Red blood cell blood circulation for about 100 to 120 days (and 80 to 90 days in a full term infant). At the end of their lifespan, they are removed from circulation. In many chronic diseases, the lifespan of the red blood cells is reduced. Erythropoiesis is the process by which new red blood cells are produced; it lasts about 7 days. Through this process red blood cells are continuously produced in the red bone marrow of large bones. (In the embryo, the liver is the main site of red blood cell production.) The production can be stimulated by the hormone erythropoietin (EPO),
Connective tissue blood vessels and in certain ligaments, particularly in the ligamenta flava. In hematopoietic and lymphatic tissues, reticular fibers made by reticular cells provide the stroma—or structural support—for the parenchyma—or functional part—of the organ. Mesenchyme is a type of connective tissue found in developing organs of embryos that is capable of differentiation into all types of mature connective tissue. Another type of relatively undifferentiated connective tissue is mucous connective tissue, found inside the umbilical cord. Various types of specialized tissues and cells are classified under the spectrum of connective tissue, and are as diverse as brown and white adipose tissue, blood,