Development

Question 1

polarity in the oocyte

Answer 1

Not visible in mammalian eggs
• animal pole
• vegetal pole - yolk

Question 2

Cytoplasmic segregation

Answer 2

• unequal distribution of cytoplasmic components in a fertilised egg
• retained in daughter cells when cell divides
• however, in subsequent divisions, the cytoplasmic contents are separated and cells now have different developmental fates

Question 3

Describe how the gray crescent forms in fertilisation

Answer 3

• occurs in amphibians
• sperm enters the egg at the animal pole
• the sperm centriole initiates the reorganisation of microtubules in the vegetal hemisphere
• the cortical cytoplasm rotates towards the site of sperm entry and grey crescent is formed

Question 4

What is beta catenin?

Answer 4

A transcription factor produced from maternal mRNA that is crucial during development

Question 5

How does fertilisation activate development?

Answer 5

1. the centriole from the sperm initiates cytoplasmic reorganisation
2. causes microtubules in the vegetal hemisphere to form a parallel array to guide cytoplasm
3. as cytoplasm moves, developmental signals are distributed
4. beta-catenin spreads throughout cytoplasm
5. GSK-3 degrades bet-catenin
6. GSK-3 inhibitor inthe vegetal cortec and prevents degrading of beta catenin
7. higher concentration of beta-catenin in dorsal region

Question 6

What is GSK-3? Where is its inhibitor found?

Answer 6

A protein kinase which phosphorylates and degrades beta-catenin
• GSK-3 inhibitor is only found in the vegetal pole

Question 7

Describe complete cleavage

Answer 7

• occurs in eggs with little yolk
• blastomeres are similar in size
• in frogs, the vegetal pole has more yolk; division is unequal
• daughter cells in animal pole is smaller

Question 8

Describe incomplete cleavage

Answer 8

Occurs in eggs with a lot of yolk when cleavage furrows do not penetrate

Question 9

What is discoidal cleavage

Answer 9

The embryo forms as a blastodisc that sits on top of the yolk

Question 10

What is superficial cleavage?

Answer 10

• A variation of incomplete cleavage in insects
• e.g. mitosis occurs without cell division, forming a syncytium - a cell with many nuclei
• the plasma membrane grows inward around nuclei and forms cells

Question 11

What are the different types of cleavage?

Answer 11

• complete cleavage

• incomplete cleavage
- superficial cleavage - insects

• discoidal cleavage
• rotational - mammals

Question 12

Describe cleavage pattern in mammals

Answer 12

• slow and asynchronous
• first cell division is parallel to the animal-vegetal axis and yields 2 blastomeres
• in 2nd division - blastomeres divide at right angles to each other (one is parallel to the axis and the other is perpendicular)

Question 13

What is mosaic development?

Answer 13

• fate of each cell is determined during early divisions

* if one cell is removed, that part of embryo will not develop

Question 14

What does the mesoderm form

Answer 14

• develops from cells that migrate between endoderm and ectoderm
• gives rise to many organs - heart, muscle, bones

Question 15

What does the Endoderm form

Answer 15

gives rise to digestive and respiratory tracts

Question 16

What does the ectoderm form

Answer 16

forms nervous systems, epidermal layer of skin

Question 17

What is regulative development?

Answer 17

• Remaining cells can compensate for cells lose in early cleavages
• in humans, one blastomere can be removed for genetic analysis without harming the embryo

Question 18

What 2 groups form at the 32-cell stage in humans

Answer 18

Inner cell mass - becomes the embryo

Trophoblast - sac that forms the outer cells
• cells secrete fluid to create the blastocoel cavity
• embryo is called a blastocyst