Head and neck formation

Head and neck

Tongue: the tongue appears at the 4th wk. in the form of 2 lateral swellings called the lateral lingual swellings and 3 median swellings called tuberculum impar, copula and the epiglottal swelling respectively. The 2 lateral swellings and the tub. Impar originate from the mesoderm of the first pharyngeal arch. The copula originates from 2nd, 3rd and part of the 4th arch. The epiglottal swelling formed by the posterior part of the 4th arch
With further development the 2 lat. Lingual swellings will proliferate and overgrow the tissue at the tub. Impar and unite with each other forming the ant. 2/3 of the tongue. Since the mucosa covering this part of the tongue originate from the 1st arch. It is innervated by the mandibular branch of trigeminal nerve.
The post.1/3 of the tongue originates from the copula which originates from the 2nd, 3rd and part of 4th arches. Since the tissue of the 3rd arch has overgrown those of the others arches, the mucosa is innervated by the glossopharyngeal nerve. The ant. 2/3 of the tongue is separated from the pos. 1/3 of the tongue by a v shaped grove called the terminal sulcus.
The most pos. part of the tongue and epiglottis are derived from the epiglottal swelling (4th arch), so they are innervated by the sup. Laryngeal nerve.
The mucosa of the tongue are derived from the myoblast originating in the occipital somites, they are innervated by hypoglossal nerve
Tongue abnormalities :
Ankyloglossia (tongue tie): the tongue is not freed from the floor of the mouth, normally extensive cell degeneration occurs and the frenulum is the only tissue persisting, tying the tongue to the floor of the mouth. In tongue tie, the frenulum extends to the tip of the tongue.
Macroglossia: large tongue, result from hypertrophy of the tongue.
Microglossia: small tongue,
Cleft tongue
bifid tongue

Thyroid gland:

It is formed by the proliferation of the epithelium Tissue of the pharynx between tub. Impar and copula at point indicated later by the foramen cecum. From this place it will migrate in front of the pharynx, hyoid bone and laryngeal cartilages, to its final position, in front of trachea in the 7th week. During its migration the gland remains connected to the tongue by a narrow canal called thyroglossal duct which become solid and disappears later.

Thyroglossal cyst and fistula:

The thyroglossal cyst it the remnant of any part of the duct which can be found anywhere along the migratory pathway. It appears close to or in the midline of the neck. The cyst sometimes is connected to the outside by a canal, the hypoglossal fistula.
Face:
At the center of the face there is a depression in the ectoderm called stomodeum (primitive mouth). The buccopharyngeal membrane lies at the floor of the stomodeum which breakdown at the 4th week. Establishing the communication between stomodeum and gut. 5 facial swelling appear during the 4th week. Around stomodeum, they are:
Frontal swelling: forms the upper border of the stomodeum
Two maxillary swellings: forms the lateral border of the stomodeum
Two mandibular swelling: From the lower border of the stomodeum.
A local thickening of the surface ectoderm on the sides of the frontal swelling appears just above the stomodeum called the nasal placode. During 5th week 2 ridges appear surrounding the nasal placode, the med. And lat. Nasal swellings. With further development, the maxillary swelling grow in med. Direction compressing the 2 med. Nasal swellings toward the midline, subsequently the clefts between the swellings is lost and swellings fuse. Hence the upper lip is formed by the 2 nasal (med.) swellings and the 2 maxillary swellings. The 2 lateral nasal swellings will form alae of the nose. The lower lip is formed from the 2 mandibular swellings. The forehead and nasal bridge are formed from frontal swellings. The crest of the nose and its tip are formed from the 2 mid. Nasal swellings.
The maxillary swellings initially are separated from the lat. Nasal swellings by the nasolacrimal grove, the ectoderm in the floor of the grove will form a solid cord that detaches from the overlying ectoderm after canalization the cords from nasolacrimal duct. The upper end widens to form the lacrimal sac. Following detachment of the cord, the maxillary and lateral nasal swellings merge with each other. The duct then runs from the med. Corner of the eye to the inferior meatus of the nasal cavity, and the maxillary swellings enlarge to form the cheek and maxillae.
Palate:
The union between the 2 med. Nasal swellings extend to a deeper level and the resulting structure is the intermaxillary segment, it consists of 3 parts:
Labial component: it forms the philtrum of the upper lip.
Upper jaw component: it gives rise to median part of the maxillary bone and incisive teeth
Palatal component: it gives rise to primary palate.
The secondary palate is formed by the union of the shell like outgrowths from the maxillary swellings.
The definitive palate is formed by the fusion of the secondary and primary palates, and the incisive foramen considered the midline landmark between the primary and secondary palates. At the sametime as the palatine shelves fuse the nasal septum grows down and joins with the upper aspect of the secondary palate.
Abnormalities :
Facial clefts: occur due to abnormal fusion of the facial swellings, include:
Anterior to the incisive foramen: like cleft lip, cleft upper jaw or cleft between the primary and secondary palates
Posterior to incisive foramen: occur due to failure of palatine shelves to fuse leading to cleft palate (secondary) and for cleft uvula.
Combined cleft
Median cleft lip: occur due to incomplete fusion between the 2 med. Nasal swelling, usually associated with a clap grove between the right and left sides of the nose.
Oblique facial cleft: occur due to failure of fusion between maxillary swelling and lateral nasal swelling. In this condition the nasolacrimal duct is usually exposed to the outside.