stem cells & differentiation

the notochord emits a signal to control the specialization

of cells in the plate & tube

cells closet to the notochord differentiate into

a structure called the floorplate

cells farther from the notochord differentiate into

precursors of motor neurons

the signal from the notochord is a protein produced by

a gene called sonic hedgehog

many differentiated cells, such as red blood cells, surface epidermal cells, & goblet cells are unable to divide

these cells are considered terminally differentiated

Terminally differentiated cells are generated from a stock of precursor cells called

stem cells that are retained in tissues along with the differentiated cells

stem cells can divide without limit; when a stem cells divides, each daughter cell

can either remain a stem cell or can begin the process leading to terminal differentiation