The postnatal development of the liver in a marsupial, Didelphis virginiana. 2. Electron microscopy.

The postnatal development of the liver has been examined with the electron microscope in 15 groups of young opossums. At birth, hepatic cells show little organization and large islands of haemopoietic elements occur between clusters of hepatic cells. Hepatocytes appear small and contain an abundance of granular endoplasmic reticulum. The cytoplasm is also characterized by scattered lipid droplets of varying sizes. Canaliculi occasionally are found between hepatocytes. Only a few scattered lining cells are found in relation to the sinusoids and extensive areas appear to lack a lining endothelium. By the 9th postnatal day large accumulations of glycogen are present in the majority of hepatic cells and large areas of the sinusoids remain devoid of a lining endothelium. Glycogen and lipid droplets remain abundant in hepatocytes of the 15 day old opossum and later stages. By the 69th postnatal day, hepatocytes are relatively mature in appearance and the principal cytological features appear similar in this and subsequent stages. The cytoplasm is characterized by the presence of glycogen, granular endoplasmic reticulum and numerous small, spherical mitochondria. A heterogeneous population of hepatic cells was noted in the majority of stages examined. This ultrastructural study confirmed the tendency for haemopoietic cells to remain in discrete foci of like kind, both with regard to cell lineage and stage of maturation.