What Are Mesenchymal Stem Cells?

MSCs are the most widely used cell type in regenerative medicine — and one of the most loosely used terms. Here's the actual scientific definition.

The Formal Definition

Mesenchymal stem cells (MSCs), also called mesenchymal stromal cells, are multipotent cells capable of self-renewal and differentiation into a defined range of mesodermal tissue types. In 2006, the International Society for Cellular Therapy (ISCT) published minimal criteria to standardize what qualifies as an MSC, given wide variability in how the term was being used across labs:

• Plastic adherence — the cells adhere to standard tissue culture plastic under normal culture conditions.
• Surface marker profile — positive expression of CD73, CD90, and CD105; negative expression of CD45, CD34, CD14 or CD11b, CD79a or CD19, and HLA-DR.
• Trilineage differentiation — demonstrated capacity to differentiate into osteoblasts (bone), adipocytes (fat), and chondroblasts (cartilage) under appropriate in vitro conditions.

A population of cells that doesn't meet all three criteria doesn't meet the ISCT definition of an MSC, regardless of what it's labeled as commercially.

Where MSCs Are Sourced From

MSCs have been isolated from a wide range of tissues, each with a somewhat different profile:

• Bone marrow — the original and most-studied source, but yield and proliferation capacity decline with donor age.
• Adipose tissue — higher yield per volume of tissue than bone marrow, obtained via lipoaspirate.
• Perinatal tissue — umbilical cord (specifically Wharton's jelly), placenta, and amniotic membrane. These sources are collected at birth from consenting donors and generally show higher proliferative capacity than adult-tissue-derived MSCs. See What Is Wharton's Jelly.

Mechanism of Action

MSCs are generally understood to exert regenerative effects less through direct differentiation into replacement tissue at the injury site and more through paracrine signaling — secreting cytokines, growth factors, and extracellular vesicles that modulate local immune activity, reduce inflammation, and support the tissue's own repair processes. This is an active area of ongoing research, and mechanisms vary by tissue source, culture conditions, and route of administration.

This page is educational reference material for licensed healthcare providers and does not constitute medical, legal, or regulatory advice. HCT/P products are not FDA-approved.