15.1. Egg to Organism Flashcards
Fertilisation…
The egg is fertilised by the sperm cell which forms the zygote.
Sperm contributes half of nuclear DNA and the centriole.
Egg contributes half of nuclear DNA and cytoplasm (containing organelles, nutrients, mRNA and proteins).
Fertilisation triggers very rapid cell division…
In early embryonic division, there is no gap phase.
This means there is a rapid increase of the number of cells, but does not increase the overall size of the embryo.
Cleavage involves modified cell cycle of S and M phases, driven by high cyclin-CDK activity.
Cleavage results in the formation of the blastula (an animal embryo in early development, where it is just a hollow ball of cells).
Drosophila have cleavage cycles that involve…
Several rounds of nuclear division (without cytokinesis) followed by cellularisation.
Mitosis occurs without cell division.
A syncytium (a single cell with many nuclei) is produced.
The nuclei migrate to the inner edge of the plasma membrane.
Cellularisation occurs creating a blastoderm.
Cleavage in mammals occurs more slowly…
Asynchronous: subsequent cell divisions don’t occur at the same time.
Rotational: the next cell division occurs on one side of the embryo.
This results in the formation of a blastocyst:
- Inner cell mass will form the embryo.
- Trophoblast will contribute to the placenta and umbilical cord.
- The blastocyst cavity contains factors for cellular differentiation.
- Blastula –> gastrula (via gastrulation) –> fully-formed organism (via organogenesis).
Gastrulation…
The process in animals whereby the blastula is transformed into an embryo with distinct tissue layers and body axes.
At the end of the blastula stage, cell division slows down and cells undergo extensive movements, establishing three germ layers:
- Ectoderm (outer layer): forms the nervous system.
- Mesoderm (intermediate layer): forms muscles, blood, bones, gonads and connective tissue.
- Endoderm (inner layer): forms the lining of the gut, liver and the lungs.
Sea urchins…
Some cells change shape and move inwards to form an archenteron.
This causes cells to break free, forming filopodia that attach these cells to the overlying ectoderm.
This causes the archenteron to elongate by contraction of the filopodia.
Frogs…
On one side of the blastula, cells change their shape and cell adhesion properties. They start to move inwards (involution) forming the dorsal lip.
Cells of the animal pole spread out across the surface and will form the ectoderm.
Cells of the vegetal pole move inwards where they will form the ectoderm.
The process of gastrulation leads to the formation of the gastrula.
Cell shape changes…
Epiboly describes the flattening and extension of cells that occurs on the outside of the blastula.
Convergent extension describes the intercalation of cells in the direction of movement that drives involution.
Initiating gastrulation…
(Including the role of the grey crescent)…
Fertilisation triggers a rotation of the cortical cytoplasm that leads to the formation of a grey crescent.
This is caused by mixing of dark pigments from the cortical cytoplasm to the white pigment from the vegetal cortical cytoplasm.
Spemann’s experiments show the grey crescent is required for gastrulation:
- His experiments showed that transplantation of the dorsal lip induced host cells around it to act as if they were dorsal cells.
- Cells communicate to coordinate their individual behaviours.
- This causes involution on both sides of the embryo and induced neurulation on the two opposite sides of the embryo.
Establishing the organiser gene…
At fertilisation, the egg has dishevelled protein (Dsh) on the vegetal side.
Fertilisation triggers a rotation of this cytoplasm, causing the Dsh to move to the dorsal site and releases it from the cortical cytoplasm into the inner cytoplasm of the embryo.
This forms a concentration gradient.
Dsh inhibits GSK3 which inhibits B-catenin by inducing its degradation.
B-catenin is a transcription factor that regulates the activity of organiser genes.
Neurulation…
Formation of the neural tube via invagination of the outer tissue layer, forming a hollow neural tube.
This is regulated by notochord, a cartilaginous rod that runs along the embryo from anterior to posterior.
This will eventually form the spine.
At this stage, it produces factors that signal to the overlying ectodermal tissue to redirect the fate of the cells towards neural tissue.
Somites…
Somites form after the neural tube.
They are mesodermal tissue that form along the length of the neural tube in segments.
This process induced body segmentation.
Each somite divides into three layers of cells:
- The upper layer contributes to the skin (the deep layer).
- The middle layer forms the muscles.
- The lower mesenchyme will form the cartilage of the vertebrae and ribs.
Body segmentation…
Controlled by hox genes (a group of related genes that control the body plan of an embryo along the anterior-posterior axis).
The location of hox genes on the genome mimics the order of expression along the anterior-posterior axis.
Hox genes direct correct cell fates within segments. They ensure the right body parts develop in the right places.
