B2.3 - Cell specialisation

DIfferentiation

cells as they develop from unspecialised to specialised

What is identical in early embryo

inner cell mass

Morphogens in early embryo

Signal molecules that control cell differentiation

What happens as morphogens diffuse outwards from source

gradient established

What helps organise and determine cells

distance from morphogen secreting cells

What does the cell “read”

Distance in concentration gradient through receptors on surface, develops accordingly

signalling molecules results in different genes expressed in different parts of embryo, different embryo, dif features

Totipotent

differentiate into any type of cell, including placental

can give rise to a complete organism

Pluripotent

Differentiate to all body cells, not whole organism

Multipotent

Few closely related types of body cells

Unipotent

differentiate into associated cell type

• liver stem cells, only make liver cells

Stem cell niche

Microenvironment within organism in which stem cells live and recieve instruction

what influences stem cells

environment and how they differentiate

Interactions that determine what happens to stem cell in niche

• cell to cell interaction & cell can interact with fluid outside cell

• signalling molecules can activate or prevent transcribing genes

  • some dormant, some make more stem cells

Most studied stem cell niches

Blood, skin, intestine, brain, muscle cells

Where are blood stem cells found

in bone marrow

What is bone marrow niche made of

combination of cells that make up blood cells

as well as supportive cells - (regulate function of the ones that generate blood cells)

Hair follicles

• stem cells that are responsible for proliferation (rapid increase in the number of something) of hair are found in an area call the bulge

• other cells in follicle responsible for breakdown of old hairs

egg cell - size

large, highly specialised, all nutrients needed for embryo development

Sperm size

doesn’t need nutrients, stays small

Long nerve cells

Nuerons in sciatic nerve, longest in human body: base of spine, ends in foot

• evolved from communication between spinal cords and more distant parts of body

Some of smallest cells

Erythrocytes (RBC)

• have no nucleus, more space for haemoglobin for binding O2

• biconcave have high flexibility, allows to be deformed and spring back in shape

White blood cells

larger than RBC

• have nucleus of various shapes, aid in identification

• able to move in an ameboid way towards site of infection: can squeeze out of blood vessel into surrounding tissue

what do cells have to ensure max size of cell type consistent with organism

cell surface receptors

growth factors in surrounding environment

SA:V ratio when cell becomes larger

ratio reduces

Larger SA to V

easier for nutrients in, excretory products out

cells that are too small

dont contain cellular components

Cells that are too large

, more organelles but SA:V may be too small, movement in and out, too slow, cell dies.

Why are eukaryotes compartamentalised

overcome difficulties with accessing nutrients and expelling waste

How to increase SA

• flatten cell, important in allowing diffusion, such as gas exchange

• invagination of membrane (mitochondria, christae, increase metabolic reaction)

• villi - increase SA of small intestine

  • microvilli further increases

• location relative to surce of nutrients and transporting waste

• how cells fit together at a specific location

why is Proximal convoluted tubule essential - kidney

Proximal convoluted tubule is a very important region for absorption of water, salts, glucose and A.A. in blood stream

• cube shaped cells packed together to use space efficiently

• lining has microvilli, increase absorption

• many mitochondria, active transport

• channels on opposite side of lumen increase SA

what does alveoli do

increase SA to maximise gas exchange

Type I pneumocytes

• cover 95%

• major function: allow gas exchange between alveoli and capillaries

• adaptations:

  • thin and flat to increase SA, decrease diffusion distance

  • shared basement membrane with endothelium (lining) of lung capillaries, minimise diffusion distance

  • tightly packed, fluid can’t enter from capillaries

Type II pneumocytes

• less than 5%

• found between type I

• Role: produce pulmonary surfactant, reduces surface tension & prevents alveoli from collapsing and sticking during breathing

• adaptations:

  • cube shape, large cytoplasmic area for organelles producing surfactant

  • microvilli oriented towards alveolar sac, increase SA

  • cytoplasm contains many organelles involved with surfactant production and secretion

  • transform into type I when needed

Adaption of skeletal muscle cell

• long, cylindrical shape

• membrane capable of impulse propagation

• multiple nuclei

• visible bands capable of shortening to produce voluntary movements

Cardiac muscle fibres in heart adaptations

• composed of branching, striated cells

• single nucleus per fibre

• connected at ends by intercalated discs

Cardiac muscle cells length

shorter in length

• because of branching cells, muscle fibres coordinate contractile pump

Why is it debated if striated muscle fibres should be considered cells

• larger than most

• do not follow cell division

  • grow when supporting cells fuse with fibres

• when damaged, dont go through usual process

Sperm cell characteristics

• smallest human cell

• flagellum, allow movement (mitochondria near)

• head and tail, speed and efficiency

• very few organelles

• specialised secretory vesicle (acrosomal) helps penetrate egg

• haploid nucleus

Egg characteristics

• largest human cell

• no flagellum

• spherical

• most cytoplasmic organelles, specialised storage structures

• all early gamete forming cells present before birth

• special secretory vesicle under plasma membrane. release contents after sperm penetrates

• haploid nucleus