Lung and metabolic development in mammals: contribution to the reconstruction of the marsupial and eutherian morphotype.

Marsupials represent only 6% of all living mammals. Marsupialia and Placentalia are distinguished mainly by their modes of reproduction. In particular, the differences in the stage of development of the neonates may be one explanation for the divergent evolutionary success. In this respect one important question is whether the survivability of the neonate depends on the degree of maturation of the respiratory system relative to the metabolic capacity at the time of birth. Therefore, this review highlights the differences in lung morphology and metabolic development of extant Marsupialia and Placentalia. The Marsupial neonate is born with a low birth weight and is highly immature. The neonatal lung is characterized by large terminal sacs, a poorly developed bronchial system and late formation of alveoli. Marsupialia have a low metabolic rate at birth and attain adult metabolic rate and thermoregulatory capacity late in postnatal development. In contrast, the eutherian neonate is born with a relative high birth weight and is always more mature than marsupial neonates. The neonatal lung has small terminal sacs, the bronchial system is well developed and the formation of alveoli begins few days after birth. Placentalia have a high metabolic rate at birth and attain adult metabolic rate and thermoregulatory capacity early in postnatal development. The differences in the developmental degree of the newborn lung between Marsupialia and Placentalia have consequences for their metabolic and thermoregulatory capacity. These differences could be advantageous for Placentalia in the changing environments in which they evolved.

Ventilatory chemosensitivity and thermogenesis of the chicken hatchling after embryonic hypercapnia.

Hypoxia during incubation results in hatchlings with a reduced thermogenic capacity and a blunted ventilatory (V (E)) chemosensitivity (Szdzuy, K., Mortola, J.P., 2007b. Ventilatory chemosensitivity of the 1-day-old chicken hatchling after embryonic hypoxia. Am. J. Physiol. (Regul. Integr. Comp. Physiol.) 293, R1640-R1649). We asked if similar effects occurred with embryonic hypercapnia, that is, with a non-hypoxic sustained stimulation of the chemoreceptors. White Leghorn chicken eggs were incubated at 38 degrees C either in air (controls, C) or in 4% CO(2) from embryonic day 5 (4% CO(2)), hatching included. The 4% CO(2) embryos hatched about 12h later than C, with similar body weight. On the day of hatching the thermogenic capacity, assessed from the changes in oxygen consumption (.V(O2)) during 1h at 30 degrees C, increased from the early (about 3h old) to the late hours (about 20 h old), and was similar between 4% CO(2) and C. Ventilatory chemosensitivity was evaluated from the changes in (.V(E)) and in ventilatory equivalent (.V(E)/.V(O2)) during acute hypoxia (15 and 10% O(2), 20 min each) or hypercapnia (2 and 4% CO(2), 20 min each). Both at the early and late hours (.V(E)) chemosensitivity was lower in 4% CO(2) than in C. The .V(E)/.V(O2) responses of 4% CO(2) in hypoxia and hypercapnia averaged, respectively, about 45 and 60% of C. A separate set of eggs incubated in 2% CO(2) gave results qualitatively intermediate between C and 4% CO(2). We conclude that prenatal hypercapnia does not compromise the newborn's thermogenesis, but, like hypoxia, affects the development of respiratory control, resulting in a blunted chemosensitivity.

Postnatal lung and metabolic development in two marsupial and four eutherian species.

Two marsupial species (Monodelphis domestica, Macropus eugenii) and four eutherian species (Mesocricetus auratus, Suncus murinus, Tupaia belangeri and Cavia aperea) were examined to compare and contrast the timing of lung and metabolic development during the postnatal maturation of the mammalian respiratory apparatus. Using light, scanning and transmission electron microscopy, the lung structural changes were correlated with indirect calorimetry to track the metabolic development. Marsupial and eutherian species followed the same pattern of mammalian lung development, but differed in the developmental pace. In the two newborn marsupial species, the lung parenchyma was at the early terminal sac stage, with large terminal air sacs, and the lung developed slowly. In contrast, the newborn eutherian species had more advanced lungs at the late terminal sac stage in altricial species (M. auratus, S. murinus) and at the alveolar stage in precocial species (T. belangeri, C. aperea). Postnatal lung development proceeded rapidly in eutherian species. The marsupial species had a low metabolic rate at birth and achieved adult metabolism late in postnatal development. In contrast, newborn eutherian species had high metabolic rates and reached adult metabolism during the first week of life. The time course of the metabolic development is thus tightly linked to the structural differentiation of the lungs and the timing of postnatal lung development. These differences in the neonatal lung structure and the timing of postnatal lung maturation between marsupial and eutherian species reflect their differing reproductive strategies.

Oxygenation and establishment of thermogenesis in the avian embryo.

The production of heat (or thermogenesis) and its response to cold improve very quickly around birth in both mammals and birds. The mechanisms for such rapid perinatal development are not fully understood. Previous experiments with hyperoxia suggested that, during the last phases of incubation, eggshell and membranes might pose a limit to oxygen availability. Hence, it was hypothesized that an improvement in oxygenation by opening the eggshell may contribute to the establishment of thermogenesis. Thermogenesis and its response to cold were measured by indirect calorimetry, in warm (38 degrees C) conditions and during 1-h exposure to 30 degrees C. Both improved throughout the various phases of the hatching process. During the latest incubation phases (internal pipping, IP, and star fracture of external pipping, EP), the removal of the eggshell in the region above the air cell raised metabolic rate both in warm and cold conditions (in IP) or the thermogenic response to cold (in EP). Adding hyperoxia after opening the eggshell caused no further increase in the thermogenic response. In cold-incubated embryos thermogenesis during the EP phase was much less than normal; in these embryos, increasing the oxygen availability did not improve thermogenesis. We conclude that oxygenation contributes to the maturation of the thermogenic mechanisms in the perinatal period as long as these mechanisms have initiated their normal developmental process.

Monitoring breathing in avian embryos and hatchlings by the barometric technique.

This communication describes the application of the barometric technique to the measurements of the breathing pattern (tidal volume and breathing rate) and pulmonary ventilation (VE) in chicken embryos and hatchlings. The chamber-plethysmograph was separated into two sections, an animal compartment, maintained at incubation temperature, and a recording compartment kept at a lower temperature. In the case of the embryos, the eggshell itself represented the animal compartment. The hatchlings were accommodated into a "nest" compartment. A flow-through system permitted simultaneous measurements of oxygen consumption (VO2) and carbon dioxide production. Values of breathing rate corresponded to those counted visually in hatchlings while resting in the incubator, and data of resting VE were similar to those obtained by airflow plethysmography, a more invasive technique applicable only to the hatchlings. At rest, the ventilatory equivalent (VE/VO2) of the hatchlings was similar to that reported for adult birds, while in embryos it was much lower. Hypoxia (15 and 10% O2) and hypercapnia (2 and 4% CO2) caused hyperventilation (increase in VE/VO2), both in the hatchlings and in the embryos, these latter using hypometabolism as the main approach to hyperventilate in hypoxia. We conclude that the barometric technique can be adapted to the study of breathing and VE responses in avian embryos and hatchlings.

Ventilatory chemosensitivity of the 1-day-old chicken hatchling after embryonic hypoxia.

We investigated the effects of sustained embryonic hypoxia on the neonatal ventilatory chemosensitivity. White Leghorn chicken eggs were incubated at 38 degrees C either in 21% O(2) throughout incubation (normoxia, Nx) or in 15% O(2) from embryonic day 5 (hypoxia, Hx), hatching time included. Hx embryos hatched approximately 11 h later than Nx, with similar body weights. Measurements of gaseous metabolism (oxygen consumption, Vo(2)) and pulmonary ventilation (Ve) were conducted either within the first 8 h (early) or later hours (late) of the first posthatching day. In resting conditions, Hx had similar Vo(2) and body temperature (Tb) and slightly higher Ve and ventilatory equivalent (Ve/Vo(2)) than Nx. Ventilatory chemosensitivity was evaluated from the degree of hyperpnea (increase in Ve) and of hyperventilation (increase in Ve/Vo(2)) during acute hypoxia (15 and 10% O(2), 20 min each) and acute hypercapnia (2 and 4% CO(2), 20 min each). The chemosensitivity differed between the early and late hours, and at either time the responses to hypoxia and hypercapnia were less in Hx than in Nx because of a lower increase in Ve and a lower hypoxic hypometabolism. In a second group of Nx and Hx hatchlings, the Ve response to 10% O(2) was tested in the same hatchlings at the early and late hours. The results confirmed the lower hypoxic chemosensitivity of Hx. We conclude that hypoxic incubation affected the development of respiratory control, resulting in a blunted ventilatory chemosensitivity.

Hypoxic incubation blunts the development of thermogenesis in chicken embryos and hatchlings.

We asked to what extent sustained hypoxia during embryonic growth might interfere with the normal development of thermogenesis. White Leghorn chicken eggs were incubated at 38 degrees C either in normoxia (Nx, 21% O2) or in hypoxia [Hx, 15% O2, from embryonic day 5 (E5) until hatching]. The Hx embryos had lower body weight (W) throughout incubation, and hatching was delayed by about 10 h. For both groups, all measurements were conducted in normoxia. At embryonic day E11, the static temperature-oxygen consumption (ambient T-Vo2) curve was typically ectothermic (Q10 = 1.92-1.94) and similar between Nx and Hx. Toward the end of incubation (E20), the Q10 averaged 1.41 +/- 0.06 in Nx and 1.79 +/- 0.08 in Hx (P < 0.005), indicating that the onset of the thermogenic response in Hx lagged behind Nx. In the 1-day-old hatchlings (H1), body weight did not significantly differ between Nx and Hx. At H1, the T-Vo2 curves were endothermic-type, and more so in the older (>8 h old) than in the newly hatched (<8 h old) chicks, whether examined statically or dynamically as a function of time. In either case, the thermogenic responses of Hx were lower than those of Nx. In a 43-31 degrees C thermocline, the preferred T of the Hx hatchlings was around 37.3 degrees C, and similar to Nx, suggesting a similar setpoint for thermoregulation. We conclude that hypoxic incubation blunted the development of thermogenesis. This could be interpreted as an example of epigenetic regulation, in which an environmental perturbation during early development alters the phenotypic expression of a regulatory system.