Functional respiratory morphology in the newborn quokka wallaby (Setonix brachyurus)

Abstract

A morphological and morphometric study of the lung of the newborn quokka wallaby (Setonix brachyurus) was undertaken to assess its morphofunctional status at birth. Additionally, skin structure and morphometry were investigated to assess the possibility of cutaneous gas exchange. The lung was at canalicular stage and comprised a few conducting airways and a parenchyma of thick-walled tubules lined by stretches of cuboidal pneumocytes alternating with squamous epithelium, with occasional portions of thin blood-gas barrier. The tubules were separated by abundant intertubular mesenchyme, aggregations of developing capillaries and mesenchymal cells. Conversion of the cuboidal pneumocytes to type I cells occurred through cell broadening and lamellar body extrusion. Superfluous cuboidal cells were lost through apoptosis and subsequent clearance by alveolar macrophages. The establishment of the thin blood-gas barrier was established through apposition of the incipient capillaries to the formative thin squamous epithelium. The absolute volume of the lung was 0.02 +/- 0.001 cm(3) with an air space surface area of 4.85 +/- 0.43 cm(2). Differentiated type I pneumocytes covered 78% of the tubular surface, the rest 22% going to long stretches of type II cells, their precursors or low cuboidal transitory cells with sparse lamellar bodies. The body weight-related diffusion capacity was 2.52 +/- 0.56 mL O(2) min(-1) kg(-1). The epidermis was poorly developed, and measured 29.97 +/- 4.88 microm in thickness, 13% of which was taken by a thin layer of stratum corneum, measuring 4.87 +/- 0.98 microm thick. Superficial capillaries were closely associated with the epidermis, showing the possibility that the skin also participated in some gaseous exchange. Qualitatively, the neonate quokka lung had the basic constituents for gas exchange but was quantitatively inadequate, implying the significance of percutaneous gas exchange.

Fig. 1

Scanning (A) and light micrographs (B–D) of the lung of a quokka joey at birth showing the morphology of the parenchyma. (A) Transverse section across the entire lobe showing a large central airway (a), tubules (t) and the thick septa delineating the tubules (arrowheads). Scale bars = 500 µm. (B) A Paraffin section of an entire lobe showing a central airway (a) and tubules (t). Some tubules (arrows) open directly into the primitive airways. Note the variable thickness of the intertubular septa (arrowheads) as well as the sizes of the tubular lumina. Scale bar = 500 µm. (C) Higher magnification of the intertubular septum showing the abundant interstitial mesenchyme with undifferentiated cells (asterisk), superficial portions of thin blood–gas barrier (black arrows), cuboidal epithelium (white arrow) and some wide portions with squamous epithelium but no capillaries (black arrowheads). Within the interstitium there are many deeply situated capillaries (white arrowheads). Scale bar = 50 µm. (D) The tubular surfaces were covered by alternating stretches of cuboidal epithelium (arrowheads) and squamous epithelium (arrow). Centrally located large capillaries (asterisk) were evident and mesenchymal tissue (me) was preponderant. Scale bar = 20 µm.

Fig. 2

Transmission electron micrographs showing the morphological changes that characterize the tubular epithelium of the neonate quokka lung with type I and type II cells and cuboidal cells in various transitional stages. (A) Micrograph showing a well-formed cuboidal epithelium endowed with lamellar bodies (white arrows). These cells notably lack microvilli and may be described as pneumoblasts with a potential to form either of the two definitive epithelial pneumocytes. Note the large blood vessel (ve) below the epithelium. Scale bar = 5 µm. (B) Some cuboidal cells may have scanty lamellar bodies while also lacking microvilli, probably showing a tendency to form type I cells (white asterisks) while others have already acquired microvilli and have abundant lamellar bodies (arrow). Scale bar = 5 µm. (C) Type II cells converting to type I pneumocytes appear to do so by extruding entire lamellar bodies (arrowhead) and flattening out (arrow). Newly formed type I pneumocytes (white arrow) may become associated with superficial capillaries (ca), thus constituting a blood–gas barrier. Scale bar = 5 µm. (D) Even at this early stage, ‘mature’ thin blood–gas barriers (arrowheads) with virtually no interstitial tissue were present and these comprised a thin epithelial layer, a common basement membrane and an endothelium. Most of the erythrocytes (erythroblasts) had a central nucleus (n). Scale bar = 5 µm.

Fig. 3

Transmission electron micrographs showing various phenotypes of the intertubular septa in the lung of the neonate quokka joey. All scale bars = 5 µm. (A) The lung of the neonate quokka has the fundamental morphological characteristics necessary for gas exchange. The presence of an alveolar macrophage (ma) next to a putatively apoptotic type II cell (ap) evinces this fact. The dying type II cells are cleared by pulmonary macrophages. Such cells are replaced by newly formed type I pneumocytes (arrowhead), which approximate the sepal capillaries (ca) to form thin blood–gas barrier portions. (B) Some of the intertubular septa are covered entirely by thin type I cells (arrowheads) but lacked capillaries and hence had no blood–gas barriers. Such septa had an abundance of mesenchymal cells (mc) and also contained dark staining cells (h), which were thought to be hemangioblasts. (C) In some cases, there occurred capillary sprouts (white arrow), represented by a narrow lumen completely filled by an intraluminal blood cell (bc). Note the thin squamous epithelium (arrowheads), the loose mesenchyme (me) and uncommitted mesenchymal cells (mc). (D) Some portions of the intertubular septa were quite thin, had well-formed squamous pneumocytes (arrowheads) but lacked capillaries and hence no blood–gas barriers.

Fig. 4

Transmission electron micrographs showing the skin covering the body surface in the neonate quokka. The epidermis (ep) measured about 30 µm in thickness and there were capillaries (c) at various levels of the dermis (d), some closely associated with the epidermis. The cells of the epidermis varied in size and shape, from rounded to elongate and narrow or even squamous towards the surface. A thin layer of keratin formed the outer covering of the skin (arrowheads), but hairs were absent. Scale bar = 5 µm.