Lecture 6: Osseous Tissue and Bone Structure

Lecture 6: Osseous Tissue and Bone Structure

Topics: Skeletal cartilage Structure and function of bone tissues
Types of bone cells Structures of the two main bone tissues Bone membranes Bone formation Minerals, recycling, and remodeling Hormones and nutrition Fracture repair The effects of aging

The Skeletal System Skeletal system includes: bones of the skeleton
cartilages, ligaments, and connective tissues

Skeletal Cartilage Contains no blood vessels or nerves
Surrounded by the perichondrium (dense irregular connective tissue) that resists outward expansion Three types – hyaline, elastic, and fibrocartilage

Hyaline Cartilage Provides support, flexibility, and resilience
Is the most abundant skeletal cartilage Is present in these cartilages: Articular – covers the ends of long bones Costal – connects the ribs to the sternum Respiratory – makes up larynx, reinforces air passages Nasal – supports the nose

Elastic Cartilage Similar to hyaline cartilage, but contains elastic fibers Found in the external ear and the epiglottis

Fibrocartilage Highly compressed with great tensile strength
Contains collagen fibers Found in menisci of the knee and in intervertebral discs

Growth of Cartilage Appositional – cells in the perichondrium secrete matrix against the external face of existing cartilage Interstitial – lacunae-bound chondrocytes inside the cartilage divide and secrete new matrix, expanding the cartilage from within Calcification of cartilage occurs During normal bone growth During old age

Bones and Cartilages of the Human Body
Figure 6.1

Functions of the Skeletal System
Support Storage of minerals (calcium) Storage of lipids (yellow marrow) Blood cell production (red marrow) Protection Leverage (force of motion)

Bone (Osseous) Tissue Supportive connective tissue Very dense
Contains specialized cells Produces solid matrix of calcium salt deposits and collagen fibers

Characteristics of Bone Tissue
Dense matrix, containing: deposits of calcium salts osteocytes within lacunae organized around blood vessels Canaliculi: form pathways for blood vessels exchange nutrients and wastes

Osteocyte and canaliculi

Characteristics of Bone Tissue
Periosteum: covers outer surfaces of bones consist of outer fibrous and inner cellular layers Contains osteblasts responsible for bone growth in thickness Endosteum Covers inner surfaces of bones

Bone Matrix Solid ground is made of mineral crystals
2/3 of bone matrix is calcium phosphate, Ca3(PO4)2: reacts with calcium hydroxide, Ca(OH)2 to form crystals of hydroxyapatite, Ca10(PO4)6(OH)2 which incorporates other calcium salts and ions

Bone Matrix Matrix Proteins:
1/3 of bone matrix is protein fibers (collagen) Question: why aren’t bones made of ALL collagen if it’s so strong?

Bone Matrix Mineral salts make bone rigid and compression resistant but would be prone to shattering Collagen fibers add extra tensile strength but mostly add tortional flexibility to resist shattering

Chemical Composition of Bone: Organic
Cells: Osteoblasts – bone-forming cells Osteocytes – mature bone cells Osteoprogenitor cells – grandfather cells Osteoclasts – large cells that resorb or break down bone matrix Osteoid – unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen; becomes calcified later

There are four major types of cells
periosteum + endo endosteum only in matrix only

1. Osteoblasts Immature bone cells that secrete matrix compounds (osteogenesis) Eventually become surrounded by calcified bone and then they become osteocytes Figure 6–3 (2 of 4)

2.Osteocytes Mature bone cells that maintain the bone matrix
Figure 6–3 (1 of 4)

Osteocytes Live in lacunae Found between layers (lamellae) of matrix
Connected by cytoplasmic extensions through canaliculi in lamellae (gap junctions) Do not divide (remember G0?) Maintain protein and mineral content of matrix Help repair damaged bone

3. Osteoprogenitor Cells
Mesenchyme stem cells that divide to produce osteoblasts Are located in inner, cellular layer of periosteum Assist in fracture repair

4. Osteoclasts Secrete acids and protein-digesting enzymes
Figure 6–3 (4 of 4)

Osteoclasts Giant, mutlinucleate cells
Dissolve bone matrix and release stored minerals (osteolysis) Often found lining in endosteum lining the marrow cavity Are derived from stem cells that produce macrophages

Homeostasis Bone building (by osteocytes and -blasts) and bone recycling (by osteoclasts) must balance: more breakdown than building, bones become weak exercise causes osteocytes to build bone

Bone cell lineage summary
Osteoprogenitor cells osteoblasts osteocytes Osteoclasts are related to macrophages (blood cell derived)

Gross Anatomy of Bones: Bone Textures
Compact bone – dense outer layer Spongy bone – honeycomb of trabeculae filled with yellow bone marrow

Compact Bone Figure 6–5

Osteon The basic structural unit of mature compact bone
Osteon = Osteocytes arranged in concentric lamellae around a central canal containing blood vessels Lamella – weight-bearing, column-like matrix tubes composed mainly of collagen

Three Lamellae Types Concentric Lamellae Circumferential Lamellae
Lamellae wrapped around the long bone line tree rings Binds inner osteons together Interstitial Lamellae Found between the osteons made up of concentric lamella They are remnants of old osteons that have been partially digested and remodeled by osteoclast/osteoblast activity

Compact Bone Figure 6–5

Microscopic Structure of Bone: Compact Bone
Figure 6.6a, b

Figure 6.6a

Figure 6.6b

Figure 6.6c

Spongy Bone Figure 6–6

Spongy Bone Tissue Makes up most of the bone tissue in short, flat, and irregularly shaped bones, and the head (epiphysis) of long bones; also found in the narrow rim around the marrow cavity of the diaphysis of long bone

Spongy Bone Does not have osteons
The matrix forms an open network of trabeculae Trabeculae have no blood vessels

Bone Marrow The space between trabeculae is filled with marrow which is highly vascular Red bone marrow supplies nutrients to osteocytes in trabeculae forms red and white blood cells Yellow bone marrow yellow because it stores fat Question: Newborns have only red marrow. Red changes into yellow marrow in some bones as we age. Why?

Location of Hematopoietic Tissue (Red Marrow)
In infants Found in the medullary cavity and all areas of spongy bone In adults Found in the diploë of flat bones, and the head of the femur and humerus

Bone Membranes Periosteum – double-layered protective membrane
Covers all bones, except parts enclosed in joint capsules (continuois w/ synovium) Made up of: outer, fibrous layer (tissue?) inner, cellular layer (osteogenic layer) is composed of osteoblasts and osteoclasts Secured to underlying bone by Sharpey’s fibers Endosteum – delicate membrane covering internal surfaces of bone

Sharpy’s (Perforating) Fibers
Collagen fibers of the outer fibrous layer of periosteum, connect with collagen fibers in bone Also connect with fibers of joint capsules, attached tendons, and ligaments Tendons are “sewn” into bone via periosteum

Periosteum Figure 6–8a

Functions of Periosteum
Isolate bone from surrounding tissues Provide a route for circulatory and nervous supply Participate in bone growth and repair

Endosteum Figure 6–8b

Endosteum An incomplete cellular layer:
lines the marrow cavity covers trabeculae of spongy bone lines central canals Contains osteoblasts, osteoprogenitor cells, and osteoclasts Is active in bone growth and repair

Bone Development Human bones grow until about age 25 Osteogenesis:
bone formation Ossification: the process of replacing other tissues with bone Osteogenesis and ossification lead to: The formation of the bony skeleton in embryos Bone growth until early adulthood Bone thickness, remodeling, and repair through life

Calcification The process of depositing calcium salts
Occurs during bone ossification and in other tissues

Formation of the Bony Skeleton
Begins at week 8 of embryo development Ossification Intramembranous ossification – bone develops from a fibrous membrane Endochondral ossification – bone forms by replacing hyaline cartilage

Intramembranous Ossification Note: you don’t have to know the steps of this process in detail
Also called dermal ossification (because it occurs in the dermis) produces dermal bones such as mandible and clavicle Formation of most of the flat bones of the skull and the clavicles Fibrous connective tissue membranes are formed by mesenchymal cells

The Birth of Bone When new bone is born, either during development or regeneration, it often starts out as spongy bone (even if it will later be remodeled into compact bone)

Endochondral Ossification Note: you DO have to know this one
Begins in the second month of development Uses hyaline cartilage “bones” as models for bone construction then ossifies cartilage into bone Common, as most bones originate as hyaline cartilage This is like a “trick” the body uses to allow long bones to grow in length when bones can only grow by appositional growth

Bone formation in a chick embryo
Stained to represent hardened bone (red) and cartilage (blue) : This image is the cover illustration from The Atlas of Chick Development by Ruth Bellairs and Mark Osmond, published by Academic Press (New York) in 1998

Fetal Primary Ossification Centers
Figure 6.15

Stages of Endochondral Ossification
Bone models form out of hyaline cartilage Formation of bone collar Cavitation of the hyaline cartilage Invasion of internal cavities by the periosteal bud, and spongy bone formation Formation of the medullary cavity; appearance of secondary ossification centers in the epiphyses Ossification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal plates

Stages of Endochondral Ossification
Formation of bone collar around hyaline cartilage model. Hyaline cartilage Cavitation of the hyaline cartilage within the Invasion of internal cavities by the periosteal bud and spongy bone formation. Formation of the medullary cavity as ossification continues; appearance of secondary ossification centers in the epiphyses in preparation for stage 5. Ossification of the epiphyses; when completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. Deteriorating matrix Epiphyseal blood vessel Spongy bone formation plate Secondary ossificaton center Blood vessel of periosteal bud Medullary cavity Articular Primary ossification Bone collar 1 2 3 4 5 Figure 6.8

Endochondral Ossification: Step 1 (Bone Collar)
Blood vessels grow around the edges of the cartilage Cells in the perichondrium change to osteoblasts: producing a layer of superficial bone (bone collar) around the shaft which will continue to grow and become compact bone (appositional growth) Figure 6–9 (Step 2)

Endochondral Ossification: Step 2 (Cavitation)
Chondrocytes in the center of the hyaline cartilage of each bone model: enlarge form struts and calcify die, leaving cavities in cartilage Figure 6–9 (Step 1)

Endochondral Ossification: Step 3 (Invasion)
Periosteal bud brings blood vessels into the cartilage: bringing osteoblasts and osteoclasts spongy bone develops at the primary ossification center Figure 6–9 (Step 3)

Endochondral Ossification: Step 4a (Remodelling)
Remodeling creates a marrow (medullary) cavity: bone replaces cartilage at the metaphyses Diaphysis elongates Figure 6–9 (Step 4)

Endochondral Ossification: Step 4b (2° Ossification)
Capillaries and osteoblasts enter the epiphyses: creating secondary ossification centers (perinatal) Figure 6–9 (Step 5)

Endochondral Ossification: Step 5 (Elongation)
Epiphyses fill with spongy bone but cartilage remains at two sites: ends of bones within the joint cavity = articular cartilage cartilage at the metaphysis = epiphyseal cartilage (plate) Figure 6–9 (Step 6)

Postnatal Bone Growth Growth in length of long bones
Cartilage on the side of the epiphyseal plate closest to the epiphysis is relatively inactive Cartilage abutting the shaft of the bone organizes into a pattern that allows fast, efficient growth Cells of the epiphyseal plate proximal to the resting cartilage form three functionally different zones: growth, transformation, and osteogenic

Functional Zones in Long Bone Growth
Growth zone – cartilage cells undergo mitosis, pushing the epiphysis away from the diaphysis Transformation zone – older cells enlarge, the matrix becomes calcified, cartilage cells die, and the matrix begins to deteriorate Osteogenic zone – new bone formation occurs

Growth in Length of Long Bone
Figure 6.9

Postnatal bone growth Remember that bone growth can only occur from the outside (appositional growth). So this type of endochondral growth is a way for bones to grow from the inside and lengthen because it is the cartilage that is growing, not the bone

Key Concept As epiphyseal cartilage grows through the division of chondrocytes it pushes the ends of the bone outward in length. At the “inner” (shaft) side of the epiphyseal plate, recently born cartilage gets turned into bone, but as long as the cartilage divides and extends as fast or faster than it gets turned into bone, the bone will grow longer

Long Bone Growth and Remodeling
Growth in length – cartilage continually grows and is replaced by bone as shown Remodeling – bone is resorbed and added by appositional growth as shown compact bone thickens and strengthens long bones with layers of circumferential lamellae

Long Bone Growth and Remodeling
Figure 6.10

Appositional Growth

Epiphyseal Lines When long bone stops growing, between the ages of 18 – 25: epiphyseal cartilage disappears epiphyseal plate closes visible on X-rays as an epiphyseal line At this point, bone has replaced all the cartilage and the bone can no longer grow in length

Epiphyseal Lines Figure 6–10

Hormonal Regulation of Bone Growth During Youth
During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone During puberty, testosterone and estrogens: Initially promote adolescent growth spurts Cause masculinization and feminization of specific parts of the skeleton Later induce epiphyseal plate closure, ending long bone growth

Remodeling Remodeling continually recycles and renews bone matrix
Turnover rate varies within and between bones If deposition is greater than removal, bones get stronger If removal is faster than replacement, bones get weaker Remodeling units – adjacent osteoblasts and osteoclasts deposit and resorb bone at periosteal and endosteal surfaces

Bone Deposition Occurs where bone is injured or added strength is needed Requires a diet rich in protein, vitamins C, D, and A, calcium, phosphorus, magnesium, and manganese Alkaline phosphatase is essential for mineralization of bone Sites of new matrix deposition are revealed by the: Osteoid seam – unmineralized band of bone matrix Calcification front – abrupt transition zone between the osteoid seam and the older mineralized bone

Effects of Exercise on Bone
Mineral recycling allows bones to adapt to stress Heavily stressed bones become thicker and stronger

Response to Mechanical Stress
Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon it Observations supporting Wolff’s law include Long bones are thickest midway along the shaft (where bending stress is greatest) Curved bones are thickest where they are most likely to buckle Trabeculae form along lines of stress Large, bony projections occur where heavy, active muscles attach

Response to Mechanical Stress
Figure 6.12

Bone Resorption Accomplished by osteoclasts
Resorption bays – grooves formed by osteoclasts as they break down bone matrix Resorption involves osteoclast secretion of: Lysosomal enzymes that digest organic matrix Acids that convert calcium salts into soluble forms Dissolved matrix is transcytosed across the osteoclast cell where it is secreted into the interstitial fluid and then into the blood

Bone Degeneration Bone degenerates quickly
Up to 1/3 of bone mass can be lost in a few weeks of inactivity

Minerals, vitamins, and nutrients
Rewired for bone growth A dietary source of calcium and phosphate salts: plus small amounts of magnesium, fluoride, iron, and manganese Protein, vitamins C, D, and A

Hormones for Bone Growth and Maintenance
Table 6–2

Calcitriol The hormone calcitriol:
synthesis requires vitamin D3 (cholecalciferol) made in the kidneys (with help from the liver) helps absorb calcium and phosphorus from digestive tract

The Skeleton as Calcium Reserve
Bones store calcium and other minerals Calcium is the most abundant mineral in the body Calcium ions in body fluids must be closely regulated because: Calcium ions are vital to: membranes neurons muscle cells, especially heart cells blood clotting

Calcium Regulation: Hormonal Control
Homeostasis is maintained by calcitonin and parathyroid hormone which control storage, absorption, and excretion Rising blood Ca2+ levels trigger the thyroid to release calcitonin Calcitonin stimulates calcium salt deposit in bone Falling blood Ca2+ levels signal the parathyroid glands to release PTH PTH signals osteoclasts to degrade bone matrix and release Ca2+ into the blood

Hormonal Control of Blood Ca
PTH; calcitonin secreted Calcitonin stimulates calcium salt deposit in bone Thyroid gland Rising blood Ca2+ levels Imbalance Calcium homeostasis of blood: 9–11 mg/100 ml Falling blood Ca2+ levels Imbalance Thyroid gland Osteoclasts degrade bone matrix and release Ca2+ into blood Parathyroid glands Parathyroid glands release parathyroid hormone (PTH) PTH Figure 6.11

Calcitonin and Parathyroid Hormone Control
Bones: where calcium is stored Digestive tract: where calcium is absorbed Kidneys: where calcium is excreted

Parathyroid Hormone (PTH)
Produced by parathyroid glands in neck Increases calcium ion levels by: stimulating osteoclasts increasing intestinal absorption of calcium decreases calcium excretion at kidneys

Calcitonin Secreted by cells in the thyroid gland
Decreases calcium ion levels by: inhibiting osteoclast activity increasing calcium excretion at kidneys Actually plays very small role in adults

Fractures Fractures: Fractures are repaired in 4 steps
cracks or breaks in bones caused by physical stress Fractures are repaired in 4 steps

Fracture Repair Step 1: Hematoma
Hematoma formation Torn blood vessels hemorrhage A mass of clotted blood (hematoma) forms at the fracture site Site becomes swollen, painful, and inflamed Bone cells in the area die Figure

Fracture Repair Step 2: Soft Callus
Cells of the endosteum and periosteum divide and migrate into fracture zone Granulation tissue (soft callus) forms a few days after the fracture from fibroblasts and endothelium Fibrocartilaginous callus forms to stabilize fracture external callus of hyaline cartilage surrounds break internal callus of cartilage and collagen develops in marrow cavity Capillaries grow into the tissue and phagocytic cells begin cleaning debris Figure

Stages in the Healing of a Bone Fracture
The fibrocartilaginous callus forms when: Osteoblasts and fibroblasts migrate to the fracture and begin reconstructing the bone Fibroblasts secrete collagen fibers that connect broken bone ends Osteoblasts begin forming spongy bone Osteoblasts furthest from capillaries secrete an externally bulging cartilaginous matrix that later calcifies

Fracture Repair Step 3: Bony Callus
Bony callus formation New spongy bone trabeculae appear in the fibrocartilaginous callus Fibrocartilaginous callus converts into a bony (hard) callus Bone callus begins 3-4 weeks after injury, and continues until firm union is formed 2-3 months later Figure

Fracture Repair Step 4: Remodeling
Bone remodeling Excess material on the bone shaft exterior and in the medullary canal is removed Compact bone is laid down to reconstruct shaft walls Remodeling for up to a year reduces bone callus may never go away completely Usually heals stronger than surrounding bone Figure

Clinical advances in bone repair
Electrical stimulation of fracture site. results in increased rapidity and completeness of bone healing electrical field may prevent parathyroid hormone from activating osteoclasts at the fracture site thereby increasing formation of bone and minimizing breakdown of bone, Ultrasound. Daily treatment results in decreased healing time of fracture by about 25% to 35% in broken arms and shinbones. Stimulates cartilage cells to make bony callus. Free vascular fibular graft technique. Uses pieces of fibula to replace bone or splint two broken ends of a bone. Fibula is a non-essential bone, meaning it does not play a role in bearing weight; however, it does help stabilize the ankle. Bone substitutes. synthetic material or crushed bones from cadavers serve as bone fillers (Can also use sea coral).

Aging and Bones Bones become thinner and weaker with age
Osteopenia begins between ages 30 and 40 Women lose 8% of bone mass per decade, men 3%

Osteoporosis Severe bone loss which affects normal function
Group of diseases in which bone reabsorption outpaces bone deposit The epiphyses, vertebrae, and jaws are most affected, resulting in fragile limbs, reduction in height, tooth loss Occurs most often in postmenopausal women Bones become so fragile that sneezing or stepping off a curb can cause fractures Over age 45, occurs in: 29% of women 18% of men

Notice what happens in osteoporosis

Osteoporosis: Treatment
Calcium and vitamin D supplements Increased weight-bearing exercise Hormone (estrogen) replacement therapy (HRT) slows bone loss Natural progesterone cream prompts new bone growth Statins increase bone mineral density PPIs may decrease density

Hormones and Bone Loss Estrogens and androgens help maintain bone mass
Bone loss in women accelerates after menopause

Cancer and Bone Loss Cancerous tissues release osteoclast-activating factor: stimulates osteoclasts produces severe osteoporosis

Paget’s Disease Characterized by excessive bone formation and breakdown An excessively high ratio of spongy to compact bone is formed Reduced mineralization causes spotty weakening of bone Osteoclast activity wanes, but osteoblast activity continues to work

Developmental Aspects of Bones
Mesoderm gives rise to embryonic mesenchymal cells, which produce membranes and cartilages that form the embryonic skeleton The embryonic skeleton ossifies in a predictable timetable that allows fetal age to be easily determined from sonograms At birth, most long bones are well ossified (except for their epiphyses)

Developmental Aspects of Bones
By age 25, nearly all bones are completely ossified In old age, bone resorption predominates A single gene that codes for vitamin D docking determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life

SUMMARY Skeletal cartilage Structure and function of bone tissues
Types of bone cells Structures of compact bone and spongy bone Bone membranes, peri- and endosteum Ossification: intramembranous and endochondral Bone minerals, recycling, and remodeling Hormones and nutrition Fracture repair The effects of aging

The Major Types of Fractures
Simple (closed): bone end does not break the skin Compound (open): bone end breaks through the skin Nondisplaced – bone ends retain their normal position Displaced – bone ends are out of normal alignment Complete – bone is broken all the way through Incomplete – bone is not broken all the way through Linear – the fracture is parallel to the long axis of the bone Transverse – the fracture is perpendicular to the long axis of the bone Comminuted – bone fragments into three or more pieces; common in the elderly Figure 6–16 (1 of 9)

Types of fractures (just FYI)

More fractures

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