Animal Cleavage Patterns, Early Nematode Development; Gastrulation In Sea Urchins Flashcards
Define cleavage
A series of rapid cell divisions after fertilisation without growth that divides the embryo up into a number of small cells. At the beginning it doesn’t increase in size, divides and cell membrane forms
Explain the reorganisation of an egg cytoplasm
. Egg cytoplasm often reorganises after fertilisation
. Egg axes:
- non-yolky - the animal pole- early development biologists saw cells developed faster on the top part of the cell
- yolky (dense granules)- the vegetal pole- cell division is less active at the bottom part of the embryo. Unpigmented
. Before fertilisation, exhaling radially symmetrical
. After fertilisation cytoplasm streams towards vegetal pole, back towards animal pole but displaced to one side
. Creates a bilateral symmetry: establishes left and right side of the embryo
(Grey crescent of the amphibian egg)
(Yellow cytoplasm of the tunicate egg)
What do the yellow crescent cytoplasm form?
Muscle tissue
Why can we track cell pigmentation?
Because it is unequally divided
What does the yellow cytoplasm contain?
mRNA for a transcription factor called Macho1
What is the Macho1 transcription factor associated with?
Associated with switching on genes associated with muscle cell development transcription factor encodes a protein
How do yellow crescent cytoplasms generate muscle cells?
The yellow cytoplasm contains mRNA for a transcription factor called Macho1 associated with switching on genes associated with muscle cell development transcription factor encodes a protein and that protein has specialised regions that allow it to bind to DNA, it will bind to the regulatory regions upstream if them predator
Describe cleavage
. The pattern of cleavage is very precise and different patterns are observed in different groups of invertebrates
. Involves cell division without cell growth- subdivides the egg and allocates regionally specific cytoplasm to specific cells
What are the 3 types of cleavage?
. Radial
. Spiral
. Superficial
What types of animals are associated with radial cleavage?
Deuterostomes (Echinodermata, tunicates)
What animals are associated with spiral cleavage?
Protosomes (Annelida, mollusca)- animals in the taxonomic groups tend to show the same cleavage pattern)
What animals are associated with superficial cleavage?
Insect
See bottom of page 60
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How do you classify cleavage?
. By if the whole embryo is going through it- complete cleavage- all cells divide
. How much yolk there is
What does it mean if there is more yolk in an area?
The cells will divide slower/ the more yolk the harder it is to divide (why the vegetal pole doesn’t divide as much)
Explain centralleficle cleavage
Large amount of yolk in the centre of the egg. Cells are only able to divide at the level of the nuclei because there is so much yolk it is not possible to put a membrane around them in the early stages- causes insitium
In drosophila- large amount of yolk in the centre of the egg is called what?
Centralleficle cleavage
Explain insitium
When you have several nuclei enclosed in a single membrane
Explain radial cleavage
2 sets of cells stacked one on top of the other
Explain spiral cleavage
One layer of cells is rotated 45 degrees in relation to the layer below- cleavage can be unequal/ daughter cells unequal cleavage patterns
Give facts about worms
. Adult 1mm long
. Easy to see cells with Nomarski microscopy (high contrast)
. 15h embryogenesis
. 959 somatic cells
. 131 cells die during development (apoptosis-born to die)
. Completely transparent- can see cells inside them and count them
. Huge densities of them in soil
. Can be pathogenic
. Easy to keep in the lab
Describe embryo cleavage
. Rotational holoblastic cleavage
. Cleavage is unequal at 2-cell stage: AB cell is larger. Small cell is called the P cell
Describe a fate map
Which cells develop into which tissues (developmental/family tree plotted). Always the same in every individual so the cells have precise instructions of what to become
Middle of page 62: cell lineage and fate maps
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Describe P granules in cytoplasm including before and after fertilisation
. Mixture of RNA and protein
. Before fertilisation they are equally distributed
. After fertilisation movements in the cytoplasm that has a concentration of these granules in the posterior if the early embryo
. Only some give rise to germ cells
. By 26-cell stage are only in P4 cell which gives rise to germ cells
. Supports mosaic development
What is the importance of cell interactions in nematodes? Use an experiment example to explain
. Embryos prodded to rearrange cell position
. Development proceeds normally (but upside down)
. ABa and ABp cell fate changed according to position. (In an experiment they just prodded the embryo to change the position of the ABa and ABp cells. Normal development (nearly) occurred
. Suggests a component of regulative development (affected by environment) in Nematoda- not as rigid as first thought
. So nematodes show a mixture between the two types of development
Why do we use sea urchin eggs in the lab?
Available in large numbers, easy to handle and fertilise, and are transparent- can see behaviour of individual cells as embryo develops. Can see through the embryo. Like other Echinoderms are deuterostomes, like vertebrates and ascidians (sea squirts)- have basic developmental features in common
What is gastrulation?
Formation of guy and main body plan
Echinoderm gastrulation: explain the formation of the archenteron
. At the start we have a ball of cells that is surrounds a fluid filled cavalry called the blastophere
. Bottle cells- the thicker cells at the bottom of the vegetal pole
. Movements of cells- some break off and form different types of cells
. Cells pull up (endoderm) and the tube pulls through and forms the gut, mouth is formed at the top
