Download Face and Skull Development

Development of Face, Palate and Skull
Pamela Knapp, Ph.D.
Professor, Dept. Anatomy & Neurobiology
MSB1 - Rm. 411
6-7570
[email protected]
Facial primordia appear in the 4th week as a series of 5
ventral swellings
NOTE: PA1
develops 2
prominences
Frontonasal prominence - a single swelling located anterior to the stomodeum. NOT derived from
pharyngeal arch tissue, but from mesenchyme proliferating ventral to the developing brain vesicles.
Maxillary prominences - are paired swellings that are part of the pharyngeal arches. They lie lateral
to the stomodeum and below the frontonasal prominence.
Mandibular prominences - paired swellings that are posterior components of the 1st pharyngeal arch.
They are found at the posterior border of the stomodeum
Frontonasal prominence develops two lateral, ectodermal swellings, the nasal placodes. They will
invaginate to form at first a nasal sac, and later a deeper nasal pit.
Merging vs. Fusion
Merging - requires a continuous mesenchyme lying
between ecto- and endoderm.
Because of the presence of a continuous
mesenchyme between the facial prominences,
MERGING DRIVES FACIAL DEVELOPMENT!
Incomplete merging?
dimples …. clefts …. complete clefts
Fusion -two separate tissue masses brought together
Incomplete fusion also results in clefting
Development of the mandibular prominences
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L & R mandibular prominences are initially separated by
a groove.
Migration and proliferation of neural crest cells within the
pharyngeal arch causes merging of the prominences
and obliterates that groove.
Adult derivatives of the mandibular prominences include:
a) lower lip; b) chin; c) outer cheek
Development of the frontonasal prominence
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FP gives rise to: forehead, the dorsum and apex of the
nose, and several other structures important in face
and palate development
Nasal placodes are small lateral swellings that develop a
deep nasal pit, with lateral and medial nasal
prominences on each side.
2 medial nasal prominences expand and merge to form
the intermaxillary segment. This segment will merge
with maxillary prominences on either side
Adult derivatives of the intermaxillary segment are:
philtrum of the lip; primary palate. The last is an
internal structure
Incomplete merging between the intermaxillary segment
and the maxillary prominences will cause cleft lip.
Lateral nasal prominences also merge with the maxillary
prominences, forming the alae (wings) of nose.
Development of the maxillary prominences
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Maxillary prominences merge with the lateral nasal
prominences and with the intermaxillary sement (the
merged medial nasal prominences).
A nasolacrimal groove originally separates the maxillary
prominence from each lateral nasal prominence.
A thick cord of tissue develops in this groove, and
migrates down into underlying tissues. The cord is
canalized, forming the open nasolacrimal duct.
Adult derivatives of the maxillary prominences include:
upper lip; inner cheek.
Palate Development
All parts of the adult hard and soft palate are formed by medial
growth of facial prominences.
Recognize that facial prominences are 3-dimensional, and extend
back into the developing head
The primary palate (medial palatine process) forms from
expansion of deeper portions of the intermaxillary segment.
Primary palate forms only a small portion of the adult hard palate.
Eventually ossifies and holds the incisor teeth.
Secondary palate is forerunner of the rest of the adult hard AND
soft palate.
Forms from 6th-10th week. Until it has fully formed, the fetus has a
common oronasal cavity.
Development of nasal cavities and secondary palate
Nasal cavity is first indicated by a small nasal pit in center
of nasal placode. Invaginates to larger nasal sac.
Up to 6 weeks, a thin oronasal membrane separates the
developing oral cavity (area of the pharynx just behind the
stomodeum) and nasal cavity (the invaginating nasal sac).
Nasal cavity enlarges and oronasal
membrane ruptures, creating a
single oronasal cavity, the
primitive choana (behind the
developing primary palate).
Olfactory epithelium by 7th week.
Secondary palate fully formed and
joined with primary palate by 12
weeks. This separates oral and
nasal cavities at forward ends.
A definitive choana lies behind the
secondary palate.
NOT SEEN IN THIS SECTION: The
nasal septum is forming - it will
grown down at midline to meet the
secondary palate. This will create
right and left nasal cavities, each of
which opens into the pharynx behind
the secondary palate (at definitive
choana).
The secondary palate is formed from 2 lateral palatine processes (palatine/palatal shelves) that extend medially from
the lateral surface of the maxillary prominence.
Same tissue forms the inner cheek and upper lip on the external surface.
At first, the LPPs grow downward, along the side of the tongue (developing on the floor of the pharynx).
Note the nasal septum, growing downward at midline from tissue of the frontonasal prominence.
LPPs slip above the tongue, and begin to grow towards each other. They normally meet and fuse rapidly.
When the LPPs have also fused with the nasal septum AND the primary palate, then the full palate has been created,
along with separate right and left nasal cavities.
The incisive foramen is the site of fusion of the primary and secondary palates.
Fusion of the LPPs occurs anterior to posterior.
Ossification occurs throughout the primary and secondary palate, except for the portion that lies
posterior to points of fusion with the nasal septum … including the uvula. The ossified
portions are the hard palate. The unossified portions (including uvula) are the soft palate.
The site of fusion of the LPPs is termed the palatal/palatine raphe.
Anomalies of face and palate development
Clefts are breaks between two tissue areas - failure to merge or fuse
completely. Underlying explanation is problem with mesenchyme
migration or proliferation (frequently a neural crest issue).
Etiology of clefting - multifactorial
-genetic
-environmental factors during critical period (7-12 weeks). Include
exposure to alcohol, Vitamin A derivatives, steroids.
Cleft lip is overall more common than cleft palate
(1 in 1000 vs. 1 in 2,500 live births)
Cleft palate is more common in female infants.
Anterior clefts - involve tissue anterior to incisive foramen (lip +/palate)
Posterior clefts - involve tissue posterior to incisive foramen
(palate only)
A. Looks like Unilateral anterior cleft - a.k.a. left side cleft lip. ((Cannot see from this view if either palate is involved.))
B. Bilateral anterior cleft with palate involvement. ((Cannot see if cleft extends to secondary palate)
C. Looks like unilateral anterior cleft lip and palate with secondary palatal cleft.
How do
these
things
happen?
Anterior Clefts: Failure of merging between intermaxillary segment and
maxillary prominence explains both lip and palate cleft. “A and B”
above show unilateral complete & bilateral complete anterior cleft.
Partial anterior cleft is lip only. (Disagreement over merging vs. fusion
IMS with secondary palate).
Posterior Clefts: Failure of fusion of the two lateral palatine processes.
Complete posterior cleft: Affects secondary palate along entire length.
Includes hard and soft portions. Shown in “C”.
Partial posterior cleft: Can affect any portion of the secondary palate
(ex: cleft uvula).
Combinations of anterior and posterior clefts are possible. “D” shows
combined complete bilateral anterior and posterior clefting.
Max
Mand
If the maxillary processes and lateral nasal processes do not merge, an oblique facial cleft is created. The
nasolacrimal ducts are exposed to the facial surface (persistant nasolacrimal grooves). Can be combined with a
cleft lip (as in “A” above) if the intermaxillary segment also does not merge with the maxillary processes.
Macrostomia (large mouth) occurs with failure of lateral merging of the maxillary and mandibular prominences. The
mouth opening can extend to the ears. Microstomia (small mouth) results from excessive merging of the same. Can
be unilateral or bilateral.
When the medial nasal prominences do not completely merge in the region of the intermaxillary segment, a median
cleft lip is formed (“C” above).
Skull Development
Two parts of the adult skull:
a. Neurocranium (encloses brain): composed of cartilaginous floor
(chondrocranium) and membranous vault.
b. Viscerocranium (encloses face)
First part of the adult skull to form is the floor of the neurocranium - the
cartilaginous chondrocranium.
Forms as a cartilaginous structure, from multiple chondrification centers.
Mesenchyme that forms the cartilage derives from occipital somites and
from somitomeres.
Multiple smaller cartilages that are generated then fuse. The cartilage is
later replaced by bone - “endochondral ossification”
Existance of separate chondrification centers is important!! They allow for
formation of foramina - the passageways for blood vessels and nerves
accessing the cranial vault.
Cartilages of the Median Cartilaginous Plate
Formation of the cartilaginous plate
a. Occipital cartilage - Formed from sclerotome cells in
several occipital somites.
b. Parachordal cartilages - Formed by cells of somitomeres
at tip of notochord.
a+b  most of occipital bone
c. Prechordal cartilages - 2 pair
c1. hypophyseal, forms around pituitary
c2. trabecular
Hypophyseals  much of body of sphenoid bone (includes
sella turcica)
Trabecular  some sphenoid & some ethmoid bone
Region of
occipital
somites
1. Additional chondrification centers form lateral cartilaginous
structures that fuse with the median cartilaginous plate (blue color
in diagrams).
Nasal capsules - develop around the nasal sacs
Otic capsules - form otic vesicles
-these later fuse with the median cartilaginous plate.
3. Alisphenoid cartilages - form greater wings of sphenoid bone
4. Orbitosphenoid cartilages - form lesser winds of sphenoid bone
NOTE: openings for nerves and blood vessels formed as median
and lateral component cartilages fuse.
Endochondral ossification - replacement of chondrocranial
cartilage models by bone, occurs in multiple centers of
ossification which appear between the 8th and 16th wks
Formation of the membranous neurocranium (cranial vault)
Membranous neurocranium (cranial vault) is made of the large, flat bones of the skull
-parietal, frontal, squamous part of temporal, interparietal part of occipital, outer part
of greater wing of sphenoid.
These form by intramembranous ossification (mesenchyme
transforms directly into bone)
-different than bones in the chondrocranium (floor), which
went through a cartilage step.
-each develop from separate ossification centers.
During fetal life and in infancy, the flat bones forming the cranial
vault are separated by fibrous connective tissue sutures.
Fontanelles are fibrous areas at the intersection of the sutures.
-largely close by end of 1st year, except for anterior fontanelle.
Utility of fontanelles and sutures is that they allow for a change
in skull shape during the birth process.
-also used clinicall for assessing hydration, intracranial P, and
assessing growth parameters by comparing to normal
ossification schedule.
Development of viscerocranium
There are components that form from mesenchyme both directly (ie, by intramembranous
ossification) and with an intermediate cartilage step (by endochondral ossification).
Membranous Viscerocranium
Multiple centers of ossification exist within the prominences of the
1st pharyngeal arch (mandibular and maxillary)
Bones: nasal, premaxilla, maxilla, lacrimal, vomer, zygomatic,
palatine, sphenoid (part of greater wing), temporal (squamous and
tympanic)
Cartilaginous Viscerocranium
Derived from cartilages of the pharyngeal arches (see chart PA lecture)
Parts of PA arch cartilages I-IV and VI persist.
Bones: incus, malleus, stapes, styloid process, hyoid bone, mandible
Cartilages of arch IV and VI remain unossified, and form cartilages of
the larynx and trachea.