In order to understand the problem of differentiation within the face,
one must understand the processes which manipulate
cranial neural crest cells into the many types of forms and tissues that
are present in the head/face region. There are three
major developmental points at which craniofacial development is analyzed.
1. Prior to migration, some neural crest cells commit themselves
to becoming craniofacial cells. This commitment influences subsequent migration
of the neural crest cells.
2. An inductive signal initiates a differentiation cascade.
3. Differentiation of neural crest cells into rudimentary craniofacial
features occurs.
When these important processes do not take place, several developmental
defects such as cleft palate and cleft lip might evidence themselves. Factors
that are known to increase the probability of having a child with a craniofacial
defect are: chronic or binge drinking during pregnancy, the use of antiseizure
medication during pregnancy, and smoking during pregnancy. It has also been
shown that genetic predisposition sometimes plays a role in these disorders.
Cleft Lip
Clefts involving the upper lip, with or without cleft
palate, occur about once in 1000 births, but their frequency varies widely
among ethnic groups. About 60-80 percent of affected infants are males.
The clefts vary from small notches of the vermilion border of the lip to
larger divisions that extend into the floor of the nostril and through the
alveolar part of the maxilla. Cleft lip can be either unilateral or
bilateral.
Unilateral Cleft Lip---This
condition results from the failure of the maxillary prominence on the affected
side to unite with the merged medial nasal prominences. This is the
consequence of failure of the mesenchymal masses to merge and the mesenchyme
to proliferate and push out the overlying epithelium. In addition,
the epithelium in the labial groove becomes stretched, and then breakdown
of tissues in the floor of the lip into medial and lateral parts.
Bilateral Cleft Lip---This
condition results from the failure of the mesenchymal masses of the maxillary
prominences to meet and unite with the merged medial nasal prominences.
The epithelium in both labial grooves becomes stretched and breaks down.
In bilateral cases, the defects may be similar or dissimilar, with varying
degrees of defect on each side. In complete bilateral cleft of the
upper lip and alveolar processes, the intermaxillary segment hangs free
and projects anteriorly. Such defects are especially deforming because
of the loss of continuity of the orbicularis oris muscle, which closes the
mouth and purses the lips as in whistling.
Cleft Palate
Cleft palate, with or without cleft lip, occurs about
once in 2500 births and is more common in females than in males. The
cleft may involve only the uvula, giving it a fishtail appearance, or it
may extend through the soft and hard palates. The embryological basis
of cleft palate is failure of the mesenchymal masses of the lateral palatine
processes to meet and fuse with each other, with the nasal septum, and/or
with the posterior margin of the median palatine process, or primary palate.
Wide Smiles
The
Library in the Sky
Sources:
1. Journal of the American Dental Association. "Meeting
the challenges of craniofacial-dental birth defects." Harold C.
Slavkin. May 1996 v127 n5 p681(2).
2. American Zoology. "Formation of the vertebrate face:
differentiation and development." Robert M. Langille. Sept 1993
v33 n4 p462(10).
3. The Developing Human. 3rd ed. Keith L. Moore.
W. B. Saunders Co., 1982.