B2.3 Cell specialisation

Stem cells

• formation of stem cells

  • diploid cell called zygote divides

  • once it becomes a solid ball of 16 to 32 cells it is called a morula

  • continues to divide and after 5 or 6 days the morula differentaties into a hollow ball of cells called a blastocyst

  • this structure has an outer layer of cells calle the throphoblast and inner cell mass (ICM)

  • trophoblast develops into placenta and ICM becomes embryo

• morphogen gradients

  • all cells within one organism have identical dna sequences with same formes of genes (alleles)

  • not all cells of individual will express same genes at the same time

  • difference in gene expression is what determines type of cell formation

  • differentation is when cells develop from unspeciaklised to specialised cells

  • in embryo ICM cell are identical

  • early embryo certain cells sercetrete from-giving molecules called morphogens

  • when the morphogens diffuse outward, a graident is established in the area

  • distance of cells from the morphogen secreting cell organizes and detrmines what those cells will be

  • cell reads diatsance in concetrayion gradient through receptors on its surface and it will develop accordingly

  • effiecnet and few genes are needed for this

• stem cell potency

  • totipoten stem cells (eight cells of the morula first eight cells formed after fertlisation) produce any cell inclduing placental cell

    • can give rise to a complate orgnaism

  • pluripotent stem cells (embryonic stems cells of blastocyst)

    • differentiate into all body cells but cannot give rise to whole orgnaism

  • multipotent stem cells (umbillical cord stem cells)

    • can differntaite into a few closely related types of body celles

  • unipotent stem cells

    • only differentiate into their associated cell type

  • as the morula devlops into the blastocyst, the cells become less adaptable

  • plurpodtent stem cells can differentation into three germ layers

    • endoderm (inner)

    • mesoderm

    • ectoderm

  • stem cells in the bone marrow are an example of multipotent stem cells.

    • they can be used for transplants anf to corect condition affecting blood cells

• stem cell niche

  • microenvrio in organism in which stem cells live and recieve instructions

  • influences the stem cells and how they differntatite

  • cell to cell interactions

  • cell can intreact with fluid outside of cells (extracellular matrix)

  • signalling molecules can activate or prevent genes from transcribing which causes some cells to be instructed to be inactive and others to be told to make more of the same stem cell

  • blood stem cells are in the bone barrow

    • bone marrow niche made up of combination of cells that make blood cells and supportive cells. spportive cells regulate function of ones that genrate blood cells

    • maintenance of blood cell nice is essentaion for constant supply of blood cells

  • hair follicle stem cells that make hair are found in area called the bulge. other cells in the follicle breakdown old hairs to make the rest stage. all cellswrok together

What determines cell size?

• human body contains between 50 and 100 trillion cells

• 7.5 um to 150um in size

• volume inside egg cell is 100 times volues of sperm cell

• the human egg cell is large because it contains all the nutrient materials needed for early developpement of the embryo

• sperm doesnt need nutritional content so its small

• some speicalised nerve cells are long and not spherical like most cells

• neuron sin the sciatic nerve are the longest in the body (runs down your leg)

  • axons can exceed 1 ,

  • evolved to communictae from spinal chord to body

• rbcs one of the smallest cell in body (7.5 um in diameter and 2 um in thickness)

  • bc they have no nucleus so that more haemoglobin can help bind oxygen

  • they have a flexible merane so that they can be repatedly deformed and go back to shape (they are bioncave because they cave in on both sides)

• wbcs are larger thanrbcs and are 10 to 20um

  • they have nuceli to aid in identication

  • they move in an amoeboid wat towards sites of inedection and squueze out of blood vessels into surrounding tissue

• striated muscle fibres are large multinucleated cells and exteremly long

• keeping cells alive

  • cells have control mechanisms like cell surface recptors and growth factoris in enviro to ensure max size of cell is oncististen with organism

  • when cells increase in size they have more rrom for organelles and metabolic reaction in cytoplasm

  • consider ratio of volume of cell to surface area of cell membrane. when the cell grows the SA:V (surface area to volume) ratio reduces

  • larger SA:V allows nutrients to pass into cell or out easier. a cells size is a balance of being lrage enought to contain necessary organelles without compromising the ability of gas and nurtient exchnage across membrane

  • when SA:V is too high it slows the movement of nutrients into the cell

Cell adaptations

• prokaryotic cells are smaller so they have a large SA:V ratio

• squamos epethelial cells are then flat or horizontal to increase surface area

• mitochondria have membranes invaginations on the inner membrane (cristae) to increase surface area and increase metabolic reactions

• in the plant photosynthetic thykaloid have mutliple membranes to enable more reactions form a larger surface area

• in a mamalls digestve system, the small intestine has villi and the cells at the end of villi absorb nutrients

  • villi increase surface area and mircro villi further increase surface area for max absorption

• in the kidney, the proximal convuluted tobule is an region for water salt and glucose absorption into bloodstream. the lining of the tubule has cuboidal shaped cells with microvilli to increase sa

• Cell adaptations in lung tissue

  • type 1 pneumocytes are very thin and flattened out

    • they have few organelles and 50um diameter

    • they are tight junctions between neighboring cells

    • make up 95 percent of alveolar surface

    • invlved in gas exchnage

  • type 2 pneomocytes

    • 5% of of alveolar surface but outnumber type 1 cells

    • cuboidal shape and tip of cell inserts into alveolar space

    • contain many organlles and have secretory vessels for secreting surfactant into alveolar space

• Cell adaptations in muscles

  • crdiac and skeletal muscles are made up of sarcomeres for contraction. cardiac muscles are involunatry control but skeletal muscles need nervous stimulation

  • cardiomyocytes (cardiac cells) for walls of heart

    • have a single nucleus (facilitates rapid sinal transmission so cells contract together)

    • 150um

    • have many mitchondria and branched fibres that connect them to other cardiomycoytes in 3D via gap juntions at intercalated disks

    • this branched pattern allos electrical impulse to pass in efficient way through heart so cells contract in synchornous way

  • skeletal muscles

    • arranged in bundles

    • up to 12cm im adult

    • fibres are unbrancehed which enables precise control of voluntary mucle contractions in one direction

    • branch points are areas of structural weakness

    • contain myofibrils of actin and mysosin

    • each fibre is a synctium bc it has many nuclei in same cytoplasm

    • being multinucleated allows tissue to conduct more protein sythesis and repair

• Cell adaptations in sperm and egg cells

  • cytoplasm in egg is rich in lipids proteins and polyssacharides

  • egg has layes of cells covering outside which prevent more than one sperm fertilising it

  • very few egg cells produced bc it takes a lot of energy

  • sperm cells are many and are way smaller

  • sperm cells are motile (can move) and have many mitchondria to give erngy for the flagellum to move and propel sperm forward

  • majority of head of sperm is haploid nucleus

  • sperm head covered in acrosome cap with digestive enzymes to penetrate egg cells “zona pellucida”

  • no storgae of nurtients for energy in sperm so it is provided by male in seminal fluid