A 5TH PHARYNGEAL ARCH ARTERY DOES NOT EXIST IN RAT

The development of the caudal pharyngeal arch arteries was studied in perfused rat embryos in the interval of embryonic day 121/2 to embryon ic day 133/4 using semithin serial sections and computer-aided 3-dimen sional reconstructions. The fourth pharyngeal arch artery was already developed at embryonic day 121/2. The sixth developed from cystic enla rged vascular sprouts at embryonic day 131/4. A real fifth pharyngeal arch artery was not found between the fourth and sixth artery. Instead a vascular plexus was present at embryonic day 121/2 which was interp osed between the epithelial sheets of the fourth pharyngeal pouch and the lower cervical sinus. At the onset of development this plexus aros e ventrally and dorsally from the fourth pharyngeal arch artery only. Subsequent to the development of the sixth pharyngeal arch artery, vas cular branches arose ventrally and dorsally from this artery, too. The plexus underwent incipient involution from embryonic day 131/4. It ar ose either from the fourth or the sixth pharyngeal arch artery only fr om embryonic day 133/4. It drained to the venous system. Branches of t he plexus which ran towards the epithelial sheet of the lower cervical sinus became prominent.