These swellings consist of
aggregations of mesodermal cells covered with ectoderm. Shallow
grooves separate adjacent swellings.
Note the proportions of the
face early in development - the laterally-placed eyes, the wide
mouth, and the low-set ears. We should not forget the important
influence that structures such as the eyes, the tongue, the brain,
the skull base, and the ear have on facial development: a defect in
one of these structures can have a profound effect on facial
development.
Recall the significant part
played by the first pharyngeal arches in development of the face. A
cartilage develops within the mandibular process - Meckel's
cartilage - but (surprisingly) most of the mandible develops by
intramembranous ossification around the outer surface of the
cartilage. (Note that most of the facial skeleton is formed by
intramembranous ossification.)
nose and nasal cavities
The first indication of nasal
development is the formation of a pair of olfactory placodes on the
frontonasal process. These invaginate and establish the nasal
cavities which eventually open into the oral cavity. At the same
time, mesodermal proliferation at the edges of each placode produces
a C-shaped ridge. The two limbs of the ridge are known as the medial
nasal swelling and the lateral nasal swelling. Note that initially
the two halves of the nose are widely spaced - they move closer and
fuse as development proceeds. (The eyes also move closer together.)
the grooves between the
building blocks
Note carefully the grooves
between the components of the developing face - several of these
have a significance in normal and abnormal development.
In the groove between the
maxillary process and the frontonasal process (including the lateral
nasal swelling) the nasolacrimal duct develops by invagination of
the surface ectoderm. (This duct links the conjuctival sac of the
eye with the nasal cavity.)
The groove between the
maxillary swelling and the medial nasal swelling is soon ‘smoothed
out' as cells from the maxillary swelling stream medially. (It
appears that the streams of cells from the left and right maxillary
swellings eventually meet in the midline of the face, forming the
upper lip.)
The midline groove between
the two mandibular swellings is also soon filled out, but a midline
junction between the two halves of the mandible persists for longer.
Don't forget that the muscles
of facial expression are contributed by the second pharyngeal arch -
the myoblasts have to migrate into the facial region.
Building blocks of the face
facial clefts
If the grooves between
adjacent mesodermal swellings fail to fill out normally as the face
develops, facial clefts arise. Cleft upper lip is the most common
form, and is usually not exactly in the midline but slightly to one
side, or sometimes bilateral. Consult your textbook and study
carefully the different forms of cleft lip, and the consequences and
treatment of these conditions. Read also about oblique facial
clefts. In each case, try and relate these abnormalities to what you
know about normal development of the face.
the palate
Many everyday activities
depend on the presence of a normal palate - for example: chewing,
swallowing, speaking. A cleft in this structure can have serious
consequences for a baby.
There has been a lot of
recent research into palatal closure, and we have a clearer idea now
of the mechanisms at work. For example, it is now clear that the
movement of the palatal shelves from a vertical to a horizontal
position (so that they can fuse) is a rapid process, taking less
than 1 second in rat embryos. An important prerequisite for this
event is movement of the tongue downwards out of the way - this
occurs when the embryo begins swallowing actions. Another fairly
recent realisation is that neural crest cells play a vital role in
palatal development, as they do in facial development generally.
Development
of the palate
Developing fetal skull
Questions on development
of the face and palate:
1 List the ‘building
blocks' from which the face develops.
2 Describe the changes
in position of the eyes and ears in relation to other facial
structures during facial development.
3 Relate the nerve
supply of the upper lip to its embryonic development.
4 How does the
lacrimal duct develop?
5 Why are tongue
movements important during palatal development? Can you think why a
baby with anencephalus might also have a cleft plate?
6 How can a feeding
bottle be modified to help a baby with cleft palate to feed?
7 Describe development
of the nose and nasal cavities. Do the nasal cavities develop
normally in a baby with cyclopia?
8 When do the
paranasal air sinuses develop?
9 What is macrostomia?
