RATHKES POUCH MORPHOGENESIS IN THE CHICK-EMBRYO

Origin of Rathke's pouch ectoderm is complex. Anterior neural ridge and plate extend and reflect ventrally, then caudad, under the head into the subcephalic pocket. At least pouch roof and part of the walls form from ventral neural ridge. Pouch roof anchors to brain floor along the ventral suture line. Pouch tip, floor, and the rest of the walls are from stomodeal ectoderm. Pouch tip anchors to prechordal plate, and floor to Seessel's pocket along the midline. The prosencephalon bulges ventrally, bending attached pouch roof at right angles to pouch floor. The pouch tip is a pivotal point about which the head of the embryo enfolds. Cranial flexure bends the head ventrally, then caudad, folding the pouch roof parallel to its floor. Prosencephalon floor elongation stretches attached pouch roof. That these forces help form the pouch is shown in experiments in which parts are surgically altered, or glass rods are placed in brain cavity or in the foregut to stop the movements and thereby affect pouch formation. Pouch wall formation is due to accumulation of mesenchyme beneath lateral, unattached ectoderm. The full length of pouch floor ectoderm is present in early embryos and is gradually incorporated into the pouch as the walls form. The pouch does not invaginate nor change its position relative to the brain floor. The true infundibulum evaginates from brain floor over the pouch tip on the fifth day of incubation, which also is when an earlier, more caudal structure called “infundibulum” disappears. Copyright © 1979 Wiley‐Liss, Inc., A Wiley Company