Stem Cells: Characterization, Differentiation, and Tissue Formation

Vocabulary and terminology covering stem cell definitions, potency levels, pluripotent characteristics, adult stem cell niches, and clinical therapies.

Stem Cell

A cell that is not itself differentiated (specialized), is capable of self-renewal, and is capable of giving rise to differentiated specialized daughter cells.

Transit Amplifying Cells

Cells that are committed to differentiate and act as middlemen, rapidly dividing to produce more cells before they become final, specialized types.

Potency

A stem cell's ability to differentiate into different types of specialized daughter cells.

Totipotent

The highest level of potency, representing a stem cell's ability to differentiate into any cell type including extra-embryonic tissues like the placenta.

Pluripotent Stem Cells

Stem cells that can differentiate into all three germ layers (ectoderm, endoderm, and mesoderm) but not extra-embryonic tissue.

Multipotent Stem Cells

Stem cells that can give rise to multiple specialized cell types within a specific lineage or tissue, such as hematopoietic stem cells.

Unipotent Stem Cells

The lowest level of potency, representing a stem cell's ability to give rise to only one specialized cell type, such as muscle satellite cells.

Blastocyst

A mammalian embryo at approximately $5$ days post-fertilization containing an inner cell mass and extra-embryonic tissue.

Inner Cell Mass (ICM)

The pink cells within a blastocyst that can be isolated and cultured as embryonic stem cells ($ES$ cells) which will become the embryo proper.

Yamanaka Factors (OSKM)

A defined set of four transcription factors ($Oct4$, $Sox2$, $Klf4$, and $Myc$) used to re-introduce pluripotency into differentiated cells to create induced pluripotent stem cells ($iPSCs$).

Pluripotency Markers

Transcription factors including $Oct4$, $Sox2$, and $Nanog$ that regulate gene expression to maintain pluripotency and control differentiation.

Embryoid Bodies (EBs)

$3D$ structures formed by pluripotent stem cells when cultured in vitro that represent early embryogenesis and differentiation into all three lineages.

Teratoma

A tumor containing cells from all three germ layers, used as a test for pluripotency by injecting cells into immunocompromised mice.

Ectoderm

The germ layer that differentiates into the spine, brain, peripheral nerves, epidermis, hair, nails, and pigmented cells.

Mesoderm

The germ layer that differentiates into cardiac muscle, skeletal muscle, red blood cells, tubule cells of the kidney, and the urinary system.

Endoderm

The germ layer that differentiates into the gastrointestinal tract, respiratory tract, and endocrine glands/organs.

Stem-Cell Niche

A specialized microenvironment or environment that supports adult stem cells and provides necessary signal molecules for self-renewal.

Paneth cells

Cells located at the bottom of intestinal crypts that secrete proteins to kill bacteria and serve as part of the niche for intestinal stem cells.

Satellite Cells

Resident adult stem cells for skeletal muscle that are unipotent and quiescent until activated by injury to re-enter the cell cycle.

Organoids

A stem cell model used to simulate tissue and organ development in vitro by modulating signaling pathways.

Hematopoietic stem cell transplantation (HSCT)

An $FDA$-approved therapy since $1956$ used to treat blood cancers like leukemia and lymphoma, as well as non-cancerous disorders like sickle cell disease.

Autologous chondrocyte implantation (ACI)

A therapy for repairing articular cartilage defects in the knee by harvesting, culturing, and re-implanting a patient's own chondrocytes.

Hepatocyte Growth Factor (HGF)

A signaling molecule that triggers cell cycle reentry in muscle satellite cells after an injury.

Enterocytes

Absorptive cells in the intestinal lining that take up nutrients and produce digestive enzymes.