AQA A Level Biology复习笔记8.2.1 Totipotent Cells

• A stem cell is a cell that can divide (by mitosis) an unlimited number of times
• Each new cell that is produced when a stem cell divides has the potential to remain a stem cell or to develop into a specialised cell such as a blood cell or a muscle cell (by a process known as differentiation)
• This ability of stem cells to differentiate into more specialised cell types is known as potency
• There are three types of potency:
  • Totipotency
  • Pluripotency
  • Multipotency

   

Totipotent Cells

• Totipotent cells can divide and produce any type of body cell
• This type of stem cell is also referred to as 'embryonic stem cells
• Totipotent cells exist for a limited time in early mammalian embryos, as well as in extra-embryonic cells (the cells that make up the placenta)
• The zygote formed when a sperm cell fertilises an egg cell is totipotent, as are the embryonic cells up to the 16-cell stage (morula) of human embryo development
• Initially, the totipotent cells in the embryo are unspecialised
• During development, totipotent cells begin to translate only part of their DNA, which results in cell specialisation
• These specialised cells then form tissues and are no longer classed as totipotent
• There are no totipotent cells present in the later stages of development as cells lose their ability to differentiate into any cell type

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