Cell Biology Quiz on Cellular Characteristics

Cellular Adaptations

• Cellular adaptations are crucial responses to changing physiological conditions or stimuli.
• Objectives for this session include describing cell signaling pathways,introducing the cell cycle, and focusing on adaptive responses in cell growth and differentiation.

Control of Cell Growth

• Cells in multicellular organisms communicate via chemical signals.
• Hormones act over long distances.
• Local mediators are secreted into the local environment (paracrine/autocrine).
• Some cells communicate directly through cell-to-cell contact.

Cell Signaling Pathways

• Extracellular signaling molecules bind to receptors to stimulate cells.
• Each receptor recognizes a specific protein (ligand).
• Receptors convert signals from one form to another.

Signaling Molecules

• Hormones, such as insulin and cortisol, are signaling molecules.
• Local mediators, including EGF, PDGF, FGF, TGFβ, cytokines (e.g., interferons, TNF), are also signaling molecules.

Receptors

• Two main receptor types are important in cell growth: G-protein-linked receptors and enzyme-linked receptors.

G-protein-linked Receptors

• These receptors activate GTP-binding proteins (G-proteins).
• G-proteins act as molecular switches, turning on for brief periods while bound to GTP.
• They switch off by hydrolyzing GTP to GDP.

Enzyme-Linked Receptors

• These receptors possess intracellular domains with enzyme functions, often receptor tyrosine-kinases.
• Activated by growth factors, playing crucial roles in cell proliferation.
• Some activate a small GTP-binding protein, Ras (important in cancer).

The Cell Cycle

• The eukaryotic cell cycle involves distinct phases, most dramatically nuclear (mitosis) and cytoplasmic (cytokinesis) division, labeled M phase.
• The rest, interphase, appears uneventful; but within it, DNA replicates, genes are transcribed, and proteins are synthesized for cell growth.
• The restriction (R) point, towards the end of G1, is a vital checkpoint.
• Progression past the R point is governed by phosphorylation of retinoblastoma protein (Rb).

Cell Populations (Labile, Stable, Permanent)

• Labile: Stem cells actively divide to replace lost cells (e.g., epithelial, hematopoietic).
• Stable: Cells resting in G0 but can rapidly proliferate when needed, including hepatocytes and fibroblasts.
• Permanent: Cells (e.g., neurons, cardiac myocytes) unable to effectively divide in response to cell loss.

Growth and Differentiation Responses

• Hyperplasia: Increase in cell numbers above normal in tissues or organs.
• Hypertrophy: Increase in cell size, leading to larger tissue or organs.
• Atrophy: Decrease in cell size and organ size due to loss of cell substance.
• Metaplasia: Reversible change of one differentiated cell type to another.

Hyperplasia: Causes

• Can only occur in labile or stable cell populations.
• May arise from physiological (e.g., hormonal) or pathological (e.g., excessive hormone/growth factor stimulation) factors.
• Physiological: hormonal effects during puberty/pregnancy; compensatory growth after tissue loss.
• Pathological: excessive hormones/growth factors, often associated with cancer risk (e.g., prostate, endometrium).

Hypertrophy: Causes

• Occurs usually in permanent cells.
• Due to synthesis of more cellular structural components.
• Results from physiological (e.g., hormonal stretching of uterus) or pathological (e.g., increased workload on heart) stimuli.

Atrophy: Causes

• Reduced workload, immobilization, loss of innervation, reduced blood supply, inadequate nutrition, loss of endocrine stimulation, and aging.

Metaplasia: Causes

• Adaptive response to certain stimuli.
• New cell type better suited to the stimulus; may later, though, develop into cancer.
• Often occurs in response to chronic irritation or injury.
• Example: squamous metaplasia in the respiratory system of smokers.

Metaplasia: Types

• Squamous metaplasia in the respiratory system of smokers.

Additional Cellular Processes

• Aplasia: Complete failure of tissue or organ development.
• Hypoplasia: Incomplete development of tissue or organ.
• Dysplasia: Abnormal maturation of cells within a tissue.