Progenitor cell

A progenitor cell is a biological cell that, like a stem cell, has a tendency to differentiate into a specific type of cell, but is already more specific than a stem cell and is pushed to differentiate into its "target" cell. The most important difference between stem cells and progenitor cells is that stem cells can replicate indefinitely, whereas progenitor cells can only divide a limited number of times. Controversy about the exact definition remains and the concept is still evolving.

The terms "progenitor cell" and "stem cell" are sometimes equated.

Properties
Most progenitors are described as oligopotent. In this point of view, they may be compared to adult stem cells. But progenitors are said to be in a further stage of cell differentiation. They are in the “center” between stem cells and fully differentiated cells. The kind of potency they have depends on the type of their "parent" stem cell and also on their niche. Like stem cells, mostly, they are formed and transported in a colony, with the right conditions for them to grow and differentiate into their target tissues. In contrast, some progenitor cells were found during research, and were isolated. After their marker was found, it was proven that these progenitor could move through the body and migrate towards the tissue where they are needed. Many properties are shared by adult stem cells and progenitor cells. But still, controversy remains because embryonic stem cells are true stem cells in that they are pluripotent and show unlimited capacity for self-renewal. In contrast, many cells termed adult stem cells would be better defined as progenitor cells, as their capacities for unlimited self-renewal and plasticity have not been comprehensively demonstrated.

Progenitor cells are found in adult organisms and they act as a repair system for the body. They replenish special cells, but also maintain the blood, skin and intestinal tissues. They can also be found in developing embryonic pancreatic tissue.

Function
The majority of progenitor cells lie dormant or possess little activity in the tissue in which they reside. They exhibit slow growth and their main role is to replace cells lost by normal attrition. In case of tissue injury, damaged or dead cells, progenitor cells can be activated. Growth factors or cytokines are two substances which trigger the progenitors to mobilize towards the damaged tissue. At the same time, they start to differentiate into the target cells. Not all progenitors are mobile and are situated near the tissue of their target differentiation. When the cytokines, growth factors and other cell division enhancing stimulators take on the progenitors, a higher rate of cell division is introduced. It leads to the recovery of the tissue.

Examples
The characterization or the defining principle of progenitor cells, in order to separate them from others, is based on the different cell markers rather than their morphological appearance.


 * Satellite cells found in muscles. They play a major role in muscle cell differentiation and injury recoveries.
 * Intermediate progenitor cells formed in the subventricular zone. Some of these transit amplifying neural progenitors migrate via rostral migratory stream to the olfactory bulb and differentiate further into specific types of neural cells.
 * Bone marrow stromal cells, basal cell of epidermis have 10% of progenitor cell, although they are often classed as stem cells due to their high plasticity and potentially unlimited capacity for self renewal.
 * Periosteum contains progenitor cells that develop into osteoblasts and chondroblasts.
 * Pancreatic progenitor cells are among the most studied progenitors. They are used in research to develop a cure against diabetes type-1.
 * Angioblasts or Endothelial progenitor cells (EPC). These are very important for research on fracture and wounds healing.

Development of the human cerebral cortices
Before E40 (embryonic day 40), progenitor cells generate other progenitor cells; after that period progenitor cells produce only dissimilar daughters. The cells from a single progenitor cell form a proliferative unit that creates on cortical column; these columns contain a variety of neurons with different shapes.