There are an estimated 37 trillion cells and hundreds of different cell types making up the human body but how did we get here from one cell?
PART A
PART B
Have you ever wondered how you came to be from a single cell? then this is the post for you! It is an unimaginable feat of science that all organisms once began as a single cell and, in our case, grew and multiplied to become a complex mass of cells which is now estimated to be over 30 trillion cells. One cell is vital for this transformation and it is aptly named the stem cell.
It all begins with the fertilization of the ovum with sperm where afterwards the fusion of the sperm and ova pronuclei produce a single cell embryo. This cell then divides generating identical daughter cells to produce a 16- 32 cell embryo called a morula, these cells are all totipotent. Next this mass of cells forms a hollow space in the centre called a blastocoel – this change in structure causes the two first distinct cell types in the embryo. The trophectoderm on the outside which will make up the placenta and, in the centre, the inner cell mass (ICM). The ICM cells are what produce the foetus. These cells are also known as embryonic stem cells and it is their pluripotent property that ensures the development and growth of the foetus.
Confused with terminology see my post -'stem cells-an introduction'
The embryonic stem cell can proliferate to produce a large mass of cells, but they are also capable of differentiating to a vast amount of different cell types. To produce these specific cell types the embryonic stem cells rely on a variety of different signals within the embryo which will allow differentiation into the specific cell lineages. The stem cell becomes more specific as certain genes necessary for a cell type are switched on and genes required in pluripotency are switched off. Overall, they can differentiate into the three germ layers necessary to produce a foetus and its specific organs. The ectoderm (central/peripheral nervous system, Skin etc), the endoderm (liver, pancreas, and the lining of organs e.g., the gut lining) and the mesoderm (muscle, bone, and the blood system). Altogether the combination of cell signalling within the embryo and the embryonic stem cells characteristics allow for the formation of a foetus.
Did you know?
The mammalian intestinal lining is renewed every 4-5 days
Once born stem cells are still crucial in the postnatal growth of a child to an adult. Of note are the chondrocytes a multipotent adult stem cell that allows for bone growth. Whilst satellite cells- a unipotent stem cell- found within muscle tissues, ensures muscle growth and repair. Stem cells are also required constantly throughout life and not just during childhood due to the substantial number of cells that are lost and need to be replenished daily. The hematopoietic stem cells are a perfect example as they provide a constant source of red and white blood cells. As, well as, resident skin stem cells that replenish the skin and organ linings constantly. These adult stem cells and many more ensure the growth and maintenance of the body to produce the 30 trillion cells that make up you!
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References
- Images were used with permission of Motifolio
- Sender, R., Fuchs, S. and Milo, R., 2016. Revised estimates for the number of human and bacteria cells in the body. PLoS biology, 14(8), p.e1002533.
- Geens, M., Mateizel, I., Sermon, K., De Rycke, M., Spits, C., Cauffman, G., Devroey, P., Tournaye, H., Liebaers, I. and Van de Velde, H., 2009. Human embryonic stem cell lines derived from single blastomeres of two 4-cell stage embryos. Human reproduction, 24(11), pp.2709-2717.
- Georgadaki, K., Khoury, N., Spandidos, D.A. and Zoumpourlis, V., 2016. The molecular basis of fertilization. International journal of molecular medicine, 38(4), pp.979-986.
- Wuelling, M. and Vortkamp, A., 2011. Chondrocyte proliferation and differentiation. Cartilage and Bone Development and Its Disorders, 21, pp.1-11.
- Rehfeldt, C., Fiedler, I., Dietl, G. and Ender, K., 2000. Myogenesis and postnatal skeletal muscle cell growth as influenced by selection. Livestock Production Science, 66(2), pp.177-188.
- Alison, M.R., Poulsom, R., Forbes, S. and Wright, N.A., 2002. An introduction to stem cells. The Journal of Pathology: A Journal of the Pathological Society of Great Britain and Ireland, 197(4), pp.419-423.
- Rippon, H.J. and Bishop, A.E., 2004. Embryonic stem cells. Cell proliferation, 37(1), pp.23-34
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