Lifespan of connective tissue cells?

Question

This post is regarding a follow up on my initial post on "Properties and life cycle of chondrocytes and tenocytes".I am elaborating on my question on the lifespan of tenocytes and chondrocytes.From what I have read, tenocytes and chondrocytes form from mature stem cells, more specifically Mesenchymal stem cells. These cells can replicate through mitosis, and are present in the body in different tissues like bone marrow. These cells are the key in the regeneration of diverse tissues. When the cells that inhabit tendons or cartilage die, do new MS cells travel to the tendon or cartilage area and diffentiate into a new tenocyte or chondrocyte? Are tenocytes and chondrocytes continuously replaced?

Answer 1

To understand this, you need to understand about the "potency" of stem cells. Cell potency refers to how diverse the options are for stem cell differentiation. There are several levels of potency in stem cells: totipotent (like a zygote - capable of becoming differentiated into anything in the organism), pluripotent (these can become just about anything, but is a step lower than totipotent), multipotent (ex. the multipotent hematopoietic stem cell that can become any type of red blood cell or white blood cell), oligopotency (multiple, but not many, options for differentiation), and unipotent (these are commonly referred to as precursor cells because they can typically only become ONE thing).

When stem cells replicate, they do so by mitosis (as you said). When mitosis is complete, it produces two exact copies of the single parent cell. One of those copies differentiates as needed and the other TYPICALLY remains a stem cell to maintain the population of stem cells.

The mesenchymal stem cells that become chondrocytes stop migrating and finish differentiating into chondrocytes during fetal development. Once those chondrocytes are at their destination, they become hypertrophic (which is the final step in their differentiation) and create cartilage while maintaining the population of chondrocytes via mitosis.

"Tenocyte" is just a specific term for fibrocyte that is found in a tendon. Fibrocytes are cells that function to maintain an extracellular matrix. The more common term for this kind of cell is "fibroblast" because the fibroblast is the activated and more commonly investigated form. The kind found in tendons would be the fibrocyte. This type of cell can undergo mitosis on its own and does not have a special population of stem cells from which it differentiates.

Sorry to take so long to answer your questions, but I felt like I should get that all out there to help with understanding.

You asked:

1) When the cells that inhabit tendons or cartilage die, do new MS cells travel to the tendon or cartilage area and diffentiate into a new tenocyte or chondrocyte?

1. No, mesenchymal stem cells do not migrate out of their set locations after fetal development is complete.

2) Are tenocytes and chondrocytes continuously replaced?

1. Yes, both of these types of cells are capable of replacing themselves via mitosis.

Answer 2

the lifespan of a chondorcyte depends on its current stage of development. however, once they become hypertrophic, they are fully differentiated. But, chondrocytes can enter senescence and last over 15 years.

drsjp