Flashcards in Clevage Deck (24)
Division of cells to form blastula
How rapid is the first division in mammals?
12-36 hours after fertilisation
How rapid is first division on sea urchins?
90 mins after fertilisation
What happens during cleavage?
rapid division of cells, however virtually no growth during this stage, so embryo doesn't enlarge significantly
What are blastomeres?
a cell formed by cleavage of a fertilized ovum.
What is a blastocoel?
fluid filled cavity that forms in the blastula
What is the blastula?
a hollow ball of cells, which is the animal at early stages of embryonic development
What defines polarity in the egg?
The uneven distribution of substances in the cytoplasm such as yolk
What is cortical rotation?
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.
What is the grey crescent?
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
Why are there different cleavage patterns?
depends on amount of yolk and orientation of spindles
What is complete cleavage?
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
How does yolk impact cleavage?
Cells are different sizes so divide asymmetrically
What is incomplete cleavage - discoidal?
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.
What is a blastodisc?
embryo-forming portion of an egg with discoidal cleavage usually appearing as a small disc on the upper surface of the yolk mass.
What is incomplete cleavage - superficial?
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.
What are mitotic spindles?
Structures composed of microtubules which segregate chromosomes into the daughter cells during mitosis. Have an effect on cleavage pattern depending on where they form
What is radial cleavage?
Mitotic spindles are at right angles or parallel to a-v axis It is typical of deuterostomes
What are deuterostomes?
Blastopore becomes anus
What is spiral cleavage?
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
What is rotational cleavage?
First division is parallel to a-v axis, second is at right angles. Common in nematodes
What type of cells occur at early development?
When is the fate of cells become fixed?