Development+of+the+Branchial+Region+and+Portions+of+the+Head+and+Neck

=**Objectives**=


 * 1. Describe a pharyngeal (branchial) arch, pharyngeal (branchial) groove, pharyngeal (brachial) pouch, and pharyngeal (branchial) membrane.**

During the fourth week of development, as the buccopharyngeal membrane covering the stomadeum is degenerating, out-pouchings of the pharyngeal foregut develop called pharyngeal pouches. Four pairs of grooves develop between the pouches called pharyngeal grooves. Interposed between the grooves and the pouches are condensations of mesenchyme called the pharyngeal arches which continue to grow until they meet at the midline and completely close off the pharyngeal portion of the foregut.

The pharyngeal arches are five bars of mesenchymal tissue forming the ventral region of the developing pharyngeal foregut. Each arch is composed initially of an outer lining of ectoderm, a core of mesoderm derived from invading neural crest cells and paraxial mesoderm, and an inner endoderm lining. Neural crest cells are responsible for giving rise to skeletal structures while the paraxial mesoderm gives rise to muscle structures and the endothelium of blood vessels. The inner endoderm lining is actually the foregut of the pharynx.

Between each pharyngeal pouch and pharyngeal groove is the pharyngeal membrane, where the mesenchyme between pharyngeal arches narrows such that the endoderm and ectoderm nearly touch.

The first pharyngeal arch is called the mandibular arch and has a maxillary and a mandibular process. The second pharyngeal arch is called the hyoid arch. The third arch is called the third arch. The fourth arch fuses with the sixth arch and is just called the fourth arch. The fifth arch is not present in humans.


 * 2. List the four structures that are found in a typical pharyngeal arch.**

A typically pharyngeal arch contains four structures: an artery branch from the dorsal aorta, a cartilaginous arch, a muscular component and a nerve.


 * 3. List the skeletal, muscular derivatives, and innervation of each pharyngeal arch.**


 * **Arch** || **Skeletal** || **Muscles** || **Nerve** || **Blood Vessel** ||
 * First Arch || Meckel’s Cartilage: regresses and forms the incus and malleus of the middle ear and part of the sphenomandibular ligament || Muscles of mastication (temporalis, masseter, medial and lateral pterygoid muscles), tensor tympani, tensor veli palatini, mylohyoid, and the anterior belly of the digastric muscle || Mandibular Nerve (CN V3) || Maxillary Artery ||
 * Second Arch || Reichert’s Cartilage: regresses and forms the styloid process, the stapes of the middle ear, stylohyoid ligament, and the superior portion of the hyoid bone || Muscles of facial expression, posterior belly of the digastric muscle, stylohyoid muscle, and stapedius || Facial Nerve (CN VII) || Stapedial Artery ||
 * Third Arch || Inferior portion of the hyoid bone, and small laryngeal cartilages || Sytlopharyngeus muscle || Glossopharyngeal nerve (CN IX) || Contribution to the Internal Carotid Artery ||
 * Fourth and Sixth Arches || Laryngeal cartilages || Muscles of the intrinsic larynx, muscles of the soft palate and pharynx || Vagus Nerve (CN X) || Aortic Arch on the left side; Subclavian Artery on the right side ||

__Comments__ The sphenomandibular ligament goes form the sphenoid bone to the lingual on the inside of the mandible.

The Stapedius is a muscle on the middle ear that dampens the stapes.


 * 4. Describe the fate of each of the branchial grooves; including the formation and fate of the cervical sinus.**

The pharyngeal grooves are depressions that separate the arches externally. Only one pair of grooves exists as an adult structure: the first pharyngeal groove.


 * **Groove** || **Derivative** ||
 * First Groove (between arch 1 and 2) || Deepens and ultimately forms the external auditory meatus and the external aspect of the tympanic membrane. ||
 * Second through Fourth Grooves || Mesoderm of the 2nd pharyngeal arch overgrows grooves 2, 3, and 4, eventually closing them all off to form a cervical cyst or sinus. The cervical cyst normally regresses. Because of the overgrowth of the 2nd pharyngeal arch, the smooth contour of the neck is formed. ||

Anomalies of pharyngeal pouches or grooves:

__Branchial Sinuses__ Small pits that are remnants of blind pouches that are primarily found at the site of the 2nd pouch. An internal branchial sinus is a pit in the tonsillar bed that can be a space for bacteria to grow and abscesses to collect. An external branchial sinus is a pit along the side of the neck that looks pretty unsightly.

__Brachial Cysts__ A persistent cervical cyst.

__Branchial Fistulas__ Patent canal formed from the internal aspect of the oral cavity and the neck. It is usually found between the 2nd pouch and the 2nd groove and along the posterior border of sternocleidomastoid muscle (SCM). Patients with this condition will present with saliva on the neck.


 * 5. Define and/or describe the fate of each of the branchial pouches or derivatives.**

The pharyngeal pouches are “balloon-like” diverticula that develop between the arches. There are 4 well defined pairs of pouches that are lined with endoderm from the developing pharyngeal foregut.


 * **Pouch** || **Location** || **Derivative** ||
 * First Pouch || Between arch 1 and 2 || Elongates dramatically to form a portion of the tympanic cavity of the middle ear, auditory tube in the nasopharyngeal tube, and inferior aspect of the tympanic membrane ||
 * Second Pouch || Between arch 2 and 3 || Remains a shallow pit that forms the tonsillar fossa (but not the tonsils) which is eventually invaded by lymphatic tissue (forming the palatine tonsil) ||
 * Third Pouch || Between arch 3 and 4 || Forms a dorsal and a ventral bud. The dorsal bud forms the inferior parathyroid glands while the ventral bud forms the thymus ||
 * Fourth Pouch || Between arch 4 and 5 || Forms a dorsal and a ventral bud. The dorsal bud forms the superior parathyroid glands while the ventral bud forms the ultimobranchial body which invades the thyroid to form the parafollicular cells. ||
 * Fifth Pouch || Between arch 5 and 6 || Rudimentary, combines with the 4th pouch. ||


 * 6. Define and/or describe the point of origin, the migratory path, and the definitive situation of the thyroid gland.**

The thyroid gland is the first endocrine organ to develop in the embryo. It begins as a thyroid diverticulum on the ventral surface of the pharynx in the area of the 2nd pharyngeal pouch and expands dragging the thyroglossal duct through the neck. The thyroglossal duct is the path along which the thyroid tissue descends. The thyroid tissue descends until it is just inferior to the thyroid cartilages. The thyroglossal duct typically remains patent until just before birth when it closes and forms a foramen cecum on the tongue.

__Thyroglossal Duct Cysts__ There are cysts that form anywhere along the former thryoglossal duct, in the tongue, anterior neck or anterior to the laryngeal cartilages.

__Ectopic Thyroid Tissue__ Left over thyroid tissue can be found at the inferior part of the thyroglossal duct or anywhere along the path of thyroid tissue migration. The pyramidal lobe is a remnant of migrating thyroid tissue.

__Patient Thyroglossal Duct__ Saliva can enter from the oral cavity and move along the path of the thryoglossal duct. Alternatively, part of the duct can remain open as a cyst will form as fluid accumulates.


 * 7. Describe the formation of the tongue.**

The tongue develops as a series of swellings in the floor of the pharynx. The median tongue bud (tuberculum impar) develops in the midline and is associated with the 1st pharyngeal arch. This stimulates the formation of the lateral lingual swellings (dorsal tongue buds), also associated with the 1st pharyngeal arch. The lateral lingual buds enlarge and overgrow the median tongue bud, forming the mucus membrane of the anterior 2/3 of the tongue.

The copula is a midline swelling associated with the 2nd pharyngeal arch which forms and stimulates the formation of the hypobrachial eminence just caudal to the copula and associated with the 3rd and 4th pharyngeal arch. The hypobrachial eminence migrates anteriorly and gives rise to the posterior 1/3 of the tongue and the epiglottis. The terminal sulcus forms where the hypobrachial eminence beets with the lateral lingual buds.

The muscles of the tongue //do not// originate from the pharyngeal wall but migrate there from occipital somites. These myotomes that invade the swellings of the tongue form the muscles of the tongue which receive motor innervation from hypoglossal nerve (CN XII). Taste is supplied to the anterior 2/3 of the tongue by the chorda tympani which is a branch of the facial nerve (CN VII) that joins with the lingual nerve, branch of the mandibular nerve (CN V3), which supplies the general sensory innervation (pain, touch, and temperature) to the anterior 2/3 of the tongue.


 * 8. Describe the formation of the face from the five face primordia.**

The face is formed between the 4th and 8th weeks of development from five face primordia: 1 frontonasal prominence, 2 maxillary prominences, and 2 mandibular prominences.

The frontonasal prominence surrounds the developing forebrain and develops into the forehead, dorsum of the nose and a portion of the upper lip (philtrum). The frontonasal prominence develops two lateral thickened areas called the nasal placodes. The lateral and medial rims of the nasal placodes enlarge and form a depression in the center called the nasal pit. The lateral rim is called the lateral nasal swellings/processes and the medial rim is called the medial nasal swellings/processes. The medial nasal swellings will meet at the midline to form the intermaxillary segment which will give rise to the philtrum of the upper lip, portions of the upper jaw, nasal septum, and palate (primary palate).

The maxillary and mandibular prominences develop from the first pharyngeal arch. The prominences are produced by a proliferation of neural crest cells that migrate there from the lower midbrain and hindbrain region of the developing neural tube. The maxillary prominences grow toward the lateral nasal swellings and meet at the nasolacrimal groove which is important to forming the lacrimal apparatus. The maxillary prominence results in the formation of the lateral portion of the upper lip, upper cheek, and most of the maxilla and the palate. The mandibular prominence meets the maxillary prominence laterally and the opposite mandibular prominence n the midnline, forming the adult lower lip, lower cheek, chin and mandible. The mesenchyme from the second pharyngeal arch invades the areas of the maxillary and mandibular prominences and forms the muscles of facial expression.


 * 9. Describe the formation of the plate and the congenital abnormalities that result in cleft palate and lip.**

The development of the palate occurs during the 5th week of embryonei development from two primordial: the primary palate and the secondary palate.

The primary palate develops from the intermaxillary segment from the frontonasal prominence. This primary palate only forms the most anterior portion of the adult hard palate, anterior to the incisive foramen.

The secondary palate develops from the two processes from the internal surface of the maxillary prominences. These are the lateral palatine processes. They gradually enlarge and grow towards the midline where the fuse with each other and with the developing nasal septum. The anterior portion of these processes contributes to the hard palate and the posterior portion to the soft palate.

The incisive foramen marks the junction between the primary and secondary palates. The midline raphe marks the junction between the two lateral palatine processes.

Cleft palate and cleft lip are two of the most common defects of the face.

The cleft lip is more common in boys than girls, occurring 1/900 births. A unilateral cleft lip is a failure of one of the maxillary and intermaxillary segments to fuse. The bilateral cleft lip is cause by failure of fusion on both sides of the intermaxillary segments with the maxillary segments.

Cleft palate is more common in girls than boys, occurring 1/2,500 births. Anterior or primary cleft palate is a failure of the primary palate to fuse with the secondary palate. Posterior or secondary cleft palate is failure of the lateral palatine processes to fuse together along with the nasal septum. Complete cleft palate is the most severe clefting, combining both the anterior and posterior cleft defects.