Pharyngeal Arches

• Gives rise to different head and neck structures.

• Appears in the 4th to 5th week

• Consists of bars of mesenchyma tissue separated by Pharyngeal clefts - mesenchyme is the embryological connective tissue.

• 1st, 2nd, 3rd, 4th, and 6th arches (5th is absent in humans but present in mammals)

Pharyngeal Pouches

• Outpocketings that appear along the lateral wall of the pharynx

• Penetrates the surrounding mesenchyme

• 4 pairs that gives rise to important organs

Mesenchyme

• Responsible for developing structures.

• Derived from:

  1. Neural crest

  2. Lateral Plate Mesoderm

  3. Paraxial Mesoderm

Neural Crest Migration Path

• From forebrain, midbrain, and hindbrain to their final locations (blue),

• Epipharyngeal placodes assist the formation of CN V, VII, IX, X sensory ganglia.

Pharyngeal Arch Development

1st Pharyngeal Arch

• Maxillary process

a. Dorsal region

b. Extends beneath eye region

c. Gives rise to premaxilla, maxilla, zygomatic, and portions of the temporal bone.

• Mandibular process

a. Contains meckel's cartilage

• Musculature

a. Muscles of mastication

b. Digastricus anterior

c. Mylohyoid

d. Tensor tympani

e. Tensor veli palatini

• Nerve Supply

a. Mandibular branch of trigeminal nerve (CN V)

Meckel's Cartilage

• Disappears except for 2 small portions at the dorsal end.

• Becomes Malleus and Incus

2nd Pharyngeal Arch

• Also known as the Hyoid arch

a. Cartilages gives rise to stapes, styloid process, stylohyoid ligament, and ventrally the lesser horn and upper corpus.

• Musculature and Nerve Supply

a. Stapedius

b. Stylohyoid

c. Digastricus posterior

d. Muscles of facial expression

e. Facial nerve (CN VII)

Reichert's Cartilage

• Divides dorsal and ventral

• Ventral: hyoid to lesser horn

• Dorsal: stapes to styloid process

3rd Pharyngeal Arch

• Structure

a. Cartilage gives rise to greater horn and lower corpus of hyoid bone.

• Musculature and Nerve Supply

a. Stylopharyngeus muscle

b. Glossopharyngeal nerve (CN IX)

4th and 6th Pharyngeal Arch

• Cartilaginous

a. Fuses to form the thyroid, arytenoid, corniculate, and cuneiform cartilages.

b. Cartilaginous because they form the laryngeal framework.

• Musculature and Nerve Supply

  1. 4th arch (superior laryngeal nerve)

a. Cricothyroid

b. Levator veli palatini

c. Pharyngeal constrictors

• 2. 6th arch

a. Intrinsic laryngeal muscles

1st Pharyngeal Pouch

• Structures

a. Forms the tubotympanic recess which forms the future external auditory meatus.

b. Distal portion widens to form the middle ear (tympanic) cavity.

c. Proximal portion forms the eustachian tube.

2nd Pharyngeal Pouch

• Structures

a. Forms the primordium of the palatine tonsils.

b. Remains to be the tonsillar fossa.

3rd Pharyngeal Pouch

• Structures

a. Dorsal region forms the inferior parathyroid gland

b. Ventral region forms the thymus gland

4th Pharyngeal Pouch

• Structures

a. Forms the superior parathyroid gland

Pharyngeal Clefts

• Between the arches are pouches internally and clefts externally

• Dorsal part of the first cleft penetrates to form the external auditory meatus

• The bottom portion epithelial lining of the meatus forms the eardrum (tympanic membrane)

Tongue Development

Appearance

• 4 weeks forms 3 swellings:

1. 2 Lateral lingual swellings

Tuberculum Impar

• Originate from the 1st pharyngeal arch

• 2nd median swelling (copula/hypobranchial eminence) formed by the mesoderm of the 2nd, 3rd, partial 4th arches.

• 3rd median swelling from the posterior 4th arch forms the epiglottis

• Behind is the laryngeal orifice with the arytenoid swellings lateral to it

Appearance

• Fusion of the lateral swelling and the tuberculum impar forms the anterior ⅔ of the tongue (body)

• Sensory innervation by the mandibular branch CN V

• Body of the tongue is separated from posterior ⅓ via the v-shaped terminal sulcus

• Epiglottis and posterior most portion innervated by superior laryngeal nerve

• Root or posterior ⅓ comes from 2nd, 3rd, 4th pharyngeal arches

• Innervated by CN IX

• Tingue musculature innervated by CN XII Hypoglossal nerve

• Special sensory innervation (taste) to anterior ⅔ is provided by chorda tympani of CN VII

Facial Structure: Appearance

• Facial prominences appear from the 1st pharyngeal arch by the 4th week

• Maxillary prominence are lateral to stomodeum

• Mandibular prominence caudal to stomodeum

• Frontonasal prominence cephalic to the stomodeum, with nasal placodes

• Nasal placodes transform to nasal pits, then this creates ridge: nasal prominence

1. Lateral nasal prominence

2. Medial nasal prominence

• Next 2 weeks:

1. Increase size of maxillary prominence

2. Grow medially compressing medial nasal prominence leading to fusion

3. Upper lip is formed by the 2 maxillary and 2 medial nasal prominences

• Mandibular prominences merge midline and form the jaw and lower lip

• Maxillary and lateral nasal prominences are separated by the nasolacrimal groove

• Solid epithelial cord from nasolacrimal duct

• Widening of the upper end forms the lacrimal sac

• Cord detached so that maxillary and lateral nasal prominence fuses together

• Enlargement of maxillary prominence leads to formation of cheeks (buccae) and maxillae

Fusion of the Nose

• The nose is formed from the 5 facial prominences

• Frontal - bridge

• Merged medial nasal - crest and tip

• Lateral nasal - sides (alae)

Intermaxillary Segments

• Medial growth of the maxillary prominences lead to its formation

• Continuous with rostral portion of the nasal septum formed by the frontal prominence

• Composed of:

a. Labial component → forms the philtrum

b. Upper jaw movement → location of 4 incisors

c. Palatal component → forms the primary plate

Secondary Palate

• Formed by the palatine shelves from the maxillary prominences that appears in Week 6

• Directed obliquely downwards on each side of the tongue

• By Week 7: palatine shelves have ascended to attain a horizontal position above the tongue

• Incisive foramen: fusion point of the primary and secondary palate

• Nasal septum grows downwards and fuses with the superior portion of the secondary palate (to separate both sides of the nasal cavity)

Inferior view of the primary and secondary palate

Frontal and Inferior view of the fused secondary palate

Nasal Cavities

• During 6th week:

a. Nasal pits deepen

b. Oronasal membrane separates from the primitive oral cavity

c. Forms the primitive choanae

• Formation of the secondary palate furthers the development of the primitive nasal chambers

• Definitive choanae lie at the nasopharynx

Dental Development

• During 6th week:

a. Basal layer forms the dental lamina

b. Lamina gives rise to the dental buds

c. Dental buds reach cap stage of tooth development

d. Mesenchyme form dental papilla

• Cap consists of layers:

a. Outer dental epithelium

b. Inner dental epithelium

c. Stellate reticulum (central core)

• Growth of the dental cap leads to the bell stage (bell-shaped appearance)

• Dental papilla differentiate into odontoblasts

• Produces dentin, making up the dentin layer

• Retreat to the papilla, leaving the dental process

• Remaining papilla cells form the tooth pulp (inner portion of the tooth)

• Inner dental epithelium differentiates into ameloblasts (enamel formers)

• Clustering of ameloblasts will form the enamel knot

• Enamel starts to cover the apex down to the neck

• Root forms the epithelial layers penetrate the mesenchyme to form the epithelial root sheath

• Pulp chamber narrows and forms a canal containing blood vessels and nerves

• Mesenchymal cells outside the tooth comes in contact with the root dentin to form cementoblasts

• Outside of cement layer, mesenchyme gives rise to the periodontal ligament (position holder and shock absorber)

• Further lengthening of root will result to tooth eruption

• Buds for permanent teeth are formed at the third month

Replacement of decidious teeth

Left anterolateral view