Craniofacial Development

Key questions

1. Q1When does tongue development begin, and which swellings appear first?

   Weeks 4–5. Arch 1 forms two lateral lingual swellings and a midline tuberculum impar.
2. Q2What swelling forms next, when, and from which arches?

   The copula (anterior hypobranchial eminence), weeks 5–10, from mesoderm under arches 2, 3 and 4.
3. Q3Which swellings build the anterior 2/3 of the tongue?

   The lateral lingual swellings, which overgrow the tuberculum impar and merge.
4. Q4What is the origin of the tongue muscle vs its epithelial lining?

   Muscle: myoblasts from the occipital somites (mesoderm). Lining: endoderm. The tongue is mesoderm surrounded by endoderm.

Key questions

1. Q5What shared developmental theme do the tongue and lung use?

   Underlying mesoderm/neural crest signals the overlying endoderm to proliferate, bud and shape the organ.
2. Q6Which molecules carry that signal?

   Endodermal BMP-7 blocks growth; mesodermal Noggin inhibits BMP-7, releasing Patched so the endoderm proliferates. This molecular detail is lower yield.
3. Q7How does thyroid budding differ from lung budding?

   The thyroid buds only twice (a bilobed diverticulum), while the lung keeps bifurcating.

Key questions

1. Q8Which four cranial nerves innervate the tongue, and which region does each serve?

   Anterior 2/3: V3 (general sensation). Taste on the anterior 2/3: chorda tympani of VII. Posterior 1/3: IX. Extreme posterior: superior laryngeal branch of X.
2. Q9Why do tongue regions follow those nerves?

   Each region keeps the nerve of the arch it came from: anterior from arch 1 (V3), posterior from arch 3 (IX), extreme back from arch 4 (X).
3. Q10Which nerves carry the two limbs of the gag reflex?

   Afferent IX, efferent X.
4. Q11A patient loses taste and sensation on the posterior 1/3 and has no gag reflex. Which nerve?

   CN IX (glossopharyngeal), the arch 3 nerve.

Key questions

1. Q12Where does the thyroid start to develop?

   At the foramen cecum, between the tuberculum impar and the hypobranchial eminence.
2. Q13What connects the thyroid to the tongue during descent, and what normally happens to it?

   The thyroglossal duct. It normally disappears.
3. Q14Where is the thyroid at week 6, and where is it at week 7?

   Week 6: in front of the hyoid and laryngeal cartilages. Week 7: in its final position anterior to the trachea.
4. Q15Which genes drive thyroid development?

   TTF-1 (thyroid transcription factor), PAX, BMPs and noggin. Low yield.

Key questions

1. Q16When does the fetal thyroid begin functioning, and what does it make?

   Weeks 14–15. Follicular cells make colloid, the source of T4 and T3, which regulate cellular metabolism.
2. Q17What do C cells secrete in the fetus, and what does it do?

   Calcitonin. It increases calcium uptake into bone and prevents resorption, which is needed for normal bone development.
3. Q18How do fetal calcitonin levels compare with maternal levels?

   Fetal calcitonin is 2–3× higher.

Key questions

1. Q19Which five prominences surround the stomodeum at weeks 4–5?

   One frontonasal, two maxillary (lateral to the stomodeum) and two mandibular (caudal to the maxillary).
2. Q20How do the nasal placodes turn into the medial and lateral nasal prominences?

   Weeks 5–8: the placodes invaginate into nasal pits, and the tissue ridges around each pit become the medial and lateral nasal prominences.
3. Q21Which prominences build the upper lip?

   Two medial nasal prominences (philtrum) + two maxillary prominences. The lateral nasal prominences do not form the lip.
4. Q22Which prominences form the bridge, the tip and the sides of the nose?

   Bridge: frontonasal. Crest and tip: merged medial nasal. Sides: lateral nasal.
5. Q23When are the cheeks identifiable, and which prominence forms them?

   Weeks 8–9, from the maxillary prominences.
6. Q24What tissue forms the facial bones, and what are the exceptions?

   Neural crest forms virtually all of them. Exceptions: thyroid and cricoid cartilages (and possibly the malleus and incus).

Key questions

1. Q25What forms the primary palate, and what are the three parts of the intermaxillary segment?

   The intermaxillary segment. Labial part → philtrum; upper jaw part → 4 incisors; palatal part → triangular primary palate.
2. Q26What forms the secondary palate, when, and where do its parts grow?

   The palatine shelves, outgrowths of the maxillary prominences, appear at weeks 6.5–7. They grow toward each other dorsal to the tongue and fuse; the uvula hangs from the caudal end.
3. Q27How does the nasal cavity come to open into the oral cavity?

   Week 7: the oronasal membrane separates them, then ruptures mid-week 7 to form the primitive choana, behind the primary palate.
4. Q28How do the paranasal sinuses form, and what do they do?

   As diverticula of the lateral nasal wall into the maxilla, ethmoid, frontal and sphenoid bones. They condition inspired air and grow through puberty, shaping the face.
5. Q29What must move out of the way for the palatine shelves to fuse?

   The tongue. The shelves grow toward each other dorsal to (above) it.

Key questions

1. Q30What is the difference between the neurocranium and the viscerocranium?

   Neurocranium surrounds the brain (cranial bones). Viscerocranium is the face and jaws. Each has a cartilaginous and a membranous part.
2. Q31How does each one mostly ossify?

   Neurocranium: mostly endochondral, from paraxial and prechordal mesoderm plus some neural crest. Viscerocranium: mostly membranous, from neural crest plus some mesoderm.
3. Q32What is the difference between endochondral and membranous ossification?

   Endochondral: bone replaces a pre-existing cartilage model. Membranous: mesenchyme or neural crest turns directly into bone, with no cartilage.
4. Q33How many ossification centers and bones are involved?

   More than 100 ossification centers form 45 bones, which fuse into the 22 bones of the mature skull.

Key questions

1. Q34A lateral cleft lip results from which failed fusion, and how common is it?

   Maxillary + medial nasal prominences (anterior cleft). About 1/1000 births, 80% male.
2. Q35What fusion defect causes a cleft secondary palate and cleft uvula?

   The palatine shelves fail to fuse.
3. Q36What causes an oblique facial cleft, and why does it come with a cleft lip?

   The maxillary prominence fails to merge with the lateral nasal prominence. The cleft lip is secondary to the cleft palate, even though the lateral nasal prominence doesn't form the lip.
4. Q37Which drugs taken in pregnancy raise the risk of facial clefts?

   Anticonvulsants (phenobarbital, lamotrigine, valproic acid) and lithium.
5. Q38Where along its path can a thyroglossal duct cyst be found?

   Anywhere on the thyroid's descent: below the hyoid, at the base of the tongue, or near the thyroid cartilage. If open to the skin, it is a thyroglossal fistula.
6. Q39What is the treatment for cleft lip and cleft palate?

   Surgery.

Key questions

1. Q40Which structures should you identify on a normal fetal face ultrasound?

   Neurocranium, viscerocranium, frontal prominence, nose and the fused maxillary process.
2. Q41Which two features do you check for abnormal development?

   Ear position relative to the eye and nose, and presence of the nasal bone.
3. Q42Which fetal face finding points to trisomy 21?

   An absent nasal bone.