Some adult tissues contain stem cells that are continuously regenerating (epidermis, hair follicles, blood cells, sperm cells, etc.), while others do not (nervous system, skeletal muscle). Stem cells produce daughter cells that can be differentiated or remain stem cells. Stem cell niches house continuously proliferating stem cells. They allow for regulated self-renewal of the stem cells and differentiation of those that leave the niche. Bone marrow is a stem cell niche, as are the placenta, the heart, the testes and the umbilical cord. The HSC niche produces many types of blood cells and is located in bone marrow. The HSC niche is able to regulate the production of blood cells, when one is at high altitudes, more red blood cells will be produced, and during an infection more white blood cells will be produced.
Sources: "Developmental Biology" Tenth Edition - Scott F. Gilbert
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