Pharyngeal Clefts

The 5-week embryo is characterized by the presence of four pharyngeal clefts (Fig. 15.6), of which only one contributes to the definitive structure of the embryo. The dorsal part of the first cleft penetrates the underlying mesenchyme and gives rise to the external auditory meatus (Figs. 15.10 and 15.11). The epithelial lining at the bottom of the meatus participates in formation of the eardrum (see Chapter 16).

Active proliferation of mesenchymal tissue in the second arch causes it to overlap the third and fourth arches. Finally, it merges with the epicardial ridge in the lower part of the neck (Fig. 15.10), and the second, third, and fourth clefts lose contact with the outside (Fig. 15.10B). The clefts form a cavity lined with ectodermal epithelium, the cervical sinus, but with further development this sinus disappears.

Was this article helpful?

0 0
Emergency Panic Remedies

Emergency Panic Remedies

Eliminate Your Panic And Anxiety Attacks Instantly With These Proven Techniques! There’s
 very
 little
 in
 this 
world
 that 
is
 as 
frightening
 and
 uncomfortable
 as 
a panic
 attack. 

Once
 they
 start,
 panic
 attacks
 can
 send
 you
 spiraling
 out
 of control.

Get My Free Ebook


Post a comment