Stem Cells in the Bone Marrow
- The stem cells found in bone marrow are multipotent adult stem cells
- This means they can only differentiate into erythrocytes (red blood cells), monocytes, neutrophils and lymphocytes
Erythrocytes
- Erythrocytes are red blood cells, the main function of which is the transport of oxygen around the body (and also the transport of carbon dioxide)
- As red blood cells lack a nucleus, they cannot divide, meaning that new erythrocytes are constantly being formed from bone marrow stem cells in order to maintain the red blood cell count in the blood
- This process is known as erythropoiesis
- The stages of this process are shown below
The differentiation of stem cells to form red blood cells by erythropoiesis
Structure and function of erythrocytes
- During erythropoiesis, changes occur that adapt the structure of the original stem cell to enable it to function as an erythrocyte
- These adaptations include:
- The changing of the cell into a biconcave shape: this shape has a larger surface area, allowing for more oxygen to be absorbed through the cell surface
- The building up of haemoglobin in the cytoplasm: haemoglobin is the pigment that binds with oxygen and only releases it when oxygen concentrations decrease below a certain level
- The ejection of the nucleus (and other organelles including mitochondria, endoplasmic reticulum and Golgi apparatus): creates more room in the cytoplasm for haemoglobin, increasing the oxygen-carrying capacity
- An elastic membrane: this allows erythrocytes to change shape and therefore squeeze through narrow capillaries
Neutrophils
- The same stem cells that form erythrocytes also form neutrophils (a type of white blood cell)
- As the stem cells differentiate into neutrophils, the main changes that occur include:
- Indentations form in the nucleus, giving it a lobed structure
- Granules accumulate (these are lysosomes that contain hydrolytic enzymes)
Structure and function of neutrophils
- Neutrophils are the first white blood cells to arrive at an infection site on or in the body
- They exit the blood through the tiny gaps in capillary walls and collect around foreign bodies (e.g. pathogens)
- They then destroy these by engulfing them (phagocytosis) and digesting them using their hydrolytic enzymes
- The adaptations of neutrophils include:
- A flexible shape and a flexible nuclear membrane: this allows neutrophils to fit between capillary wall cells and to form pseudopodia (the extensions of the cytoplasm that engulf foreign bodies during phagocytosis)
- Containing many lysosomes: these contain digestive enzymes that destroy invading cells