Cell Differentiation
Cell differentiation is the process by which unspecialised cells, known as stem cells, develop into specialised cells with distinct structures and functions. This process is fundamental to the development of multicellular organisms, allowing a single fertilised egg to give rise to a complex organism with various tissues, organs, and systems.
Stem Cells
Stem cells originate through the process of mitosis, where a single stem cell divides to produce two daughter cells. These daughter cells can either remain as stem cells or begin the process of differentiation, where they gradually take on the characteristics of specific cell types, such as muscle cells, nerve cells or blood cells. Differentiation is guided by gene expression, where certain genes are turned on or off in response to signals from the cell’s environment, leading to the formation of specialised cells.
Stem cells are classified based on their potential to differentiate into various cell types:
Totipotent
These are the most versatile stem cells and have the ability to differentiate into any cell type in the body, including both embryonic and extraembryonic tissues (such as the placenta). Totipotent cells are found in the earliest stages of development, specifically in the fertilised egg (zygote) and the cells formed immediately after the first few divisions.
Pluripotent
After fertilisation, the zygote undergoes rapid cell divisions, forming a blastocyst about five to six days later. Pluripotent stem cells are found in the embryoblast, or inner cell mass (ICM) of the blastocyst. These cells can differentiate into almost any cell type within the body, forming the three germ layers (ectoderm, mesoderm, endoderm) that give rise to all tissues and organs of the developing embryo. The trophoblast is the outer cell layer that supports the embryo’s development. Although not pluripotent, trophoblast cells differentiate into structures like the placenta, which facilitates nutrient and waste exchange between the mother and embryo.
Multipotent
These cells can differentiate into a limited range of cell types within a particular tissue or organ. For example, haematopoietic stem cells in the bone marrow are multipotent and can give rise to various types of blood cells, but not other cell types like neurons or muscle cells.
Unipotent
These cells are the most limited in their potential, as they can only produce one type of specialised cell. An example are germ line stem cells, which can only differentiate into sperm cells.
