B2.3 - Cell Specialization

Fertilization

Fusion of a male and female gamete to produce 1 cell

→ for multicellular organisms, this cell repeatedly divides to get an embryo of many cells, mitosis ensure all these cells are genetically identical

Gamete

An organism’s reproductive cells

Early-stage embryo

• Contains unspecialised cells

• As it grows, its cells develop along different paths and become specialised for specific functions → efficient

• Cells can develop the ideal structure, with enzymes for chemical reaction

Differentiation

Development of cells in different ways to carry out specific functions

→ humans have 220 distinctively different highly specialised cell types

Gene expression

• When a gene is being used in a cell

• Information in a gene is used to make a protein or other gene product

• Cell development involves expressing certain genes but not others

• Differentiation happens as different gene sequences are expressed in different cell types

Location of cells of a muticellular organism

• For multicellular organisms, they need enough cells of each type and they must be positioned where they are needed in the body

• A cell’s position in the embryo determines how they differentiate

• Chemicals here regulate gene expression, and their gradients indicate cell’s position in the embryo

Stem cells

• Cells with potential to develop into many different types of cells

• Either undifferentiated or partly differentiated → always able to differentiate

• Once fully differentiated, it is no longer a stem cell

• They differentiate based off genetic expression and environmental stimuli

Stem cell division

• Can divide repeatedly to replace lost, dead or damaged cells

• Cells produced by division may remain as stem cells or differentiate

Stem cells in adult humans

• Some stem cells can remain in adults

• Present in human tissues e.g bone marrow, skin, liver

• They give these tissues better capability for regeneration and repair

How do stem cells differentiate?

1. Cell differentiation

2. Different genes are expressed/ switched on

3. Different concentrations of signalling molecules/ molecules determine position of cells in embryo;

Niche

Precise location of stem cells in a tissue

How do stem cells survive in a tissue?

• Tissues must provide a microenvironment for stem cells to remain inactive/undifferentiated for a long time

• And to multiply rapidly and differentiate when needed

Striated (skeletal) muscle

• There are stem cells that remain inactive unless there is muscle injury

• Changes in niche cause these cells to multiply and differentiate to replace damaged muscle tissue

• Highly regenerative after damage

Bone marrow and hair follicles

• Both are stem cell niches that has a microenvironment that promotes continuous stem cell proliferation and differentiation

• Results in production of replacement blood cells and hair growth

Proliferation

Multiply, reproduce rapidly

Research interest on stem cell niches

• Niches could potentially generate human tissue in vitro to use in restorative surgery

• May be non-therapeutic uses for stem cells e.g producing large quantities of striated muscle fibres for eating meat

Vitro

Outside the living body and in an artificial environment/laboratory

Totipotent

Can differentiate into any cell type → useful for growth of whole replacement hearts, kidneys etc

Pluripotent

Capable of differentiating into a range of cell types but not every 

→ stem cells change from Toti to Pluri during embryo development

Multipotent

Adult stem cells that can differentiate into several types of mature cell

→ adult stem cells are more restricted in potential

Cell size

The size of a mature differentiated cell is one way of how it is adapted to perform its functions

Sperm (function of its size)

• 50 micrometres

• Narrow and small volume → reduces resistance and allows them to swim to egg more easily

Egg (function of its size)

• 110 micrometres in diameter

• Spherical and larger volume → allows large quantities of food to be stored in cytoplasm

Red blood cells (function of its size)

• 6-8 micrometres in diameter

• Indented on both sides

• Small size and shape allow passage in narrow capillaries

• Large surface area to volume ratio → un/loading of oxygen is faster

White blood cells (function of its size)

• B-lymphocytes are 10 micrometers in diameter when inactive

• Active → 30 micrometer, extra volume is cytoplasm with rER and Golgi apparatus for protein synthesis

Cerebellar granule cells (function of its size)

• Cell body is 4 micrometres in diameter

• Twin axons extend for about 3 milimetres

• Small volume of neurons allows cerebellum to have 50 billion of them

Motor neurons

• Cell body is 20 micrometres in diameter → large for enough proteins to be synthesised to maintain long axon

• Can extend long -? can carry signals to CNS from faraway muscles

Striated muscle fibres

• 20-100 micrometres in diameter

• Long (over 100 millimetres)

• Allows greater force and contractions by greater lengths

Effect of small surface area: volume ratio

• Substances enter slower and waste accumulates as they aren’t excreted in time

• Cell may overheat as well as metabolism produces heat faster than it is lost over cell’s surface 

Red blood cell (adaptation to increase SA:V)

• Bioncave disc shape → lower volume of sphere with same diameter

→ Smaller max. distance from anywhere in its cytoplasm to plasma membrane

Proximal convoluted tubule cells (adaptation to increase SA:V)

• Near outer surface of kidney → narrow coiled tubes

  • Receives fluids filtered out of blood in the kidney

  • Reabsorbs most of this filtrate which are useful e.g glucose

• One cell thick wall

  • 1. Inner apical membrane: touches filtrate, lots of microvilli

  • 2. Outer basal: close to blood capillaries, has infoldinfgs

SA:Vol. ratio

• Metabolic rate of cell is proportional to volume

• For metabolism to continue, reacting substances must be absorbed by cell and waste removed

• Rate of substances moving in and out of cell depends on SA

SA: Vol. formula

Surface area (mm²)/Volume (mm³)

Alveolar epithelium

• The alveolus wall, one-cell thick

• Contains 2 types of cells (pneumocytes)

Type 1 pneumocytes (AT1 cells)

• For diffusion of oxygen and CO2

• Passive → little need for organelles

• Small cytoplasm volume

• Wide but thin

• Wall of adjacent capillaries has single layer of thin cells → diffusion distance is small

Type 2 pneumocytes (AT2 cells)

• More numerous than AT1 (90% of alveolar cells, but takes only 5% of surface area)

• Cytoplasm has mitochondria, rER and lysosomes

• Lots of phospholipid synthesises in the cytoplasm and stored in lamellar bodies

Lamellar bodies

Vesicles of many layers of phospholipid and some proteins, contents are secreted by exocytosis

Muscle tissue

• Contractile → can shorten in length

  • Muscles exert pulling force to shorten

  • To return to original, a pulling force is exerted on the muscle → usually by another muscle

Antagonistic pairs

Contraction of one muscle causes lengthening of the other

Skeletal muscle

Muscles attached to bones and help move the body

Striated muscle

• Stripes are seen when structure is viewed under light microscopes

• Composed of many long, unbranched cylindrical structures called muscle fibres arranged in parallel

Microfibres in striated muscle

• Single membrane per fibre

• Many nuclei present

• Long

• Each fibre has many parallel cylindrical structures (myofibrils)

Myofibrils

• Inside microfibres

• Has alternating light and dark bands

• Centre of each light band is disc shaped structure → Z line

Cardiac muscle

• Forms wall of the heart

• Has striated appearance → but shorter cells than elongated fibres 

• Where end of 1 cell contacts end of another, there is a specialised junction called intercalated disc

• Branched → discs can connect each end with several other ends

• Electric signals propagate rapidly between cells as there are connections between membrane and cytoplasm of adjacent CM cells

Sperm (structures to swim fast)

1. Tail → long flagellum with 9+2 microtubules that generates forward motion with force with beating action

2. Mid piece with mitochondria → mitochon. is wound around microtubules at base of tail to supply ATP for motion

3. Head → streamlined in shape and narrow due to tightly packed chromosomes in nucleus

4. Little cytoplasm in head → less resisted movement

Sperm (structures to insert nucleus in egg)

1. Receptors in membrane for ZP3 glycoproteins in zona pellucida to which it binds to

2. Acrosome → enzyme sac that digests proteins and polysaccharides in zona pellucida so it can reach egg’s membrane

3. Binding proteins in inner acrosomal membrane → reveal after exocytosis of acrosome, which binds to proteins in egg’s membrane → fuses sperm’s membrane to egg’s and sperm enters

Egg cell features

• Moves passively and slowly

• Has food reserves for embryo development

Egg cell (how it allows only 1 sperm in)

1. Zona pellucida → layer of glycoproteins containing ZP3, sperm binds to it and can penetrate it, but it chemically alters later to prevent more sperm from entering

2. Binding proteins in membrane helps fuse with sperm’s membrane

3. Cortical granules → vesicles of enzymes near membrane of egg are released in to zona pellucida and make it impenetrable after sperm enters

Egg cells (structures that provide resources)

1. Yolk → large volume of cytoplasm that has lipid stores and food

2. Mitochondria → produces ATP and divides repeatedly

3. Centrioles → needed for mitosis