Skin Stem Cells Flashcards
What are the layers of the skin?
• Epidermis
− outer stratum corneum
− stratum lucidum
− stratum granulosum
− stratum spinosum
− stratum basale
− Epidermal cells advance outwards through these layers and become more differentiated as they do.
− Skin stem cells localize to the stratum basale, although not all cells in the basale are stem cells (only 10-20%).
• Dermis
− papillary region – project into the epidermis
− reticular region
• Hypodermis
− Not so clearly divided as the epidermis and dermis
− Fat cells are here
What are the cell types of the skin?
- Epidermis → keratinocytes: full of keratin. Adhere to each other and the BM tightly to form a protective barrier.
- Dermis → fibroblasts: main mesenchymal component. Produce collagen and endothelial cells, and form the vasculature
- Hypodermis → adipocytes: energy storage, thermal regulation, niche for immune cells and MSCs
How does skin stratification occur during development?
• Occurs during embryonic development
- Early on, there are ectodermal cells. They are kind of like tissue skin stem cells. There is only one layer.
- Gets more and more stratified during embryogenesis. The basal layer remains the more immature. Each subsequent layer is more and more differentiated
- The top layer (cornified layer) is really just dead cells.
• p63 important – in p63 KO mice, no epidermis forms, ectoderm doesn’t differentiate. Loss of epidermal development also effects dermal development.
How does the epidermis effect dermal development?
• Epidermis is an epithelial layer, dermis is a mesenchymal layer
• Epithelia and mesenchyme communicate continuously → reciprocal induction.
• Lack of reciprocal induction can have profound effects on organ development.
• Other organs that depend on epithelial-mesenchymal signaling include: − Teeth − Salivary gland − Mamamry gland − Lung − Pancreas − Liver • The epithelium usually contains the specialized cells of the organ, but it gets its instructions from the mesenchyme.
How do skin appendages form?
- Require E-M communication
- All begin as a placode → the early beginning during embryogenesis where the skin appendage is going to form
- The same epithelial cells are competent to make a tooth, hair, gland etc…
- The information to differentiate comes from the mesenchyme
Initial formation of hair follicles
• Formed before birth
• Stage 0 → cant distinguish where the follicle will form, all cells look the same
• Stage 1 → Get thickening of cells at one point – cell condensation. Both the epidermal and dermal cells condense
• Stage 2 → invagination of the epidermal cells into the derm region
What factors are involved?
• BMP/TGF-B signaling from the epidermis to the dermis required for follicle formation
• Wnt signaling from the dermis to the epidermis required for correct spacing of the follicles
− More Wnt signaling gives more spaces between follicles
− Wnt signals repress the neighbours from making a follicle
What are the 3 stem cell niches of a hair follicle?
- Bulge region of the hair follicle – regenerates the hair follicle
- Basal layer of interfollicular epidermis – between the hair follicles. These SCs are different from bulge SCs. Every 5th cell or so in this region is a SC, the rest aren’t.
- Sebaceous gland of the hair follicle.
What initial experiments suggest that bulge stem cells can migrate to repair injury?
• Under normal circumstances in mice, after 3 weeks of age, each stem cell repopulates only the region in which it is found
• Newborn mice have more migration outside their region.
• In neonatal mice, bulge stem cells can repopulate the epidermis – could be because the skin is growing rapidly and interfollicular cells cant keep up on their own.
• In adult mice, this only happens in response to injury:
− Injury induces stem cell mobilization
− Label cells for sonic hedgehog:
− In unwounded skin, label remains around the follicle
− In wounded skin, the label moves out of the follicle and contributes to the healing region.
Can you heal a wound without follicle bulge cells?
Denis Headon (Here in Manchester)
• Mutant mice with mutation in Edaradd pathway – necessary for hair follicle formation. They have no follicles on the tails.
• Would their tails and see if the wound heals
− An absence of hair follciles delays wound healing
− Does eventually heal, but takes longer
− So hair follicles are not essential but are important.
How is the hair cycle controlled?
• Cycling Wnt/beta-catenin and BMP activity regulate the hair cycle
• Propagating anagen phase:
− Epithelial Wnt/b-catenin activity, dermal noggin and so on induces hair follicle stem cell activation and follicle growth
• Autonomous anagen phase:
− Again, epithelial Wnt/b-catenin activity, dermal noggin and so on needed
− BMP activity is now needed
• Refractory telogen phase:
− Wnt/b-catenin activity stops and this marks the end of the anagen phase
− You no longer have any signals promoting follicle growth
− BMP activity now becomes inhibitory – inhibits stem cell activation and hair follicle growth at high concentrations.
• Competent telogen phase:
− Wnt/b-catenin activity is then turned on again at the start of a new cycle
− It overlaps with the cycling of BMP and counteracts its inhibitor effects.
− Wnt signaling from the papilla (which has moved towards the stem cells) stabilizes b-catenin, which leads to gene expression changes that activat bulge stem cells during the hair cycle
− This counteracts BMP inhibitory effect on bulge SCs.
How do IFE cells work to replenish skin and help with wound healing?
• Skin is replaced every 4 weeks
• If bulge stem cells do not contribute to other regions of the skin, which stem cells do?
• Interfollicular stem cells:
− Basal layer stem cells divide asymmetrically to give rise to daughter cells that repelenish the stem cell but also give rise to a cell that moves upwards, and differentiates into a mature keratinocyte.
− Numb is a cortically localized polarity protein → when a cell goes to divide, it localizes to one side so that you get asymmetric division.
• Normally, IFE stem cells can only contribute to the IFE
Describe the bone marrow chimera analysis used to see how bone marrow cells contribute to wound healing? (Dermal stem cells originate from the bone marrow)
• Used to track BM-derived cells during tissue homeostasis and in response to injury
• If cells express GFP they can be followed in the periphery
1. Harvest bone marrow cells
2. GFP tag them
3. Put them in an irradiated mouse
4. It will reconstitute
5. Wound them after the marrow has reconstituted for > 4 weeks
6. Wound tissue harvested after a set period of time
7. Wound and surrounding healthy tissue sectioned and probed with antibodies against bone marrow cell markers.
8. → After wounding goes from 10% to 50% GFP positive cells
What cell types are present?
• Haematopoietic lineages decline after 21 days
• Non-haematopoetic cells remain steady at 20% at 28 days → so many of these cells are probably MSCs
− MSCs contribute directly by differentiating into endothelial cells
− Also secrete paracrine factors to promote wound healing
How does diabetes affect wound healing?
• Diabetes and old age can effect marrow stem cell recruitment
• Normal:
− Little GFP+ cells at baseline
− Increase after wounding
− After around 14 days this increase begins to reduce
• Diabetes:
− Same initial amount of GFP+ cells in unwounded skin
− After wounding they have a huge influx of cells → unfortunately most of these are inflammatory.
− Diabetics have a heightened inflammatory response
− Inflammatory cells are recruited at the expense of the stem cells
− Endothelial/progenitor cells are ‘under recruited’
− → overexpression of pro-angiogenic factors such as Hoxa3 can modulate this
Dysfunctional bone marrow derived cells contribute to the development of chronic wounds
• Leads to chronic inflammation
• Impaired neovascularization
• Patients with diabetes account for more than 60% of all non-traumatic amputations performed in the US each year.
Diabetic mice have severely impared wound healing:
• Like humans, they have chronic inflammation, aberrant recruitment of bone marrow derived stem cells and severely impaired neovascularization
• Most of these problems stem from dysfunctional bone marrow derived stem cells
− HSCs can give rise to inflammatory cells
− Some types of inflammatory cell secrete destructive molecules → diabetics have skewed differentiation, and are more likely to produce this type of inflammatory cell.
− Diabetics produce little anti-inflammatory cells
Efficient tissue repair and regeneration requires the right balance of BMDC types in the wound environment: • Wild type: − Around 15% EPCs − The rest is an equal split of BMDCs and LKs • Diabetic: − Over 75% LKs − Around 20% BMDCs − Only around 5% EPCs
