BRANCHIAL APPARATUS

James C. McKenzie, Ph.D.

OBJECTIVES:

The student should be able to:

1. Recognize the developmental contributions of the branchial apparatus to adult head and neck structures.

2. Know the skeletal, muscular, arterial and nervous components of each branchial arch.

3. Know the structures derived from each branchial pouch.

4. List the derivatives of the branchial clefts

5. Describe the development of the thyroid gland and tongue

Required reading: Sweeney, pp 200-222, 230.

OUTLINE:

INTRODUCTION

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).

FIG. 1

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

a). The stomodeum is at first separated from the primitive pharynx by the buccopharyngeal membrane

FIG.2 Section viewed as in Fig. 1

B. Branchial Arch Components

1. Each arch has a mesodermal core covered externally by ectoderm and internally by endoderm

a. Neural crest cells which migrate into the arches give rise to skeletal and connective tissue components

b. The mesoderm of each arch gives rise to muscles

2. A typical arch contains the following:

a. A cartilaginous bar

1). arches 1-3

2). arches 4 & 6

b. A muscle element

1). arches 1-3

2). arches 4 & 6

 

FIG. 3

c. An artery

1). arches 1 & 2

2). arch 3

3). arch 4

4). arch 6

FIG. 4

d. A nerve

1). 1st arch

2). 2nd arch

3). 3rd arch

4). 4th and 6th arches

II. FATE OF THE BRANCHIAL ARCHES

A. First Branchial Arch

1. Cartilage derivatives

a. The first arch contains two processes, the maxillary and the mandibular

1). palatopterygoquadrate

2). mandibular (Meckel's)

a). the dorsal end

b). the middle portion

c). The ventral portion

2. 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

3. 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

1). Absence of the thymus

2). Malformations of the mouth

3). Nasal clefts

4). Cardiac abnormalities

B. The Second Branchial Arch

1. Cartilage derivatives (Reichart's cartilage)

a. The dorsal part

b. The middle part

c. The ventral portion

2. Muscle derivatives:

a. Muscles of facial expression

b. Stapedius

c. Stylohyoid

d. Posterior belly of digastric

1). These muscles are innervated by cranial nerve VII but migrate into the area of the first arch

C. The Third Branchial Arch

1. Cartilage derivatives

2. Muscle derivatives

D. The Fourth and Sixth Branchial Arches

1. Cartilage derivatives

2. Muscular derivatives

III. THE PHARYNGEAL POUCHES

A. Introduction

1. The endoderm lining the pouches is in contact with the ectoderm lining the branchial grooves forming a thin branchial membrane

a. Analogous to gill slits

FIG. 5

B. The First Pharyngeal Pouch

1. Extends into a tubotympanic recess enveloping the ear bones

a. Tympanic membrane

b. Tympanic cavity, the mastoid antrum and the auditory tube

C. The Second Pharyngeal Pouch

1. The 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 invades and forms the connective tissue framework

c. Populated predominantly by B lymphocytes

D. The Third Pharyngeal Pouch

1. Solid dorsal portion differentiates into the inferior parathyroid gland

2. Hollow ventral portion forms the primordia of the thymus

a. The two thymus primordia meet and fuse in the midline

3. The parathyroid and thymic primordia migrate caudally

E. The Fourth Pharyngeal Pouch

1. Solid dorsal portion becomes the superior parathyroid gland

2. Hollow ventral portion develops into the ultimobranchial body

 

FIG. 6

IV. BRANCHIAL CLEFTS

A. Although Four Branchial Clefts Form, Only One Contributes to the Definitive Structure of the Embryo

B. The First Cleft

1. Dorsal part of the enlarges to form the external auditory meatus

a. The ectoderm contributes to formation of the eardrum (tympanic membrane)

C. The Second through Fourth Clefts

1. The mesenchyme of the 2nd arch proliferates, causing it to overlap the remaining clefts

a. The clefts lose contact with the exterior and form a cavity lined by ectoderm, the cervical sinus

2. 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

a. The fistula drains a lateral cervical cyst

V. THE TONGUE

FIG. 7

A. Origin

1. Tuberculum impar

2. Lateral lingual swellings

a. The lateral lingual swellings grow rapidly, merge with each other and overgrow the tuberculum impar

1). Form the anterior 2/3 of the tongue

2). Median sulcus

3. The posterior third of the tongue

a. The copula develops from the ventromedial parts of the 2nd arches

b. The hypobranchial eminence develops caudal to the copula from ventromedial parts of the 3rd and 4th arches

4. The terminal sulcus

5. Branchial arch mesoderm forms the connective tissue, lymphatic and blood vessels while the muscles are mostly derived from the occipital somites

6. Papillae and taste buds

a. The foliate and vallate papillae appear during the 8th week in relationship to terminal branches of the glossopharyngeal nerve (CN IX)

b. The fungiform papillae appear later and are induced by the chorda tympani branch of the facial nerve (CN VII)

d. The filliform papillae develop during the early fetal period and are not associated with nerves

VI. THE THYROID GLAND

A. Origin

1. Develops from a median endodermal thickening which appears in the floor of the primitive pharynx caudal to the future site of the tuberculum impar

a. Forms a downgrowth called the thyroid diverticulum

2. The developing thyroid grows and descends in the front of the neck, connected to the tongue by the thyroglossal duct

a. Its opening in the tongue is called the foramen cecum

3. The diverticulum grows rapidly and divides into two lobes

a. The thyroglossal duct disappears but a ` remnant may remain as the pyramidal lobe

FIG. 8