Embryological Development Flashcards
What are the embryological divisions of the neural tube?
Expansion of cranial neural tube forms three primary vesicles at 3-4wks:
- FOREBRAIN (Prosencephalon)
- MIDBRAIN (Mesencephalon)
- HINDBRAIN (Rhombencephalon)
5wks there are five secondary vesicles:
- Prosencephalon splits into telencephalon (becomes cerebrum) and diencephalon (optic vesicles develop from)
- Mesencephalon remains the same
- Rhombencephalon splits into metencephalon (becomes pons and cerebellum) and myelencephalon (becomes medulla oblongata)
What are the embryological components of the facial skeleton?
Frontal nasal prominence (unpaired; there due to presence of developing brain)
Paired pharyngeal arches: pharyngeal arch 1 forms the maxillary and mandibular arches
Outline how the cranial nerves are formed.
Week 5:
- pharyngeal arches present (Max 1 & Man 1, 2, 3, 4, 6)
- pharyngeal clefts between the pharyngeal arches (1st cleft = external acoustic meatus, 2nd cleft grows down to obliterate all other clefts)
- pharyngeal pouches present (pharyngeal gut tube - glandular derivatives)
The segmental arrangement of the cranial nerves is lost
note: all cranial nerves are derived from the midbrain and hindbrain except for the olfactory nerve (CNI) & the optic nerve (CNII)
What are the nerves of the pharyngeal arches? What function do they serve in general?
ARCH 1 = trigeminal nerve (CNV)
ARCH 2 = facial nerve (CNVII)
ARCH 3 = glossopharyngeal nerve (CNIX)
ARCH 4 & 6 = vagus nerve (CNX)
- 4th = superior laryngeal nerve
- 6th = recurrent laryngeal nerve
+ cranial accessory nerve (CNXI) and hypoglossal nerve (CNXII) have a relationship with the pharyngeal arch system
Supply derivatives of pharyngeal arches, have mixed sensory and motor functions
What are the muscles which develop from each pair of pharyngeal arches?
ARCH 1 = muscles of mastication (trigeminal nerve - CNV)
+ mylohyoid, stylohyoid, anterior & posterior digastric, tensor tympani (dampens sounds made by chewing), tensor veli palatine (tenses soft palate), stapedius
ARCH 2 = muscles of facial expression (facial nerve - CNVII)
ARCH 3 = stylopharyngeus (elevates the larynx & pharynx to facilitate swallowing) (glossopharyngeal nerve - CNIX)
ARCH 4 = cricothyroid (elongates vocal cords by increasing tension & drawing back the cricoid cartilage) (superior laryngeal nerve - branch of vagus nerve CNX)
+ levator palatini (draws back soft palate to prevent food entering nasopharynx during swallowing)
+ constriction of pharynx (superior laryngeal nerve)
ARCH 6 = intrinsic muscles of larynx (recurrent laryngeal nerve - branch of vagus nerve CNX)
What disorders can occur when the cervical sinus is not obliterated by the down growth of the 2nd pharyngeal arch?
Branchial cysts (opening to internal surface)
Branchial fistulae (opening to external surface)
note: can occur anywhere along the anterior border of sternocleidomastoid
What cartilage is associated with each pharyngeal arch?
ARCH 1 = mandibular prominence develops Meckel’s cartilage (template for mandible during membranous ossification)
+ sphenomandibular ligament + malleus & incus
ARCH 2 = Reichert’s cartilage which contributes to middle ear development (stapes), styloid process, stylohyoid ligament, and part of the hyoid bone
ARCH 3 = remainder of hyoid bone
ARCHES 4 & 6 = thyroid, arytenoid, and cricoid cartilages
+ epiglottis (pharyngeal floor)
Reminder: what does each aortic arch develop into?
ARCH 1 = disappear
ARCH 2 = disappear
ARCH 3 = internal carotid artery
ARCH 4 = aortic arch & brachiocephalic artery
ARCH 6 = pulmonary arch
What does each pharyngeal pouch develop into?
POUCH 1:
- VENTRAL = obliterated
- DORSAL = Eustachian/pharyngotympanic tube & middle ear cavity
POUCH 2:
- VENTRAL = partially obliterated; colonised by lymphoid precursors which form the crypts of the palatine tonsil
- DORSAL = contributes to Eustachian tube
POUCH 3:
- VENTRAL = thymus
- DORSAL = inferior parathyroid gland
POUCH 4:
- VENTRAL = ultimobranchial bodies (C-cells of thyroid gland)
- DORSAL = superior parathyroid gland
What are the pharyngeal arches?
Mesodermal (specifically mesenchymal) outpouchings in the neck region (bilateral) invaded by neural crest cells
Each pharyngeal arch has an associated artery (from the aortic arches), vein, cranial nerve, cartilage bar (mesenchyme), ectodermal covering, endodermal lining (derived from gut tube of pharynx)
Outline the development of the eyes.
- Outgrowth of surface ectoderm on each side of the diencephalon (optic vesicles: give rise to retina, iris, ciliary body)
- Signalling molecules released by optic vesicle stimulate changes in ectoderm —> localised thickening opposite the optic vesicle —> lens placode formed
- Optic vesicle & lens placode invaginate —> lens becomes closed vesicle and sinks below surface ectoderm —> becomes cornea
- Mesenchyme cells arrange themselves around developing lens & retina —> form choroid (layer of eyeball between retina & sclera, contains blood vessels and pigment) and sclera
Muscles which move the eye are derived from somites
Eyes originally on sides of the head; growth of maxillary prominences towards the midline shifts the eyes to the front
Give some examples of foetal eye abnormalities.
Cyclopia = single midline eye
Anopthalmia = absence of eyes
Microphthalmia = abnormally small eyes
Outline the development of the nose.
- Nasal placodes on frontonasal prominence sink to become nasal pits (medial and lateral prominences form either side)
- Maxillary prominences grow medially, pushing the nasal prominences closer together in the midline
- Maxillary prominences fuse with medial nasal prominences and medial nasal prominences fuse in the midline —> creates intermaxillary segment:
- labial component —> philtrum
- upper jaw —> 4 incisors
- palate —> primary palate - Oronasal membrane between nasal pit and oral cavity ruptures —> creates nasal cavities
Outline the development of the secondary palate.
- Maxillary prominences give rise to palatal shelves which grow vertically downwards into the oral cavity on each side of the developing tongue
- Mandible grows larger enough to allow tongue to “drop”
- Palatal shelves grow towards each other and fuse in the midline
- Nasal septum develops as a midline down-growth and ultimately fuses with the palatal shelves
What are the different types of cleft lip?
Lateral cleft lip = failure of fusion of medial nasal prominence and maxillary prominence
Lateral cleft lip involving primary palate
Cleft lip & cleft palate = failure of palatal shelves to meet in the midline
