Skin development in the gray short-tailed opossum (Monodelphis domestica)-From skin respiration to thermoregulation

Abstract

Marsupials are born at an early stage of development, and compared to eutherians, skin development is slow, and a functional change during skin ontogenesis occurs. The skin development in 36 gray short-tailed opossums (Monodelphis domestica) has been examined using histological, morphometric, and μCT methods during postnatal development from neonate to adult. The aim of the study is to follow the structural and functional transition of the skin in this immature marsupial species. Additionally, the postnatal development of the external appearance and the cardiac and respiratory systems is looked at to assess skin development in relation to the general development. The skin of the newborn gray short-tailed opossum is thin and undifferentiated (no hair follicles, no sebaceous and sweat glands). Numerous subepidermal capillaries allow for gaseous exchange via the skin. A dense cutaneous capillary net with a high capillary volume density (0.25 ± 0.04) is present at term, indicating significant cutaneous gas exchange in the neonate. The capillary volume density decreases markedly during the first postnatal week (0.08 ± 0.01). In the same time period, the skin diffusion barrier increases from 27 ± 4 to 87 ± 1 μm. From this age on, the skin development is characterized by thickening of the different cutaneous layers and beginning formation of hair follicles. First, hair covering the skin, sweat glands, and subcutaneous fat are observed by day 28, indicating the onset of thermoregulation. The total skin thickness in the gray short-tailed opossum increases from 58 μm at birth to 726 μm by day 35, when the pelage is fully developed. The cardiac and respiratory systems are immature at birth. A fenestrated interatrial septum is present for the first 4 days, allowing skin respiration. Between day 4 and day 7, the lung enters the saccular stage of lung development and is mature enough to meet the respiratory needs of the growing organism. During a long period of postnatal development, the structural differentiation of the skin results in a functional shift from transcutaneous gas exchange to thermoregulation in later life.

Keywords: cutaneous respiration; dermal ontogenesis; marsupials; skin development; thermoregulation.

FIGURE 1

Postnatal development of Monodelphis domestica. Macrographs reflect the morphological transformation from neonate to adult: neonate (a), 4 dpn (b), 7 dpn (c), 11 dpn (d), 14 dpn (e), 21 dpn (f), 28 dpn (g), 35 dpn (h), 49 dpn (i), 57 dpn (j), adult (k). The marsupial young are firmly attached to the maternal teat immediately after birth and remain permanently attached until day 11 (see top panel). The highly immature neonate has strongly developed forelimbs and rudimentary hind limbs, an oral shield, and large prominent nostrils. The eye primordium shows retinal pigmentation, and ear primordia are not visible. During the first week, the body posture is still curled up. Around day 11, the hind limbs start to differentiate, and digits become recognizable. A clear developmental progress was observed between day 14 and day 21, where the area of the later mystacial vibrissae and eyelids, and auricle become visible but were still fused. The oral fissure is formed, and the young are not permanently attached anymore. The morphological transformation of forelimbs and hindlimbs to their final posture progresses. Pinnae became free from the head around day 28; eyes and ears are open now. First hairs erupt on the head around day 21, reaching the middle back by day 28. The young are completely furred by day 35. During the late postnatal period, the young become larger, and except for the proportionally bigger heads, they resemble the adult animals in external appearance. Scale bar = 1 cm. Details of the specimens are provided in Table 1. dpn, day post natum.

FIGURE 2

Light micrographs of a transversal (a) and a longitudinal (b) histological section through a newborn Monodelphis domestica. Numerous superficial capillaries (arrowheads) are closely associated with the epidermis, providing an extremely thin diffusion barrier. Cutaneous capillaries are present in the dorsal, lateral, and ventral sides; however, capillary density is highest in the skin of the dorsolateral side of the trunk (b, insets). The general developmental degree of M. domestica is highly immature at birth, showing cartilaginous skeletal elements, lungs at the canalicular period, and poor development of the gut and associated organs (i.e., liver, intestine, mesonephric kidney). e, esophagus; en, encephalon; h, heart; i, intestine; k, kidney; lu, lung; m, musculature; nc, nasal cavity; r, rib; sc, spinal cord; t, tongue; v, vertebrae.

FIGURE 3

μCT images and 3D reconstructions of the cutaneous capillaries in the skin of the gray short‐tailed opossum by near term (a–f; 13 dpc), in the neonate (g–j), and by 2 dpn (k) and 4 dpn (l–n). The skin in the near‐term embryo has numerous subepidermal capillaries (a–b, indicated by arrowhead). 3D reconstruction of μCT scans of the skin reveals a dense cutaneous capillary net all over the body (d–f). In the newborn, capillaries are still numerous and densely connected, but they do not appear as a compact capillary net anymore (g–j). The subepidermal capillaries communicate with larger transport vessels, which join into larger blood vessels that communicate with the arterial and venous system (i). By day 2, cutaneous capillaries appear still dense in the dermis (k). By day 4, cutaneous capillaries can be seen less often; the capillary volume density of the skin decreases, and the diffusion distance increases due to dermal growth (l–n). bv, blood vessel; d, dermis; dpc, days post coitum; dpn, day post natum; epidermis; f, forelimb; h, heart; lu, lung; r, rib; s, spine; sc, spinal cord; v, vertebrae.

FIGURE 4

Morphometric methods applied for analyzing skin development in the gray short‐tailed opossum. First, the entire specimen (a, neonate‐21 days) or the dorsal skin samples (b, 35 days‐adult) were serial sectioned, followed by systematic selection of a known fraction of the whole (fractionator). The total length of the specimen or dorsal skin sample divided by 10 gave the sampling thickness for the fractionator. In the small specimen, an orientator clock was applied to choose random sections of the dorsolateral side of the torso. As a result, 10 selected skin sections of each specimen were captured with a magnification of 200× (torso) or 100× (skin) on a light microscope equipped with a digital camera. Measurement of total skin thickness and of its different components (dermis, epidermis, stratum corneum, periderm) as well as the diffusion distance was performed at 5 different points on the image (fractionator principle), yielding 50 measurements per specimen. Using morphometric point‐counting methods, the volume densities of capillaries (Vvc), hair follicles (Vvh), and sweat glands (Vvs) were obtained in the 10 selected skin sections per specimen. d, dermis; e, epidermis; p, periderm; pap, papillary dermis; Pc, points on capillaries; Ph, points on hair follicles; Ps, points on sweat glands; Ptot, total points on skin tissue; ret., reticular dermis; s, stratum corneum.

FIGURE 5

Histological sections comparing the structural skin development of Monodelphis domestica shortly before birth and in the postnatal period: 13 dpc (a), neonate (b), 2 dpn (c), 4 dpn (d), and 7 dpn (e). In the newborn and 2‐day‐old M. domestica, capillaries (arrowheads) are numerous and closely associated with the epidermis. A lower capillary density and increasing diffusion distance become noticeable with progressive age. Up to the age of 4 days, the outer layer of the epidermis is formed by the periderm. An additional thin layer of keratin (stratum corneum) is visible between the periderm and the basal region of the epidermis (see b, right side with higher magnification). First epidermal placodes appear by day 7 (indicated by the arrow). d, dermis; e, epidermis; p, periderm; s, stratum corneum. Magnification: Left photographs: ×200; right photographs: ×400.

FIGURE 6

Histological sections comparing the structural skin development of Monodelphis domestica in the middle postnatal period: 11 dpn (a), 14 dpn (b), 21 dpn (c), and 28 dpn (d). Capillaries (filled arrowheads) become less numerous, and the diffusion distance between cutaneous capillaries and the skin increases further. Epidermal placodes and developing hair follicles (arrows) become more numerous by day 11. Starting by day 11, but more pronounced by day 14, the dermis differentiates into papillary and reticular layers. A multilayer of keratin (stratum corneum) forms the outer rim of the epidermis, providing cover and protection for the skin. The epidermis increases in thickness, reaching its maximal depth by day 21. The formation of hair follicles progresses, and mature hair follicles with cells of sebaceous glands (asterisk) and first sweat glands (open arrowhead) are visible by day 28. d, dermis; e, epidermis; pd, papillary dermis; rd, reticular dermis; s, stratum corneum. Magnification: Left photographs: ×200 (a–c), ×100 (d); right photographs: ×400.

FIGURE 7

Histological sections comparing the structural skin development of Monodelphis domestica in the late postnatal period: 35 dpn (a), 49 dpn (b), 57 dpn (c), and adult (d). The highest hair follicle volume density during the entire skin development can be seen by day 35. At this time, the pelage is fully developed. Mature hair follicles (arrows) with sebaceous glands (asterisk) and numerous small sweat glands (open arrowhead) are present, indicating the onset of thermoregulation. Cutaneous capillaries (filled arrowheads) are rare and found only in the proximal part of the dermis. The epidermis becomes successively thinner, reaching a final depth of 14 μm in the adult skin. In the matured skin, the density of hair follicles decreases. Hair follicles are confined to the papillary dermis, and sweat glands are located on the border of the papillary and reticular dermis. e, epidermis; pd, papillary dermis; rd, reticular dermis. Magnification: Left photographs: ×100; right photographs: ×400.

FIGURE 8

Light micrographs of the cardiorespiratory system and 3D reconstruction of the interatrial septum of the neonate (a–f), 2‐day‐old (g), 4‐day‐old (h–j), 7‐day‐old (k), and 11‐day‐old (l) Monodelphis domestica. In the newborn gray short‐tailed opossum numerous subepidermal capillaries (indicated by arrowheads) facilitate cutaneous gas exchange. They communicate with deeper‐lying capillaries running through the dermis (arrows), indicating a transport of blood to and from the cutaneous capillaries (a, b). These capillaries merge into larger blood vessels, which are drawing from peripheral regions towards the venous system (c). The heart of the newborn was immature with a fenestrated interatrial septum (d–f), permitting communication between the left and right sides of the heart (interatrial opening indicated by red arrows). By day 2, there are still small interatrial fenestrated openings in the thin septum (g). In the 4‐day‐old young, the interatrial septum is nearly complete (h–j). In the 7‐day‐old young, the heart is fully subdivided into left and right atria and ventricles (k). By 11 days the heart is muscular, and interatrial and interventricular septa increase in thickness (l). bv, blood vessel; c, capillary; e, esophagus; ias, interatrial septum; ivs, interventricular septum; la, left atrium; ll, left lung; lv, left ventricle; ra, right atrium; rl, right lung; rv, right ventricle; t, trachea.

FIGURE 9

Postnatal lung development of Monodelphis domestica. Histological sections of a neonate (a) and by day 7 (b), 14 (c), 21 (d), 28 (e), 35 (f), 49 (g), 57 (h) and in the adult (i). The lung in the neonate M. domestica is at the canalicular stage. The bronchial tree is simple and consists of short wide lobar bronchioles supplying large terminal air spaces (a). During the first postnatal week, the air spaces become more and more subdivided by septal ridges. By day 7 (b) the lung enters the saccular period, characterized by a double capillary septum (c). Until day 28, the lung is still at the saccular stage, and newly formed septa are still double capillary (d–f). Alveolarization starts by day 28 (f), and by day 35 (g), the alveolar period, characterized by the presence of a single capillary bed, is fully attained. During the late postnatal development, alveolarization continues until the adult mature lung structure is established (g–i). Insets show the airspace septa in a higher magnification.

FIGURE 10

Timeline graph summarizing the important steps in cutaneous, pulmonary, cardiovascular, and general development in Monodelphis domestica during the postnatal period.