Stem Cells Overview

Stem Cells

• Stem cells possess abilities to self-renew and differentiate into diverse cell types.

Types of Stem Cells

• Embryonic Stem Cells: Pluripotent, able to generate all cell types in an organism.
• Adult Stem Cells (Somatic): Found in specific organs, differentiate only into cell types of that tissue.
  • Examples: Hematopoietic Stem Cells (HSC) in blood, Mesenchymal Stem Cells (MSC) in bone marrow.

Hematopoietic Stem Cells (HSC)

• Differentiates into all blood cell types; less than 1 in 500,000 bone marrow cells.
• Generally quiescent but can proliferate significantly when required.
• Hematopoiesis: The process by which HSCs differentiate into various blood cells.
• Hematopoiesis rates depend on the surrounding environment.

Human Blood Corpuscle Types

• Two lineage paths: Common Myeloid Progenitor (CMP) and Common Lymphoid Progenitor (CLP).

Myeloid-Erythroid Lineage

• First responders to infections.
• CMP Differentiation:
  • Erythroid Progenitor:
    • Generates Erythrocytes (red blood cells) and Platelets.
  • Myeloid Progenitor:
    • Differentiates into Granulocytes (Neutrophils, Basophils, Eosinophils, Mast cells) and Myeloid Antigen Presenting Cells (Monocytes, Macrophages, Dendritic cells).

Granulocytes

• Neutrophils:
  • Lifespan of a few days; primary responders during infections; increased number seen during leukocytosis.
  • Kills pathogens through phagocytosis and antimicrobial protein release.
• Basophils:
  • Contain granules with histamine; involved in allergic reactions; releases cytokines.
• Mast Cells:
  • Found in various tissues; similar functions to basophils; released undifferentiated, differentiate in blood.
• Eosinophils:
  • Phagocytotic and cytokine-secreting cells; important in defense against multicellular organisms; involved in asthma and allergy responses.

Myeloid Antigen-Presenting Cells

• Phagocytic cells capable of antigen presentation, bridging innate and adaptive immunity.

Monocytes

• Heterogeneous, differentiate into various tissue resident cells.
• Two categories: Inflammatory (enter tissue during infection) and Patrolling (resident along blood vessels).

Macrophages

• Tissue-resident monocytes involved in tissue repair and regeneration.
• Can also act as inflammatory macrophages and antigen-presenting cells (APC).

Dendritic Cells

• Arise from both myeloid and lymphoid lineages; most efficient at activating T cells.

Cells of Lymphoid Lineage

• Lymphocytes play a central role in adaptive immunity; make up 20-40% of circulating blood lymphocytes.

Types of Lymphocytes

• Subdivided into B cells, T cells, and Natural Killer (NK) cells; differentiated by surface proteins (Cluster of Differentiation - CD).

B-Lymphocytes

• Mature in the bone marrow (mammals) and Bursa fabricus (birds).
• Differentiate into Plasma cells (secrete antibodies) and Memory cells.

T-Lymphocytes

• Mature in the thymus; express T-cell receptors (TCR).
• Differentiated into T-helper cells, T-cytotoxic cells, and T-regulatory cells.

T Cytotoxic Lymphocytes (TC)

• Express CD8 recognizing antigen-MHC1 complex; eliminate infected, cancer, or foreign graft cells.

T Helper Lymphocytes (TH)

• Express CD4, recognizing antigen-MHCII; further differentiated into TH1, TH2, TH17, and T follicular Helper Cells involved in B cell development.

T Regulatory Lymphocytes (TREG)

• Inhibit immune responses; identified by CD4, CD25, and FoxP3 expression.

Natural Killer Cells (NK)

• Part of innate immunity with non-specific receptors; account for 5-10% of circulating lymphocytes.
• Target abnormal cells by recognizing absent MHC-1 on cell surfaces and expressing receptors for immunoglobulins.

NKT Cells

• Share traits of T cells and NK cells; express TCR, recognize specific lipids/glycolipids on CD1, and release cytotoxic granules.