Date of Award

1-1-1982

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Anatomy and Cell Biology

Abstract

This study describes the ultrastructure of the amphibian atrium and corroborates light microscopic evidence that atrial myocytes are stimulated to proliferate after ventricular wounding. In addition, the atrial response to a direct injury is described at light and electron microscopic levels.Normal atrial tissue from adult newts was processed for TEM. Fifteen days after ventricular wounding animals were injected with ('3)H-thymidine and atrial tissue was processed for TEM autoradiography. Direct atrial injury (DAI) was achieved by removal of a piece of the atrial wall. DAI animals received a ('3)H-thymidine injection at intervals of 5, 10, 15, 20 and 25 days after injury. Tissue was processed for light and TEM autoradiography. Mitotic and thymidine indices were calculated for mesothelial, connective tissue and myocardial cells of left and right atria within 500 (mu)m's from the wound edge. Sham-operated and unoperated animals served as controls.The atrial wall was 25-75 (mu)m thick and possessed no coronary vessels. The endocardial endothelium possessed 50-60 nm fenestrae with diaphragms and, like the epicardial mesothelium, was continuous. Interstitial cells included fibroblasts, macrophages, melanophores, xanthophores, and mast cells. Unmyelinated nerves were present in the subendothelial interstitium. The myocardium was 3-5 cell layers thick. Myocytes were 8-12 (mu)m wide and contained 2-5 myofibrillae. T-tubules were not seen but peripheral couplings were present. Fasciae adherents and desmosomes were located on lateral cell surfaces and at intercalated disks.TEM autoradiography confirmed that atrial myocytes which possessed straited myofibrillae were able to synthesize DNA following ventricular and atrial wounding. Myofilaments were present in mitotic myocytes but were not organized into myofibrillae with Z-bands.The atrial response to DAI occurred within a 500 (mu)m distance of the wound with no difference between right and left atria. The maximum myocardinal thymidine index of 9.84% occurred ten days after wounding with a mitotic index of 0.23%. Maximum mitotic and thymidine indices occurred five days after wounding for mesothelial cells and at ten days for connective tissue cells. Despite high levels of myocyte proliferation the atrial wound heals primarily by a connective tissue scar.

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