All bones consist of outer compact bone and inner spongy bone

Bone as an organ

Bones are organs because they contain several types of tissue

Compact bone

External dense outer layer that looks smooth and solid to the naked eye

Spongy bone

Internal to compact bone, is a porous lightweight bone tissue of small needle like or flat pieces called trabeculae

Short, irregular and flat bones all share the same thing which is…

They all consist of thin plates of spongy bone covered by compact bone.

With few exceptions, all longs bones have..

They all have the same general structure , a shaft, bone ends, and membranes.

Diaphysis

The long, tubular shaft of a bone made of a thick layer of compact bone surrounding a central medullary or (marrow) cavity. The cavity contains yellow bone marrow in adults and may have a thin layer of spongy bone between the marrow and compact bone.

Epiphyses

The end of a long bone, made of an outer layer of compact bone and inner spongy bone. Covered with articular cartilage to cushion joints and absorb stress.

Epiphyseal plate and epiphyseal line

The epiphyseal plate (growth plate) is cartilage that allows bones to grow in length during childhood. It become the epiphyseal line in adults.

Metaphysis

the flared region where the diaphysis (shaft) meets the epiphysis (bone end).

Periosteum

A tough , double layered membrane that covers the outside of the bone, expect at point surfaces . contains blood vessels, nerves, and bone cells.

Layers and functions of the periosteum

the outer fibrous layer anchors tendons and ligaments, while the inner osteogenic layer contains cells that form and break down bone. Perforating fibers attach the membrane to the bone.

Endosteum

A thin connective tissue membrane that covers the internal surfaces of bones, including spongy bone trabeculae and canals in compact bone. Contains the same cell types as the inner periosteum.

Blood supply to bones

Bones are well vascularized (rich in blood vessels). the diaphysis receives blood through a nutrient artery and vein.

Nutrient foramen

An opening in rhetoric diaphysis where blood vessels enter to supply the bone marrow, spongy bone, and compact bone.

Blood vessels and nerves of long bones

Epiphyseal arteries and veins supply the epiphysis. Nerves enter bones alongside blood vessels through the nutrient foramen.

Hematopoietic tissue (red bone marrow)

Blood forming tissue found in spongy bone that produces blood cells

red bone marrow in infants Vs adults

In infants, red marrow fills the medullary cavity and spongy bone. in adults, much of it is replaced by yellow marrow.

Location of red bone marrow in adults

Found mainly in the spongy bone of flat bones, irregular bones and the heads of the femur and humorous.

Yellow bone marrow

Stores fat in the medullary cavity of adult bones and can change back into red marrow when the body needs more red blood cells, such as severe anemia.

Bone marking

Visible features on the bones surface that show where muscles , ligaments , joints, blood vessels and nerves connect or pass through.

three categories of bone markings

Projections for muscle and ligaments attachment, surfaces that form joints , and depressions or openings for blood vessels and nerves.

Five major Bone cell types

Osteoprogenitor cells, osteoblasts, osteocytes, bone lining cells, and osteoclasts. All originate from embryonic connective tissue cells expect osteoclasts.

Bone remodeling

The continuous process of breaking down (resorption) and building new bone, keeping bone tissue alive and changing

Osteoprogenitor cells (osteogenic cells)

Stem cells found in the periosteum and endosperm that divide and develop into osteoblasts when stimulated. In growing bones they are flattened or squamous cells.

Osteoblasts

Bone forming cells that produce and secrete bone matrix (osteoid), which contains collagen and calcium binding proteins. They help build and mineralize bone.

Oseteoblasts vs osteocytes

Osteoblasts become oceocytes when they are completely surrounded by the bone matrix they produce. inactive osteoblasts can also become bone lining cells.

osteocytes

Mature bone cells that maintain the bone matrix , detect mechanical stress, and signal other cells to build or break down bone as needed. they also help maintain calcium balance in the body.

Bone lining cells

Are flat cells found on bone surfaces where bone remolding is not occurring . like osteocytes , they are thought to help maintain the matrix.

Osteoclasts

Large, multinucleate bone cells that break down bone using enzymes. They have a ruffled border that increases surface area for bone breakdown.

What is an osteon?

The structural unit of compact bone. Also called the Haversian system .

What is the shape and orientation of an osteon?

An elongated cylinder Running parallel to the bones long axis

Function of osteons

Act as tiny weight-bearing pillars that help support the bone

Lamellae

Layers of bone matrix arranged in hollow tubes one around another like the rings of a tree trunk. Compact bone is also called a Miller bone because it contains these layers.

Arrangement of collagen, fibers and bone salt crystals

Collagen fibers run in One Direction within each Lamella and alternate directions in adjacent layers tiny bone salt crystals align between the collagen fibers and also alternate directions

How osteons resist, twisting

The alternating arrangement of collagen, fibers and bone slot crystals, reinforces adjacent Lalay helping compact bone with stand twisting forces

Central Canal (Haversian Canal)

Runs through the center of each osteon and contains small blood vessels and nerve fibers that supply the bone cells.

Perforating Canals (Volkmann's Canals)

Run at right angles to the bone's long axis and connect blood vessels and nerves from the medullary cavity to the central canals. They are not surrounded by concentric lamellae.

Endosteum

A membrane that lines the internal cavities of bones, including the central and perforating canals.

Lacunae and Osteocytes

Osteocytes are mature bone cells that occupy small spaces called lacunae, located between the lamellae.

Canaliculi

Tiny, hair-like canals that radiate from lacunae, connecting osteocytes to each other and to the central canal. They contain tissue fluid and osteocyte extensions.

Function of Canaliculi

Allow osteocytes to communicate and exchange nutrients and wastes through gap junctions. This keeps bone cells nourished even though the hard bone matrix is impermeable to nutrients.

Interstitial Lamellae

Incomplete layers of bone matrix found between intact osteons. They fill gaps between forming osteons or are remnants of older osteons broken down during bone remodeling.

Circumferential Lamellae

Layers of bone matrix located just beneath the periosteum and just above the endosteum. They extend around the entire circumference of the diaphysis (shaft) of the bone.

Function of Circumferential Lamellae

Help resist twisting forces and strengthen the long bone by wrapping around its entire circumference.

Trabeculae

Thin, branching structures that make up spongy bone. They are carefully arranged along lines of stress to help the bone resist pressure and forces.

Structure of Trabeculae

Contain irregularly arranged lamellae and osteocytes connected by canaliculi. They are only a few cells thick.

Osteons in Spongy Bone

Spongy bone does not contain osteons, unlike compact bone.

How Do Osteocytes in Spongy Bone Receive Nutrients?

Nutrients diffuse through canaliculi from capillaries in the endosteum surrounding the trabeculae.

Chemical Composition of Bone

Bone contains both organic and inorganic substances that work together to make it strong, durable, and resistant to brittleness.

Organic Components of Bone

Include bone cells and osteoid (the soft components of bone). They allow bone to resist tension (stretching).

Inorganic Components of Bone

Consist of mineral salts, which give bone hardness and allow it to resist compression.

Strength of Healthy Bone

Bone is half as strong as steel when resisting compression and just as strong as steel when resisting tension.

Organic Components of Bone

Include bone cells and osteoid, which is the organic part of the bone matrix.

Osteoid

Makes up about one-third of the bone matrix and contains ground substance and collagen fibers produced by osteoblasts.

Function of Collagen Fibers

Provide bone structure, flexibility, and tensile strength, allowing bone to resist stretching and twisting.

Bone Resilience and Sacrificial Bonds

Stretch and break upon impact to absorb energy and help prevent fractures. Most bonds reform afterward.

Inorganic Components of Bone

Consist of mineral salts called hydroxyapatites, mainly calcium phosphate, found as tiny crystals around collagen fibers.

Function of Mineral Salts

Give bone its hardness and allow it to resist compression.

Mineral Salts and Bone Preservation

Help bones last long after death, allowing skeletal remains to reveal information about ancient people's bodies, activities, and diseases.