Embryology Flashcards
When does the trilaminar embryo form?
-First 3 weeks of development
What are the three layers of the trilaminar embryo?
- Ectoderm
- Mesoderm
- Endoderm
What arises from the ectoderm?
- Neural tissue
- Surface of epithelium
What arises form the mesoderm?
-Most connective tissues of the body: muscle, bone, cartilage
What arises from the endoderm?
-Lining epithelium of the gastrointestinal, urogenital, & respiratory tracts
What layers of the trilaminar embryo give rise to the sense of hearing?
-All three
What does the otic vesicle give rise to?
- Give rise to all inner ear structures, including:
- Cochleovestibular neurons from future CN VIII
- Auditory system (cochlear duct)
- Vestibular system (SSCs)
Describe early otic vesicle development.
- Undergoes evagination (folding out) and takes on elongated shape
- Divides into an utricular-saccular area and a tubular extension (endolymphatic duct)
The inner ear arises from what layer of the trilaminar embryo?
-Ectoderm
The middle ear arises from what layer of the trilaminar embryo?
-Endoderm
What do the pharyngeal arches (bracchial arches) form?
-Main anatomical structures of the head & neck, including components of the middle and outer ear
Describe pharyngeal (bracchial) arch development.
- By week 4, 5 bracchial grooves or “gill slits” appear in the lower head and neck region on the lateral surfaces
- Inside the embryo, corresponding pharyngeal pouches develop
- The collective structures are identified as “arches” (series of 5)
- Pharyngeal arches 1 and 2 are main contributors to the middle and outer ear
Describe where the ossicles arise from.
- 1st branchial arch:
- Responsible for most of the body structure of the malleus and incus
- 2nd branchial arch:
- Lenticular process of the incus
- Handle of the malleus
- Stapes
Describe ossicle development.
MALLEUS & INCUS
- By 8.5 weeks, the incus and malleus have a cartilaginous adult-like form
- By weeks 15-16, ossification begins to occur in the malleus and incus, which have nearly reached completion by the 32rd week
STAPES
- Stapes continues to develop as a cartilaginous structure until week 15
- Stapes does not begin to ossify until week 18 and continues to develop through life even after ossification is complete
Describe auricle development.
- The auricle develops during the 3rd or 4th week from the 1st and 2nd bracchial arches
- The auricle is derived primarily from the 2nd branchial arch (tragus arises from 1st)
- Week 6: 6 tissue hillocks (thickenings) form which impacts the ultimate shape and configuration of the adult auricle
- Weeks 7-20: Auricle continues to develop, moving form original ventromedial position to be slowly displaced laterally by the growth of the mandible and face
- Week 20: Auricle is in the adult shape but continues to grow in size until 9 years of age
Describe external auditory meatus development.
- Weeks 4-5: derived from 1st bracchial groove
- Week 8: primary auditory meatus sinks toward the middle ear cavity and becomes the outer 1/3 of the auditory canal, surrounded ultimately by cartilage
- The ectodermal groove deepens towards the tympanic cavity from the external surface until it meets a meatal plug
- Mesenchyme grows between the meatal plug and the epithelial cells of the tympanic cavity
- The solid meatal plug keeps the EAC closed until week 21
- Meatal plug disintegrates and forms a canal, with the innermost layer of meatal plug epithelium becoming the squamous epithelial layer of the TM
- The EAC continues to develop until the 9th year
- At birth, the floor of the EAC has no bony portion (not completed until ~7 year)
- In infants, the EAC is short and straight
- In adults, the EAC is longer and curved
What are the 3 categories of abnormalities?
- Genetic
- Environmental
- Unknown
Describe genetic abnormalities.
- In human development, most major genetic abnormalities are thought to be within 1st 2 weeks of development as the zygote:
- Chromosomal
- Translocation of chromosomal segments
- Single gene mutation
Describe environmental abnormalities.
- Teratogens (maternal effects)
- Maternal behavioral factors (i.e. smoking, alcohol, drugs)
- Bacteria, virus, parasites
- Hyperthermia
- Environmental contaminents
- Radiation
What external ear abnormalities could be indicative of middle ear bone abnormalities?
- Thickened lobes: abnormal incus and stapes
- Small cupped ears with absent cartilage: mennonite genetic absences of incus and stapes
- Absent superior crus: congenital ossicle fixation
What is anotia?
- Unformed ear canal
- Due to pharyngeal arch hillocks failing to form
- Also impacts EAC and middle ear bones
- Known genetic associations:
- Manibulofacial dyostosis (Treacher Collins)
- Hemifacial microsomia
- Congenital aural atresia
What are some physical features associated with Treacher Collins?
- Downward slanting eyes
- Small jaw and chin
- Alters development of facial bones and tissues
What is microtia?
- Smaller cup ear or overfolded auricle
- Over 40 different associations including syndromes:
- Hearing and vision abnormalities
- Neurological abnormalities
- Clefting of the face and palate
- Musculoskeletal abnormalities
- May also be caused by maternal drugs (thalidomide, retinoic acid)
What are some less common external ear abnormalities?
- Auricular fistulas and sinuses
- Auricular appendages
- Stenosis of EAC
What external ear characteristics are indicative of Fetal Alcohol Syndrome?
- Lower or uneven external ear position
- “Railroad track” ear
What is “railroad track” ear?
-Auricle has a curve at the top part of the ear, which is underdeveloped and folded over parallel to the curve beneath
Describe absent ossicles.
- Can be associated with a range of conditions/syndromes (i.e. dwarfism, achondrogenesis)
- Most common bones to be absent: incus, stapes
What cochlear abnormalities of the oval window are associated with stapes fixation?
- X linked deafness
- Branchio-oto-renal syndrome
- Beckwith-Wiedemann Syndrome
What prenatal infections could lead to inner ear abnormalities?
- Rubella: cochlear duct, saccular development
- Cytomegalovirus (CMV)
- Toxoplasmosis
What are some genetic conditions associated with cochlear and SSC abnormalities?
- Trisomies
- Michel aplasia: absence of inner ear labyrinth
- Pendred syndrome: abnormalities of the cochlea, enlarged vestibular aqueduct, thyroid enlargement (goiter)
How is persistent stapedial artery associated with middle ear abnormalities?
- The stapedial artery initially lies between the foramen of the stapes and is lost before birth
- If the regression fails, the persistent artery can affect conduction through the ossicular chain
- May be seen in hemifacial microsomia
- Middle ear cavity can be delayed in formation or size, leading to various conduction effects
- May occur with BOR syndrome
