Bone Tissue

Part 2: Support & Movement

Function of the Human Skeleton

Provides support for the body Protection to the internal organs Provides & framework for the connection of

skeletal muscles Allows for movement

Contains red bone marrow for blood formation and storage Produces blood cells

Primary storage site for minerals & fats Including calcium!

Bone Tissue

Osseous (Bone) Tissue: Dynamic (ever-changing) living tissue that makes up the skeletal structure.Bone is a type of connective tissue with a

matrix made of hydoxyapatite (a calcium salt).

Two types of bone: Compact BoneSpongy Bone

Bone Tissue

Osteon: The functional unit of compact bone. Contains many types of bone cells.Osteogenic Cells: “Stem Cells” that divide to

provide a constant supply of the other bone cells.

Osteoblasts: “Bone-building” cells that transport calcium from the bloodstream to the bone matrix.

Osteoclasts: “Bone-chewing” cells that reabsorb bone & transport calcium from the bone to the blood.

Structure of Long Bone

Diaphysis: The central shaft of a long bone. Contains the medullary (marrow) cavity – yellow

bone marrow in adults.

Epiphysis: The ends (proximal and distal) of long bone.

Metaphyses: Where the diaphysis and epiphysis means. Contains the epipheseal plate (growth plate) where

cartilage is replaced by bone as the bone grows longer.

Structure of Long Bone

Articular Cartilage: The layer of hyaline cartilage surrounding the epiphysis where bones connect at joints.

Periosteum: The tough covering on the outside of the surface of bone, composed primarily of dense irregular connective tissue.

Endosteum: Thin membrane lining medullary cavity.Contains a single layer of bone-forming cells.

Histology of Compact Bone

Osteons: The type of tissue that makes up compact bones.

Made up of…Central Canal aka Haversian System

Contains concentric lamellae containing lacunae and canaliculi

Perforating Canals aka Volkmann’s Canal Interstitial Lamaellae

Components of Osteons

Central Canal aka Haversian Systems: The functional unit of compact bone. Located in the center of the osteon.Provides a passageway for blood vessels and

nerves through the length of the bone. Surrounded by calcified rings called

concentric lamellae Lacunae: Spaces in the concentric lamellae

containing osteocytes. Canaliculi: Tiny canals between the lamellae.

Components of Osteons

Perforating Canal aka Volkmann’s Canal: Passageways with blood & lymph vessels and nerves running at a right angle to the Haversian canals.Connects blood vessels from Periosteum to

Haversian canals. Interstitial Lamellae: The area between

the osteonsConsists of fragments of old, partially

destroyed osteons

Spongy Bone

Spongy Bone: Consists of a latticework of columns of bones called trabiculae. Named for the “sponge-like” appearance Permeated spaced filled with bone marrow, NOT

osteons Trabeculae develop along lines of stress

Wolf’s Law: Bone mass & density will increase in areas of stress, so bone will be thickest where it is under the greatest stress.

Much lighter than compact bone Found in irregularly shaped bones in places with less

stress or stress from multiple directions Found at the epiphyses of long bones

Bone Marrow

Red Marrow: Found throughout the skeleton in infants and children.Produces all types of blood cellsPrimary found in flat bones in adults & the

ends of long bones Sternum, ribs, skull bones, pelvis

Yellow Marrow: Fills most marrow cavities in adultsFilled with triglycerides (fat) as an energy

reserve

Osseous Tissue Homeostasis

Bone Tissue serves as a reservoir for 99% of calcium & phosphorous in addition to other minerals. Calcium: Important to bone, muscle, & nerve tissue. Phosphate Groups: Important in the formation of

DNA, RNA, ATP, & Phospholipids. Mineralization (Deposition): The process of

depositing calcium and phosphate in bone via osteoblasts.

Mineral Resorption: The process of disolving bone tissue to release minerals into the bloodstream via osteoclasts.

Hypocalcaemia

Hypocalcaemia: Low calcium concentration in the blood. Can result in adverse effects on the nervous

and muscle systems.

Endocrine Regulation of Blood Calcium Level

Parathyroid Hormone (PTH): Raises blood calcium when it drops too low.

Calcitonin: Lowers blood levels of calcium. Produced by the thyroid gland.

Calcitriol: Causes reduced excretion of Ca++ by the kidneys & increased absorption by the GI tract A form of Vitamin D! Manufactured in the skin from a precursor of

cholesterol – stimulated by ultraviolet light Growth Hormone (GH): Affects the epiphyseal

plate

Bone Formation & Development

Ossification: The process by which bones are formed.

TWO Methods of Ossification: Intramembranous OssificationEndochondral Ossification

Intramembranous Ossification

Produces flat bones…. Replaces connective tissue membranes with bone in

the skull, forms the mandible (the lower jaw) and the breastbone

Starts as embryonic connective tissue called mesenchyme

Osteogenic (stem) cells develop into osteoblasts, which secrete bone matrix

Calcification: Osteoblasts deposit osteoid tissue, then calcium phosphate is deposited. Osteoblasts then become osteocytes

Intramembranous Ossification

Bone matrix develops into trabiculae to form spongy bone.

Compact bone tissue then forms around spongy bone as the mesenchyme condenses into periosteum

Spongy bone remains in the center, surrounded by a thin layer of compact bone.

Endochondral Ossification

Develops from a hyaline cartilage modelCartilage model is “invaded” by blood vessels

which start to form a central marrow cavity.Perichondrium: The membrane that forms

around the cartilage model. Perichondrium cells become osteogenic

and begin to produce bone on the otuside of the model, which then grows in length as chondrocytes (cartilage cells) divide.

Endochondral Ossification

Ossification moves inward from the external surface forming a primary ossification center. Cartilage is then “remodeled” into bone by

osteoblasts with mineral deposits. Perichondrium becomes periosteum

Secondary ossifaction proceeds outward from the center of the epiphyses

Hyaline cartilage remains in the growth plates

Cartilage Growth

Cartilage growth occurs in two ways: Interstitial growth involves increasing the internal

cartilage matrix and typically results in an increase in bone length.

Long bones grow in length due to the addition of bone at the growth plates (on the diaphyseal side)

Appositional Growth involves depositing material on the cartilage surface to increase bone thickness.

Due to deposition of bone by osteoblasts at the outer surface while osteoclasts in the endeosteum destroy bone to increase the cavity & diameter of the bone.

Bone stops growing in length by approximately age 25, but may continue to thicken throughout life.

Hormonal & Chemical Factors Influencing Bone Development

Homeostasis Factors: Parathyroid Hormone (PTH)Calcitonin

Bone Growth: Growth Hormone (GH)Thyroxine (Thyroid Hormone)

Sex HormonesEstrogenTestosterone

Hormonal & Chemical Factors Influencing Bone Development Promotion of osteoclast activity

Calcitriol Vitamins:

C K B12 A

Minerals: Calcium Phosphorous Magnesium (Mg) Iron (Fe) Manganese (Mn) Fluorine (F)

Reminder: Cartilage

Types of Cartilage: Hyaline: Found in articular surfaces, growth plates in

long bones Fibrocartilage: Absorbs shock, found in

intervertebral discs Elastic: Found in the framework of the outer ear &

the epiglottis

Chondroblasts: Cells which actively produce cartilage

Chondrocytes: Cells trapped in lacunae of cartilage

Types of Bones

Long Bones: Bones that are longer than they are wide. E.g. humerous, radius, femer, etc.

Short Bones: Closer to equal in length and width. E.g. wrist & ankle bones

Flat Bones: Enclose/protect organs E.g. cranium, breastbone

Irregular Bones: Elaborate shapes that don’t fit the other types E.g. Vertebral bodies, ethmoid and sphenoid bones

Sesamoid Bones: Bone contained in a tendon E.g. patella (kneecap)

Sutures: Small bones located within suture joints.

Bone Processes

Bone Processes: The markings found on the bones.TubercleTuberosityFoveaForamenCondylesFossa

Tubercle

Tubercle: A small, rounded projection. Area where muscle

tendon attaches.

Tuberosity

Tuberosity: A large, rounded projection. Area where tendon

attaches.

Fovea

Fovea: Depression or pit. Example: Fovea

capitis on the head of the femur.

Foramen

Foramen: An opening in a bone. Example: The

foramen magnum in the base of the skull.

Condyles

Condyles: A large, rounded protrusion at articular surfaces that form joins.

Fossa

Fossa: Shallow depressions.

Bone Remodeling

Bone Remodeling: The process of bone rebuilding itself.Used as a means of constant renewal and to

redistribute the bone matrix (such as at areas of increased stress or to heal injuries).

Typically occurs as a balance between resorption by osteoclasts and deposition of osteoblasts.

Types of Fractures

Bone Healing

Stages of Bone Healing/Fracture Healing: Reactive PhaseReparative PhaseRemodeling Phase

Fracture Healing

Reactive Phase: Blood vessels near the fracture constrict to

stop further bleedingExtravascular blood cells (“hematoma”) form a

blood clot within a few hoursThe blood cells within the clot die, while

fibroblasts survive & replicate to create a loose aggregate of cells interspersed with small blood vessels – known as Granulation Tissue

Fracture Healing

Reparative Phase: Days after the fracture…

Fibroblasts within the granulation tissue & the periosteal cells proximal to the fracture replicate into chondroblasts to form hyaline cartilage

the periosteal cells distal to the fracture develop into osteoblasts and form new woven bone.

Both tissues grow in size until they unite to form a fracture callus.

Endochondral ossification occurs: Lamellar bone replaces woven bone and then hyaline catilage.

Eventually, all of the lamellar bone is replaced by trabecular bone, restoring most, if not all, of the bone’s original strength.

Fracture Healing

Remodeling Phase: Trabecular bone is resorbed by osteoclasts,

creating the Hallowship’s Lacuna (the shallow resorption pit) and replaced with compact bone.

The fracture callus is remodeled into a new shape closely duplicating the bone’s original shape and strength.