Stem Cells

All cells in plants and animals begin as undifferentiated cells and originate from mitosis or meiosis. They’re not adapted to any particular function and have potential to differentiate to become a specialised cell.

They continually divide via cell division and are the source of new cells necessary for growth, development and repair. once stem cells are specialised, can no longer divide, entering the G0 phase of cell cycle.

If stem cells don’t divide fast enough, tissues not efficiently replaced = ageing. If there is uncontrolled division = masses of cells formed called tumours = potential cancer.

Stem Cell Potency

Potency = stem cell’s ability to differentiate into different cell types. Greater number of cell types it can differentiate into = greater potency.

  • Totipotent→ stem cells can differentiate into any type of cell (example→ fertilised egg, zygote etc). Totipotent cells occur only for a limited time in early mammalian embryos

  • Pluripotent→ stem cells that can form all tissue types but not whole organisms

  • Multipotent→ stem cells that can only form a range of cells within a certain type of tissue.

Differentation

In multicellular organisms, cells have to specialise to take on different roles in tissues and organs; such as - form barriers such as skin etc.

Erythrocytes (red blood cells) and neutrophils (white blood cells) are both present in the blood. They look different due to different functions. When cells differentiate, they become adapted to their specific role and this is dependent on the function of the tissue, organ and organ system the cell belongs to.

All blood cells are derived from stem cells in the bone marrow.

Replacement of Red and White blood cells

Mammalian erythrocytes are essential for transport of oxygen around the body and are adapted to maximise their oxygen-carrying capacity - only having few organelles and structure.

Due to lack of organelles and nucleus, Lifespan = 120 days. Therefore replaced constantly via stem cell colonies in the bone marrow (produce over 3bn erythrocytes per kg of body mass per day)

Neutrophils = Essential role in immune system. Lifespan = 6 hours and replaced via colonies of stem cells in bone marrow (producing 1.6bn per kg per hour). That figure will increase if an infection.

Sources of animal stem cells

  • Embryonic stem cells→ cells present at a very early stage of embryo development and are Totipotent. 7 days in, a mass of cells (called a blastocyst) is formed and cells are now pluripotent - remaining this state in the feetus until birth

  • tissue (adult) stem cells→ cells present throughout life from birth. Found in specific areas such as bone marrow. They’re multipotent however could be artificially triggered to become pluripotent. Stem Cells can be harvested via umbilical cords of babies. Advantages = plentiful supply, no invasive surgery needed, Umbilical chords can be stored for later use with no chance of rejection

Sources of Plant stem cells

  • Meristematic tissue (Meristems)→ in plants. Tissue found wherever growth is occurring in plants. Meristematic Tissue is also located sandwiched between phloem and xylem tissues called ‘Vascular Cambium’→ Cells in this region differentiate into different cells present in xylem and phloem tissues. They are pluripotent.

Uses of stem cells

Stem cells transplanted into areas to potentially treat certain diseases.

  • Heart Disease → muscle tissue in the heart may be damaged

  • Type 1 diabetes → insulin dependent diabetes

  • Parkinson’s Disease → delay the death of dopamine producing cells

  • Alzheimer’s Disease → somewhat repair brain cells that are destroyed

  • Macular degeneration → Potentially treat the blindness caused

  • Birth defects → reverse untreatable birth defects

  • Spinal Injuries → help restore some movement by repairing damaged spinal cords

Areas already used

  • The treatment of burns → produce new skin for burn patients.

  • Drug trials

  • Developmental biology

Ethics

Use of embryonic stem cells in therapies and research has lead to controversy and debates regarding ethics

  • Embryonic Destruction → Morally wrong - life begins at conception, therefore murder

  • However could lead to treatments of incurable diseases if used.

  • Development made into producing artificial pluripotent stem cells as an alternative.