Embryology (finished)

Question 1

Cell layers in the neural tube

Answer 1

Ventricular layer-lines neural canal

a. Initially makes up entire wall of neural tube
b. Rapidly dividing neuroepithelial cells that cause the wall of the tube to thicken.
c. Source of neuroblasts (neurons) and glioblasts (glial cells).
d. After all neurons and glia are formed, this ventricular layer becomes the ependyma, i.e. a single layer of cells lining the central canal of the spinal cord.

2. Intermediate layer
   a. Becomes populated by neuroblasts formed in the ventricular layer.
   b. Will become the gray matter of the spinal cord (dorsal, ventral, lateral horns); containing all spinal cord nerve cell bodies.
3. Marginal layer
   a. Includes the axons of neuroblasts whose cell bodies are in the intermediate layer and in the spinal ganglia.
   b. Becomes the white matter of the spinal cord.
4. Mitotic ability
   a. Neurons are generally considered to be non-mitotic cells since neuroblasts lose their mitotic potential once they develop processes. There are some limited examples of neurons dividing in the adult.
   b. Glial cells retain their mitotic ability.

Question 2

Alar and Basal Plates

Answer 2

1. Within the intermediate layer, the rapidly forming neuroblasts form two mirror image accumulations of cells – the alar and basal plates.
2. Sulcus limitans
   a. Longitudinal groove in the lumen, marking the division between alar and basal plates
   b. Continues along the length of the neural tube; seen early in development
3. Alar plates
   a. Dorsal location
   b. Form sensory (afferent) neurons primarily of the dorsal horn
4. Basal plates
   a. Ventral location
   b. Form motor neurons of the:
   • ventral horns–large motor neurons which innervate skeletal muscles
   • lateral horns–preganglionic neurons of the sympathetic nervous system in certain segments

Question 3

The neurons of the sensory ganglia are derived from

Answer 3

neural crest .

Question 4

There are no synapses in the

Answer 4

dorsal root ganglion

Question 5

Myelinating cells

Answer 5

Myelinating cells are glial cells

a. Schwann cells in the PNS
b. Oligodendrocytes in the CNS

Question 6

Primary brain vesicles

Answer 6

form from the cranial end of the neural tube during wk 4.
1. Forebrain or Prosencephalon

2. Midbrain or Mesencephalon
3. Hindbrain or Rhombencephalon

Question 7

Secondary brain vesicles

Answer 7

form by Wk 5/6

1. Forebrain or Prosencephalon:
   a. Telencephalon -> cerebral hemispheres
   b. Diencephalon ->thalamus, etc.
2. Midbrain or Mesencephalon
   a. Does not subdivide
   b. Mesencephalon -> midbrain
3. Hindbrain or Rhombencephalon: (divided by pontine flexure)
   a. Metencephalon -> pons and cerebellum
   b. Myelencephalon -> medulla

Question 8

brain developments of the three layers of neural tube

Answer 8

a. Ventricular layer -> ependymal lining of the ventricles
b. Intermediate layer -> alar and basal plates -> Gray Matter
c. Marginal layer -> pathways or nerve tracts -> White Matter

Question 9

Autonomic post-ganglionic neurons

Answer 9

*Preganglionic autonomic neurons (sympathetic and parasympathetic) are located in the CNS and are derived from the Neural Tube.

a. sympathetic ganglia
• sympathetic chain ganglia (paravertebral ganglia)
• cardiac, celiac and mesenteric ganglia (preaortic group)
• abdominal/pelvic plexuses related to heart, lungs and gut

b. parasympathetic ganglia in the organs
• Auerbach’s and Meissner’s plexuses in gut (terminal ganglia)

c.	parasympathetic ganglia in the head may or may not form from crest; origin is uncertain or controversial. 	
•	ciliary ganglia
•	pterygopalatine ganglia
•	submandibular ganglia
•	otic ganglia

Question 10

congenital megacolon or Hirschsprung’s disease

Answer 10

Neural crest cells fail to migrate into the colon; the parasympathetic ganglia do not form resulting in congenital megacolon or Hirschsprung’s disease. (MP&T fig. 11-27)

1. Usually involves sigmoid colon and rectum
2. Fecal retention with ballooning of colon and abdomen proximal to obstruction
3. Secondary to obstruction, lack of peristalsis in the colon distal to the dilatation.

Question 11

Waardenburg syndrome

Answer 11

occurs with diffuse disruption of migration of neural crest cells

1. Abnormal appearance of face, deafness, due to head neural crest cell involvement
2. Lack of pigmentation (melanocytes)
3. Digestive problems (enteric ganglia)

Question 12

Craniorachischisis

Answer 12

complete failure of the neural tube to close resulting in exposure of the malformed tissue to the outside of the head and body.

Question 13

Anterior defects

Answer 13

Anencephaly or Meroanencepaly: Failure of the rostral neuropore to close and subsequent failure of the cranial vault to form
• Anencephaly = total absence of brain tissue
• Meroanencephaly = remnants of brainstem tissue may be present
b. Encephalocele: protrusion of brain through a defect in the skull, often in the occipital area

Question 14

Posterior defects

Answer 14

Spina bifida is the general term for failure of the caudal neuropore to close

Question 15

Spina bifida occulta

Answer 15

• Mildest form in which there is malformation of the vertebral arch; no obvious involvement of the spinal cord, meninges or skin. Only evidence may be a tuft of hair on the skin surface overlying the defect. In some cases, the spinal cord might be tethered to the subcutaneous tissue.

Question 16

Spina bifida cystica

Answer 16

• Meningocele: Spinal cord is intact but meninges filled with CSF herniate through the defect in the vertebral arch and skin – usually in the lumbosacral region.
• Myelomeningocele (often used interchangeably with spina bifida)
o Most severe form; malformed spinal cord and the meninges herniate through the opening – usually in the lumbosacral region.
o May be accompanied by an Arnold-Chiari malformation where the cerebellum and caudal brainstem are elongated and protrude caudally into the foramen magnum. In this situation, CSF is often obstructed resulting in hydrocephalus.

Question 17

Meningocele

Answer 17

(often used interchangeably with spina bifida)
o Most severe form; malformed spinal cord and the meninges herniate through the opening – usually in the lumbosacral region

May be accompanied by an Arnold-Chiari malformation where the cerebellum and caudal brainstem are elongated and protrude caudally into the foramen magnum. In this situation, CSF is often obstructed resulting in hydrocephalus.

Question 18

Arnold-Chiari malformation

Answer 18

where the cerebellum and caudal brainstem are elongated and protrude caudally into the foramen magnum. In this situation, CSF is often obstructed resulting in hydrocephalus.

Question 19

Holoprosencephaly

Answer 19

failure of the normal development of the forebrain such that it is not divided into two hemispheres

Question 20

Hydrocephalus

Answer 20

a. Dilation of the cerebral ventricles by CSF due to CSF overproduction, obstruction of flow or failure of CSF reabsorption.
b. Often seen as part of other malformations such as Dandy-Walker or Chiari malformations

Question 21

Arnold-Chiari malformations

Answer 21

Chiari type II: Syndrome or association of anomalies that includes a NTD such as a lumbosacral meningomyelocele, displacement of part of the cerebellum and brain stem into the cervical spinal canal, hydrocephalus.

Chiari type I: Milder variant of Type II. Many of these patients have syringomyelia and some have hydrocephalus. May be asymptomatic.

Question 22

Dandy-Walker malformations

Answer 22

1. Posterior fossa abnormalities that include agenesis of the cerebellar vermis, formation of a large posterior fossa cyst, blockage of foramina of Luschka and Magendie, and resultant hydrocephalus.
2. Symptoms: in infants–slow motor development, irritability, vomiting; in older children –lack of muscle coordination, jerky eye movements

Question 23

Syringomyelia

Answer 23

1. Tubular cavitation of the spinal cord, usually in the cervical/upper thoracic region
2. Sequentially involves the area of the central canal, the fibers crossing it the ventral white commissure and eventually the ventral horns and corticospinal tracts.
3. Initial signs of a problem are loss of pain and temperature sensation over the shoulders and down the arms due to damage of the fibers crossing in the ventral white commissure.
4. Often associated with Chiari type I malformation
5. Pathogenesis is unknown and probably diverse