Ch. 3

Gestation period

The period of development from fertilisation to birth.

• In humans it lasts about 40 weeks.

• It includes major growth from a single cell into a fully developed baby with organ systems.

Early embryological development process

Conception (fallopian tube): fertilisation occurs when a sperm fuses with an ovum, forming a diploid zygote (1 cell) Cleavage (mitosis without growth): rapid cell division increases cell number but not overall size:  2-cell stage → 4-cell → 8-cell → 16-32 cell stage Morula (~16-32 cells): solid ball of cells formed after cleavage

travels to uterus Blastocyst formation (in uterus): hollow structure with cell differentiation Early blastocyst (~50-60 cells): initial cavity forms Late blastocyst (~100-120 cells): fully formed structure Inner cell mass: develops into the embryo Trophoblast: develops into placenta and supporting membranes Implantation (~day 6-7): blastocyst embeds into uterine wall for nutrient exchange Gastrulation (~week 3): formation of three germ layers: ectoderm, mesoderm, endoderm → gives rise to all tissues and organs Embryo stage (weeks 3-8): major organs and body structures begin forming (organogenesis) Foetus stage (week 9-birth): growth, refinement, and functional maturation of all organs

development continues until birth (~38-40 weeks total gestation)

Gastrulation

the process that occurs after implantation of blastocyst, in which the embryo differentiates into three primary germ layers—ectoderm, mesoderm, and endoderm—forming a gastrula. These germ layers give rise to all tissues and organs of the body. Gastrula → Embryo occurs when the ectoderm, mesoderm, and endoderm begin differentiating into specialised cells that form the body's tissues and organ systems.

Embryonic germ layers

the three layers of differentiated cells formed from the inner cell mass of the blastocyst Ectoderm - outer layer Mesoderm - middle layer Endoderm - inner layer

Each germ layer forms specific tissues: Ectoderm: skin, nervous system Mesoderm: muscles, bones, blood Endoderm: digestive and respiratory systems

• These layers produce all specialised body tissues.

• This differentiation allows the development of complex tissues and organs during embryonic development

What are the three stages of the human gestation period?

Germinal stage (Weeks 1-2)

• Begins at fertilisation.

• Extends until the blastocyst implants into the uterine wall.  Includes the formation of the zygote, morula, and blastocyst. Embryonic stage (Weeks 2-8)

• Begins after implantation and lasts until about 2 months after fertilisation.

• The developing organism is called an embryo.

• Major organs and body structures form during this stage.

• This is the most critical and vulnerable period, as exposure to harmful substances can cause severe developmental abnormalities or death. Foetal stage (Week 9 to birth)

• Begins when the embryo becomes recognisably human.

• The developing organism is called a foetus.

• Existing organs and structures continue to grow, mature, and become fully functional until birth.

Critical periods of development

specific stages of development when organs and body systems are forming and are most sensitive to environmental influences, making the embryo or foetus particularly vulnerable to developmental abnormalities.

• Exposure to harmful factors (e.g. chemicals, drugs, alcohol, smoking, viruses) during this stage can cause serious birth defects or death

• The longer a body system takes to develop, the more vulnerable it is to abnormalities during its formation

Stem cells

Stem cells are unspecialised cells that can self-renew (divide to make more stem cells) and differentiate into specialised cells for growth, development, and tissue repair.

Self renewal: The ability of stem cells to regenerate by giving rise to the exact copies of themselves

Sources of stem cells

Can be classified into two types according to where they can be sourced from

• Embryonic stem cells

• Adult stem cells

Embryonic Stem Cells

• Stem cells found in the early embryo (zygote to blastocyst stage) before implantation

• Located in the inner cell mass of the blastocyst (~day 5) Pluripotent: can form almost any body cell type

• Early embryonic cells have very high developmental potential, which becomes restricted as development progresses

• After implantation and gastrulation, cells form the three germ layers and begin to specialise

• As differentiation increases, cells lose broad potency and become more specialised

• Capable of self-renewal via mitosis

• Important for development, research, and regenerative medicine

Adult stem cells

• Undifferentiated stem cells found in specific tissues throughout life

• Located in tissues such as bone marrow, brain, skin, liver, blood vessels, heart, and spinal cord

• Function mainly in repair, replacement, and maintenance of damaged or old cells (e.g. skin healing after injury) Multipotent: can differentiate into a limited range of cell types

• Their differentiation is restricted to the tissue they originate from

• More specialised (less potent) than embryonic stem cells

• Example: haematopoietic stem cells (bone marrow) → produce all types of blood cells but not other tissue types

Embryonic VS Adult stem cells

Types of stem cells

Stem cells are classified according to their potency, which is their ability to differentiate into other cell types. The greater the potency, the greater the variety of cells that can be formed.

• totipotent

• pluripotent

• multipotent

• unipotent Potency decreases in the order: Totipotent → Pluripotent → Multipotent → Unipotent

Totipotent stem cells

• Can differentiate into all possible cell types

• Have the highest potency

• Include embryonic stem cells from the zygote to morula stage

• Are the only cells that are totipotent

• Can produce an entire embryo

Pluripotent stem cells

• Can differentiate into almost any type of cell

• Include the embryonic stem cells of the inner cell mass of the blastocyst

• Give rise to the three primary germ layers of the foetus

• Cannot form the placenta or embryonic membrane layers

• The placenta and membranes are formed by the trophoblast

Multipotent stem cells

• Can differentiate into a variety of closely related cell types

• Have a lower potency than pluripotent stem cells

• Haematopoietic stem cells in bone marrow are multipotent

• These cells can produce different types of blood cells

• Stem cells of the differentiated germ layers are also multipotent

Unipotent stem cells

• Can produce only one type of cell: their own

• Have the lowest potency

• Still capable of self-renewal

• Found in somatic tissues

• Examples include skin stem cells and muscle stem cells

• Self-renewal allows these cells to repair and maintain tissues

What is stem cell therapy?

• Treatment and prevention of disease using stem cells

• Aims to repair, replace, or regenerate damaged cells and tissues

• Important area of regenerative medicine

• Has potential to treat many currently incurable diseases

How are embryonic stem cells used in therapy?

Obtained from early embryos (~5 days post-fertilisation, blastocyst stage) with consent

• Grown in lab culture with nutrients where they can self-renew indefinitely

• Can be directed to differentiate into specific cells (e.g. heart, liver, muscle cells) Differentiation controlled by:

• Chemical changes in growth medium

• Nutrient control

• Gene modification Highly valuable for repairing or replacing damaged tissues

How are adult stem cells used in therapy?

• Easier to source and ethically less controversial Bone marrow (haematopoietic stem cells):

• Produce all blood cell types

• Used in treatment of leukaemia and blood disorders after chemotherapy

• Collected via bone marrow extraction or blood cell separation - after stimulation injections Umbilical cord blood:

• Contains same stem cells as bone marrow

• Easy, safe collection at birth

• Can be stored for future use

• Lower risk of immune rejection if using own cells

Induced pluripotent stem cells

Adult cells genetically reprogrammed to an embryonic-like pluripotent state

• Created by activating specific genes

• First developed in 2007

• Can become almost any cell type  Advantages:

  • Avoid ethical issues of embryos

• Potential for personalised medicine

• Major future role in regenerative therapy

Applications of stem cell therapy

Stem cells are valuable in medicine because they can differentiate into many cell types, allowing treatment of a wide range of diseases Central field: regenerative medicine, which aims to replace, regenerate, or engineer cells, tissues, or organs to restore function Main applications: Tissue regeneration:

• Used in burn treatment (skin repair) using patient's epithelial stem cells grown in labs

• Can form new skin for grafting

• Experimental use in growing blood vessels for potential cardiovascular disease treatment Cell replacement therapy:

• Used when diseases involve loss of specific cells Examples:  ~Type 1 diabetes → insulin-producing pancreatic cells  ~Alzheimer's → replacement of damaged brain cells  ~Macular degeneration → retinal cell replacement Blood disorders (haematopoietic stem cells):

• From bone marrow or cord blood

• Used in leukaemia treatment after chemotherapy

• Restore blood and immune cell production Organ regeneration:

• Potential to grow organs in the lab to solve donor shortages

• Using patient's own cells may reduce rejection risk

• Example: 2015 mini beating heart grown using iPSCs Research applications:

• Studying gene control of differentiation

• Investigating cancer development

• Testing drugs on lab-grown tissues

• Research into mental health disorders and ageing

What are the ethical issues in stem cell therapy?

Main ethical concern involves embryonic stem cells, as they are taken from early embryos (blastocysts), which some argue is the destruction of potential human life

• Although techniques have improved, embryonic stem cells are still sourced from early embryos

in some cases a single cell can be removed without destroying the embryo iPSCs (induced pluripotent stem cells) reduce ethical concerns by reprogramming adult cells to behave like embryonic stem cells, avoiding embryo use or destruction Animal research concerns arise when human stem cells are inserted into animals (e.g. mice), raising questions about creating human-animal hybrids Adult stem cells are ethically less controversial because they are obtained from donors with informed consent and do not involve embryos

Advantages and disadvantages of stem cell use

DNA structure

• DNA (deoxyribonucleic acid) is a nucleic acid that stores genetic information

• It has a double helix structure made of two antiparallel strands

• Each strand is made of repeating nucleotides

• The outside is a sugar-phosphate backbone

• The inside has nitrogenous bases forming the "rungs" of the ladder

• Bases are held together by hydrogen bonds

What is a nucleotide and how do DNA bases pair?

A nucleotide is the basic unit of DNA, made of: ~ deoxyribose sugar ~ nitrogenous base (A, T, C, G) Bases follow complementary base pairing: ~ A pairs with T ~ C pairs with G Purines (A, G) are double-ring Pyrimidines (C, T) are single-ring Purines always pair with pyrimidines for stability

How is DNA packaged inside the nucleus?

• Each cell contains ~2 metres of DNA in a nucleus ~6 µm wide

• DNA is compacted through DNA packaging

• DNA + histone proteins form chromatin

• DNA wraps around histones forming nucleosomes (8 histones + DNA)

• This allows DNA to be compact but still functional

What is the relationship between chromatin and chromosomes?

• Chromatin is DNA loosely/condensed with histone proteins in non-dividing cells

• During cell division, chromatin becomes highly condensed

• This forms chromosomes, the most compact form of DNA

• Chromosomes are visible under a microscope and often X-shaped during division

What is the purpose of cell division in organisms?

• Produces two genetically identical daughter cells

• Main role is replication and passing on of DNA

• In eukaryotes, this is called mitosis

• In prokaryotes, cell division occurs by binary fission

• In multicellular organisms, mitosis is used for growth and repair

How does mitosis contribute to growth and repair?

Growth:

• All organisms begin as a single cell

• Humans develop from one fertilised egg into ~37 trillion cells and ~210 specialised cell types Mitosis is responsible for:

• Producing new cells for growth

• Forming specialised cells during development

• Maintaining tissues through cell replacement Repair:

• Different cells have different lifespans:

• Neurons last a lifetime

• Stomach cells last ~5 days due to acid

• Skin cells last ~4 weeks but are often damaged earlier

• Mitosis replaces damaged or worn-out cells

• Maintains healthy tissues and organ function

How do unicellular organisms use cell division?

• Do NOT use mitosis for growth or repair

• Use cell division for asexual reproduction

• In bacteria, this is called binary fission

• Produces a new individual organism directly

What is binary fission and why is it simpler than mitosis?

• A form of asexual reproduction in prokaryotes (e.g. bacteria)

• Prokaryotes lack membrane-bound organelles, making division simpler than mitosis

• Produces two identical daughter cells

What are the stages of binary fission?

Replication o The bacterial DNA is a single circular chromosome in the nucleoid region o It attaches to the plasma membrane near the cell's midpoint o The DNA is copied, producing two identical circular chromosomes Elongation o The parent cell elongates (lengthens) o This helps separate the two chromosomes Division o Chromosomes move to opposite ends of the cell o A septum (new cell wall) forms in the middle o The membrane pinches inward until the cell splits into two identical daughter cells

What are the advantages and disadvantages of binary fission?

Advantages:

• Only one parent needed (no mate required)

• Very fast reproduction (e.g. E. coli every ~20 minutes) Disadvantages:

• Offspring are genetically identical

• Very low genetic variation

• Population is vulnerable to environmental change or disease outbreaks

Why is eukaryotic cell division more complex than prokaryotic division?

• Eukaryotic cells contain membrane-bound organelles that must be accurately copied and separated

• Cell division is organised into a cell cycle with multiple regulated stages

• Produces two genetically identical daughter cells

• Used for growth, development, and repair

• One full cycle in human cells takes about 24 hours (varies by cell type)

• Daughter cells immediately enter a new cycle, creating a continuous process

What is the cell cycle?

A series of ordered stages that a eukaryotic cell undergoes to grow and divide

• Produces two genetically identical daughter cells Includes:

• Interphase (G1, S, G2, sometimes G0)

• M phase (mitosis)

• C phase (cytokinesis) Ensures accurate DNA replication and cell division

What happens during interphase?

Longest stage of the cell cycle

• Cell is metabolically active and prepares for division

• Includes G1, S, and G2 phases (and sometimes G0) During interphase, the cell: ~ Grows ~ Replicates DNA ~ Increases organelles DNA exists as chromatin (not visible chromosomes)

What happens in the G1 phase?

• First growth stage of interphase ("gap 1")

• Cell increases in size (almost doubles)

• Organelles are replicated

• Ensures daughter cells will have enough cell machinery

• Often the longest phase, so many cells are seen in G1

• Maintains normal cell size after division

What is the G0 phase?

• A resting phase entered from G1

• Cells leave the active cell cycle and do not divide Types: Temporary G0: cells can re-enter cycle later Permanent G0: cells never divide again (e.g. neurons) Explains why nervous system repair is limited

What happens in the S phase?

DNA replication occurs (synthesis phase)

• Each chromosome is copied to form two identical sister chromatids

• Chromatids are joined at a centromere

• Genetic material is duplicated in preparation for division Human chromosome count remains 46, but DNA content doubles: Before: 46 single chromosomes After: 46 double chromosomes Key terms: Chromatid: one copy of a replicated chromosome Sister chromatids: identical copies Centromere: attachment point for spindle fibres

What happens in the G2 phase?

• Final stage of interphase

• Cell grows further and prepares for division

• Increased energy storage and metabolic activity

• Proteins needed for mitosis are produced

• Ensures cell is ready for accurate division

What is the M phase? (Mitosis)

• Stage where mitosis occurs (nuclear division)

• Ensures DNA is divided equally into two nuclei

• Essential for growth, repair, and replacement

• Occurs after G2 phase

• Followed by cytokinesis

• consits of 7 distinct stages

C Phase (Cytokinesis)

• The C phase occurs immediately after mitosis.

• It involves cytokinesis, where the cytoplasm divides.

• This results in the formation of two genetically identical daughter cells.

Mitosis

Mitosis is division of the nucleus

• Produces two genetically identical diploid cells

• Maintains chromosome number (2n → 2n)

• Before division: cell has homologous chromosome pairs ~ One maternal set ~ One paternal set Ensures genetic stability Homologous chromosomes: matching chromosome pairs with same genes and structure

What are the stages of mitosis (PMAT)?

P - Prophase: chromosomes condense, nuclear membrane breaks down M - Metaphase: chromosomes line up at equator A - Anaphase: sister chromatids pulled apart T - Telophase: nuclear membranes reform, chromosomes decondense Purpose: ensures accurate separation of genetic material

What is cytokinesis?

Final stage of cell division

• Division of the cytoplasm after mitosis

• Produces two genetically identical daughter cells

• Restores normal cell size and SA:V ratio

• Distributes organelles between cells

How does cytokinesis differ in animal and plant cells?

Animal cells:

• Plasma membrane pinches inward

• Forms a cleavage furrow

• Furrow deepens until cell splits Plant cells:

• Cannot pinch due to rigid cell wall

• Form a cell plate instead

• Vesicles from Golgi build the cell plate

• Becomes a new cell wall and membrane

How is the eukaryotic cell cycle regulated?

• The eukaryotic cell cycle is tightly controlled to ensure accurate division

• Cell division produces two genetically identical daughter cells

• Progression depends on internal and external signals

• These signals ensure cells only divide when conditions are favourable and safe

• Regulation prevents damaged cells from dividing and passing on faulty DNA

What are cell cycle checkpoints?

Checkpoints are control points in the cell cycle They:

• Monitor if the cell is ready to proceed

• Stop division if conditions are unsuitable If errors are detected, the cycle can be:

• Paused for repair

• Stopped completely (G0 or apoptosis)

• Ensure only healthy cells divide Contain G1 checkpoint, G2 checkpoint, M checkpoint

G1 Checkpoint

Occurs between G1 → S phase Most important checkpoint (commitment point) Checks: ~ Cell size ~ Nutrient and energy availability ~ DNA damage ~ Growth factors present Outcomes: ~ if all conditions are met→ cell enters S phase ~ If not met→ repair, or enter G0 (temporary or permanent)  Permanent G0 includes neurons

G2 checkpoint

Occurs before mitosis (M phase) Checks: ~ DNA damage ~ Accuracy of DNA replication ~ Cell size Outcomes: ~ If correct → enter mitosis ~ If damaged → pause for repair or undergo apoptosis Prevents damaged DNA being passed to daughter cells

M Checkpoint (Spindle Checkpoint)

Occurs during mitosis (metaphase → anaphase)

• Ensures chromosomes are correctly attached to spindle fibres Checks: ~ Chromosomes aligned at metaphase plate ~ Proper attachment to centromeres via kinetochores If incorrect: ~ Mitosis pauses for correction ~ If unfixable → apoptosis  Ensures accurate separation of sister chromatids

What is the role of checkpoint regulatory proteins?

• Control progression through the cell cycle

• Respond to internal and external signals

• If conditions are normal → allow progression

• If errors detected → halt cycle or trigger apoptosis

• Prevent damaged cells from dividing

• Dysfunction can lead to cancer

What is apoptosis and why is it important?

Apoptosis is programmed cell death, a highly regulated "cellular suicide" process in eukaryotic cells

• It is controlled, orderly, and energy-dependent, unlike necrosis

• occurs when a cell receives specific internal or external signals indicating it is damaged, infected, or no longer needed Key features of the process:

• Cell shrinks and condenses

• DNA is fragmented in a controlled manner

• Cell membrane remains intact (prevents leakage)

• Cell breaks into membrane-bound apoptotic bodies

• These are removed by phagocytes via phagocytosis

• No inflammation occurs because contents are not released into surrounding tissue

Roles of Apoptosis

1. Protection

• Removes damaged, infected, or mutated cells

• Prevents cancer by eliminating cells with dangerous DNA errors

• Destroys virus-infected cells to stop viral replication

• Prevents defective cells from passing on faulty genetic material

1. Development (Morphogenesis)

• Shapes structures during embryonic development

• Example: removal of interdigital webbing to form fingers and toes

• Failure leads to conditions such as syndactyly (fused digits)

• Other examples:  Tadpole tail removal during metamorphosis

• Elimination of excess neurons during brain development

• Removal of uterine lining during menstrual cycle

1. Tissue Homeostasis (Balance)

• Maintains correct cell number in tissues

• Replaces old or damaged cells with new ones

• Ensures organs maintain correct size and function

• Prevents overcrowding of cells that would disrupt tissue function

Apoptosis Pathways

Two types of programmed cell death pathways: Intrinsic (internal signals) Extrinsic (external signals)

• Both activate caspases which control cell breakdown

• Cell is broken into apoptotic bodies

• Fragments are removed by phagocytes

• No inflammation occurs Purpose: remove damaged, infected, or unnecessary cells in a controlled way

Intrinsic Pathway

Triggered by internal stress signals inside the cell (DNA damage, toxins, lack of growth factors) Mitochondria control the process

• Bcl-2 normally prevents apoptosis

• Stress activates Bax, which creates pores in mitochondria

• Cytochrome c is released

• Activates caspases → cell breakdown → apoptotic bodies → phagocytosis Outcome:

• Caspases trigger full cell breakdown

• Cell is dismantled into apoptotic bodies

• Phagocytes remove debris through phagocytosis

Extrinsic Pathway

Triggered by external signals outside the cell

• These signals bind to death receptors on the cell membrane

• Often used by immune system to remove damaged or infected cells

• Signal activates caspases directly

• Caspases initiate controlled breakdown of the cell

• Final steps match intrinsic pathway:  cell dismantling

• apoptotic body formation

• removal by phagocytes

Necrosis vs Apoptosis

Necrosis is cell death that occurs as a result of trauma or injury. Unlike apoptosis, necrosis is not controlled. During necrosis: · The cell swells · The cell bursts · Cellular contents are released into the extracellular space This uncontrolled release causes: · Inflammation · Damage to surrounding cells and tissues · Harm to the overall health and wellbeing of the organism

Cell Cycle Malfunction

Cell cycle checkpoints normally prevent damaged cells from dividing  Regulatory proteins detect errors and stop the cycle or trigger apoptosis If regulatory proteins are damaged or faulty:

• Errors are not detected

• Apoptosis is not triggered

• Cell cycle continues unchecked This leads to:

• Division of defective cells

• Accumulation of mutations

• Potential development of cancer

What is the role of the p53 protein in the cell cycle?

p53 is a key regulatory protein at G1 and G2 checkpoints

• Detects DNA damage before mitosis

• In healthy cells, p53 levels are low When DNA damage occurs, p53 increases and can: ~ Stop the cell cycle ~ Activate DNA repair enzymes ~ Trigger apoptosis if damage is irreparable If p53 is damaged: ~ DNA damage is not repaired or stopped ~ Apoptosis is not triggered ~ Mutated cells continue dividing ~ Leads to cancer development Causes of p53 damage: ~ UV radiation (major cause of skin cancer) ~ Pollution ~ Ageing and genetic factors

Cancer

Cancer is a disease resulting from the uncontrolled division of abnormal cells.

• It occurs when mutations accumulate and cells bypass normal cell cycle checkpoints and avoid apoptosis.

• As a result, defective cells continue dividing and form a tumour, an abnormal mass of tissue.

Development of cancer

• Normally, damaged cells are repaired or eliminated by apoptosis.

• Cancer develops when regulatory proteins are faulty or cells acquire mutations that allow them to bypass cell cycle checkpoints.

• Mutations accumulate over time, causing abnormal cells to divide uncontrollably.

• This can lead to the formation of a benign tumour, which remains localised and does not invade surrounding tissues.

• Further mutations may transform the tumour into a malignant tumour, which can invade nearby tissues and impair organ function.

Progression of Cancer

• As cancer progresses, mutations accumulate more rapidly.

• Cancer cells may contain up to 60 different DNA mutations.

• These additional mutations increase the rate of cell division and the severity of the disease.

• In advanced stages, major genomic changes may occur, including the loss of entire chromosomes.

• Progression increases the likelihood of invasion and spread to other parts of the body (metastasis).

Characteristics of cancer cells

Do not require growth factor signals: they can divide rapidly without the external signals needed by healthy cells. Random arrangement of cell layers: unlike normal cells, which stop dividing when crowded, cancer cells continue dividing and form irregular masses called tumours. Increased division: cancer cells divide much more frequently and many more times than normal cells. Metastasis: cancer cells can spread from their original site through the blood or lymphatic system, forming secondary tumours elsewhere in the body. Angiogenesis: cancer cells stimulate the formation of new blood vessels, providing the tumour with oxygen and nutrients needed for continued growth.

Structural features visible in cancer cells