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I. Pharyngeal apparatus

A. Fates of pharyngeal clefts

The pharyngeal clefts are ectodermal-lined recesses that appear on the OUTSIDE of the pharnyx between the arches; cleft 1 is between arch 1 and 2, cleft 2 is between arches 2 and 3, etc.

1. pharyngeal cleft 1: develops into the external auditory meatus (the corresponding 1st pharyngeal pouch develops into the auditory (or Eustacian) tube, and the intervening membrane develops into the tympanic membrane).

Defects in the development of pharyngeal cleft 1 can result in preauricular (i.e. in front of the pinna of the ear) cysts and/or fistulas.

2. pharyngeal clefts 2, 3, and 4 are overgrown by expansion of the 2nd pharyngeal arch and usually obliterated

Remnants of pharyngeal clefts 2-4 can appear in the form of cervical cysts or fistulas found along the anterior border of the sternocleidomastoid muscle.

B. Fates of pharyngeal arches

1. Pharyngeal Arch 1 (mandibular arch)

2. Pharyngeal Arch 2 (hyoid arch)

3. Pharyngeal Arch 3

4. Pharyngeal Arch 4

5. Pharyngeal Arch 6

The fates of the pharyngeal arches and their derivative structures can be summarized by the two figures below:

C. Fates of pharyngeal pouches

The pharyngeal pouches are endodermal-lined pockets that form on the INSIDE of the pharynx between the arches; pouch 1 forms between arch 1 and arch 2, pouch 2 forms between arch 2 and arch 3, etc.

1. Pharyngeal Pouch 1 develops into the auditory tube and middle ear cavity

2. Pharyngeal Pouch 2 forms numerous infoldings that become the crypts of the palatine tonsil; later, lymphocytes (from the thymus and bone marrow) infiltrate the underlying lamina propria to establish the definitive palatine tonsil.

3. Pharyngeal Pouch 3 divides into a superior (or dorsal) and inferior (or ventral) portion:

dorsal portion of pouch 3: forms the inferior parathyroid glands the chief (or principal) and oxyphil cells are derived from the endodermal lining of the pouch ventral portion of pouch 3: forms the thymus the epithelial reticular cells (including those that comprise the thymic or Hassall's corpuscles) are derived from the endodermal lining of the pouch. T-cell progenitors from the bone marrow infiltrate the cortex to establish the definitive thymus.

4. Pharyngeal Pouch 4 also divides into a superior (or dorsal) and inferior (or ventral) portion:

dorsal portion of pouch 4: forms the superior parathyroid glands the chief (or principal) and oxyphil cells are derived from the endodermal lining of the pouch

ventral portion of pouch 4: forms a diverticulum called the ultimobranchial body, the cells of which migrate into the thyroid gland and differentiate into parafollicular (C) cells of the thyroid gland.

Anomalous development of the derivatives of pouches 3 and/or 4 can result in ectopic or absent parathyroid, thymic, or parafollicular thyroid tissue. The most common disorder in which this occurs is DiGeorge syndrome, caused by a deletion in the long (or "q") arm of chromosome 22, leading to a hypoplasia of 2nd and 3rd pharyngeal pouch derivatives. Symptoms and signs of DiGeorge often include:

Interestingly, the hypoplasia of the 2nd and 3rd arches can also disrupt the 1st and arch, leading to the following additional findings:

II. Development of the tongue

A. Anterior 2/3 of the tongue:

1. Formation: the anterior 2/3 of the tongue is derived from median and lateral tongue buds that arise from the floor of the 1st pharyngeal arch and then grow rostrally. The tongue buds are then invaded by occipital myoblasts that form the intrinsic muscles of the tongue.

2. Innervation of the anterior 2/3 of the tongue:

B. Posterior 1/3 of the tongue:

1. Formation: swellings from the floor of the 3rd and 4th pharyngeal arches overgrow the 2nd arch and fuse with the anterior 2/3 of the tongue. Thus, the posterior 1/3 of the tongue is derived from the 3rd and 4th arches and there is NO contribution of the 2nd pharyngeal arch in the adult tongue. Intrinsic musculature is also derived from occipital myoblasts. The line of fusion of the anterior 2/3 and posterior 1/3 of the tongue is indicated by the terminal sulcus.

2. Innervation of the posterior 1/3 of the tongue:

III. Development of the thyroid gland

Anomalies in thyroid development can result in ectopic thyroid tissue and/or cysts present along the course of the thyroglossal duct, which is a midline structure (as opposed to cervical cysts, which are remnants of pharyngeal clefts 2-4 and are found lateral to the sternocleidomastoid muscles).

IV. Development of the skull

Because the brain continues to grow in size up until 6-7 years of age, premature fusion of the sutures or fontanelles will result in abnormal shaping of the head as the brain will cause displacement of the bones that remain unfused.

V. Development of the face

Below is a summary of the contributions of the prominences to the adult face:

Disruption of the development of any of the facial prominences can result in a variety of facial anomalies, such as (from left to right in figures below):

VI. Development of the palate

A. Primary palate

B. Secondary palate

Complete fusion of the primary and secondary palate is a complex process involving growth of the component tissues, epithelial to mesenchymal transformation, cell migration, and programmed cell death at fusion sites disruption of any part of this process can result in cleft palate. Given the involvement of the maxillary and nasal prominences, cleft palate is often (but NOT always) accompanied by cleft lip.

Practice Questions

1. In craniofacial development, paraxial mesoderm contributes to which of the following?

A. occipital bone B. muscles of the tongue C. extraocular muscles D. NONE of the above E. ALL of the above

ANSWER

2. The craniofacial defect illustrated in the figure below was most likely caused by which of the following?

ANSWER

3. The condition shown in the figure below was most likely caused by:

A. failure of the medial and lateral nasal processes to fuse with the maxillary process. B. incomplete merging of the maxillary and mandibular processes. C. incomplete fusion of the medial nasal processes. D. overgrowth of the frontonasal process. E. incomplete growth of the mandibular process.

ANSWER

4. The thyroid gland is derived primarily from the:

A. 1st pharyngeal pouch. B. 2nd pharngeal pouch. C. ventral portion of the 3rd pharyngeal pouch. D. dorsal portion of the 4th pharyngeal pouch. E. foramen cecum at the base of the tongue.

Read more:
Duke Embryology - Craniofacial Development