Development of Aortic Arches in frog

Blood vessels arise by the confluence or fusion of blood islands which develop first in the splanchnic mesoderm. The cells of the blood islands merge to form blood vessels which become continuous with the heart. The embryonic blood cells also originate from the blood islands but later it derives from the spleen and bone marrow. A pair of lateral dorsal aortae develop from splanchnic mesoderm on each side dorsal to the region of the pharynx . Simultaneously, with the development of these, the anterior region of embryonic heart gives rise to the ventral aorta which becomes the external carotids. The first two pairs of visceral arches, the mandibular and hyoid arches, in fact, give rise to aortic arches in the embryonic stage, but they disappear immediately. The third, fourth, fifth and sixth aortic arches develop greatly and the intermediate portion of each vessel grows out into the external gills as afferent and efferent branchial loops. They coil through the filaments of developing external gills. The third, fourth and filth visceral arches now give rise to secondary blood vessels or capillary loops (branchial loops). These become connected both dorsally and ventrally with the corresponding aortic arches. That portion of the capillary loop which is connected with the ventral part of the aortic arch is known as the afferent bronchial loop. The other end which is connected with the dorsal part of the aortic arch is the efferent branchial loop. The irtermediate section of each capillary loop then grows out into the extemal gill. At this stage the blood may flow through either of Iht, two courses, ie, through the aortic arch or branchiaI loop. As the external gills get fully developed, the aortic arches tend to become relatively non - functional since the external gills handle major volume of the blood flow in this region. Later, with the degeneration of external gills, there develops a short circuit, a connection between the afferent and efferent ends of thy branchial loops. This is also aided by the remnant of the original mill arches. This short circuit supplies the newly forming internal gill. All the blood from the ventral aorta now passes through the filaments of nu. internal gills, until the lung circulation develops. During metamorphosi.. internal gills and the gill circulation altogether degenerate. As metamorphosis advances, the third aortic arch differentiates into carotid arch connected laterally with the lateral dorsal aorta by loops surrounding the pharynx. These are known as aortic arches. Aortic arches originate within the mesenchyme cf the visceral arches as blood vessels . The afferent branchial portion of each arch joins with ventral aorta while the efferent brar chial portion connects with dorsal aorta.The original paired ventral aorae proceeds into the head as the external carotids. The aortic arches which develop within the mesenchyme of III visceral arch give rise to the internal carotids. The aortic arches which develop within the IV visceral arches become the main systemic trunk. The V aortic arch will degenerate and the VI forms the pulmocutaneous of the adult frog.