###immature RBC has nucleus...but at maturation it lacks nucleus for providing More space to gaseous exchange...

Immature RBCs in mammals
Immature red blood cells (RBCs) in mammals, also known as reticulocytes, undergo a process called erythropoiesis in the bone marrow before they mature into fully functional red blood cells. During this process, the RBCs go through various stages of development.

Single nucleus
At the immature stage, mammalian RBCs have a single nucleus. This nucleus eventually gets expelled from the cell as it matures, allowing more space for hemoglobin, the oxygen-carrying protein, to be packed into the cell. Once the nucleus is removed, the cell becomes a mature, biconcave disc-shaped red blood cell capable of efficiently transporting oxygen throughout the body.

Function of the nucleus
The presence of a nucleus in immature RBCs is essential for the cell's ability to undergo cell division and replicate. However, in mature RBCs, the absence of a nucleus allows for a larger surface area for oxygen exchange and increased flexibility to pass through narrow blood vessels.

Significance of a single nucleus
Having a single nucleus in immature RBCs ensures that the cell retains its ability to divide and differentiate during the early stages of erythropoiesis. This process is crucial for maintaining a constant supply of red blood cells in the body to support oxygen transport and overall physiological function.
In conclusion, the presence of a single nucleus in immature RBCs of mammals is a key characteristic that distinguishes them from mature red blood cells and plays a vital role in the cell's development and function.