Chapter 23

DEVELOPMENT OF FACE, MOUTH, and PHARYNX

Development of Face (Mouth and Nasal cavity) involves the following structures:

- Frontal process

- Medial and lateral nasal processes

- Mandibular arch I

- Maxillary process

Formation of Mouth:

Stomodeum: Depression between the 1st pair of branchial (pharyngeal) arches. Ectoderm comes in direct contact with the endoderm of pharynx and forms a membrane: oropharyngeal membrane

- Epithelial parts of oral cavity rostral to palatoglossal arch are ectodermal (palatine tonsil)

- Elongation of mandibular arch and maxillary process results in the elongation of oral cavity

Formation of Nasal Cavity

- Bilateral nasal pits bounded by lateral nasal process and medial nasal process.

- Deepening of pits form the nasal cavity which is initially separated from the oral cavity

by the oronasal membrane. This membrane ruptures causing the confluence of nasal cavity with oral cavity.

- Division of Nasal Cavity into Two Halves: The medial nasal prominences merge which

results in a nasal septum separating two nasal cavities. Prominences also form vomer bone upon which nasal septum rests.

- Rostrally, medial nasal septum fuses with maxillary process and thus forms incisive bone

Formation of the Nasolacrimal Duct

- Lateral Nasal Process fuses with Maxillary Prominence. During fusion a line of ectoderm persists which later grows and canalizes and thus forms a nasolacrimal duct - medial canthus to nasal cavity

Separation of Nasal Cavity from Oral Cavity

- Palatine processes of the maxilla grow medially and caudally to form hard palate and soft palate

Derivatives of Embryonic Facial Structures

See Table 15-2

Development of Oral Structures

Upper Lip and Gingiva: Medial part of medial nasal prominence, lateral maxillary prominence

 

Lower Lip and Gingiva: Mandibular prominence

Cheeks: Lateral maxillary and mandibular prominences

Salivary Glands

Epithelial Part: Ectoderm/endoderm

Connective Tissue: Mesenchyme of branchial arch I

Pituitary Gland: Will be discussed under endocrine glands

Tongue

- Two lateral swellings (from mandibular arch).

- Medial tuberculum impar (tubercle between arch I & 11).

- Proximal lingual swelling (enlargement from III & IV branchial

(Copula) arches)

- Body of tongue, from lateral and medial swelling (Ectoderm)

- Root of tongue, from copula (Endoderm)

 

Muscles of Tongue: Occipital somites

C.T. Branchial mesenchyme

 

Innervation of Tongue

Part Origin Nerve

Body Branchial arch I Trigeminal

Root Branchial Arch III +IV Glossopharyngeal + ----------------------------------------------------------------------------------------------------Vagus

Muscles Occipital Somites Hypoglossal

Taste Buds (Sensory) Facial

 

Development of Teeth

Enamel Ameloblasts Inner Enamel Epith. Ectoderm

 

Dentin Odontoblasts Pulp Cavity Neural Crest or

----------------------------Mesenchyme

Pulp Cavity -------- -------- Mesenchyme

Cementum Cementocyte Dental Sac Mesenchyme

Periodontal Cementocyte Dental Sac Mesenchyme

Alveolar Bone

 

Pharynx and Branchial Apparatus

- Branchial apparatus consists of 6 bilateral branchial arches. But only 4 arches become

visible in higher animals.

- Each succeeding pair of arches is separated caudally by a pharyngeal pouch (endoderm)

and branchial groove (ectoderm) combination.

- Interfacing epithelia between the groove and the pouch form the branchial plate.

- In fish, plates break and form gill slits.

- In mammals, they break down momentarily and then reform.

- Derivatives of the Pharyngeal Pouches (see Table 7-5, Latshaw)

- Each arch contains an aortic arch and a nerve.

Only the first branchial groove (cleft) persists in mammals - it forms the external auditory meatus. The remaining branchial grooves form a bilateral cervical sinus which disappears with further development. Branchial grooves may persist as vestiges known as branchial fistulas or lateral cervical cysts.

Musculoskeletal Derivatives of Branchial Arches (see Table 7-4, Latshaw).

Derivatives of the Branchial Grooves:

- Branchial groove-1: Auricle, ext acoustic meatus.

Anomalies of the Face, Mouth, and Pharynx

1. Cleft Lip

- Rare in domestic animals

- Cause: Lack of fusion of medial nasal prominence with each other (middle cleft) or with the maxillary prominence (lateral); one side or bilateral. May be associated with cleft palate.

- Note: Medial cleft in sheep and incomplete cleft in carnivores are normal.

2. Cleft Palate

- Defect in the incisive bone due to the failure of medal nasal prominences to fuse (primary).

- Failure of the palatine processes of maxilla to fuse (secondary).

3. Facial Cleft

- Extends from nasal cavity to medial canthus of eye.

- Caused by failure of maxillary prominence to fuse with lateral nasal prominence (rare) .

4. Atresia of Nasolacrimal Duct

- Failure of epithelial cells between maxillary and lateral nasal prominences to canalize.

- Result: Overflow of lacrimal gland secretion

5. Agnathia (absent) and Brachygnathia (subnormal) Inferior

- Subnormal development of lower jaw.

6. Brachygnathia Superior

- Subnormal development of the upper jaw

7. Supernumery Teeth, Adontia (no teeth), Enamel Hypoplasia

8. Tongue Abnormalities: Rare

9. Branchial Cysts: Found under skin in the pharyngeal region. Failure of branchial goove II or other grooves to obliterate. (Same as cervical cyst.)

10. Thyroglossal cyst

11. Bird tongue - seen in bassett hound. Results from lack of development of lateral lingual primordia.

DEFINE THE FOLLOWING:

patent

stenosis

achalasia

vestigial

cyst

sinus

fistula

atresia

aberrant

ectopic

SHOULD BE ABLE TO EXPLAIN -

Reasons for innervation of the tongue from four (really 5) sources

Reasons for the digastric muscle to get innervation from 2 sources

Reasons for thymic tissue in the cervical area

Reasons for the presence of parathyroid tissue under the capsule of the thyroid gland (NORMAL)

Reasons for the presence of thymic tissue under the capsule of the thyroid gland (ABNORMAL)

Reasons for the presence of parafollicular cells in the thyroid gland

Reasons for the presence of foramen cecum between the body and the root of the tongue

Reasons for the presence of a thyroglossyl cyst or fistula in the middle of the neck or in the path of the thyroid gland

TABLE 7-1 Derivatives of Embryonic Facial Structures

 

Embryonic Structure Mesenchymal/Skeletal Other Derivatives

Derivatives

Frontal prominence Frontal bone Forehead area

 

Lateral nasal prominence Nasal and lacrimal bones Sides and alae of nose

 

Medial nasal prominence Vomer and incisive bones Medial part of upper lip

and gingiva; nasal septum (including part of ethmoid bone)

 

Maxillary prominence Maxillae, palatine, Upper lip, gingival, and

pterygoid. zygomatic cheek region

and squamous part of

temporal bones

 

Mandibular prominence Mandible Lower lip, gingiva and

cheek region

 

 

TABLE 7-4 Musculoskeletal Derivatives of Branchial Arches amd their Innervation

 

Branchial

Arch Muscular Derivative Skeletal Derivative Cranial Nerve

I Rostral part of digastric. Mandible, maxilla, malleus, Trigeminal

(Mandibular) Muscles of mastication incus, pterygoid. Palatine, (V)

Tensor tympani squamous temporal,

Tensor of palate zygomatic

 

II Muscles of facial Stapes: Bulla/tympanica Facial

(Hyoid) expression Stylo-, tympano-, (VII)

Caudal digastric epi., and ceratohyoid

Stapedus bones;

Lingual process and part

of basihyoid

 

III Stylopharyngeal Caudal part of basihyoid, Glossopharyngeal

Other pharyngeal? thyrohyoid, and laryn- (IX)

geal cartilages

 

IV-VI Pharyngeal constrictors Laryngeal cartilages Vagus (X)

(IV)

Laryngeal muscles

 

 

TABLE 7-5 Derivatives of the Pharyngeal Pouches

 

Pouch Derivative

I

Dorsal Part Auditory tube

Tympanic cavity (includes gutteral

pouch in horse)

 

Ventral part None, obliterated by tongue growth

 

II

Dorsal part Crypt of palatine tonsil? (none in horse)

 

Ventral pan None. - obliterates

 

III

Dorsal part External parathyroid (Parathyroid III)

 

Ventral part Reticular cells of thymus

 

IV

Dorsal part Internal parathyroid (Parathyroid IV).

None in pig

 

Ventral part None. (thymic remnant in thyroid?)

 

V

Ultimobranchial Thyroid parafollicular cells**

body

 

**May actually be from neural crest.