Red Blood Corpuscle enucleation and in vitro synthesis

Riding on the red road: Red blood cells are also known as RBCs, red cells, red blood corpuscles (an archaic term), haematids, erythroid cells or erythrocytes (from Greek erythros for "red" and kytos for "hollow", with cyte translated as "cell" in modern usage).

These cells' cytoplasm is rich in haemoglobin, an iron-containing biomolecule that can bind oxygen and is responsible for the blood's red color.

As red blood cells contain no nucleus, protein biosynthesis is currently assumed to be absent in these cells, although a recent study indicates the presence of all the necessary biomachinery in the cells to do so.

In humans, mature red blood cells are oval and flexible biconcave disks. They lack a cell nucleus and most organelles to accommodate maximum space for haemoglobin. 2.4 million new erythrocytes are produced per second. The cells develop in the bone marrow and circulate for about 100–120 days in the body before their components are recycled by macrophages. Each circulation takes about 20 seconds. Approximately a quarter of the cells in the human body are red blood cells.

Scanning electron micrograph of blood cells.
From left to right: human erythrocyte,
thrombocyte (platelet), leukocyte.

RBCs are specialized bags of hemoglobin Red blood cells are designed to pick up oxygen from the lungs and release it into the tissues of the body.  This is their major function.  The only reason that they can pick up and carry oxygen around is because they contain a protein within them called hemoglobin.

If red blood cells (RBCs, or erythrocytes) have to carry oxygen, yet it is the hemoglobin inside them that carries the oxygen, we might expect that RBCs should be loaded with hemoglobin. And in fact, they are!  They load themselves up so much with hemoglobin to carry oxygen that their nuclei get in the way.  So, RBCs don't have any nuclei!  1/3 of their entire volume is due to hemoglobin!

Production of RBCs: This process is called erythropoiesis.  Hemocytoblasts and RBC precursors have nuclei.  But, as cell division continues toward producing the RBC, a cell called a normoblast is made.   The normoblast continues to manufacture hemoglobin (at this point, it looks like a normal cell, but somewhat red).  Then, the normoblast ejects its nucleus.  The cytoplasm continues to make hemoglobin, even without a nucleus, because the instructions from the nucleus are still in the cytoplasm, even though the nucleus is gone.   Eventually, enough hemoglobin is made and the cell is released into the blood as an RBC.

    The odd-looking shape of the RBC actually allows lots of surface area to exist on this cell. That is another important feature of the RBC because it helps there be more room on the RBC for gas exchange across its membrane. The RBCs are rather small, averaging about 8 mm in diameter; a small size also helps them increase their surface area.

Life of RBC: Red blood cells can't last forever.  They have no nucleus. They have to squeeze through the teeniest of blood vessels all day long!  The odds are against them-- either they will run out of materials they need because they have no nucleus to make more, or they will squeeze through one too many teeny blood vessels and burst.  They can only last so long.

    On average, a red blood cell will survive 120 days (that's about 4 months... which is pretty good, considering their difficulties). Over 120 days, each RBC wll travel through the entire body about 75,000 times!  Wow!

Regulation of RBC production: Some people have a hard time with the change in altitude for a couple of days.  That's because at higher elevations there is less oxygen in the air... so to get enough oxygen, we actually need more RBCs in our body.  

    A drop in oxygen levels in one's body triggers the release of erythropoietin. Erythropoietin is released from the kidneys and liver, and it triggers erythropoiesis to occur.  Within a couple days, new RBCs are in the blood, and a person's RBC level increase.  

    Just like with the other hormones, there is a negative feedback loop to control erythropoiesis.  As oxygen levels return to normal, the kidneys (and liver) stop making erythropoietin.  The extra RBC production declines as long as oxygen levels remain normal.

Why RBCs are enucleated: Because they were descended from ancestors that had evolved red blood cells that had the ability to enucleate themselves as part of the process of red blood cell maturation. That's about it. This is useful because:

1. The enucleated red blood cell has more room for carrying hemoglobin and thus oxygen if it's not carrying around a nucleus

2. The enucleated red blood cell doesn't need an aerobic metabolism to support a nucleus, and therefore isn't itself using up the oxygen that it's carrying

3. The enucleated red blood cell is more bendy than a nucleated one, and can thus fit through narrower capillaries, which is more efficient since more of the body mass can be given over to cells of the body rather than capillary volume.

Enucleation less understood: Prior to entering the blood stream, developing erythroid or red blood cells condense and expel their nucleus. This unusual process occurs millions of times per minute in a healthy adult, and is unique to mammals. However, our understanding of this process is very limited.

Mammalian erythrocytes or red blood cells circulate without a nucleus. During development in the bone marrow, erythroid progenitors expand, mature and condense and expel their nucleus. This is estimated to occur 2 millions/second in a healthy adult human. The processes that regulate this event are extremely poorly defined. Interestingly, enucleation is restricted to mammals. All mammals possess enucleated red blood cells whereas erythrocytes of birds, reptiles, amphibians and fish all possess their nuclei. We are very interested in exploring the processes by which mammalian erythroid cells condense and expel their nuclei and asking why other vertebrates do not.

Enucleation is the hallmark of erythropoiesis in mammals. Previously, we determined that yolk sac–derived primitive erythroblasts mature in the bloodstream and enucleate between embryonic day (E)14.5 and E16.5 of mouse gestation. While definitive erythroblasts enucleate by nuclear extrusion, generating reticulocytes and small, nucleated cells with a thin rim of cytoplasm (“pyrenocytes”), it is unclear by what mechanism primitive erythroblasts enucleate. Immunohistochemical examination of fetal blood revealed primitive pyrenocytes that were confirmed by multispectral imaging flow cytometry to constitute a distinct, transient cell population. The frequency of primitive erythroblasts was higher in the liver than the bloodstream, suggesting that they enucleate in the liver, a possibility supported by their proximity to liver macrophages and the isolation of erythroblast islands containing primitive erythroblasts. Furthermore, primitive erythroblasts can reconstitute erythroblast islands in vitro by attaching to fetal liver–derived macrophages, an association mediated in part by α4 integrin. Late-stage primitive erythroblasts fail to enucleate in vitro unless cocultured with macrophage cells. Our studies indicate that primitive erythroblasts enucleate by nuclear extrusion to generate erythrocytes and pyrenocytes and suggest this occurs in the fetal liver in association with macrophages. Continued studies comparing primitive and definitive erythropoiesis will lead to an improved understanding of terminal erythroid maturation.

Human erythrocytes are produced through a process named erythropoiesis, developing from committed stem cells to mature erythrocytes in about 7 days. When matured, these cells live in blood circulation for about 100 to 120 days (and 80 to 90 days in a full term infant). At the end of their lifespan, they become senescent, and are removed from circulation.

Erythropoiesis is the development process by which new erythrocytes are produced; it lasts about 7 days. Through this process erythrocytes are continuously produced in the red bone marrow of large bones, at a rate of about 2 million per second in a healthy adult. (In the embryo, the liver is the main site of red blood cell production.) The production can be stimulated by the hormone erythropoietin (EPO), synthesised by the kidney. Just before and after leaving the bone marrow, the developing cells are known as reticulocytes; these comprise about 1% of circulating red blood cells.

Blood in a dish: in vitro synthesis of red blood cells

In vitro production of RBCs: the 2-step erythroid culture system

Red blood cells, currently obtained from donors, represent the most common form of cell-based therapy. A better understanding of normal erythropoiesis is leading to improved multi-step protocols for the in vitro generation of fully mature red cells. The extensive in vitro expansion of embryonic erythroblasts and development of erythroid precursors as a potential transfusion product may help to deal with issues of scale and eventually find a place in the treatment of patients with acute and chronic anemias.

# pictures thankfully shared from wikipedia.org and other internet resources.