BIOL 2P95 Lecture Notes - Hyaloid Artery, Progenitor Cell, Hypoplasia

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Head and Neck Development
I. Review
a. critical time for craniofacial development is wk 3-4 (same as gastrulation)
b. Neural crest cells bones of face & anterior skull
c. Mesoderm posterior bones & base of skull
d. Lateral Plate Mesoderm laryngeal cartilage
II. Somitomeres
a. Week 3: paraxial mesoderm differentiates into somitomeres
i. differentiate in cranial to caudal fashion
ii. 7 somitomeres
iii. key in segmenting brain into fore- mid- hind- brain segments by inducing
neural tube (via chemogradients)
b. somitomere = incompletely differentiated (in head)
c. somite = completely differentiated (found in lower body)
d. somitomeres form all voluntary muscles in head/neck; but scaffold (patterning)
is by connective tissue elements
III. Neural Crest Cells and Pharyngeal Arches
a. NCCs: migrate from neural folds in cranial-to-caudal sequence to somitomeres
pharyngeal arches (brachial arches)
i. directed by HOX genes populate specific regions of head/neck
ii. MSX-2 genes: specific “suicide” signal to NCCs along rhombomeres 3 &
5
1. produces gaps (brachial grooves)
2. ensures non-mixing between populations of NCCs
iii. Rhombomere: subdivision of hindbrain (segment of neural tube)
1. each rhombomere has its own HOX code
2. NCCs in each rhombomere migrate to brachial arches with
compatible HOX code specific link between face & brain
development
iv. NCCs provide link between development of face & brain; key BUT also
very sensitive to teratogens
b. Pharyngeal Arches (Brachial Arches)
i. General:
1. develop during 4th week of gestation
2. lateral swellings on either side of head; result from local
proliferation of NCC
3. give rise to specific cartilages and bones
4. pharyngeal grooves—external delineations between brachial
arches; ectoderm covered
5. pharyngeal pouches—internal deliniation between brachial arches;
endoderm lined
6. closing membranes—ectoderm/endoderm contact points ensuring
non-mixing between brachial arches
ii. Aortic Arch Arteries
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1. blood vessel development follows path of least resistance (thru
mesenchyme) => many run through brachial arches during early
embryogenesis
iii. Cranial Nerves—each is associated with a specific pharyngeal arch &
innervates all its muscles, etc
iv. Muscles—each pharyngeal arch differentiates into a specific set of
muscles which are innervated by the CN in that arch
Arch Artery Nerve Skeletal Elements Muscles
1 Maxillary V(V2 & V3
only)
(NCCs) Maxilla,
mandible, zygomatic
portions of temporal,
mm. of mastication (temporalis, masseter,
pterygoids) myelohyoid, ant. belly of
digastric, tensor tympani, & veli palatini
2
Stapedial
(practically
degenerates)
VII (NCCs) Stapes, styloid
process, part of hyoid
muscles of facial expression, post. belly
of digastric, stylohyoid, stapedius
3 Carotid IX (NCCs) Part of hyoid Stylopharyngeus
4
Right:
subclavian,
Left: arch of
the aorta
X (superior
laryngeal br)
(LPM) Laryngeal
cartilages
Cricothyroid, levator veli palatine,
pharynx constrictors
5 - - - -
6 Pulmonary X (recurrent
laryngeal br)
(LPM) Laryngeal
cartilages Intrinsic larynx mm.
IV. Pharyngeal Pouches & Grooves
a. Pharyngeal Pouches (Glands)
i. First Pharyngeal Pouch
1. forms the auditory tube (narrow proximal part) & tympanic
cavity (distal sac-like structure)
2. tympanic membrane—forms from lining of tympanic cav.
3. later comes into contact with epithelial lining of 1st pharyngeal
cleft (future EAM)
ii. Second Pharyngeal Pouch
1. proliferates to form palatine tonsil
iii. Third Pharyngeal Pouch
1. forms thymus and inferior parathyroid glands
2. thymus descends during development, pulling superior/ inferior
parathyroid glands with it
iv. Fourth Pharyngeal Pouch
1. forms superior parathyroid glands
2. attach to dorsal surface of thyroid
v. Fifth Pharyngeal Pouch
1. forms ultimobrachial body—regulates Ca2+ in body & is
embedded in thyroid
b. Pharyngeal Grooves (Clefts)
i. four recognizable clefts; all but one disappear during development
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