2. Bone Calcium + Homeostasis

the skeletal system is composed of

bones, cartilage, teeth, tendons, ligaments, and other tissues that interconnect bones

the skeletal system is about ___% of body mass

about 20% of body mass

reinforce joints

ligaments

cartilage

reduces friction between bones and reduces impact on bones

provides structure

tendons

connect muscles to bones

functions of the skeletal system (5)

support - structural framework for organs and tissues

protect soft tissues and organs

leverage - provides a lever on which muscles can act to generate force (and movement)

mineral storage - largest storage of calcium in the body… also stores phosphate

blood cell production - red and white blood cells as well las platelets are produced in red blood marrow

bone types

flat, irregular, long, short

sesamoid bones

grow in tendons

reduce friction, provide leverage and support

commonly found in: feet, hands, wrists, knee

Flat bones

layer of spongy bone bw two thin layers of compact bone

have marrow, but no bone marrow cavity

irregular bones

no easily characterized shape

layer of spongy bone bw two thin layers of compact bone

short bones

roughly cube shaped

mostly spongy bone, surrounded by a thin layer of compact bone

parts of the long bone (9)

• Diaphysis

• Medullary cavity

• Epiphyses (2)

• Epiphyseal plate or line

• Articular cartilage

• Metaphyses (2)

• Periosteum

• Endosteum

• Bone marrow

diaphysis

tubular, long shaft of the bone

medullary cavity

hollow portion of long bone contains the bone marrow

endosteum

membrane that covers the internal bone surfaces

epiphyses

the ends of the bone

articular cartilage (hyaline)

covers each epiphysis, provides shock absorption and reducing friction in the joint

metaphysis

the flared portion between the diaphysis and the epiphysis

epiphyseal line

bw the epiphysis and the metaphysis

epiphyseal plate

aka “growth plate”

layer of cartilage that grows and is replaced by bone

periosteum

double layered membrane that covers the external surface of the bone

• very vascularized and innervated

• tendons, ligaments, and aponeuroses insert into periosteum

• two parts, fibrous outer layer and the inner osteogenic layer

Fibrous outer layer of periosteum

dense irregular connective tissue

inner osteogenic layer of the periosteum

lays adjacent to bone and houses osteoprogenitor cells, osteoclasts, and osteoblasts

perforating fibres (sharpey’s fibers)

attach the periosteum to the bone matrix and are extra dense and entheses

compact bone (aka cortical bone)

the dense outer layer of bone, provides protection and rigidity to structure

Spongy bone (aka cancellous bone)

thin small pieces of bone called trabeculae that are arranged like a honeycomb

reduces wight of bone, but still provides structure

usually found at the ends of long bones

where bone marrow is found

fills the medullary cavity as well as the spaces between the trabeculae in spongy bone

red bone marrow

produces RBCs, WBCs, and platelets

yellow bone marrow

adipose tissue storage

adipose tissue

basically body fat

can be found under the skin (subcutaneous fat), packed around internal organs (visceral fat), between muscles, within bone marrow, and in breast tissue

blood is supplied to the diaphysis and bone marrow by

at least one nutrient artery and vein

(nerves are often packaged wiht these)

how the nutrient arteries / veins access the inner bone (and sometimes also nerves)

through hole in wall of diaphysis called the nutrient foramen

periosteal arteries and veins supply…

the outer surface of the bone and periosteum

epiphyseal arteries and veins perfuse ….

the epiphyses

the metaphysical arteries and veins perfuse the…

metaphyses

structural unit of compact bone

osteon

central (Haversian) canal

part of the osteon

contains vessels surrounded by concentric lamellae

concentric lamellae

thin layers of bone matrix

formed by many osteons grouped together

compact bone

circumferential lamellae

surround the osteons and form the outermost layers of the compact bone

lacunae

small cavities between lamellae that house osteocytes

one osteocyte in each one

canaliculi

narrow passageways extending from lacunae into the lamellae

connect lacunate to vascular passageways, thus allowing the osteocytes a source of nutrients and a way to dispose of waste products

osteoprogenitor cell (osteogenic)

• type of stem cell

• found in inner layer of periosteum and endosteum

• can divide into osteoblasts

• huge role in healing fractures

osteoblast

• bone building cell

• premature bone cell found periosteum and endosteum

• secrete collagen and chondroitin into matrix of bone

the stuff secreted by osteoblasts

collagen and chondroitin

osteocyte

mature bone cell found in lacunae

important for bone turnover and repair

lacunae

small cavity in bone tissue where osteocytes live

osteoclast

• different cell base than the other bone cells

• found on surface of bone

• breaks down bone matrix for resorption

4 main reasons for osteogenesis

1. embryonic and fetal development

2. bone growth before adulthood (lengthening)

3. bone remodeling (throughout life)

4. fracture healing

what the embryonic skeleton is formed of

fibrous connective tissue and hyaline cartilage

ossification or osteogenesis

the building or remodeling of bone

both processes of ossification begin with this type of cell

mesenchymal cells

intramembranous ossification

the differentiation of (sheets of) mesenchymal cells directly into bone cells

examples of bones formed by intramembranous ossification

cranial bones, mandible, clavicles, sesamoid bones

endochondral ossification is when… (and how does it happen)

bone develops by replacing hyaline cartilage model

• mesenchymal cells first transform into hyaline cartilage model or template, which is later replaced by bone as ossification proceeds

bones formed by endochondral ossification

most bones, except those formed by intramembranous ossification (cranial, mandibles, clavicles, and sesamoid bones)

stages of intramembranous ossification (4)

1. development of ossification centre

2. calcification

3. formation of trabeculae and periosteum

4. formation of compact bone development

when does intramembranous ossification begin?

during embryonic development

what happens during the development of ossification centre in intramembranous ossification

• mesenchymal cells cluster together and differentiate into osteoblasts

• this site is called the ossification centre

what happens during calcification in intramembranous ossification

• osteoblasts secrete osteoid (bone ECM) until they are surrounded it

• these trapped osteoblasts differentiate into osteocytes

• osteocytes extend cytoplasmic processes from lacunae into canaliculi that radiate in all directions

• within a few days, mineral salts (calcium phosphates) are deposited into the osteoid, which hardens or calcifies (aka bone mineralization)

what happens during the formation of trabeculae and periosteum in intramembranous ossification

• accumulating osteoid is laid down between blood vessels forming thin, rodlike bony tissue called trabeculae

• the trabeculae fuse together and form a honeycomb structure of spongy bone around the blood vessels

• mesenchymal cells cluster on the external surface of the bone and differentiate into periosteum

what happens during the formation of compact bone and marrow in intramembranous ossification

• deep to the periosteum, the trabechulae are remodeled and replaced by a layer of compact bone

• spongy bone remains at the centre of the bone

• blood vessels that are crowded together around the trabeculae differentiate into red bone marrow, which fills the spaces between the spongy bone

the steps of endochondral ossification

1. development of cartilage model

2. development of bone collar and primary ossification centre

3. development of medullary cavity and secondary ossification center(s)

4. development of articular cartilage and epiphyseal plates

what happens during the development of the cartilage model

• mesenchyme cells crowd together in the shape of future bones

  • these cells then differentiate into chondroblasts

• chondroblasts secrete an ECM, which forms a hyaline cartilage model

• more mesenchyme cells condense on the surface of the model and form the perichondrium

• the now encased chondroblasts differentiate into chondrocytes

• chondrocytes undergo continuing cell division, thus increasing the bone in length (also in width by chondroblasts secreting more ECM)

what happens during the development of bone collar and primary ossification centre?

what happens during the development of the medullary cavity and secondary ossification centre(s) in endochondral ossification

• osteoclasts break down some of the trabecular bone, forming the medullary cavity

• surrounding bone is replaced by compact bone

• epiphyseal capillaries grow into the epiphyses and invading cells form a secondary ossification center in one or both ends of the bone (in the spongy parts)

  • in these centers, the cartilage is replaced by bone, but no cavity forms

what happens during the development of articular cartilage and epiphyseal growth plate in endochondral ossification

• hyaline cartilage on the ends of bone becomes articular cartilage which is a specialized type of hyaline cartilage

  • articular cartilage forms a protective cap over the epiphyses

• a layer of hyaline cartilage remains healthy between either end of the diaphyses and the epiphyses

  • these chondrocytes remain active and continue to divide and build new tissue, thus progressing growth in bone length

  • this area is called the epiphyseal growth plate

• the growth plates remain active until about age 25, when they ossify and become the epiphyseal line

explain each step

differentiate between the two types of post-natal bone growth

1. interstitial growth

   • growth in length from within the tissue

   • chondrocytes divide and secrete new matrix from within the lacunae, expanding the cartilage and causing the cartilage model to grow

   • same process happens in bone, but with the bone “chasing” newly formed cartilage, replacing it as it grows. this forces the cartilage to grow unidirectionally towards the ends, thus elongating it

2. appositional growth

   • growth in width or diameter through the addition of new layers

   • osteoblasts secrete new matrix agains existing external face of bone

     • cartilage grows in the same manner, but chondroblasts secrete matrix onto existing cartilage model

steps of appositional growth

1. Osteoblasts on the periosteum form new bone tissue — ridges develop around periosteal capillaries that grow and fuse into a tunnel around the blood vessels

2. Osteoblasts in the endosteum of those canals build new concentric lamellae toward the tunnel which forms a new osteon

3. Osteoblasts in the periosteum build new circumferential lamellae furthering the outward growth

4. Osteoclasts on the inside of the bone break down tissue, widening the medullary cavity

appositional growth is stimulated by ___ and requires ___

stimulated by chronic loading

requires adequate nutrient intake

______ load is a major determinant of bone growth

mechanical load

• anatomy of the bone reflects the stresses placed on it, and is continuously remodeled (Wolff’s Law)

• bones become thicker and stronger to resist forces (gravity, load, muscle contraction), and thinner and weaker if there are no forces placed on them

____ triggers affect bone health

chemical triggers

• If calcium or phosphorus are in short supply, regulating hormones will take them out of the bone to serve vital functions in other systems of the body” (Office of the Surgeon General (US), 2004)

peak bone mass

our maximum bone size and strength

determinants of peak bone mass

genes and lifestyle factors (exercise level and type, smoking, alcohol, drug use, diet and nutrition, etc)`

bone mass loss can be significantly slowed by…

regular exercise and adequate nutrition

calcium is crucial in ___ and ___ function

neurological and muscular function

what happens when plasma calcium is too high

hormones are released to stimulate osteoblast activity

what happens when plasma calcium is too low

hormones are released to stimulate osteoclast activity

organs involved in calcium homeostasis

bones kidneys and gastro-intestinal tract

hormones involved in calcium homeostasis

parathyroid hormone

vitamin D

calcitonin

what does parathyroid hormone stimulate?

• osteoclast activity

• reabsorption of calcium by the kidneys

• synthesis of Vitamin D

• osteoblast activity

• released by parathyroid

what releases parathyroid hormone

parathyroid

what does Vitamin D do

Stimulates calcium absorption by the gut

how does vitamin D enter the body

ingested via the gut as food

synthesized via the skin (sun)

what does calcitonin do

inhibits osteoclast activity

decreases Ca2+ reabsorption by kidneys

released by thyroid

what releases calcitonin

the thyroid

parathyroid hormones are released when..

blood calcium levels are too low

vitamin D is most active when

blood calcium levels are too low

calcitonin is released at a higher rate when

blood calcium levels are too high

osteoporosis

low bone bass and therefore weakened bone

• bone becomes brittle light and porous

  • leads to fragility fractures

osteoclast activity outpaces osteoblast activity

commonly affected bones = hip spine wrist and scapula

no early signs or symptoms

treatment : mostly focused on reducing impagrs or slowing disease process

prevention: resistance training and adequate nutrition

treatment for osteoporosis

mainly focused on reducing impacts or slowing disease progress

prevention of osteoporosis

resistance training and adequate nutrition

process of fracture healing

1. hematoma forms

   • blood vessels and periosteum are torn w a fracture

   • hemorrhaged blood clots at fracture site

2. fibrocartilage (soft) callous forms

   • wishing days, new blood vessels grow into the clot

   • fibroblasts and chondrocytes invade site

   • fibroblasts produce collagen fibres that connect ends of broken bones

   • chondroblasts secrete cartilaginous matrix that calcifies to form the soft callus

3. bony callous forms

   • wishing a week, osteoblasts lay down new bone around soft callous

   • gradually replaces soft callus with hard, bony callus

4. bone remodeling

   • for several months, bony callus is continually remodeled

   • excess callus material on diaphysis and medullary cavity is removed

   • compact bone laid down to reconstruct shaft walls