The ectoderm forms the epidermis, the epidermal appendages, and the nervous system. This has clinical relevance in the healing process in partial-thickness wounds as the ectodermally derived skin appendages such as hair follicles, eccrine and apocrine sweat glands, and sebaceous glands are lined with epithelium and provide a source of keratinocytes for re-epithelialization. This knowledge also helps to understand how certain conditions present. For example, the fact that skin and neural tissue share embryologic origin explains why patients with neurofibromatosis develop both skin and nerve tissue tumors. The mesodermal derivatives are also clinically relevant in plastic surgery as this embryologic layer forms bone, cartilage, muscle, connective tissue, and blood vessels. The endoderm has less applicability in plastic surgery as it forms the lining of the gastrointestinal and respiratory tracts and the digestive organ parenchyma. The endoderm does not however form the genitourinary tract. The heart and great vessels are derived from mesoderm, but not ectoderm.1–3
2. Carlson BM, ed. Human Embryology and Developmental Biology. 5th ed. Philadelphia: Elsevier; 2013
3. Hopper RA. Cleft lip and palate: embryology, principles, and treatment. In: Thorne CH, Chung KC, Gosain A, Guntner GC, Mehrara BJ, eds. Grabb and Smith’s Plastic Surgery. 7th ed. Philadelphia: Lippincott Williams & Wilkins; 2014 PHARYNGEAL (BRANCHIAL) ARCHES 2. Which one of the following statements is correct regarding the pharyngeal arches in embryological development? There are six pharyngeal arches and each comprises a mixture of endoderm, mesoderm, and ectoderm such that all layers of the head and neck can be formed (Table 24.1). Knowledge of the embryologic origins of the head and neck has clinical relevance to plastic surgery. For example, each of the digastric muscles has an anterior and a posterior belly, which have different embryologic origins. The anterior belly develops from the first arch along with the muscles of mastication and tensor veli palatini. The main nerve derived from this arch is the trigeminal (CN V) and this supplies these structures. The posterior belly develops from the second arch, which also forms the muscles of facial expression and the stapes. The main nerve derived from this arch is the facial nerve (CN VII) and this supplies these structures (Fig. 24.1). When the facial nerve is damaged, the muscles of facial expression become paralyzed but the trigeminally supplied muscles of mastication and anterior belly of digastric remain functional. Therefore, the digastric Table 24.1 Pharyngeal Arch Derivatives Arch Nerve Artery Bone Muscle I CN V Maxillary Greater wing of sphenoid, incus, malleus, maxilla, zygomatic, temporal (squamous), mandible Muscles of mastication, anterior digastric, mylohyoid, tensor tympani, tensor veli palatini II CN VII Stapedial (corticotympanic) Stapes, styloid process, stylohyoid ligament, lesser horn, and upper body of hyoid Muscles of facial expression, posterior digastric, stylohyoid stapedius III CN IX Common carotid, proximal internal carotid Greater horns and lower body of hyoid Stylopharyngeus IV/VI CN X Aortic arch, right subclavian, origin of pulmonary arteries, ductus arteriosus Laryngeal cartilages Pharyngeal constrictors, levator veli palatini, palatoglossus, striated upper esophageal muscles, laryngeal muscles muscle can be transferred to the lower lip to compensate for marginal nerve branch weakness and the temporalis muscle can be transferred to compensate for buccal and zygomatic nerve branch weakness. Of further interest is that facial nerve paralysis also results in a loss of sound dampening and again this can be explained by embryologic development because the nerve to the stapedius, the stapedius itself, and the stapes are all derived from the second arch. The other two ossicles (incus and malleus) are derived from the first arch. A common exam question relates to the innervation of the palatal musculature. All but one are innervated by the pharyngeal plexus as they are derived from arches four to six. The tensor veli palatini opens the Eustachian tube and is derived from the first arch and so is trigeminal innervation.1–5
4. Moore KL, Persaud TV, Torchia MD, eds. The Developing Human: Clinically Oriented Embryology. 10th ed. Philadelphia: Elsevier; 2016
5. Sadler TW, ed. Langman’s Medical Embryology. 13th ed. Philadelphia: Wolters Kluwer Health; 2014 PHARYNGEAL (BRANCHIAL) GROOVES 3. Which one of the following statements is correct regarding the pharyngeal grooves in embryological development? branchial sinus tracts. There are five pharyngeal grooves (also known as clefts), and the cervical sinus is formed from grooves II through IV. Failure of the sinus to obliterate leads to the formation of a branchial sinus tract, cyst, or fistula. These are often detected in the second (not fourth) decade of life as neck swellings and those formed from groove II (not IV) are most common. Groove I forms the external auditory meatus and tympanic membrane, not the internal auditory meatus, which is formed by the first pharyngeal pouch. Grooves II and III differ in their relationship to the internal and external carotids (not the internal jugular vein). A groove II anomaly runs between the external and internal carotid arteries toward the tonsillar fossa, and a groove III anomaly passes under the internal carotid. Knowledge of this anatomy can be useful intraoperatively when excising branchial sinuses.1–5
The face develops from the paired maxillary and mandibular prominences along with the frontonasal prominence, which comprises the medial and lateral nasal prominences. The medial nasal prominence forms the primary palate, the midmaxilla, midlip, philtrum, central nose, and nasal septum. The lateral nasal prominence forms the nasal alae. The maxillary prominences form the secondary palate, lateral maxilla, and lateral lip. The mandibular prominences form the mandible, lower lip, and face. Knowledge of the embryologic development and fusion of these prominences provides the basis for understanding the concepts of cleft lip, cleft palate, and Tessier clefts. Unilateral cleft lips result from failure of fusion of a medial nasal prominence with the corresponding maxillary prominence (Fig. 24.2).1,2 Fig. 24.2 Embryonic development of the human face. A, Week 5. B, Week 6. C, Week 7. A B C Frontonasal prominence Medial nasal prominence Lateral nasal prominence Maxillary prominence Medial nasal prominence Lateral nasal prominence Maxillary prominence Mandibular prominence Mandibular prominence First branchial groove First branchial groove
7. Which cranial nerve is responsible for taste in the anterior part of the tongue? The tongue has multiple embryologic origins and therefore has multiple different nerve supplies. These elements are often tested in written and clinical examinations. The anterior two-thirds of the tongue originates from the first pharyngeal arch and sensation is supplied by the lingual nerve from the trigeminal nerve (CN V3). However, the innervation for taste comes from the chorda tympani branch of the facial nerve (CN VII). The posterior third of the tongue originates from the third and fourth pharyngeal arches and taste and sensation are supplied by the glossopharyngeal nerve (CN IX) and the vagus nerve (CN X). Motor innervation for the tongue is largely from the hypoglossal nerve (CN VII) with some innervation from the vagus (CN X).1,2