1.3:  Early Embryology

see also : additional branchial anatomy notes here.


Is an embryo for the first 8 weeks

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

-           Rathke’s 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) 

Derivatives of branchial arches

1st (mandibular) Arch

- 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, ).

2nd (hyoid) Arch

- bones: stapes, styloid process, stylohyoid ligament, lesser horn & superior part of body of hyoid bone.

- all muscles supplied by facial nerve. 

3rd (thyrohyoid) Arch

- 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.

4th & 6th Arches

(5th disappears without trace).

- make up muscles & cartilage of larynx.
-
4th arch artery = R subclavian on R, arch of aorta on L. 

- vagus nerve

Lateral derivatives of branchial pouches

(eating toast is such pleasure)

1st Pouch (ear)

Tympanic membrane, middle ear & mastoid antrum.  External cleft - external acoustic meatus. 

2nd Pouch (tonsil)

Dorsal part forms tympanic cavity with 1st pouch.  Ventral part - tonsillar crypts (endoderm) and lymphoid tissue of palatine tonsil (mesoderm). 

3rd Pouch (inferior parathyroids & thymus)

Dorsum - inferior parathyroids, ventrally ® thymus.  Descent of thymus drags parathyroids with it \ parathyroids III come to lie inferior to parathyroids IV. 

4th Pouch (superior parathyroids)

Dorsum - superior parathyroids, ventrally attached to thyroid gland which prevents descent of parathyroids IV. 

5th Pouch (parafollicular cells)

- ultimobranchial body, producing parafollicular C cells of thyroid. 

 

Cervical sinus

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. 

 

Ventral derivatives from the floor of the pharynx

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

 

Branchial arch arteries

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

 

Anomalies of the great vessels

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.

 

Face and Mouth

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

Defects

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

 

Development of the cloaca

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.

 

Heart

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.

 

Development of veins

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.

 

Fetal circulation

See physiology notes.