stem cells

• pluripotent stem cells


  can develop into most, but not all, of the body's cell types

  found in inner mass cells of blastocyst
• totipotent stem cells


  can develop into any other cell type

  found in early stages of fertilised egg
• multipotent stem cells


  can only develop into select types of cells

  found in many tissues
• blastocyst


  a ball of cells found in an early embryo -> central mass = pluripotent cells
• sources of stem cells

  - embryonic

  - umbilical cord blood

  - adult stem cells (blood, brain, muscle, bones, tissue)
• uses of stem cells

  - bone marrow transplants

  - drug research (test drugs on these tissues)

  - developmental biology (study how they develop to make different cell types)

  - repair of damaged tissues (regenerative medicine)
• features of cells in meristems

  - thin walls (very little cellulose)

  - no chloroplasts

  - don't have a large vacuole
• how cambium cells differentiate into xylem vessels

  - lignin is deposited in their cell walls -> reinforce/waterproof them + kills cells

  - ends of cells break down -> forms continuous columns with wide lumens to carry water + dissolved minerals
• how cambium cells differentiate into phloem sieve tubes

  - sieves tubes lose most of organelles

  - sieve plates develop between them
• how cambium cells differentiate into companion cells

  - retain organelles

  - continue metabolic functions -> provide ATP for active loading of sugars