**3. Bone marrow anatomical structure**

The bone is an organ composed of cortical and trabecular bone, cartilage, and hematopoietic and connective tissues. The bone tissue has an essential role in the structure and protection of the human body. Spongy, or trabecular bone, is composed of a lattice of fine bone plates filled with hematopoietic marrow, fatcontaining marrow, and arterial-venous sinusoidal blood vessels. Furthermore, it consists of bone cells at different developmental stages (including pre-osteoblasts, osteoblasts, and osteocytes), collagen fibrils, and calcium and phosphate deposits [8]. Arterial vessels enter the marrow through foramina nutricia and then divide into several arterioles. Small arterioles and capillaries from these vessels span throughout the bone marrow and supply sinusoids, which are interconnected by inter-sinusoidal capillaries [9]. The BM tissue is soft, similar to the peripheral blood, flexible connective tissue comprising the center and the epiphysis of bones, referred to as the BM cavity. In this place a variety of new blood cells are produced and ultimately released to the peripheral circulation.

### **3.1 Red and yellow bone marrow**

We recognize two categories of bone marrow tissue: the red and yellow marrow. Depending on age, teh red marrow is replaced by the yellow marrow. In adults, the

red bone marrow is a rich source of bone marrow-derived cells and present in most skeletal system bones of the iliac crest, tibia, spine vertebrae, humerus, calcaneus, ribs, and near point of attachment of long bones of legs and arms. In this wellshielded environment, an estimate of 500 billion cells per day can be produced, in particular erythrocytes, granulocytes, and platelets [10]. Regenerative medicine applications have a focus on the use of the red bone marrow as it contains myeloid and lymphoid stem cells and MSCs. In contrast the yellow marrow consists primarily of fat cells with poor vascularity and is deprived of the multipotential MSCs [11].
