2.6.10 Stem Cells in Animals & Plants

• 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