Structural and functional development of the marsupial respiratory system

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."