Phase contrast imaging reveals low lung volumes and surface areas in the developing marsupial

Abstract

Marsupials are born with immature lungs when compared to eutherian mammals and rely, to various extents, on cutaneous gas exchange in order to meet metabolic requirements. Indeed, the fat-tailed dunnart is born with lungs in the canalicular stage of development and relies almost entirely on the skin for gas exchange at birth; consequently undergoing the majority of lung development in air. Plane radiographs and computed tomography data sets were acquired using phase contrast imaging with a synchrotron radiation source for two marsupial species, the fat-tailed dunnart and the larger tammar wallaby, during the first weeks of postnatal life. Phase contrast imaging revealed that only two lung sacs contain air after the first hour of life in the fat-tailed dunnart. While the lung of the tammar wallaby was comparatively more developed, both species demonstrated massive increases in air sac number and architectural complexity during the postnatal period. In addition, both the tammar wallaby and fat-tailed dunnart had lower lung volumes and parenchymal surface areas than were expected from morphometrically determined allometric equations relating these variables to body mass during the neonatal period. However, lung volume is predicted to scale with mass as expected after the neonatal marsupial reaches a body mass of ∼1 g and no longer relies on the skin for gas exchange. Decreased lung volume in the marsupial neonate further supports the maxim that cutaneous gas exchange occurs in the marsupial neonate because the respiratory apparatus is not yet capable of meeting the gas exchange requirements of the newborn.

Figure 1. Phase contrast X-ray imaging of the developing tammar wallaby using a synchrotron radiation source.

Anterior-posterior (top) and lateral (bottom) views of tammar wallaby pouch young aged 36 hours (A), 10 (B) and 20 (C) postpartum days. All animals are positioned such that the skull is at the top of each image. White areas indicate the less dense, air-filled lung and trachea. Thirty-six hours after birth, the lungs of the tammar wallaby are simple and saccular in structure (A). The lung increases in complexity by P10 (B) and further again by P20 (C) with an increase in the number of air sacs and a decrease in the size of these air sacs. Scale bar represents 1 mm.

Figure 2. Phase contrast X-ray imaging of the developing fat-tailed dunnart using a synchrotron radiation source.

Anterior-posterior (top) and lateral (bottom) views of the developing fat-tailed dunnart from birth (A) to 40 postpartum days (F). White areas indicate the less dense air-filled lung and trachea. Some air may also be noted in the nares (located at the top of the images) or on the body surface. The fat-tailed dunnart undergoes a substantial degree of extra-uterine lung development from just 2 ‘air bubbles’ visible after parturition (A). At P10 (D), the lungs are still characterised by large open circular sacs. By P20 (E) the size, shape and number of air sacs have substantially altered. Scale bar represents 1 mm.

Figure 3. 3-Dimensional volume rendering from fat-tailed dunnart computed tomography data sets.

Shown are three frames captured over 90 degrees of lung rotation. The series consists of an anterior view (A), a lateral view (C) and the mid-point between them (B), further demonstrating the increase in air sac number, and changes to air sac shape throughout the first weeks of lung development in the fat-tailed dunnart. Scale bar represents 200 µm.

Figure 4. Lung volume (A) and mass specific lung volume (B) as a function of body mass during postnatal development.

Open circles represent both adult and newborn eutherian mammals. Open squares represent newborn and adult marsupials, including the tammar wallaby (square containing cross). Black triangles represent tammar wallaby joeys measured in this study aged from birth to P30. Grey inverted triangles represent fat-tailed dunnart pouch young measured in this study from P1–P20. Each triangle represents a single marsupial. The dotted regression line relates to the marsupials in this study while the solid regression line is derived from all other data points. The newborn marsupials measured in this study (with synchrotron imaging) show a lower than expected lung volume, until a body mass greater than 1 g, when the lungs become the main (only) source of gas exchange. Data taken from , , , , , , , , , , .

Figure 5. Surface area of the lung as a function of body mass during postnatal development.

The air-tissue interface during postnatal lung development of the fat-tailed dunnart and tammar wallaby is less than that predicted from the allometric equation. This is in line with other marsupials, and most likely due to the fact that alveolarisation and the subsequent increase in surface area by septation does not commence until after P70 in the tammar wallaby and P45 in the fat-tailed dunnart. Symbols, lines and references as in Figure 4.