The biconcave cells in blood that lack nuclei when they mature are called red blood cells or erythrocytes.

Here's why:

* Biconcave shape: Red blood cells have a distinctive biconcave shape, like a flattened disc with a depression in the center on both sides. This shape increases their surface area for efficient oxygen transport.

* Lack of nucleus: When red blood cells mature, they expel their nucleus and other organelles to make room for more hemoglobin. Hemoglobin is the protein responsible for carrying oxygen.

* Function: Their primary function is to transport oxygen from the lungs to the body's tissues and carry carbon dioxide back to the lungs to be exhaled.

Let me know if you have any other questions about blood cells!