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why do red blood cells have no nucleus

Shown to be in mice rats (and sick humans), the cell-cell interaction between a macrophage (this is a big engulfing cell required for immunity) and young red blood cells (RBC), is known as the erythroblastic island (commonly known as EBI). If you googled it, there is a scientific review in 2008 that describes this structure. At the embryonic stage (in humans), we still retain our RBC nuclei. But as we developed into fetus and adult, we no longer have RBC nuclei. This is thought to be related to the EBI present (in the fetal liver and adult bone marrow respectively). Currently, there is a lack of information of the EBI in other mammals. The only proven ones are mice, rats and sick humans. It is widely assumed (not proven) that mammals have EBIs. Besides mammals, some other animals (e. g. birds) have enucleated RBC, and some don't. It is unclear why is it so. Our lab thinks that it might be related to the formation of the EBI. In addition to engulfing the RBC nuclei, it is believed that the macrophage acts as a "nurse" cell as proposed in the 50s.

In other words, possibly providing iron, and possibly providing some proteins required for young RBC to mature. In early 2013, for the first time, it was showed that these macrophages are important in animal models (published by 2 research groups in nature medicine journal). As for enucleation (the removal of erythroid nuclei), the exact mechanisms are unknown. But cytoskeleton proteins are important players in enucleation. However, there isn't enough information, as these proteins are essential for other important cellular activities as well. For example, bringing in nutrients, development and cellular migration. Most animal models that lack these proteins are unavailable for studies, and these animals usually die at the embryonic stage. The research mentioned by EdoDodo is a proposed model on how enucleation takes place, and is a widely accepted model. Currently, our lab are working on another model that could partially explain how enucleation is being triggered. In addition to better oxygen diffusion across the membranes, some older scientific papers mentioned that it lightened the cardiac workload.

Each extruded RBC nuclei is approximately 40 picograms. A normal healthy adult individual would produce about 2 million RBC per second. That would be 0. 08 milligrams of weight per second are required to be removed. However, I couldn't trace the scientific evidence for this claim, but this have been cited by some scientific papers. The other advantage would be to reduce risk of hemolysis when transversing through the microvasculature. In other words, mature RBC can move along tiny blood capillaries by changing their biconcave shape (to bell-shaped I think), so that they will not rupture (and die). Also, not all RBCs have similar shapes and sizes. You might want to google it for more information. I think camels have slightly different RBC morphology.
As explained in the previous answer, the RBC loses its nucleus only at maturity. The nucleus contains the DNA and which can in turn produce protein.

No nucleus means - no protein/ mRNA synthesis. Also, obviously, the cell loses its capability to divide. Hemoglobin too is a protein. Knowing that it would lose its nucleus, hemoglobin is synthesized in the precursor stages of RBC. A mature RBC can not synthesize new Hb. Hence, the RBC keeps ready its store of enzymes and structural proteins (not only Hb, all proteins it would need for normal functioning) since it can not form new stuff once it has lost its nuclues. The lack of a nucleus also limits the cell's repair capabilities. So human RBCs are cleared from circulation once they are about 4 months old. Damage (mostly from oxidative stress) to their structural proteins leads to loss in membrane flexibility. Since, the RBC can't replace the stuff, it gets gulped in by macrophages. It is not surprising that the cell doesn't have a nucleus. The loss of a nucleus renders it better oxygen carrying capability. Even platelets donot have a nucleus. There too, enzymes are prepared and stored beforehand. Hope that helps!

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