Blastema

A blastema (Greek βλάστημα, "offspring"^[1]) is a mass of cells capable of growth and regeneration into organs or body parts. The changing definition of the word "blastema" has been reviewed by Holland (2021).^[2] A broad survey of how blastema has been used over time brings to light a somewhat involved history. The word entered the biomedical vocabulary in 1799 to designate a sinister acellular slime that was the starting point for the growth of cancers, themselves, at the time, thought to be acellular, as reviewed by Hajdu (2011, Cancer 118: 1155-1168). Then, during the early nineteenth century, the definition broadened to include growth zones (still considered acellular) in healthy, normally developing plant and animal embryos. Contemporaneously, cancer specialists dropped the term from their vocabulary, perhaps because they felt a term connoting a state of health and normalcy was not appropriate for describing a pathological condition. During the middle decades of the nineteenth century, Schleiden and Schwann proposed the cell theory, and Remak and Virchow insisted that cells can only be generated by division of existing ones. Consequently, the conception of the blastema changed from acellular to cellular. More specifically, the term came to designate a population of embryonic cells that gave rise to a particular tissue. In short, the term blastema started being used to refer to what modern embryologists increasingly began calling a rudiment or Anlage. Importantly, the term blastema did not yet refer to a mass of undifferentiated-looking cells that accumulates relatively early in a regenerating body part. For instance, Morgan (1900), does not use the term even once in his classic book, “Regeneration.” It was not until the eve of World War 1 that Fritsch (1911, Zool. Jb. Zool. Physiol. 30: 377-472) introduced the term blastema in the modern sense, as now used by contemporary students of regeneration. Currently, the old usage of blastema to refer to a normal embryological rudiment has largely disappeared (except for describing aspects of development of the kidney and, to a lesser extent, the adrenal gland).

During the last century, blastemas were thought to be composed of undifferentiated pluripotent cells, but recent research indicates that in some organisms blastemas may retain memory of tissue origin.^[3] They 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.^[4]

Some amphibians and certain species of fish and two species of African spiny mice can produce blastemas as adults.^[5] For example, salamanders can regenerate many organs after their amputation, including their limbs, tail, retina and intestine.^[6] Most animals, however, cannot produce blastemas.