Posthatching development of the rete ophthalmicum in relation to brain temperature of mallard ducks (Anas platyrhynchos)

Abstract

In posthatching mallard ducks (Anas platyrhynchos), brain cooling improves with growth. To determine whether this may be correlated with growth-related changes in morphology of the rete ophthalmicum, we studied the development of this rete in immature mallards from hatching to 29 days of age. We found that the number of arteries and veins was fixed at hatching. The rete continued to grow, however, in length and vessel diameter during body and brain growth. The vascular surface area for heat exchange in the rete therefore also increased with body and brain mass. The increase in retial heat-exchange area was faster than the simultaneous increase in brain mass. The increase in body-to-brain temperature difference (delta T) described previously occurred nearly in direct proportion to heat-exchange area, such that the ratio of delta T to exchange area was nearly constant at about 0.1 degrees C per mm2 during growth. It is concluded that the increase in heat-exchange area of the rete ophthalmicum plays a major role in the development of brain cooling capacity of posthatching ducks.

Vasculature of the orbital rete in the Japanese deer (Cervus nippon)

“…These connections, involving the juxtapositioning of arterial and venous blood, may function as a countercurrent heat exchanger, as seen in some organs (e.g. the pampiniform vessels of the testis, 17 the carotid rete, 9 and the avian orbital rete 10 ).…”

“…Venous blood cooled in the heat‐loss surfaces of the head and nasal and palatine mucosa flows into the cavernous sinus, which surrounds the carotid and orbital retia 5 . 7 –9 In the duckling and fowl, the orbital rete functions as a heat exchanger to prevent overheating of the brain and ocular tissue 10 . Similar structures in other organs seem to produce major alterations in the nature of blood flow to the brain.…”

“…5,7±9 In the duckling and fowl, the orbital rete functions as a heat exchanger to prevent overheating of the brain and ocular tissue. 10 Similar structures in other organs seem to produce major alterations in the nature of blood flow to the brain. For example, the carotid rete is held responsible for regulating blood flow to the brain and for dampening the pressure pulse.…”

Ontogeny of avian thermoregulation from a neural point of view

“…Arad et al (1987) demonstrated a power function increase in the available heat exchange area of the rete ophthalmicum with post hatching growth. The number of blood vessels in the retial network, on the other hand, was fixed at the time of hatching.…”

Posthatching Development of Vascularization, Arteriovenous Anastomoses and Seromucous Glands in the Avian Nasal Mucosa

“…It is believed that the venous blood returning from the nasal mucosa cools the arterial blood to the brain by countercurrent heat exchange in the ophthalmic retia (Kilgore et al 1979). Experiments with ducklings, chickens and pigeons have shown that the effectiveness of brain cooling improves with age (Arad et al 1984, personal communication) and it has been suggested that this is due to growth of the heat exchange area in the ophthalmic rete during posthatching development (Arad et al 1987). The present study has shown an increase in the vascularity and density of AVAs in the nasal mucosa and, if the capacity for evaporative cooling of the mucosa also increases with age, as suggested above, this might contribute to the improved effectiveness of brain cooling during early posthatching development.…”