Specification: Snails + C.elegans (lec 11)

Question 1

The two main patterns of cleavage are Holoblastic and Meroblastic. Differentiate both.

Answer 1

Holoblastic is a complete cleavage. There are two types: ISOLECITHAL (evenly distributed yolk) and MESOLECITHAL (moderate yolk distribution). There are 4 types of isolecithal distribution: Radical, Spiral, Bilateral, and Rotational.

Meroblastic is an incomplete cleavage. There are two types: TELOLECITHAL (dense yolk throughout the cell) and CENTROLECITHAL (yolk in center of egg). There are two types of telolecithal distribution: Bilateral and Discoidal.

Question 2

What kind of cleavage is seen in the molluscan (snail) embryo?

Answer 2

Spiral (isolecithal –> holoblastic)

Question 3

Fill in the blanks regarding the spiral cleavage of snails.

The _______ ______ divide the cells unequally and at an angle to the ______ and ______ axes.

Each successive quartet of _________ is displaced clockwise (_______) or counterclockwise (_______) relative to its sister _________.

Answer 3

Mitotic spindles; vertical; horizontal

Micromeres; dextral; sinistral; macromere

Question 4

1) What corresponds to cell fates of ectomesoderm, visceral mesoderm and endoderm origins in a snail blastula?

2) Where does the oral opening develop in the blastula of a snail?

Answer 4

1) A clear distribution of DETERMINANTS along the ANIMAL-TO-VEGETAL AXIS corresponds to cell fates of ectomesoderm, visceral mesoderm and endoderm origins in a snail blastula.

2) The oral opening develops just ANTERIOR TO THE blastopore (protostome) and forms at the ventral surface.

Question 5

The origin of sinistral and dextral coiling can be traced to the:

Answer 5

Orientation of the mitotic spindle at the third cleavage.

Question 6

What causes a snail embryo to become sinistral vs dextral?

Answer 6

Maternally deposited factor (formin mRNA /protein) before DIVISION/CLEAVAGE influences coiling pattern!

A sinistral coiling of the snail shows a complete LOSS of FORMIN mRNA!

A dextral coiling of the snail embryo has FORMIN!

Question 7

(T/F) If a mother has a dextral coiling and the father has a sinistral, all of the offsprings will be sinistral like the father!

Answer 7

False!

All offsprings will be dextral! Offsprings display what the mother is!

Question 8

(T/F) At the gastrula stage, formin is no longer seen in dextral embryos.

Answer 8

True!

Seen in the 1 cell, 4 cell, 24 cell and late blastula stage!

Question 9

There are other factors than formin that influence the development of some organs in one region vs others of the two different coiling of snails.

For example, ______ is expressed in the left side in sinistral while is it expressed in the right side in dextral.

Similarly, _____ is expressed in the left side in sinistral and in the right side in the dextral.

Answer 9

Nodal; Pitx

Question 10

What is DPP (decapentaplegic)?

Answer 10

DPP is a BMP-like morphogen that is critical to molluscan (snail) development.

As mitosis continues, DPP mRNA is seen to attend the centrosome in the MACROMERE rather than the centrosome in the MICROMERE of each cell.

Question 11

B-catenin is essential for the formation of the ____, _____, and _____ of ___ derived cells in a snail balstomere.

_______ is also critical for ______ cells derived from ____ cells.

Answer 11

Heart; muscles; hindgut; 4d

Nanos; germ; 4d

*these are the cell fate specifying determinants in the 4d

Question 12

Why are snails a model system for studying embryogenesis?

Answer 12

Snails are a model system for studying embryogenesis because they give a high resolution insight during the very important steps of embryogenesis and how they give rise to the important tissues that are ANALOGOUS to ours.

Question 13

Fill in the blanks regarding the GASTRULATION in the snail Crepidula.

There is _________ of endoderm of snails which is derived from the ________ and the _______ tier of micromeres.

The ectoderm undergoes _______ from the animal pole and _______ the other cells of the embryo.

Answer 13

Internalization; macromeres; fourth

Epiboly; envelops

Question 14

What kind of cleavage is seen in C. elegans?

Answer 14

Rotational cleavage (isolecithal –> holoblastic)

Question 15

What kind of cell lineage is seen in C.elegans and what does it mean?

Answer 15

Invariant cell lineage is seen in C.elegans.

Invariant cell lineage: each cell gives rise to the same number and types of cells in EVERY embryo. This allows us to trace where a cell originated and its progeny.

Question 16

There are _____ cells present in the newly hatched larva of all C.elegans!

Answer 16

558

*it is a very controlled system

Question 17

(T/F) Holoblastic rotational cleavage occurs as the egg is fertilized and moves towards the vulva. The P lineage consists of stem cells that will eventually form the germ cells in C. elegans. All of these stages occur in the ADULTS!

Answer 17

True!

Question 18

Briefly answer the following questions regarding PAR proteins:

1) What are PAR proteins?

2) What kind of PAR proteins are found in C.elegans and where?

Answer 18

1) PAR proteins form and stabilize cortical asymmetries to regulate polarity. Both CYTOSKELETON CONTRACTIONS and PHOSPHORYLATION STATUS of the PAR proteins influence their polarity role.

2)
The cortical cytoplasm (anterior of the cell): PAR 3 and PAR 6

The internal cytoplasm (posterior of the cell): PAR 1 and PAR 2

Question 19

1) What is PEI-1 protien?

2) What do Mex 5 and Mex 6 do to PEI-1?

3) What happens to Mex 5 and 6 in the anterior part of the cell compared to posterior part of the cell?

Answer 19

1) PEI-1+: germ cells, PEI-1-: somatic cells. Presence of PEI-1 leads to formation of germ cells.

2) Mex 5 and Mex 6 proteins degrade PEI-1, so cells can become somatic.

3) In the posterior end, PAR 1 and PAR 2 inhibit MEX 5 and MEX 6, so that PEI-1 is not degraded and there can be formation of germ cells. In the anterior end, PEI-1 is degraded by MEX 5 and MEX 6, leading to formation of somatic cells.

Question 20

What are the two signals produced by P2 cell in C.elegans?

Answer 20

1) JUXTACRINE protein APX-1 (delta) is bound by GLP-1 (notch) on the Abp cell

2) PARACRINE protein MOM-2 (wnt) which is bound by MOM-5 (frizzled) protein on the EMS cell

Question 21

(T/F) Cells become internalized during gastrulation in C.elegans.

Answer 21

True!