When an animal develops from the fusion of an egg and sperm cell, the first cell will divide and divide until a large ball of cells is formed. These cells will go on to produce the entire animal, the placenta and membranes that surround it during development. The organs and tissues formed from these cells are all very different from the original cells – we say that these cells have differentiated. The cells can go down three possible routes: The innermost layer of cells will form all of the guts, lungs and liver. The middle layer of cells will form the skeleton, muscles, heart, blood and kidneys. The outermost layer will form the skin and nervous system. Eventually the developing animal will have a vast array of different cell types e.g. nerve cells, muscle cells, liver cells etc.
All of these cells have the potential to go wrong and form tumours e.g. brain tumours, muscle tumours and liver tumours. Generally, these tumours will be made up of one cell type, e.g. the haemangiosarcoma we saw in a previous post was a tumour originating from the blood vessels, so the tumour itself was made up of a mish-mash of badly formed blood vessels.
In the ovary and testicle there are lots of cells which form the egg and sperm cells (germ cells) of the animal. The job of these germ cells is to make complete new animals, so these cells have the potential to become ANY cell in the body during development of the foetus. If this development goes wrong, and a tumour forms, the cells which make up the tumour could become any cells imaginable!
A teratoma (‘terato’ comes from the greek for monster, and ‘-oma’ comes from the greek for tumour) is a tumour which consists of cells from all three layers of cells found during development. We can find whole teeth, skin, glands, cartilage, bone, hair, muscle, fat, spleen, nerves and many more tissues inside these monstrous tumours. These tumours originate from cells which have the potential to form other types of cells (stem cells), and stem cells are found all over the body, but they are in their largest numbers in the testicles and ovaries. It goes without saying, that these are the most common sites for teratomas.
They cause a problem because they can grow quite large and interfere with the normal function of the organ in which they are growing. Generally they are benign (teratoma), as they contain well-formed cells which don’t behave very aggressively. However, sometimes some of the cell types within the tumour might not form fully or may be altered so that they become invasive and the tumour can be malignant (teratocarcinoma).
In this case, a tumour was found behind the eye of a bird. It was composed of a very bizarre mixture of lots of different tissues: