Cell Turnover and Growth Processes

Cell Turnover

• Cell turnover is a presentation on the topic of cell turnover, growth, differentiation, and morphogenesis.
• The presentation was given by Dr N Buthelezi in the Department of Anatomical Pathology.
• The date of the presentation was January 20, 2025.

Learning Objectives

• The learning objectives cover definitions of growth, differentiation, and morphogenesis.
• The objectives also cover the cell cycle, types of stem cells, and various tissue types.

Growth, Differentiation, and Morphogenesis

• Growth is the process of increasing size through the synthesis of specific tissue components.
• Types of growth include:
  • Multiplicative: An increase in the number of cells via mitosis, crucial during embryogenesis and in all tissues.
  • Auxetic: An increase in the size of individual cells (e.g., skeletal muscle).
  • Accretionary: An increase in intercellular tissue components (e.g., bone, cartilage).
  • Combined: A combination of these growth types.

Maintaining Cell Populations

• Cell proliferation is vital for development and maintaining homeostasis in steady-state tissues.
• Cell proliferation also replaces dead or damaged cells.
• Cell populations depend on proliferation rates and apoptosis.
• Cell proliferation can be stimulated by pathological and physiological conditions.
• Physiological examples include endometrial tissue during the menstrual cycle and thyroid tissue during pregnancy.
• Pathological stimulation can include after cell death or tissue injury.

Cell Cycle

• The cell cycle is a sequence of events leading to cell division.
• The cell cycle comprises the following phases:
  • G1: Presynthetic growth phase
  • Restriction point
  • G1/S checkpoint
  • S: DNA synthesis
  • G2: Premitotic growth
  • G2/M checkpoint
  • M: Mitosis
• The cell cycle's phases have checkpoints to ensure proper DNA replication and prevent damage.

Cell Cycle Regulation

• The cell cycle is controlled by activators and inhibitors.
  • Cyclins and Cyclin-Dependent Kinases (CDKs)
  • CDK inhibitors (CDKIs).
• CDKs phosphorylate protein substrates, regulated by cyclin activity.
• Cyclin levels fluctuate, affecting CDK activity.

Cyclin-Dependent Kinase Inhibitors

• p21 (CDKN1A), p27 (CDKN1B), and p57 (CDKN1C) are broadly inhibitory to CDKs, stopping their activity, which prevents cell cycle progression.
• p15 (CDKN2B) p16 (CDKN2A), p18 (CDKN2C), and p19 (CDKN2D) are more selective, impacting CDK4 and CDK6 specifically.

Stem Cells

• Stem cells are undifferentiated cells capable of differentiating into various cell types.
• Characteristics of stem cells:
  • Self-renewal: Maintaining the stem cell pool.
  • Asymmetric division: One daughter cell differentiates, while the other retains self-renewal capacity.
• Types of stem cells:
  • Embryonic stem cells (ES cells): From the inner cell mass of the blastocyst, have virtually limitless self-renewal capacity and can become any cell type (totipotent).
  • Tissue stem cells (adult stem cells): Located in differentiated tissues, producing only cell types specific to that tissue (pluripotent, multipotent).
• Examples of tissues/locations for stem cells: Bone marrow, hair follicle bulge, eye limbus, canals of Haller in the liver, and supraventricular zone in the brain.
• Labile stem cells proliferate continuously, have a brief lifespan, and render tissues susceptible to radiation or drugs (e.g., hematopoietic cells).
• Stable stem cells divide infrequently under normal conditions, rapidly dividing when cells are lost (e.g., liver, endocrine glands, bone)
• Permanent stem cells' active stem cells do not stay in the system long after birth and cannot be replaced when lost. (e.g., neurons, cardiac muscle).

References

• Kumar V, Abbas AK, Fausto N, Aster JC. Robbins and Cotran pathologic basis of disease.
• Cross S. Underwood's Pathology: A Clinical Approach. Elsevier Health Sciences; 2018 Mar 9.

This quiz explores the concepts of cell turnover, growth, differentiation, and morphogenesis as presented by Dr. N Buthelezi in the Department of Anatomical Pathology. It covers essential learning objectives related to the cell cycle, types of stem cells, and various tissue types necessary for understanding these biological processes.