Fertilization has three functions:
- transmission of genes from both parents to offspring
- restoration of the diploid number of chromosomes reduced during meiosis
- initiation of development in offspring
The egg and sperm. Sperm are color enhanced (green) while the egg is color enhanced to gold. The above image is modified from http://130.102.208.100/FMRes/FMPro?-db=images.fp3&key=32931&-img.
Sperm on the surface of a human egg. This image is from http://130.102.208.100/FMRes/FMPro?-db=images.fp3&key=32932&-img.
Steps in Fertilization
- Contact between sperm and egg
- Entry of sperm into the egg
- Fusion of egg and sperm nuclei
- Activation of development
Cleavage
Cleavage is the first step in development of ALL multicelled organisms. Cleavage converts a single-celled zygote into a multicelled embryo by mitosis. Usually, the zygotic cytoplasm is divided among the newly formed cells. Frog embryos divide to produce 37,000 cells in a little over 40 hours.
The blastula is produced by mitosis of the zygote, and is a ball of cells surrounding a fluid-filled cavity (the blastocoel). The decreasing size of cells increases their surface to volume ratio, allowing for more efficient oxygen exchange between cells and their environment. RNA and information carrying molecules are distributed to various parts of the blastula, and this molecular differentiation sets the stage for the layering of the body in the next phases of development.
Early events in development of a fertilized egg in amphibians, reptiles, and mammals. The above image is from http://www.biosci.uga.edu/almanac/bio_103/notes/apr_11.html.
Early development compared in a sea urchin and frog. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Information provided by: http://gened.emc.maricopa.edu