Cells grown in laboratory culture dishes undergo only a fixed number of division before dying. The number of possible divisions drops depending on the age of the donor organism. This phenomenon is called the

Hayflick limit

Explanation:

The Hayflick limit, or Hayflick phenomenon, is the number of times a normal human cell population will divide before cell division stops. It was postulated by Leonard Hayflick in 1961,

it is called the mitotic spindle