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Flashcards in Embryo VI Deck (84)

Where is the pharyngeal apparatus located?

in the lateral walls of the developing pharynx


What are the three components of the pharyngeal apparatus?

pharyngeal arches, pharyngeal grooves (clefts), and pharyngeal pouches.


The four pharyngeal grooves (on each side) are covered by what?

general ectoderm


The four pharyngeal pouches (on each side) are covered by what?



What germ layer are the pharyngeal arches composed of?

pharyngeal arch mesoderm

note that the pharyngeal arches are another important site for the migration of neural crest cells.


The pharyngeal mesoderm/neural crest cells of each of the pharyngeal arches form what three major components?

1. A central rod of pre‐cartilaginous mesenchyme that will give rise to adult skeletal structures including bone/cartilage/connective tissue. Migration of neural crest cells to this region is essential to the formation of these skeletal structures.

2. A major artery, referred to as an aortic arch

3. Mesoderm that will form skeletal muscles of head and neck (BE muscles)

There is also a cranial nerve associated with each arch (note however that the CN does not develop from the arch, these are derived from neuroectoderm).


At what stage in development will the mesodermal layer have differentiated into paraxial (somite) mesoderm, intermediate mesoderm, and lateral plate mesoderm?

5th week


By the 5th week, the somite (paraxial) mesoderm has further re‐organized itself into a series of mesodermal blocks called?

somites. These appear in pairs along either side of the midline in a cranial to caudal fashion


More cranially, the somites begin to thin out and are referred to by the term ______.

occipital somites


What structures will the occipital somites give rise to?

they migrates to the floor of the mouth and oral cavity, and will give rise to skeletal muscles of the tongue and the seven extraocular muscles of the orbit.

The remaining skeletal muscles of the head will develop from the mesoderm of the pharyngeal arches of the pharyngeal apparatus


The adult structures derived from occipital somite mesoderm receives motor innervation from what?

These muscles receive GSE innervation from cranial nerves III, IV, VI, and XII.


Which aortic arches regress and do not form any adult structures?

only the 5th


What does the 3rd aortic arch give rise to?

the major arterial system of the neck, the right and left common carotid arteries and most of the right and left internal carotid arteries.


The cranial nerve associated with each pharyngeal arch is derived from which germ layer?

neuroectoderm. The nerves DO NOT develop from the arch itself. However, it is true that the skeletal muscles derived from a particular arch are innervated by the CN associated with that arch.


What do the neural crest cells of the 1st pharyngeal arch make?

involved in the development of the major bones of the anterior face (mandible, maxilla, zygomatic bones, part of temporal bone) and two of the small bones of the middle ear (malleus, incus)


What do the malleus and the incus derive from specifically?

a structure called Meckel’s cartilage.


The mesoderm of arch 1 develops what structures?

the four muscles of mastication (masseter, temporalis, lateral pterygoid, medial pterygoid) plus the mylohyoid, anterior digastric, tensor tympani, and tensor veli palatine (MAST‐MATT)

The role of arch 1 mesoderm in blood vessel development is very small. It give rise to a few small branches of vessels.


The first arch is associated with which cranial nerve?

the mandibular nerve (CN V3)


Defects in the development of arch 1, will result in what?

facial abnormalities and/or difficulty chewing.


What do the neural crest cells of the 2nd pharyngeal arch make?

stapes of the middle ear, styloid process, lesser horn of the hyoid, and the upper body of the hyoid

Note: the three bones of the middle ear play an important role in conduction of sound and amplification of sound. NC defects associated with Arches 1 & 2 can give rise to hearing problems in the newborn. i.e. conductive hearing loss.


The mesoderm of arch 2 develops what?

the muscles of facial expression along with the stapedius, stylohyoid, and posterior digastric

The mesoderm of the second arch does not play a significant role in aortic arch development


What are the muscles of facial expression responsible for?

primarily to closing the eye and moving the mouth


What is the role of the stapedius?

The stapedius has an important role (along with the tensor tympani) in protecting the inner ear from loud noises. They inhibit sound by stilling the stapes and the eardrum itself


The 2nd arch is associated with which cranial nerve?

facial n.


What do the neural crest cells of the 3rd pharyngeal arch make?

greater horn of hyoid and lower part of the body of the hyoid bone


The mesoderm of arch 3 develops what?

muscles: stylopharyngeus

aortic arch: right/left common carotid arteries and right/left internal carotid arteries


The 3rd arch is associated with which cranial nerve?

glossopharyngeal nerve (CN IX)


What do the neural crest cells of the 4th pharyngeal arch make?

thyroid cartilage


The mesoderm of arch 4 develops what?

muscles: the muscles of the soft palate (uvula), and the remaining five muscles of the pharynx (the constrictors and the other two longitudinal muscles). The muscles of this arch are critical for swallowing

aortic arches: right subclavian artery and aortic arch


The 4th arch is associated with which cranial nerve?

superior laryngeal branch of vagus


What do the neural crest cells of the 6th pharyngeal arch make?

laryngeal cartilages


The mesoderm of arch 6 develops what?

muscles: muscles of the larynx, and the striated muscles of the upper esophagus. These muscles are important in vocalization.

aortic arches: the right pulmonary artery (from the right 6th arch) and the left pulmonary artery and the ductus arterious (from the left 6th arch)


The 6th arch is associated with which cranial nerve?

the recurrent laryngeal branch of the vagus nerve (CN X)

Take note of the fact the vagus nerve innervates three major groups of muscles (laryngeal, pharyngeal, & soft palate) that are involved in vocalization, swallowing, and uvula movement


Defects are most common with which pharyngeal arch?



What is first arch syndrome caused by?

First arch syndrome usually results from failure of NC cells to migrate to the first pharyngeal arch, causing facial abnormalities


What are the two of the most common presentations of first arch syndrome?

Treacher Collins syndrome and Pierre Robin sequence


What is Treacher Collins syndrome characterized by?

underdevelopment of the zygomatic arch, hypoplasia of the mandible,
downward slanting palpebral fissues, and malformed, malpositioned ears


What can Treacher collins be caused by?

may be inherited as an autosomal dominant trait or may be caused by exposure to teratogens. NC defect!


What is Pierre Robin syndrome characterized by?

underdevelopment, primarily, of the mandible. It may be associated with a cleft palate and a posteriorly placed tongue (glossoptosis). It may be due to either genetic or environmental factors. NC defect!


What does pharyngeal pouch 1 give rise to?

epithelial lining of the auditory tube and the middle ear


What does pharyngeal pouch 2 give rise to?

epithelial lining of the crypts of the palatine tonsils


What does pharyngeal pouch 3 give rise to?

secretory cells of the inferior thyroid gland and the thymus gland

Pouches 3 and 4 have dorsal and ventral divisions


What does pharyngeal pouch 4 give rise to?

secretory cells of the superior parathyroid gland and the ultimobranchial body. The ultimobranchial body gives rise to the parafollicular (C) cells of the thyroid gland. This is dependent upon NC cell migration to pouch 4.

Pouches 3 and 4 have dorsal and ventral divisions


The classic developmental defect of pharyngeal pouch development is?

DiGeorge anomaly


What causes DiGeorge anomaly?

arises when pouches 3 and 4 fail to differentiate in the absence of NC cells


What does DiGeorge anomaly result in?

-thymic hypoplasia (severe immunodeficiency),

-failure of C cells to develop (defects in calcium metabolism),

-cardiovascular defects (persistent TA or T of F), and

-facial defects (shortened philtrum, low set ears).


What happens to the 2nd-4th pharyngeal grooves?

In humans, the 2nd, 3rd, and 4th grooves get filled in with other tissues and smooth over, resulting in the smooth appearance of the human neck (i.e. no appearance of gills in humans)


The first pharyngeal groove is retained, and it gives rise to what?

the external auditory meatus and external auditory canal.

Note the dual development of the auditory canal from ectoderm of the 1st groove and endoderm of the 1st pouch.


A potential space is captured by the filling in of the lower grooves. What is this called?

the lateral cervical sinus


What are Cervical (branchial) cysts?

can form when the lateral cervical sinuses of the pouches become filled with fluid and enlarge

These fluid‐filled masses on the lateral neck line up in a characteristic position along the anterior border of the SCM. They are commonly under the angle of the mandible and usually don’t enlarge until later in life.

The cervical cyst may also open externally as a cervical (branchial) fistula


What does the thyroid gland develop from?

develops from endoderm, but not endoderm of one of the pouches. The thyroid gland develops from an endodermal thickening in the midline floor of the developing oral cavity (at the foramen cecum). It must then migrate to its definitive position surrounding the upper tracheal cartilages.


How does the thyroid gland reach its final position in the neck?

It descends the anterior midline of the neck, following along a structure called the thyroglossal duct to its final position. Once the thyroid gland reaches its final position, the thyroglossal duct normally closes down.


Developmental defects associated with the thyroglossal duct include?

the presence of ectopic thyroid tissue and the formation of thyroglossal duct cysts.


Where are Ectopic thyroid tissue or thyroglossal cysts usually found?

in the floor of the mouth (foramen cecum), near the hyoid bone, or near the thyroid cartilage

Before removing ectopic thyroid tissue, you have to make certain that there is functional thyroid tissue in the proper location. Sometimes the ectopic tissue is all that develops.


Where are thyroglossal duct cysts found and how are they differentiated from cervical (branchial) cysts?

location in the anterior midline of the neck. They are differentiated from cervical (branchial) cysts by location.


The tongue is divided into three parts. What are they?

the anterior two‐thirds (the body), the posterior one‐third, and the root of the tongue (or epiglottic region)


What marks the boundary between the anterior 2/3rds and the posterior 1/3rd?

The sulcus terminalis (a V‐shaped line)


Which pharyngeal are related to the development of the mucosa of the anterior two‐thirds of the tongue?

Arches 1 and 2. Arches 3 and 4 are related to development of the mucosa of the posterior one‐third of the tongue and the epiglottic region.


The mucosa of the anterior two‐thirds of the tongue are initially represented by three masses of mesoderm in the pharyngeal floor that derive primarily from arch 1. What are these three masses and how are they relevant to the development of the anterior 2/3rds of the tongue mucosa?

the two lateral lingual swellings and the midline tuberculum impar. The two lateral lingual swellings increase in size and overgrow the tuberculum impar. They eventually merge in the midline, forming the mucosa of the anterior two‐thirds of the tongue. The tuberculum impar is not important to tongue development.


What structure of arch 3 gives rise to the mucosa of the posterior one‐third of the tongue?

The copula


What structure of arch 4 gives rise to the mucosa of the root of the tongue?

the hypobranchial eminence

So, for development of the mucosa of the tongue, the main players are the lateral lingual swellings (arch 1), the copula (arch 3) and the hypobranchial eminence (arch 4).


Skeletal muscles of the tongue are derived from what?

myoblasts that originated from the mesoderm of the occipital somites.


General sensation (pain, temperature, etc.) to the anterior two-thirds of the tongue is via?

the lingual branch of CN V3 (mandibular)

Taste (special sensation) is conveyed via the chorda tympani branch of CN VII (facial). CN V3 and CN VII are the nerves of arches 1 and 2, respectively.


Both general sensation and taste of the posterior 1/3rd of the tongue are conveyed via?

branches of CN IX (glossopharyngeal). CN IX is the nerve of the 3rd pharyngeal arch.


Both general sensation and taste of the root of the tongue are conveyed via?

The mucosa of the root of the tongue derived from the hypobranchial eminence of arch 4.

Both general sensation and taste are conveyed via branches of CN X (vagus). CN X is the nerve of the 4th pharyngeal arch.


GSE innervation of the skeletal muscle of the tongue (derived from the mesoderm of the occipital somites), is via which CN nerve?

CN XII (hypoglossal).


When does facial development begin and go through?

Facial development begins in the latter 4th to 5th week and continues through the 10th week of development


The entire skeletal structure of the face is developed from the mesoderm and neural crest cells of which pharyngeal arch?

arch 1.


Arch 1 forms which five facial prominences (or swellings) that are comprised of mesoderm and neural crest cells?

1. A single frontonasal prominence
2. Right and left maxillary prominences
3. Right and left mandibular prominences


What is the first part of the face to form? How?

The right and left mandibular prominences come across the face and fuse in the midline, forming the lower face (mandible, all of the lower lip)


What does the frontonasal prominence form?

the upper face (top of cranium, forehead down to the bridge of the nose).


What structures make the nasal cavity?

On each side of the lateral mass of the frontonasal prominence, there is a surface thickening of ectoderm called the nasal pit or the nasal placode. The nasal pits are formed by an invagination of surface ectoderm


On the surface of the frontonasal prominence, each nasal pit is surrounded by two elevations of mesoderm/NC called?

the medial and lateral nasal prominences.


How does the middle face (and upper lip) develop?

The growth of the maxillary prominences pushes them toward the midline, forcing the medial and lateral nasal prominences more medially.

The two medial prominences eventually fuse in the midline, forming the philtrum of the upper lip, the crest of the nose, and the tip of the nose. The upper lip is completed by the fusion of the (maxillary prominences with the medial nasal prominences. The lateral part of the upper lip is formed from the maxillary prominences, the philtrum is formed by the medial nasal prominence.

The lateral nasal prominences will form the lateral sides of the nose. The nasolacrimal duct (for tear drainage) will form in the junction between the maxillary prominence and the lateral nasal prominence.


What structures do the mandibular prominences form?

fuse to form the lower jaw and lower lip


What structures does the frontonasal prominence form?

forms the forehead, top of the cranium, upper eyelid, and the bridge of
the nose.


What structures do the medial nasal prominences form?

fuse to form the philtrum of the upper lip, tip of the nose, crest of the nose, and intermaxillary process.


What structures do the lateral nasal prominences form?

form the lateral sides (alae) of the nose


What structures do the maxillary prominences form?

form the central face, lower eyelid, and upper lateral lip

The maxillary prominences and the medial nasal prominences are involved in the formation of the hard palate as well


What causes cleft lip?

Cleft lip results when the maxillary prominence fails to fuse with the medial nasal prominence. These may be unilateral or bilateral. Cleft lip may or may not be associated with cleft palate.


What developmental structures play a role in development of the hard palate?

The maxillary and medial nasal prominences. These are the same two tissues that formed the upper lip


The medial nasal prominences form what part of the hard palate?

the intermaxillary segment (primary palate). The intermaxillary segment consists of a wedge‐shaped piece of bone (part of the maxilla) surrounding the central incisors, and the gum tissue surrounding them.


The maxillary prominences form what part of the hard palate?

form the palatine shelves which comprise the remainder of the hard palate (secondary palate) by fusing with one another and with the intermaxillary segment.


What causes cleft palate?

Cleft palate occurs when the palatine shelves fail to fuse with one another in the midline, or fail to fuse with the intermaxillary segment.

Cleft palates can occur in various locations as indicated on the right (bifid uvula, complete cleft, partial cleft, etc.).

Often cleft palate will occur in common with cleft lip since both structures derive from the same source.