Prenatal Development and Cell Growth
Timeline of Prenatal Development
Blastocyst
Embryonic Development
Fetal Development
Parturition
Noteworthy time frames include 16 weeks (4 months) and 9 months (full term).
Early embryo development and regulation noted to be similar across species.
Fertilization marks the start of development.
Basics of Cell Division
How Do Animals Grow?
Involves multiple processes, primarily cell growth and division, also known as cell proliferation.
Processes Necessary for Growth
Cell Proliferation
Process synonymous with hyperplasia; the increase in the number of cells.
Comprises two key steps:
DNA replication: Duplication of the cell's DNA.
Cell division: The separation of the duplicated DNA into daughter cells.
Cell Hypertrophy
Involves growth of individual cells, making them larger.
Critical for daughter cells to maintain size and function.
Process includes:
Transcription: Converting DNA to RNA (gene expression).
Translation: Synthesizing proteins based on RNA information.
Protein Degradation
Essential for removing improperly made, damaged, or unneeded proteins.
Cell Differentiation
Process by which cells acquire specialized structures and functions, becoming distinct cell types.
Cell Death (Apoptosis)
Involves programmed cell death which is necessary for development, remodeling, and turnover of cells.
Cell Proliferation in Early Embryonic Development
High rates of cell proliferation are evident in embryos.
Observed Processes:
Cell Hypertrophy: Interestingly, cells can appear to shrink during divisions in this phase.
Protein Synthesis and Degradation: Continuous cycles to maintain cellular function and homeostasis.
Cell Migration: Movement of cells, is also crucial in development.
Cell Differentiation: Development of specialized cells targeted for specific roles.
Possible indications of Cell Death are worth noting, though observed responses vary.
The Cell Cycle and Proliferation
To proliferate, cells undergo the Cell Cycle:
G1 Phase: Involves cell hypertrophy and a commitment to proliferate. It can vary in duration and the size of the cell is weakly linked to proliferation.
S Phase: Represents DNA Synthesis, during which DNA content doubles from 2n to 4n.
G2 Phase: Involves checks for DNA integrity alongside preparation for division, also resulting in increased organelles and RNA production.
M Phase: Marks division, resulting in two daughter cells.
Mitosis: Nuclear division that is generally symmetric.
Cytokinesis: The process of separating the cell, which may be asymmetric or not closely coordinated with mitosis.
Overview of Mitosis and Chromosomal Behavior
M Phase Overview:
Early mitotic spindles form, chromatin is duplicated, and processes occur in several phases:
Prophase: Chromatin condenses into visible chromosomes.
Prometaphase: The nuclear envelope breaks down, and spindle fibers attach to chromosomes.
Metaphase: Chromosomes align at the cell's equatorial plane.
Anaphase: Sister chromatids are pulled apart towards opposite poles of the cell.
Telophase: Nuclear membranes begin to reform, and chromosomes de-condense.
Result: 46 chromosomes in each daughter cell, maintaining genetic fidelity.
DNA Packaging and Transcription During Mitosis
Transcription occurs only from unfolded DNA, while condensed (histone-bound) DNA cannot be transcribed.
All DNA condenses during M phase; hence, active transcription is not possible during this phase.
RNA necessary for mitosis and division must be synthesized in the G2 phase prior to M.
Growth Factors and Their Roles
Growth Factors (GF)
Secretions by cells which act locally (paracrine function).
Regulate the processes of hypertrophy and proliferation, and can either inhibit or activate these pathways.
Activate two primary intracellular signaling pathways:
MAPK Pathway (Mitogen-activated protein kinase).
PI-3K Pathway (Phosphoinositide 3-kinase).
Outcomes of GF Signaling:
Alters gene expression where hundreds of genes are transiently induced or repressed, regulating the cell cycle.
Alters overall protein synthesis and cellular metabolism.
Bio Signaling Mechanisms
Key Steps in Bio Signaling:
Reception: The signal (molecule) interacts with a cell receptor found within the plasma membrane or cytoplasm.
Transduction: Signal transduction pathways relay information downstream, leading to cellular responses.
Response: The end result of signaling pathways such as altered gene expression or cellular behavior.
Action of Extracellular Growth Factors on Signaling Pathways
MAPK Pathway Actions:
A receptor binds a growth factor, which causes a conformational change in the protein.
This change activates kinase activity.
Successive phosphorylation of target proteins ensues.
The activated target proteins further bind and cause new conformational changes and kinase activations.
This process repeats, amplifying the signal.
Outcome of MAPK Signaling:
Triggers transcription of cyclins capable of binding cyclin-dependent kinases (CDK).
Cyclin-CDK dimers act as transcription factors that guide the progression through the cell cycle.
Stimuli for Activation:
Various stimuli such as growth factors, adhesion to extracellular matrix (ECM), hormones, inflammatory cytokines, and environmental stresses activate MAPK pathways, which are conserved across eukaryotic cells.
MAPK Pathway Complexity
MAPK Scaffold: Various components and modules that connect different pathways and stimuli to the MAPK signaling cascade, emphasizing its complexity.
Notable proteins include:
MAPKK, MEKK1, various receptors (GPCR, RTK, TNF receptors), and others.
Role of scaffolds in organizing signaling events and informing specific pathways.
Action of Extracellular Growth Factors on the PI-3K Pathway
PI-3K Pathway Dynamics:
Stepwise actions initiated by growth factor binding and Ras activation (GDP to GTP conversion).
Pathway leads to PIP3 formation and activates downstream effectors like mTOR and AKT, which modulate protein synthesis and proliferation responses.
Proliferation Modification:
Activation of protein synthesis through components such as 4EBP, EIF-4e, S6K occurs following PI3K pathway activation.
Cell Growth Dynamics
Cell Growth Factors:
Both MAPK and PI-3K signaling converge on mTor (mammalian Target of Rapamycin), integrating nutrient and energy signals to facilitate cell proliferation.
mTor as a critical regulator of growth in response to availability of amino acids and energy.
Rapamycin is noted as an inhibitor used in research, originally derived from Rapa Nui (Easter Island).
Protein Degradation Mechanisms
Cell Cycle Dependency:
Protein degradation is crucial for the cell cycle, removing regulatory proteins promptly to ensure proper progression through phases.
Varying half-lives of proteins are influenced by E3 'targeting' enzymes, determining their stability.
ATP-dependent degradation is a major consumer of metabolic energy, while some forms of degradation do not require energy (e.g., digestive proteases).
Ubiquitin-Proteasome Pathway:
Major mechanism accounting for 80-90% of intracellular protein degradation.
E3 Ubiquitin Ligases: These add ubiquitin to proteins to tag them for degradation, ensuring specificity among numerous possible targets.
Cell Death Mechanisms
Apoptosis:
Defined as programmed cell death, which is deliberate and organized.
Contrasts with necrosis, which occurs due to injury and is uncontrolled.
Triggers of apoptosis include:
Death ligands, withdrawal of growth factors, attachment signals, DNA damage, and mitochondrial dysfunction.
Both extrinsic (external signal trigger) and intrinsic (internal stress-induced) pathways lead to cell death via activated proteases known as caspases.
Role of Apoptosis in Cancer:
The ability to induce apoptosis in cancer cells is a potential mechanism for cancer treatment strategies, as triggering death pathways can reduce tumor viability.
Genetic Variations in Cell Behavior
Sources of Variation:
Single nucleotide sequence differences (SNPs) between animals.
Variability in DNA copy numbers and RNA processing effects.
Micro RNA effects on translation can modify protein synthesis outcomes.
Genes can lead to altered protein structure and function, ultimately culminating in different expressions and responses in cells.
Epigenetic Modifications:
Critical modifiers of gene expression, capable of rendering specific genes ‘on’ or ‘off’ without altering the DNA sequence itself.
Such modifications may include DNA methylation, histone acetylation or phosphorylation, and can influence tissue differentiation.