Fine Structure of the Gas Bladder of Alligator Gar, Atractosteus spatula

Abstract

Anthropogenic factors seriously affect water quality and adversely affect fish populations. Agricultural runoff accumulates in the Mississippi River and the coastal estuaries. Alligator gar (Atractosteus spatula) inhabits these waters and is impacted by agricultural pollution, petrochemical contaminants and oil spills. These fish are bimodal air breathers, and use a primitive lung as an accessory organ. The gas bladder, or Air Breathing Organ (ABO) of alligator gar (Atractosteus spatula), is a vascularized air sac located dorsal to the body lumen. It has characteristics of amphibian lungs. Alligator gar air-breathing organs function to supplement branchial respiratory exchange with aerial gas exchange. The alligator gar gas bladder is an elongated air sac that originates dorsal to the pharynx. Air enters through a pharyngeal-esophageal duct that is controlled by two glottal ridges. The pharyngeo-esophageal duct is approximately 5mm long and is lined by ciliated columnar epithelium that is continuous with the central canal epithelium. The gas bladder has a central canal that subdivides the bladder into right and left lobes. Each lobe is further divided by septa into series of air spaces. The septa consist of blood vessels and smooth and striated muscles. Air circulates throughout the central canal, lobes and air spaces. The thickness of the septa is determined by underlying, supportive striated muscle. The internal surface of the gas bladder is a continuous respiratory epithelial layer that includes mucus cells, pneumocytes, and ciliated epithelial cells. An understanding of the normal tissue characteristics of this air breathing organ provides a baseline for studying the effects of environmental toxins on this organ.