1.2_Cell differentiation, morphogenesis, gene regulation

Human development

is the process in which a single fertilized egg (zygote) becomes a complex multicellular organism.

Cell Differentiation, Morphogenesis, and Gene Regulation

Human development involves three major processes:

Cell Differentiation

Human development involves three major processes:

• Cells become specialized for specific functions.

• is the process by which unspecialized cells become specialized in structure and function

• Process in which unspecialized cells become specialized in structure and function.

Morphogenesis

Human development involves three major processes:

• Cells organize into tissues, organs, and body structures.

• gives the embryo its shape and structure.

• Biological process that generates the shape and organization of tissues and organs.

Gene Regulation

Human development involves three major processes:

• Controls which genes are turned on/off, guiding development.

• Control of gene expression that determines cell identity and orchestrates developmental processes.

Totipotent, Pluripotent, Multipotent, and Terminally Differentiated

Levels of Developmental Potency:

Totipotent

Levels of Developmental Potency:

• Can form all cell types + extraembryonic tissues (e.g., zygote, early blastomeres).

• Cells with the capacity to form all cell types, including both embryonic and extraembryonic tissues (e.g., zygote, early blastomeres).

• represent the highest level of developmental potential

• a single ___ can form both the embryo and support structures such as the placenta and chorion

Examples:

• Zygote (day 0)

• Early blastomeres (up to 8-cell stage)

Pluripotent

Levels of Developmental Potency:

• Cells that can produce all three germ layers but not extraembryonic tissues.

• Can form all three germ layers (ectoderm, mesoderm, endoderm).

• lose the ability to form extraembryonic tissues but retain the ability to form any cell type in the body.

• they originate from the inner cell mass (ICM) of the blastocyst

Example:

• Embryonic stem cells (ESCs)

• Induced pluripotent stem cells (iPSC) (artificially reprogrammed adult cells)

Ectoderm

Differentiation potential: Pluripotent cells can generate:

• skin, brain, spinal cord

• Outer germ layer that forms skin, brain, spinal cord, and sensory organs.

Mesoderm

Differentiation potential: Pluripotent cells can generate:

• muscle bone blood, heart

• Middle germ layer forming muscle, bone, blood, heart, and connective tissues.

Endoderm

Differentiation potential: Pluripotent cells can generate:

• digestive lining, liver, pancreas

• Innermost germ layer that forms digestive tract lining, liver, pancreas, and associated structures.

Multipotent

Levels of Developmental Potency:

• Can form multiple related cell types within a tissue system (e.g., hematopoietic stem cells).

• These cells can form several related cell types within one tissue system

• They function in growth, repair, and maintenance of tissues

• Stem cells capable of forming several related cell types within one tissue lineage (e.g., blood stem cells).

Examples:

• Hematopoietic stem cells (HSCs): give rise to all blood cell types

• Neural stem cells: neurons, astrocytes, oligodendrocytes

• Mesenchymal stem cells: bone, cartilage, fat cells

Terminally Differentiated Cells

Levels of Developmental Potency:

• Fully specialized; usually do not divide (e.g., neurons, RBCs).

• These are fully mature cells with specific structures and functions

• They usually have a limited ability to divide, since their role is functional rather than developmental

Examples:

• Neurons: electrically excitable, communication-focused

• Red blood cells: efficient oxygen transport; lack nuclei

• Muscle fibers: specialized for contraction

• Goblet cells: mucous secretion

Cytoplasmic Determinants, Induction (Cell Signaling), and Epigenetic Regulation

Mechanisms of Differentiation:

Cytoplasmic Determinants

Mechanisms of Differentiation:

• Molecules unevenly distributed in the egg give cells positional information.

• These are molecules (RNAs, proteins, transcription factors) stored in specific regions of the egg.

• When the zygote divides, daughter cells receive different cytoplasmic contents, leading to different gene expression patterns.

• This mechanism is critical during very early embryogenesis, before extensive signaling begins.

Result

• Cells inherit positional information that determines their initial developmental fate.

Induction (Cell Signaling)

Mechanisms of Differentiation:

• Process where one group of cells influences the developmental fate of neighboring cells through paracrine or juxtacrine signals.

• means cells instruct neighboring cells to adopt specific fates. It can occur through:

• Paracrine (long range: diffusible signals: FGF, Hedgehog, Wnt)

  • Diffusible molecules influence cells at a distance.

• Juxtacrine (direct contact: Notch–Delta)

  • Requires direct membrane-to-membrane contact.

Functions:

• Guides tissue patterning

• Coordinates organ formation

• Ensures spatial organization (e.g., which cells become epidermis vs neural plate)

Paracrine Signaling (long-range)

Induction (Cell–Cell Signaling):

• Diffusible molecules influence cells at a distance.

• Cell communication using diffusible molecules that act on nearby cells.

Major pathways (from your document):

• FGF: brain development, limb bud formation

• Hedgehog (especially SHH): neural tube patterning, limb polarity

• Wnt: dorsal-ventral axis patterning, cell proliferation

Juxtacrine Signaling (contact-dependent)

Induction (Cell–Cell Signaling):

• Requires direct membrane-to-membrane contact.

Function of Induction

• Guides tissue patterning

• Coordinates organ formation

• Ensures spatial organization (e.g., which cells become epidermis vs neural plate)

Notch-Delta pathway

Juxtacrine Signaling (contact-dependent):

• Essential in nervous system development

• Controls lateral inhibition (deciding which cells become neurons vs support cells)

Epigenetic Regulation/Mechanisms

Mechanisms of Differentiation:

• regulate gene activity without altering DNA sequence.

• determines how open or closed regions of DNA are, affecting gene activation.

• Mechanisms (DNA methylation, histone modification, chromatin remodeling) that alter gene activity without changing DNA sequence.

Key Mechanisms:

• DNA methylation (gene silencing) - Typically silences genes.

  • Adds methyl groups to cytosines.

  • Critical for maintaining stable cell identities.

• Histone modification - affects chromatin structure (open vs closed)

  • Acetylation → opens chromatin → activates genes

  • Methylation → activates or represses depending on site

  • Other modifications: phosphorylation, ubiquitination

• These changes affect how tightly DNA is wrapped around histones.

• Chromatin remodeling - nucleosomes repositioned to expose or hide DNA

  • Large protein complexes reposition nucleosomes.

  • Opens previously inaccessible DNA regions.

  • Allows transcription factors to bind target genes

Regulation Mechanisms:

• Maintain long-term cell identity

• Determines how open or closed regions of DNA are, affecting gene activation

shape and structure

Morphogenesis gives the embryo its ___

Gastrulation, Neurulation, and Organogenesis

Major Developmental Movements:

Gastrulation

Major Developmental Movements in Morphogenesis:

• formation of ectoderm, mesoderm, endoderm

• Major morphogenetic event forming the three germ layers: ectoderm, mesoderm, and endoderm.

Neurulation

Major Developmental Movements in Morphogenesis:

• Formation of the neural tube (precursor of brain and spinal cord).

Organogenesis

Major Developmental Movements in Morphogenesis:

• formation of major organs

• Formation of organs through:

  • Folding

  • Branching (e.g., lungs)

  • Budding (e.g., limbs)

Cell migration • Cell shape changes • Controlled proliferation and apoptosis • Remodeling of the extracellular matrix (ECM)

Key Cellular Behaviors in Morphogenesis:

different genes are active or silent

Even with identical DNA, cells differ because?

Gene Regulation in Development

• Establishes cell identity (e.g., neuron vs skin cell)

• Directs developmental pathways

• Controls morphogenesis, timing, and structure formation

• Maintains cellular specialization over time

Maternal-Effect Genes

Hierarchy of Gene Regulation:

• Establish body axes (anterior–posterior, dorsal–ventral).

• These genes are expressed in the mother, not the embryo. Their mRNAs and proteins are deposited into the egg during oogenesis.

Functions:

• Establish the primary body axes (anterior-posterior, doorsal-ventral) of the embryo

• Provide initial spatial cues before the embryo’s own genome is activated

Examples:

• Bicoid (anterior structures)

• Nanos (posterior structures)

Relevance:

• ____set the starting conditions for all later developmental gene regulation.

Segmentation Genes

Hierarchy of Gene Regulation:

• Organize broad regions and repeated segments.

• These genes refine the body pattern established by maternal-effect genes.

• Cells that can produce all three germ layers but not extraembryonic tissues.

Importance:

• These genes divide the embryo into modular, repeated segments, which later give rise to structures like vertebrae, ribs, etc.

Gap genes, Pair-rule genes, segment polarity genes

Types of Segmentation genes:

Gap genes

Types of Segmentation genes:

• define broad regions (head, thorax, abdomen)

Pair-rule genes

Types of Segmentation genes:

• create repeated segment patterns

Segment polarity genes

Types of Segmentation genes:

• define anterior vs posterior within each segment

Homeotic (HOX) Genes

Hierarchy of Gene Regulation:

• are master developmental regulators that specify the identity of each segment along the anterior-posterior axis.

• Assign segment identity (e.g., where limbs form).

• Master regulatory genes that assign identity to body regions along the anterior–posterior axis.

Key Characteristics:

• Organized in clusters on chromosomes

• Exhibit colinearity: gene order corresponds to body position

• Evolutionarily conserved (found in flies, mice, humans)

Examples:

• A leg emerging from the position of an antenna (in insects)

• Vertebral transformations (lumbar → thoracic) in mammals

Transcription Factors, Developmental Signaling Pathways, MicroRNAs, Epigenetic Mechanisms

Regulation Mechanisms:

Transcription Factors (e.g., PAX6 for eye development)

Regulation Mechanisms:

• bind to DNA and regulate the expression of specific target genes

Role in development:

• Activate whole gene programs needed for certain cell types

• Coordinate timing and spatial expression

• Interact with other TFs and epigenetic marks

Example:

• PAX6: essential for eye development

  • A mutation can lead to aniridia in humans

  • Demonstrates how a single TF can control an entire organ’s formation

Developmental Signaling Pathways

Regulation Mechanisms:

• These pathways allow cells to communicate and influence each other’s fate

• Wnt, FGF, Hedgehog (SHH), BMP

• These pathways coordinate morphogenesis, tissue patterning, and organ formation.

• Molecular communication systems (e.g., Wnt, FGF, SHH, BMP) that guide cell fate, proliferation, and tissue organization.

MicroRNAs

Regulation Mechanisms:

• Fine-tune gene expression

• are short, non-coding RNAs that bind to mRNAs and: Prevent translation and Trigger mRNA degradation

• Small noncoding RNA molecules that regulate gene expression by inhibiting translation or promoting mRNA degradation.

Functions in development:

• “Fine-tuning” cell responses to signaling pathways

• Ensuring precise timing of differentiation

• Stabilizing cell identity after commitment

DNA methylation

Key mechanisms of Epigenetic Mechanisms:

• typically silences genes

• Adds methyl groups to cytosines.

• Typically silences genes.

• Critical for maintaining stable cell identities.

Histone modification

Key mechanisms of Epigenetic Mechanisms:

• affects chromatin structure (open vs closed)

• Acetylation → opens chromatin → activates genes

• Methylation → activates or represses depending on site

• Other modifications: phosphorylation, ubiquitination

• Chemical changes to histone proteins that influence chromatin structure and gene expression.

Chromatin remodeling

Key mechanisms of Epigenetic Mechanisms:

• - nucleosomes repositioned to expose or hide DNA

• Large protein complexes reposition nucleosomes.

• Opens previously inaccessible DNA regions.

• Allows transcription factors to bind target genes.

• Reorganization of chromatin to open or close regions of DNA, affecting gene accessibility and expression.

Wnt, FGF, Hedgehog (SHH), BMP

Major developmental Signaling pathways:

Wnt signaling

Major developmental Signaling pathways:

• Critical developmental pathway involved in axis formation, cell proliferation, and organ patterning.

• Axis formation

• Cell proliferation

• dorsal-ventral axis patterning, cell proliferation

FGF (Fibroblast Growth Factor)

Major developmental Signaling pathways:

• Limb development

• Neural induction

• brain development, limb bud formation

• A cell–cell signaling pathway essential for limb development, neural induction, and cell proliferation.

Hedgehog (SHH)

Major developmental Signaling pathways:

• Neural tube patterning

• Limb polarity

• Key developmental pathway involved in neural tube patterning and limb polarity.

BMP signaling

Major developmental Signaling pathways:

• Bone formation (bone morphogenic pattern)

• Dorsal-ventral patterning

Fertilization → Cleavage → Morula → Blastocyst → Implantation → Gastrulation → Neurulation → Organogenesis → Fetal Period

Stages from Zygote to Body Plan:

Fertilization

From Zygote to Body Plan (Timeline):

• Time: Day 0

• Key events: Zygote formed (totipotent)

What Stage?

Cleavage

From Zygote to Body Plan (Timeline):

• Time: Day 1-3

• Key events: Rapid cell division

What Stage?

Morula

From Zygote to Body Plan (Timeline):

• Time: Day 3

• Key events: 16–32 cells

What Stage?

Blastocyst

From Zygote to Body Plan (Timeline):

• Time: Day 5

• Key events: ICM + trophoblast

What Stage?

Implantation

From Zygote to Body Plan (Timeline):

• Time: Day 6-10

• Key events: Embeds in uterus

What Stage?

Gastrulation

From Zygote to Body Plan (Timeline):

• Time: Week 3

• Key events: Germ layers form

What Stage?

Neurulation

From Zygote to Body Plan (Timeline):

• Time: Week 4

• Key events: Neural tube

What Stage?

Organogenesis

From Zygote to Body Plan (Timeline):

• Time: Weeks 4–8

• Key events: Major organs

What Stage?

Fetal Period

From Zygote to Body Plan (Timeline):

• Time: Week 9–birth

• Key events: Growth & maturation

What Stage?

set up initial polarity

How the Processes Work Together: Development unfolds through coordinated gene regulation:

• Maternal-effect genes →

divide embryo into regions

How the Processes Work Together: Development unfolds through coordinated gene regulation:

• Segmentation genes →

assign identity to each region

How the Processes Work Together: Development unfolds through coordinated gene regulation:

• HOX genes →

specify cell types

How the Processes Work Together: Development unfolds through coordinated gene regulation:

• Transcription factors →

allow cell-cell communication

How the Processes Work Together: Development unfolds through coordinated gene regulation:

• Signaling factors →

fine tune responses

How the Processes Work Together: Development unfolds through coordinated gene regulation:

• microRNAs →

stabilize cell fate long-term

How the Processes Work Together: Development unfolds through coordinated gene regulation:

• Epigenetics →

defines identity

How Differentiation, Morphogenesis, and Gene Regulation Interact:

• Gene regulation →

creates specialized cells; perform specialized roles in shaping tissues

How Differentiation, Morphogenesis, and Gene Regulation Interact:

• Differentiation →

assembles the body’s structure; driven by regulated cell behaviors

How Differentiation, Morphogenesis, and Gene Regulation Interact:

• Morphogenesis →

Coordinate development

How Differentiation, Morphogenesis, and Gene Regulation Interact:

• Feedback loops →

Neural Tube Defects (NTDs)

Major Disorders:

• These occur when the neural tube fails to close properly during early embryogenesis (around week 3–4 of development).

• Spina bifida, anencephaly

• Caused by failed neural tube closure (week 3–4)

Congenital Heart Defects (CHDs)

Major Disorders:

• Septal defects, outflow defects

• Arise during cardiac morphogenesis (weeks 4–8)

• Gene Mutations (e.g., SHH, HOX genes)

• Failed Morphogenetic Movements (e.g., failed fusion/closure)

• Environmental Teratogens

Causes of Developmental Disorders:
Contribution Factors of Cleft Lip and/or Palate:

Organoids and Tissue Engineering

Modern Applications of Developmental Biology

• Mini-organs grown from stem cells for research, disease modeling

Core Developmental Principle: Self-organization, signaling

Example Uses: Brain organoids for research

Cancer Biology

Modern Applications of Developmental Biology

• Many cancers reactivate embryonic pathways (Wnt, SHH, Notch)

Totipotency

explains phenomena like monozygotic twinning, where early blastomeres separate and each develops into a full individual.

Importance of Epigenetics

• Locks cells into specific lineages during differentiation.

• Helps define differences between skin cells, neurons, muscle cells—despite identical DNA.

• Enables long-term maintenance of cell identity.

• Allows pluripotent cells to become specialized

• Explains why identical DNA yields different cell types

Folding, Branching, (e.g., lungs), and Budding (e.g., limbs)

In organogenesis, Formation of organs through:

Cell migration (e.g., neural crest cells) • Cell shape changes (e.g., apical constriction) • Cell proliferation vs apoptosis (e.g., digit formation) • ECM remodeling

Cellular Processes Involved in Morphogenesis:

Spina bifida

Common Forms of Neural Tube Defects (NTDs):

• Lower portion of the neural tube fails to close

• Can lead to paralysis, motor impairment, or neurological deficits

Anencephaly

Common Forms of Neural Tube Defects (NTDs):

• Failure of the anterior neural tube to close

• Leads to absence of major portions of the brain and skull (usually fatal)

Gene mutations • Environmental teratogens (e.g., alcohol)

Causes of Neural Tube Disorders':
Causes of Congenital Heart Defects (CHDs):

Congenital Heart Defects (CHDs)

Major Developmental Disorders:

• These are structural abnormalities in the heart arising during weeks 4–8, the period of organogenesis.

Examples:

• Septal defects (ASD, VSD)

• Outflow tract defects (Tetralogy of Fallot)

• Valve malformations

Developmental Basis:

• Involves failed morphogenetic movements and errors in cardiac looping, septation, or neural crest migration.

Causes:

• Gene mutations

• Teratogens

Significance:

• These defects are among the most common birth abnormalities, making them important to include in lectures on morphogenesis.

Cleft Lip and/or Palate

Major Developmental Disorders:

• This occurs when facial prominences fail to fuse properly during weeks 4–7.

• Failure of facial prominences to fuse (weeks 4–7)

Mechanism:

• Incomplete fusion of the maxillary and medial nasal processes

Can be:

• Cleft lip only

• Cleft palate only

• Cleft lip with cleft palate

Contribution Factors:

• Gene mutations

• Failed morphogenetic movements (facial merging)

• Environmental teratogens (e.g., alcohol, thalidomide)

Clinical Impact

• May affect feeding, speech, hearing

• Requires surgical correction

Gene Mutations

Causes of Developmental Disorders:

• These disrupt pathways responsible for:

  • Cell differentiation

  • Tissue patterning

  • Organogenesis

Examples

• Mutations in SHH → holoprosencephaly

• HOX gene mutations → limb or vertebral malformations

• Genes controlling cardiac septation → congenital heart diseases

Mutations can be inherited or arise spontaneously during embryogenesis.

Failed Morphogenetic Movements

Causes of Developmental Disorders:

• Morphogenesis depends on cell migration, shape changes, folding, and fusion. When these movements malfunction, structural defects arise.

Examples

• Failure of neural tube closure → spina bifida

• Failure of cardiac septation → ventricular septal defect

• Failure of facial prominence fusion → cleft lip/palate

Underlying causes may include:

• Disrupted signaling (e.g., Wnt, FGF, BMP)

• Abnormal ECM remodeling

• Defective cytoskeletal dynamics

Environmental Teratogens

Causes of Developmental Disorders:

• Environmental agent (e.g., alcohol, thalidomide) that disrupts fetal development and causes birth defects.

Effects of Teratogens

• Interrupt cell proliferation

• Alter signaling pathways

• Induce apoptosis

• Interfere with organ development

Examples

• Alcohol → Fetal Alcohol Spectrum Disorders (FASD), facial anomalies, neural defects

• Thalidomide → limb malformations due to disrupted angiogenesis

Timing matters:

• Embryos are most vulnerable during organogenesis (weeks 3–8).

holoprosencephaly

Mutations in SHH →

limb or vertebral malformations

HOX gene mutations →

congenital heart diseases

Genes controlling cardiac septation →

cell migration, shape changes, folding, and fusion

Morphogenesis depends on _____

ventricular septal defect

Failure of cardiac septation →

Alcohol

→ Fetal Alcohol Spectrum Disorders (FASD), facial anomalies, neural defects

Thalidomide

→ limb malformations due to disrupted angiogenesis

Gene mutations → incorrect protein signals → leads to abnormal cell differentiation → causes faulty morphogenesis (e.g., improper folding or fusion) → leads to structural defects (e.g., NTDs, CHDs, clefts)

Pathways of how Gene Regulation Errors Lead to Developmental Disorders

Stem cell therapy

Modern Applications:

• involves using pluripotent or multipotent stem cells to replace or repair damaged tissues.

• These cells' ability to self-renew and differentiate makes them ideal therapeutic tools.

• Uses pluripotent or multipotent stem cells

• Treats blood cancers, regenerates damaged tissues

• Core Developmental Principle: Cell potency, differentiation

• Example Use: Treating blood cancers

Embryonic Stem Cells (ESCs)

Types of Stem Cells Used:

• pluripotent, able to form all body cell types

Adult Stem Cells

Types of Stem Cells Used:

• multipotent, used for tissue-specific repairs

Induced Pluripotent Stem Cells (iPSCs)

Types of Stem Cells Used:

• reprogrammed adult cells with ESC-like potency

dopaminergic neuron replacement

Emerging uses of Stem Cells: Parkinson’s disease

insulin-producing β-cell regeneration

Emerging uses of Stem Cells: Diabetes

developmental potency

The foundation of stem cell therapy is ____, making this a direct application of developmental biology principles.