Note: The head consists of a cranium and a face. The cranium, which contains the brain and pharynx,
develops first. The face, which contains the mouth and nasal cavity, grows outward from the
cranium. Because the face develops separately from the brain, it can vary greatly among breeds.

Face (overview) — outward growth produces upper & lower jaws, the nose, and the forehead
• the first pharyngeal arch forms the lower jaw & most of the upper jaw via growth of two processes:
- the mandibular process forms the mandible and soft tissue of the lower jaw; right & left
mandibular processes fuse rostrally to form the mandibular symphysis
- the maxillary process forms the upper jaw caudal to the incisor bone & teeth
• dorsal to the first pharyngeal arch, a frontonasal prominence grows outward and divides into:
- the frontal prominence, which forms frontal bone and forehead; and
- the medial and lateral nasal processes, which form nose and primary palate (incisors)

About pharyngeal arches — pharyngeal arch (mammal) = branchial arch (fish; branchial = gills)
• bilaterally, six pharyngeal arches develop, but only the first three are visible in mammals
(pharyngeal arch V atrophies early and arches IV & VI merge)
- externally, each arch is demarcated by a pharyngeal cleft (groove)
- internally, each external cleft is apposed by a pharyngeal pouch
• the mesenchyme within pharyngeal arches (and the frontonasal prominence) is ectomesenchyme,
which is derived from neural crest; ectomesenchyme forms intramembranous bone and fascia of the face and cranium
• skeletal muscle of the head is derived from either somite or somitomere myotomes that migrate into
pharyngeal arches or the frontonasal prominence
• in general, each pharyngeal arch receives innervation from one cranial nerve which innervates all
structures derived from the arch
Note: Somitomeres are less developed than somites. They originate from paraxial mesoderm located
rostral to the notochord (which induces somite formation). There are seven pairs of somitomeres.
Their myotomes migrate to the orbit or to pharyngeal arches and give rise to skeletal muscles
innervated by cranial nerves 3 - 9 (extra-ocular, masticator, facial, plus one pharyngeal muscle).
(Myotomes from occipital somites give rise to skeletal muscles innervated by the vagus &
hypoglossal cranial nerves: tongue, pharynx, larynx, & esophageal muscles.)

Pharyngeal pouch derivatives — these include: middle ear, auditory tube, & palatine tonsil fossa
• each pharyngeal pouch is a lateral evagination of pharyngeal endoderm that is apposed to the
ectoderm of the corresponding pharyngeal cleft; clefts separate adjacent pharyngeal arches
(in fish, the apposed endoderm-ectoderm at a branchial cleft degenerates, becoming a gill slit)
• there are five bilateral pharyngeal pouches (in mammals the fifth pair is rudimentary)
• derivatives of pharyngeal pouches include:
- tympanic cavity (middle ear) and auditory tube [first pouch]
- fossa for the palatine tonsil [second pouch]
- external parathyroid gland and thymus [third pouch]
- internal parathyroid gland [fourth pouch]
- parafollicular cells of thyroid gland [fifth pouch]
• the thyroid gland develops from endoderm on the floor of the pharynx; depending on species, the
thyroid may remain single (pig) or split into bilateral lobes connected by an isthmus (horse)
or become separate paired lobes (dog)
Note: The thyroid gland originates as a diverticulum connected to the pharynx by a duct.
The duct degenerates because the thyroid is an endocrine gland. Rarely a remnant of the duct
persists as a cyst that can enlarge and interfere with breathing by compressing the pharynx.


Nasal cavity — develops by outgrowth of nasal processes from the frontonasal prominence
• initially, a nasal placode (ectoderm thickening) appears bilaterally at the rostral end of the
frontonasal prominence.
• medial and lateral nasal processes surround each nasal placode and grow outward creating a nasal pit
• continued growth of nasal pits (plus oronasal membrane erosion) produces a primitive nasal cavity
that communicates with the oral cavity (formed by maxillary & mandibular processes)

• the bilateral rostral openings of the nasal cavity are external nares (nostrils) and ectomesenchyme
surrounding them forms cartilage of the nose; each lateral nasal process give rise to
alar cartilage of the nose, nasal bone and lacrimal bone
Note: The nasolacrimal duct is formed by ectoderm along the seam where the lateral nasal process
meets the maxillary process
• right & left medial nasal processes fuse, forming a primary palate rostrally and nasal septum caudally:
- the primary palate gives rise to the incisive bone, upper incisor teeth and the rostral upper lip
- the nasal septum consists of bone, cartilage, and a patch of soft tissue membrane that separates
right & left halves of the nasal cavity
• the following features develop in association with the nasal cavity:
- a concha is a mucosal covered scroll of thin bone that grows into the nasal cavity; dorsal & ventral
conchae originate bilaterally as cartilaginous ridges from lateral wall bones of the nasal cavity
- vomeronasal organ is an olfactory sense organ located rostrally in the floor of each nasal cavity;
the organ is produced by an outgrowth of nasal epithelium that forms a caudally-closed tube
- paranasal sinuses arise as epithelial diverticula from the lining of the nasal cavity; the extent of
sinus development varies with species
Note: Most paranasal sinus development occurs postnatally. Newborn animals have cute, rounded heads
which become angular with age as paranasal sinuses develop during head growth

Palate — palates separate nasal cavity from oral cavity & nasopharynx from oropharynx
• the primary palate, formed first by medial nasal processes, becomes incisive bone, upper
incisor teeth and the rostral upper lip
- failure of medial nasal processes to fuse (cleft primary palate), produces hare lip (cheiloschisis)
and often incisor bone & teeth defects (a hare lip is normal in hares, sheep, etc.).
• the secondary palate is formed by medial extensions of maxillary processes (palatine processes);
the bilateral extensions meet at the midline, merging dorsally with nasal septum and rostrally
with primary palate
• the secondary palate (hard palate) partitions the oral cavity from the nasal cavity;
an extension of the secondary palate (soft palate) grows caudally into the pharynx and
divides the rostral pharynx into dorsal (nasopharynx) and ventral (oropharynx) chambers
• the typical cleft palate is the result of failure of the secondeary palate to close along the midline, leaving a gap or cleft that is often fatal in animals due to inability to suckle or because of aspiration of milk into the lungs (aspiration pneumonia)

Note: A cleft palate involving the secondary palate is more common than that of the primary palate.


Mouth (oral cavity) — develops during upper and lower jaw formation
• the first evidence of a mouth is the stomodeum, created during upper and lower jaw formation
• the stomodeal cavity is lined by ectoderm and the deep wall of the stomodeum (oropharyngeal
membrane) is composed of surface ectoderm apposed to endoderm (rostral wall of the pharynx);
• the oropharyngeal membrane becomes fenestrated and disappears, allowing the stomadeum to
communicate with the pharynx (the adult palatoglossal fold marks the membrane location)

• the stomodeal cavity and nasal pits communicate freely (following degeneration of an initial
oronasal membrane) until the secondary palate develops, when oral-nasal communication is shifted
caudally into the pharynx
• a labiogingival lamina (an arc of thickened ectoderm) forms on the lining of the stomodeal cavity
of both upper and lower jaws; the lamina invaginates into underlying ectomesenchyme, forming
a labiogingival groove which become the vestibule of the mouth
• the labiogingival groove (vestibule) separates lips from gingivae; caudal fusion of upper and lower
lips forms cheeks

Teeth — deciduous and dormant permanent teeth originate simultaneously in the embryo
• a dental lamina (arc of thickened ectoderm) forms inside the labiogingival lamina on stomodeal
surfaces of both upper and lower jaws; periodically laminar cells proliferate & invaginate forming
dental buds (in the case of deciduous teeth, an additional bud for the permanent tooth develops
superficial and medial to each deciduous tooth bud)
• each dental bud develops into a tooth in the following way:
- the bud assumes a cup-shaped configuration becoming an enamel organ; condensation of
ectomesenchyme within the concavity of the cup forms a dental papilla
- the concave epithelial layer of the enamel organ induces dental papilla to form an epithelial layer
of odontoblasts that deposit the dentin of the tooth
- the odontoblasts induces the concave epithelium of the enamel organ to differentiate into
ameloblasts that form the enamel that covers the dentine of the crown of a typical tooth
- a dental sac, which condenses around the enamel organ & dental papilla, gives rise to three layers:
1] outer cells differentiate into osteoblasts that deposit bone of the alveolus (tooth socket)
2] the dental sac middle layer forms periodontal ligament (which anchors the tooth)
3] inner cells of the dental sac become cementoblasts, producing cementum (modified bone)
which adheres to the dentin surface of the root of a typical tooth
Note: Osteoclasts must re-absorb encasing superficial bone in preparation for tooth eruption.


Tongue — develops from four swellings on the floor of the pharynx
• the body & apex of the tongue are formed by paired distal (lateral) swellings that fuse along the
midline and grow forward into the oral cavity, acquiring an ectodermal coat from the stomadeal
lining; the body of the tongue arises predominantly from the first pharyngeal arch, the second arch
also contributes [general sensation is provided by the trigeminal nerve (V); taste sensation is
provided by the facial nerve (VII)]
• the root of the tongue is formed by the proximal swelling and is covered by endoderm; it arises from
the third pharyngeal arch [sensation is supplied by the glossopharyngeal nerve (IX)]
• the median swelling contributes significantly to tongue development only in ungulates (in cattle it
forms a prominent bulge on the dorsum of the tongue)
• both intrinsic and extrinsic muscles of the tongue originate from occipital somites [innervated by
the hypoglossal nerve (XII)]

Salivary glands — water-like or mucous-like secretion pending innervation and the particular gland
• salivary glands are derived from ectoderm (parotid, zygomatic, labial & buccal) or endoderm
(mandibular and sublingual).
• the process by which salivary glands are formed is typical of exocrine gland development in general:
- localized proliferation of surface epithelial cells produces a cellular cord that invades underlying
ectomesenchyme; the initial site of proliferation becomes the duct opening to the surface
- the invading cord of cells begins to branch, ultimately becoming the main duct and branched ducts
of the gland
- enlarged masses of epithelial cells accumulate at the ends of terminal branches, each forming a
secretory acinus of the gland
- the epithelial cords and terminal enlargements canalize (become hollow) so the gland can become
functional (canalization developmental defects can result in duct atresia and cystic glands)
- growth of the jaw re-positions surface openings and causes main duct elongation
Note: A polystomatic gland (e.g., sublingual) is one that has many duct openings to the surface. Such
glands arise as a series of independent epithelial cords. Although they are independent glands,
they appear to form a single mass and in gross anatomy they are collectively identified as a
single gland.


Adenohypophysis—the hypophysis (pituitary gland) consists of: neurohypophysis & adenohypophysis
• the adenohypophysis develops from an ectodermal thickening (placode) in the roof of the stomodeum
• the placode evaginates, forming a hypophyseal pouch (Rathke’s pouch); the pouch separates from
stomadeal ectoderm and wraps around the neurohypophysis, an outgrowth of the hypothalamus
Note: Both neurohypophysis & adenohypophysis are controlled by the hypothalamus of the brain.
The neurohypophysis is connected to the hypothalamus by means of an infundibulum that
contains axons from hypothalamic neurons. Hypothalamic neurons release molecules
into the blood stream to control release of hormones by the adenohypophysis.