Vertebrate from egg to embryo

Question 1

What makes animals vertebrates?

Answer 1

They’re all:
- Chordate ~ head, dorsal nerve cord, notochord (skeletal structure)
- Cartilage
- Vertebral column
- Jaw
- Bone
- Lungs
- Tetrapod
all vertebrates don’t develop same way, but all have similarities at some point
Similar stage called Pharyngula stage

Question 2

Observations of Ernst Haeckel

Answer 2

Similarities probably due to “bottle-necks” during development that are evolutionarily conserved
Metameric structures - repeated modules, pharyngeal pouches around 5-7 ish and give rise to facial regions and somites
Vertebrates go through very segmented embryonic stage
Think of embryos in all vertebrates as a sand timer:
- Very different so there’s a large spread at the top during early embryogenesis
- Very similar during Pharyngula stage so tight in (bottle-neck)
- Different again so large spread during late embryogenesis

Question 3

Idealised early development (need to update once gone back over lecture)

Answer 3

Egg and sperm haploid, once nuclei fuse to form zygote becomes diploid
Blastula stage, has centre that’s hollow (blastocoel) as all cells moved to the outside (blastoderm)
As divide, cells getting smaller but overall size not increasing due to fact cytoplasm dividing in half for each split into a daughter cell

Question 4

Early cell proliferation is exponential

Answer 4

Nstart * s^(t*f) = Nfinish
N = number of cells
t = time
f = frequency of division
Early stage known by very quick rates of cell division

Question 5

Fertilisation: egg activation (need to update once gone over)

Answer 5

Ca controls rate cell division post fertilisation
Wave of Ca referred to as cell …
Are ways to block Ca wave and if happens development doesn’t begin
If trick unfertilised egg with wave calcium will start developing but won’t go well as it is haploid
After sperm entry wave of free Ca^2+ ions travel across egg, now development can proceed
Ca acts on proteins that control cell cycle to initiate cleavage (cell division)
Waves of Ca go away and then come back and keep repeating this - oscillation of Ca2+
Return of calcium causes new cell division
Oscillation in levels during early development synchronise cell divisions, divisions are very rapid

Question 6

Cell cycle in early embryo

Answer 6

Rapid synchronous cleavages
S and M phases only - no G phase
Transcription suppressed as result lack G phases (specifically G1)
and fact DNA replication and transcription cannot happen at same time
Maternal stores provide RNA and proteins for DNA synthesis and growth

Question 7

Zygotic genome activation

Answer 7

Timing:
- 2 cell stage in mice
- 5,000 cell stage in frogs
Characteristics:
- Cell cycle slows
- Cell cycle becomes asynchronous, no longer coordinated by waves of Ca
- Cell movement begins, stops just being ball of cells
Process sometimes called de novo activation
As time progresses the relative levels of maternal RNA decrease (from the formation) while zygotic RNA increase *(after zygotic genome activation)

Question 8

Gastrulation - formation of three germ layers

Answer 8

Movement cells to inside embryo to form endoderm and mesoderm
Cells that remain on surface form ectoderm
Establishment of true AP and DV axes
Germ layer derivatives:
- Ectoderm
- Mesoderm
- Endoderm
Involves the complex rearrangement of tissues

Question 9

Ectoderm

Answer 9

Neurons
Glia
Epidermis
Pigment cells

Question 10

Mesoderm

Answer 10

Muscle
Cartilage, bone
Dermis
Kidney
Heart
Blood
Other tissues

Question 11

Endoderm

Answer 11

Gut
Lungs
Associated organs

Question 12

Epithelium and mesenchyme are first cell types

Answer 12

Mesenchymal cells move easily and are amorphous (no defined shape)
Epithelial cells more structured and tend to be cuboidal
Can migrate by usually move en masse as a sheet or cluster

Question 13

Cell behaviours in embryo (go back over a check this one)

Answer 13

Mesenchyme can go through condensation - get closer
Epithelium can go to mesenchyme (where cells leave)
Mesenchyme to epithelium (form tube)
Involution - when epithelium folds outwards and disconnects
Invagination - when the epithelium folds outwards but doesn’t disconnect

Question 14

Forces that drive cell and tissue rearrangement

Answer 14

Cell shape changes, cytoskeletal rearrangements
Changes in expression of cell surface proteins, cell adhesion molecules
Migration
Localised cell proliferation
Cell death
These all act during process called Morphogenesis - the creation of shape

Question 15

Somitogenesis

Answer 15

Zebrafish: Early cleavage -> Gastrulation -> Somitogenesis
Somites form from anterior to posterior

Question 16

The vertebrate body is segmental

Answer 16

Somites are a transient structures form following gastrulation in mesoderm
At end gastrulation, mesenchymal cells gather dorsally
Cell on outside edge of somite epithelialize to make it distinct from neighbouring tissue
The somites then disassemble and revert to mesenchymal cells

Question 17

Mesodermal progenitors in somite

Answer 17

Dermomyotome -> Dermis and skeletal muscle
Sclerotome -> Vertebrae and ribs
Segmentation of these structures in adult result of segmentation during somitogenesis
One somite doesn’t give rise to one vertebrae, gives rise to two halves of a vertebrate, posterior of one and anterior of other
Trunk muscle and skeleton are highly segmented

Question 18

Neurulation

Answer 18

Neural tube (brain and spinal cord) arise from ectoderm
Morphogenesis takes place by cell shape changes
Process called neurulation
Initially neural tube just full empty space