Lamellar inclusions and trilaminar substance in the parabronchial epithelium of the quail (Coturnix coturnix)

Abstract

The fine structure of the epithelial cells of the parabronchus and their secretory products have been the subject of many studies and have given rise to considerable controversy about their configuration and ultrastructure. The aim of the present study was to investigate the mode of formation and discharge of lamellar bodies of granular cells and the trilaminar substance produced and discharged by the embryologically related squamous atrial and respiratory epithelial cells. The material for light and transmission electron microscopic analysis was collected from 10 mature quail and 3 individuals aged 2 days. The parabronchial atria harbour two ultrastructurally distinct types of epithelial cells. The granular cells (analogous to type II cells of the mammalian pulmonary alveolus) produce and discharge balls of lamellar bodies. The squamous atrial cells produce and discharge sheets of trilaminar substance sandwiched between long tentacle-like processes, viz. the microvilli. The infundibula and air capillaries of the gas exchange tissue are invested with squamous respiratory cells which extend very thin, long processes that cover the air capillaries and constitute, together with the blood capillaries, the blood-air barrier. The squamous respiratory cells produce and discharge trilaminar substance as an extracellularly located acellular lining layer which is found in close contact with their cell membrane. Both squamous atrial and respiratory cells hence synthetize and discharge trilaminar substance, the basic unit of which has the ultrastructural appearance of a 7.5 to 8.0 nm membrane unit. The formation of trilaminar substance originates in the agranular endoplasmic reticulum, while the origin of the lamellar bodies of granular cells is related to the granular endoplasmic reticulum, the Golgi complex and the multivesicular bodies. Their structural unit is composed of a 4.5 to 5.0 nm thread-like structure which is concentrically arranged around a spherical core of granular substance.