Describe the phases/stages of a stem cell. What is the most likely type of cells that form during the first few divisions?
Stem cells can be broadly classified into two main types: embryonic stem cells (derived from developing embryos) and adult or somatic stem cells (found in various tissues of the body). The stages or phases of a stem cell's life cycle can vary depending on the type of stem cell. Here, we will focus on the stages of embryonic stem cells.
1. Fertilization and Cleavage: Embryonic stem cells start their journey after fertilization occurs, resulting in the formation of a zygote. The zygote then undergoes a series of rapid cell divisions called cleavage, resulting in the formation of a solid ball of cells called a morula.
2. Blastocyst Formation: The morula develops into a blastocyst around 4-5 days after fertilization. This stage is characterized by the formation of two distinct cell types - the outer layer of cells called the trophectoderm, which will develop into extra-embryonic tissues, and a group of cells inside called the inner cell mass (ICM) or pluripotent cells.
3. Pluripotent Stage: The inner cell mass (ICM) within the blastocyst contains pluripotent stem cells. These cells have the ability to differentiate into any cell type in the body, making them highly versatile. Pluripotent stem cells are typically considered to be in the "naïve" stage, as they have the potential to give rise to all cell types within the body.
4. Commitment and Differentiation: During subsequent divisions, the pluripotent stem cells within the ICM start to commit to specific cell lineages. This commitment gives rise to different types of precursor or progenitor cells that have a more limited potential for differentiation. For example, some cells may commit to becoming neural stem cells, while others may commit to forming cardiac muscle cells.
5. Tissue-Specific Stem Cells: As development progresses, the committed precursor cells further differentiate into specific tissue types, giving rise to tissue-specific stem cells. These tissue-specific stem cells possess the ability to self-renew and differentiate into various specialized cells within their respective tissues. For instance, within the bone marrow, hematopoietic stem cells are responsible for generating different types of blood cells, while neural stem cells in the brain give rise to neurons and glial cells.
During the first few divisions of embryonic stem cells, the most likely types of cells that form are the pluripotent cells. These cells have the ability to develop into any cell type, allowing for the potential formation of all tissues and organs in the developing embryo.