Animal Cleavage Patterns, Early Nematode Development; Gastrulation In Sea Urchins

Question 1

Define cleavage

Answer 1

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

Question 2

Explain the reorganisation of an egg cytoplasm

Answer 2

. 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)

Question 3

What do the yellow crescent cytoplasm form?

Answer 3

Muscle tissue

Question 4

Why can we track cell pigmentation?

Answer 4

Because it is unequally divided

Question 5

What does the yellow cytoplasm contain?

Answer 5

mRNA for a transcription factor called Macho1

Question 6

What is the Macho1 transcription factor associated with?

Answer 6

Associated with switching on genes associated with muscle cell development transcription factor encodes a protein

Question 7

How do yellow crescent cytoplasms generate muscle cells?

Answer 7

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

Question 8

Describe cleavage

Answer 8

. 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

Question 9

What are the 3 types of cleavage?

Answer 9

. Radial
. Spiral
. Superficial

Question 10

What types of animals are associated with radial cleavage?

Answer 10

Deuterostomes (Echinodermata, tunicates)

Question 11

What animals are associated with spiral cleavage?

Answer 11

Protosomes (Annelida, mollusca)- animals in the taxonomic groups tend to show the same cleavage pattern)

Question 12

What animals are associated with superficial cleavage?

Answer 12

Insect

Question 13

See bottom of page 60

Answer 13

…

Question 14

How do you classify cleavage?

Answer 14

. By if the whole embryo is going through it- complete cleavage- all cells divide
. How much yolk there is

Question 15

What does it mean if there is more yolk in an area?

Answer 15

The cells will divide slower/ the more yolk the harder it is to divide (why the vegetal pole doesn’t divide as much)

Question 16

Explain centralleficle cleavage

Answer 16

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

Question 17

In drosophila- large amount of yolk in the centre of the egg is called what?

Answer 17

Centralleficle cleavage

Question 18

Explain insitium

Answer 18

When you have several nuclei enclosed in a single membrane

Question 19

Explain radial cleavage

Answer 19

2 sets of cells stacked one on top of the other

Question 20

Explain spiral cleavage

Answer 20

One layer of cells is rotated 45 degrees in relation to the layer below- cleavage can be unequal/ daughter cells unequal cleavage patterns

Question 21

Give facts about worms

Answer 21

. 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

Question 22

Describe embryo cleavage

Answer 22

. Rotational holoblastic cleavage

. Cleavage is unequal at 2-cell stage: AB cell is larger. Small cell is called the P cell

Question 23

Describe a fate map

Answer 23

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

Question 24

Middle of page 62: cell lineage and fate maps

Answer 24

…

Question 25

Describe P granules in cytoplasm including before and after fertilisation

Answer 25

. 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

Question 26

What is the importance of cell interactions in nematodes? Use an experiment example to explain

Answer 26

. 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

Question 27

Why do we use sea urchin eggs in the lab?

Answer 27

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

Question 28

What is gastrulation?

Answer 28

Formation of guy and main body plan

Question 29

Echinoderm gastrulation: explain the formation of the archenteron

Answer 29

. 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