Pharyngeal Arches – Clefts – Pouches

Head, Neck Face Embryology Anatomy Notes And Important Questions With Answers

Evolution of Pharyngeal Arches

1. In fihes, the region of the foregut is marked by slits.
2. These are caudal to th stomodeum and th oral membrane.
3. These slits are called gill slits and gill-bearing arches.
4. With th evolution of lungs amongst land vertebrates, the region of th branchial (gill-bearing) arches underwent extensive modifiations.
5. This gives rise to the regions of th face, th pharynx and th associated structures.
6. Hence, thy are called branchial arches.

Body Fluids	Muscle Physiology	Digestive System
Endocrinology	Face Anatomy	Neck Anatomy
Lower Limb	Upper Limb	Nervous System

Read And Learn More: Face Anatomy Notes And Important Questions

1. Their vascular supply is derived from a series of ventrodorsal aortic arches.
2. Because of the peculiar nature of the blood supply, the mesoderm of the pharyngeal region shows localized thckenings.
3. These are in the form of a series ofdorsoventrally disposed bars.
4. They are numbered from cranium to caudal as first to sixth.
5. These bars appear temporarily as prominent “arches” on either side of the developing pharynx.
6. Hence, they are called “pharygeal arches”.

First pharyngeal arch

First pharyngeal arch is also called mandibular arch.

Anomalies of first pharyngeal arch

1. Agnathia: It is the most extreme form of 1st arch hypplasia.

1. There is failure of formation of lower jaw.
2. In severe agnathia, the external ears remain in the ventral cervical region and may even join the ventral midline.

2. Pierre Robin syndrome: It consists of

1. Extreme micrognathia (small jaw)
2. Relatively large tongue
3. Cleft palate
4. Defects of the ear
5. Leads to respiratory distress

3. Treacher Collins syndrome (mandibulofacial dysostosis-defective ossification of foetal cartilage)

1. It is typically inherited as an autosomal dominant condition.
2. Asymmetry of facial skeleton
3. Responsible gene: Treacle.
4. Manifestation

1. Coloboma-type defects of the lower eyelid
2. High cleft palate
3. Faulty dentition
4. Malformations of the external and middle ears
5. Hypoplasia of the mandible and facial bones.

4. Maxillar process defect is recognized but with variable conditions with cleft lips and after facial fissures.

Meckel’s cartilage

1. Definition: Skeletal element of the 1st pharyngeal arch (mandibular arch) is called Meckel’s cartilage. ”

2. Extent: It extends from the developing otic capsule to mandibular prominence.

3. Derivatives: Following are the structures given by different parts of Meckel’s cartilage.

Give the persistent structures of fibrous envelope of Meckel’s cartilage.

1. The cartilaginusbar of th mandibularpart is known as Meckel’scartilage. It extends dorsally up to the cartilaginous ear capsule.

2. The dorsal part of the cartilage is ossified to form the malleus and incus.
The part of the cartilage extending from the region of the middle ear to mandible disappears.
The sheath (perichondrium) forms

1. Anterior ligament of the malleus
2. Sphenomandibular ligament.

3. The fibrous membrane of ventral part of the Meckel’s cartilage is ossified to form the body of the mandible.
It extends from the inferior alveolar foramen to mental foramen. The cartilage cells disappear.
The ossified remnants of the cartilage are mental ossicle of symphysis menti.

Second pharyngeal arch

Reichert’s cartilage and its pericondrium form the stapes, styloid process, stylohyoid
ligament, lesser comu and probably the upper part of the body of the hyoid bone.

The word “second” and the derivatives of the second pharyngeal arch begins with the letter “S”.

Pharyngeal pouches

The mesoderm between any two successive arches is very thin. This leads the ectoderm being drawn inwards to form clefts (grooves) and the ectoderm being pulled outwards to form pouches. Endodermal invaginations of each pharyngeal arch is called pharyngeal pouch. Following are the structures given by the respective pouches…

Abnormalities of pharyngeal pouches

1. Nezelof syndrome

1. Definition: It is failure of development of only 3rd pharyngeal pouch.
2. Manifestations

1. Superior parathyroid glands are present.
2. Inferior parathyroid gland and thymus are absent.
3. There is normal function of parathyroid glands.

2. DiGeorge syndrome (3rd and 4th pharyngeal pouch syndrome) results from failure of development of 3rd and 4th pouches.

1. Manifestations

1. Thymic hypoplasia (resulting in selective T cell deficiency and greatly reduced cell-mediated immunity)
2. Absence of parathyroid glands (resulting in hypocalcaemia and tetany).
3. Immunodeficiency.

3. Catch 22 spectrum
1. Aetiology: Deletion of long arm of chromosome 22 is the reason of the multiple anomalies.

2. Manifestations

1. Thymic hypoplasia (resulting in selective T cell deficiency and greatly reduced cell-mediated immunity)
2. Absence of parathyroid glands (resulting in hypocalcemia and tetany).
3. Cardiac defects
4. Abnormalities of face and cleft palate.

Pharyngeal cleft

Introduction: The depression between two arches on outer side is called pharyngeal cleft.

1. 1st pharyngeal cleft: Dorsal part of 1st pharyngeal cleft forms

1. External acoustic meatus.
2. Outer layer of tympanic membrane.
3. Pinna of ear: Swellings on 1st and 2nd arches are called hillocks.
   These hillocks combine and form pinn of ear.

2. 2nd pharyngeal arch: It grows caudally over the 2nd and 3rd pharyngeal clefts.
It encloses space lined by ectoderm called cervical sinus.
The overhanging 2nd arch joins the epicardial ridge.
Cervical sinus disappears.

3. Anomalies: Branchial cyst is the persistence of cervical space.
It is located below and behind the angle of mandible.
Cholesterol crystal in the aspirate is diagnostic feature of branchial cyst.

Derivatives of 4th and 6th pharyngeal arches

1. Muscles

1. Muscles of larynx

1. Extrinsic-cricothyroid.
2. Intrinsic muscles of larynx

2. Constrictor of pharynx.

3. Striated muscle of oesophagus.

2. Skeletal element: The 4th and 6th pharyngeal arches fuse together and form following cartilages:

1. Thyroid,
2. Cricoid,
3. Arytenoid,
4. Corniculate, and
5. Cuneiform.

3. Nerves

1. Nerve of the 4th pharyngeal arch is superior laryngeal nerve, a branch of vagus nerve.
2. Nerve of the 6th pharyngeal arch is recurrent laryngeal nerve, a branch of vagus nerve.

4. Artery
1. 4th arch

1. Right side: Forms the proximal part of right subclavian artery.
2. Left side: Arch of the aorta.

2. 6th arch

1. Ventral part forms pulmonary artery.
2. Dorsal part on right side forms ductus arteriosus which degenerates and forms ligamentum arteriosum.