The Control of Growth of the Chick Embryo Liver Studied by the Method of Parabiosis.

A CONSIDERABLE amount of work has been done on the growth and regeneration of the adult mammal liver and theories of liver growth control have been put forward, the most far-reaching of which was advanced by Glinos (1958). According to this theory, the growth of the liver is regulated by a feed-back mechanism, involving diffusible chemical substances secreted into the blood by the liver itself. The higher the concentration of the substances in circulation, the less will be the rate of liver growth. Much descriptive work has been done on the growth of embryonic chick organs (Romanoff, 1960), but none on the problem of growth control of the liver. It is possible that the postulated feed-back mechanism of liver growth control in the adult rat also operates in the chick embryo during development. But there is hardly any evidence for this. In the adult rat, Bucher et ah (1951) found that 72 hr. after partial hepatectomy of one parabiotic twin, the mitotic rate had increased six times in the intact liver of the other partner. Glinos & Gey (1952) diluted the plasma of rats and found that cell division was induced in resting liver cells and was intensified with increasing rate and duration of plasmapheresis. Conversely, Glinos (1958) reported that fluid restriction, which increases the concentration of the plasma, inhibited regeneration in partially hepatectomized rats. He argued that serum proteins were largely responsible for these effects and were the real growth regulators of the liver. Partial hepatectomy results in a lowering of the serum protein concentration in the blood and this serves to remove the mitotic inhibition. It was considered that if the same mechanism applies to the chicken embryo, then the observed decline in growth rate with age (Romanoff, 1960; Lovlie, 1959) may be due to an increasing concentration of blood-borne liver inhibitor. Thus when a bigger, older, embryo is parabiosed with a smaller, younger one,