Totipotent cells are the fertilized oocyte and the cells of the first few divisions, capable of developing into a complete organism, including both embryonic and extraembryonic tissues (like the placenta).
In more detail, totipotency refers to the ability of a single cell to divide and differentiate into all of the different cell types in an organism. This includes all the somatic cells and the germ cells. Totipotency is the "highest" level of pluripotency. Here's a breakdown:
- Zygote (Fertilized Oocyte): The single cell formed from the fusion of a sperm and an egg is totipotent. It has the potential to create an entire organism.
- Early Blastomeres (Cells of the First Few Divisions): After fertilization, the zygote undergoes cell division (cleavage). The resulting cells, called blastomeres, are also totipotent up to approximately the 4-8 cell stage in mammals. Each of these cells, if isolated, could potentially develop into a complete organism. This is how identical twins can form.
Totipotency vs. Pluripotency vs. Multipotency: It's important to distinguish totipotency from other related terms:
-
Pluripotency: Pluripotent cells can differentiate into any cell type derived from the three germ layers (ectoderm, mesoderm, and endoderm) – essentially, all cells of the body. However, they cannot form extraembryonic tissues like the placenta. Embryonic stem cells (ESCs) are a classic example of pluripotent cells.
-
Multipotency: Multipotent cells can differentiate into a limited number of cell types within a particular lineage. For instance, hematopoietic stem cells in the bone marrow are multipotent, as they can differentiate into various types of blood cells (red blood cells, white blood cells, platelets) but not into, say, brain cells.
Examples:
- A zygote is totipotent
- Blastomeres (early embryo) are totipotent
- Embryonic stem cells are pluripotent
- Hematopoietic stem cells are multipotent
In summary, totipotency is a transient state in early embryonic development, representing the maximum developmental potential of a cell.
