Skeletal System

Osteopenia

A condition that occurs when the body doesn't make new bone as quickly as it reabsorbs old bone.

Calcification

Calcification is the accumulation of calcium salts in a body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue, causing it to harden.

Calcitonin

Calcitonin, a hormone secreted by the thyroid gland, has some effects that counteract those of PTH. Calcitonin inhibits osteoclast activity and stimulates calcium uptake by the bones, thus reducing the concentration of calcium ions in the blood.

Lamellae

Each osteon is composed of concentric rings of calcified matrix called lamellae (singular = lamella).

Ossification

In the early stages of embryonic development, the embryo's skeleton consists of fibrous membranes and hyaline cartilage. By the sixth or seventh week of embryonic life, the actual process of bone development, ossification (osteogenesis), begins.

Articular cartilage

In this region, the epiphyses are covered with articular cartilage, a thin layer of cartilage that reduces friction and acts as a shock absorber.

Osteoclasts activating factor

Osteoclasts resorb old bone that lines the medullary cavity, while osteoblasts, via intramembranous ossification, produce new bone tissue beneath the periosteum.

Osteogenesis

Osteogenesis imperfecta (OI) is a genetic disease in which bones do not form properly and therefore are fragile and break easily. It is also called brittle bone disease. The disease is present from birth and affects a person throughout life.

Osteoid

Osteoid (unmineralized bone matrix) secreted around the capillaries results in a trabecular matrix, while osteoblasts on the surface of the spongy bone become the periosteum.

Red bone marrow

Red marrow fills the spaces in the spongy bone.

Osteoclasts

The cell responsible for bone resorption, or breakdown, is the osteoclast. They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone.

Medullary cavity

The hollow region in the diaphysis is called the medullary cavity, which is filled with yellow bone marrow.

Bone markings

The surface features of bones vary considerably, depending on the function and location in the body.

Epiphysis

The wider section at each end of the bone is called the epiphysis (plural = epiphyses), which is filled with spongy bone. Red bone marrow fills the spaces in the spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the narrow area that contains the epiphyseal plate (growth plate), a layer of hyaline (transparent) cartilage in a growing bone.

Canals of Volkmann

Vessels and nerves branch off at right angles through a perforating canal, also known as Volkmann's canals, to extend to the periosteum and endosteum.

Epiphyseal cartilage

(also, growth plate) sheet of hyaline cartilage in the metaphysis of an immature bone; replaced by bone tissue as the organ grows in length

Canaliculi

Canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system allows nutrients to be transported to the osteocytes and wastes to be removed from them.

Osteolysis

Osteolysis is an active resorption of bone matrix by osteoclasts and can be interpreted as the reverse of ossification. Although osteoclasts are active during the natural formation of healthy bone the term "osteolysis" specifically refers to a pathological process.

Osteoporosis

Osteoporosis is a disease characterized by a decrease in bone mass that occurs when the rate of bone resorption exceeds the rate of bone formation, a common occurrence as the body ages.

Haversian canal

Running down the center of each osteon is the central canal, or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels.

Diaphysis

The diaphysis is the tubular shaft that runs between the proximal and distal ends of the bone. The hollow region in the diaphysis is called the medullary cavity, which is filled with yellow bone marrow. The walls of the diaphysis are composed of dense and hard compact bone.

Yellow bone marrow

The hollow region in the diaphysis is called the medullary cavity, which is filled with yellow marrow.

Trabeculae

The lacunae and osteocytes are found in a lattice-like network of matrix spikes called trabeculae (singular = trabecula). The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone.

Endosteum

The medullary cavity has a delicate membranous lining called the endosteum (end- = "inside"; oste- = "bone"), where bone growth, repair, and remodeling occur.

Remodeling

The medullary cavity has a delicate membranous lining called the endosteum (end- = "inside"; oste- = "bone"), where bone growth, repair, and remodeling occur.

Osteon

The microscopic structural unit of compact bone is called an osteon, or Haversian system. Each osteon is composed of concentric rings of calcified matrix called lamellae (singular = lamella).

Metaphysis

The narrow area that contains the epiphyseal plate (growth plate), a layer of hyaline (transparent) cartilage in a growing bone.

Osteoblasts

The osteoblast is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts.

Periosteum

The outer surface of the bone is covered with a fibrous membrane called the periosteum (peri- = "around" or "surrounding"). The periosteum contains blood vessels, nerves, and lymphatic vessels that nourish compact bone. The periosteum covers the entire outer surface except where the epiphyses meet other bones to form joints .

Epiphyseal line

When the bone stops growing in early adulthood (approximately 18-21 years), the cartilage is replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal line.

Osteocytes

located in a space called a lacuna and is surrounded by bone tissue. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes.