Every cell in a multicellular organism comes from a single fertilised egg, the zygote. The zygote is totipotent: it can give rise to every cell type in the body and also the placenta and membranes around the embryo.The zygote divides repeatedly by mitosis. In mammals this produces a solid ball of cells and then, after about five days, a hollow ball called the blastocyst. The outer layer of the blastocyst forms the placenta, while the inner cell mass forms the embryo itself. Cells of the inner cell mass are pluripotent: they can become any cell of the body, but not the placenta.Development involves three linked processes:
• growth: an increase in cell number by mitosis and in cell size
• differentiation: cells become specialised for a particular function
• morphogenesis: cells move, change shape and are removed by programmed cell death (apoptosis) to build tissues and organs.Almost all body cells contain the same genes, so specialisation must come from differential gene expression: in each cell type only some genes are switched on. Genes are switched on or off by transcription factors, proteins that bind to the promoter of a gene and start or block transcription.Some genes control the overall body plan. Hox (homeobox) genes code for transcription factors that are expressed in order along the head-to-tail axis of the embryo, telling each region which structures to build. As development proceeds, cells lose potency until most are fully specialised.
• growth: an increase in cell number by mitosis and in cell size
• differentiation: cells become specialised for a particular function
• morphogenesis: cells move, change shape and are removed by programmed cell death (apoptosis) to build tissues and organs.Almost all body cells contain the same genes, so specialisation must come from differential gene expression: in each cell type only some genes are switched on. Genes are switched on or off by transcription factors, proteins that bind to the promoter of a gene and start or block transcription.Some genes control the overall body plan. Hox (homeobox) genes code for transcription factors that are expressed in order along the head-to-tail axis of the embryo, telling each region which structures to build. As development proceeds, cells lose potency until most are fully specialised.