Embryology 2 Head and Neck - Basic Science Core

Basic Science Core

Gastrulation week three

• Cells of the epiblast begin to invaginate

• 1st wave of cells migrating through the streak interdigitate with the hypoblast layer → ENDODERM

• 2nd wave → MESODERM

• Remaining cells in the epiblast layer for the → ECTODERM

The Notochord

• Originates from mesoderm

• Some mesoderm cells condense and form the notochord

• The notochord is essential for guiding adjacent cell differentiation

• Rod like structure

• Formed on the dorsal side

• Present during Embryonic development

• Know as the back bone

Mesoderm cells differentiate into specialized types (4)

Paraxial mesoderm

• Intermediate mesoderm

• Lateral plate mesoderm

• Extraembryonic mesoderm

Paraxial mesoderm

• Closest to the notochord

• Forms structures of the head and somites

• 3 pairs form per day from

  cranial to caudal direction

  • 42-44 pairs total

  • Resulting structures

   Vertebrae, ribs, rib tendons

   Muscles of the back, ribs, and

  limbs

   Parts of the dermis

How does the Notocord Effect Endoderm?

• Notochord signals neural tube formation and results in the ectoderm dividing into 3 sets of cells:

  • Neural tube

    • Neural crest cells

     HIGHLIGHT

     Cranial neural crest  cartilage, bone, cranial ganglia &

    connective tissue of the face

    • Epidermis

Main event during head and neck development:

FORMATION OF PHARYNGEAL APPARATUS

PHARYNGEAL APPARATUS

• Consists of Pharyngeal (Branchial) ARCHES, POUCHES, and CLEFTS

• Appear in the 4th-5 th week of development

• Consist of 5 paired prominences with 3 layers:

  • an outer covering of ectoderm

  • an inner covering of endoderm

  • A middle core of mesenchyme

Pharyngeal Apparatus Morphology

• 1) Pharyngeal clefts derive from the ectoderm

• 2) Pharyngeal pouches arise from endoderm cells

• 3) Pharyngeal arches arises from mesoderm +

Pharyngeal arches (6)

• Mesoderm tissue

• No “5th” arch!/Fifth arch is transient

• Involutes and produces no adult structures

• Each arch has core of mesenchyme that develops into arterial supply, nerve supply, cartilage, musculature & bone

Mesenchyme

• Mesenchyme is an embryonic precursor tissue that generates a range of structures in vertebrates including cartilage, bone, muscle, and the erythropoietic system

Major cell contributors to the mesenchyme of head and neck development:

• Praxial and lateral plate mesoderm

• Neural crest cells

• Thickened regions of ectoderm called ectodermal placodes

Cranial neural crest cells

• Migrate in the pharyngeal arches and face

The face is initially made up of five prominences (5)

• 1 central frontonasal prominence

• 2 paired maxillary prominences

• 2 paired mandibular prominences

Stomodeum

• Structures surround the primitive mouth

• Stomodeum is initially sealed by the oropharyngeal membrane which breaks down ~day 30

The 1st pharyngeal arch

• Mandibular Arch

• Maxillary and Mandibular prominences arise from the 1st pharyngeal arch

The 1st pharyngeal arch (2 Processes)

1) Maxillary process

2) Mandibular process

Maxillary process

Mesenchyme here → premaxilla, maxilla, zygomatic bone and part of the temporal bone

Mandibular process

• Contains Meckel cartilage

• Mesenchyme here ^→ mandibular bone, part

  of external ear, incus and malleus bones of the

  middle ear and external auditory meatus

Meckel cartilage

• In the Mandibular process

• Contributes to incus and malleus and serves as a template for mandible formation

Muscles of the 1st pharyngeal:

• Masseter,

• Temporalis, Lateral

pterygoid, Medial pterygoid], anterior belly of the

digastric, mylohyoid, tensor tympani, and tensor

palatini

Nerve supply of the 1st pharyngeal:

• motor and sensory from branches of Trigeminal nerve (CN V)

The 2nd pharyngeal arch

• The HYOID arch

• Has Reichert cartilage

• Precursor to the stapes, the styloid process,

  the stylohyoid ligament and parts of the hyoid

  bone & most of external ear

Muscles of the 2nd pharyngeal:

• stapedius,

stylohyoid, posterior belly of the

digastric, auricular, and muscles of

facial expression

Nerve supply of the 2nd pharyngeal:

Nerve supply: Facial nerve (CN 7)

The 3rd pharyngeal arch

• Small components of the hyoid bone

Muscles of the 3rd arch: (NOT CRITICAL TO memeroize)

• stylopharyngeus muscle

• Muscle of the pharynx

• Functions in swallowing

Nerve supply of the 3rd pharyngeal arch

glossopharyngeal nerve (CN IX)

The 4th & 6th pharyngeal arches

Cartilaginous components of the fourth and

sixth pharyngeal arches fuse to form:

• Thyroid, cricoid, arytenoid, corniculate, and

cuneiform CARTILAGES of the larynx

Muscles of the fourth arch

• cricothyroid, levato veli palatini,

  and constrictors of the pharynx

Nerves of the fourth arch

Nerve: branches of Vagus (CN X)

The pharyngeal pouches

• Lined by epithelial from the endoderm layer

• 4 pairs of pharyngeal pouches:

• 1) 1st pharyngeal pouches

• 2) 2nd pharyngeal pouches

• 3) 3rd pharyngeal pouches

• 4) 4th pharyngeal pouches

1st pharyngeal pouches

Structures arising from 1st pouch:

• Middle ear cavity

• Auditory (eustachian) tube

• Tympanic membrane (eardrum)

2nd pharyngeal pouches

Endoderm of the pouch

proliferates and forms buds that

penetrate surrounding

mesenchyme

• Mesodermal tissue enters buds and

forms palatine tonsil

• Months 3-5 tonsil is infiltrated by

lymphatic tissue

• Remnants of pouch create tonsillar

fossa

4th pharyngeal pouches

Dorsal region of the

fourth pharyngeal

pouch  superior

parathyroid gland

Ventral region 

Ultimobranchial body

• Ultimobranchial body later

incorporated into the thyroid

gland

• Cells of the ultimobranchial

body gives rise to

the parafollicular cells (C

cells) of the thyroid gland

that secrete calcitonin

3rd pharyngeal pouches

Structures arising from the 3rd

pouch:

• Dorsal region of the third pouch

differentiates into the inferior

parathyroid gland

• Ventral region of the third pouch

differentiates into the thymus

Pharyngeal Clefts (4)

• 1 st cleft  external auditory meatus (No permejnt structures)

• Second third and fourth clefts merge and form the

  cervical sinus

Cervical Sinus

Second third and fourth clefts merge and form the

cervical sinus

• Cervical sinus normally disappears during development

Development of the TONGUE

Tongue appears in the embryo ~4 weeks

• Origins of tissue for the tongue:

 FROM FIRST ARCH: formation of anterior 2/3 of tongue aka

body of the tongue

• 2 lateral swellings: Lingual swellings

• 1 medial swelling: Tuberculum impar

 FROM SECOND, THIRD, & FOURTH ARCH: posterior 1/3

tongue

• 1 medial swelling: Copula [hypobranchial

eminence

Development of the TONGUE PT 2

Anterior 2/3 of the tongue

 Sensory innervation is a branch of CN V because these

structures originate from 1 st pharyngeal arch

 Anterior 2/3 is separated from posterior

1/3 by a V-shaped groove called

terminal sulcus

 Posterior 1/3 of the tongue

• Sensory innervation by glossopharyngeal nerve,

CN IX (supplies 3rd pharyngeal arch)

 Additional special sensory innervation (taste)

Oral cavity proper versus Oropharynx

The oral cavity is inferior to the nasal cavities ( Fig. 8.247A ). It has a roof and floor

and lateral walls, opens onto the face through the oral fissure, and is continuous with

the cavity of the pharynx at the oropharyngeal isthmus.

 The roof of the oral cavity consists of the hard and soft palates. The floor is formed

mainly of soft tissues, which include a muscular diaphragm and the tongue. The lateral

walls (cheeks) are muscular and merge anteriorly with the lips surrounding the oral

fissure (the anterior opening of the oral cavity).

 The posterior aperture of the oral cavity is the oropharyngeal isthmus, which opens

into the oral part of the pharynx.

The EPIGLOTTIS

Originates from tissue of 4th pharyngeal arch

Originates from Epiglottal swelling, swelling posterior to copula (posterior 1/3 of tongue)

• Innervation: branch of Vagus nerve, CN X

Development of the THYROID

Tissue for thyroid gland originates from:

 Endoderm between first and second pharyngeal pouches

• Development steps

 Proliferation begins at junction of anterior 2/3 tongue and

posterior 1/3 of tongue

• Leaves behind a depression called foramen cecum

Thyroid descends from tongue to final position in the neck ~7th

week

 Thyroid remains connected to the tongue by a narrow canal,

the thyroglossal duct which should disappear during

development

• Gland begins to function at approximately the end of the

third month (10-12 weeks)

Birth defects & the Pharyngeal Apparatus WILL NOT BE ON THE EXAM