The lungs of Polypterus senegalus and Erpetoichthys calabaricus: Insights into the structure and functional distribution of the pulmonary epithelial cells.

The present article is a comparative, structural study of the lung of Polypterus senegalus and Erpetoichthys calabaricus, two species representative of the two genera that constitute the Polypteriformes. The lung of the two species is an asymmetric, bi-lobed organ that arises from a slit-like opening in the ventral side of the pharynx. The wall is organized into layers, being thicker in P. senegalus. The inner epithelium contains ciliated and non-ciliated bands. The latter constitute the respiratory surface and are wider in E. calabaricus. The air-blood barrier is thin and uniform in P. senegalus and thicker and irregular in E. calabaricus. In the two species, the ciliated areas contain ciliated cells, mucous cells and cells with lamellar bodies. Additionally, P. senegalus contains polymorphous granular cells (PGCs) and neuroendocrine cells (NECs) while E. calabaricus lacks PGCs but shows granular leukocytes and a different type of NEC. Interestingly, ciliated cells and secretory cells show a dual morphology in E. calabaricus indicating the presence of cellular subtypes and suggesting more complex secretory activity. Also in E. calabaricus, cilia show a novel doublet-membrane interaction that may control the displacement of the microtubule doublets. The subepithelium is a connective layer that appears thicker in P. senegalus and contains, in the two species, fibroblasts and granulocytes. The outer layer contains bundles of richly innervated striated muscle. This layer is likely involved in the control of lung motion. In the two species, smooth muscle cells constitute a limiting layer between the subepithelium and the striated muscle compartment. The role of this layer is unclear.