Formation of the pharyngeal arch arteries in the chick embryo. Observations of corrosion casts by scanning electron microscopy.

An investigation was made of the formation of the pharyngeal arch arteries (PAAs) in developing chick embryos that ranged from Hamburger-Hamilton stage (st) 12 to st44 and 1-day-old chicks after hatching. Corrosion casts of the vasculature were made by injecting resin and examined by scanning electron microscopy. The relationship between the PAAs and associated organs was then demonstrated by computer-aided reconstruction. The ventral aorta connected with the dorsal aorta through a loop in front of the foregut at st12. We named this loop the primitive aortic arch to distinguish it from the true first PAA. The second PAA was found to form at st14 and the third PAA at st15, while the true first PAA to connect the dorsal and ventral aortae was found at st16. The aortic regions anterior to the first PAA then fused. By st19, the first PAA had disappeared and the fourth PAA had appeared. The fifth and sixth PAA were observed as a capillary plexus at st20 and st21. The fifth PAA consisted of a bypass of the sixth PAA during st22 to st25. The second PAA was observed to be very slender at st23 and to rupture by st25. At the same time, the proximal parts of the first and second PAAs and the ventral aorta changed into the primary external carotid artery (ECA) and gave off branches to the upper and lower jaws. Furthermore, the distal part of the second PAA remained to become the two branches of the dorsal aorta giving rise to the stapedial artery and the root of the secondary ECA.(ABSTRACT TRUNCATED AT 250 WORDS)