Flashcards in Pharyngeal Apparatus and Face Deck (77):
What are pharyngeal arches?
A series of four bulges along the foregut (pharynx) in the neck region of the embryo.
Arches + pouches + clefts = pharyngeal apparatus
What are pharyngeal clefts, grooves, and pouches?
Cleft - ectoderm in groove, caudal to corresponding arch
Groove - Invaginations between arches
Pouch - outpockets of pharyngeal endoderm that lies close to to cleft in the grooves (making a membrane). Numbered for the arch cranial to them
What is a pharyngeal membrane?
Wherever ectoderm and endoderm are closely associated, as in at grooves of the pharyngeal apparatus (cleft + pouch are in close proximity)
How many pharyngeal arches are there, and how are they numbered?
There are 5 total, numbered 1-6 (there is no 5th arch in humans). They run cranial to caudal. The arches bulge into the pharynx as well, giving them a structure like a donut segment
What are the internal components of a pharyngeal arch?
1. Aortic arch - artery
2. Muscle - derived from paraxial mesoderm which migrates in during week 3
3. Skeletal - originates from cartilage (neural crest cells)
5. Mesenchymal connective tissue - neural crest cells which undergo EMT to become mesoderm
What are the subdivisions of the first pharyngeal arch and what do they outline?
1. Maxillary prominence (Cranial)
2. Mandibular prominence (Caudal)
Outlines the stomodeum (primitive mouth)
What muscles are formed by arch 1? What innervates them?
Muscles of mastication, mylohyoid, anterior belly of digastric, tensor tympani and tensor palatini
All sensory / motor innervation of arch 1 comes from trigeminal nerve (CNV)
What bones form by endochondral ossification from arch 1? What is the function of the rest of the cartilage?
Malleus and incus only
Rest of mandibular cartilage just functions transiently to reinforce the developing strucutures
What bones from arch 1 form by intramembranous ossification?
Palantine, maxilla, mandible, zygomatic arch, part of temporal bone
What is the second pharyngeal arch called and what muscles develop from it?
Muscles of facial expression, posterior belly of digastric, stylohyoid, stapedius (to stapes)
all innervated by Facial nerve (CN7)
What are the skeletal derivatives of the second pharyngeal arch? What type of ossification is it?
Stapes, and all bones associated with stylohyoid muscle, including:
Styloid process of temporal bone, stylohyoid ligament, lesser horn and upper part of body of hyoid bone
All endochondral ossification!
What is the third pharyngeal arch called and what muscle arises from it?
Innervation: Glossopharyngeal nerve (CN9)
What skeletal components arise from the third pharyngeal arch, and by what process?
Lower part of body and greater horn of hyoid bone
(All but most of arch 1 is EO)
What muscles are derived from the 4th + 6th arches?
Pharyngeal constrictors (innervation: CN10 - pharyngeal and superior laryngeal branches) and intrinsic muscles of the larynx (innervation: CN10 - recurrent laryngeal)
All innervated by vagus nerve
What skeletal components do the 4th + 6th arches give rise to?
Just the laryngeal cartilages - thyroid + cricothyroid
What is the function of the first pharyngeal pouch (endoderm)?
Elongates to form auditory tube, middle ear epithelium, and inner surface of tympanic membrane
What is the function of the second pharyngeal pouch (endoderm)?
Forms epithelium of palatine tonsils
What is the function of the third pharyngeal pouch (endoderm)?
Dorsal portion - inferior parathyroid gland
Ventral portion - thymus (immune organ), epithelial reticular cells
How does the third pharyngeal pouch migrate?
Migrates caudally with the thymus, apparently guiding the inferior parathyroid gland to its position inferior to the superior parathyroid gland.
What is the function of the fourth pharyngeal pouch (endoderm)?
Dorsal portion - superior parathyroid
Ventral portion - Parafollicular cells of thyroid
What is the function of the first pharyngeal cleft (ectoderm)?
Elongates to form external auditory meatus and outer lining of tympanic membrane
What is the cervical sinus?
It is the fate of ectodermal clefts 2-4, as Arch 2 overgrows all these spaces and closes around arch 4 (which gives rise to thyroid cartilages). This overgrowth creates the smooth contours of the neck and should ultimately disappear
What is arch 1 syndrome? What causes it?
Also called Treacher Collins syndrome - small lower jaw, facial defects, malformed ears. It is insufficient neural crest cells to Arch 1. It is genetic, but may be induced by too much retinoic acid (Vitamin A)
What is Di George anomaly?
Anamoly affecting neural crest migration and pouches 3 & 4, some of which is destined for the heart.
Leads to immune deficiency (thymus), hypocalcemia (parathyroid), and heart vessel deformities
What are cervical cysts?
Bulges remaining from cervical sinus which did not obliterate. They are typically placed laterally, just anterior to sternocleidomastoid muscle.
What are cervical fistulas?
When cervical sinus does not obliterate, and the cyst opens to outside world (laterally, anterior to SCM muscle)
What are the lateral lingual swellings derived from?
Two swellings derived from arch 1, around 4 weeks
What is the tuberculum impar?
The median lingual swelling, derived from arch 1
What is the hypopharyngeal eminence derived from?
The most posterior swelling, arises from arches 3-4.
Also called the hypobranchial eminence.
What is the foramen cecum related to?
Shows the boundary of the tuberculum impar (median lingual swelling) and the copula
What is the copula?
The posterior midline swelling, derived from arch 2. Borders the foramen cecum anteriorly
What happens to the lateral lingual swellings?
They increase in size and fuse, to overgrow the tuberculum impar. Since they are derived from arch 1, sensory innervation to anterior 2/3 of tongue is via the lingual branch of CN V.
How do taste buds (fungiform papillae) come to be derived from CN7?
The second pharyngeal arch tissue of the tongue migrates under the tongue as the lateral lingual swellings fuse. Taste buds of anterior 2/3 are provided by chorda tympani (CN7)
What happens to the hypobranchial eminence?
Expands mostly from arch 3, overgrows arch 2, to form posterior 1/3 of tongue.
What separates anterior and posterior tongue?
The sulcus terminalis (between the lateral lingual swellings and the hypobranchial eminence)
What gives sensory / taste bud innervation of posterior 1/3 of tongue?
CN 9 - hypoglossal (derived from arch 3 / hypobranchial eminence) - including circumvallate papillae
How does the epiglottis form?
From arch 4, general sensation and taste buds are innervated by CN10 at this point
Where do the muscles of the tongue come from?
Myoblasts that migrate from the occipital somites. CN12 (hypoglossal) accompanies them during migration and innervates the tongue muscles
What is ankyloglossia?
Literally "fused tongue", it's when the frenulum of the tongue extends to tip of mouth, keeping tongue attached to floor. Extension of tongue is critical for latching / feeding, and needs to be snipped at birth.
What causes a bifid tongue?
When the lateral tongue swellings fail to fuse - fairly rare.
How is the thyroid diverticulum made?
Proliferation of columnar cells in floor of pharynx between arches 1 and 2 (between tuberculum impar and hypobranchnial eminence)
How does the thyroid diverticulum migrate?
Elongates inferiorly, anterior to the pharynx and along MIDLINE, passing anterior to hyoid bone and laryngeal cartilages until reaching trachea.
What is the thyroglossal duct?
Canal which is the top of the thyroid diverticulum. Opens to foramen cecum in tongue and extends to thyroid gland on trachea. It will degenerate and leave behind pyramidal lobe of thyroid + foramen cecum.
What is a thyroglossal cyst?
Remnants of the thyroglossal duct, forms along midline and can happen anywhere the duct was. Most often, just inferior to hyoid bone. (Cervical cysts are lateral, not at midline)
What is aberrant thyroid tissue?
Thyroid gland found anywhere along migratory route of thyroid, but most common around foramen cecum. Don't want to remove because it could be the MAIN thyroid gland of the person.
What is congenital hypothyroidism?
Cretinism - thyroid deficiency leading to retarded growth and mental retardation. You have to check thyroid hormone levels at birth
What lines the stomodeum?
It is the primitive mouth. It is lined with ectoderm which extends inward to the level of the buccopharyngeal membrane.
What lines the buccopharyngeal membrane?
Ectoderm of the stomodeum, and endoderm. The pharynx starts here, and it is part of the foregut.
What is Rathke's pouch? What does it do?
A diverticulum in the roof of the stomodeum. It is encircled by a downgrowth of the diencephalon called the infundibulum. It ultimately separates from the oral cavity to form the epithelial anterior pituitary (adenohypophysis).
What does the infundibulum become?
Neurohypophysis (posterior pituitary), made of neuroectoderm. It's what comes and relates to Rathke's pouch which will form the adenohypophysis from ectodermal epithelial cells of the mouth
What happens after Rathke's pouch associates with the infundibulum?
The sphenoid bone closes off beneath them when the Rathke's pouch is cut, and the pituitary gland now sits in the sella turcica
How do the submandibular and sublingual glands develop?
Ingrowths of the ventral stomodeum ectoderm
How does the parotid gland develop?
Ingrowth of the pharynx ENDODERM
Where do nasal placodes lie around end of week 4? What are they?
On both sides of frontonasal prominence. They are aggregates of placodal ectoderm cells within the surface ectoderm. They change shape due to the migration and proliferation of underlying neural crest / mesenchyme
What is the shape of the nasal placodes?
Lateral and medial nasal prominences surrounding a nasal pit on each side. The lateral nasal prominences touch the maxillary prominences of arch 1 on both sides.
What happens at the junction of the lateral nasal prominence and the maxillary prominence?
The nasolacrimal groove is formed, which runs between the eye and the stomodeum (connects tears to mouth). It will form the NLD and widen dorsally into the lacrimal sac.
What forms the intermaxillary segment?
When the two medial nasal prominences enlarge and fuse medially (~7 weeks)
What are the three components of the intermaxillary segment?
1. Philtrum - labial component
2. Part of upper jaw and gum containing 4 incisor teeth
3. Palatal component producing the triangular primary palate
What is the primary palate?
Triangular formation of the intermaxillary segment. It is continuous with the nasal septum, which is derived from the frontonasal prominence
Where does the primary palate meet the maxillary prominence?
The secondary palate. The triangular primary palate meets both sides when the palatal plates (palatine shelves) of the maxillary prominence fuse. The cleft is obliterated on fusion.
What forms the cheek?
What forms the chin and mandible?
Merging of mandibular prominences
What forms the upper lip?
Two maxillary prominences laterally fuse with the philtrum medially (which is the merged medial nasal prominences, from nasal placode)
What forms the lower lip?
Merged mandibular prominences
What are the palatine shelves? How do they grow initially?
They are parts of the maxillary process that initially grow downward alongside the tongue during week 6. They re-orient to grow horizontally above the tongue to fuse with the primary palate during week 7
What is the junction of the primary and secondary palate?
Incisive foramen - not actually a hole, but would be a hole if there wasn't fusion.
What happens as the palatal shelves fuse?
The downward-growing nasal septum also fuses with them along the midline.
What forms on the posterior midline of where the palatal shelves fuse?
What is lateral cleft lip (and jaw)?
When fusion failure is anterior to incisive foramen. This is caused by poor fusion between medial nasal prominence and maxillary prominence. Multifactorial in etiology, but generally lack of neural crest cells
What is median cleft lip?
Occurs due to failure of merging of two medial nasal prominences
What is oblique facial cleft?
Occurs due to failure of merging maxillary prominence with lateral nasal prominence (NLD opens to space_
What is cleft secondary palate (cleft uvula)?
Failure of fusion of palatine shelves of maxilla. The cleft is posterior to incisive foramen. Increased in frequency due to seizure meds (anticonvulsants)
What is the oronasal membrane?
The thin layer of connective tissue found temporarily between the nasal pits and the oral cavity, while the nasal septum is forming
What is the primitive connecting space left whenever the oronasal membrane breaks down?
The primitive choana - a funnel between the nasal and oral cavities
What are nasal conchae?
Expansions of the nasal cavity walls forming shortly after the oronasal membrane breaks down.
What happens to the primitive choana?
It is displaced anteriorly to posteriorly while the secondary palate forms and the nasal septum fuses , forming the definitive choana (funnel between nose and pharynx)