Stem Cells

Cytokinesis

• Nuclear division ends with telophase.

• During telophase, the cytoplasm begins to divide.

• A furrow develops in the cytoplasm between the two nuclei.

• Cytokinesis is complete when the cytoplasm splits into two parts (cells), each with its own nucleus.

• The two daughter cells now enter interphase.

• They are identical to the parent cell - same number of chromosomes, and same genetic information.

Stem cells

• Cells in the body can be classified as either specialised cells or stem cells.

• Specialised cells = unable to divide, but can undergo differentiation.

• Stem cells = undergo mitosis to produce either:

  • New stem cells (proliferation), or

  • Cells that differentiate into specialised cells.

• Stem cells are capable of repeated mitotic divisions as they have no particular role.

Types of stem cells

Stem cells originate from either embryonic, adult, or chord blood. They are classified by where they originate or the type of cell they can differentiate into:

• Totipotent stem cells

• Pluripotent stem cells

• Multipotent stem cells.

Totipotent stem cells

Can create any type of cell for embryonic development.

• Form the embryo and the membranes that surround and nourish the embryo.

• Embryonic cells within the first couple of mitotic divisions after fertilisation.

Pluripotent stem cells

Can create most cells and tissues of an organism.

• Inner cell mass of embryo.

Multipotent stem cells

Give rise to cells that have a specific function.

• E.g. blood stem cell, which then differentiates into platelets, erythrocytes, and leukocytes.

• Embryonic stem cells, adult stem cells, and cord blood stem cells are multipotent.

Stem cell differentiation

The process by which cells become specialised cells.

• As the cells undergo mitosis, different genes can be activated.

• This activation causes the cells to differentiate into specialised cells, performing particular functions.

E.g. muscle cells (can contract), red blood cells (can carry oxygen), and stomach cells (secrete enzymes).

Hayflick limit

There is a limit to the amount of times a specialised cell can divide (old age). Specialised somatic cells can only undergo mitotic divisions 40-60 times before cell death.

• Chromosomes contain telomeres which consist of the same sequence of six nucleotides repeated over and over again. With age, telomeres shorten in length, which has been associated with aging.

However, this concept is still being investigated, and is subject to further research.

• The only exceptions to the limit of mitotic divisions are stem cells and cancer cells.

Cancer

A malignant growth; one that has the capability of spreading to other body parts.

Tumour

An abnormal mass of tissue resulting from uncontrolled division of cells.

Malignant

Able to spread to other parts of the body.

Metastasis

The spreading of tumour cells to form secondary tumours in different parts of the body.

Secondary tumours

Cancer that has spread from the original cancer.

Benign

Not able to spread to other parts of the body.

Carcinogen

A cancer causing agent.