Aortic Arches: Embryological Development and Clinical Significance
The aortic arches, also known as pharyngeal arch arteries, are a series of six paired vascular structures that develop within the embryo. These temporary vessels serve as the blueprint for the great arteries of the head and neck. Located ventral to the dorsal aorta and arising from the aortic sac, these arches begin as symmetrical structures but undergo a complex process of remodeling to create the asymmetrical arterial system found in adults.
[ไม่มีภาพประกอบ]
Key Facts
- Six Pairs: The embryo initially develops six pairs of aortic arches.
- Remodeling: Most arches regress or transform into specific adult arteries.
- Asymmetry: While they start symmetrically, the final adult configuration is asymmetrical.
- Clinical Impact: Failure of arches to regress or develop correctly leads to congenital vascular defects.
- Fetal Shunt: The sixth left arch forms the ductus arteriosus, a critical shunt that closes after birth.
The Development of the Six Arches
The transformation of the aortic arches is a sequential process where specific vessels persist while others disappear.
Arches 1 and 2
The first and second arches disappear early in development. However, they leave behind small remnants. A portion of the 1st arch contributes to the maxillary artery. The 2nd arch's ventral end becomes the ascending pharyngeal artery, while its dorsal end forms the stapedial artery. In humans, the stapedial artery typically atrophies, though its remnants contribute to the internal maxillary artery and the middle meningeal artery.
Arch 3 (The Carotid Arch)
The third arch is fundamental to cranial blood supply. It develops into the common carotid arteries and the proximal portions of the internal carotid arteries on both sides of the body.
Arch 4 (The Systemic Arch)
The fourth arch develops differently on each side to create the systemic circulation:
- Right side: Forms the proximal segment of the right subclavian artery.
- Left side: Forms a segment of the arch of the aorta, situated between the left common carotid and left subclavian arteries.
Arch 5
The fifth arch is rudimentary; it either never forms or regresses shortly after appearing.
Arch 6 (The Pulmonary Arch)
The sixth arch is critical for the transition from fetal to neonatal circulation. The proximal part of the right arch becomes the proximal right pulmonary artery. The left arch forms the left pulmonary artery and the ductus arteriosus.
The ductus arteriosus allows blood to bypass the lungs during fetal life. Shortly after birth, increased oxygen concentration triggers the production of bradykinin, causing the ductus to constrict. Within one to three months, it is completely obliterated and becomes the ligamentum arteriosum.
Summary of Aortic Arch Derivatives
| Arch | Right Side Derivative | Left Side Derivative |
|---|---|---|
| 1st | Maxillary artery (part) | Maxillary artery (part) |
| 2nd | Stapedial and hyoid arteries | Stapedial and hyoid arteries |
| 3rd | Common and internal carotid arteries | Common and internal carotid arteries |
| 4th | Proximal right subclavian artery | Arch of the aorta |
| 5th | Regresses/Does not form | Regresses/Does not form |
| 6th | Proximal right pulmonary artery | Left pulmonary artery & ductus arteriosus |
Clinical Significance
Congenital defects of the great arteries typically occur when arches that should regress persist, or when arches that should persist regress.
- Aberrant Subclavian Artery: Occurs when the right 4th arch and right dorsal aorta regress, forcing the right subclavian artery to originate on the left and cross behind the trachea and esophagus.
- Double Aortic Arch: An abnormal right aortic arch persists alongside the left, creating a vascular ring that can compress the trachea and esophagus, leading to breathing and swallowing difficulties.
- Patent Ductus Arteriosus: A condition where the ductus arteriosus fails to close after birth.
- Other Defects: Including right-sided aortic arch and coarctation of the aorta.
Frequently Asked Questions
What happens to the ductus arteriosus after birth?
After birth, the increase in blood oxygen levels triggers the release of bradykinin, which causes the ductus arteriosus to constrict and close. Over the next one to three months, it transforms into a fibrous cord called the ligamentum arteriosum.
Why is the 4th aortic arch called the systemic arch?
It is called the systemic arch because it contributes to the primary vessels that distribute oxygenated blood to the rest of the body: the arch of the aorta on the left and the right subclavian artery on the right.
What causes a double aortic arch?
A double aortic arch occurs when the right aortic arch fails to regress and develops alongside the left aortic arch, forming a ring of blood vessels around the trachea and esophagus.
Which arch is responsible for the carotid arteries?
The third aortic arch, often referred to as the carotid arch, gives rise to the common carotid arteries and the proximal portions of the internal carotid arteries.