Growth, mitosis and morphogenesis of the simple liver acinus in neonatal rats

Abstract

Radioautography after 3H-thymidine injection, blockage of mitosis due to colchicine and camera lucida drawings were used to study growth, mitosis and morphogenesis in the simple liver acinus of Rappaport in neonatal rats. Hepatic parenchymal cell plates are irregularly arranged and thick from birth to 4 days postpartum. By 10 days the plates begin to assume the adult configuration, irregular and thick in acinar zone 1 (periportal) but straight and thin in acinar zone 3 (pericentral). After a single injection of 3H-thymidine at birth the distribution of labeled nuclei among the hepatic acinar zones was such that zone 1 contained most, zone 3 least, and zone 2 an intermediate amount. This relationship remained constant out to 10 days postpartum. The amount of labeled cells within each zone varied, reaching its highest values at 2 days in zones 1 and 2 but not until 4 days in zone 3. Similar to the distribution of labeled nuclei, frequency of mitosis also exhibited a constant relationship of zone 1 > zone 2 > zone 3, but peaks of cell division within each zone were not always present. All three zones displayed a peak of mitosis at 4 days, whereas a second mitotic peak at 10 days was attained only by cells in zones 1 and 2. Conclusions are: (1) The neonatal liver is an expanding cell population with most of the expansion confined to acinar zones 1 and 2, (2) the period of 4–6 days postpartum is critical and could be the time when acinar metabolic zones are forming, (3) the irregular arrangement of cell plates in the center of the acinus (zone 1) may act to dampen arterial pulsations and allow adequate mixing of arterial and venous blood, (4) immediate postnatal growth of the liver acinus, as shown by an increase in its width, is due primarily to an increase in cell number since individual cell size does not increase.