Chapter 60: Highlights and Pearls

Embryology

1. Branchial arch derivatives: nerve, muscles, cartilage, artery

   1. First arch (mandibular)

      1. Nerve: trigeminal (V)

      2. Muscle: muscles of mastication, tensor veli palatini, mylohyoid, anterior digastrics, tensor tympani

      3. Skeletal structure: sphenomandibular ligament, anterior malleal ligament, mandible, malleus, incus

      4. Artery: maxillary

      5. Pouch and derivative: eustachian tube, middle ear

   2. Second arch (hyoid)

      1. Nerve: facial (VII)

      2. Muscle: stapedius, posterior digastric, stylohyoid, muscles of facial expression

      3. Skeletal structure: stapes superstructure, styloid process, stylohyoid ligament, lesser cornu/upper portion of hyoid

      4. Artery: stapedial

      5. Pouch and derivative: palatine tonsil

   3. Third arch

      1. Nerve: glossopharyngeal (IX)

      2. Muscle: stylopharyngeus

      3. Skeletal structure: greater cornu of the hyoid, lower body of hyoid

      4. Artery: common and internal carotid

      5. Pouch and derivative: thymus and inferior parathyroid

   4. Fourth arch

      1. Nerve: superior laryngeal (X)

      2. Muscle: constrictors of the pharynx, cricothyroid

      3. Skeletal structure: laryngeal cartilages

      4. Artery: subclavian on right, arch of aorta on left

      5. Pouch and derivative: superior parathyroid, parafollicular cells of thyroid

   5. Sixth arch

      1. Nerve: recurrent laryngeal (X)

      2. Muscle: intrinsic laryngeal muscles

      3. Skeletal structure: laryngeal cartilages

      4. Artery: pulmonary artery on right, ductus arteriosus on left

   6. Branchial cleft cysts/sinuses/fistulas form within its branchial arch derivates, and form superficial to the next branchial arch structures.

2. Hillocks of His

   1. Fifth week of gestation.

   2. External ear develops from six small buds of mesenchyme.

   3. Hillocks of His fuse during the 12th week.

   4. 1-3 (from first arch) develop into tragus, helical crus, helix.

   5. 4-6 (from second arch) develop into antihelix, antitragus, and lobule.

3. Tongue

   1. Anterior two-thirds of tongue form from:

      1. Tuberculum impar in midline

      2. Two lateral lingual prominences

   2. Posterior one-third of tongue forms from copula.

4. Thyroid

   1. At 3 to 4 week’s gestation, starts as epithelial proliferation at the base of tongue (foramen cecum) between tuberculum impar and cupola.

   2. Connected to foregut by thyroglossal duct as it descends into neck.

   3. Thyroglossal duct obliterates.

   4. Ultimobranchial body contributes parafollicular C cells of thyroid.

5. Nasal dermal sinus

   1. Foramen cecum: anterior neuropore at anterior floor of cranial vault.

   2. Prenasal space: potential space beneath nasal bone running from anterior aspect of nasal bone to frontal bone/foramen cecum area.

   3. Fonticulus frontalis: embryologic gap between nasal and frontal bones.

   4. Dura and nasal skin lie in close proximity and become separated with foramen cecum closure.

      1. Persistent dural-dermal connection via foramen cecum and prenasal space (or less frequently via fonticulus frontalis) produces gliomas, meningoceles, or encephaloceles projecting from above. Projecting from below may be dermal sinuses or dermoids.

Anatomy

1. Skin

   1. Epidermis: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, stratum basale

   2. Dermis

   3. Subcutis

2. Contents of skull base foramina (Figures 60-1, 60-2, 60-3, 60-4, 60-5, 60-6, 60-7, 60-8)

   1. Cribriform plate—olfactory nerves

   2. Optic canal

      1. Optic nerve

      2. Ophthalmic artery

      3. Central retinal artery

   3. Superior orbital fissure

      1. Cranial nerve (CN) III, IV, V1 (frontal nerve, lacrimal nerve, nasociliary nerve), VI

      2. Superior ophthalmic vein

      3. Inferior ophthalmic vein branch

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