Correlation Between Age and Size of Embryo
|Weeks ||Millimeters |
|2.5 ||1.5 |
|3.5 ||2.5 |
|4 ||5 |
|5 ||8 |
|6 ||12 |
|7 ||17 |
|8 ||23 |
|10 ||40 |
|12 ||56 |
|16 ||112 |
|5-10 mo ||160-350 |
Development of the Branchial Arches
The first 8 weeks constitute the period of greatest embryonic development of the head and neck. There are five arches that are named either pharyngeal or branchial. Between these arches are the grooves or clefts externally and the pouches internally. Each pouch has a ventral or dorsal wing. The derivatives of the arches are usually of mesoderm origin. The groove is lined by ectoderm, and the pouch is lined by entoderm (Figure 51-1).
Each arch has an artery, nerve, and cartilage bar. These nerves are anterior to their respective arteries, except in the fifth arch where the nerve is posterior to the artery. (Embryologically, the arch after the fourth is called the fifth or sixth arch depending on the theory one follows. For simplicity in this synopsis, it is referred to as the fifth arch.) Caudal to all the arches lies the 12th nerve. The sternocleidomastoid muscles are derived from the cervical somites posterior and inferior to the above arches.
There are two ventral and two dorsal aortas in early embryonic life. The two ventral aortas fuse completely, whereas the two dorsal ones only fuse caudally (Figure 51-2A). During the course of embryonic development, the first and second arch arteries degenerate. The second arch artery has an upper branch that passes through a mass of mesoderm, which later chondrifies and ossifies as the stapes. This stapedial artery usually degenerates during late fetal life but occasionally persists in the adult. The third arch artery is the precursor of the carotid artery in both left and right sides. The left fourth arch artery becomes the arch of the aorta. The right fourth arch artery becomes the proximal subclavian. The rest of the right subclavian and the left subclavian are derivatives of the seventh segmental arteries. The left fifth arch artery becomes the pulmonary artery and ductus arteriosus. The right fifth arch artery becomes the pulmonary artery with degeneration of the rest of this arch vessel (Figure 51-2B).
Development of the embryonic arteries.
Should the right fourth arch artery degenerate and the right subclavian arise from the dorsal aorta instead, as shown in Figure 51-2C, the right subclavian becomes posterior to the esophagus, thus causing a constriction of the esophagus without any effect on the trachea (dysphagia lusoria). The innominate artery arises ventrally. Hence when ...