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path from egg to sea urchin

spawning L. pictus urchin


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1) Spawn Sea Urchins: play the Spawn Game


animation of potassium chloride being injected into a sea urchin to induce spawning

A 2cc syringe filled with 0.5M potassium chloride is inserted into the sea urchin between the teeth and the hard outer shell. A small amount of potassium chloride is injected into the urchin on each side (amount varies with size of urchin, 0.1cc/inch/side). This induces the urchin to spawn IF it has any eggs or sperm left.

WARNING: ONLY TEACHERS should do this.

Potassium chloride is potentially lethal if used improperly.


animation of male sea urching spawning and sperm being collected

The injected male urchin is placed on a dry surface mouth side down. The sperm are collected with a glass or plastic pipet and stored in a test tube at 4C for later use.

gear  FEMALE SPAWNINGanimation of a female sea urching spawning and eggs collecting in a beaker

Female sea urchin spawning:The injected female urchin is placed mouth side up over a beaker filled with sea water. The eggs will then be shed into the sea water and settle down to the bottom of the beaker. Store the eggs at the same temperature as would be best for development in this urchin.

see details

2) Observe Sperm Motility

Use a fairly high concentration of sperm to maximise viewing of living sperm under a microscope, 40x objective is best.

gear  2D SPERM MOVEMENT2D animation of sperm moving

The sperm moves by rotating its tail in a spiral motion through the water. This induces waves of force backward propelling the sperm forward.

If the sperm hits a hard surface, like an egg, the spiral motion will cause the entire sperm to rotate.

gear  3D SPERM MOVEMENT Sperm movement 3D

The sperm moves by rotating its tail in a spiral motion through the water. This induces waves of force backward propelling the sperm forward.

If the sperm hits a hard surface, like an egg, the spiral motion will cause the entire sperm to rotate. This is somewhat easier to see in three dimensions.

3) Sperm Events

gear  ACROSOME REACTIONanimation of sperm undergoing the acroscome reaction and fusing with the egg membrane

The red receptors on the sperm come into contact with the egg jelly, yellow. This induces the acrosome reaction causing the acrosome in green to fuse with the plasma membrane of the sperm. The actin in pink goes from a globular state to a filamentous state pushing the front of the sperm outward exposing the binding receptors, blue. The binding receptors can now bind with the egg.

4) Just the Right Amount of Sperm

If too many sperm then you get Polyspermic Development. Use a low concentration of sperm to fertilize eggs.

gear  TOO FEW SPERMtoo little sperm to fertilize the eggs

Too few and the eggs will not be fertilized.

gear  JUST RIGHTJust the right amount of sperm

With just the right number of sperm the egg get fertilized and development occurs normally.

gear  POLYSPERMYanimation of what happens with polyspermy

If two or more sperm make it into the egg polyspermy occurs. In this case some of the genetic material condensed into chromosome is attracted to the extra sperm causing abnormal development and ultimately the death of the embryo. Too many sperm and toxic conditions can lead to polyspermy.

5) When Sperm Meets Egg

sperm contacts the eggsperm fuses with eggfertilization membrane rises

gear  CORTICAL GRANULESthe fusion process

Upon activation the cortical granules fuse with the plasma membrane of the egg. Note the surface proteins on the plasma membrane and the vesicle membrane. When the vesicle fuses with the plasma membrane the proteins on the plasma membrane are displaced and proteins inside the vesicle are now exposed to the outside of the cell. The contents of the vesicle are expelled into the environment. In the case of the special "cortical granules" of the sea urchin egg, this expelling of contents is what raises the fertilization membrane, preventing further sperm entry.

gear  EARLY EVENTSEarly development: fusion, cortical granual release, sperm enters egg

Sperm undergoes the acrosome reaction and binds with the egg. Fusion occurs, actin polymerizes in the egg, the cortical granules fuse with plasma membrane and release their contents. This causes the fertilization membrane to rise. The sperm is pulled into the egg with help from the fertilization cone. Microtubules started at the male centrosome help push the sperm to the center of the egg to fuse with the female pro-nucleus.

6) Cell Division

gear  ZERO-4CELLanimation of fertilization to four cell stage

The sperm enters and binds to the egg surface. This induces a calcium wave that causes the cortical granules, white, to fuse with the plasma membrane, releasing their contents and causing the fertilization membrane to rise. The sperm is then pulled into the egg where it moves to the egg nucleus and fuses. DNA synthesis occurs along with duplication of the centrosome, black. Nuclear envelope breakdown and chromosome condensation leads to the metaphase plate. Anaphase leads to telophase and cell division. The process is repeated.

gear  MITOSIScloseup animation of mitosis

Mitosis is part of the cell cycle. Cycle implying no end. Between fertilization and the blastula stage this is true. Ultimately though, most cells "rest" at "G0", often times for the rest of their lives. This animation concentrates of the roles of the chromosomes and the centrosome (black). The "G" phases are growth phases. "S" is when DNA synthesis occurs. The nuclear envelope is pictured as translucent white when it is present. The rest of the cell and its components have been left out for clarity.


camera  NUCLEAR MIGRATIONVideo of Nuclear Migration

This time-lapse video shows the female pronucleus moving to the center of the embryo where it will fuse with the male pronucleus. The numbers in the lower left corner are minutes after fertilization. Species is Lytichinus pictus at about 18°C. Red arrow points to the nucleus.

camera  1-4 CELLvideo of single cell to four cellSingle Cell to Four Cell

This time-lapse video shows the embryo from 30 minutes to the 4 cell stage. The numbers in the lower left are minutes after fertilization. Species is Lytichinus pictus at about 18°C.

7) Later Development

gear  WHOLE DEVELOPMENTAnimation: from single cell to baby urchin

This animation traces the development of the sea urchin from the single fertilized cell to the baby sea urchin.

Some of the steps along the way:
At the forth division the micromeres form
The swimming blastula "hatches"
Gastrulation forms the mouth from the anus
Prism stage. Embryo starts to feed
Early Pluteus, about 3 days from start
Late Pluteus, about one week
Metamorphosis happens at several months
Producing the baby sea urchin


gear  3D DEVELOPMENT3D animation of single cell to late prism stage

A 3D representation of development in the sea urchin from the one cell to the late prism stage. Note the MICROMERES are highlighted in RED. The micromeres are essential for gastrulation. At the prism stage the free swimming embryo starts to feed. Remember, in deuterostomes, the anus forms first and then the mouth.

See also this page on Virtual Urchin

gear  CROSS ECTION VIEWanimation show steps in gastrolation

Yellow is ectoderm, orange is endoderm, red is primary mesenchyme cells and skeleton, orange "dots" are secondary mesenchyme cells. Process from "blastula" to "pluteus" takes about 48 hours. Note: Anus is formed before the mouth.

video  SLIDE SHOW - URCHINquick slide show of pluteus to sea urchin

The pluteus larva grows over the period of 4-6 weeks, till it is big enough and developed enough to metamorphose. Metamorphosis itself takes place in only a few hours time. The "skeleton" of the pluteus becomes the spines of the new sea urchin. The gut transforms into the test [shell] of the sea urchin. Many thanks to Margaret and Greg for this contribution. If you use this animation, please give them credit. © 2000 Margaret Pizer and Greg Wray, Duke University