Lecture 3

Question 1

What is the difference between fate, determination and specififcation?

Answer 1

Fate is what is likely to happen to a cell based on its location
Determination is what will happen to a cell as it has reached a point where it can no longer change its fate
Specification is when a cell will turn into a specific cell even if ti recieves no more signalling but its fate can still be changed by new signalling

Question 2

What are the series of steps that begin in a developing embryo after the fusion of the pronuclei?

Answer 2

It first enters the phase of cleavage where it undergos rapid mitotic division
The embryo will then undergo gastrulation which is a dramatic rearrangement of the cells in the embryo to establish the three germ layers
During these phases the major axes of the body are formed and cell acquire their fates
(axial specification can begin earlier however cleavage ALWAYS proceeds gastrulation

Question 3

What occurs in cleavage?

Answer 3

A series of rapid mitotic divisions in which the egg is divided into smaller and smaller cells known as blasotmeres and there is no overall increase in the embryos mass

Question 4

How is the synchrony of cell division during cleavage controlled?

Answer 4

In most organisms both this and the placement of blastomeres relative to each other is under the control of proteins and mRNAs stoed in the oocyte as only later does this process come under control of the zygotic genome

Question 5

How can the mitotic divisions during cleavage occur so much more rapidly than typical cell divisions?

Answer 5

The gap periods G1 and G2 re abolished from the cell cycle resulting in a biphasic cell cycle of M and S phase controlled by MPF

Question 6

What is MPF?

Answer 6

Mitosis promoting factor which controls cleavage, it is a complex of cyclin dependent kinase 2 and cyclin B
MPF is high in the M phase and undectable in the S phase
When cdc2 is bound to cyclin B it is active and phosphorylates key target proteins including histones, nuclear envelope lamin proteins and the regulatory unit of cytoplasmic myosin

Question 7

What controls the levels of cyclin B in the cell during cleavage?

Answer 7

It is controlled by proteins in the egg cytoplasm which regulate its periodic synthesis and degradation
Eventually the cytoplamsic regulators will be used up and the cell will need to begin to produce its own allowing it to enter the mid-blastula transition

Question 8

What occurs in the mid-blastula transition?

Answer 8

the cell cycle lengthens by gaining the G1 and G2 phases and synchrony of division is lost
New mRNAs are transcribed resulting in new proteins being synthesized many of which will initiate gastrulation and specify cell fate

Question 9

What occurs in the mid-blastula transition?

Answer 9

the cell cycle lengthens by gaining the G1 and G2 phases and synchrony of division is lost
New mRNAs are transcribed resulting in new proteins being synthesized many of which will initiate gastrulation and specify cell fate

Question 10

What are the two co-ordinated processes that control cleavage?

Answer 10

karyokinesis which is the mitotic division of the nucleus

cytokinesis which is the division of the cytoplasm of the cell

Question 11

What is the mechanical agent for karyokinesis?

Answer 11

the microtubular mitotic spindle

Question 12

What is the plane of cleavage in spiral cleavage?

Answer 12

There are successive division planes at slight angles to each other resulting in a spiral pattern of blastomeres

Question 13

What are the two parameters that control the cleavage pattern?

Answer 13

The amount and distribution of yolk which inhibits cleavage and time of formation and positioning of the mitotic spindle

Question 14

What is the pattern of cleavage in isolecithal eggs?

Answer 14

Isolechithal eggs have even yolk distribution and cleavage can be radial, bilateral or rotational
it is also is typically holoblastic cleavage

Question 15

What is the patterno of embryone cleavage in mesolechital eggs?

Answer 15

These eggs have a moderate yolk and undergo cleavage which is holoblastic but displaced

Question 16

What is the pattern of cleavage in telolecithal eggs?

Answer 16

These eggs have a dense yolk and undergo meroblastic cleavage

Question 17

What is the pattern of cleavage in centrolecithal eggs?

Answer 17

centrolecithal eggs have a central yolk and cleavage is meroblastic

Question 18

What is the difference between holoblastic and meroblastic cleavage?

Answer 18

Holoblastic is total or entire cleavage while meroblastic is only partial cleavage

Question 19

What is the plane of cleavage in radial cleavage?

Answer 19

The cleavage occurs at right angles to the cell surface giving tiers of blastomeres

Question 20

What is the plane of cleavage in spiral cleavage?

Answer 20

There are successive division planes at slgith angles to each other resulting in a spiral pattern of blastomeres

Question 21

How does the mitotic spindle function during cell division?

Answer 21

During the S phase the centrosome of the cell (a microtubular organizing centre) is composed to two centrioles at right angles this strucure duplicates and forms asters by moving to opposite sides of the nucleus
Spindle fibers then attach to the kinetocore on the chromosome to pull it apart during cell division

Question 22

How is the blastocoel formed?

Answer 22

Fluid accumlates in the blastocoel due to the Na/K pump located on one side of the embryo this drices osmotic water movement to the blastocoel through aquaporin channels
tight junctions on the other side of the cell prevent the water from escaping causing hydrostatic pressure to build resluting in the cells stretching to form the fluid filled cavity known as the blastocoel

Question 23

What are the early cleavage events that go on in the sea urchin?

Answer 23

The first and second cleavages are meridional thrugh the animal and vegetable poles at right angles to each other
the third cleavage is equilateral seperating the animal and vegetable hemispheres
The fourth cleavage results in the animal half cells being divided meridonally into eight equal balstomeres and the vegetable half divides equatorially but unequally to give four large macromeres and four small micromeres

Question 24

How does gastrulation begin in the sea urchin?

Answer 24

With an epithelial to mesenchymal transition with most vegetable cells becomging motile and mesenchymal in form they migrate into the blastocoel as single primary mesenchymal cells and later lay down the spicules of the skeleton.

Question 25

What are the requirements for cellular internalization?

Answer 25

loss of cell-cell adhesion and is associated with repression of cadherin expression and its removal from the cell surface by endocytosis, and the loss of alpha and beta catenins.

Question 26

What follows Internalization of the primary mesenchyme in the sea urchin?

Answer 26

invagination and extension of the endoderm to form the embryonic gut (archenteron)

Question 27

How is the archenteron formed in the sea urchin?

Answer 27

Cells at the tip of the invaginating endoderm (which later detach as the secondary mesenchyme and form pigment and muscle cells and immunocytes) extend long filopodia that adhere to the blastocoel wall and then contract to pull the elongating gut across the blastocoel until it comes into contact and fuses with the mouth region (deuterostome)
Extension is aided by rearrangement of cells in the endodermal sheet, leading to the formation of a tube