Ach -- Face/Ear Development

Question 1

Five facial prominences form as neural crest cells proliferate and migrate into the pharyngeal arches.

Answer 1

1. Frontonasal (1) → Midline structures of the face and head
2. Maxillary (2) → Maxillary region of the face and head
3. Mandibular (2) → Mandibular region of the face and head

Question 2

Placodes

Answer 2

• ectodermal thickenings with neurogenic potential that appear in pairs on the developing head.
• They contribute important components to the special senses.
• Their location on the embryonic head changes significantly during development.

a. Lens placodes → lens of the eye
b. Otic placodes → inner ear
c. Nasal placodes → nasal cavity and part of upper lip

Question 3

Describe the formation of the palate…

Answer 3

The palate forms between weeks 5 and 12. The most critical period of development is from the end of week 6 to the beginning of week 9.

Primary palate

• Develops from the palatal component of the intermaxillary segment

Secondary palate

• Palatine shelves grow toward the midline from the medial aspects of the maxillary prominences.

Primary and secondary palates fuse; incisive foramen marks this fusion in midline.

Question 4

Describe the formation of the nasal cavities

Answer 4

1. Nasal placodes invaginate forming nasal pits
2. Oronasal membrane separates the nasal pits from the primitive oral cavity (newborns are obligate nose breathers)
3. Nasal septum formed by a medial downgrowth from the medial nasal prominences
4. Nasal conchae (superior, middle and inferior) develop as elevations in the lateral wall of each nasal cavity.
5. Olfactory epithelium develops from surface ectoderm in the superior aspect of each nasal cavity (CN I)

Question 5

Cleft lip

Answer 5

**Anterior Defect: **Result from failure of fusion of maxillary prominences with the fused medial nasal prominence.

_Unilateral: _ranges in severity from cleft in vermillion border to a cleft that is continuous with cleft palate

For example: Unilateral cleft lip and nose

Bilateral
Intermaxillary segment projects anteriorly
Less common than unilateral

Question 6

Cleft palate

Answer 6

• Involve hard and soft palate
• Result from failure of fusion of the palatine shelves (lateral palatine processes) with one another, with the nasal septum (from the intermaxillary segment) and/or with the posterior margin of the primary palate.

Question 7

Fetal Alcohol Spectrum Disorder (FASD)

Answer 7

Low nasal bridge
Wide set eyes
Short palpebral fissures
Underdeveloped mandible (micrognathia)
Thin upper lip, indistinct philtrum
Ear abnormalities, hearing impairment
Short up-turned nose

Question 8

Describe the time of appearance and derivatives of the otic placode

Answer 8

Otic Placodes ==> Otic Pits ==> Otic Vesicles ==> Membranous Labyrinth (i.e. the epithelial structures of the internal ear)

Otic placodes appear approx d 22 as thickenings of the surface ectoderm on each side of the rhombencephalon (hindbrain)

Question 9

membranous labyrinth

Answer 9

• formed as the otic vesicle and differentiates into dorsal and ventral parts
• Dorsal, utricular part of otic vesicle gives rise to:
  a. semicircular ducts
  b. utricle
  c. endolymphatic duct
• Ventral, saccular part of otic vesicle gives rise to:
  a. saccule
  b. cochlear duct
  c. ductus reuniens

Question 10

Osseous or Bony labyrinth

Answer 10

• forms from the mesenchyme (mesoderm) surrounding the membranous labyrinth within petrous temporal bone.
• Bony labyrinth ossifies (endochondral) within the petrous temporal bone to form semicircular canals, cochlea and vestibule

Question 11

development of the tympanic cavity

Answer 11

Tympanic cavity and auditory (pharyngotympanic or Eustachian tube) are of endodermal origin from first pharyngeal pouch (Tubotympanic recess)

Question 12

middle ear ossicle developement

Answer 12

formation of the ear bones…

Malleus and incus arrise from dorsal tip of Meckle’s cartilage/first arch

** Stapes** arrises from Reichert’s cartilage/second arch

Question 13

Tensor tympani muscle

Answer 13

• Mandibular branch of trigeminal nerve (CN V3)(SVE)
• First arch derivative
• Attaches to the manubrium of the malleus

Question 14

Stapedius muscle

Answer 14

• Facial nerve (CN VII) (SVE)
• Second arch derivative
• Attaches to the neck of the stapes

Question 15

Neurosensory deafness

Answer 15

• Abnormal development of cochlea, cranial nerve VIII or brain stem
• Rubella in first trimester can cause defects in spiral of the organ of Corti in 30% of cases

Question 16

congenital conduction deafness

Answer 16

• Persistent meatal plug; seen in first arch syndrome
• Fixation of footplate of stapes
• Malleus and incus often involved in first arch syndrome

Question 17

Treacher Collins Syndrome

Answer 17

Deficiency of neural crest cells migrating into the arch.

Abnormal development of structures derived from the first arch

Question 18

Pierre Robin Syndrome

Answer 18

Deficiency of neural crest cells migrating into the first arch.

Micrognathia + large, posteriorly placed tongue > airway obstruction (glossoptosis)

Question 19

Second arch cartilage is AKA…

Answer 19

Reichert’s cartilage

stapes, styloid process, stylohyoid ligament, hyoid bone (part of it)

Question 20

Third arch cartilage…

Answer 20

• gives rise to Glossopharyngeal nerve CN (IX) (SVE)
• innervates one muslce: Stylopharyngeus

Question 21

Fourth and Sixth arch derivatives…

Answer 21

Fourth arch: Superior laryngeal branch of Vagus (CN X) (SVE) innervates:

• Muscles of the soft palate (except tensor veli palatini)
• Muscles of the pharynx (except stylopharyngeus)
• Cricothyroid
• Cricopharyngeus

Sixth arch: Recurrent laryngeal branch of Vagus (CN X) (SVE) innervates:

• Intrinsic muscles of larynx (except cricothyroid)
• Striated (upper) muscle of esophagus

Question 22

The second pharyngeal pouch gives rise to the …

Answer 22

Palatine tonsils (epithelial component)

Question 23

Third and fourth pouch derivatives…

Answer 23

Third Pouch

• Thymus (ventral)
• Inferior parathyroid glands (dorsal)

Fourth Pouch

• Superior parathyroid glands (dorsal)
• Ultimobranchial bodies (ventral) AKA parafollicular cells of the thyroid

Question 24

Disorder causing failure of neural crest migration into arches; 3rd & 4th pouch failure & hypoplasia of 1st arch (cleft palate, low set ears, poor feeding, delayed speech)

• Also, heart defects, poor circulation, poor muscle tone

Answer 24

DiGeorge Syndrome: Genetic and/or environmental (microdeletion of Chr. 22q11.2)

No thymus or parathyroids (diminished immunity, hypocalcemia)

Question 25

Normally the remaining 2nd, 3rd & 4th clefts disappear as the 2nd arch overgrows them to give a smooth surface to the lateral aspect of the neck. What is the fate of the 1st cleft?

Answer 25

External auditory meatus (epithelial lining)

Tympanic membrane!!!

Question 26

If the 2rd, 3th, and 4th clefts fail to dissapear, then …

Answer 26

archs leave a transitory CERVICAL SINUS lined with ectodermally derived epithelium.

Forms lateral cervical (branchial) cyst or fistula when it persists

Question 27

Endodermal evagination from floor of 4th pharyngeal pouches developes into?

Answer 27

Thyroid Gland Development

first endocrine gland to appear: 24 days (midway in the 4th week)

Question 28

Most of the tongue muscles are derived from ______ that migrate from the _________ with the ______ nerve.

Answer 28

myoblasts

occipital myotomes

hypoglossal