CIE A Level Biology复习笔记5.1.5 The Role of Stem Cells

• A stem cell is a cell that can divide (by mitosis) an unlimited number of times
• Each new cell (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 – totipotent stem cells are stem cells that can differentiate into any cell type found in an embryo, as well as 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 of human embryo development
  • Pluripotency – pluripotent stem cells are embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into extra-embryonic cells (the cells that make up the placenta)
  • Multipotency – multipotent stem cells are adult stem cells that have lost some of the potency associated with embryonic stem cells and are no longer pluripotent

Multipotent adult stem cells

• As tissues, organs and organ systems develop, cells become more and more specialised
• Having differentiated and specialised to fulfil particular roles, most adult cells gradually lose the ability to divide until, eventually, they are no longer able to divide
• However, small numbers of stem cells (known as adult stem cells) remain to produce new cells for the essential processes of growth, cell replacement and tissue repair
• Although these adult stem cells can divide (by mitosis) an unlimited number of times, they are only able to produce a limited range of cell types – they are multipotent
• For example, the stem cells found in bone marrow are multipotent adult stem cells – they can only differentiate into blood cells (red blood cells, monocytes, neutrophils and lymphocytes)
• In adults, stem cells can be found throughout the body (eg. in the bone marrow, skin, gut, heart and brain)
• Research is being carried out on stem cell therapy, which is the introduction of adult stem cells into damaged tissue to treat diseases (eg. leukemia) and injuries (eg. skin burns)

Stem cells can be totipotent, pluripotent or multipotent