Stem Cells
Stem Cells and Tissue Renewal
1
The Multicellular “Habitat”
• Cells of multicellular organisms
– highly specialized – over 200 types in human body
• Arranged into tissues, organs, organ systems • Unable to survive outside body “habitat” skin wont survive in heart tissues. They all start up identicals
2
Specialized Cell Types
• Cells of early embryo identical • Differentiation results in specialized cell types
– some differentiated cells continue to divide – many others are “terminally differentiated”
• Tissue stem cells supply replacement cells
– in those tissues where differentiated cells do not divide
3
What is a Stem Cell?
• Not terminally differentiated • Never lose capacity to divide • Daughter cells assume one of two fates – remain a stem cell – commit to terminal differentiation and stop dividing – often pass through intermediate cell types
4
Stem Cells
• Stem cell “systems”—tissues which rely on stem cells for renewal • Basic stem cell questions
5
– what are the molecular characteristics of a stem cell? We don’t know – what determines how and when it divides?we cant predeict when they divide – what determines more stem cells vs differentiated cells?al have identicsl dna seq – what determines which of several differentiated cell types is produced? – complete molecular definition of a stem cell is unknown!
Stem Cell Division
• Environmental asymmetry
– two identical daughter cells produced – some remain stem, some become differentiated – ratio of stem to differentiated controlled by environmental factors – each stem cell division always results in one new stem cell and one cell destined for differentiation – less flexible approach – nonetheless seen in some tissues
• Divisional asymmetry
• Examples of both are known
6
Stem Cell Division Strategies
7
Stem Cells
• To illustrate the behavior of stem cells we will look at two different stem cell tissue systems
– skin – blood
8
Skin
1
The Multicellular “Habitat”
• Cells of multicellular organisms
– highly specialized – over 200 types in human body
• Arranged into tissues, organs, organ systems • Unable to survive outside body “habitat” skin wont survive in heart tissues. They all start up identicals
2
Specialized Cell Types
• Cells of early embryo identical • Differentiation results in specialized cell types
– some differentiated cells continue to divide – many others are “terminally differentiated”
• Tissue stem cells supply replacement cells
– in those tissues where differentiated cells do not divide
3
What is a Stem Cell?
• Not terminally differentiated • Never lose capacity to divide • Daughter cells assume one of two fates – remain a stem cell – commit to terminal differentiation and stop dividing – often pass through intermediate cell types
4
Stem Cells
• Stem cell “systems”—tissues which rely on stem cells for renewal • Basic stem cell questions
5
– what are the molecular characteristics of a stem cell? We don’t know – what determines how and when it divides?we cant predeict when they divide – what determines more stem cells vs differentiated cells?al have identicsl dna seq – what determines which of several differentiated cell types is produced? – complete molecular definition of a stem cell is unknown!
Stem Cell Division
• Environmental asymmetry
– two identical daughter cells produced – some remain stem, some become differentiated – ratio of stem to differentiated controlled by environmental factors – each stem cell division always results in one new stem cell and one cell destined for differentiation – less flexible approach – nonetheless seen in some tissues
• Divisional asymmetry
• Examples of both are known
6
Stem Cell Division Strategies
7
Stem Cells
• To illustrate the behavior of stem cells we will look at two different stem cell tissue systems
– skin – blood
8
Skin