Erythropoiesis in the newt, Triturus cristatus Laur. II. Characteristics of the erythropoitic process.

Abstract

ABSTRACT In splenectomized newts (Triturus cristatus Laur.) rendered anaemic by acetylphenylhydrazine (APH), an erythropoietic response is delayed so that the animals are completely devoid of the erythron, including erythrocytes. At 11–14 days after APH, ‘erythroid precursor cells’ (EPC) in the blood signal the occurrence of an erythropoietic response. Ultrastructural studies have shown few or no ribosomes in EPCs, but well developed nucleoli and intense RNA synthesis are seen in these cells. Correlated morphological and cytochemical data indicate the production of ribosomes in EPCs, a process culminating in the formation of basophilic erythroblasts (BE). Microphotometric studies show the accumulation of haem during this interval. Thus, in EPCs and BEs, both ribosomal (rRNA) and messenger (m) RNA are synthesized, making possible the early synthesis of haemoglobin. In subsequent stages, nucleoli exhibit a size decrease, most evident in the particulate component, while all RNA synthesis ceases during the mid-poly chromatophilic erythroblast (MPE) stage. Coupled with the gradual loss of ribosomes characteristic of this developmental period, the results suggest that rRNA synthesis occurs in EPCs, BEs, and in early polychromatophilic erythroblasts, where it is completed. Haemoglobin mRNA is also formed in these stages since ultrastructural and microphotometric data show the accumulation of haemoglobin. Beyond the MPE, haemoglobin production is dependent upon stable messenger RNA since no RNA synthesis is detected in this period. Ultrastructural studies demonstrate that autophagy may play an important role in the loss of cytoplasmic organelles characteristic of the erythropoietic process. The occurrence of swollen cristae and outer compartment in mitochondria coupled with the presence of myelin-like membrane whorls in association with mitochondria is seen in all erythroid cells except EPCs. Membrane profiles derived from endoplasmic reticulum are frequently encountered in the vicinity of mitochondria and appear to encircle these organelles and adjacent cytoplasm to form autophagic vacuoles. In the process, the limiting membranes assume a dense laminar appearance. Elements of the Golgi complex also display morphological alterations suggestive of degeneration. Cytoplasmic bodies of varying appearance and content, some of which are similar to lysosomes in other cell types, are seen in all erythroid cells. Ferritin is seen in some bodies, in vacuoles, or in aggre gates, but does not appear in micropinocytotic vesicles.