Modifications of Aortic Arches in Vertebrates

 

Modifications of Aortic Arches in Vertebrates

Typical Plan of Aortic Arches:

When the heart is being formed in a vertebrate embryo, a blood vessel called ventral aorta appears mid-ventrally below the pharynx, which soon gets connected to the conus arteriosus. The ventral aorta arising from the heart runs forward beneath the pharynx and divides anteriorly into a pair of external carotid arteries into head.

The ventral aorta gives off 6 pairs of lateral aortic arches at equidistance which run through the visceral arches. Each aortic arch has a ventral afferent branchial artery carrying venous blood to the gill and a dorsal efferent branchial artery taking oxygeneted blood from the gill. The efferent branchial arteries of either side join dorsally with the lateral dorsal aorta or radix aorta which enters into the head as internal carotid artery.

The first aortic arch is a mandibular aortic arch, the second is a hyoid aortic arch, the remaining ones are called third, fourth, fifth and sixth aortic arches. The lateral dossal aorta fuse behind the pharynx to form a dorsal aorta which is continued mid-dorsally into the tail as a caudal artery.

From the dorsal aorta paired and unpaired arteries arise which supply various organs of the body. In an embryo with a yolk sac, a pair of vitelline arteries arises from the dorsal aorta and supplies the yolk sac. In embryos of amniotes a pair of umbilical or allantoic arteries arises from the dorsal aorta supplying blood to the allantois.

In an adult the vitelline arteries fuse to form the main mesenteric artery, the major part of the allantoic arteries is lost, but their remnants form hypogastric or internal iliac arteries.

Aortic Arches in Vertebrates:

In various adult vertebrates, the arterial system appears to be different, but they are built on the same basic fundamental plan. The difference is due to increasing complexity of heart due to a change in respiration from gills to lungs. There is a progressive reduction in the number of aortic arches in the vertebrate series.

Aortic arches in  Cyclostomata:

In Petromyzon, there are 7 pairs of aortic arches. In other cyclostomes these vary from 6 pairs in Myxine and 15 pairs in Eptatretus.

Aortic arches in Pisces:

Although six is considered to have been the basic number of aortic arches for fishes. This number is reduced to five even in sharks and rays with the loss of the first pair, the mandibular aortic arch or it is represented by an efferent pseudobranchial artery. In most bony fishes both the mandibular (i) and hyoid (ii) aortic arches disappear or are much reduced.

In Polypterus and Dipnoi (lung-fishes), gills are not well developed. So the pulmonary artery arises from the efferent part of 6th arch on either side which supplies blood to the air bladder or lung. In elasmobranchs and Dipnoi each arch has one afferent and two efferent branchial arteries (formed by splitting) in each gill. In bony fishes each gill has one afferent and one efferent artery.

In tetrapoda aortic arches do not break up into afferent and efferent parts because true internal gills are absent. In all tetrapoda the first and second arches disappear.

Aortic arches in Amphibia:

1.     Here the aortic arches show modification due to loss of gills and appearance of the lungs.

2.     In urodeles there are external gills present as respiratory organs in addition to lungs.

3.     The III, IV, V and VI aortic arches are present,

4.     The lateral dorsal aortae between the III and IV aortic arches persist as a vascular connection, the ductus caroticus.

5.     The VI aortic arch forms the pulmo-cutaneous arch or artery on either side taking blood to the lung and skin.

6.     It also retains a connection with the lateral dorsal aorta known as a ductus arteriosus (duct of Botalli).

7.     In the larva of anuran (frog tadpole), arrangement of aortic arches are like an adult urodele due to presence of gills. At metamorphosis, with the loss of gills, I, II, and V aortic arches disappear completely, only the IIIrd, IVth and Vlth aortic arches are present.

8.     The fourth aortic arch along its lateral dorsal aorta forms the systemic arch.

9.     The sixth aortic arch becomes the pulmocutaneous arch supplying venous blood to lungs and skin.

10.                        The ductus arteriosus disappears during metamorphosis.

11.                        Thus, adult anurans have only III, IV and VI aortic arches. These are also retained by amniotes.

Aortic arches in Reptilia:

1.     In reptiles, the gills are fully replaced by lungs.

2.     Only III, IV and VI aortic arches are present. With the partial separation of the ventricle into two parts, the distal portion of the conus arteriosus

3.     the entire ventral aorta are split into three vessels, i.e., two aortic or systemic and one pulmonary.

4.     Right systemic arch (IV) arise from the left ventricle carrying oxygenated blood to the carotid arch (III).

5.     The left systemic (IV) and pulmonary aortae (VI) take their origin from the right ventricle.

6.     The left systemic carries deoxygenated or mixed blood to the body through dorsal aorta.

7.     While the pulmonary artery takes deoxygenated blood to the lungs.

8.     The ductus caroticus disappears, but it persists in snakes and some lizards (Uromastix).

9.     The ductus arteriosus disappears in most reptiles though it persists in a reduced form in Sphenodon and some turtles.

10.                        Due to mixing of blood, reptiles are cold blooded animals like fishes and amphibians.

Aortic arches in Aves:

In birds, the III, IV and VI aortic arches are present. They follow the general pattern of reptiles with some differences. With the complete division of the ventricle into two parts, the conus arteriosus and ventral aorta have split to form two vessels, systemic aorta arising from the left ventricle and a pulmonary aorta from the right ventricle.

Third aortic arch with remnants of lateral and ventral aortae forms the carotids which arise from systemic aorta. Fourth aortic arch forms the systemic aorta on the right side only. It unites with the lateral aorta of its own side and forms the dorsal aorta. Part of the fourth aortic arch of the left side forms the left subclavian artery, the rest along with its lateral dorsal aorta disappears. The sixth aortic arch forms the pulmonary aorta. Ductus caroticus and ductus arteriosus disappear.

Aortic arches in Mammalia:

In mammals also the III, IV and VI aoric arches persist. The ventricle is divided completely into two parts.

The conus arteriosus and ventral aorta split to form two vessels:

(i) A systemic aorta arising from the left ventricle, and

(ii) A pulmonary aorta from the right ventricle. Third aortic arch with remnants of lateral and ventral aortae forms the carotid arch.

Fourth aortic arch forms the systemic aorta on the left side only, while on the right side its proximal portion forms an innominate and right subclavian artery, the rest along with its lateral dorsal aorta disappears. Sixth aortic arch forms the pulmonary aorta. The ductus arteriosus degenerates but it persists in some until hatching or birth in a reduced form on the left side as a thin ligamentum arteriosum.