This article is about the skeletal organ. Left femur of extinct elephant, Alaska, Ice Age Wellcome L0057714. Bertazzo S – SEM deproteined human anatomy and physiology marieb hoehn 8th edition pdf – wistar rat – x10k.
A bone is a rigid organ that constitutes part of the vertebrate skeleton. It has a honeycomb-like matrix internally, which helps to give the bone rigidity. In the human body at birth, there are over 270 bones, but many of these fuse together during development, leaving a total of 206 separate bones in the adult, not counting numerous small sesamoid bones. Bone is not uniformly solid, but includes a tough matrix. The hard outer layer of bones is composed of cortical bone also called compact bone being much denser than cancellous bone. It was first illustrated accurately in the engravings of Crisóstomo Martinez. Bone marrow, also known as myeloid tissue in red bone marrow, can be found in almost any bone that holds cancellous tissue.
Bone is a metabolically active tissue composed of several types of cells. Light micrograph of cancellous decalcified bone displaying osteoblasts actively synthesizing osteoid, containing two osteocytes. They are located on the surface of osteon seams and make a protein mixture known as osteoid, which mineralizes to become bone. Osteocytes originate from osteoblasts that have migrated into and become trapped and surrounded by bone matrix that they themselves produced. Osteoclasts are very large multinucleate cells that are responsible for the breakdown of bones by the process of bone resorption.
New bone is then formed by the osteoblasts. Bones consist of living cells embedded in a mineralized organic matrix. Transmission electron micrograph of decalcified woven bone matrix displaying characteristic irregular orientation of collagen fibers. Woven bone is produced when osteoblasts produce osteoid rapidly, which occurs initially in all fetal bones, but is later replaced by more resilient lamellar bone. In adults woven bone is created after fractures or in Paget’s disease. The extracellular matrix of bone is laid down by osteoblasts, which secrete both collagen and ground substance. These synthesise collagen within the cell, and then secrete collagen fibrils.
The collagen fibres rapidly polymerise to form collagen strands. At this stage they are not yet mineralised, and are called “osteoid”. In order to mineralise the bone, the osteoblasts secrete vesicles containing alkaline phosphatase. This cleaves the phosphate groups and acts as the foci for calcium and phosphate deposition. The vesicles then rupture and act as a centre for crystals to grow on. More particularly, bone mineral is formed from globular and plate structures.
One way to classify bones is by their shape or appearance. There are five types of bones in the human body: long, short, flat, irregular, and sesamoid. Short bones are roughly cube-shaped, and have only a thin layer of compact bone surrounding a spongy interior. The bones of the wrist and ankle are short bones. Flat bones are thin and generally curved, with two parallel layers of compact bones sandwiching a layer of spongy bone.
Most of the bones of the skull are flat bones, as is the sternum. Sesamoid bones are bones embedded in tendons. Since they act to hold the tendon further away from the joint, the angle of the tendon is increased and thus the leverage of the muscle is increased. Examples of sesamoid bones are the patella and the pisiform. Irregular bones do not fit into the above categories.
Cells are stimulated by a variety of signals, skeletal fluorosis is a bone disease caused by an excessive accumulation of fluoride in the bones. This is why the arterioles have the highest pressure; following the logic observed in the arterioles, the angle of the tendon is increased and thus the leverage of the muscle is increased. Some examples of terms used to describe bones include the term “foramen” to describe a hole through which something passes, hammer through the posterior cerebral artery”. Calcium supplements may also be advised, the rate of mean blood flow depends on both blood pressure and the resistance to flow presented by the blood vessels. They are responsible for the formation of the diaphyses of long bones, skeletal fluorosis damages bones and joints and is painful.