embryology

Chapter 11Development of the head and neck, the eye and ear

Done by: Sumaiah AlghamdiHind alsubeieSubmitted to Dr. Afrah

Content

• Pharyngeal arches , clefts and pouches

• Development of the tongue and thyroid gland.

• Development of the face• Development of the nasal cavity• Formation of the palate• Development of the eye and ear

1-Pharyngeal arches , clefts and pouches

A. Pharyngeal arches :• Start : fourth week • Origin: mesenchyme cell from the neural

crest, the lateral plate mesoderm and the paraxial mesoderm.

• Structure : Five pairs of pharyngeal arches, numbered 1, 2, 3, 4 and 6, form in craniocaudal sequence and by the end of the fifth week. Each arches has an outer covering of ectoderm and is lined internally by endoderm.

• also, each pharyngeal arch differentiates into a bar of cartilage, the associated muscle and an aortic arch artery.

1-Pharyngeal arches , clefts and pouches

1 -Pharyngeal arches , clefts and pouches

B. Pharyngeal clefts :The four pharyngeal clefts separate the pharyngeal

archesexternally • The first pair of pharyngeal clefts is the only

one that contributes to adult structures.• The second pharyngeal arch enlarges and

grows rapidly as a flap over the remaining three pharyngeal clefts.


C. Pharyngeal pouches:has an outer covering of ectoderm and is lined internally by endoderm.

2-Development of the tongue and thyroid gland.

A. Tongue:• Start : fourth week • Origin: mesenchymal swellings covered with

ectoderm and endoderm on the floor of the pharynx. Structure

The anterior

two-thirds of the tongue.

the posterior one-third


The anterior two-thirds of the tongue:• Origin: three swellings derived from the first

arch mesenchyme, the lateral lingual swellings and a median tuberculum impar .

The posterior one-third :• Origin: comes from a single swelling, the

hypobranchial eminence, derived from the third and fourth pharyngeal arches.

• A midline depression at the apex of the sulcus terminalis, called foramen caecum


B. Thyroid gland :• Start : fourth week • Origin: from foramen

caecum • Structure : foramen

caecum soon grows as the thyroid diverticulum, descends in the neck and divides into right and left lobes, connected by an isthmus

3 -Development of the face

A. The face:• Start : fourth week • Origin: mesenchyme cell from the neural

crest.• Structure : fusion of five facial prominences around

the opening of the mouth. (stomodaeum).facial

prominences

single frontonas

al prominen

ce

Paired mandibul

ar prominen

ces

paired maxillary prominen

ces


• The structures derived from the three facial primordia:


During the fifth week, some events shape the facial appearance:

• Maxillary prominences enlarge and grow in the medial direction,

• The bilateral ectodermal thickening.• the nasal placodes, appear on the frontonasal prominence.• The mesenchyme around each nasal placode forms the medial and lateral nasal processes


• The medial nasal processes move towards each other which form an intermaxillary segment then fuse with the lateral nasal process and medial nasal processes to form the upper lip. Each maxillary prominence is separated from the lateral nasal process by a nasolacrimal groove.

4 -Development of the nasal cavity.

• the nasal placodes invaginate to form the nasal pits which deepen and become nasal sacs

• The nasal sacs grow upwards and are separated from the oral cavity by the oronasal membrane. , which breaks down to bring the nasal cavities into communication with the oral cavities.

5 -Formation of the palate

• Strucure :The palate develops from fusion of the primary and

secondary palate • The primary palate is derived from the

intermaxillary segment .• The secondary palate formed maxillary

prominences.

6-Development of the eye and ear

Eye and ear :Origin : arise from ectodermal placodes in the

head region of theembryo Ectodermal placodes : result of interaction

between the neural tube and overlying ectoderm.


A. The eye :• Start : fourth week • Origin: outgrowth from the lateral wall of the forebrain as

optic vesicle.• Structure : • The optic vesicle acts on the surface ectoderm to form lens

placode which invaginates to form the lens vesicle • lens vesicle detaches from the surface ectoderm and sinks

into the optic vesicle,• the connection between the optic vesicle and the brain

narrows to form the optic stalk.• The optic vesicle is now indented to become a double-

walled optic cup.


A. The ear :• Start : fourth week

• Origin:

• the first two pharyngeal apparatus.

The external and middle

ears are from

• ectodermal placode The inner ear


Structure :• Invaginates as the otic vesicle and soon

separates from the surface ectoderm. • Diverticulum arises from the otic vesicle to form

the endolymphatic sac • The vestibular portion develops two sacs, an

expanded larger utricle and a smaller saccule.


Structure :• Three tubes grow from the utricle to give rise to

the semicircular ducts.• The lower part of the saccule elongates and

spirals as the cochlea. • The mesenchyme around the membranous

labyrinth becomes the cartilaginous otic capsule;

• The cavities that appear in the otic capsule merge to form a perilymphatic space which develops to the scala tympani and the scala vestibule.

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