A fifth 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 12 1/2 to embryonic day 13 3/4 using semithin serial sections and computer-aided 3-dimensional reconstructions. The fourth pharyngeal arch artery was already developed at embryonic day 12 1/2. The sixth developed from cystic enlarged vascular sprouts at embryonic day 13 1/4. A real fifth pharyngeal arch artery was not found between the fourth and sixth artery. Instead a vascular plexus was present at embryonic day 12 1/2 which was interposed between the epithelial sheets of the fourth pharyngeal pouch and the lower cervical sinus. At the onset of development this plexus arose ventrally and dorsally from the fourth pharyngeal arch artery only. Subsequent to the development of the sixth pharyngeal arch artery, vascular branches arose ventrally and dorsally from this artery, too. The plexus underwent incipient involution from embryonic day 13 1/4. It arose either from the fourth or the sixth pharyngeal arch artery only from embryonic day 13 3/4. It drained to the venous system. Branches of the plexus which ran towards the epithelial sheet of the lower cervical sinus became prominent.