BSC Embryo 2 Flashcards
what is the overall event happening during week 3
- cells of the epiblast begin to invaginate
what are the 2 waves of gastrulation during week 3
- 1st wave: cells migrate through the streak interdigitate with the hypoblast layer -> ENDODERM
- 2nd wave -> MESODERM
- remaining cells in the epiblast layer for the ECTODERM
what is the name of process occurring with the ectoderm
neurelation
what is neurulation
notochord signals neural tube formation and results in the ectoderm dividing into 3 sets of cells
what are the three sets of cells that the ectoderm divides into
1) neural tube
2) neural crest cells
3) epidermis
what is the highlight of neural crest cells
cranial neural crest leads to cartilage, bone, cranial ganglia & CT of the face
which cells form the mesoderm
cells migrating in the 2nd wave of gastrulation
what forms the notochord*
some mesoderm cells condense to form it
what is the notochord essential for?
guiding adjacent mesoderm cells to differentiate into specialized types
what are the 4 specialized types that adjacent mesoderm cells differentiate into
1) paraxial mesoderm*
2) intermediate mesoderm
3) lateral plate mesoderm*
4) extraembryonic mesoderm
what is the location of the paraxial mesoderm?
closest to the notochord
what structure does the paraxial mesoderm form
forms structures of the head and somites
what are somites?
paired tissue blocks forming adjacent to the notochord
how often are somites formed? how many are there total?
~ 3 pairs form per day from cranial to caudal
42-44 pairs total
what are the resulting structures of the paraxial mesoderm
- vertebrae, ribs, rib tendons*
- muscles of the back, ribs, and limbs*
- parts of the dermis*
what is the main event during head and neck development
FORMATION OF PHARYNGEAL APPARATUS
- CONSISTS OF PHARYNGEAL (BRANCHIAL) ARCHES, POUCHES, AND CLEFTS
when does the pharyngeal apparatus appear?
4th- 5th week of development
what are the prominences and layers of the pharyngeal apparatus
- 5 paired prominences
- 3 layers: an outer covering of ectoderm, an inner covering of endoderm, and a middle core of mesenchyme*
where do the pharyngeal clefts derive from
ectoderm
where do pharyngeal pouches arise from
endoderm
where do pharyngeal arches arise from
mesoderm
how many pharyngeal arches are there? what’s the catch?
- 6 arches
- no 5th arch! the 5th arch is transient. it involutes and produces no adult structures*
what does each pharyngeal arch have?
- core of mesenchyme that develops into arterial supply, nerve supply, cartilage, musculature & bone
define mesenchyme
an embryonic precursor tissue that generates a range of structures in vertebrates including cartilage, bone, muscle, and the erythropoietic system
which are major contributors to the mesenchyme of head and neck development
- paraxial and lateral plate mesoderm
- neural crest cells
- thickened regions of ectoderm called ectodermal placodes
what is the clinical connection of pharyngeal arches
- “tumors of mesenchymal origin”
- a tumor arising from the supporting mesenchymal (ex: fibrous, CT, blood vessels, lymphatics, nerves, adipose tissues and muscle)
where do cranial neural crests cells migrate
in the pharyngeal arches and face
what are 2 alternative names for ectodermal thickenings
1) ectodermal placodes
2) epipharyngeal placodes
what is the job of ectodermal thickenings
assist crest cells in formation of the fifth (V), seventh (VII), ninth (IX), and tenth (X) cranial sensory ganglia [group of neuronal cell bodies]
what does each pharyngeal arch get supplied by
its own cranial nerve
what 5 prominences initially make up the face
*1 central frontonasal prominence
*2 paired maxillary prominences
*2 paired mandibular prominences
what is the stomodeum
structures that surround the primitive mouth
where is the structure of the stomodeum initially
sealed by the oropharyngeal membrane which breaks down by day 30
what is the name of the 1st
pharyngeal arch
*the mandibular arch
which prominences arise from the 1st pharyngeal arch
maxillary and mandibular prominences*
what is the *maxillary process
mesenchyme here -> premaxilla, maxilla, zygomatic bone and part of the temporal bone
what is the mandibular process*
- contains meckel cartilage* -> contributes to incus and malleus and serves as a template for mandible formation
- mesenchyme here -> mandibular bone*, part of external ear, *incus and *malleus bones of the middle ear and *external auditory meatus
what muscles belong to the 1st mandibular arch
**muscles of mastication (Masseter, Temporalis, Lateral pterygoid, Medial pterygoid)
*what is the nerve supply of the 1st pharyngeal arch
motor and sensory from branches of *trigeminal nerve (CN V)
what is the name of the 2nd pharyngeal arch
*hyoid arch
what is the cartilage component of the 2nd pharyngeal arch
- *Reichert cartilage
describe *Reichert cartilage
precursor to the stapes, the styloid process, the stylohyoid ligament and parts of the hyoid bone & most of external ear
what are the *muscles of the 2nd arch
muscles of facial expression, stapedius, stylohyoid, posterior belly of the digastric, and auricular
what is the nerve supply for the 2nd pharyngeal arch
Facial nerve (CN 7)
what are the components of the 3rd pharyngeal arch
small components of the hyoid bone
what are the muscles of the 3rd arch*
stylopharyngeus muscle *
what is the stylopharyngeus muscle
- muscles of the pharynx
- functions in swallowing
what is the nerve supply for the 3rd arch*
*glossopharyngeal nerve (CN IX)
what are the components of the 4th and 6th arch
cartilaginous components
what do the cartilaginous components of the 4th and 6th arch fuse to form*
thyroid, cricoid, arytenoid, corniculate, and cuneiform CARTILAGES of the larynx *
what are the muscles of the fourth arch
cricothyroid, levato veli palatini, and constrictors of the pharynx
what is the nerve that supplies the 4th and 6th arches*
branches of vagus (CN X)
how many pharyngeal pouches are there
4 pairs of pharyngeal pouches
what are the pharyngeal pouches lined by
epithelial from the endoderm layer
what are the structures that result from the 1st pharyngeal pouch
- middle ear cavity*
- auditory (eustachian) tube*
- tympanic membrane (eardrum)*
what occurs with the 2nd pharyngeal pouch
endoderm of the pouch proliferates and forms buds that penetrate surrounding mesenchyme
what are 3 features of the 2nd pharyngeal pouch
- mesodermal tissue enters buds and forms palatine tonsil*
- months 3-5 tonsil is infiltrated by lymphatic tissue*
- remnants of pouch create tonsillar fossa
which structures arise from the 3rd pharyngeal pouch
- dorsal region of the third pouch differentiates into the inferior parathyroid gland*
- ventral region of the third pouch differentiates into the thymus*
what do the dorsal and ventral region of the 4th pharyngeal pouch develop into
dorsal -> superior parathyroid gland
ventral -> ultimobranchial body
what are 2 features of the ultimobranchial body
- ultimobranchial body later incorporated into the thyroid gland
- cells of the ultimobranchial body gives rise to the parafollicular cells (C cells) of the thyroid gland that secrete calcitonin
what do some texts call the 1st cleft
external auditory meatus
what do the 2nd, 3rd, and 4th clefts merge to form? what occurs to this?
- cervical sinus
- cervical sinus normally disappears during development
when does the tongue appear in the embryo
~ 4weeks
what are the origins of tissue for the tongue
*from first arch: formation of anterior 2/3 of tongue aka body of the tongue
* from 2nd, 3rd, & 4th arch: posterior 1/3 tongue
what are the 3 swellings of the tongue
these are all anterior and come from 1st arch
- 2 lateral swellings: lingual swellings
- 1 medial swelling: tuberculum impar
what is the swelling that comes from the 2nd, 3rd, and 4th arch
1 medial swelling: copula (hypobranchial eminence)
describe the innervation of the anterior 2/3 of the tongue
*sensory innervation is a branch of CN V because these structures originate from the 1st pharyngeal arch
what separates the anterior 2/3 of the tongue from the posterior 1/3
V-shaped groove called terminal sulcus
describe the innervation of the posterior 1/3 of the tongue
*sensory innervation by glossopharyngeal nerve, CN IX
what is an important sensory feature of the tongue
*additional special sensory innervation (taste)
where does the epiglottis originate from
- tissue of the 4th pharyngeal arch*
- epiglottal swelling*, swelling posterior to copula (posterior 1/3 of tongue)
what innervates the epiglottis
branch of Vagus nerve, CN X
where does the tissue for the thyroid gland originate from
- endoderm between first and second pharyngeal pouches*
what are the 3 development steps for the thyroid
1) proliferation begins at juction of anterior 2/3 tongue and posterior 1/3 of tongue
- leaves behind a depression called foramen cecum
2) thyroid descends from tongue to final position in the neck ~7th week
3) thyroid remains connected to the tongue by a narrow canal, the thyroglossal duct, which should disappear during development
when does the thyroid gland begin to function
- at approx. the end of the third month (10-12 wks)
what are the 5 birth defects*
1) hemifacial microsomia (oculoauriculovertebral spectrum or Goldenhar syndrome)
2) branchial cleft cyst (lateral cervical cyst/ cervical lymphoepithelial cyst)
3) thyroglossal duct cyst and fistula
4) lingual thyroid
5) tongue tie (ankyloglossia)
what are a couple of features of hemifacial microsomia
- abnormal development in 1st and 2nd pharyngeal arch structures
-*cause is unknown
what are the clinical features of hemifacial microsomia
- *under-development of the oral-facial area, usually one-side only
- other defects in ~50% of cases
->* other cardiac defects: tetralogy of Fallot, ventricular septal defects
how does branchial cleft cyst develop
improper involution of any of the branchial clefts leaves behind epithelium that can -> branchial cleft cyst formation
where can branchial cleft cyst occur
*anywhere along the anterior sternocleidomastoid muscle
- depends on which cleft fails to involute
- *most commonly the second cleft
how does the branchial cleft cyst develop over time
frequently not visible at birth but becomes evident as it enlarges during childhood
what is the clinical presentation of the branchial cleft cyst
- painless, compressible, *lateral neck mass
- cyst may form a channel to the surface to drain= *draining sinus tract
what can cause a branchial cleft cyst to enlarge
can be secondarily infected and cause enlargement in rare cases
why does the thyroglossal duct cyst occur
- if thyroglossal tract doesn’t involute entirely
-> *remnants of tissue can develop into cysts along the tract
what are clinical features of the thyroglossal duct cyst
- *MIDLINE neck mass, *occurs anywhere along the descending tract
- *typically present in 1st-2nd decade of life
- *painless, fluctuant, moveable swelling
- *moves with swallowing and elevates with protrusion of the tongue
what is a requirement of they thyroglossal duct cyst
MUST BE IN MIDLINE
what are two alternative names for the lingual thyroid
1) ectopic thyroid
2) aberrant thyroid tissue
what is the etiology of a lingual thyroid
- primitive thyroid gland doesn’t descend normally/ remnants of gland are left behind
- of all ectopic thyroids, *90% are found between foramen cecum and epiglottis
- on autopsy: small remnants of thyroid tissue present present on the posterior dorsal tongue in ~10% of both men and women (asympt.)
how common are clinically evident/ symptomatic lingual thyroids?
uncommon
what is the frequency of lingual thyroid?
4-7x more frequent in females
what are symptoms of lingual thyroid? when do they start?
-*dysphagia, dysphonia, and dyspnea
- start during hormone shifts (ex: puberty, adolescence, pregnancy, or menopause)
what happens for 70% of cases for lingual thyroid?
ectopic gland is the patient’s *only thyroid tissue
what is ankyloglossia
condition in which degeneration does not occur properly causing “tongue-tie”
describe important details of ankyloglossia
- *extensive cell degeneration should occur at the floor of the month during development to release the tongue
- *only a small frenulum should remain to attach tongue
what are examples of the ankyloglossia
- kotlow classification
- functional classification systems
what are the symptoms in adults for tongue tie
mostly speech difficulties
what are the symptoms in infants for tongue tie
- frequent, unfulfilling attempts at nursing
- colic, reflux and gassy infant
- failure to gain weight and thrive
- calloused or blistered upper and lower lips
- unstained latch
