Cell Differentiation, Determination and Replacement

Cell determination - process which a previously undifferentiated cell already programmed to become a specific cell type by following specificed path to differentation

Developmental choice is taken and cell commited to specialised development = self-perpetuating change of internal character (cell different from others)

Cell differentiation - normal process by which less specialised cell develops or matures to process more distinct form and function

In cell differentiation, overt cell specialisation apparent

CELL USUALLY DETERMINED BEFORE DIFFERENTION

Fish and crustaceans - growth continous = organisms larger and larger until death

Birds and mamamls - growth determinate = somatic cells replaced in adulthood

Flies and nematodes - growth determinate = somatic cells not replaced adulthood

Some cells permanent - cannot be repaired if damaged

Examples - eye lenses, mammal auditory hair cells, eye rod cells

Lens of eye lacks of organelles and nucles - not replaced if damaged

Eye rod photoreceptors - not be structurally replaces but photosensitive rhodopsin continually replaced (show using 3H-leucine radioactive pulse)

Auditory hair cells convert mechanical stimuli to electrical signals in brain 

• In mammals damaged hair cells not replaced
• Other vertebrates - supporting cells triggered to behave as stem cells and generate replacement hair cells

Simple Duplication:

• Example - endothelial cells lining blood vessels
• Capable of dividing to replace lost/damaged cells and develop into new blood vessels
• Some cells too specialised for this so replacement via stem cells

Undifferentiated Stem Cells:

• Stem cells - not terminally differentiated
• Can divide without limit
• Divide into another stem cell or irreversible terminal differentiation

Stem cell forms a differentiated cell

Unipotent - only replace 1 type of cell

• Olfactory sensory neurones, spermatozoa

Oligopotent/Multipotent - replace small number of cells

• Absorptive, goblet, paneth, enteroendocrine cells

Pluripotent/Multipotent - replace large number of cells

• Blood cells

Totipotent - replace all types of cells

• Embryo cells
• Plant cells

Olfactory basal cell -> Olfactory sensory neurones

Survive 1-2 months and then replacements come from basal cells

Spermatogonium -> Spermatozoa

1 stem cell can replace multiple cells but need to be similar locality

Example - epithelial cells of small intestine - form paneth, goblet, absorptive cells

1 stem cell type replace mutiple cells but need to be similar locality (IN BONE MARROW)

Example - haematopoiesis (blood cells)

Form red blood cells, white blood cells etc

Stimulate cell division

Example - erythropoietin production in kidney stimulated by low oxygen

CSFs bind to cytokine receptors on target cells

Target cells begin to differentiate 

Cell type stimulated by specific colony stimulating factors

When differentiated cells removed from body and maintained in culture they usually maintain differentiated form = have a memory

Stem cells in culture divide and capable of producing limited number of differentiated cell types

Totipotent cells potentially produce any type of cell/whole organs e.g. early embryonic stem cells maintained in culture with stimulating factors have potential to produce all cell types for adult organism

Stem cells also transplanted into other organisms but chance of rejection by donor as DNA from different individual

Most somatic cells reverted back to totipotency in right environment

Plants can be cloned in synthetic environment - take cells and put in correct environment

Natural environments - remove nucleus from one cell and using another unfertilised enucleated egg as host

Problem when embryo develops - all cells determined at 16 cell stage in humans