Clevage

Question 1

summarise cleavage

Answer 1

Division of cells to form blastula

Question 2

How rapid is the first division in mammals?

Answer 2

12-36 hours after fertilisation

Question 3

How rapid is first division on sea urchins?

Answer 3

90 mins after fertilisation

Question 4

What happens during cleavage?

Answer 4

rapid division of cells, however virtually no growth during this stage, so embryo doesn’t enlarge significantly

Question 5

What are blastomeres?

Answer 5

a cell formed by cleavage of a fertilized ovum.

Question 6

What is a blastocoel?

Answer 6

fluid filled cavity that forms in the blastula

Question 7

What is the blastula?

Answer 7

a hollow ball of cells, which is the animal at early stages of embryonic development

Question 8

What defines polarity in the egg?

Answer 8

The uneven distribution of substances in the cytoplasm such as yolk

Question 9

What is cortical rotation?

Answer 9

Determines the dorsal ventral axis, plasma membrane and associated cortex rotate relative to the inner cytoplasm, animal hemisphere moves towards the vegetal inner cytoplasm in the side where the sperm entered.

Question 10

What is the grey crescent?

Answer 10

Exposed in cortical rotation in some species, light grey area, opposite point of sperm entry, proteins in the gray crescent control what form the cells will take. All becomes the marker for future dorsal side of the embryo

Question 11

Why are there different cleavage patterns?

Answer 11

depends on amount of yolk and orientation of spindles

Question 12

What is complete cleavage?

Answer 12

There is no yolk, so all cells are equal in size. Blastocoel is centrally located and cleavage furrow passes all the way through the cells

Question 13

How does yolk impact cleavage?

Answer 13

Cells are different sizes so divide asymmetrically

Question 14

What is incomplete cleavage - discoidal?

Answer 14

With lots of yolk. the cleavage doesn’t penetrate through, so is incomplete and a blastodisc forms on top of the yolk. Is common in birds, reptiles and insects.

Question 15

What is a blastodisc?

Answer 15

embryo-forming portion of an egg with discoidal cleavage usually appearing as a small disc on the upper surface of the yolk mass.

Question 16

What is incomplete cleavage - superficial?

Answer 16

The zygote nucleus is situated within a mass of yolk. Cleavage begins with nucleus undergoing mitotic division that isn’t accompanied by cytokinesis, eventually migrate to the edge and plasma membranes form, resulting in wall of cells surrounding the yolk.

Question 17

What are mitotic spindles?

Answer 17

Structures composed of microtubules which segregate chromosomes into the daughter cells during mitosis. Have an effect on cleavage pattern depending on where they form

Question 18

What is radial cleavage?

Answer 18

Mitotic spindles are at right angles or parallel to a-v axis It is typical of deuterostomes

Question 19

What are deuterostomes?

Answer 19

Blastopore becomes anus

Question 20

What is spiral cleavage?

Answer 20

Mitotic spindles are at oblique angles to a-v axis, typical of protostomes, and is characterised by by arrangement of blastomeres of upper cell junctions spiralling around pole axis to lower

Question 21

What is rotational cleavage?

Answer 21

First division is parallel to a-v axis, second is at right angles. Common in nematodes

Question 22

What type of cells occur at early development?

Answer 22

Totipotent

Question 23

When is the fate of cells become fixed?

Answer 23

Determination

Question 24

When do cells become structurally and functionally specialised?

Answer 24

Differentiation