Midterm 1 Flashcards
Stages of (frog) Development
- Gametogenesis
- Fertilization
- Cleavage
- Gastrulation
- Organogenesis
- Larval Stages
- Maturity
Blastula forms during …
Cleavage
Blastocoel, blastopore, germ layers and body topology form during …
Gastrulation
Oocyte Content and Cleavage
- Oocyte size and yolk content depends on the needs of the growing embryo
- Cleavage pattern is affected by oocyte structure (yoke content)
T or F: amphibians and amniotes show differences in gastrulation and embryo patterning
T
Sperm vs. Oocytes
Sperm: divide into 4 haploid cells
Oocytes: uneven division of cytoplasm leading to one, large haploid cell that usually has all cytoplasm/material in it
Takeaway: Meiosis between sperm cells and oocytes is different
Why are frog embryos a good model for studying development?
- Easy to understand in 3D
- Produce a lot of eggs
- Larger eggs (relatively)
Blastula
- Forms during fertilization
- Where germ cells are located
Zygote
A fertilized egg, containing a full set of chromosomes from each parent
Blastomere
Cell that results from division of zygote
Cell divisions are ___________ (complete) but unequal
holoblastic
This means the cytoplasm is cut completely
What does the vegetal pole contain? Explain what it is and what it does.
The yolk: mixture of proteins, lipids, carbs, and vitamins that support embryonic growth. Inhibits cell division.
T or F: The animal pole divides faster than the vegetal pole, so there are more cells at the animal pole.
T
Where is the blastocoel and what does it look like?
The blastocoel forms inside the blastula (which forms from the morula), and it is like a liquid-filled center
The 12th Division
Up until the 12th division: Embryo is running on maternally derived RNAs. Zygotic genes are not active yet.
After the 12th division: Zygotic genes transcribed and embryo runs on its own genes. This leads to individual variation.
Mechanism for activating zygotic gene transcription involve epigenetic changes that affect chromatin structure.
Important Purposes of Gastrulation. Generally, what happens?
- Layers
- Planes of symmetry
Cells migrate into interior of developing embryo in a process called involution. From this, topological differences emerge.
Explain topologically inside vs. topologically outside
Topologically inside is when you cannot come in contact with something from the outside
Phylogenetic Classification: Protostomes vs. Deuterostomes
.. and then Amniotes
Protostomes: mouth first
Deuterostomes: mouth second
(Division based on what the first hole gives rise to in gastrulation)
Amniotes have an amniotic sac (i.e., chickens, mice, and humans
Amniotes and their common extra-embryonic membranes
Food: Yolk
Waste management: Allantois
Blanket: Amniotic cavity
Chicken vs. Human embryo
In humans, the embryo gets nutrients from the mother, replacing the function of the allantois and yolk (even though they are still present)
Amniotes (mammals and birds): similar and different patterns
Different early cleavage patterns but similar gastrulation and embryo patterning and overall embryo structure
Where is CNS derived from? Where is PNS derived from?
CNS: ectoderm via neurulation (forms neural tube)
PNS: neural crest cells and placodes (ectodermal structures)
Match the animal to the cleavage pattern.
Mammal
Chick
Holoblastic
Meroblastic
Mammal: holoblastic (complete cleavage)
Chick: meroblastic (incomplete cleavage)
Meroblastic Cleavage
Blastomeres are still partially connected (incomplete cleavage)
Zona Pellucida
- Tough ECM shell that covers early embryo
- Prevents embryo from implanting ectopically
- Blastula digests and “hatches” from zona pellucida, usually in uterus
Ectoderm, Mesoderm, Endoderm and what they give rise to
Ectoderm: nervous system, epidermis, lining of mouth and anus
Mesoderm: dermis, muscle, vasculature, skeleton, gonads, kidneys
Endoderm: stomach, intestine, bladder, lungs
Induction
Influencing cell fate through cell-cell interaction
In what direction does chick neurulation proceed and what end develops earlier?
Anterior to posterior
The anterior end develops earlier
Neural tube closure in mammals
- Tube “zips up” along the middle from anterior to posterior
- Neuropores are holes on either end of the tube filled with amniotic fluid and are eventually closed and form a separate compartment
T or F: The brain cannot develop in the presence of amniotic fluid
T
Craniorachischisis
- Open brain and spinal cord
- Incompatible with life
- Soft tissue doesn’t develop properly - Brain degenerates
Anencephaly
- Failed closure of anterior neuropore
- Brain protrudes from cranium and then degenerates
- Inside skull
Spina Bifida
- Failed closure of posterior neuropore
- Can be covered or exposed
- Can range from mild to severe
Neural tube defects have been related to ___________ deficiencies.
Folate (Vitamin B9)
Match the neuron to the horn.
Sensory neurons
Motor neurons
Dorsal horn
Ventral horn
Sensory neurons: dorsal horn
Motor neurons: ventral horn
Neural tube gives rise to
CNS (brain, spinal cord) and retina
Neural crest gives rise to
PNS (sensory and autonomic neurons, Schwann cells)
Ectodermal placodes give rise to
Special sensory structures (olfactory epithelium, vestibular and auditory inner ear)
Placode
Specialized portions of ectoderm that thicken and differentiate into neural and non-neural structures (stay in epithelium). Forms just outside developing neural plate
Olfactory system/placodes
- Form along with neural plate
- Derived from cells that reside at edge of neural tube from same tissue as CNS
Development of olfactory placode
Curls in/invaginates and then becomes complex (epithelium filled with neurons)
When does neural cell fate specification happen around?
Fertilization
Hans Spemann (1920s) Two-Cell Blastomere Separation
- Zygote has patch of cytoplasm called gray crescent
- Separated blastomeres containing gray crescent produce normal embryos BUT separated blastomeres without gray crescent led to abnormalities: no dorsal tissues, disorganized ventral belly piece
- So, it was concluded that cells from the gray crescent form the dorsal lip of the blastopore
Spemann and Mangold (1920s) Dorsal Blastopore Transplantation
- Transplanted dorsal lip of one newt embryo into ventral surface of another (donor was pigmented for detection and newt embryo was albino)
- Second site of gastrulation and second body axis induced
- Dorsal lip transplant becomes mesodermal structure called notochord
- You get conjoined twins with their own nervous systems, d/v axes, and a/p axes
What is “The Organizer?”
Dorsal lip of blastopore
The Organizer: has dorsal lip cells that…
- Initiate gastrulation
- Dorsalize central tissues (neural induction: ventral ectoderm to neural ectoderm, ventral mesoderm to dorsal mesoderm/somites)
- Define complete body axes (d/v, a/p)
Induction
Process by which embryonic cells in one part of the embryo influence the developmental fate of surrounding cells
Plays crucial role in development of neural ectoderm, eyes/lens, heart
What organizes The Organizer?
The oocyte
- Gray crescent is positioned opposite the point of sperm entry through rotation of oocyte cytoplasm
B-catenin
Transcription factor that initiates dorsal fate
Stabilized in dorsal embryo by protein complex
Acts as dorsal signal (dorsal lip -> notochord -> mesoderm)
Nodal
Protein secreted from vegetal cells that induces mesoderm to form in neighboring cells by interacting with cells just above them
B-catenin and nodal signaling overlap in ______________
The Organizer
What is the blastocoel doing when nodal is at work?
Preventing nodal from affecting the ectoderm
Cells become what before gastrulation?
Mesoderm
Dorsal mesoderm is where gastrulation begins
What about changes in ectodermal cell fate?
The newly formed dorsal mesoderm may interact with the ectoderm, leading it to form the nervous system
Spemann (1918) Ectodermal Transplant in Early vs. Late Gastrula
EARLY:
1. Remove ectoderm (that would become nervous system)
2. Transplant to region that would become ventral epidermis
Result: transplanted ectoderm became epidermis, so dorsal ectoderm takes on identity of final transplant location
LATE:
^ Same procedure as above
Result: transplanted ectoderm becomes nervous system, so dorsal ectoderm restricted to nervous system during gastrulation
Ectoderm and its induction
Ectoderm no induction = epidermis
Ectoderm induction = neural
Ectoderm single cells = neural
Takeaway: Neural cell fate is the default
BMP4
- Produced by ectodermal cells onto each other
- Acts on TGFB receptors to inhibit neural differentiation/neural fate (they become epidermis)
noggin, follistatin, and chordin
- BMP inhibitors released by newly formed dorsal mesoderm
- Disrupt BMP4 signaling in overlying ectoderm, which becomes neural plate
Prosencephalon and its vesicles
Forebrain (most anterior)
1. Telencephalon
2. Diencephalon
Mesencephalon and its vesicles
Midbrain
1. Mesencephalon
Rhombencephalon and its vesicles
Hindbrain (most posterior)
1. Metencephalon
2. Myelencephalon
T or F: Drosophila’s body plan is defined in the embryo (early on in development)
T
AP patterning in flies: A? P?
A: bicoid
P: nanos
These are RNAs unequally deposited into the oocyte by the mother
Bicoid and nanos
- Locally translated after fertilization
- Are morphogens that form opposing concentration gradients
Morphogen
Protein that is non-uniformly distributed and acts in a concentration-dependent manner to determine cell identity
Bicoid LOF vs. GOF
LOF: two tails
GOF: two heads
Bicoid and nanos and the subsequent cascade
Bicoid and nanos set up a cascade of TFs that further divides the embryo into increasingly smaller segments with well-defined borders
The types of genes in the bicoid-nanos cascade
Maternal polarity
Gap genes
Pair-rule genes
Segmented polarity genes
Homeotic/Hox genes
Hox genes
- Encode TFs
- Turn on/off other genes to define identity of fly segments
- Found in clustered arrays
Colinearity
Spatial expression matches genomic location. Anterior-posterior follows 3’-5’ arrangement of genes in cluster
Specific Hox genes and what they do
Ubx: in 3rd thoracic segment, deletion converts T3 to T2 and duplication of T2 makes 2 sets of wings
Antp: expressed in T2 and T3
abd-A: not expressed in T2 nor T3
Hox Code Hypothesis
The identity of a segment can be determined by a unique combination of TFs that are expressed in that cell
T or F: Effects of gene mutations are typically more subtle than in flies
T
T or F: Hox genes are duplicated in multiple clusters in mammals
T
Homolog vs. Paralog vs. Ortholog
Homolog: gene that looks like another gene (related to common ancestor gene)
Paralog: Homologs in same organism
Ortholog: Homologue in another species
A/P patterning in mammals?
Wnt (high in posterior end)
Early gradient of Wnt parses ectoderm into different domains
Wnt-B catenin pathway
- Wnt activates frizzled and LRP5/6
- Protein complex with B-catenin is recruited to receptor complex
- B-catenin released and translocated to nucleus
- B-catenin binds to TCF to form activating TF
Ultimately: regulates gene expression.
Segmentation of the rhombencephalon (hindbrain)
8 rhombomeres each giving ice to unique set of neurons and brain regions (unique combination Hox genes). Transient segments in developing rhombencephalon
What happens when you delete a Hox gene in rhombomere segments?
Facial motor neurons (normal) to trigeminal-like motor neurons
D/V patterning
Also determined by morphogens (gradient of identity)
Recall: dorsal sensory, ventral motor
Notochord
- Transient structure made of axial mesoderm (chordamesoderm), so presence defines chordates
- Induces ectoderm to become neural tube
- Morphogen source of Sonic Hedgehog protein to specify ventral neural tube
- Forms central part of invertebrate discs of spinal column in adult vertebrates
Morphogens and their 3 characteristics
- Emanate from a source
- Diffuse to form non-uniform distribution
- Induce concentration-specific changes in gene expression
Cells closer to morphogen have more of a response
T or F: You don’t need multiple morphogens to give rise to multiple cellular identities–it can be achieved with different concentrations of the same morphogen
T
BMP and Shh
BMP: dorsal to ventral. Roof plate; these cells can now secrete BMP.
Shh: ventral to dorsal. Floor plate; these cells can now secrete Shh.
Shh and other developmental mechanisms
- AP axis of limbs
- Split prosencephalon into left and right hemispheres (affects facial development)
Holoprosencephaly
Failure of prosencephalon to form 2 distinct hemispheres (genesis of corpus callous to cyclopia)
BMP: MAP Kinase and SMAD pathways
BMP receptor gets phosphorylated and activates pathways to affect gene expression
TF Cross-Repression
Expression of one TF inhibits the transcription of another (Olig2 represses Nkx22 and vice versa)
- Happens in single cells (conversion occurs in “winner takes all” process)
- Initial gradient of expression give sharp boundary
Pia
Innermost meningeal layer
Neuroepithelial cells
- Rapidly dividing multipoint stem cells
- Give rise to more neuroepithelial cells
- Give rise to radial glia
Totipotent, pluripotent, and multipotent stem cells
Totipotent: cells in extra embryonic membranes, early cell type, can become anything
Pluripotent: cells in embryo proper, give rise to any cell in embryo proper
Multipotent: give rise to several cells types (neurons and glia arise from radial glia)
Neural progenitor cells
Limited capacity to divide and restricted to becoming on of a few cells types. Not self-renewing.
Neuroblast
No longer dividing cell and differentiates into neuron
Nuclear migration in neural tube directionality
Apical to basal
Stem cell mitosis in neuroepithelium
Go to apical surface to divide and then come back up to basal surface
Symmetric vs asymmetric division of cells in neuroepithelium
Self-renew stem cells vs. give rise to other cell types (neuroblasts)
Asymmetric inheritance of par protein complex
Radial glia can give rise to
Neuroblasts, intermediate progenitors, glia through symmetric and asymmetric divisions
Radial glia are transient cell type
Processes of a glial cell
Highway for daughter cells to migrate
Intermediate Progenitors
Migrate up to subventricular zone (SVZ), resulting daughter cells become neurons and intermediate progenitors
Intermediate progenitors greatly amplify the number of neurons that can be produced by radial glia
Cortical plate
Developing cortex
T or F: The cortex is built for inside out
T
Building cortex with early and late born neurons
Early born neurons migrate to cortical plate and start differentiating. Late born neurons migrate past them into superficial layers. Think: inside first, outside last
Type 1/Classical Lissencephalies (smooth brain, less cortex)
Mutations in several genes that impact neuronal migration/microtubule function
Symptoms: hypotonia, seizures, mental disability, often fatal. Relatively rare.
Proneural genes vs Hes genes
Proneural: trigger neuronal and inhibit stem cell identity
Hes genes: promote stem cell and inhibit neuronal identity
Notch Pathway in radial glial cells
Ligand: delta
Receptor: notch
1. Binding of delta to notch activates He’s genes (stem cell fate promotion)
2. Hes turns off pro neural genes
3. Proneural genes turn on delta expression
What happens to cortex size if you inhibit Notch in radial glia?
Cortex gets smaller because you make all neurons early and don’t invest in the future by creating stem cells that can alter give rise to more neurons). Without notch, you can’t reinforce radial glial cells, so you just get neuroblasts.
NOTCH2NL Deletion
Deletion: microcephaly
Duplication: macrocephaly
Overtime, NOTCH2NL was repaired to correct a disrupted gene, and we now have an extra copy that changed cortical development in modern humans
Neural Crest and the cells there
Cells at neural plate border undergo an epithelial-to-mesenchymal transition–this process is called delamination (neural tube folds over)
Head vs. Trunk neural crest
Head: Sensory neurons, Schwann Cells, Melanocytes, Bone & Cartilage
Trunk: Autonomic neurons (sympathetic and parasympathetic), Sensory neurons Schwann cells, Melanocytes, Adrenal Medullary cells
Post vs Pre ganglionic cells
Post: cell bodies in ganglion (neural crest)
Pre: projections to periphery from spinal cord (neural tube)
Parasympathetic vs Sympathetic neurons and their derivations
Para: vagal and sacral neural crest
Sympathetic: trunk neural crest
LeDouarin’s Quail-Chick Chimeras
Trunk neural crest placed in head became cholinergic and head neural crest placed in trunk became adrenergic
Takeaway: transplanted neural crest took on identity of new position (potential to become other things)
Fate of neural crest cells is strongly influenced by
Environment (e.g., chemical cues)
Dorsolateral pathway vs ventral pathway
Dorsal: melanoctyes
Ventral: Schwann cells, sensory, autonomic neurons, adrenal medullary cells
Adrenal medulla
A modified postganglionic sympathetic ganglion
Ventrally migrating sympathy-adrenal neural crest cells become sympathetic and adrenal chromatin cells (driven by local glucocorticoids from develop adrenal cortex)
