Echinoderms

Question 1

when a sperm and egg join in fertilization, what do they produce? by which process does this divide to form a multicellular embryo?

Answer 1

• a zygote

- mitosis

Question 2

why are echinoderms good models for the study of fertilization and embryonic development?

Answer 2

• because they practice external fertilization and shed they’re gametes into the water
• ie everything happens out in the open and not inside of a reproductive tract

Question 3

what are the main parts of a sperm? what is contained in these?

Answer 3

• sperm head –> contains haploid nucleus and specialized vesicle called the acrosome
• the mid-piece –> contains numerous mitochondria
• flagellum

Question 4

what is an acrosome, what does it contain? what is the acrosomal reaction?

Answer 4

• specialized vesicle in the sperm head that contains hydrolytic enzymes that aid the sperm by digesting the thick jelly coat which surrounds the egg in order to reach plasma membrane
• the acrosomal reaction is the fast block to polyspermy by:
  1. aiding in breaking down jelly
  2. proteins on surface of sperm and egg bind to recognize
  3. receptors allow fusion of the 2 plasma membranes

Question 5

why do sperms have numerous mitochondria?

Answer 5

-to synthesize ATP to power the flagellum

Question 6

what are sea urchin/sand dollar egg cells (ovums) surrounded by? what colour is it?

Answer 6

• surrounded by thick jelly coat

- transparent

Question 7

how long does it take the complex series of events to take place in fertilization? what part are you able to actually witness?

Answer 7

• a period of several minutes

- formation of the fertilization envelope from the vitelline layer

Question 8

how does an egg ensure the incoming sperm is of the same species?

Answer 8

Molecules on sperm surface bind to receptors on the egg surface, allowing for identification

Question 9

what is polyspermy? how does an egg stop polyspermy?

Answer 9

• fertilization of an egg by more than one sperm.

- egg stops polyspermy by forming fertilization envelope

Question 10

how is the fertilization envelope made/what triggers it? what reaction is this?

Answer 10

• contact with the sperm triggers the release of calcium within the egg cell –> this causes exocytosis of cortical granules –> this turns the vitelline layer into a hardened layer (the envelope)
• Cortical Reaction

Question 11

what are the 2 reactions to aid against polyspermy? (according to slides)

Answer 11

1. The fast block (Acrosomal Reaction)

2. slow block (Cortical Reaction) to polyspermy

Question 12

how does a fertilized egg appear vs an unfertilized egg?

Answer 12

fert –> has ring around it “halo”

unfert –> does not

Question 13

what happens when calcium is released inside the oocyte?

Answer 13

the cell rsumes cell division by mitosis –> this is called egg activation

Question 14

what are ionophores? what do they do? what does the ionophore in this experiment do?

Answer 14

they are lipophilic molecules that can carry certain ions across cell membranes
-carries calcium ions across the membrane from area of higher concentration to area of lower concentration

Question 15

in which direction will the ionophore carry Ca+2 across the egg’s plasma membrane? why?

Answer 15

will carry Ca+2 from the seawater side (approx 10mM Ca+2) to the inside of ova (approx 0.1mM of Ca+2) because the ionophore works to Ca+2 from area of higher concentration to area of lower concentration

Question 16

what happens after fertilization? what is this called?

Answer 16

• cleavage

- a period of rapid cell division without growth

Question 17

what does cleavage do? what are these known as?

Answer 17

cleavage partitions the cytoplasm of one large cell (the zygote) into many smaller cells
-smaller cells are known as blastomeres

Question 18

what does the 1st cleavage produce? 2nd? 3rd?

how long does this process go on for?

Answer 18

1st –> 2-cell embryo
2nd –> 4-cell embryo
3rd –> 8-cell embryo

continues until the seventh cleavage produces an embryo called a blastula

Question 19

what is a blastula? is this still contained in the fertilization envelope? what is the next step for a blastula?

Answer 19

• a hollow ball of cells with a central fluid filled blastocoel
• yes
• begins to rotate using cilia it develops on its outersurface
• then releases enzymes to digest fert envelope to emerge as a free sswimming “hatched” blastula

Question 20

what does the process of gastrulation do?

Answer 20

rearranges the cells of a blastula into a more complex, three-layered embryo, called a gastrula

Question 21

what are the 3 layers that develop during in a gastrula?

Answer 21

ectoderm –> outside layer
-forms the outer epidermis and its derivatives- brain, nerves, etc.
endoderm –> inside layer
-lines the digestive tract and its derivatives- liver, pancreas, etc
mesoderm –> middle layer
-partly fills the space between the endoderm and ectoderm- the muscles, skeleton, connective tissues, etc.

Question 22

what is the internal cavity that gastrulation forms? what does this eventually form? what is the opening to this cavity called? what does it turn into in echinoderms?

Answer 22

archenteron –> later develops into the gut

blastopore –> eventually turns into the anus

Question 23

what are two kinds of developement?

Answer 23

protostome –> blastopore turns into mouth

deuterostome –> blastopore turns into anus

Question 24

what marks the transition of the gastrula to the larva?

Answer 24

• the completion of the gut –> means the archenteron fully elongates and makes contact with the outerwall where the mouth will develop
• this is the stage at which the animal will begin to feed and grow

Question 25

what kind of symmetry do echinoderms have?

Answer 25

• as larva –> bilateral symmetry

- as adults –> radial symmetry