4. STEM CELLS

Stem cell (SC) is an undifferentiated cell capable of producing a differentiated progeny and its own copy (i.e. it is capable of a self-renewal). Stem cells give rise to all tissues (they have a broad differential potential). In adulthood, they are responsible for maintenance of tissue homeostasis and for tissue regeneration. These functions are performed by the stem cells throughout the entire lifespan, i.e. in all developmental stages. In the course of development, different populations of stem cells are generated. The generations of stem cells are hierarchically arranged – older (i.e. more primitive) stem cells have a broader developmental potential.

FUNCTION OF STEM CELLS:

• In embryonic development: establishment and development of tissues (units of development)
• Postnatal development: tissue homeostatis (units of tissue maintenance)
• In injury: units of regeneration
• In therapy: SCs can be used in stem cell therapy (units of transplantation)
• In evolution: units of selection

STEM CELL CHARACTERISTICS

SCs differ from other somatic cells in their biological properties:

  • Unlimited self-renewal (asymmetric division)
  • Production of differentiated elements
  • High proliferative capacity (telomerase)
  • Broad differentiation potential (toti-, pluri-, multipotency)
  • SC plasticity (repertoire of their progeny can be changed, e.g. neural SC can produce blood cells )

SCs are the oldest (earliest) cellular elements produced during ontogenesis.

SCs have primitive morphology (no characteristic structures, large nucleo/cytoplasmic ratio, they lack markers of specialized cells). The tissues contain a limited pool of SCs (population of a defined size); SCs occupy certain morphological microenvironment the so called niche (basis of Lieberkühn crypt, subependymal zone of the forebrain, bulge region of the hair follicle). Within a tissue there are several SC generations (their properties might differ) hierarchically arranged – their origin is explained in Figure below:

SC origin