Observation of the reproductive system of sea urchins

     The sea urchin is an animal that was related to the starfish.  Some people have described the sea urchin as a round thing that looks like a pincushion.  The fertilization of sea urchin eggs takes place outside the sea urchin's body.  This is how we can collect eggs and sperm from sea urchins and observe in the laboratory how fertilization is carried out. We will also be able to observe how the zygote of the sea urchin divides, as well as the early stages of its development.

MATERIALS:

• Sea urchin eggs and sperm are in separate containers.
• Sea urchin zygotes
• Compound Microscope
• Slides,
•  Coverslips,
•  Excavated Slides, 
• Dropper

METHOD

PART A: Observation of eggs and sperm

1. Place a drop of sea urchin egg suspension on an excavated slide.  Look through the microscope at the eggs with the lowest magnification objective. Make a multi-egg outline.  When you're done with your drawing, save the preparation for later use.

2. Place a drop of sea urchin sperm suspension on a flat slide.  Protect it with a coverslip.  Look through the microscope at the highest magnification of the preparation.  If you can't clearly see the sperm, adjust the lighting through the diaphragm until it looks more accurate.  If the cells you see are moving, you can be sure that you are looking at live sperm.

PART B: Fertilization

1. To the egg preparation you did in part A of point 1, add a drop of sperm suspension. Look through the microscope with the lowest magnification objective of the mixture of the two suspensions.  Look carefully at an egg and focus your attention on the sperm around it. How can you tell when the egg has been fertilized?

PART C: Observation of eggs and sperm

1. Prepare excavated slides using fertilized egg suspensions given to you by your master.  Examine them under the microscope with the lowest magnification objective and look for various phases of their development.  Compare the phases you encounter in your preparation using the drawings in the figure.

2. Look at various egg suspensions and try to find as many phases of sea urchin embryo development as you can.  You may need to observe these suspensions on the second day.

Almost all sexually reproducing organisms have sperm and eggs.  These two cells join together during fertilization and form a zygote.  These are cells and processes that you've observed in the lab. Why is the egg fertilized by a single sperm? In fact, more sperm are produced than eggs. The moment the first sperm penetrates the egg, a membrane is formed around it, called a fertilization membrane, and prevents another sperm from entering the egg.  The zygote divides into two cells, which in turn form four, and so on.  These divisions are made by mitosis.  Based on what you know about mitosis, what must be happening inside the nucleus of each of these cells before they divide? How can you prove that your idea is correct?

From Biología: Modelos y procesos 

Harold G. Liebherr