Lungs of Polypterus and Erpetoichthys

Abstract

The wall of the asymmetrical saclike lungs of the fishes Polypterus and Erpetoichthys consists of several functionally different tissue layers. Their lumen is lined by a surface epithelium composed of (1) highly attenuated cells, termed pneumocytes I; (2) pneumocytes II with lamellar bodies, presumably indicating surfactant production; (3) mucous cells; and (4) ciliated cells. Underlying the pneumocytes I is a dense capillary net. The thin continuous endothelium of this net, together with the pneumocytes I, constitute the very thin blood-air barrier. The basement membrane of epithelium and endothelium fuse in the area of the blood-air barrier (thickness 210 m̈m). Secretory and ciliary cells form longitudinal rows in the epithelium. Below the zone with a gas-exchanging tissue, a layer of connective tissue containing collagen and special elastic fibers occurs. The blood vessels that give rise to or drain the superficial capillary plexus are located in this connective tissue. The outermost layer of the lung consists of muscle cells, a narrow inner zone with smooth muscle cells, and an outer, broader zone with cross-striated muscle cells. The lung is innervated by myelinated and nonmyelinated nerve fibers. The morphology of the gas-exchange tissue in the lungs of these primitive bony fish is fundamentally very similar to that of the lungs of tetrapod vertebrates. The morphologic observations are in close agreement with physiologic data, disclosing well-developed respiratory capacities. Structural simplicity can be regarded as a model from which the lungs of the higher vertebrates derived. In addition to respiratory function, the lungs seem also to have hydrostatic tasks.

Innervation and Neurotransmitter Localization in the Lung of the Nile bichir Polypterus bichir bichir

“…These pulmonary branches reach the lungs on the dorsal surface and run to the distal end of each lung (Diagram 1d). The right lung is innervated by the pulmonary branch of the left ramus intestinalis, whereas the left lung receives an innervation from a branch of the right ramus intestinalis (Lechleuthner et al, 1989). Pulmonary arteries and veins run along the length of the wall of the lungs and branch perpendicular to the axis.…”

“…In comparison to the lungs of dipnoan fishes, there appears to be a complexity of the ANS of lung tissue in bichirs associated with a highly developed sympathetic adrenergic system. A vagal innervation of the lungs and gill arches in bichirs has only been inferred from anatomical studies (Lechleuthner et al, 1989; Piotrowski and Northcutt, 1996), and there is no information on the spinal autonomic system or functional studies on the role of innervation in polypterids. The lungs of the bichirs receive output from both cranial and spinal autonomic sources, but the location of these nerves and their functional significance await pharmacological testing.…”

“…In a few teleosts and holosteans, including the bichirs (Lechleuthner et al, 1989), the swimbladder is primarily a respiratory organ analogous to the lung. In most actinopterygians with a swimbladder its function is to regulate buoyancy.…”

“…It seems that the evolutionary history of the actinopterygian lung followed a different course by its gradual transformation from a respiratory glass bladder into a nonrespiratory physostomous gas bladder, and finally to a physoclistous gas bladder primarily specialized for sound reception, buoyancy control, or both (Graham, 1997). In the bichir lung, the gas‐exchange area is located on very flat folds containing the respiratory cells (pneumocytes I), which constitutes the very thin blood–air barrier (Lechleuthner et al, 1989). The airway cells in the mucociliated epithelium of the furrowed epithelium are both ciliated cells, pneumocytes II (Lechleuthner et al, 1989), and NECs.…”

Polypterus and the evolution of fish pectoral musculature

“…1). In addition, Polypterus have a suite of features, including a paired lung (Lechleuthner et al 1989), lobed pectoral limbs, and the ability to breathe through their spiracle (Graham et al 2014), as well as observed terrestrial excursions both in the wild (Klaatsch, 1896) and in the laboratory (Standen et al 2014, E.M. Standen, T.Y. Du, P. Laroche & H.C.E Larsson, unpubl.…”