Embryology1 Head, Neck, Neuroanatomy

Question 1

How does the embryo start and end week 4 in terms of facial, head and neck structures?

Answer 1

Early week 4: no distinguishing external facial features but
Head and neck represent half length of embryo

Further into week 4:
Pharyngeal and branchial arches develop which are a complex tissue system involves many systems notably: brain, CVS, special sensory organs - each arch has a neurovascular plan develops muscles and skeletal elements and a specific nerve

Question 2

What are the pharyngeal arches?

Answer 2

Comprised of a system of mesenchymal proliferations in the neck region

Made up of layer of ectoderm laterally, mesenchyme cord centrally, endoderm medially

5 in total numbered 1-6 (5th not formed in humans) get progressively smaller

Together with frontal nasal prominence (FNP) the building blocks for the head and neck

Question 3

What’s between each pharyngeal arch? What do these structures become?

Answer 3

Pharyngeal clefts between each arch laterally from ectoderm. 1st cleft all that remains -> external acoustic meatus, 2nd -> grows down to cover overs. If these are remnants branchial cysts or fistulae can occur

Pouches medially made from endoderm
First pouch largest -> tympanic cavity
Development results in formation of palatine tonsils, parathyroid glands and thymus (regressed post childhood)

Question 4

Associations of cranial nerves with pharyngeal apparatus

Answer 4

CN 5,7,9,10 mixed functions supply derivatives of pharyngeal arches

ONLY CN 11 & 12 have relationship with pharyngeal arch system

Question 5

Muscular derivatives of the pharyngeal arches and cranial nerve supply of each

Answer 5

Pharyngeal arch 1 supplied 5Th CN -> muscles of mastication

PA2 supplied 7th CN -> muscles of facial expression

PA3 supplied CN9 -> stylopharyngeus (elevates the larynx and pharynx in swallowing)

PA4 CN10 -> cricothyroid (helps tense vocal cords), levator palatini (elevates soft palate swallowing), constrictors of pharynx

PA6 CN10 -> intrinsic muscles of larynx

Question 6

Cartilages of the pharyngeal arches

Answer 6

Each PA develops a neural crest derived cartilage bar

PA1 - meckel’s: malleus & incus (inner ear) plus a template for formation of the mandible

PA2- reichert’s: stapes (middle ear) plus upper part hyoid bone

PA3- remainder of hyoid bone

PA4&6 - cartilages of larynx

Question 7

Aortic arches arteries

Answer 7

1st & 2nd arteries disappear

3rd- internal carotid
4th - arch of aorta (L) & brachiocephalic (R)
6th- pulmonary arch

Question 8

What drives development of the face?

Answer 8

Expansion of the cranial nerve tube -> appearance of a complex tissue system associated with the cranial gut tube and outflow of the developing heart

Development of the sense organs

Question 9

First structures that the face develops from

Answer 9

Facial primordial from 1st PA (maxillary and mandibular arch) + FNP surrounds ventral-lateral part of the forebrain

Primordial of eyes on side of head

Question 10

At the end of 4th week what are the components of the face? What do they each become?

Answer 10

Stomadeum centraly closed off at cranial end

FNP superiorly -> forehead, bridge nose, nose, philtrum (indentation lip)

1stPA - maxillary prominence (cheeks, lateral upper lip, lateral upper jaw) middle both sides of stomatodeum & mandibular prominence (lower lip and jaw) most inferior both sides

Question 11

How does the nose develop?

Answer 11

Nasal placoderms appear on the frontonasal prominence then sink to become the nasal pits

Medial and lateral nasal prominences form on either side of the pits

Maxillary prominences grow medially pushing the nasal prominences closer
(-> intermaxiallry segment becomes the groove of the upper lip (philtrum)/ upper jaw 4 incisors/ primary palate)

Maxillary prominences fuse with medial nasal prominences which then fuse in midline

Placode- thickening of ectoderm

Question 12

Describe how the mandible, tongue, palate and nasal spectrum all grow in relation to one another

Answer 12

Main part of the definitive palate is secondary palate derived from palatal shelves (derived form maxillary prominences) grow vertically down into the oral cavity on each side of developing tongue

Mandible grows large enough to allow tongue to drop

PALATAL shelves grow and fuse in midline

Nasal septum develops as a midline down growth and fuses with palatal shelves

Question 13

Explain how lateral cleft lip forms

Answer 13

Failure of fusion of medial nasal prominence and maxillary prominences

Nasal cavity connected to oral cavity through normal lip space

Question 14

Explain how cleft lip and cleft palate forms

Answer 14

Failure of fusion of medial nasal prominences and maxillary prominences to fuse = cleft lip AND

Failure of palatal shelves to meet in midline = cleft palate

Slide 32

Question 15

What structures develop into what parts of the ear?

Answer 15

Externally auditory meatus develops from the 1st pharyngeal cleft (the rest are obliterated by the 2nd)

Middle ear cavity and ossicles develop from 1st pharyngeal pouch and cartilages of 1st and 2nd arches respectively

Auricles (external ear) develop from proliferation within the 1st and 2nd pharyngeal arches surrounding the meatus

Question 16

How do the ears change position during development? What can cause problems with this positioning?

Answer 16

External ears develop on the neck -> mandible grows and ears ascend to lie in line with the eyes

All common chromosomal abnormalities have associated external ear abnormalities

Hypoplasia/ underdevelopment of the mandible -> lower- set ears

Question 17

Explain fetal alcohol syndrome occurs. What’s the safe limit of alcohol in pregnancy?

Answer 17

Facial skeleton is derived from neural crest cells populating the pharyngeal arches - neural crest migration as well as development of the brain are know to be extremely sensitive to alcohol (kills neural crest cells)

No known safe level

Incidence of fatal alcohol syndrome and alcohol related neuro-developmental delay = 1/100 births

Question 18

What facial features develop with fetal alcohol syndrome?

Answer 18

Small head (microcephaly)

Low nasal bridge

Small eye openings

Flat midface

Epicanthal folds (skin of upper eyelid covers inner corners of eye)

Short nose

Smooth philtrum

Thin upper lip

Underdeveloped jaw