1 - Embryology Flashcards
Describe the process of neurulation.
1) Beginning of 3rd wk: ectoderm overlying notochord thickens to form neural plate; lateral edges form neural folds; neural groove forms between folds; folds + groove = neuroectoderm
2) folds join midline and form neural tube; cells from the apices of the folds migrate to form neural crest
3) tube is first formed at the occipital region and then proceeds cranially and caudally-> cranial @25th; caudal @28th
4) The rostral portion enlarges to become the brain, the caudal end thins to become the cord
What happens if the neural tube fails to close?
Neural Tube Defects
Caudal:
-Spina bifida occulta: vertebral arch is defective, skin may be covered by tuft of hair
-Spina bifida cystica:
w/ meningocele -> meninges protrude through vertebral arch and skin
w/ meningomyelocele -> both meninges and neural tissue protrude through vertebral arch and skin
-Rachischisis: neural folds fail to elevate or close, forming an exposed flattened mass of neural tissue
Cranial NTD:
-Meningocele: only meninges bulge through an occipital bone defect
-Meningoencephalocele: meninges and brain tissue bulge through an occipital bone defect
-Meningohydrencephalocele: meninges, brain tissue and part of the ventricular system bulge through an occipital bone defect
-Anencephaly: failure of anterior neuropore closure with most of brian and skull roof absent
What type of cells develop from neural crest cells?
- pigment cells
- dorsal root ganglia, cranial nerve ganglia
- schwann cells and satellite cells
- sympathetic and parasympathetic ganglia
- adrenal medulla chromaffin cells
- c-cells of thyroid gland
- dentin
- facial skeleton
Describe the histogenesis of neurons.
1) Neuroepithelial cells comprise ventricular zone of the neural tube
2) These develop into primitive neuroblasts after tube closes
3) These migrate to Mantle Layer surrounding ventricular layer where they differentiate into neurons
4) This continues until ~6mo gestation, and ~1yr postnatal in the cerebellum
5) Mature neurons migrate within the brain via radial glial cells
6) Axons from the maturing neurons form a Marginal Layer around Mantle layer
Describe the histogenesis of glial cells.
1) Neuroepithelial cells comprise the ventricular zone on the inside of the neural tube
2) After neuron genesis is complete, the neuroepithelial cells differentiate into glioblasts
3) These in turn differentiate into astrocytes and oligodendrocytes
4) Once glial cells are formed, the remaining neuroepithelium develop into ependymal cells, which will line the ventricles of the adult brain
5) Microglia are thought to develop from mononuclear hematogenesis in the bone marrow and migrate to the brain during development
What are Alar and Basal Plates? What do they develop into?
The mantle layer of the spinal cord thickens and is divided by the development of the Sulcus limitans; creating the Alar and Basal Plates (Dorsal and Ventral horns).
The mantle layer becomes the white matter of the CNS and the center the ventricle of the adult brian.
Describe the development of the brainstem.
- Develops similar to the spinal cord with alar(dorsal) and basal(ventral) plates developing.
- In the region of the pons/medulla, the central canal opens to form the 4th ventricle
- This opening displaces the plates so that the alar plates are now lateral and the basal plates medial to the sulcus limitans
- This arrangement is seen in the pyramids and olives of the adult medulla which retain their motor and sensory nuclei
Describe the development of the cerebellum.
- metencephalon and dorsolateral portion of alar plate expand to form Rhombic Lips
- these grow into 4th ventricle and are separated by a thin Roof Plate
- the rhombic lips overgrow the roof plate and meet to form the respective cerebellar hemispheres
- the roof plate represents the Vermis of the adult cerebellum
What is the Arnold-Chiari malformation?
Congenital hypoplasia of the posterior cranial fossa does not allow adequate room for the growing cerebellum. This causes “herniation” of the tonsillar cerebellum through the foramen magma. However, the foramen typically expands during development allow with herniation, therefore it is frequently asymptomatic or presents late with only mild symptoms. Tx with surgery.
Describe the development of the telencephalon.
1) Rostral proencephalon develops two hollow bulges that develop into the telencephalic vesicle/cerebral hemispheres
2) The cavities of the vesicles will become the lateral ventricles
3) The vesicles continue to grow and “wrap” around the diencephalon in a C-shape that is seen in several adult brain structures
4) The major commissures (corpus colostrum, anterior and posterior commissure) develop from the region of the lamina terminalis
5) The cortex develops as maturing neurons travel outward in Radial or Tangential Migration guided by Radial Glial Cells
6) The migration causes the cortex to go from smooth (lissencephalic) to pitted by sulci to increase surface area -> continues into 5-7th prenatal month
What are the cranial nerves associated with each of the pharygeal arches?
- 1st Arch -> CN V3 (mandibular division of trigeminal)
- 2nd Arch -> CN VII (facial)
- 3rd Arch -> CN IX (glossopharyngeal)
- 4th Arch -> CN X (Vagus)
- 6th Arch -> CN X (recurrent laryngeal nerve)
What are the adult derivatives of the aortic arches?
- 1st and 2nd Arches degenerate.
- 3rd Arch develops into the Common carotids and join with the part of the Dorsal Artery to form the Internal and External Carotids
- 4th Arch forms the subclavian on the right and the aortic arch on the left
- 5th (doesn’t exist)
- 6th Arch forms the pulmonary arteries
What are the muscular derivatives of the pharyngeal arches?
1) MM-MATT: Muscle of Mastication, Mylohyoid, Anterior belly of Digastric, Tensor Tympani, Tensor Veli Palatini
2) FESSP: Fascial Expression, Stapedius, Sylohyoid, Posterior belly of Digastric
3) Stylopharyngeus
4) Palate Muscles(NOT tensor veli palatini), pharynx muscles(NOT stylopharyngeus), cricothyroid
6) Layngeal muscles (not cricothyroid)
What are the skeletal derivatives of the pharyngeal arches?
1) Maxilla, mandible, malleus, incus, squamous temporal, sphenomandibular ligament
2) Stapes, styloid, hyoid (top), stylohyoid ligament
3) hyoid (lower)
4) Laryngeal cartilage
6) Laryngeal cartilage
What is the embryonic origin of the eardrum?
- The external auditory canal is formed from the 1st pharyngeal cleft
- the tympanic membrane (eardrum) is formed from the layering of the ectoderm and endoderm (and some mesoderm) from the 1st pharyngeal cleft
- the pharyngotympanic membrane is formed from the outgrowth of the 1st/2nd pharyngeal pouches
