The authors thank Ann Angel and Richard Walmsley for their support of, and contribution to, this project. They also gratefully thank all participants in the trial and Adrian Esterman, Department of Clinical Epidemiology at Flinders Medical Centre, for statistical assistance.

Background Marsupials are born at an early stage of development, and lung development from an air‐sac stage to maturity occurs in the air‐breathing environment, the pouch. Methods The morphology of the lung parenchyma in pouch young of the tammar wallaby, Macropus eugenii, at four stages of development (0, 30, 70, and 180 days) after birth and in the adult animal was investigated using light, scanning, and transmission electron microscopy, and developmental changes were documented. Results The lung at birth is at the terminal‐sac stage of development. In the first 70 days, development involves tissue proliferation and septal development leading to subdivision of the terminal sacs, terminal‐sac expansion, and septal thinning. Between 70 and 180 days, when the pouch young make the transition from ectothermy to endothermy, remodelling of the septa and microvascular maturation occurs. There is an overlap with further tissue proliferation and terminal‐sac subdivision and expansion. Alveoli are found at 180 days, but they are small in comparison to those in the adult lung. Conclusions Lung growth occurs slowly, and the lung remains at the terminal‐sac stage of development during the ectothermic period. True alveoli appear only during the transition from ectothermy to endothermy. © 1996 Wiley‐Liss, Inc.

The aim of the present study was to examine the organization of lymph fluid and cellular pathways and distribution of the membrane water channel Aquaporin-1 (AQP-1) in rat lymph nodes. Lymph fluid and cellular pathways within lymph nodes were examined by fluorescent protein tracer/confocal microscopy and by scanning electron microscopy (SEM), While the distribution of AQP-1 was studied immunohistochemically. Tracer studies showed the subcapsular sinuses continued directly at the hilum or via the intermediate sinuses to the medullary sinuses, and lymphatic labyrinths originating with blind-ends in the deep cortex drained into medullary sinuses. Afferent lymph tracers were also observed in node cortex interstitium. By SEM, lymphatic labyrinths appeared densely filled with lymphocytes and had few intraluminal sinus reticular cells, while medullary sinuses possessed well-developed networks of sinus reticular cells. The presence of many lymphocytes wedged in the walls of the lymphatic labyrinth suggested that lymphocytes migrate between the node parenchyma and lymphatic labyrinths. AQP-1 was distributed on the membrane of lymphatic endothelium and reticular cells as well as on both luminal and abluminal cell membranes of high endothelial venules (HEVs). Our SEM findings support the concept that lymphocytes migrate from the node parenchyma into lymphatic labyrinths in the deep cortex. The nodal distribution of AQP-1 plus the presence of a polarized distribution of ion pumps and/or ion channels in the HEV endothelium hypothesized in our discussion could explain the mechanism of the reported lymph-to-plasma fluid flux in lymph nodes and also facilitate the entry of afferent lymph antigens into the node cortex interstitium.

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The aquaporins (AQ-s) are a group of intrinsic membrane proteins which facilitate movement of water across cell membranes; their recent identification in the kidney has led to the reappraisal of the mechanisms and pathways of water movement across epithelia. Aquaporin-1, (CHIP-28) is reported distributed in cardiac myocytes and vascular smooth muscle cells of large arteries. A related protein, AQ-4, has been identified in the sarcolemma of skeletal muscle fibres. We report aquaporin expression in the cell membrane of smooth muscle cells of the rat genital tract; fluorescence immunohistochemistry of rat uterine (fallopian) tube and vagina demonstrated AQ-1 in visceral smooth muscle of these tissues. In the uterine tube, AQ-1 labelling is most pronounced in the innermost longitudinal and the inner cells of the circular muscle layer and is absent from the outer longitudinal muscle layer of the myosalpinx. The possibility of a specific role for AQ-1 in tubal transport by altering the tubal luminal diameter during the estrus cycle is suggested.

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In adapting from intra-uterine life, it seems that marsupials have adopted different, but equally effective strategies, with regard to the circulatory system.

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