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Head and Neck > Embryology > Flashcards

Embryology Flashcards

0
Q

What are the neural tube defects?

A

Spina bifida occulta

  • no cyst in spinal cord
  • lack of fusion of vertebral arches (posterior pore)

Spina bifidida cystica

  • cyst on spinal cord
  • lack of fusion of posterior pore

Anencephaly

  • anterior pore not closed
  • incompatible with life

Rachischisis

  • failure of neural folds to elevate
  • incompatible with life

Defects can be detected by USS, or by screening for alpha fetoprotein present due to open pore

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1
Q

How does the nervous system develop?

A
  • production of notochord by prenotochordal cells from primitive pit
  • neurulation causes ectoderm differentiation- lateral edges of neural plate elevate and fuse to form neural tube
  • opening at each end (neuropores): anterior closes at d25, posterior at d28
  • defects in closure are neural tube defects
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2
Q

What are the divisions of the embryonic nervous system?

A
  1. Primary vesicle (4/52): three divisions
    - prosencephalon
    - mesencephalon
    - rhombencephalon
  2. Secondary vesicles (5/52): five divisions
    - prosencephalon divides into telencephalon and diencephalon
    - mesencephalon remains as the mesencephalon
    - rhombencephalon divides into metencephalon and myelencephalon
  3. Mature structures: 5 divisions
    - telencephalon forms cerebral hemispheres
    - diencephalon forms thalamus
    - mesencephalon forms midbrain
    - metencephalon forms pons cerebellum
    - myelencephalon forms medulla
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3
Q

When does the head and neck develop embryologically?

A

Week 4

- head and neck are half the length of the embryo

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4
Q

What are the pharyngeal (branchial arches)?

A

Ridges that form the lateral walls of the embryonic pharynx
- external is ectoderm
- internal is endoderm
Development is linked to cranial nerve, blood supply, CVS, brain and sensory organs

  • branchial arches are made up of artery, vein and cranial nerve with some mesenchyme tissue of mixed mesoderm (muscle) and neural crest cells (bone/cartilage)

In theory there are 6 arches but in practice, 4th/5th merge

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5
Q

Define branchial clefts, pouches and membrane

A
  • Brachial clefts are external invaginations that separate the branchial arches
  • branchial pouches are internal invaginations that separate the arches
  • branchial membrane separates grooves and pouches, with ectoderm externally, and endoderm internally
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6
Q

Discuss the different branchial arches

A

1st/2nd branchial arches are the largest

  • tissue flap from 2nd arch grows down to cover lower arches (cervical sinus)
  • the cervical sinus is usually obliterated but if it remains, cysts in the neck occur along the anterior border of sternocleidomastoid
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7
Q

What are the nerves of the pharyngeal arches?

A

Trigeminal (CN V) - 1st Arch

  • sensory: face and head
  • motor: mastication/mandible

Facial (CN VII) - 2nd Arch

  • sensory: anterior two thirds of tongue (taste)
  • motor: facial expression

Glossopharyngeal (CN IX) - 3rd Arch

  • sensory: posterior third of tongue
  • motor: stylopharyngeus

These all develop with common carotid, ECA, ICA, and their branches

Vagus nerve (CN X)

  • 4th Arch: superior laryngeal nerve (aortic arch and right subclavian)
  • 6th Arch: recurrent laryngeal nerve (pulmonary arteries)
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8
Q

How does the facial skeleton develop?

A

Skeleton

  • frontonasal prominence (FNP) with developing brain beneath
  • 1st arch develop into maxillary and mandibular prominences

1st arch:

  • muscle of mastication
  • Meckels cartilage, malleus and incus, mandible template
  • sensory to skin of face and oral/nasal lining

2nd arch

  • muscle of facial expression
  • reicharts cartilage, stapes, upper hyoid and lesser horn, styloid process, stylohyoid ligament
  • sensory to anterior two thirds of tongue

3rd arch

  • muscle: stylopharyngeus
  • lower hyoid and greater horn
  • sensory to posterior third of tongue

4th arch

  • pharyngeal muscles, cricothyroid
  • thyroid and cricoid cartilage, epiglottis

6th arch

  • intrinsic laryngeal muscles
  • aretynoid cartilage
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9
Q

What are the pharyngeal pouches?

A

Endothermal lining: denotes important organs

  • pouch 1: Eustachian tube and middle ear
  • pouch 2: crypts of palatine tonsils
  • pouch 3: inferior parathyroid (dorsal) and thymus (ventral)
  • pouch 4: superior parathyroid (dorsal) and ultimobranchial body and thyroid C cells (ventral)
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10
Q

How does the nose develop?

A
  • bilateral ectodermal thickening (nasal placodes)
  • placodes invaginate to form nasal pits
  • horseshoe shape ring forms around each future nostril (medial and lateral nasal prominences)
  • nasal pits separated by oronasal membrane which eventually disappears allowing oral and nasal cavity connection
  • maxillary prominences grow medially, pushing nasal prominences together
  • maxillary prominences fuse with lateral nasal prominence whilst medial nasal prominences fuse in midline
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11
Q

How does the palate develop?

A

Palate formation: separates oral and nasal cavities following obliteration of the oronasal membrane

  • medial nasal prominences fuse at midline separates nostril from mouth
  • results in philtrum, median maxillary bone and teeth plus primary palate formation
  • a palatal shelf grows medially from each maxillary prominence to midline, fusing with each other and primary plate = secondary plate which separates nasal and oral cavities
  • mandible growth allows tongue to drop out of the way allowing palatal fusion
  • nasal septum develops as midline grows down, fusing with palatal shelf
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12
Q

What is a cleft palate and how does it form?

A

Failure of fusion during palate development

Lateral cleft palate: failure of medial nasal prominences and maxillary prominence to fuse

Cleft lip and palate: as above but with failure of palatal shelves to meet up in midline

These defects can result in speech and suckling issues

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13
Q

Briefly describe the embryological development of the eyes

A
  • begin at 4/52 with out pockets of forebrain (optic vesicles) which develop and make contact with overlying ectoderm (lens placodes)
  • lens placode invaginates into out pocket and pinches off
  • optic vesicle stretches out (central fissure along stalk- hyoid artery)
  • hyoid artery degenerates distally creating central artery of retina
  • rim of optic vesicle = ciliary body musculature and iris
  • retina develops from optic cup (intra retinal space develops as retina formed from two layers)
  • optic stalk degenerates to become optic nerve
  • primordia originally positioned on size of head, move medially as facial prominences grow = binocular vision
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14
Q

What are embryological eye defects?

A

Congenital cataracts: defect due to teratogenic exposure

Congenital rubella syndrome: pregnant women contract rubella, producing classic triad of defects

  • Sensoneurial deafness
  • cataracts/retinopathy
  • congenital heart disease

Prevented due to mass immunisation

Detached retina can occur if the intra retinal space opens up again!

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15
Q

Briefly describe embryological ear development

A

External ear

  • External auditory meatus develops from 1st cleft
  • auricles develop from 1st/2nd arches
  • sensory innervation by CN V

Inner ear

  • otic placodes invaginate to form auditory vesicles, closed over by ectoderm
  • morphological change occurs: saccule becomes cochlear (hearing) whilst utricle become the semicircular canals (balance)

Middle ear

  • tympanic cavity and auditory tube develop from 1st pouch, with ossicle serviced from cartilage bar derivatives (1st/2nd arch)
  • proximal end of tube remains narrow: Eustachian tube
  • ossicle become suspended: sensor tympanii (CN V3), stapedius (CN VII)
  • ear originally develops in the neck, ascending as mandible grows until they are in line with the eyes
16
Q

What are the congenital ear issues?

A

Congenital deafness

  1. Middle ear deafness: 1st/2nd arch problems
  2. Inner ear deafness: maldeformed organ of corti generally due to teratogens (rubella)
17
Q

What is foetal alcohol syndrome?

A

Developmental abnormalities due to ethanol freely crossing the placenta

  • small eye/nose opening, thin philtrum and underdeveloped jaw
  • neural crest tissue pis alcohol sensitive
  • alcohol related neuro developmental delay occurs of there is no morphological change, but developmental delay
18
Q

How does the pituitary gland develop?

A

Sits in sella turcica (pituitary fossa) of sphenoid bone, and has two compartments

ANTERIOR:

  • adenohypophysis: Rathkes pouch, ectodermal tissue and endocrine functions
  • develops form outpouch of mouth (Rathkes pouch)
  • breaks away from mouth and wraps around pituitary stalk
  • anterior to infundibulum

POSTERIOR:

  • neurohypophysis: infundibulum, neuro ectodermal tissue with neuroendocrine function
  • develops from down growth of diencephalon called infundibulum
  • pituitary stalk connects diencephalon and infundibulum
  • nerve tract develops from hypothalamus

Anterior and posterior pituitary are linked by hypophyseal portal blod supply

19
Q

How does the tongue develop?

A

Appears at 4/52 (same time as palate)

  • two lateral lingual swellings (1st arch) tip of tongue
  • three medial lingual swellings
    1. 1st arch: tuberculum impar
    2. 2nd/3rd arch: cupola
    3. 4th arch: epiglottal swelling
  • lateral lingual swellings overgrow tuberculum impar whilst 3rd arch (cupola) over grows 2nd arch component
  • apoptosis frees tongue from floor of oral cavity leaving lingual frenulum
20
Q

What is the innervation of the tongue?

A

Sensory:
Anterior two thirds: lingual nerve CN V3, 1st arch
Posterior third: CN IX 3rd arch

Special sense:
Anterior two thirds: chorda tympanii CN VII 2nd arch
Posterior third: CN IX 3rd arch

Motor:
palatoglossus CN X
other muscles CN XII

Chorda tympanii is nerve of 2nd arch, but passes into 1st through middle ear (1st pouch)

21
Q

Describe the development of the thyroid gland

A

First endocrine gland to develop

  • on pharynx floor between tuberculum impar and cupola between 1st/2nd arches
  • marked by foramen cecum in adult
  • starts at back of tongue, bifurcates and descends
  • two lobes connected by isthmus
  • connected to tongue during descent by thyroglossal duct
  • 1st/2nd arch: thyroid diverticulum (T3/4) by follicular cells
  • 4th pouch: ultimobranchial body (calcitonin) by para follicular cells which migrates into thyroid gland
22
Q

What are the thyroid abnormalities?

A

Thyroglossal cysts: occur at any point along migratory path

Thyroglossal fistula: cyst connected to outside

Ectopic thyroid tissue: occur at any point along migratory path (common at base of tongue behind foramen cecum)

First arch syndrome: eye, ear and mandible defects due to failure of neural crest tissue to colonise 1st arch
Eg. Treacher Collins syndrome: inherited auto dominant resulting in hypoplasia of mandible/facial bones

Di George syndrome: congenital thymol aplasia and parathyroid absence due to deletion of chromosome 22 (catch 22)

Charge syndrome: CHD7 heterozygous mutation. Protein is essential for production of multipotent neural crest cells

Head and Neck (8 decks)

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