Gross Anatomy  Long Bone

Basic Structure

Superior view  Transverse section, diaphysis of an immature long bone

Terminology Periosteum - fibrous connective tissue covering the bone, it contains blood vessels and nerves and serves as an attachment site for ligaments and tendons Compact bone – higher density bone, contains a larger percentage of mineral salts. The amount of compact bone varies depending on the individual and the particular stresses placed on the bone

Terminology Spongy / cancellous bone– found at the ends (epiphyses) of long bones and in the flat bones. Lower density bone, offers more flexibility. Contains red marrow. Marrow cavity – found in the center of the diaphysis (shaft). In a developing skeleton the marrow is red, as the skeleton matures the marrow in the cavity changes to yellow marrow.

Note : Red marrow is found in the the epiphysis of mature long bones (within the cancellous bone) and in flat bones

The Function of Bone Marrow Red marrow is the site of blood cell formation (red blood cells, platelets and white blood cells) Found mainly in flat bones, but also in the epiphyses of the long bones Yellow marrow is found in the marrow cavities of long bones Chiefly adipose tissue, yellow bone marrow is a storage site for fat (energy) Yellow marrow can change to red marrow in times of need

Take a closer look…

Bone Formation & Remodelling Ossification – the process by which new bone is produced Osteoblasts – bone forming cells (secrete protein matrix called osteoid) Osteoid – gelatin-like substance, initial step in bone formation Osteoclast – bone-resorbing cells (remove old bone by releasing acids and enzymes) **Note – bone is continually remodelled (created, destroyed, created etc.)

Bone formation Compact bone begins as cartilage Cartilage is a specialized connective tissue composed of collagen and several other fibres Cancellous bone begins as a fibrous membrane, and development proceeds as in compact bone Osteoblasts deposit osteoid into the cartilage or fibrous membrane, minerals are then slowly deposited into the osteoid as cartilage / fibrous membrane converts to bone Osteoclasts work to remodel the bones as they form

Bone formation Compact bone formation occurs at ossification centers: Long bones have one in the middle and one at either end Short bones have a single ossification center near the middle Cancellous bone formation occurs at ossification centers in the fibrous membrane (roughly in the middle of the bone)

Ossification centers (growth plates)

Compact bone formation As a person ages, and bone growth ceases the ossification centers gradually change to epiphyseal plates

Adult or child??

Infant skeleton  all blue areas represent cartilage

Bone Remodelling Bones remodelling occurs in two phases: 1. Osteoclasts remove old bone by releasing acids and enzymes 2. Osteoblasts deposit new tissue by secreting osteoid thus allowing for mineral deposit, and eventually new bone growth

Bone Remodelling During early years (age 0 – 35) remodelling is most active  bone deposit heavily outweighs bone resorption Remodelling declines by around age 35 and by around age 40, the process reverses (resorption outpaces deposit) Result  loss of 5 to 10% of bone mass each decade after age 40

Bone Remodelling What can you do??? Nothing…and something Bone resoprtion will outpace bone deposit, but ensuring you have strong, healthy bones when you are younger will keep your bones healthier as you age

What’s the big deal about calcium?? The bones store 99% of the body’s calcium, a vital ion in many cellular functions If your calcium intake is not high enough, your body will signal your bones to release calcium (via osteoclast action)

Calcium balance Rising blood Ca 2+ levels trigger the thyroid to release calcitonin Calcitonin stimulates calcium salt deposit in bone

Calcium balance Falling blood Ca 2+ levels signal the parathyroid glands to release PTH PTH signals osteoclasts to degrade bone matrix and release Ca 2+ into the blood