Maturation of the Rabbit Alveolar Macrophage during Animal Development. I. Perinatal Influx into Alveoli and Ultrastructural Differentiation

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Abstract
Summary: Rabbit alveolar macrophages (AM's) were collected by tracheobronehial lavage at sequential times during animal development. The total number of cells recovered by this technique was found to increase markedly shortly before birth (Fig. 4). This apparent influx of macrophages into the alveoli continued during the first postnatal week, and, at a reduced rate, throughout the first postnatal month of life (Fig. 3). Ultrastructurally, AM's of the prenatal period resembled their monocytc precursors, and contained modest numbers of lysosomes and mitochondria, scant lamellae of ribosome-studded endoplasmic reticulum (RER), and a small Golgi apparatus (Fig. 12). A considerable amount of phagocytosed material was observed in these AM's, and consisted largely of cellular debris and two forms of surfactant-related phospholipids, termed tubular and lamellar myelin (Figs. 12–15). The quantity of these ingested phospholipids increased dramatically shortly after birth, in correlation with the known release of similar material from type II pneumocytes at this time. (Figs. 16 and 19). During the first postnatal week AM's showed a considerable increase in number of mitochondria and in the development of the RER and Golgi apparatus (Fig. 22). Increased accumulations of lipid droplets were also noted during this period. Ingested material continued to consist largely of surfactant-related phospholipids, but was less abundant at this time. By 28 days after birth, AM's were nearly morphologically mature (Fig. 25). They showed large numbers of lysosomes and mitochondria, and well developed RER and Golgi apparatuses. Ingested phospholipid material was still visualized, and the in complete degradation of this material appeared to give rise to the dense intraphagolysosomal whorls charcteristic of the mature AM. Speculation: The finding of a marked increase in lavageable Am's may serve to prepare the alveoli for early postnatal events. Possible factors which could contribute to the induction of this prenatal influx include hormonal stimulation involving estrogen and/or thyroid hormones, and chemotactic stimulation, possibly resulting from intra-alveolar surfactant. The quantity of surfactant-realated phospholipids ingested by AM's From all time periods studied suggests that scavenging of inactive and/or excess intra-alvcolar surfactant may be an important homecostatic funtion of the AM. Such a phospholipid scavenging role may be especially significant in the newborn animal when large quantities of surfactant are rapidly released into the alveoli.