Cell specialisation

stem cells.

Stem cells are unspecialised cells from which all other cell types may be derived in a multicellular organism.

They possess two key traits which allow them to form the myriad of different tissues and organs in a body:

• Self-Renewal: They have an unlimited capacity for growth (they can continuously divide and replicate)

• Potency: They retain the capacity to differentiate into specialised cell types (they are progenitor cells)

types of stem cells

Totipotent

• only exist in very early-stage embryo

• differentiate into all cell types, including those that support embryonic development (e.g. placenta)

• very useful in stem cell therapies

Pluripotent

• only exist in early-stage embryo

• can differentiate into most cell types

• cannot produce a complete organism

Multipotent

• occur later in the development of an embryo and remain present throughout life

• differentiate into a limited number of cell types

• bone marrow is an example

Unipotent

• form late in the embryonic stage and exist in the functioning organism

• self-renewal distinguishes them from non-stem cells

• only form a single cell type (e.g. sperm cells in mammals)

stem cell niche

Stem cell niche (IB Biology definition):

A stem cell niche is the microenvironment that surrounds stem cells and provides the chemical and physical signals needed to maintain them, control their self-renewal, and regulate their differentiation.

EMBRYO DEVELOPMENT

EMBRYO DEVELOPMENT

Following fertilisation, an unspecialised zygote will divide and develop into a mass of specialised cells (early embryo) via differentiation.

→This process is driven by the release of gene regulating chemicals (transcription factors) called morphogens.

The impact of the morphogen will be determined by its relative concentration (which decreases as the morphogen diffuses from the source cell).

Cells closer to the morphogen source will receive higher concentrations of morphogen, resulting in the activation of more genes.

Cells further away from the morphogen source receive lower concentrations of morphogen, resulting in the expression of fewer genes.

Hence morphogen gradients control the expression of genes within an early-stage embryo.

DIFFERENTIATION

DIFFERENTIATION

All cells of a multicellular organism share an identical genome - each cell contains the entire set of genes for that organism. Differentiation is the process whereby new cells become more specialised and distinct as they mature. Differentiation is caused by the activation of different genes within a given cell (triggered by chemical signals). Within the nucleus of eukaryotes, the DNA is packaged to form chromatin. The active genes are packaged in an expanded form (euchromatin), whereas inactive genes are packaged in a condensed form (heterochromatin). Specialised cells have their identical genomes packaged differently according to their function.

EMBRYONIC STEM CELLS

ADULT STEM CELLS

ADULT STEM CELLS

Adult stem cells are partially differentiated (multipotent) and have a limited capacity to differentiate. They can form a number of closely related cell types and function as a stock source of cells from which specific tissues can be replenished. Stem cell niches are sites within the body where pools of adult stem cells are maintained in preparation for future proliferation. Niche locations include the bone marrow (which gives rise to the different blood cells) and hair follicles (produces the cells for hair growth and vascularisation).

STEM CELL THERAPY

STEM CELL THERAPY

Embryonic and adult stem cells can be used therapeutically to treat diseases by replacing damaged cells with healthy ones. Embryonic stem cells have a greater potency (can treat more conditions) but there are ethical issues associated with their use (involves the generation and destruction on an embryo). Adult stem cells have less ethical issues and a lower chance of graft rejection (involves use of patient's own cells), but have lower potency and are therefore limited in their potential use (can only treat very specific conditions).