Re: We know how mitosis works, but how are the first cells in your body formed?

Hi!

Basically, everything starts (at least in sexually reproducing species, e.g. most animals) when the sperm and the egg cell unite and consequently fuse (you can see this in the animations which you'll find if you follow the WWW links I listed). This results in the so-called activation of egg metabolism which is a critical and necessary step if further development is to occur. Lots of things (which I won't go into detail about) happen at the periphery (cell membrane) and inside the egg which ready the egg for the cell divisions which turn the zygote (=fertilized egg) into a lump of cells, the embryo. I should mention that the genetic material (=DNA) from the sperm and egg cell, contained in so called pronuclei (which can be seen as 2 separate partial 'nuclei' in the light microscope), also merges to form a true nucleus.

After that many mitotic cell divisions take place. They are in this case called cleavage divisions because they subdivide the egg and all the following cells into 2 new cells (so called blastomeres - that's Greek and literally means parts of the seedling/embryo) either of same or different size (also called symmetric or asymmetric division) depending upon the species one studies and the stage of development.

Of course you can analyze these processes down to the microscopic or even submicroscopic level (genetics) but if you know how mitosis works you're all set to understand in principle the earliest stages in the development of any organism be it an animal or a plant.

Some (I hope) interesting links:

• Bill Wassermann's Developmental Biology Page. Has some interesting pictures and even animations. Different levels.
• StudyWeb. Different levels, various fields including early development.
• Tutorial and overview of human development
• Sea urchin development. Animation!!!
• Sea urchin development. Time lapse video!!!
• Both of the above links can be found here!
• I did a Web search with the Google Search Engine on fertilization, development, and animation and that's what I came up with (please note though that not all the content in the links is suited for your needs. Some of it might be to complicated or detailed to understand right now. But I hope the animations make up for that, at least a little bit.)