Therapeutic applications of stem cells in human diseases

Stem-cell-based interventions encompass biologically distinct therapeutic strategies with markedly different levels of clinical maturity. Hematopoietic stem cell transplantation is an established treatment for selected hematologic disorders, whereas many other approaches remain investigational and require more rigorous evidence of durable efficacy and long-term safety. This review critically examines the major cellular platforms used in regenerative medicine, including embryonic stem cells (ESCs), tissue-specific adult stem cells, hematopoietic stem cells (HSCs), mesenchymal stromal cells (MSCs), and induced pluripotent stem cells (iPSCs). We compare their biological properties, therapeutic rationale, translational suitability, and principal limitations across cardiovascular, ocular, hepatic, oncologic, neurodegenerative, gastrointestinal, and autoimmune diseases. Particular attention is given to the distinction between direct cell replacement and indirect reparative effects mediated by immunomodulation, trophic signaling, angiogenesis, extracellular vesicles, and remodeling of the tissue microenvironment. We also discuss enabling technologies, including genome editing, organoid systems, and stem-cell-derived extracellular vesicles, as well as the major barriers to clinical translation. These include donor- and source-dependent heterogeneity, limited engraftment and persistence, incomplete differentiation or maturation, tumorigenic and immunologic risks, manufacturing reproducibility, quality-control requirements, and regulatory complexity. Rather than functioning as interchangeable therapeutic products, different stem-cell platforms are likely to have disease-specific roles that depend on their mechanism of action and the biological requirements of the target tissue. Greater mechanistic precision, standardized product characterization, and carefully designed clinical studies will be essential for broader and more reliable implementation.