Cancer doesn’t arise from just any DNA damage.  We are exposed to many insults in our environment, food, water, and activities and our DNA can be easily damaged.  As well, there are naturally occurring errors that occur during DNA replication.  So why doesn’t all of this DNA damage lead to cancer right away? The answer is – it depends on which gene the DNA damage is located.

^{Overview of Genetic Basis of Cancer by Lyz Boyd, licensed under  CC-BY-NC}

Metastasis

Metastasis is when the cancerous growth spreads beyond its original site. With the exception of cancers of blood tissue, most tumours begin in their local site, but is still confined to its basement membrane.  By staying within this localized site, the tumour can tap into blood supply to feed its growth.  However, when the tumour’s growth disrupt and then invade the basement membrane, the tumour now has access to even more blood and lymphatic vessels.   Similarly cancerous cells that aren’t attached well to each other can now detach and join the lymphatic or the blood flow.  These traveling cancerous cells will eventually enter the systemic blood circulation, allowing it to reach every organ and tissue in the body. The cells that enter the lymphatic vessels encounter the lymphnodes on their way to the blood circulation and will often colonize the lymphnodes making lymphnode involvement a handy marker for how far the cancer has spread.

^{Metastasis by Lyz Boyd, licensed under CC-BY-NC}

The site of metastasis depends upon the following factors:

• Type of cancer: currently, certain cancers tend to metastasize to a specific organ.  The mechanism as not yet been elucidated.
• An organ’s blood supply:  the liver, lungs, & brain are highly vascularized with tiny vessels which allow for slow blood flow, thus allowing a cancerous cell to leave the blood and attach to its new metastatic site.
• Geographic location:  neighbouring structures are frequently invaded by groups of traveling cells or cells that have been rubbed off on to neighbouring structures, (e.g. peritoneal metastasis of ovarian cancer).
• The local biochemical environment of the target organ interacting with selective metastatic cancer cells. This is known as  “seed and soil” hypothesis of why cells stick to particular place.

Section Review

Cancer is uncontrolled growth in tissue.  This uncontrolled growth begins with damage to the DNA, particularly in areas that have genes that encode for pro-growth (proto-oncogene) and/or tumour suppression.  Some common features of cancerous tissue – regardless of the tissue are:

• Rapid replication of cells.  May look like disorganized growth and invading surrounding structures (e.g. basement membrane).
• Large nuclei of various sizes  to represent the high nuclear activity and different levels of differentiation and replication.
• Tissue and organs with cancerous changes usually have loss of function due to loss of differentiation. In rarer cases there may be a gain of function (i.e. produces an unusual product).
• Their rapid growth often compress surrounding structures thus affecting their function.
• Cancerous growth has a high demand for nutrients and blood flow due to it’s uncontrolled cell division & growth. The more tumours, the more demand.
• Metastasis is the primary cause of death from cancer because it compromises the functions of the target organ.

Review Questions