The biggest cell by diameter in the human body is the oocyte; the human egg cell (also known as an ovum). The reason why I mention diameter is because there are some neurons in our body that can be up to a meter long – the ones that go from our spinal cord and extend into our feet. The ovum however is 1000 micrometers in diameter, which is visible to the unaided eye.
The oocyte is released during the female menstrual cycle during a process called ovulation when a mature follicle ruptures and releases the oocyte into the fallopian tube where it is moved along by sweeping hairs called cilia. If the egg becomes fertilized by a sperm, it will implant in the uterine wall. If not, it will be dissolved.
The oocyte is so large because it contributes more material to the developing zygote. Almost all mitochondrial DNA comes from the oocyte. Mitochondria are structures within the cell that produce the energy that the cell needs to function; as a result, many mitochondrial diseases are passed down through the mother.
I would like to make a brief note about Human Stem Cell Research and where the Stem Cells are gathered from. Stem cells are taken from a 5 to 6 day old, fertilized embryo, called a blastocyst. At this stage, the embryo is between 50 to 100 cells large. The reason why these cells are so valuable, is because they are pluripotent; meaning they have not yet differentiated into the different organ systems that make a fetus. These cells can become anything, and therefore have great potential in health research.
Want to know what is the smallest cell in the human body?