Comparison of the respiratory transition at birth or hatching in viviparous and oviparous amniote vertebrates

Development of ventilatory chemoreflexes in Coturnix quail chicks

Compared to mammals, little is known about the development of the respiratory control system in birds. In the present study, ventilation and metabolism were measured in Coturnix quail chicks exposed to room air, hypoxia (11 % O₂), and hypercapnia (4% CO₂) at 0-1, 3-4, and 6-7 days posthatching (dph). Mass-specific ventilation and metabolic rate tended to increase between 0-1 and 3-4 dph and then decrease again between 3-4 and 6-7 dph. The magnitude of the hypoxic ventilatory response (HVR) increased with age. The HVR also exhibited a biphasic shape in younger quail: after the initial increase in ventilation, ventilation declined back to (0-1 dph), or toward (4 dph), baseline. Older chicks (6-7 dph) had a "sustained HVR" in which ventilation remained high throughout the hypoxic challenge. The biphasic HVR did not appear to be caused by a decline in metabolic rate; although hypoxic hypometabolism was observed in quail chicks in all three age groups, the metabolic response appeared to occur more slowly than the biphasic HVR. The biphasic ventilatory response was also specific to hypoxia since the hypercapnic ventilatory response (HCVR) was characterized by a sustained increase in ventilation in all three age groups. The magnitude of the HCVR decreased with age. These results point to several similarities in the development of ventilatory chemorflexes between Coturnix quail and newborn mammals, including age-dependent (1) increases in the HVR, (2) transitions from a biphasic to a sustained HVR, and (3) decreases in the HCVR. Whether homologous mechanisms underlie these developmental changes remains to be determined.

Comparison of the 3-D patterns of the parasympathetic nervous system in the lung at late developmental stages between mouse and chicken

Although the basic schema of the body plan is similar among different species of amniotes (mammals, birds, and reptiles), the lung is an exception. Here, anatomy and physiology are considerably different, particularly between mammals and birds. In mammals, inhaled and exhaled airs mix in the airways, whereas in birds the inspired air flows unidirectionally without mixing with the expired air. This bird-specific respiration system is enabled by the complex tubular structures called parabronchi where gas exchange takes place, and also by the bellow-like air sacs appended to the main part of the lung. That the lung is predominantly governed by the parasympathetic nervous system has been shown mostly by physiological studies in mammals. However, how the parasympathetic nervous system in the lung is established during late development has largely been unexplored both in mammals and birds. In this study, by combining immunocytochemistry, the tissue-clearing CUBIC method, and ink-injection to airways, we have visualized the 3-D distribution patterns of parasympathetic nerves and ganglia in the lung at late developmental stages of mice and chickens. These patterns were further compared between these species, and three prominent similarities emerged: (1) parasympathetic postganglionic fibers and ganglia are widely distributed in the lung covering the proximal and distal portions, (2) the gas exchange units, alveoli in mice and parabronchi in chickens, are devoid of parasympathetic nerves, (3) parasympathetic nerves are in close association with smooth muscle cells, particularly at the base of the gas exchange units. These observations suggest that despite gross differences in anatomy, the basic mechanisms underlying parasympathetic control of smooth muscles and gas exchange might be conserved between mammals and birds.

Embryonic water uptake during pregnancy is stage- and fecundity-dependent in the snake Vipera aspis

Water is a crucial resource that can profoundly impact the biology of terrestrial organisms. Early life stages are particularly sensitive to hydric constraints because water uptake is an important component of embryonic development. While amniotic eggs constitute a key innovation to terrestrial life, many vertebrates are viviparous wherein the mother must be the source of water for her developing embryos. Since most viviparous squamates are lecithotrophic (i.e., energy is supplied to the offspring as yolk deposited into pre-ovulated follicles), water is the predominant resource allocated from the mother to the offspring during development. Contrary to energy that can be stored (e.g., as fat reserves), water typically cannot be acquired in advance. Therefore, the embryos' need for water can impose significant constraints on the pregnant female. We detailed water flux during pregnancy in a viviparous snake, the aspic viper (Vipera aspis). We found that embryonic water uptake occurred mostly during the second half of pregnancy-a period dominated by somatic growth. We also found that, somewhat unexpectedly, changes in female plasma osmolality were negatively related to fecundity. This latter result suggests that water consumption by the female is especially important for large litter sizes, and thus may suggest an important sensitivity of reproductive females to environmental water availability.