Cells can be categorized by their behavior and location. Many are “resident,” meaning they remain in a fixed position within a tissue. In contrast, “transient cells” are defined by their temporary presence and ability to move, either passing through a tissue or existing for a limited time before being eliminated.
They act like temporary workers, moving into an area to perform a specific job and then leaving once it is complete. This is distinct from resident cells, which form a tissue’s stable structure and long-term function. This temporary nature allows transient cells to play specialized roles throughout the body.
The Role of Transient Cells in Development
During embryonic formation, coordinated cellular migration is required to build the organism. Transient cells are central to this phase, acting as mobile construction crews that travel to precise locations to help form tissues and organs. After their task is accomplished, these cells often undergo programmed cell death, or apoptosis, making their presence temporary.
A prime example involves neural crest cells, which originate along the embryonic neural tube. From this starting point, neural crest cells migrate throughout the developing embryo. Their regulated pathways guide them to various destinations where they differentiate into a variety of cell types.
These transient cells contribute to forming the bones and cartilage of the skull, neurons of the peripheral nervous system, and pigment-producing melanocytes in the skin. This process demonstrates how transient cells build permanent structures before disappearing or integrating in a new form.
Transient Cells in the Immune System
In a mature organism, the role of transient cells shifts to protection and maintenance. The immune system relies on cells that are constantly moving, acting as first responders to injury or infection. These cells are not permanently stationed in tissues but are deployed from central locations, like bone marrow, when needed.
Neutrophils are a primary example of transient immune cells. As the most abundant type of white blood cell, they circulate in the bloodstream awaiting chemical signals of inflammation. When an infection occurs, neutrophils exit blood vessels and migrate to the site. There, they engulf and destroy bacteria before dying within a few days.
Other immune cells like certain types of macrophages also exhibit transient behavior. While some macrophages are resident, others are recruited to sites of inflammation to clear away dead cells and debris. This cleanup crew helps resolve inflammation and prepare the tissue for repair.
When Transient Cell Behavior is Harmful
The migratory capabilities of transient cells can also be exploited in disease. The same mechanisms that allow developmental or immune cells to travel can be hijacked by cancer cells. This adoption of transient characteristics is a feature of cancer metastasis, the process by which cancer spreads to other parts of the body.
For a tumor to metastasize, cancer cells must break away from the primary mass by becoming less adhesive and more mobile, mimicking the behavior of normal transient cells. Once free, these malignant cells can penetrate blood vessels or lymphatic channels to travel to distant organs.
Upon arriving at a new location, cancer cells exit the vessels, invade the surrounding tissue, and form a new tumor, or metastasis. This ability to migrate and establish new colonies makes metastatic cancer dangerous and difficult to treat, representing the harmful side of transient cellular behavior.