are carried out with fetuses in the third trimester. They are performed on fetuses obtained from voluntary interruption or termination of pregnancies. In some countries, use of the fetus in such cases is considered an organ donation, and this enables bypass of many ethical issues and concerns. Besides stem cells, cells with specific lineage commitments are also isolated for other research purposes.
Dental Pulp
There is a constant search for new isolation strategies that yield stem cells of great quality and quantity. Periodontal ligament, deciduous and permanent teeth are also great sources of stem cells. In sum, dental pulp is a source of pluripotent stem cells, and it is highly accessible. These cells are called pulp derived stem cells or PDSCs. The isolation process is very simple, and PDSCs are shown to be adherent to plastic surfaces. This facilitates easy propagation in vitro. They are proven to self-renew and possess plasticity, making them the ideal candidate for stem cell therapy. They are multipotent and have the capacity to differentiate into several cell types, such as adipocytes, chondrocytes, osteoblasts, neural cell progenitors, and myotubes. Thus, these cells are a great source of stem cells for osteoinduction.
The different sources of stem cells for differentiation into bone shows the advancement of cellular therapy in the field of regeneration. While this is exciting, one important consideration is that PDSCs have a malignant tendency with their capability to self-renew and differentiate into other cell types. The viability of cells with respect to genome stability, absence of mutations, and intact cellular pathways such as DNA repair pathways must be tested before they can be used for therapy purposes. Stem cells are double-edged swords; while they are extremely useful tools, even a small amount of imprudence leads to dangerous outcomes.
Sharanya Kumar
Independent Scholar
See Also: Bone: Cell Types Composing the Tissue; Bone: Development and Regeneration Potential; Bone: Existing or Potential Regenerative Medicine Strategies; Bone: Major Pathologies; Bone: Stem and Progenitor Cells in Adults; Bone Marrow Transplants.
Further Readings
Arvidson, K., B. M. Abdallah, L. A. Applegate, et al. “Bone Regeneration and Stem Cells.” Journal of Cellular and Molecular Medicine, v.1/4 (2011).
Dawson, J. I., et al. “Concise Review: Bridging the Gap: Bone Regeneration Using Skeletal Stem Cell-Based Strategies—Where Are We Now?” Stem Cells, v.32/1 (2014).
Bone: Development and Regeneration Potential
Bone: Development and Regeneration Potential
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Bone: Development and Regeneration Potential
Bones are the solid, firm structures that make up the vertebrate skeleton. They are complex living organs that form the supportive framework of the body and are comprised of mineral matrix, marrow, blood vessels, nerves, and cartilage. A typical adult human body has 206 distinct bones with the largest bone (femur) in the thigh and smallest (stapes) inside the ear.
Bones, together with skeletal muscles, tendons, and joints, allow three-dimensional movement of the body. They provide protection to multiple internal organs, for example, lungs and heart inside the rib cage. Bone marrow is the site of production of all types of blood cells (a process called hematopoiesis). Another function is the storage of nutrients: lipids, serving as a reservoir for energy; and minerals, especially calcium and phosphorus, which are required for multiple cellular actions throughout the body. Bones also have a role in transduction of sound, maintenance of acid base balance, and detoxification of heavy metals.
Structure
Compact, or cortical, bone forms the outer covering of most bones. It is denser than trabecular bone and is responsible for facilitating bones’ main functions. It makes up 80 percent of the total bone mass in the body. Trabecular, or cancellous, bone is a spongy network that is highly vascularized. It contains the bone marrow, which is the primary site of blood cell formation. Examples include ends of long bones.
Cells
Osteoblasts are the immature cell types and are responsible for formation of new bone. They produce certain chemicals (alkaline phosphatase and prostaglandins), which favor the mineralization of bone, and they themselves mature into adult bone cells. Osteocytes are the mature cells. They manufacture the bone matrix in the surrounding space and maintain the calcium stores. The primary function of osteoclasts is the recycling of bone. They produce certain enzymes like acid phosphatase, which break down the mineral surface and allow for the remodeling of bone by osteoblasts.
Types
Depending upon anatomy, the following is true:
Long bones have a central shaft, known as the diaphysis, and the ends with the growth plates, known as epiphyses. The compact bone is thick and covers the spongy bone. Their characteristic feature is that they are much longer than they are wide. Long bones include most of the limb bones like the humerus, radius, and ulna (of the upper limb) and the femur, tibia, and fibula (of the lower limb), including those of the hands and feet, namely, metacarpals and meta tarsals.
Short bones have a thin, compact bone surrounding the spongy bone. They have roughly the same width as their length. Short bones include those of the ankle and wrist joints, called carpals and tarsals.
Flat bones have two parallel layers of compact bone covering the spongy bone. These include the skull bones (cranium), breast bone (sternum), hip bone (pelvis), and ribs.
Irregular bones are mostly made up of cancellous bone and have a thin covering of compact bone. The bones of spine (vertebrae), sacrum and lower jaw (mandible) are irregular bones.
Sesamoid bones are fixed within tendons. They are either short bones or irregular. The knee cap (patella) is a sesamoid bone and it is embedded in the quadriceps tendon.
Depending upon embryonic development, the following is true:
Intramembranous bones are the flat bones (for example, skull bones) and some irregular bones (e.g., the jaw bone, “mandible”).
Endochondrial bones can be long (limb bones, e.g., femur), short (wrist and ankle bones), and irregular bones (vertebrae); all undergo endochondrial ossification.
Development
Bone development, known as osteogenesis, starts early on in the fetal age by unspecialized mesenchymal stem cells (MSCs), which later develop into the precursors of bone cells known as osteoprogenitor cells. By the end of week eight of gestation, the basic structure of the skeleton is laid down. At this stage the skeletal structure is either made up of thin membranes or soft flexible cartilage. During the third month, the hardening of this fragile fetal skeleton begins through ossification. Bone formation and remodeling continues throughout life.
Ossification can occur by one of two processes. The first is intramembranous ossification, which involves mineralization of the sheet-like membranes made up of connective tissue.
