B2.3 Cell specialization SL (copy)

How is a zygote formed?

Fusion of m + f gametes

What produces unspecialised cells?

After fertilization

Embryo

Ball of undifferentiated (unspecialised) stem cells

• Produced when zygote undergoes mitosis several times (to produce genetically identical cells)

Differentiation

Process by which unspecialised cells become specialized for specific functions by acquiring specific structures

• Unspecialised develop into specialised cells

What determines what type of tissue unspecialised cells will form?

Their location in the embryo

What is common betw all cells in the body?

Same genome

Are gene expression patterns the same across all cell types?

No

• GE patterns are diff in diff cell types

What affects gene expression in an early stage embryo?

Gradients

• Eg chemical gradients- morphogens

• Influence which genes activated / suppressed in embryonic cells

Morphogens

Signalling molecules that form conc gradients in an embryo + control patterns of gene expression

Impact of gradients on gene expression within an early-stage embryo

1. In early embryo, morphogens are distributed unevenly

2. Cells in diff positions within the gradient are exposed to diff concs

3. Varying concs trigger diff patterns of gene expression

4. Leads to cell differentiation

What happens to cells closer to where morphogens get releaed?

Get higher concs of morphogen → activates more genes

2 properties of stem cells

Capacity to:

1. Divide endlessly

   • To make more stem cells

2. Differentiate along diff pathways

Potency

The capacity to differentiate along diff pathways to become specialized

Endless self-renewal

• Stem cell = unspecialized

• But can go thru numerous cycles of cell division while maintaining the undifferentiated state

Stem cell niches

1. Locations that have SC in high concs

2. Provide an environment that allow SC to regenerate +/ differentiate

Location of stem cell niches in adult humans

1. Bone marrow

2. Hair follicles

Proliferation

Rapid increase in cell number

Function of stem cell niches in adult humans

SC niche can:

1. Maintain the cells

2. Promote their proliferation + differentiation

3 diff potencies of cells

1. Toti

2. Pluri

3. Multi

Totipotent

• Can differentiate into any cell type

  • Including extraembryonic cells

• Stem cells in early-stage animal

Pluripotent

• Can differentiate into many (not all) body cell types

  • Not extraembryonic cells

• SC in early-stage animal embryos soon become pluripotent

Multipotent

• Can form a limited range (few) cell types

• SC in adult tissue (eg bone marrow → blood cells)

Aspect of specialisation

Cell size

• Cells have diff sizes, shapes functions

• Closely related to function

Cell shapes in humans

1. Male gamete (sperm)

2. Female game (egg)

3. RBC

4. WBC

5. Neuron

6. Striated muscle fibres

1. Male gamete (sperm)- long + narrow

2. Female game (egg)- huge + rounded

3. RBC- 8μm (small), biconcave disc shape

4. WBC- larger, grow from 10-30μm when activated

5. Neuron- large cell body, long axon

6. Striated muscle fibres- long, multinucleated

Cell sizes in humans

1. Male gamete (sperm)

2. Female game (egg)

3. RBC

4. WBC

5. Neuron

6. Striated muscle fibres

1. RBC

2. WBC

3. Sperm

4. Egg

5. Neuron

6. Striated Muscle fibres

What limits cell size?

SA:V

What does the exchange of materials across a cell surface depend on?

Its area

What does the need for exchange of materials depend on (metabolism)?

Its volume

What happens to SA:V as a cell grows + why?

SA:V decreases bc V increases faster than SA

What does it mean if cells have a higher SA:V?

More efficient exchange → higher metabolism

• Bc can exchange materials (O2, nutrients, waste) more efficiently across cell membrane

Models

Simplified versions of complex systems

Small cells = higher SA:V so more efficient. So why aren’t all cells as small as possible?

Need the correct no. + types of organelles to complete their specific roles

Risk of low SA:V (large cells)

• If metabolic rate exceeds the rate of exchange of vital materials + wastes

• Cell will eventually die

Why do growing cells tend to divide?

To remain small → can maintain high SA:V suitable for survival

Equation for a cube:

1. SA

2. V

1. SA = SA of 1 face x 6

2. V = LWH

Why are egg cells large?

So it contains all the nutrient materials needed for the early development of the embryo

Why are sperm cells small?

Easy to move (need less energy)

Why are RBC small + flexible?

Need to squeeze thru narrow capillaries

Adaptations of RBC

1. Small

2. Flexible

3. No nucleus – more space for haemoglobin

4. Biconcave – larger SA:V

Why are WBC larger than RBC?

So WBC can carry out phagocytosis on pathogens

Why are neurons long?

Allows for communication betw spinal cord + other distant parts of the body

How many nuclei do striated muscle fibres have?

Multinucleated

Stem cell

An undifferentiated cell which can give rise to more cells of the same type + differentiate along diff pathways

2 types of stem cells in humans

• Embryonic

• Adult