NeuroBio Exam 1

Neural Induction

1st step; specification of the neural tissue in the early embryo

Neural Patterning

Step 2; specification of the regions of the nervous system

Neurogenesis and gliogenesis

step 3; birth of neurons and glial cells

Neuronal Migration

Step 4; migration of neurons to their correct place in the brain

Neuronal Morphogenesis

Step 5; acquisition of a specific shape and properties

Formation of synapses and neuronal circuits

step 6

Morula

“mulberry,” stage of embryonic cell division w/ 32 divided cells

blastulation

forms a blastocyst that forms the embryo (embryoblasts) and extra-embryonic tissues, ie placenta (trophoblasts)

Bilaminar disk

-epiblasts move from the top of the bilaminar disk, undergoes GASTRULATION, and forms the 3 germ layers

Trilaminar Disk

3 germ layers

-ectoderm

-mesoderm

-endoderm

When is the nervous system induced?

During Neurulation, after gastrulation

Notochord

forms in the mesoderm, secretes morphogens that cause the formation of the ventral side of the neural tube

Neurulation

1) Neural Induction: specification of the neural tissue (neural plate+borders)

2) Formation of the neural tube (invagination) and the neural crests (in vertebrates)

How does Neurulation occur?

-The neural plate invaginates to form the neural fold and the neural plate borders form the neural folds.

-The neural crests are formed from the neural folds and the neural tube is formed from the neural groove.

Spemann and Mangold Experiments

Does the mesoderm induce the neural tissue?

Gastrulation in the frog as a model system

Fate acquisition during gastrulation in frogs

• when the animal cap (ectoderm) was removed pre-gastrula and placed into culture, epidermal cells formed

• when the animal cap (ectoderm) was removed after gastrulation and placed into culture, neural tissue formed

  • the mesoderm is sufficient for the formation of a nervous system

The Spemann Organizer

-the mesoderm cells act as a neural inducer

-the mesoderm cells organize the host cells to form a new embryo: they act as an organizer

Molecular Nature of the Neural Inducer

Neuralizing activity still present after:

• cells are killed by heat, cold, or alcohol

• freezing, boiling, or acid treatment

• neuralizing activity lost at 150 C

  • activity caused by a protein

NOGGIN, CHORDIN, and FOLLISTATIN

• secreted by the mesoderm

• BLOCK ACTIVIN PATHWAY

• blocking activin causes specification of neural cells

  • if activin is not blocked, no neural fate, embryo does not develop

• blocking the activin pathway (using 1/3 neural inducers or a truncated receptor to the ectoderm) induces early neural fate

Neural Fate is Inhibited in the Ectoderm & Studies

• Isolated animal cap (intact)

  • epidermal cells form

• isolated animal cap (all factors removed)

  • neural tissue forms

• isolated animal cap (activin added)

  • epidermal cells form

Neural Tube Defects: Anencephaly

• impaired closure of the neural tube in the anterior region

• exposition of the developing nervous system to the amniotic fluid leads to degeneration

• prenatal or perinatal death

• 1 in 10,000 births in the US

Neural Tube Defect: Spina Bifida

• impaired closure of the neural tube in the posterior region

• leads to paralysis (severity depending on location)

  • 1 in 2,500 births w/ this condition worldwide

How do we prevent neural tube defects?

Vitamin B12 (folic acid) supplementation during the 1st trimester

Regionalization of Neural Tube: Primary Vesicles

Anterior- Forebrain, midbrain, hindbrain, spinal column- Posterior

Regionalization of the Neural Tube: Secondary Vesicles

Anterior- Telencephalon, Diencephalon, Neural Retina (forebrain), Mesencephalon (midbrain), Metencephalon, Myelencephalon (hindbrain), spinal cord (spinal column)- Posterior

What do the secondary brain vesicles become?

The central nervous system

Homeotic Genes

encode homeodomain transcription factors

HOX Genes

homeotic genes that specify rhombomeres (segments) of the hindbrain

Hereditary Congenital Facial Paresis 3

• symptoms

  • congenital facial palsy, hearing loss, strabismus, upturned nose, midface retrusion, feeding difficulties, speech delay

• Agenesis of Facial Nerve 7

• Mutation in the HoxB1 gene

  • C replaced by T in DNA → Protein R207 replaced by C

Otx2 and Gbx2

• define the midbrain-hindbrain border

  • homeodomain transcription factors that inhibit each other

Midbrain-Hindbrain Border

• signaling organizing center

  • FGF8 and WNT1 are secreted factors expressed at the midbrain-hindbrain boundary

FGF8

• secreted factors expressed at the midbrain-hindbrain boundary

• triggers a transduction signal inside cells

  • FGF8 is a critical factor for the organizer activity

Regionalization of the Nervous System

• defined by a combination of homeodomain transcription factors that regulate the secretion of factors that act on neighboring cells to control their fate

midbrain-hindbrain boundary (genes and factors)

Otx2 & Wnt 1, Gbx2 & FgF8

Patterning along the antero-posterior axis

defined by a combination of homeodomain transcription factors regulating each other

The neural tube gets organized along the…

dorso-ventral axis

How are the different D-V regions specified?

transcription factors

peripheral sensory neurons are located on the ___ of the spinal cord

dorsal side

motor neurons are located on the ___ of the spinal cord

ventral side

Holtfreter experiment (1934)

determined that the notochord is necessary and sufficient for the development of the DV axis of the spinal cord

Shh- Sonic Hedgehog

-morphogens that specify the ventral identity of the neural tube

morphogen

signaling molecule that is:

1. produced locally

2. secreted

3. acts at a distance in a concentration dependent manner

4. specifies the fate/identity of a target cell

sensory neurons come from…

NCCs

Shh signaling w/ Shh

1. Shh binds w/ ___

2. ___ is degraded and SMO is activated

3. leads to the activation of ____

4. ____ activates target genes

1. PTC1

2. PTC1

3. GliA

4. GliA

Shh signaling w/o Shh

1. ___ is active w/o Shh

2. leads to the activation of ___

3. ___ represses transcription

1. PTC 1

2. GliR

3. GliR

Shh diffuses into the ___ in the neural tube from the ___

1. CSF

2. notochord

Concentrations in Neural tube

1. Shh does not reach all the way to the dorsal side

2. GliR is activated on dorsal side, dorsal cells will form

3. GliA is activated on ventral side and ventral cells will form

What does the epidermis secrete?

secretes BMP and Wnt1 (morphogens)

• adding to the neural tube makes cells more dorsal and makes neural crest cells

the epidermis is ___ for the specification of dorsal side

• sufficient

Shh, BMP, and Wnt 1 establish D-V polarity

-epidermis secretes BMP and Wnt1

-notochord secretes Shh

Joubert Syndrome

symptoms

• muscle hypotonia, ataxia, delayed development, intellectual disability, brain defects, TOOTH SIGN

syndromes are ciliopathies: caused by a defect in cillia development or function

No normal Shh or Wnt1 signaling

• abnormal D-V patterning

  • abnormal nervous system development

Ciliopathies

1. defects in cilia formation or function

2. abnormal Shh & Wnt signaling

3. abnormal dorso-ventral patterning

4. abnormal nervous system development

The neural tube contains:

progenitor cells

pial surface

outer surface of the neural tube

ventricular surface

interior side of the neural tube

Shh:

promotes progenitor cell divisions

Progenitor cells

-undergo cell cycles

-located in ventricular zone

-give rise to cells in the brain

what are the regulators of the cell cycle?

Cyclins/CDK

growth and thickness of cerebral cortex depends on:

regulation of cyclins

regulation of the progenitor cell cycle

• overall length of the progenitor cycle increases during development

  • progenitors shift from expansion phase to a neurogenic phase

expansion phase its this one oh my god

fast cell cycle, proliferation

neurogenic phase

slower cell cycle, differentiation

BrdU Labeling

BrdU: Bromodeoxyuridine (synthetic analogue to thymidine)

• incorporated in DNA during DNA synthesis (S Phase)

  • marks cells that divide in the entire animal

  • cells born at the time of injection “birth marking”

• “marked” mitotic cells and their progeny can be visualized by detecting BrdU at later stages

• Pros: easy

• cons: can’t tell morphology

Retroviral Labeling Technique

• modified retrovirus is injected into the neural tube

  • virus can only inject one cell

  • GFB or RFB is placed into code

• cells that are infected and their progeny will glow with GFB

• pros: can tell cell morphology

• cons: very difficult

Main Neural Cell Types in CNS

neuron

oligodendrocyte

astrocyte

ependymal cell

oligodendrocytes

OLIG1 & OLIG2

myelinates axons in CNS

astrocytes

HES &HEY

provide structural and nutritional support, formation and function of synapses

ependymal cells

line ventricles, homeostasis of CSF

microglia

not formed by progenitors, instead formed by mesoderm

progenitors

• can give rise to all 4 cell types early on

• become restricted w/ time

  • self-renew, multipotent

what regulates the differentiation of progenitors?

morphogens and growth factors

How do progenitors differentiate?

• early on, progenitors will show all 3 TFs in a cyclic manner

• 1 TF will be stabilized at the expense of the other 2

CDKi

CDK inhibitor

• inhibits transition between the G1 and S phases by inhibiting CDKs

what inhibits CDKi?

Shh

• SHH promotes transition between G1 and S

if CDKi is inhibited:

• cell cycle is faster

• more progenitors are present

  • cortex is thicker, fewer neurons

If CDKi is increased

• cortex is thinner

• cell cycle is slower

  • less progenitors, more neurons

the overall length of the progenitor cycle:

increases during development

expansion phase

large no. of progenitors transitions to neurogenic phase

bHLH transcription factors

form hetero- or homo-dimers and activate or repress transcription of target genes

bHLH domain

Basic Helix-loop-helix domain for DNA binding and dimer formation

Autosomal Recessive Primary Microcephaly (MCPH)

Smaller but normal brain

• mental retardation, non-progressive

• rare

mitotic spindle doesn’t form, mitosis is impaired

six-layered neocortex

structure unique to mammals

• relative number of neurons in each layer varies among cortical regions

  • cortical layers have various types of neurons w/ different projections

excitatory glutamatergic neurons

generated locally by progenitors in the ventricular zone

inhibitory GABAergic neurons

generated by progenitors in the ventricular zone of the medial ganglionic eminence, then migrate to the cortex

betz/pyramidal cells

only present in level 5 of the primary motor cortex

earliest progenitors give rise to:

neurons in the deepest layers

• timing of differentiation dictates which layer a neuron will end up in

when progenitors are transplanted early:

fate is extrinsically determined

when progenitors are transplanted late:

fate is intrinsically determined

environmental cues

influence early progenitors to produce different types of neurons

competence of progenitors to respond to extrinsic signals becomes:

restricted over time

CSF

produced by choroid plexus in the ventricle

pericytes

cells that form the blood/brain barrier

Neural crest cells

come from the neural plate border

• neural plate invaginates to form the neural groove

  • neural crest cells migrate ventrally to form ventricular structures

SOX10

causes the specification of the ventral Neural Crest Cells

EMT transition

breaks adhesions btwn epithelial cells

• cells then develop migratory properties

the identity of NCCS is established:

along the A-P axis before migration occurs

• can use RVL to trace NCCS

  • extrinsic signaling factors regulate the final differentiation of NCCs (influenced by location)

NCCs give rise to:

• melanocytes

• neurons, glia

• cartilage, bone

• connective tissue

• adrenal gland

• sensory neurons, glia

NCCs form the:

PNS

PNS sensory system

Somatic: sensory

visceral: organs

PNS motor system

• somatic (skeletal muscle innervation)

• autonomic nervous system (innervation of smooth and cardiac muscles, glands)

  • parasympathetic (non-emergency)

  • sympathetic (emergency)

ganglions

contain all cell bodies of the PNS, located along the sides of the spine

• sympathetic: thoracic and lumbar

• parasympathetic: cranial and sacral