Embryo 1st lecture

Question 1

Describe the primary axes of the CNS

Answer 1

refer to diagram

Question 2

Describe the significance of the rhombomeres, in terms of the segmental development of the hindbrain and its relationship to specific cranial nerves. 


Answer 2

rhombomeres are 8 distinct segments of the rhomboencephalon. they develop distinctly in terms of morphology, axonal trajectories, neurotransmitter synthesis, neurotransmitter selectivity, firing properties, synapse specificity. eventually they don’t have distinct boundaries, but cranial nerves originate from distinct rhombomeres (eg trochlear = rhombomere 1, abducens = 5 and 6). Each expresses a specific combiation of Hox genes.

Question 3

1) Describe the general scheme of dorsoventral patterning of the neural tube into alar and basal plates, and 2) how this scheme is modified at the level of the midbrain, pons, medulla, and spinal cord. 


Answer 3

BMPs initially permeate the entire flat embryonic disc. The midlien primitive node inhibits BMPs, so that midline is BMP-poor. When the neural plate folds into the neural tube, the lateral lips come together, bringing the BMP-rich lateral ectoderm into a position on the dorsal aspect of the closed neural tube. So this process leads to distinct zones and distinct populations of cells. Those on ventral aspect develop into motor neurons, and those in the dorsal aspect develop into neurons that receive inputs from DRG cells. A crease (the sulcus limitans) separates these 2 populations. 2) The brainstem follows the same scheme but the scheme isn’t preserved in the mesencephalon and rhomboencephalon.

Question 4

Describe the basic scheme of dorsoventral patterning of the prosencephalon.

Answer 4

before the telencephalic vesicles begin to form, the rostral neural tube develops regionally restricted DV markers that cause the formation of three discrete proliferative zones: cortex dorsally, the lateral and medial ganglionic eminences, and the basal forebrain most ventrally.

Question 5

How does the basic scheme of dorsoventral patterning of the prosencephalon relate to the adult three-dimensional structure?

Answer 5

Lateral surface of telencephalic vesicle folds over itself to form the sylvian fissure (producing insular cortex). This also distorts the telencephalic vesicle AND the caudate nucleus into a “C” shape. So structures in the telencephalon generally follow a “C” shaped pattern so that a coronal slice will cut through the caudate in 2 places. This also explains why the internal capsule splits fibers from the cortex split the basal ganglia into the globus pallidus and caudate.

Question 6

What are the primary cerebral vesicles?

Answer 6

prosencephalon (forebrain), mesencephalon (midbrain), rhomboencepahlon (hindbrain)

Question 7

prosencephalic vesicle segments into..

Answer 7

telencephalic vesicles (2) + diencephalic vesicle (1)

Question 8

mesencephalic segments into..

Answer 8

doesn’t segment anymore.

Question 9

rhomboencephalic vesicle segments into..

Answer 9

cranial metencephalon + myelencephalon (more caudal)

Question 10

what is the bend in the neural tube referred to as? where does this occur?

Answer 10

cephalic flexure. between the midbrain and the diencephalon.

Question 11

what do the telencephalic vesicles correspond to in the adult brain?

Answer 11

cerebral hemispheres and the lateral ventricles

Question 12

what does the diencephalic vesicle give rise to?

Answer 12

thalamus, hypothalamus, subthalamus, and epithalamus. and lumen of 3rd ventricle.

Question 13

what does the mesencephalic vesicle give rise to?

Answer 13

mesencephalon/midbrain + aqueduct of sylvius

Question 14

what does the metencephalon give rise to?

Answer 14

pons + cerebellum + upper part of 4th ventricle

Question 15

what does the myelencephalon give rise to?

Answer 15

medulla oblongata + lower part of 4th ventricle

Question 16

role of homeobox/hox genes in development?

Answer 16

AP patterning of nervous tissue

Question 17

morphogens

Answer 17

There is a graded concentration of morphogens along the rostrocaudal axis that promotes neural tube differentiation.

Question 18

exception to general rule that all CNS cells are derived from the neuroepithelial layer

Answer 18

microglia (hematopoeitic lineage)

Question 19

number of cell layers in the cerebral cortex

Answer 19

6

Question 20

sulcus limitans

Answer 20

crease in the neural that separates the ventral population from the dorsal population.

Question 21

basal plate

Answer 21

ventral population of the spinal cord

Question 22

alar plate

Answer 22

dorsal population of the alar plate

Question 23

How is the forebrain gradient established?

Answer 23

Shh (produced by more ventral structures) establishes dorsoventral gradient.

Question 24

Discrete proliferative zones in DV axis of telencephalon

Answer 24

(remember this axis is flipped up) cortex dorsally, lateral ganglionic eminence below, medial ganglionic emininence below that, basal forebrain most ventrally.

Question 25

How does the 4th ventricle communicate with the subarachnoid space?

Answer 25

Foramen of Luscka and foramen of Magendia (FA pg 468)

Question 26

what factors influence formation of embryonic axes?

Answer 26

1) first axis is caused by sperm entry, which breaks radial symmetry of egg. This axis is defined by the mitotic spindle and called the embryonic/abembryonic axis 2) blastomere where sperm enters divides first, so this blastomere defines the embryonic pole where ICM forms 3) RC axis is determined by implantation: the ICM enters the uterine wall at an agle relative to the embryonic/abembryonic axis. Leading egg becomes the caudal end. 4) signal from the implanting trophoblast induces a “head organizer” in the anterior hypoblast cell, which secrets Cerebrus. Cerebrus inhibits NOdal creating a Nodal gradient (high caudally, low rostrally). 5) epiblast responds to high Nodal signaling by making the primitive node, where secretion of gooscoid, and BMP inhibitors (chordin, noggin, and follastatin) occurs. **I think 4 and 5 are really in reference to notochord. which defines the primitive axis of the embryo.

Question 27

origin of embryonic ectoderm

Answer 27

epiblast

Question 28

what cell layer is the notochordal process derived from?

Answer 28

mesoderm

Question 29

how is the neural plate formed?

Answer 29

notochord releases Shh, which induces overlying ectoderm to divide more rapidly, forming a thickened mass.

Question 30

outline primary neurulation

Answer 30

notochordal canal regresses –> notochordal plate forms –> notochordal plate infolds to form notochord

Question 31

neuropores

Answer 31

anterior or posterior openings in the neural tube that eventually close.

Question 32

what is secondary neurulation?

Answer 32

an aggregate of undifferentiated cells at the caudal end of the embryo that eventually contacts the central canal of the neural tube.

Question 33

what does the caudal cell mass of secondary neurulation give rise to?

Answer 33

conus medullaris + filum terminale

Question 34

Where are morphogen gradients involved in RC patterning produced?

Answer 34

anterior visceral endoderm rostrally and primitive node caudally.

Question 35

notochordal process?

Answer 35

hollow tube that is the precursor to the notochord

Question 36

what does the notochord give rise to?

Answer 36

nucleus pulpusos + intervertebral disc

Question 37

gastrulation?

Answer 37

This is the phase where the single-layered blastula is reorganized into a trilaminer structure known as the gastrula (consisting of ectoderm, mesoderm, and endoderm).

Question 38

role of sonic hedgehog?

Answer 38

causes overlying ectoderm over notochordal process to differentiate into neuroectoderm. This is how the neural plate forms.

Question 39

where do first folds form during primary neurulation?

Answer 39

caudal rhomoboencephalon

Question 40

what does secondary neurulation give rise to in the adult?

Answer 40

S2 segments and downward (conus medullaris and filum terminale)

Question 41

what is homeotic transformation?

Answer 41

developmental malformation cause by hox gene loss or mis-expression. One bady part develops the characteristics normally found in another part at another site.

Question 42

Where do neural cells undergo cell division

Answer 42

lumenal side of neural tube. they loss their process that attach to opposite side.

Question 43

how is the insular cortex formed?

Answer 43

lateral surface of the telencephalic vesicle folds over itself to form the Sylvian fissure. This “buries” a patch of cortex within the fissure, producing the insular cortex.

Question 44

what is the central canal an embryologic remnant of ?

Answer 44

the neural tube

Question 45

what cell layer is the neural tube derived from?

Answer 45

ectoderm

Question 46

embryological derivation of virtually all cells of the CNS

Answer 46

neuroepithelial layer