharge of some bile constituents from the hepatocyte into the bile canalicule and of endocytosis of unknown substance by means of bristle-coated micropinocytotic caveolae and vesicles into the hepatocyte from the content of the bile canalicule.
The most conspicuous feature of the avian sinusoidal endothelium consisted in that the perikaryonal cytoplasm was so rich in organelles, and comparable to that of the Kupffer cell; it contained numerous fine smooth-surfaced tubules filled with dense material, coated micropinocytotic caveolae and vesicles, macropinocytotic vesicles and many lysosomes. The distinction between thinner and thicker portions in the membraneous cytoplasmic extension was not conspicuous and sieve plates limited to the thinner portions were not numerous.
The most conspicuous and reliable cytological differences between the Kupffer cell and endothelial cell were found in cell coat, pseudopodia and mitochondria, the last of which were larger in the former. The worm-like body was found in the Kupffer cells of chicken liver. In the initial stage of the erythrophagocytosis, the Kupffer cell sends out pseudopodia not only to embrace the adhered erythrocyte but also to penetrate into the latter, so that a complex interdigitation of cytoplasm of both cell types was observable.
The avian Ito cell, located in the Disse's space, contained multiple small lipid droplets and exhibited well-known essential cytological features as revealed in the mammalian liver. Segments of the so-called subendothelial processes were found here and there in the Disse's space. The diplosome was demonstrated within the Golgi area and the distal centriole proved to be provided with a cap-like ciliary vesicle, a cross-striated basal foot and a cross-striated rootlet, and a microtubule arose from the tip of the basal foot. The sequent developmental processes of a single cilium from the distal centriole first into the ciliary vesicle and then further into the Disse's space or into the sinusoid through a fenestrum of the endothelial lining were traced. Thus, the single cilium was established to be a common meta-plasmic structure of the Ito cell in vertebrates. Not only Ito cell, but also endothelial and Kupffer cell of birds contained the spheridy (nuclear body) in the nucleus as reported in mammalian sinusoidal cells.
Besides sporadical lymphocytes and plasma cells, many macrophages of various differentiation stages were found in the hepatic parenchyma of the birds examined, especially of chickens. Those in an advanced differentiation resembled Kupffer cells in ultrastructure. Occasionally morphological sign suggestive of migration of the macrophage into the sinusoid was observed, and the macrophage even in a low differentiation stage was actually found in the sinusoidal lumen. Macrophages derived from the hepatic parenchyme were thought to transform into Kupffer cells after being fixed to sinusoidal endothelial lining. Therefore, the presence of organellepoor immature Kupffer cells might be explained. Thus, macrophages in the hepatic parenchyma may be an origin or a source of the Kupffer cell in the sinusoid.