BIOL110 Lecture Notes - Hox Gene, Aortic Arch, Trochlear Nerve
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30 Jan 2013
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Embryology: Head and Neck
Begins at start of fourth week
Neural crest (cells at junction between surface ectoderm and neural plate) deposits in
cranial region as neural folds form
•mesenchyme of head: paraxial mesoderm (somitomeres and somites) + neural
crest
•while in neural folds, neural crest cells receive precise info (via HOX gene
code) as to where to go after they leavepopulate very specific region of
head
•some neural crest cells (along rhombomere 3 and 5), receive instructions from
MSX-2 genes to commit suicide to produce gaps to prevent mixing of neural
crest cell population
•free gaps produce clefts between the upper arches = branchial grooves or
branchial clefts
neural crest = bones of face and anterior skull
mesoderm: posterior bones and base of skull
laryngeal cartilages in neck: lateral plate mesoderm
mesoderm from primitive streak accumulates on both sides of midline; head region: not
completely segmentedsomitomeres
•seven somitomeres
•form from cranial to caudal sequence
•from region of prosencephalon to occipital region
•each one associated with specific portion of cranial neural plate; forebrain,
midbrain and hindbrain presaged by one or more somitomeres
osomitomeres tell neural tube segment what to become
•somitomeres set up initial organization of brain and form all voluntary muscle of
head and neck
•scaffold or patterning of muscles determined by connective tissue of head a neck
(neural crest)
somitomere primitive brain subdivision cranial nerve
craniofacial muscle
Origin Craniofacial muscles Innvervation
Somitomere 1,2 Sup., med., ventral recti CN III
Somitomere 3 Sup. Oblique CN IV
Somitomere 4 Jaw-closing mm. CN V
Somitomere 5 Lat. Rectus CN VI
Somitomere 6 Jaw-opening mm.
Other 2nd arch mm.
CN VII
Somitomere 7 Stylopharyngeus CN IX
Somites 1,2 Intrinsic laryngeals CN X
Somites 2-5 Tongue mm. CN XII
(Note: Raymond has replaced a table image with equivalent text)
hindbrain undergoes further segmentation: rhombomeres; each has unique genetic
identity
pharyngeal (branchial, visceral) arches (six arches, #5 is degenerate)
•lateral swellings on either side of head
•apparent during 5th week
•result of local proliferation of neural crest cell populations as they take
position
•made up of mesenchyme
•delineated from each other by pharyngeal grooves, covered by ectoderm
(external) and by pharyngeal pouches, lined by endoderm (internal)
•closing membranes (ectoderm/endoderm contact points) seal off archesno
mixing
•each arch has unique genetic identityspecific cartilages and bones
•with connective tissue in place, somitomeres and occipital somites can
organize the formation of the voluntary musculature associate with their
respective connective tissues
Summary of info, thus far:
•each segment of neural tube has unique genetic identity (HOX)carried into
pharyngeal arches through neural crest cellssome cells die to for grooves
•coordinate brain and facial development
Aortic arch arteries
•pharyngeal arches initially serve as conduits for aortic arch arteries
oarteries migrate thru path of least resistance: mesenchyme
•early in embryogenesis, pattern of arterial supply is symmetrical, but each pair of
arch arteries has different fates; some arteries grow, some degenerate
•
(Note: Raymond has replaced a table image with equivalent text below)
Aortic arch
1st Maxillary a.
2nd Stapedius a.
3rd Carotid a.
4th Proximal subclavian a. (right), aortic arch (left)
5th None
6th Pulmonary a.
Cranial Nerve
•each pharyngeal arch associated with specific cranial nerve
•CN emerge from specific regions of brain and supply pharyngeal arch with neural
crest that came from same region (same HOX code)
•To know which arch a muscle came from, know the CN
(Raymond has removed a table image from the original document)
Pharyngeal pouchesglands/organs
•5 pairs
•first pouch
oauditory tube
ocomes in contact with epithelial lining of first pharyngeal cleftfuture
EAM
odistal portiontympanic cavity (lining will become eardrum)
oproximal portionauditory tube
•second pouch
oforms buds that penetrate surrounding mesenchyme
opalatine tonsil
ostructure infiltrated by lymphatic tissue
•third pouch
oforms thymus and inferior parathyroid glands
oas thymus descends into thoracic region, pulls inferior parathyroid glands
with it
oinferior parathyroid glands become embedded into gland
•fourth pouch
osuperior parathyroid glands
oas thyroid gland moves caudally, glands attach to dorsal surface
•fifth pouch
outlimobranchial body
oregulates calcium levels in blood
oembedded into thyroid gland when pulled caudally
pharyngeal clefts/grooves
•initially, four clefts exist
•only one gives rise to definite structure
•1st pharyngeal cleft penetrates underlying mesenchyme and forms EAM
obottom of EAM forms lateral aspect of tympanic cavity
•2nd cleft
oundergoes active proliferation and overlaps remaining clefts
omerges with ectoderm of lower neckremaining cleft have no contact
with outside