Development and regulation of breathing rhythms in embryonic and hatchling birds

For many, if not all, air-breathing vertebrates, breathing-like movements begin while the embryo is still ensconced in an aqueous environment. This is because primordial regions of the CNS become spontaneously active during early gestation and then must functionally transform and specialize once air breathing commences. The degree to which the embryonic ventilatory control system is established and competent at birth is variable, however, even between different components of the respiratory system. Moreover, the embryological experiences of an individual can also affect the outcomes and responsiveness of ventilation to respiratory stimuli and these details have major clinical implications. The broad field of respiratory neurobiology still has much to learn about the ontogeny of breathing control systems, and the oviparity of birds provides a unique model to examine how early rhythms transform day-to-day as they become functional. This hybrid review and research article will highlight the contributions of birds to the study of breathing control during early development. We will detail what is currently known about the onset and maturation of respiratory rhythm generation and also provide novel data about the development of central chemosensitivity. Finally, we will review data regarding the development of peripheral afferent inputs during early development and discuss whole-animal reflex responsiveness to common respiratory stimuli, both chronic and acute, during the incubation period and following hatching.