• 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

   

• Xylem vessels?and?phloem?sieve tubes?form the transport systems of plants and are found throughout their?roots?and?stems
• The xylem and phloem are formed from?stem cells?that are found in the tissue between them
  • This tissue is known as the?cambium

   

• The cambium is a?meristem, which is the term given to any?undifferentiated tissue?in a?plant?that has the ability to give rise to?new cells
  • For example, there are also meristems located at the?tips?of?shoots?and?roots?that provide new cells to these growing parts of the plant

   

The Cambium

• In the roots and stems of plants, the stem cells at the?inner edge?of the?cambium?differentiate into?xylem cells?and the stem cells at the?outer edge?of the?cambium?differentiate into?phloem cells
  • Cambium cells that differentiate to form the xylem?lose their cytoplasm, deposit?lignin?in their?cell walls?and?lose their end cell walls
  • Cambium cells that differentiate to form the phloem lose?some?of their?cytoplasm?and?organelles, and develop?sieve plates?(located at?ends of the cells)

   

• This cell differentiation is stimulated by?hormones?(the balance of different hormones can determine whether xylem or phloem tissue is produced)

Cross-sections of the stem and root of a plant, showing the locations of the xylem and phloem tissues, as well as the cambium

Table showing different types of stem cells and their sources