Stem Cells

Topics: Stem cell, Cellular differentiation, Bone marrow Pages: 10 (1360 words) Published: June 6, 2013
Stem Cells

Tissue Renewal


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


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


– 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

Stem Cell Division Strategies


Stem Cells
• To illustrate the behavior of stem cells we will look at two different stem cell tissue systems – skin – blood


Skin Tissue
• Epidermis
– keratinocytes – melanocytes – dendritic cells

• Extracellular Matrix
– specialized cells – proteins

• Hypodermis
– fats – proteins


• Fundamental component of skin • Derived structures
– fingernails, hair – horns, antlers, feathers – glands

• Interfollicular epidermis

– simple architecture

Interfollicular Epidermis
• Comprised of multiple layers of cells called


– also keratin proteins synthesized and secreted by keratinocytes

• Keratinocytes in different layers are different


Keratinocyte Types
• Basal cells
– attached to basal lamina – basil lamina attached to extracellular matrix

• Prickle cells (named for visible desmosomes) • Granular cells – last layer of living cells

• Keratinized squames • Shedding squames

– dead cells, no organelles

Basal Cells
• Dividing cells of the epidermis • New basal cells have one of two fates – add to population of basal cells – begin differentiation into other epidermal types

• Differentiation from basal to shedded squame • Keratinized squame or shedded squame – terminally differentiated epidermal cell

• Basal cells = skin tissue stem cells

Basal Skin Cells
• There are different kinds of basal skin cells

• Seemingly identical basal cells exhibit different characteristics in culture – – – – –

non-dividing stop dividing after several divisions continuous division, form large colonies basal and terminally differentiated produced

• Proliferation appears related to synthesis of integrin protein(atach the extracellular matrix to the cytoskeleton, cell signaling, communicating

– required for adhesion to basal lamina

Transit Amplifying Cells
• Skin Stem Cells
– basal skin cells, but only rarely divide – when dividing, exhibit high expression of integrin – given the name, “transit amplifying cells” – – – – – – derived from division of stem cells undergo several rounds of division leave basal...
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