anucleate

Anucleate is a biological term describing cells or cell fragments that lack a nucleus. The word derives from Greek an- meaning without and nucleus meaning kernel or center.

Representative examples include mature red blood cells (erythrocytes) of mammals and blood platelets (thrombocytes). In mammals, erythrocytes lose their nucleus during maturation in the bone marrow, and mature erythrocytes lack a nucleus and most organelles, maximizing space for hemoglobin and enabling efficient gas transport. Platelets are small, anucleate cytoplasmic fragments derived from megakaryocytes and contain granules vital for clot formation.

In other vertebrates, red blood cells may remain nucleated; birds, reptiles, amphibians, and many fish retain nucleated erythrocytes. This contrasts with the mammalian condition, reflecting evolutionary variation in red blood cell structure and function.

The absence of a nucleus precludes DNA transcription and replication, as well as de novo protein synthesis. Consequently, anucleate cells rely on pre-existing proteins and metabolic pathways for their function. Platelets, for example, cannot divide and have a limited lifespan, but they rapidly respond to vascular injury through granule-mediated signaling and secretion. The presence or absence of a nucleus in red blood cells is a notable physiological and phylogenetic trait across vertebrates.

In summary, anucleate describes cells without a nucleus, most prominently mammalian erythrocytes and platelets, with distribution and implications that vary across taxa.