MOUSE EPICYTES AND THEIR INTRAMURAL PROCESSES

Abstract

Thin paraffin sections of mouse lungs fixed by bronchial filling and stained intensely with iron haematoxylin show numerous and conspicuous epicytes of varying size, form, internal features, and density. Many have a dark lacelike cytoplasm with numerous small clear vacuoloids and a rounded opaque nucleus, while others stain more lightly and have large irregular vacuoloids, and a generally achromatic nucleus. Between these apparent extremes are graded intermediate forms. These epicytes may be arranged in a series perhaps connoting a functional or age sequence. Metabolic, endocrinic, and exocrinic roles have been suggested for them as well as the commonly attributed phagocytic function. The nuclei are often indented by contiguous vacuoloids. In partitions between alveoli, epicytes may have two, three, or even more air faces. Most of them are in the angles where septa between contiguous alveoli join. In many epicytes an extension may be seen arising from a larger nucleated part; and not infrequently two such extensions appear in the section. These processes may be more or less blunt and short, or long and thin; and their ends, of varying area, often form part of the wall of an alveolus, lying directly against the air space. On some of the narrow free ends were seen irregular spicules, which may be rudimentary cilia. In cross sections the processes are of circular or oval outline and are partially or completely buried in the connective tissue of the alveolar wall. Like the juxtanuclear part of the epicyte, the processes are bounded by a delicate cell membrane and show mitochondria in the cytoplasm between the vacuoloids. These processes presumably afford anchorages for the cells to the alveolar walls, and also surfaces through which nutriment may be absorbed from the contiguous capillaries. Epicytes that abut upon the pleura, interlobular septa, or sheaths of air and blood channels usually have only one air face and often appear without a process, partly sunken into the connective tissue, with a mass of frothy cytoplasm bulging into the alveolar space; but more rarely a process is visible arising from them. Release of epicytes from the alveolar walls may be possible, to create the relatively scarce free foam cells. Curious extended epicytes, some looking like dumb-bells, were often found.