Cell-cell Adhesion and Cell Adhesion Molecules

Cell-Cell Adhesion

• Cell-cell adhesion is selective, meaning cells only adhere to specific types.
• It is mediated by transmembrane adhesion proteins known as cell adhesion molecules (CAMs).
• CAMs span the cell membrane, with one end linking to the cytoskeleton and the other end linking to structures outside it.
• There are four groups of CAMs: cadherins, selectins, integrins, and immunoglobulin (Ig superfamily).

Cell-Cell Adhesion - Anchoring Junctions

• Anchoring junctions are a type of cell-cell adhesion that provides strength and shape to cells.
• Cadherins, integrins, and selectins are examples of anchoring junctions.
• These junctions require Ca2+, Mg2+, or Mn2+ to function.

Cell-Cell Adhesion - Cadherins

• Cadherins are a type of anchoring junction that plays a crucial role in cell polarity.
• They are highly selective, allowing for the formation of many different tissues.
• There are many different types of cadherins, forming a superfamily of 180 members.
• Cadherins require Ca2+ to function, hence they are also known as "calcium adhering".
• They bind directly or indirectly to adapter proteins that bind to actin filaments.

Cell-Cell Adhesion - Tight Junctions

• Tight junctions are specialized contacts between epithelial cells that are important for epithelial cell sheet function.
• They act as barriers between fluid compartments, such as the blood-brain barrier.
• Tight junctions have minimal adhesive strength, so they associate with adherens junctions and desmosomes.

Cell-Cell Adhesion - Gap Junctions

• Gap junctions are channels that allow direct communication between the cytoplasm of adjacent cells.
• They are regulated channels that allow the passage of ions and small molecules.
• Gap junctions are important for cell-to-cell communication and coordination.

Cell-Matrix Adhesion Proteins - Integrins

• Integrins are transmembrane receptors that mediate the linkage between the extracellular matrix and the cytoskeleton.
• They allow the external cellular environment to influence intracellular functions.
• Integrins are important for cell-matrix adhesion and signaling.

Cell-Matrix Adhesion Proteins - Fibronectin

• Fibronectin is a matrix adhesion protein that links matrix components to one another and to the surfaces of cells.
• It is a dimeric glycoprotein that is cross-linked into fibrils in the ECM.
• Fibronectin has binding sites for collagen and GAGs, and is a major binding site for cell surface receptors, such as integrins.

Cell-Matrix Adhesion Proteins - Laminins and Nidogen

• Laminins are principal components of and major organizers of basal laminae.
• They can self-assemble into mesh-like networks and have binding sites for cell surface receptors and other ECM components.
• Nidogen is tightly associated with laminins and binds to type IV collagen.

Tissue Types

• Tissue is composed of cells and extracellular matrix.
• The four main types of tissues are: connective, nervous, muscle, and epithelial.
• Tissue structures and shapes can be formed by the organization of groups of cells into polarized arrangements and by coordinating their polarity in space and time.

Tissue Types - Connective Tissue

• Connective tissue is a type of tissue that provides support, structure, and connectivity to the body.
• It is composed of cells and ECM, with a high percentage of ECM.
• Examples of connective tissue include loose connective tissue, dense irregular connective tissue, and dense regular connective tissue.

ECM Polysaccharides

• The ECM is composed of fibrous collagen and elastin structural proteins, which are embedded in gels formed from polysaccharides.
• Glycosaminoglycans (GAGs) are highly negatively charged and attract lots of cations, which in turn attract water, forming a porous, hydrated gel.
• The gel provides mechanical support to the ECM.

Cell Plasticity and Differentiation

• Totipotent cells can give rise to a complete organism
• Pluripotent cells can give rise to all cell types in an organism
• Multipotent cells can give rise to multiple cell types within a specific tissue or organ
• Unipotent cells can give rise to only one cell type

Cell Differentiation and Determination

• Cellular differentiation is the process by which a stem cell changes into a specialized cell, losing its precursor properties and acquiring new characteristics
• Involves a change in gene expression
• Cell type is characterized by the cell’s protein expression profile
• Cellular determination is the process by which initially identical cells become committed to different pathways of development
• Determination is an irreversible process

Stem Cell Fate

• Stem cell fate is controlled by asymmetric cell division, where determinants are segregated during cell division, resulting in daughter cells with different fates
• Stem cell fate is also influenced by extrinsic (external) signals, such as those from the cell niche, and intrinsic (internal) signals, such as changes in DNA methylation or chromatin modification

Embryogenesis

• During gastrulation, three germ layers are formed: ectoderm, endoderm, and mesoderm
• These germ layers can differentiate into all adult cells in the body

Types of Stem Cells

• Somatic stem cells are found in tissues and organs of the juvenile and adult body
• Somatic stem cells are multipotent or unipotent, and maintain homeostasis and regenerate local tissue
• Unipotent somatic stem cells can give rise to a single mature cell type
• Progenitor cells are less plastic and more differentiated, giving rise to multiple but limited number of lineages, usually a restricted/related group of cells

Factors Affecting Stem Cell Fate

• Inductive signals, such as those from the cell niche, can influence stem cell fate
• Extrinsic signals, such as those from the environment, can also influence stem cell fate
• Intrinsic signals, such as changes in DNA methylation or chromatin modification, can also affect stem cell fate