Stem Cells: Introduction and Cell Biology

Exam 3 and Introduction to Stem Cells

Exam 3 was graded and will be distributed.
The lecture will focus on stem cells, the first unit of cell biology for the final exam (Exam 4).
Exam 4 is semi-cumulative, focusing on unit four but requiring knowledge from units one, two, and three.
Several students will take a late version of exam three; students are asked to observe the honor code and not share their graded exams.
The lecture recording will be made available for those who can't attend in person.

The Biological Importance of Stem Cells

Stem cells are crucial to development, tissue maintenance, and therapeutics.
Understanding stem cell biology is fundamental for engineering them for therapeutic applications.
Stem cells operate through various signaling pathways and execute the central dogma.
Stem cells should be considered not in isolation but in the context of other cells and the extracellular matrix (ECM).

Cells in Context

A shift in scale is needed from individual biomolecules to thinking about cells in their environment.
Textbook schematics may have scale issues, but they help visualize cells in context.

Stem Cells of the Skin: A Model

The skin has multiple layers, including a dead skin cell layer full of keratin.
Dead skin cells are robustly coupled, forming a waterproof barrier.
The skin continually produces cells via proliferation in the basal layer, where stem cells reside.
Stem cells in the basal layer sit on the basement membrane (basal lamina), an extracellular matrix.
Contact with the basal lamina provides information to the stem cells.

Signals from the Basal Lamina

Stem cells receive information from the basal lamina and cells in the dermis.
This information includes contact with the extracellular matrix and soluble ligands.
Stem cells reside in a niche, which influences their behavior.
Anaphase figures indicate proliferative cells in the basal layer.

Stem Cell Niche

A stem cell niche is the microenvironment around a stem cell that provides signals to regulate its behavior.
A stem cell divides into two daughter cells: one remains a stem cell, and the other differentiates.
The stem cell retains its proliferative capacity and does not specialize.

Implications of Stem Cell Division

If a stem cell divides to give two differentiating cells, the stem cell population is eliminated, impairing tissue renewal.
If both cells differentiate, a tumor might not form if differentiation leads to being post-proliferative.
As long as one daughter cell remains a stem cell, the tissue can renew itself.

What Defines a Stem Cell?

The niche dictates whether a cell remains a stem cell.
A niche can be another cell, extracellular matrix, a soluble protein (growth factor), or a combination.
Contact with the niche correlates with being a stem cell.

Examples of Niches

The niche can be a cell, ECM, or a protein gradient.
The field of stem cell biology is dynamic, and understanding niches is a work in progress.
Cells from the dermis provide the niche for epidermal cells in the skin.
The dermis and epidermis are separated by an extracellular matrix.
EGF (Epidermal Growth Factor) is a small diffusible protein made in the dermis that acts as a niche signal.
Skin stem cells express receptors (EGFR) for this growth factor.
Integrins on stem cells contact the basement membrane ECM.

Asymmetric Division

Asymmetric division results in daughter cells that are genetically identical but have different fates.
The geometry and physical orientation of division matter for stem cell asymmetric division.
The size and ultimate fate of the daughter cells can differ.
S phase occurs with high fidelity, and both daughter cells get an equal copy of the genome.
Epigenetic changes can occur rapidly after the differentiating daughter cell leaves the niche.

Cytoskeletal Dynamics in Asymmetric Division

Microtubules reach out to the plasma membrane and cortex, interacting with dynein.
Dynein, a minus-end directed motor, reels the microtubule toward the centrosome.
Dynein can be strategically placed to tug at microtubules from one part of the spindle.

Parallel vs. Perpendicular Divisions

Parallel division: Spindle is parallel to the basement membrane, giving rise to two stem cells that inherit contacts with the niche.
Perpendicular division: Spindle is perpendicular to the basement membrane, resulting in a differentiating cell.
The ratio of parallel to perpendicular divisions depends on whether the organism needs to increase the surface area of the tissue or perform differentiation.

True Statement About Skin Cells

Both daughter cells from a parallel division can be stem cells because they inherit contact with the niche.
The daughter cell born out of contact with the ECM becomes a differentiating cell.
Parallel divisions expand the stem cell population.

Differentiation

When a cell is not on the basement membrane, it stops expressing niche receptors.
Delta-notch signaling promotes differentiation.
Differentiation means increasing desmosome proteins and keratin.
A mutually exclusive identity exists between stem cells (EGF receptive) and keratinocytes (delta receptive).

Multiple Puzzle Pieces Assembled

Skin protects us from UV, water, air, and abrasion.
Growth factor signaling, delta-notch signaling, and mechanical cell-cell junctions are at play.
Different skin cell layers exist.

Stem Cells of the Small Intestines

The intestine is highly convoluted to increase surface area for secretion and absorption.
Cells have a short lifetime (four days) due to wear and tear.
Cell proliferation occurs at the bottom of the crypts.
Stem cells are protected in the crypts.
Panneth cells provide niche signals to stem cells.

The Niche in the Small Intestine

The niche in this case is another cell (Paneth cells).
Paneth cells tell a cell to be a stem cell and to be proliferative.
Direct cell-cell contact is required between Paneth cells and stem cells.
The daughter cell born out of contact with a Paneth cell becomes a non-stem cell.

Transit Amplifying Cells

Transit amplifying cells are proliferative precursor cells that are not yet specialized for secretion or absorption.
They divide rapidly before differentiating into intestinal cells.
There are three steps: stem cell state, proliferative cell state, and differentiation.
Wnt signaling is important for maintaining the stem cell state.

Blood Cells

Circulating blood is another tissue with wear and tear and finite cell lifetime.
Stem cells of our circulatory blood cell types are continually proliferative throughout our lifetime.
A single stem cell type gives rise to a diverse array of blood cell types.

Why Learn About Stem Cells?

To understand what makes a stem cell stem-like.
To work with differentiated cells.
To differentiate stem cells.
To treat stem cells and genetically modify them.
All are advantages to better understanding stem cell biology.

Classifications

Totipotent: Can give rise to any cell type in the body plus extraembryonic tissues.
Pluripotent: Can give rise to any cell type in the body.
Multipotent: Can give rise to a limited range of cell types.
Unipotent: Can give rise to only one cell type.

Obtaining Stem Cells

The zygote is totipotent.
Hematopoietic stem cells are multipotent.
Pluripotent stem cells can be obtained from early embryos or induced.

Embryonic Stem Cells vs. Induced Pluripotent Stem Cells

Embryonic stem cells (ESCs) are pluripotent cells derived from the inner cell mass of a blastocyst.
Induced pluripotent stem cells (iPSCs) are generated by introducing specific genes into somatic cells, inducing them to revert to a pluripotent state.
IPSCs offer personalized medicine potential but are currently expensive and time-consuming.

In Vitro Fertilization and Stem Cells

In vitro fertilization (IVF) creates embryos outside a person's reproductive tract.
Embryos are genotyped to isolate desired alleles.
Unused embryos can be a source of multipotent or pluripotent stem cells.

Applications for Stem Cells

Stem cells can be used for controlled experiments, tissue engineering, and mechanistic cell biology.
Dedifferentiated cells, transformed to induced pluripotent stem cells (iPSCs) by three transcription factors (Yamanaka factors--Oct4, Sox2, Klf4).

Yamanaka Factors

Yamanaka factors are sufficient to cause dedifferentiation.
They control the genome and regulate each other's expression through feedback loops.