A combined biochemical and morphometric study on tissue changes inXenopuslarvae during induced metamorphosis

Abstract

The metamorphic tissue changes in liver and tail muscle of Xenepus larvae have been followed with the aid of biochemical and morphc metric characteristics. After induction cf metamorphosis by thyroxir.e the larval liver shows an increase and then a marked decrease in weight, but no change in water content. Coincident with this, there are similar, but differential fluctuations in the amount of DNA, total RNA and acid precipitable protein. The concentiation of RNA and protein (per unit fresh weight) remains constant throughout metamorphosis, whereas that of DNA decreases during the growth period and increases during late metamorphosis. In tail muscle, which undergoes involution at metamorphosis, there is no change in water content. The concentration of macromolecular constituents does not show any significant change, except for DNA the concentration of which increases during later stages of metamorphosis. The morphometric analysis of the liver tissue reveals that the initial growth response occurs without any significant change in the volume proportion between extrahepatocytic space and hepatocytes, and that liver atrophy during later stages of metamorphosis coincides with similar decreases in the fractional volumes of hepatocytes and their cytoplasmic constituents (mitochondria, ER, ground substance). It is deduced that the larval liver of Xenopus — in contrast to tail muscle — responds to thyroid hormone by transient growth followed by a marked atrophy of the hepatocytes. Biochemical and morphometric data support the view that this response primarily involves cell mass, and possibly to some extent also cell number, but does not affect the volume proportions of cytoplasmic constituents of the hepatocytes.