Blastema

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Blastema cells surrounded by transparent cystic spaces.

A blastema is a mass of undifferentiated cells capable of growth and regeneration into organs or body parts. Blastemata are typically found in the early stages of an organism's development such as in embryos, and in the regeneration of tissues, organs and bone.

"Primitive" Organisms such as salamanders can still produce blastema when adult; more developed organisms such as frogs can't. That's the reason why salamanders can regenerate an amputated leg and frogs aren't able to do so.

When the limb of the salamander cut off a layer of epidermis covers the surface of the amputation site. During the first few days after injury, this so-called wound epidermis transforms into a layer of signaling cells called the Apical Epithelial Cap (AEC), which has a vital role in regeneration. In the meantime, fibroblasts from the connective tissue migrate across the amputation surface to meet at the center of the wound.They multiply to form a blastema which are the progenitors for the new limb.