Functional development of the prenatal brain. II. Ontogeny of the hypothalamo-neurohypophysial axis in the pre- and perinatal chicken brain.

The present paper investigates the ontogeny of connective patterns in the brain of the 17-, 17.5-, and 18-day-old chicken embryo and in the newborn chicken as well. From the onset of electrical activity, neurophysiological properties of neurones undergo significant changes during further development. Total duration of action potentials decreases (p less than 0.001) between day 17 of incubation and day one of life. The percentage of spontaneously active neurones increases significantly (p less than 0.01) from 20% on day 17 to 69% in the newborn chick. For the same time period mean conduction velocity of presumed non-myelinated fibers in the hypothalamo-neurohypophysial-tract increases by about 50%. On the other hand, mean constant latency decreases from 20 ms in the 17-day-old embryo to 15.5 ms in the newborn chick (p less than 0.001). Changes in threshold intensity to antidromic stimulation and the increasing number of identifiable neurones per embryo indicate that new cell populations may become excitable during development. The results show, that considerable maturational events take place in action potential properties of the magnocellular system during late embryonal and early postnatal life of the chicken.