Cell Signalling and Receptors

Cellular Adaptation

• Cellular adaptation occurs when cells respond to increased workload or stress, leading to reversible or irreversible injury.

Control of Cell Growth

• Cells communicate through chemical signals, hormones, and local mediators.
• Hormones act over a long range, while local mediators are secreted into the local environment.
• Some cells communicate through direct cell-cell contact.
• Cells are stimulated when extracellular signaling molecules bind to a receptor, which recognizes a specific protein (ligand).
• Receptors act as transducers, converting the signal from one physical form to another.

Signaling Molecules

• Hormones: insulin, cortisol, etc.
• Local mediators: epidermal growth factor (EGF), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), TGFβ, cytokines (e.g., interferons, tumor necrosis factor (TNF)).
• Signaling molecules cannot pass through the cell membrane, so their receptors are located in the cell membrane or are cytoplasmic or nuclear.

Receptors

• There are three main classes of receptors: ion-channel-linked, G-protein-linked, and enzyme-linked receptors.
• Ion-channel-linked receptors are important in neural signaling.
• G-protein and enzyme-linked receptors respond by activating cascades of intracellular signals, altering cell behavior.

Cell Signaling and Proliferation

• Cells proliferate when stimulated by growth factors, which bind to receptor tyrosine kinases.
• These signaling pathways override the normal brakes on proliferation, which are part of the cell cycle control system.
• This ensures that cells divide only under appropriate circumstances.

Phases of the Cell Cycle

• S phase: DNA replication.
• M phase: nucleus divides (mitosis) and cytoplasm divides (cytokinesis).
• G1 phase: gap between M and S phase.
• G2 phase: between S and M phase.

Cell Cycle and Adaptation

• Cells in the body divide into labile, stable, and permanent cells, affecting the adaptive response.

Cellular Adaptations

• Cellular adaptations include hypertrophy, hyperplasia, atrophy, and metaplasia.

Hypertrophy

• An increase in cell size, resulting in an increase in organ size.
• Causes of hypertrophy include physiological need, pathological conditions, and compensatory mechanisms.

Hyperplasia

• An increase in the number of cells.
• Physiological hyperplasia occurs in response to hormonal stimulation, while compensatory hyperplasia occurs in response to tissue damage.
• Pathological hyperplasia can occur in response to abnormal growth factors or mutations.

Atrophy

• A reduction in cell size and number.
• Causes of atrophy include reduced workload, loss of innervation, reduced blood supply, inadequate nutrition, loss of endocrine stimulation, and aging.

Metaplasia

• A replacement of one mature adult tissue with another, often in response to injury or stress.
• Examples include the replacement of pseudo-stratified columnar epithelium with stratified squamous epithelium in the lungs, and the replacement of soft tissues with mature bone.