Ach -- Face/Ear Development Flashcards
Five facial prominences form as neural crest cells proliferate and migrate into the pharyngeal arches.
- Frontonasal (1) → Midline structures of the face and head
- Maxillary (2) → Maxillary region of the face and head
- Mandibular (2) → Mandibular region of the face and head
Placodes
- 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
Describe the formation of the palate…
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.
Describe the formation of the nasal cavities
- Nasal placodes invaginate forming nasal pits
- Oronasal membrane separates the nasal pits from the primitive oral cavity (newborns are obligate nose breathers)
- Nasal septum formed by a medial downgrowth from the medial nasal prominences
- Nasal conchae (superior, middle and inferior) develop as elevations in the lateral wall of each nasal cavity.
- Olfactory epithelium develops from surface ectoderm in the superior aspect of each nasal cavity (CN I)
Cleft lip
**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
Cleft palate
- 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.
Fetal Alcohol Spectrum Disorder (FASD)
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
Describe the time of appearance and derivatives of the otic placode
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)
membranous labyrinth
- 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
Osseous or Bony labyrinth
- 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
development of the tympanic cavity
Tympanic cavity and auditory (pharyngotympanic or Eustachian tube) are of endodermal origin from first pharyngeal pouch (Tubotympanic recess)
middle ear ossicle developement
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
Tensor tympani muscle
- Mandibular branch of trigeminal nerve (CN V3)(SVE)
- First arch derivative
- Attaches to the manubrium of the malleus
Stapedius muscle
- Facial nerve (CN VII) (SVE)
- Second arch derivative
- Attaches to the neck of the stapes
Neurosensory deafness
- 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
congenital conduction deafness
- Persistent meatal plug; seen in first arch syndrome
- Fixation of footplate of stapes
- Malleus and incus often involved in first arch syndrome
Treacher Collins Syndrome
Deficiency of neural crest cells migrating into the arch.
Abnormal development of structures derived from the first arch
Pierre Robin Syndrome
Deficiency of neural crest cells migrating into the first arch.
Micrognathia + large, posteriorly placed tongue > airway obstruction (glossoptosis)
Second arch cartilage is AKA…
Reichert’s cartilage
stapes, styloid process, stylohyoid ligament, hyoid bone (part of it)
Third arch cartilage…
- gives rise to Glossopharyngeal nerve CN (IX) (SVE)
- innervates one muslce: Stylopharyngeus
Fourth and Sixth arch derivatives…
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
The second pharyngeal pouch gives rise to the …
Palatine tonsils (epithelial component)
Third and fourth pouch derivatives…
Third Pouch
- Thymus (ventral)
- Inferior parathyroid glands (dorsal)
Fourth Pouch
- Superior parathyroid glands (dorsal)
- Ultimobranchial bodies (ventral) AKA parafollicular cells of the thyroid
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
DiGeorge Syndrome: Genetic and/or environmental (microdeletion of Chr. 22q11.2)
No thymus or parathyroids (diminished immunity, hypocalcemia)
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?
External auditory meatus (epithelial lining)
Tympanic membrane!!!
If the 2rd, 3th, and 4th clefts fail to dissapear, then …
archs leave a transitory CERVICAL SINUS lined with ectodermally derived epithelium.
Forms lateral cervical (branchial) cyst or fistula when it persists
Endodermal evagination from floor of 4th pharyngeal pouches developes into?
Thyroid Gland Development
first endocrine gland to appear: 24 days (midway in the 4th week)
Most of the tongue muscles are derived from ______ that migrate from the _________ with the ______ nerve.
myoblasts
occipital myotomes
hypoglossal
