Branchial Apparatus
Introduction
Branchial apparatus contributes to the develppment of head and neck structures.
The structures contributed to are skeletal, muscular, arterial and nervous components of each branchial arch.
Structures are derived from each branchial pouches and clefts.
The thyroid gland and tongue are developed from the branchia apparatus
The branchial apparatus consists of:
1) branchial arches (mesoderm),
2) branchial grooves or clefts (ectoderm),
3) pharyngeal pouches (endoderm), and
4) branchial membranes (ectoderm and endoderm).
I. BRANCHIAL ARCHES
A. General
1. Arches begin to develop during the 4th week as rounded ridges on each side of the future head and neck region
2. Four arches appear by the end of the 4th week
3. The arches are separated from each other externally by branchial grooves
4. The arches support the lateral walls of the primitive pharynx
5. The mouth initially appears as a slight depression in the surface ectoderm called the stomodeum
B. Branchial Arch Components
1. Each arch has a mesodermal core covered externally by ectoderm and internally by endoderm
The mesoderm of each arch gives rise to muscles
2. A typical arch contains the following:
a. A cartilaginous bar
b. A muscle element
c. An artery
d. A nerve
DERIVATIVES OF THE BRANCHIAL ARCHES
First Branchial Arch
Contains two processes, the maxillary and the mandibular
1). palatopterygoquadrate
2). mandibular (Meckel's)
Muscle derivatives
a. Muscles of mastication
Temporalis
Masseter
Medial pterygoid
Lateral pterygoid
b. Mylohyoid and anterior belly of digastric
c. Tensor tympani
d. Tensor veli palatini
First arch syndromes
a. Consist of a number of malformations resulting from deficiencies in components of the arch (primarly neural crest)
b. Treacher-Collins Syndrome (mandibulofacial dyostosis)
1). Abnormal external, middle and inner ear
2). Malar and mandibular hypoplasia
3). Lower eyelid defects
c. Pierre Robin Syndrome
1). Mandibular hypoplasia
2). Cleft palate
3). Eye and ear defects
4). DiGeorge Syndrome
Absence of the thymus
Malformations of the mouth
Nasal clefts
Cardiac abnormalities
Second Branchial Arch
1. Cartilage derivatives (Reichart's cartilage)
2. Muscle derivatives:
a. Muscles of facial expression
b. Stapedius
c. Stylohyoid
d. Posterior belly of digastric
These muscles are innervated by cranial nerve VII but migrate into the area of the first arch
Third Branchial Arch
1. Cartilage derivatives
2. Muscle derivatives
Fourth and Sixth Branchial Arches
1. Cartilage derivatives
2. Muscular derivatives
II. PHARYNGEAL POUCHES
The endoderm lining the pouches is in contact with the ectoderm lining the branchial grooves forming a thin branchial membrane
First Pharyngeal Pouch
Extends into a tubotympanic recess enveloping the ear bones
a. Tympanic membrane
b. Tympanic cavity, the mastoid antrum and the auditory tube
Second Pharyngeal Pouch
Forms the tonsillar fossa (palatine tonsils)
a. The endoderm lining the pouch proliferates and sends buds into the surrounding mesenchyme to form tonsillar crypts
b. Mesenchyme forms the connective tissue framework
Third Pharyngeal Pouch
1.Inferior parathyroid gland
2. Thymus
3. The parathyroid and thymic primordia migrate caudally
Fourth Pharyngeal Pouch
1. Superior parathyroid gland
III. BRANCHIAL CLEFTS
Although Four Branchial Clefts Form, Only One Contributes to the Definitive Structure of the Embryo
First Cleft
Dorsal part forms the external auditory meatus
The ectoderm forms the tympanic membrane
Second to Fourth Clefts
The mesenchyme of the 2nd arch proliferates, causing it to overlap the remaining clefts
The clefts lose contact with the exterior and form a cavity lined by ectoderm, the cervical sinus
When overgrowth of the 2nd arch is not complete, the 2nd, 3rd, and 4th clefts remain in contact with the outside via a narrow canal, the branchial fistula
The fistula drains a lateral cervical cyst
THE TONGUE
Origin :
1. Tuberculum impar
2. Lateral lingual swellings
The lateral lingual swellings grow rapidly, merge with each other and overgrow the tuberculum impar. Form the anterior 2/3 of the tongue
Median sulcus
The posterior third of the tongue
THE THYROID GLAND
Origin : -
Develops from a median endodermal thickening in the floor of the primitive pharynx
Forms a downgrowth called the thyroid diverticulum
The developing thyroid grows and descends in the front of the neck, connected to the tongue by the thyroglossal duct
Its opening in the tongue is called the foramen cecum
The diverticulum grows rapidly and divides into two lobes
The thyroglossal duct disappears but a remnant may remain as the pyramidal lobe
