Embryology and Genetics Flashcards
At which weeks do the Zygotic, Embryonic, and Fetal stages occur?
Zygotic = 1-2; Embryonic = 3-8; Fetal = 9-40
What are the 3 layers of cells in an embryo?
Endoderm = inner cell layer; Mesoderm = middle cell layer; ectoderm = outer cell layer
What is Mesenchyme?
connective tissue derived from all three germ layers. Capable of developing into connective tissue, bone, cartilage, the lymphatic system, and the circulatory system
Explain development (particularly important to the ear) in terms or the Carnegie stages
1-4 (week1) - fertilisation -> blastocyst 5-6 (week2) - implantation of the embryo 7-9 (week3) 10-12 (week4) 12-13 (4 or so weeks) - pharyngeal(branchial) arches develop and the site of the otic placode (which will develop into the inner ear) 16 - development of the auricular Hillocks on the first and second pharyngeal (branchial) arches that will from the outer ear (specifically the pinna) 19 - external auditory meatus beginning to develop 23 - (the final stage in embryogenesis) shows the external ear almost completely formed
What are the derivatives of the first branchial arch, cleft and pouch?
Where do the 6 Hillocks and Concha form from?
Hillocks 1-3 = first branchial arch; Hillocks 4-6 = second branchial arch; Concha = first branchial groove
Arrested development between 4-12 weeks may lead to:
Microtia, Atresia, Anotia or Cryptotia.
The earlier, the more prominent the malformation
Microtia CANNOT occure without atresia, however atresia can occur without microtia
Describe the development of the ear canal
EC develops by deepening of the 1st branchial cleft at approx. 6 weeks.
Ectoderm forms the meatal plug (fills the inner portion of the canal)
Epithelial core hollows at 24 weeks and begins canalisation at 28 weeks.
Normal canalisation leaves a patent ear canal and a 3 layered tympanic membrane.
The ear canal does not reach its final length until 9-10years of age.
Describe the development of the middle ear
The first branchial pouch forms the tubotympanic recess that forms the eustachian tube and middle ear.
The Malleus and Incus form from the 1st branchial arch.
The stapes forms from the 2nd branchial arch.
The bones are enclosed in the middle ear space in the last month of gestation.
Describe the development of the ossicles
During the 7th week the cartilaginous precursors of the ossicles condense in the mesenchyme of the 1st and 2nd arches near the tympanic cavity
Cartilage of the mandibular process form the malleus
Of the maxillary process forms the incus
Of the second branchial arch the stapes
Therefore, jaw formation disorders and possible ossicle maldevelopment go hand in hand
Name and describe two syndromes/sequences that are involve maldevelopment of the first branchial arch derivatives
Treacher Collins Syndrome: congenital malformation of the eyes, ears, palate and mandible. Not likely to have SNHL, same risk level as normal population. Genetic.
Pierre Robin Sequence: Mandibular malformations (underdeveloped), cleft palate, tongue retraction and sometimes eye and ear defects
External and middle ear issues are often associated with other cranio-facial issues because they are developing at the same time
Second branchial arch, cleft and pouch derivatives
Arch (mesoderm) = stapes superstructure, stapedial atery, cranial nerve VII, styloid process, root of tongue
Pouch (endoderm) = palatine tonsil, supratonsillar fossa
Describe the development of the inner ear
Develops from an epidermal otic placode that invaginates at the end of the 3rd week to form the otic vesicle in the mesenchyme
- the otic pit closes off to move inward to form the cochlea
It then very rapidly differentiates into three sections - the endolymphatic duct, a utricle and a saccule.
During weeks 4-7 the semicircular canals develop from the utricle and the ventral end of the saccule forms the cochlea
Late in the 5th week (or slightly later) the ventral tip of the saccule begins to elongate and coil, forming the cochlear duct.
By the end of the 8th week 1.5turns present, by the 10th week all 2.5turns are present
From weeks 9-23 the mesechyme surrounding the membranous labyrinth ossifies to form the bony labyrinth within the petrous portion of the temporal bone
Describe the malformation that occurs if insult occurs during the 5th/6th week of development?
Mondini Malformation = spectrum malformation, the development of the cochlea can be arrested and you may therefore not see the full 2.5turns developed
Can have a spectrum of effects on hearing: can sometimes get good thresholds but can’t tell anything about their functional hearing; some can’t hear at all; some can get CIs some can’t.
More likely to be unilateral
Timing of ear development
Week 3 = otic placode, otic vesicle
Week 5 = cochlear part of the otic vesicle elongates (2.5turns)
Week 9 = Mesenchyme surrounding membranous labyrinth (otic capsule) becomes cartilaginous
Week 12-16 = Capsule adjacent to membranous labyrinth forms a cavity (perilymphatic space) around membranous labyrinth and fills with perilymph
Week 16-24 = Centres of ossification appear in remaining cartilage of otic capsule and form petrous portion of temporal bone. Continues to ossify to form mastoid process of temporal bone.
3rd trimester = vibration acoustically of maternal abdominal wall induces startle response in foestus
Therefore, in general earlier insult = conductive, later insult = SNHL
