Did You Know?
Camel is an exception to the mammalian feature of having non-nucleated red blood cells.
In almost all vertebrates, red blood cells or erythrocytes are responsible for carrying oxygen to the different tissues of the body. However, the red blood cells present in mammals are devoid of nucleus as well as other organelles, like mitochondria and endoplasmic reticulum.
Red blood cells are formed through the process of erythropoiesis that occurs in the bone marrow. The immature red blood cells formed during erythropoiesis are nucleated. The nucleus is expelled out of the cell during maturation. Such expelled nuclei and other organelles are engulfed and degraded by the phagocytic cells of the body.
This Buzzle write-up provides a brief overview of some of the reasons why red blood cells do not have nuclei, and how the absence of nucleus is essential to improve oxygen transport.
How Does Absence of Nucleus Help Red Blood Cells
Creates Space for Hemoglobin
The sole function of red blood cells is to carry oxygen molecules from the lung, and deliver it to the peripheral tissues. This is achieved through proteins called hemoglobin, which have the ability to irreversibly bind to oxygen molecules. The hemoglobin structure is such that it can bind to four oxygen molecules at a time. Being so, logic dictates that more the number of hemoglobin molecules, more is the number of oxygen molecules to be carried.
The absence of nucleus (the largest organelle of a cell), mitochondria, and other organelles enable the red blood cells to carry more number of hemoglobin molecules. As a result, more oxygen can be carried per cycle of oxygen transport, thus, increasing the efficiency of the process. Each red blood cell has about 250 million molecules of hemoglobin, which account for one-third volume of the entire cell. Moreover, the concentration of hemoglobin in red blood cells influences its shape and may also affect the viscosity of blood.
Attains Biconcave Shape
The shape of red blood cells influences the surface area that is available for gaseous exchange with the tissue. A disc-like biconcave shape provides high surface to volume ratio, as against the usual spherical shape of cells. This helps increase the efficiency of diffusion of oxygen molecules. In order to attain this shape, the red blood cells get rid of the nucleus and other large organelles.
Moreover, it has been suggested that the evolution of mammals was accompanied with an increased need for efficient oxygen transport, as well as increased accumulation of non-coding DNA. A higher amount of DNA implied a larger nucleus, necessitating a larger cell size. On the contrary, the shape and size of the red blood cell must be such that it can easily pass through the narrow capillaries of peripheral tissues.
The nucleus, being the largest organelle, hampers the flexibility of red blood cells, and needs to be expelled out of the cell. However, an opposing school of thought states that the nucleus is too small to hinder the ability to change shape.
Nevertheless, an interesting fact is that the average diameter of mammalian nucleus is 6 µm, and the average diameter of mammalian red blood cells is in the range of 4 - 8 µm.
Adapts to High Oxidative Stress
Nucleus and mitochondria are sites for cellular processes that involve reduction of oxygen, and hence, may lead to formation reactive oxygen species (ROS). ROS are known to damage the structural integrity of several macromolecules of the cell, including proteins and RNA. This may even affect the viability of a cell. The extrusion of nucleus and mitochondria limits such oxidative stress in red blood cells, thus, enabling better adaptation to the high hemoglobin conditions in the cell.
Moreover, the most disastrous consequence of oxidative stress is DNA damage and cancer. Red blood cells account for one-fourth of the total number of cells present in the body, and 2 - 3 million red blood cells are produced in the bone marrow per second. The presence of nucleus and the ability of cell division will highly increase the risk for genetic anomalies.
The removal of nucleus during the final stage of development into a mature red blood cell is an adaptation to excel in oxygen transport, and combat the risks associated with it.