Question

Discuss the biological importance of stem cells in relation to development, regeneration and tissue homeostasis in animals

Answer 1

Stem cells are undifferentiated (unspecialized) cells that have the potential to differentiate, or develop into, a variety of specific cell types like liver cells, muscle cells, blood cells, and other cells by a process called differentiation which have specific functions.

On division, stem cell produces one cell that retains its undifferentiated character; and a second cell that can undergo one or more paths of differentiation. Thus, a stem cell has the potential to renew itself at each division.

The two major divisions of stem cells are 1. Zygotic stem cells have the ability to generate an entire organism. They are called totipotent stem cells. This totipotency is retained by early progeny of the zygote up to the eight-cell stage of the morula. 2. Embryonic stem cells are derived from the inner cell mass of mammalian blastocysts. They are capable to form all cells of body except trophoblast. They are also called pluripotent stem cells. 3. Adult stem cells are found in the tissues of matured organs. They are involved in replacing and repairing tissues of that particular organ. They are called multipotent or unipotent stem cells.

Both zygotic and embryonic stem cells play major role in development. By cleavage, a series of rapid mitotic divisions, the single celled zygote converted into a multicellular structure called morula which is a 16-celled stage. This process is possible due totipotency of zygote. Later, morula is converted into blastula (blastocyst).

Blastocyst consists of inner cell mass and trophoblast. The inner cell mass contains cells called stem cells which have potency to give rise to all tissues and organs. These cells form three germ layers- ectoderm, mesoderm and endoderm. From these germ layers, all tissues, organs and organ-systems are formed.

Adult stem cells play role in major role in regeneration. Regeneration refers to the proliferation of cells to replace lost structures, such as the growth of an amputated limb in amphibians. In mammals, whole organs and complex tissues rarely regenerate after injury, but tissues with high proliferative capacity, such as the hematopoietic system and the epithelia of the skin and gastrointestinal tract, renew themselves continuously and can regenerate after injury, as long as the stem cells of these tissues are not destroyed.

The hematopoietic stem cell can form the granulocyte, platelet, and red blood cell lineages. Blood-forming (hematopoietic) stem cell is located in the bone marrow and generates a second stem cell that is capable of becoming either a lymphocyte progenitor cell (which divides to form the cells of the immune system) or a myeloid stem cell (which forms the blood cell precursors). Some adult stem cells are unipotent stem cells, which are found in particular tissues and are involved in regenerating a particular type of cell. Spermatogonia, for example, are stem cells that give rise only to sperm.

Each tissue/organ responds to injury and maintains the tissue homeostatis by quickly repairing damaged cells. The homeostatic replacement of cells varies among different tissues. The epithelium of the intestine is one of the most rapidly self-renewing tissue in adult mammals and it completely self-renews in around 5 days. Interfollicular epidermis takes 4 weeks to renew whereas the lung epithelium can take as long as 6 months to be replaced. Repair most often consist of a combination of regeneration and scar formation by the deposition of collagen which relative contribution depends on the ability of the tissue to regenerate and the extent of the injury. For instance, in superficial injury of the skin, wound can heal through the regeneration of the surface epithelium. However, scar formation is the predominant healing process that occurs when the extracellular matrix framework is damaged by severe injury.