Chapter 3 The Cell Cycle and growth, death and differentiation

What is necrosis?

• Accidental cell death that occurs due to uncontrolled external factors (trauma) in the external environment of the cell

• Involves messy cell death that leads to the spillage of cell structures as well as inflammation

What is the purpose of apoptosis?

It is essential for development, shaping organs and tissues, and removing cells that are old, infected, damaged or no longer needed

What is apoptosis?

Programmed cell death or "cellular suicide" where the cell actively destroys itself to maintain a smooth function in the body

What is the process of apoptosis?

• The specific sequence of events that result in the ordered dismantling of the internal contents of the cell
  • Dismantled pieces (apoptotic bodies) are then cleaned up by macrophages and other phagocytes

How is apoptosis a part of development?

• It allows an organism to shape tissues and remove unneeded cells
• It ensures proper development from the embryo to the adult stage

Examples of apoptosis as a part of development

Hand development: As embyros, hands start out as a paddle-like block of tissue that are 'carved' into fingers by apoptosis of cells in between developing fingers Frog development: Loss of a tadpoles tail as it turns into a frog

Why does apoptosis removed damaged cells?

• Cells can have errors in replication during interphase
• Cells with DNA damage, pre-cancerous cells, and cells infected by viruses can pose a threat
• Apoptosis removes the threat to the organism (cancer, spread, viral infection)
• Damaged DNA triggers the cell to repair it or use apoptosis if unsuccessful
  • When apoptosis fails, cells can potentially develop into cancer

Caspases

Protein enzymes that dismantle cell components as part of apoptosis

What is the caspase cascade in apoptosis?

• The sequential activation of caspases by internal or external triggers that result in apoptosis
• Initiator caspases activate executioner caspases which dismantle the cell, leading to cell death

Triggers of apoptosis: Intrinsic/mitochondrial cytochrome c pathway

• An internal signal triggered by DNA damage, lack of nutrients, growth factors or presence of toxins

1. Internal stress signal occurs
2. Cytochrome c leaks from the mitochondria into the cytoplasm which activates caspases
3. Caspases begin breaking down the cell, cell shrinks and membrane begins to bleb
4. Chromatin condenses and nucleus fragments
5. Nucleus and organelles collapse
6. Apoptotic bodies form

Triggers of apoptosis: Extrinsic/death receptor pathway

• An external signal mediated by the immune system

1. T cell releases death ligand and binds to death receptor on infected cell
2. Death receptor activates caspases
3. Caspases begin breaking down the cell, cell shrinks and membrane begins to bleb
4. Chromatin condenses and nucleus fragments
   5. Nucleus and organelles collapse
5. Apoptotic bodies form

Death receptor

Specific receptors located on the plasma membrane of cells that detect extracellular signals (EG death ligand) and initiates the extrinsic pathway of apoptosis

Cytochrome c

• A mitochondrial protein that is triggered by internal or external signals to be released into the cytosol during apoptosis
• This causes the caspase cascade to initiate programmed cell death

Stages/process of apoptosis

1. - Internal or external stress initiates apoptosis (DNA damage, lack of growth factors, death receptors)
2. - Caspases are activated (intrinsic: Cytochrome c Extrinsic: death receptors) and cytochrome c leaks from the mitochondria into the cytoplasm
3. - Caspases break down cellular components, cell shrinks and membrane begins to bleb
4. - Chromatin condenses and the nucleus fragments
5. - Nucleus and organelles collapse
6. - Apoptotic bodies form

Bleb

The irregular 'blister-like' bulge of the cell membrane during apoptosis as it separates from the cytoskeleton

What occurs after apoptosis?

Macrophages engulf apoptotic bodies, maintaining homeostasis

Apoptotic bodies

Vesicles formed during the final stage of apoptosis which are engulfed by phagocytes

Why do cell cycle checkpoints do?

• Mistakes in DNA replication can result in mutations or faulty cells
• Checkpoints recognise and repair mistakes
• Checkpoints ensure the cell is growing well, replicating DNA and completing it's cellular functions correctly before allowing to pass to the next stage

When are the cell cycle checkpoints?

At the end of G1 and G2 and during mitosis between metaphase and anaphase

Cylcins

• Proteins that regulate the progression of cells through the cell cycle by activating CDK enzymes
• Act as gatekeepers of the checkpoints

CDK

• Cyclin-dependent kinases
• Some promote cell cycle progression and others inhibit it

Growth factors

Proteins that enable cell growth

G1 checkpoint

• Cell cycle progress through early G1 phase if growth factors are present Checks for: Nutrients, growth factors and DNA damage
• CDK inhibitors stop the cell cycle to repair DNA or import nutrients when conditions aren't met
• Cannot be fixed: Apoptosis occurs

G2 checkpoint

• G2 checkpoint is prolonged as DNA is assessed for damage to determine whether DNA replication was completed without errors Checks for: Successful DNA replication, cell size Faulty DNA: Cell cycle paused to repair, apoptosis follows if it fails

Mitosis checkpoint

• Occurs between metaphase and anaphase Checks for: Spindle fibre defects in chromosome attachment
• Chromosomes must be aligned correctly to separate chromatids to opposite poles
• Otherwise anaphase occurs and daughter cells may have extra or missing chromosomes

What two genes are major controllers of the cell cycle?

• Proto-oncogenes
• Tumour-suppressor genes

Wht are proto-oncogenes?

Gene that codes for proteins which stimulate cell division and regulated apoptosis

What are tumour suppressor genes?

Gene that inhibits cell division by slowing it down, repairing DNA mistakes or instructing cells on when to die

What is the p53 gene?

• Gene activated when DNA damage is detected or cell injury occurs, halting the cell cycle to allow for repairs
• Many cancer types involve mutated p53 gene
• The loss/faulty mutation of the protein results in rapid, unchecked cell growth (through cell division) and evasion of apoptosis

What are the functions and mutations of BRCA 1 and 2?

• Produce proteins that protect DNA by repairing it
• Harmful mutations to BRCA 1 or 2 gene result in faulty or absent proteins where the cell doesn't successfully repair damaged DNA
• This can result in increased risk of breast cancer

What does a mutated RB1 gene result in?

• Retinoblastoma, cancer in the retina of the eye

What do mutations in proto-oncogenes and tumour result in?

• Results in uncontrolled division of abnormal cells - Cells continue to grow and evade apoptosis forming cancer cells and tumours - Therefore increased likelihood of cancer because pf of impaired mechanisms that suppress cancer
• This results in a genetic predisposition to cancer

What is genetic predisposition to cancer?

The increased risk of developing a specific cancer due to inheritance of a mutant allele in certain genes

What do inherited mutations affect?

DNA repair, cell growth and apoptosis

Mutation

A permanent change in the DNA sequence of a gene

Mutagens

Agents that induce or increase the frequency of mutation in DNA

What are the 3 types of mutagens?

Radiation, chemical, biological (infectious agent)

Radiation mutagens

UV rays from sunlight, X-rays for medical uses

Chemical mutagens

Carcinogens such as cigarettes, processed foods and preservatives, cosmetics and cleaning products

Biological (infectious agent) mutagens

Viruses and bacteria

Carcogenesis

• The process by which normal cells are transformed into cancer cells, resulting in the formation of a tumor
• Starts with mutagens that can lead to the activation of proto-oncogenes, and inactivation of tumor suppressor genes
• Results in altered genes and consequent proteins

What are the 6 characteristics of cancer cells?

1. Self sufficient
2. Bypass growth suppressors
3. Evades apoptosis:
4. Replicative immortality
5. Nutrient access
6. Metastasis:

• Cells only need to meet 5 to be considered cancer cells

Characteristics of cancer cells: Self sufficent

Cells can initiate their own cell division without external signals

Characteristics of cancer cells: Bypass growth suppressors

• Cells block signals that control the checkpoints and halt division in damaged or abnormal cells (p53)
• Mutation in p53 gene allows for division at any time

Characteristics of cancer cells: Evasion of apoptosis

Cell contains mutations in DNA that control the cell's apoptotic pathway

Characteristics of cancer cells: Replicative immortality

Cells can undergo limitless cell cycles divide unlimitedly whereas healthy cells have a limit to number of divisions

Characteristics of cancer cells: Nutrient access

• Cells have sufficient nutrient access through a good blood supply
• Encourage growth of blood vessels around to supply

Characteristics of cancer cells: Metastasis

Cells can migrate around the body through leader cells that detach from the primary tumour and form secondary tumours in healthy tissue

Leader cells

Specialised cells that detach from the primary tumour and begin to metastasize as cancer cells follow and colonise healthy tissue

Metastasis

The spread of cancer cells from a primary tumour to distant organs or tissues that forms new secondary tumours

Stem cells

Unspecialised cells that can divide to produce more stem cells or differentiate into specialised cells with distinct functions

What can stem cells do?

Self-renewal: Divide repeatedly to produce more stem cells Differentiate: Specialise into different types of stem cells

Specialised cells

Cells that have specialised to develop a specific structure that enables it to carry out its particular function within a an organism

What are the types of stem cells?

• Embryonic stem cells
• Adult (somatic) stem cells
• Induced pluripotent stem cells

Fertilisation

The fusion of an egg and sperm cell that forms a zygote

Zygote

The fertilized egg formed from the fusion of egg and sperm cells that develops into an embryo

• A type of totipotent stem cell as it can differentiate into every cell type needed to complete an organism

Embryo

The early stage of a developing organism, following fertilisation

• Develops from the inner cell mass of the blastocyst as it implants into the uterus around the 1 week after fertlisation

What are the 3 stages of embryonic development?

Germinal, embryonic, fetal

How is the embryo formed?

• Cleavage occurs in the germinal stage
• Morula then blastocyst form in the embryonic stage
• Gastrulation occurs in the embryonic stage where the embryo is formed with the 3 germ layers as part of the gastrula

Gestation

Growth process from fertlisation to birth

Germinal stage

• 1st stage of development (0-2 weeks) from fertilization to implantation
• Involves rapid, undifferentiated cell division (cleavage)
• Day 4 morula (16 cells), after wk 1 Blastocyst

Embyronic stage

• 2nd stage of development (3 to 8 weeks)
• Week 3: Blastocyst differentiates into 3 germ layers
• Involves rapid growth and development of major body systems and organs

Fetal stage

• 3rd stage of development (9 weeks to birth)
• Involves rapid size growth and maturation of basic organs

Gatrulation

• Occurs in the 3rd week in early embryonic stage
• Blastocyst is reorganised into the gastrula
• Process of gastrulation involves major cell movement to form the 3 germ layers: ectoderm, the mesoderm, and the endoderm and the embryo

Morula

A solid ball of cells formed as the zygote undergoes cleavage through cell division consisting of 16 cells

Blastomeres

Undifferentiated, totipotent cells formed by the rapid mitotic cleavage of a zygote during the earliest stages of embryonic development

• Morula follows

Blastocyst

• Fluid-filled ball of cells containing an inner cell mass of pluripotent stem cells that form the fetus
• Outer layer of blastocyst forms the placenta
• Blastocyst is formed in early embryonic development from a morula about 5-6 days after fertilization
• Marks differentiation from totipotent to pluripotent

Gastrula

Embyro with 3 primary germ layers: ectoderm, mesoderm, and endoderm

• Formed through the process of gastrulation

What are the 3 germ layers?

• Ectoderm (outer), mesoderm (middle), endoderm (inner)
• Lay the foundation for development of organs and tissues in the organism

Germ layer

Any of the 3 layers of cells differentiated in embryos during gastrulation

Ectoderm

• Outermost germ layer
• Gives rise to the epidermis (skin), hair, peripheral nerves, brain and spinal cord (PNS)

Endoderm

• Innermost germ layer
• Gives rise to the linings of the digestive and respiratory system as well as other organs including the liver, pancreas and gallbladder

Mesoderm

• Middle germ layer
• Gives rise to muscle, cartilage, kidneys and gonad cells (sex cells)

Adult (somatic) stem cells

Where and when: Undifferentiated, multipotent stem cells found amongst differentiated cells in a tissue or organ Role: They maintain and repair tissue they're in

Embryonic stem cells

Where and when: Undifferentiated, pluripotent cells found in the blastocyst's inner mass (early stage embryo) Role: Gives rise to all cells in the body except for placenta cells (outer cell layer does that)

Totipotent stem cells

When and where: Undifferentiated cells within the zygote to morula stage (2-8 cell stage) of development Potency: Differentiate into any type of specialised cells including both placental and embryonic cells

Pluripotent stem cells

Where and when: Undifferentiated cells found in the inner mass of the blastocyst (embryonic stem cells) Potency: Can self-renew and differentiate into almost all cell types except for placental cells

Multipotent stem cells

Where and when: Undifferentiated self-renewing cells found within the 3 germ layers of the gastrula (adult/somatic stem cells) Potency: Differentiate into multiple but limited cell types restricted to a single germ layer or tissue type

Unipotent stem cells

Cells with the lowest potency that can self-renew but only differentiate into one specialized cell type EG Skin or muscle stem cells

Stem cell potency

• Stem cells differentiation potential Ranked: Totipotent, pluripotent, multipotent and unipotent

Induced Pluripotent Stem Cells (iPSCs)

• Adult cell derived from skin/blood cells that have been reprogrammed back into an pluripotent embyronic-like state
• Enables development of all cell types that could be used for therapeutic purposes (medicine, diseases) - However artificial induction can result in cancer development as cells can become any type