see also : additional branchial anatomy
notes here.
Morula = mass of
developing cells
Blastocyst = morula with
a cystic space appearing
(the extraembryonic coelom);
implants at day 6.
Trophoblast = outer layer
of cells in blastocyst (¨ placenta).
Inner cell
mass attaches to inner
layer of trophoblast; two further cavities appear within it: amnion & yolk
sac with intervening embryonic plate (¨ organs & tissues).
Amniotic aspect
of embryonic plate = ectoderm,
yolk sac aspect = endoderm;
in between is mesoderm. Neural tube (¨ nervous system) develops from neural
groove on ectoderm (over the notocord).
-
neural
crest forms from cells
beside tube (¨: ganglia,
schwann cells, meninges, bones of skull & face, sclera and
choroid,
dentine, parafollicular & chromaffin cells and
melanocytes.
Mesoderm lies alongside
neural tube in 3
longitudinal strips: (1) paraxial mesoderm segments into
somites (¨ sclerotome and myotome, forming
bones and muscles of body
wall respectively) (2)
intermediate
cell mass (¨ urogenital
system) (3)
unsegmented lateral plate.
More rapid
growth of dorsal (ectodermal) surface ¨ curling of embryo.
A space appears in the centre of the mesoderm, = intraembryonic
coelom.
-
Inner layer
= splanchnopleure (¨
vitellointestinal duct and alimentary
canal)
-
Outer layer
= somatopleure (paraxial
myotomes migrate into it (¨
muscles of body wall.
Pleura and
peritoneum are initially continuous; lining is mesodermal. Limb bids develop
from lateral plate
mesoderm.
Septum
transversum = mass of
mesoderm lying on cranial aspect of coelomic cavity.
-
cranial
part ¨ pericardium
& part of diaphragm
(invaded by muscles of cervical myotome)
-
caudal part
surrounds liver as ventral mesogastrium.
Mouth pit (stomoderm) develops
after 2 weeks, caudally lined
with endoderm, cranially with ectoderm
-
RathkeÕs
pouch arises from
ectoderm (¨anterior
pituitary)
Mesodermal
condensations in walls of primitive pharynx form 6 branchial
arches which grow ventrally to fuse in
midline. Internal
grooves between
these are 4 branchial pouches,
external grooves form clefts.
-
there are
patterns to the derivatives of these:
-
each arch
has a cartilage bar and muscle differentiates around it
-
each also
has an artery and nerve allocated to it (though vascular
supply may
change)
- incus,
malleus, anterior ligament of malleus, sphenomandibular
ligament, mandible
& maxilla.
- all muscles
supplied by mandibular nerve
- mucosa
& glands of anterior 2/3 of tongue
(but not the
muscle: note that nerve supply of anterior 2/3
= chorda
tympani from facial nerve which is the nerve of the 2nd
arch, ).
- bones: stapes, styloid
process, stylohyoid ligament, lesser horn &
superior part of body of hyoid bone.
- all muscles
supplied by facial nerve.
- greater horn &
inferior part of body of hyoid bone
- mucosa
& glands of posterior 1/3 of
tongue.
- all muscles
supplied by glossopharyngeal nerve (ie stylopharyngeus)
- artery
persists as internal carotid.
(5th
disappears without trace).
- make up
muscles & cartilage of larynx.
- 4th
arch artery = R
subclavian on R, arch
of aorta on L.
- vagus nerve
Tympanic membrane,
middle ear & mastoid antrum.
External cleft -
external acoustic meatus.
Dorsal part
forms tympanic cavity with 1st pouch. Ventral part - tonsillar crypts (endoderm) and
lymphoid tissue of palatine
tonsil (mesoderm).
Dorsum - inferior
parathyroids, ventrally ¨ thymus. Descent of thymus drags parathyroids
with it \ parathyroids
III come to lie inferior to
parathyroids IV.
Dorsum - superior
parathyroids, ventrally
attached to thyroid gland which prevents descent of
parathyroids IV.
- ultimobranchial body, producing
parafollicular C cells of thyroid.
2nd
arch grows caudally and covers 3rd - 6th
arches; resulting pit =
cervical sinus. Margins
of this
pit fuse & imprison ectoderm which then disappears;
persistence - branchial
cyst.
Breaking down of
endoderm - branchial
fistula, usually in 2nd
pouch forming a track from tonsillar fossa to anterior neck
near lower end of
sternomastoid; this track runs between internal & external
carotids.
The muscle of
the tongue is formed by occipital myotomes, which carry their nerve (XII)
with them.
Evagination from
base of tongue at foramen caecum
forms thyroglossal duct;
thyroid develops from distal end.
Pyramidal lobe arises from persistence of distal
extremity.
-
Duct passes
most commonly anterior to hyoid but may pass behind or even
through it.
-
Remnants
may persist as thyroglossal cysts;
failure of descent is lingual thyroid
The larynx forms
from the ventral wall of the pharynx at the laryngotracheal
groove.
-
the groove
forms into a tube, which becomes the trachea
-
failure of
proper separation results in tracheobronchial fistula
Ventral aorta
from primitive heart divides into R
& L branches
which curve back dorsally as dorsal aortae.
-
these
continue as the two umbilical arteries
Each developing
arch has a vessel which joins ventral to dorsal aortae.
1st
& 2nd disappear early (maxillary is remnant of
1st). 3rd
remains as internal
carotid artery. 4th
persists on R as R subclavian,
and on L as arch of aorta. 5th disappears
entirely. 6th
persists
as the pulmonary artery ventrally & ductus arteriosus
dorsally on L.
-
this is why
the recurrent laryngeal nerve (sixth arch nerve) hooks around
the ligamentum
arteriosum
Most common is the patent ductus arteriosus (see heart notes)
Coarctation is due to a shelf in the media, which projects into the lumen, usually near the ductus.
Abnormal origin of the right subclavian artery from the aortic arch, may cause dysphagia by passing behind the oesophagus; associated with a non-recurrent right laryngeal nerve (a hazard in 1% of thyroidectomies.
The mandibular bits of the 1st arch surround the stomodeum to produce the lower jaw, lip and mouth.
The frontonasal prominence grows down from the forebrain capsule, indented by nasal pits.
Maxillary prominences grow ventrally from each mandibular prominence
- each side gives a palatal process that meet centrally and unite, separating mouth and nose.
All of these structures get supply from the trigeminal nerve
Cleft lip more frequently lateral from nostril
Cleft palate may be partial or complete, accompanied by irregular formations of teeth
- both caused by arrest of union
At the caudal embryo, hindgut and allantois (ie derivative of yolk sac) meet at the cloaca
The allantois ¨ urorectal
septum, which grows down, dividing the cloaca
into two (the front is the urogenital sinus; the back is
the urogenital membrane).
Urogenital sinus has
three parts: i) vesicourethral (upper) part ¨
bladder epithelium (mesoderm contribute muscle and c.t),
female urethra, and
trigone in males; ii) middle or pelvic part: ¨ most of male urethra, epithelium
of vagina; iii) phallic
(lower) part ¨ dorsal
penis and penile urethra, lower vagina.
Urogenital
membrane ¨ genital
tubercle (¨clitoris or
glans) and urogenital folds (¨labia minora
and scrotum)
-
failure of
the urogenital folds to unite in the male leads to
hypospadias.
Primitive vessels appear on the yolk sac; two fuse to make the early heart tube
-
differentiates into four: bulb
(¨truncus
arteriosus and so aorta & R ventricle),
ventricle, atrium (mostly
¨ auricles),
sinus venosis (mostly
¨ R atrium).
Bends because it grows faster than pericardium; bulb and ventricle come to lie in front of the atrium and sinus venosis.
The sinus venosis receives blood from three sources:
i)
placenta via umbilical veins (left one shunts
blood over liver to IVC via ductus venosis; ¨ ligamentum
teres and ligamentum venosum
ii) yolk sac (later alimentary canal) via vitelline veins
iii) general embryo tissue via cardinal veins (anterior and posterior)
- initially the R and L anterior cardinals are analogous to a R and L SVC, and a R and L posterior cardinals are analogous to a R and L IVC.
- the L ones obliterate, leaving the left brachiocephalic and longer left common iliac.
- the azygous and hemi-azygous veins develop from the right posterior cardinal vein.
See physiology notes.