53. The Branchial Apparatus: The Floor of The Pharynx -- Tongue and Associated Structures

  1. The tongue appears in the floor of the pharynx, in embryos of about 4 weeks of age, in the form of 2 oval lateral lingual swellings (distal tongue buds) and 1 median triangular swelling, the tuberculum impar. They are the result of proliferation of mesenchyme in the ventromedial part of the first pair of branchial (mandibular) arches
    1. THE LATERAL LINGUAL SWELLINGS increase in size, merge together, and overgrow the tuberculum impar and form the anterior two-thirds or body of the tongue
      1. Their fusion is superficially marked by the median sulcus of the tongue and internally by the fibrous median septum. The tuberculum impar forms no significant portion of the adult tongue
    2. THE POSTERIOR THIRD OR ROOT of the tongue first is seen as two elevations caudal to the foramen cecum
      1. A median, copula (connector), is formed by fusion of the ventromedial parts of the second branchial arches, and a large hypobranchial eminence, caudal to the copula is formed from mesoderm of arches III and IV
        1. The copula is overgrown by the hypobranchial eminence and disappears. Thus, the posterior third of the tongue comes from the cranial portion of the hypobranchial eminence
    3. THE LINE OF FUSION of the anterior two-thirds and the posterior third of the tongue is marked by the V-shaped terminal sulcus
    4. BRANCHIAL ARCH MESODERM forms the connective tissue, lymphatics, and blood vessels of the tongue and some of its muscle fibers. However, most tongue muscle is derived from myoblasts that migrate from the myotomes of the occipital somites
    5. PAPILLAE OF THE TONGUE appear about day 54 with the vallate and foliate seen first in relation to the terminal branches of cranial nerve IX and the fungiform seen later, induced by the chorda tympani nerve of cranial VII
      1. All develop taste buds
      2. Reflex pathways between taste buds and facial muscles are present by weeks 26 to 28
    6. NERVE SUPPLY OF TONGUE is explained by the branchial arch development. The sensory supply to the mucosa of the entire anterior two-thirds of the tongue is via the lingual branch of the mandibular division of cranial V
      1. The chorda tympani branch of VII supplies the taste buds in the anterior two-thirds of the tongue except for the vallate papilla Cranial VII does not supply any tongue mucosa since arch III overgrows arch II
      2. The vallate papillae in the anterior two-thirds of the tongue and the posterior third of the tongue are innervated by cranial IX, with the superior laryngeal of X (arch IV) supplying a small area of the tongue anterior to the epiglottis
      3. Nerve XII supplies all the extrinsic and intrinsic muscles of the tongue since it follows the occipital myotomes
  2. Salivary glands develop as solid proliferations of cells from the epithelium of the primitive mouth during weeks 6 and 7
    1. THE PAROTID GLAND: from buds of ectodermal lining of the stomodeum. Duct is Stensen's
    2. THE SUBMANDIBULAR GLANDS: from entoderm in floor of mouth. Duct is Wharton's
    3. THE SUBLINGUAL GLANDS: from multiple buds of entoderm in the paralingual sulcus
  3. Congenital malformations of the tongue
    1. ANKYLOGLOSSIA (tongue-tie) occurs in 1/300 North American infants. There is a shortening of the lingual frenulum so tip of tongue is tied to floor of mouth
    2. MACROGLOSSIA: an excessively large tongue resulting from generalized hypertrophy
    3. MICROGLOSSIA: an abnormally small tongue associated with micrognathia
    4. CLEFT TONGUE: incomplete fusion of the lateral lingual swellings posteriorly
    5. BIFID TONGUE: complete failure of fusion of lateral lingual swellings

the branchial apparatus:  the floor of the pharynx -- tongue and associated structures: image #1