Embryology Flashcards
What are pharyngeal arches
Pharyngeal arches are a sequence of ridges that form in the lateral walls of embryonic pharynx at the cranial end
Arches appear after 4wks and form the structures of the head and neck
Arches are formed of a mesenchyme core covered externally by ectoderm and internally by endoderm
What are found between pharyngeal arches externally
Pharyngeal clefts are found between pharyngeal arches
Clefts disappear as the 2nd arch grows down to over the others
1st cleft remains - forms external auditory meatus
Brachial clefts/fistulae can occur if there are remnants of the clefts - are found medial to SCM
What are found between pharyngeal arches internally
Pharyngeal pouches are found between pharyngeal arches
Glandular structures arise form these pouches
How are pharyngeal arches named
Pharyngeal arches are numbered 1-6 - 5th arch does not form in humans
1st arch is split into maxillary and mandibular prominences
What is associated with each arch
Artery
Nerve
Cartilage bar - 1st arch’s cartilage bar (Meckel’s cartilage) gives rise to mandible, malleus and incus bones
Cartilage becomes skeletal element of head and neck
What arteries and nerves are associated with each arch
Common, external and internal carotid arteries and branches are assocaited with 1st, 2nd and 3rd arches
Aortic arch and subclavian artery are assocaited with 4th arch
Pulmonary arteries are assocaited with 6th arch
Trigeminal nerve is associated with 1st arch
Facial nerve is associated with 2nd arch
Glossopharyngeal nerve is associated with 3rd arch
Vagus nerve is associated with 4th and 6th arches
What are the structural derivatives of the 1st arch
Muscles of mastication
Mylohyoid
Anterior belly of digastric
Tensor tympani
Tensor veli palatine
Maxillae and zygomatic bones
Meckel’s cartilage - mandible, malleus and inucs
What are the structural derivates of the 2nd arch
Muscles of facial expression
Stapedius
Stylohyoid
Posterior belly of digastric
Reichert’s cartilage - superior part of hyoid, stapes, styloid process, stylohyoid ligament
What are the structural derivates of the 3rd arch and what do the 3rd and 4th arches form
3rd arch: stylopharyngeus (pharynx muscle) and inferior part of hyoid body
3rd and 4th arch: epiglottis
What are the structural derivates of the 4th and 6th arches
Constrictor muscles of pharynx and muscles of soft palate (4th)
Levator palatini (4th)
Cricothyroid (4th)
Intrinsic muscles of larynx (6th)
Striated muscle of upper oesophagus
Cartilage of larynx
What structures are formed form the pharyngeal pouches
1st: tympanic caivty -> pharyngotympanic tube and middle ear cavity
2nd: palatine tonsil
3rd: inferior parathyroid and thymus
4th: superior parathyroid, C cells of thyroid
What does the face develop from
Face develops from facial primordia: frontonasal prominence, two maxillary prominences and two mandibular prominences
What do the parts of the facial primordia form
Frontonasal prominence: forehead, bridge of nose, upper eyelids, centre of upper lip
Maxillary prominences: middle third of face, upper jaw, most of the lip and sides of the nose
Mandibular prominences: lower third of face including the jaw and lip
What is some of the first evidence of facial development
The stomodaeum - depression in ectoderm on the ventral aspect of the head (future site of the mouth)
What is the first evidence of nose formation
Appearance of nasal placodes on either side of the midline, on ventrolateral aspect of frontonasal prominence
What are placodes
Placodes - area of ectoderm that starts to thicken and differentiate itself to form sensory structures
How do nasal pits develop
Nasal placodes invaginate and form nasal pits
What separates the nasal pits from the stomodaeum
The oronasal membrane - disappears so the oral and nasal cavities becomes continuous
Describe the development of the palate
Maxillary prominences grow medially, pushing nasal prominences closer together in the midline
Maxillary prominences fuse with medial nasal prominences
Medial nasal prominences merge in the midline separating the nostrils from the mouth - forms the philtrum
Fusion of medial nasal prominences forms intermaxillary segment
Palatal shelves grow from each maxillary prominence towards the midline
Mandible grows large enough to allow tongue to drop so that palatal shelves can fuse in midline
Palatal shelves fuse together and with the primary palate to form secondary palate - fusion separates nasal cavity from oral cavity
Nasal septum develops as a midline down growth and fuses with palatal shelves to divide nasal cavity in two
Where do the parts of the ear develop from
External auditory meatus develops from 1st pharyngeal cleft
Middle ear cavity and ossicles develop from 1st pharyngeal pouch and cartilage of 1st and 2nd arches respectively
Auricles develop from proliferation within 1st and 2nd arches surrouding the meatus
External ears develop initially in neck and then as mandible grows, ears ascend
Name some congenital abnormalities of the face
Lateral cleft lip
Cleft lip
Cleft palate
What is a lateral cleft lip
Failure of fusion of medial nasal prominece and maxillary prominence
One grows further over to compensate
Only affects primitive plate
What is a cleft lip
Failure of fusion of medial nasal prominence with maxillary prominence
Have a cleft in the centre
Affects primary palate
What is a cleft palate
Where there is failure of the palatal shelves to meet and fuse in the midline
Affects both primary and secondary palates
What parts of the foetal system are particularly sensitive to alcohol
Neural crest migration and development
What are two conditions caused by alcohol intake during pregnancy
Foetal alcohol syndrome - significant and prologned alcohol exposure
Alcohol related neurodegenerative delay - seen with lower levels of alcohol comsumption than FAS
Describe the development of the pituitary gland
Posterior pituitary begins as downward growth of the forebrain twoards pharynx
Anterior pituitary is formed from Rathke’s pouch which is an invagination of the ectoderm that grows upwards towards developing forebrain. Ossification of the sphenoid pinches off Rathke’s pouch from the ectoderm
Describe the formation of the tongue
Tongue begins as two lateral swellings and 3 median lingual swellings within arches 1-4
Lateral lingual swellings over-grow the tuberculum impar while at the same time the 3rd arch component over-grows the 2nd arch component, helping to form the tongue
Extensive degeneration occurs to free the tongue from the oral cavity floor though leaves the lingual frenulum
How does the sensory innervation of the tongue relate to the pharyngeal arches that form the tongue
Mucosa of anterior 2/3s derived from pharyngeal arches 1 and 3 -> general sensory by CN V and XI
Posterior 1/3 is from 3rd and 4th arches -> general sensory and special sensory by CN IX and X
What is the name given to the boundary between the anterior and posterior tongue
Sulcus terminalis
Describe the development of the thyroid gland
Primordium of the thyroid gland appears in the floor of the pharynx, between the tuberculum impar and cupola
The gland bifurcates and descends, becoming bi-lobed diverticulum joined in the midline by the isthmus
Thryoid gland origin is foramen cecum
What is Di-George syndome
Congetial thymic aplasia and absence of thyroid glands
Syndromes include additional defects
Have disruption of 3rd and 4th pharyngeal arches and abnormal development of neural crest cells
What is CATCH 22
Deletion on chromosome 22 causing Di George syndrome - CATCH 22 is mnemonic
Cardiac - aortic arch abnormalities, conotruncal, tricuspid atresia
Abnormal faces - hypertelorism, ear abnormalities
Thymic hypoplasia - cellular immune deficiency
Cellular immune deficiency, Cleft palate
Hypoparathyroidism with hypocalcaemia
22 chromosomal deletion
What is CHARGE syndrome
CHD7 heteroxygous mutation. CHD7 required for production of multipotent NC
C - coloboma
H - heart defects
A - choanal atresia
R - growth and developmental retardation
G - genital hypoplasia
E - ear defects
