Q.(a) Write the characteristics of 'stem cells'.

Stem Cells

Concept: Stem cells are undifferentiated cells with unique regenerative properties.

1. Characteristics of stem cells: Stem cells possess two defining features. First, they are unspecialized — they lack tissue-specific structures and functions. Second, they exhibit self-renewal, dividing to produce more stem cells indefinitely. Most importantly, they show potency — the ability to differentiate into specialized cell types under appropriate physiological or experimental conditions. Depending on their differentiation potential, they are classified as totipotent (can form all cell types including extraembryonic tissues), pluripotent (can form all three germ layers), or multipotent (limited to specific lineages).
2. Sources of stem cells in humans: Stem cells can be obtained from embryos (embryonic stem cells from the inner cell mass of blastocysts), umbilical cord blood (collected at birth, rich in hematopoietic stem cells), bone marrow (adult stem cells that produce blood cells), and various adult tissues such as adipose tissue, dental pulp, and the brain (tissue-specific adult stem cells).
3. Applications in curing diseases: …

Understanding Stem Cells

Stem cells represent one of biology's most remarkable phenomena: cells that remain in an undifferentiated state yet hold the potential to become almost any specialised cell type in the body. Think of them as biological "blank slates" that can be programmed to replace damaged or diseased tissues. This dual capacity—to replicate themselves indefinitely while retaining the ability to specialise—makes them fundamentally different from ordinary body cells.

(a) Characteristics of Stem Cells

Stem cells possess three defining properties that set them apart:

1. Self-renewal: Stem cells can divide and produce more stem cells through mitosis, maintaining a reservoir of undifferentiated cells. This isn't just ordinary cell division—the daughter cells retain the same stem-cell properties as the parent, allowing the population to persist throughout an organism's lifetime.

2. Unspecialised nature: Unlike muscle cells, nerve cells, or blood cells, stem cells have no specific structural or functional features. They lack tissue-specific structures and don't perform specialised functions until they receive appropriate signals to differentiate.

3. Potency (differentiation capacity): Under suitable physiological or experimental conditions, stem cells can give rise to specialised cell types. The extent of this ability varies:

   • Totipotent cells (like a zygote) can form an entire organism
   • Pluripotent cells (embryonic stem cells) can become any cell type in the body
   • Multipotent cells (adult stem cells) can differentiate into a limited range of cell types within a particular tissue lineage

(b) Sources of Stem Cells in Humans

Stem cells can be harvested from multiple sources, each with distinct advantages:

The umbilical cord and placenta, once discarded as medical waste, are now recognised as valuable sources of stem cells that can be banked for future therapeutic use.

(c) Applications in Treating Human Diseases

Stem cell therapy has moved from experimental promise to clinical reality in several areas:

1. Treatment of blood and immune system disorders

   Haematopoietic stem cell transplantation (bone marrow transplant) is an established treatment for:

   • Leukaemia (blood cancer): Healthy stem cells replace cancerous blood-forming cells after chemotherapy destroys the patient's bone marrow
   • Thalassemia and sickle cell anaemia: Transplanted stem cells produce normal haemoglobin, curing these genetic blood disorders
   • Severe combined immunodeficiency (SCID): Stem cells restore a functional immune system in patients born without one …