What Kind of Doctor Do I Need? Slideshow Dental (Oral) Health Quiz Causes of a Heart Attack Slideshow Tissue: A group or layer of cells that perform specific functions. For example, muscle tissue is a group of muscle cells. From Healthy Resources When Prostate Cancer SpreadsHow Well Are You Living With AS?Protecting Your Immune System Featured Centers When Prostate Cancer SpreadsEssential Tips to Manage DiabetesWhat Is Cystic Fibrosis? What Causes It? Health Solutions From Our Sponsors Kept Your Wisdom Teeth? What is TAVR?