Week 1 Flashcards
Embryology & Genetics
Trilaminar embryonic disc has what three parts?
- Endoderm
- Mesoderm
- Ectoderm
Disc thickening happens at which end?
The cranial end
Ectoderm
all contact with outside world + CNS, PSN - some cells are neuroectodermal cells
What are neuroectodermal cells?
Form the neural plate
Invagination
Inpocking which forms the neural groove, neural folds, and neural tube
Subcutaneous neural crest cells are found where?
Top most level of the dorsal side
What happens in the 4th week in utero in terms of head/neck development?
Differentiated cells develop - placodes, somites, and pharyngeal apparatus
What are placodes?
Thickened areas of embryonic ectoderm that the sensory organs arise from
What are the three kinds of placodes?
Nasal, otic, and optic placodes
What are somites?
Paired masses of cells that develop caudally (from top to bottom) from the mesoderm at about day 20
What do somites become?
The axial skeleton, muscles, and dermis
Describe how somites develop
Caudally to about 42 pairs on the sides of the neural tube
What is the pharyngeal apparatus?
Pharyngeal arches that correspond to primitive gill bars/branchial arches. They consist of a core of mesenchyme covered externally by ectoderm and internally by endoderm.
Pharyngeal arches are separated externally by…
pharyngeal clefts
Pharyngeal aches are separated internally by…
pharyngeal pouches
When are ectodermal structures evident?
Around the fourth week
What do ectodermal structures participate in?
Ectodermal structures participate in the formation of nasal and oral structures
Where is mesenchymal tissue derived from?
Mesenchymal tissue is derived from mesoderm
What does mesenchymal tissue form?
Mesenchymal tissue forms the muscles associated with each arch
What creates the mesenchymal tissue?
Neural crest cells create mesenchymal tissue
Describe neural crest cell migration.
These cells migrate to form the skeletal portion arising from each arch. They’re underneath the ectodermal level dorsally and they migrate around what will be the face.
Pharyngeal Arch #1
“Mandibular” arch - associated with…
CN V
Muscles of mastication
Malleus, incus, maxillary process, and part of the mandible
Pharyngeal arch #2
“Hyoid” arch - associated with…
CN VII
Muscles of facial expression
Stapes & upper hyoid
Pharyngeal arch #3
Associated with…
CN IX
Stylopharyngeus
Lower hyoid
Which pharyngeal arches are somewhat joined together in terms of function?
Pharyngeal arch #4, #5, and #6
Pharyngeal arches #4-6
Associated with…
CN X
Cricothyroid, levator palatini, pharyngeal constrictors, intrinsic laryngeal muscles
Larynx
Pharyngeal pouches associated with pharyngeal arch #1
Tympanic cavity, auditory tube, eustachian tube
Pharyngeal groove associated with pharyngeal arch #1
External auditory meatus
Pharyngeal membrane associated with pharyngeal arch #1
Tympanic membrane
Pharyngeal puches, grooves, and/or membranes associated with pharyngeal arch #2
Tonsils
Pharyngeal pouches, grooves, and/or membranes associated with pharyngeal arch #3
Parathyroid gland, thymus
Pharyngeal pouches, grooves, and/or membranes associated with pharyngeal arch #4
Sup. parathyroid gland
When does facial development occur?
Facial development occurs between weeks 4-9
How is facial development organized?
Facial development is organized around central opening (stomodeum)
Describe how neural crest cells migrate in terms of facial development.
Neural crest cells migrate to stomodeal area from neural folds to form facial primordial.
What structures do the neural crest cells develop in facial development?
- Frontonasal prominence (forebrain behind, nasal placodes on sides)
- Maxillary prominences (upper part of arch #1)
- Mandibular prominences
How many facial primordia are there?
5
- Frontonasal prominence
- Paired maxillary prominences
- Paired mandibular prominences
The medial nasal prominences merge with…
The medial nasal prominences merge with each other and with lateral nasal and maxillary prominences
What is the nasolacrimal groove?
The nasolacrimal groove is between the lateral nasal and maxillary prominences and becomes the nasolacrimal duct.
What is the intermaxillary segment
The inter maxillary segment is the merger of the medial nasal prominences and it gives rise to the philtrum, premaxillary bones, and primary palate
Frontonasal prominence turns into… (3)
The forehead, dosum, & apex of the nose
The lateral nasal prominence turns into… (1)
The sides of the nose
The medial nasal prominence turns into… (4)
The nasal septum, philtrum, premaxilla, and primary palate
The maxillary prominence turns into… (3)
The upper cheek, most of the upper jaw, and the lip
The mandibular prominence turns into… (4)
The lower jaw, chin, lower lip, and lower cheek
Excess tissue in the frontonasal prominence (or frontonasal dysplasia) results in:
A broad nasal bridge, hypertelorism (wide set eyes), cleft nose, & median cleft lip. - often associated with other defects (cognitive, neurological, etc.)
Deficient tissue in the frontonasal prominence (or holoprosencephaly) results in:
Defective formation of forbrain (prosencephalon) which manifests as mid facial deficits. Wide range of facial defects ranging from short, upturned nose, deficient philtrum, arched palate, and microcephaly to medial nasal prominences, inter maxillary process fail to form, absence of nasal septum & ethmoid bone, single nostril (cebocephaly), hypotelorism or even cyclopia with proboscis
Describe the development of the nose
Around the 4th week, the nasal placodes form on frontonasal process. The medial and lateral nasal prominences with the depression in between will be the nostril. Nasolacriminal groove separates the lateral prominences from the maxillary prominences.
Describe the development of the inner ear.
~4th week the otic placodes invaginate and form otic pits which pinch themselves off from the ectodermal surface. - These auditory vesicles will form the membranous labyrinth of the inner ear and eventually the receptor cells of the inner ear.
~7th week: the auditory vesicle will differentiate into 3 parts
In the ~7th week, the auditory vesicle of the inner ear will differentiate into what? (3)
- The endolymphatic duct with the endolymphatic sac at the end.
- The utricle as well as utricular diverticula
- The saccule, the cochlear duct, and the spiral organ of corti.
Describe the development of the middle ear.
The endoderm of the 1st pharyngeal pouch will line the ME cavity & auditory tube - bones of the ME are derived from the cartilage of the 1st & 2nd pharyngeal arches
When does the middle ear start to develop?
As the inner ear differentiates (~week 7)
What does the middle ear develop from?
The 1st and 2nd pharyngeal arches
When does the outer ear begin to develop?
~week 7
Describe the development of the outer ear
Six auricular hillocks arise on the pharyngeal arches. They begin to enlarge, differentiate, & fuse together to take the early form of the pinna. They move during the fetal period from the side of the neck to the sides of the head
When does the formation of the primary palate take place?
Formation of the primary palate takes place at the end of the 6th week (begins in week 5 but weeks 6-9 are most critical)
Describe the development of the primary palate
The maxillary and lateral nasal prominences merge at the nasolacrimal groove. Medial nasal prominences merge together and with maxillary and lateral nasal prominences to form inter maxillary segment. The segment produces the philtrum, pre maxilla, alveolar ridge - the primary palate
What forms the primary palate?
The inter-maillary segment (produces the philtrum, pre maxilla, & alveolar ridge)
What are the lateral palatine processes?
Ingrowths from maxillary prominences - eventually project horizontally above the tongue - fuse with each other, primary palate, and nasal septum
What is the nasal septum?
Downgrowth of medial nasal prominence - fusion with lateral palatine processes starts anteriorly, then moves back
What is the hard palate?
Includes part of the primary palate (pre maxilla) and the lateral palatine processes (maxilla)
What is the soft palate?
Unossified portion of lateral palatine processes.
Where does the PRIMARY palate end?
At the incisive foramen
What are the boarders of the SECONDARY palate?
SECONDARY palate consists of hard & soft palates, starting at the posterior boarder of the pre maxilla (incisive foramen) to the uvula.
Describe the development of the SECONDARY palate.
~6-12 weeks: lateral palatine processes (shelves) are formed - mandible creates space for tongue to drop (from nasal to oral cavities) - shelves become horizontal & fuse first with the pre maxilla at incisive foramen and then from front to back along the median palatine suture line (~8-10 weeks)- vomer descends and fuses w/superior palate - velum and uvula follow
Causes of clefts (most common birth defect) (4)
- Chromosomal disorders
- Genetic disorders
- Environmental teratogens
- Mechanical factors (some kind of pressing in utero)
Cleft lip & palate has a : ratio, males:females
2:1 (males:females)
Cleft palate has a : ratio, females:males
2:1 (females:males) - females are more likely to have cleft palate ALONE because their palate takes longer to form, prolonging their sensitive period
A unilateral cleft means what?
One shelf is attached to vomer
Kernahan classification: prolabium
Separated philtral tissue
Kernahan classification: Premaxilla
Extends to bone beneath (alveolus and anterior portion of the maxilla - 4 front teeth)
Kernahan classification: Fistulae through dehiscence
After an attempt to close the palate - it’s too snug so it tears back open (tight dress example)
Incidence of cleft lip (+/- cleft palate)
1 in 750
Incidence of cleft palate (only)
1 in 2500