Streaming tissues

The organism is made of cells. Cells aggregate into tissues units, that assemble into tissues. Several tissues make an organ, Nearly all cells live less than the organism does. Epithelia in the gut live two days, skin cells, three weeks. Red blood cells live 120 days, and bone cells, several years. Dying cells are replaced with newly formed cells, a process known as cell turnover.

The entire surface of our skin, known as epidermis, is replaced every three weeks. Skin cell population consists of young , adult and old cells. Just like the human population. One would assume that once a cell is formed it will stay in the place of it birth. Eventually die there, and replaced by a new cell. Yet this is not the case. Cells in the body are formed at one place and literally stream to their graveyard.

Tissue unit

During growth, organism does not pile up cells like bricks. Its building blocks consist of cell clusters, called tissue units.
The epithelial tissue unit is made of five components, embedded in a gel matrix:

-Epithelium cells
-Connective tissue
-Nerve fibers
-Blood vessel
-Lymph vessel

Epithelium cells are responsible for tasks executed by the unit.. Connective tissue cells contribute the scaffold and matrix. Nerve fibers provide communication with other units. Resources arecarried by blood carrying vessels. Lymph vessels drain tissue fluid to other units.

At the base of the tissue unit is a stem cell whose progeny make the unit

Click here to read more on tissue units