Chapter7-bone tissue- sesay emphasized

Skeletal system

bones, cartilages, ligaments

osteology

study of bone

hyaline cartilage is

forerunner of most bones, and covers joint surfaces.

3x types of cartilage

hylaline

elastic

fibrocartilage

ligament

bone to bone at joints (collagen, a dense regular connective tissue)

tendon

attach muscle to bone

dense regular connective tissue

What do healthy bones do?

1-regulate blood sugar, so unhealthy bones increase risk of diabetes.

bones secrete osteocalcin hormone that affects action of insulin and moderate stresss response.

2- Electrolyte balance- calcium and phosphate levels

3-Acid Base balance- buffers blood against large PH changes by altering phosphate and carboante levels

4- blood formation- red bone marrow is the chief producer of blood cells

healthy bones release

osteocalcin- stimulates pancreas to release insulin

mineralization and calcification

the hardening/stengthening of bone

individual bone consists of

bone tissue,

bone marrow

cartilage

adipose tissue

nervous tissue

fibrous connective tissue

nervous, fibrous, and connective tissues are integrated primarily within the outer protective membranes and the internal channels that run through the mineralized matrix

Compact bone

dense/cortical bone- dense outer shell of bone

compact bone encloses the

marrow/ medullary cavity.

spongy bone/ cancellous

loosely organized bone tissue

Diaphysis

shaft for leverage

epiphysis

enlarged end of a long bone. strengthens joint, anchors ligaments and tendons.

Epiphyseal line and plate

line- found in adults. remnant of childhood growth zone of hyaline cartilage epiphyseal plate.

plate- found in kids. ontop of epiphysis, contains hyaline cartialge

Osteoblasts

Bone forming cell in endosteum ( The endosteum is a thin, delicate layer of active connective tissue that lines the internal surfaces of all bones.)and inner layer of periosteum(periosteum is a tough, double-layered membrane of fibrous and cellular connective tissue that covers the entire outer surface of all bones, except at the joints where articular cartilage is present.)

osteogenic cell

stem cells found in the endosteum and inner layer of periosteum

Osteocyte

Mature bone cell: former osteoblasts that have become embedded in the matrix they deposit

octeocalcin (not on test)

osteoblasts and osteocytes, under physical stress, secrete osteocalcin. response to stress, inhibits parasympathetic, and allows the sympathetic to work unnopposed. stimulates pancreatic release of insulin, increases insulin sensitivty, promotes energy availability.

Osteoclast

dissolves bone. found on bone surface. performs osteolysis/ resorption or bone breakdown/remodeling.

often reside in pits called resorption bays.

develop from same bone marrow stem cells that give rise to blood cells

Bone matrix

collagen and minerals of bone form a composite. mineral portion allows body to support weight.

Rickets

children with rickets- a disease caused by mineral deficiency results in soft deformed bones.

osteomalacia

vitamin D deficiency in adults that also leads to softening bones and joint pain

Bone marrow

soft tissue occupying marrow cavities of long bones and small spaces of spongy bone

Red bone marrow (myeloid tissue)

contains multiple tissues, including hematopoietic tissue- tissue that produces blood cells

Red bone marrow is hemopoietic (blood-producing) tissue, meaning it is the manufacturing site for all formed elements of the blood. It contains pluripotent stem cells that differentiate not just into erythrocytes (RBCs), but also into all types of leukocytes (white blood cells) and megakaryocytes (which shatter to form platelets).

formed blood elements:

• Erythrocytes (Red Blood Cells): Cells responsible for binding and transporting oxygen from the lungs to the body's tissues, and carrying carbon dioxide back to the lungs.

• Leukocytes (White Blood Cells): Immune cells

• Thrombocytes (Platelets): Cell fragments that play a critical role in blood clotting (hemostasis) to repair damaged blood vessels a

infant bone marrow is

100 percent red in long bones of children

adults bone marrow

70 percent red (In adults, found in skull, vertebrae, ribs, sternum, part of pelvic

girdle, and proximal heads of humerus and femur) and 30 percent yellow.

yellow bone marrow

Fatty marrow that does not produce blood

Can transform back to red marrow in the event of chronic anemia

helps with energy and immunity

ossification or osteogenesis:

Formation of bone:

Occurs in two ways that begin with embryonic mesenchyme:

1-Intramembranous ossification

2- Endochondral ossification

Intramembranous ossification

Produces flat bones of the skull, most of the clavicle, and part of the mandible

Stages (not on test):

1- deposition of osteoid tissue into embryonic mesenchyme

2- calcification of osteoid tissue and entrapment of osteocytes

3-honeycomb of spongy bone with developing periosteum

4- filling of space to form compact bone at surfaces, leaving spongy bone in the middle

key for lifelong thickening and remodeling of long bones

Endochondral ossification

requires hyaline cartilage.

Bone develops from hyaline cartilage: produces most bones of the body (limbs, vertebrae, ribs, sternum, scapula, pelvic girdle.

summary of stages (metacarpal example)

1- Mesenchyme forms early hyaline cartilage model.

2- formation of primary ossification center, bony collar, and periosteum

3- vascular invasion, formation of primary marrow cavity. and appearence of secondary ossification center.

4- Bone at birth contains enlarged marrow cavity and secondary marrow cavity.

5- child has epiphyseal plate

6- adult bone contains single marrow cavity and close epiphyseal plate (becomes epiphyseal line)

*Mesenchyme is a type of loosely organized, undifferentiated embryonic connective tissue. It is made up of star- or spindle-shaped cells floating in a watery, gel-like extracellular matrix

Stages of endochondral ossification

Metaphysis

Epiphyseal plate serves as growth zone. The transitional zone on each side, where cartilage is replaced by bone is called metaphysis.

The structure of metaphysis corresponds to steps of cartilage replacement

steps of metaphysis/ cartilage replacement

Epiphyseal plate serves as growth zone

Transitional zone on each side, where cartilage is replaced by bone is called metaphysis

Steps correspond to steps of cartilage replacement:

1- Zone of reserve cartilage: 100 cells

2- Zone of cell proliferation: 1,000 cells hyaline cartilage

3- Zone of hypertrophy- cells get bigger

4- zone of calcification- cells fill with minerals

5- zone of bone deposition

Zones of metaphysis

1- zone of reserve cartilage: histology of resting hylaine cartilage

2- Zone of cell proliferation: chondrocytes multiplying and lining up in rows of small flattened lanunae

3-zone of cell hypertrophy: cessation of mitosis, enlargment of chondrocytes and thining of lacuna walls

4- Zone of calcification: temporary calcification of cartilage matrix between columns of lacunae

5- zone of deposition: Breakdown of lacuna walls, leaving open channels- death of chondrocytes, bone deposition by osteoblasts, forming spongy bone of trabeculae

Achondroplastic Dwarfism:

Theres a problem with epiphyseal plate

long bones stop growing in childhood; results in normal torso, short limbs

failure of cartilage growth in metaphysis

spontaneous mutation produces mutant dominant allele

pituitary dwarfism

lack growth hormone

normal proportions with short stature

Wolff’s law

the architecture of bone is determined by the forces put on it- the bone adapts to withstand it.

osteoblasts and osteoclasts

if a bone is little used osteoclasts remove matrix

calcium Homestasis

calcium is needed for bone structure, neuron communication, muscle contraction, blood clotting, and exocytosis.

hypocalcemia

calcium deficiency- leads to excessive excitability of nervous system and muscles

hypercalcemia

very rare- excess is sucked into bone. calcium excess. causes nerve and muscle cells to be less excitable than normal.

hypocalcemia causes

low vitamin D, diarrhea, thyroid tumors, underactive parathyroid glands

calcium homestasis is regulated by three hormones

calcitriol

calcitonin

parathyroid hormone

calcitriol

step 1- vitamin D from sun or food… travels to liver and turns into calcidiol

step 2- calcidiol travels to the kidneys where an enzyme converts in into calcitriol- the hormonal active form of Vit D.

necessary for bone deposition- helping provide adequate calcium and phosphate

inadequate calcitriol results in abnormal softness of bones (rickets in children and osteomalacia in adults)

1-It then increases calcium absorption by the small intestine

2- increases calcium resorption from skeleton (stimulates formation of osteoclasts)

3- weakly promotes absorption of calcium from the kidneys (reduced extretion of Ca+)

Calcitonin

If blood calcium concentration goes over 10.4 (9.2-10.4):

The Thyroid senses this- too much calcium in the blood! and releases Calcitonin. calcitonin :Osteoclast inhibition: Rapidly reducing the rate of bone breakdown. Osteoblast stimulation: Promoting calcium deposition into the bone matrix by increasing osteoblast activity (mostly in children). Relevance in the Textbook: Saladin emphasizes that this mechanism is significantly active in children—whose bones are rapidly remodeling and mineralizing—but has a weak or negligible effect in healthy adults

secreted when blood calcium levels are too high.

lowers blood calcium conentration in two ways:

osteoblast inhibition: reducing bone resorption

osteoblst stimulation; to deposit calcium into bone

important in children/weak affect in adults

PTH-Parathyroid Hormone

If too little claclium in blood (under 9.2-10.4):

Parathyroid senses this! it releases PTH, which stimulates osteoclasts in bone to break down osseus tissue and put the calcium back into the blood stream.

Some PTH also goes to the kidneys and said “ speed up activation of vitamin D into Calcitriol. Calcittriol goes to the intestines/gi tract, and gathers calcium to bring into the blood stream.

Direct kidney reabsorption: In addition to speeding up calcitriol production, PTH acts directly on the kidneys, signaling them to stop letting calcium escape in the urine and to reabsorb it straight back into the blood.

The Phosphate Dump: When osteoclasts break down osseous tissue, they release both calcium and phosphate into the blood. If both levels rise together, they will bind and form solid calcium-phosphate crystals (ectopic calcification) instead of staying dissolved as usable ions. To prevent this, PTH forces the kidneys to excrete extra phosphate into the urine.

The Osteoblast Middleman: While PTH does ultimately stimulate osteoclasts, it doesn't bind to them directly. PTH actually binds to osteoblasts (the bone builders). The osteoblasts then release a chemical signal (RANKL) that wakes up the osteoclasts to start resorbing bone.

PTH effect on calcium blood levels:

PTH raises calcium blood level by four mechanisms:

Binds to receptors on osteoblasts, which then stimulate osteoclasts

to resorb bone

Promotes calcium reabsorption by kidneys, so less lost in urine

Promotes the final step of calcitriol synthesis in the kidneys,

enhancing calcium-raising effect of calcitriol

Inhibits collagen synthesis by osteoblasts, inhibiting bone

deposition

calcium intake and excretion image

phosphate homeostais:

calcitriol raises phosphate levels by promoting its absorption by the small intestine

PTH lowers blood phosphate levels, in the blood stream, by promoting its urinary excretion

if phosphate is too high, means kidney has a problem

osteopenia

measurable decline in bone density, may advance to osteoporosis

osteoporosis

severe loss of bone density. bones easily fracture.