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Abstract

Marsupials are born with structurally immature lungs and rely, to varying degrees, on cutaneous gas exchange. The fat-tailed dunnart (Sminthopsis crassicaudata) is one of the smallest and most immature marsupial newborns having a gestation period of just 13 days, a birth weight of 13 mg and a delay in the onset of ventilation. This thesis (1) documents the structure and function of the respiratory system in the fat-tailed dunnart throughout the first weeks of life; (2) investigates possible causes for the necessity of cutaneous gas exchange including structural, neural, and mechanical constraints, and (3) explores changes in the control of breathing.
The skin is almost solely responsible for gas exchange in the newborn fat-tailed dunnart.
Indeed, less than 35 % of newborn dunnarts were observed to make any respiratory
effort on the day of birth. "Breathing", if seen, was usually accompanied by gross body
movements, prolonged periods of apnoea, and tidal volumes and frequencies unlikely to
result in efficient gas exchange. As a result of a poor breathing pattern, pulmonary
ventilation did not meet the demand for oxygen until approximately 35 days
postpartum.
Electron microscopy demonstrated that despite a general absence of breathing on the
day of birth, the respiratory epithelium was well developed, containing both Type-I and
Type-II (surfactant producing) alveolar epithelial cells. While surfactant coils were
detected in the airways of the newborn marsupial it is likely that low diffusibility for
oxygen contributes to the functional inadequacy of the lungs in the newborn; low
diffusibility resulting from high diffusion distance caused by underdeveloped
vasculature, small surface area and volumes available for gas exchange, and thickened
singular cytoplasmic extensions of the gas exchanging Type-I epithelial cells. In addition,
poor muscle co-ordination, chest wall distortion, and the absence of alveoli until after 40
days postpartum impede efficient pulmonary gas exchange in the newborn. All the
above factors force the neonatal fat-tailed dunnart to rely predominately on its skin for
gas exchange which is supported by a low metabolic rate and small size, hence large
general surface area. In addition, the afferent input from chemoreceptors, and
subsequent ability to mount a ventilatory response when challenged with hypoxia or
hypercapnia, seems poorly developed in the neonatal period, and may contribute to the
low convective requirement and need for cutaneous gas exchange in these neonates.
With much of the structural and functional development of the respiratory system
occurring in the extra-uterine environment, the newborn marsupial challenges the
traditional view that the mammalian respiratory system must be adequately developed
to act as the sole organ of gas exchange at birth.

Item Type:	Thesis - PhD
Authors/Creators:	Simpson, SJ
Keywords:	Marsupials, Respiratory system
Copyright Information:	Copyright 2010 the author
Additional Information:	Chapter 3 appears to be the equivalent of a pre or post-print version of an article published as: Simpson, S. J., Flecknoe, S. J., Clugston, R.D., Greer, J. .J, Hooper, S. B., Frappell, P. B. 2011, Structural and functional development of the respiratory system in a newborn marsupial with cutaneous gas exchange, Physiological and biochemical zoology: ecological and evolutionary approaches, 84(6), 634-649. Copyright 2011 The University of Chicago. Chapter 4 appears to be the equivalent of a pre-print version of an article published as: Simpson, S. J., Siu, K K. W., Yagi, N., Whitley, J. C., Lewis, R. A., Frappell, P. B. 2013, Phase contrast imaging reveals low lung volumes and surface areas in the developing marsupial, Plos ONE, 8(12), e53805, 1-8 Chapter 5 appears to be the equivalent of a post-print version of an article published as: Simpson, S. J., Fong, A. Y., Cummings, K. J., Frappell, P. B. 2012, The ventilatory response to hypoxia and hypercapnia is absent in the neonatal fat-tailed dunnart, Journal of experimental biology, 215, 4242-4247
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