Question

2. (a) Write down the definition of stem cell in your word. (b) Describe the four potency levels of stem cells in (with examples) relation to the development process from a zygote to mature somatic cells.

Answer 1

Stem cells are the cells which has the ability to differentiate into different type of cells. As everyone know that human body loses and replaces a large number of cells daily the cells for replacement come from stem cells. Stem cells are generally of two types i) embryonic stem cells ii) Adult stem cells.

On the basis of potency stem cells are of 4 types

1) Totipotency ( totipotent stem cells) Totipotency means total potential. These are stem cells which have the ability to differentiate into any cell type including cells of extraembryonic tissues. Totipotent cells differentiate into pluripotent cells.

Example fertilized egg and zygote divided upto first 4 celled stage are totipotent cells.

2) Pluripotency ( Pluripotent stem cells) Pluri means many, which means these cells have potential to differentiate into many cells types but not all. These cells can differentiate into any cell types but not into cells of extraembryonic tissues. These cells can differentiate into three germ layer ecdoderm, mesoderm and endoderm.

Example Inner cell mass of Blastocyst stage of embryonic development can different into three germ layers.

3) Multipotent stem cells  these cell type can differentiate into several cell types but can differentiate into limited cell types.

Example Hematopoietic stem cell ( a blood stem cell) is a Multipotent stem cell they can differentiate into all types of blood cells but cannot develop into brain cells.

4) Oligopotency (oligopotent stem cell ) These cells can differentiate into ony few cell types.

Example It includes lymphoid and myeloid cells formed from differentiation of Hematopoietic stem cells. lymphoid cell can give rise to B lymphocytess and T lymphocytes.

Sometimes unipotency term is also used. It includes cells which can differentiate into single type. Like hepatocytes which form cells of liver.

upto Murula Stage totipotent stem Lells, Mitosis MOS Mitosis 2 celled stage 4 Celled Stage minosis 2 8 celled Stage Morula Inner celle Blastocyst (Pluripotent Relis Mesoderm contains blood. Step cells CSYVY Hematopiotic stero cells (Potrop multipotent) Lymphoid (oligopotent) myloich X-B cells - monocytes Neutoophilis L T cell - Eosinophilss L Basophills.