Lecture 10

What is bone (osseous) tissue, and what does the skeleton do?

Bone tissue forms most of the skeleton: a framework that supports and protects organs and allows movement.

What are the six functions of bone in this lecture?

Support; protection; movement/leverage; mineral homeostasis; blood cell production; energy storage.

How do bones provide support?

They support soft tissues and provide muscle attachment sites, creating a framework for the body.

How do bones protect organs? Give the lecture example.

Bones surrounding internal organs protect them from injury; cranial bones protect the brain.

How do bones facilitate movement?

They provide leverage for muscle contraction.

What connects bone to bone, and what connects muscle to bone?

Ligaments connect bone to bone; tendons connect muscle to bone.

How do bones contribute to mineral homeostasis? Which ions are emphasized?

They store minerals, especially calcium and phosphate ions, and mobilize them when needed elsewhere in the body.

What is hemopoiesis (hematopoiesis), and where does it occur?

Blood cell production in red bone marrow, a connective tissue found in certain bones.

What do the stem cells in red marrow produce?

Mature blood cells.

What daily white blood cell production figure does the lecture give, and why are replacements needed?

More than a billion white blood cells are produced and released daily, replacing cells that die defending the body or from old age.

How does yellow marrow store energy?

It consists mostly of adipose cells; their stored lipids can serve as an energy source.

What determines classification as compact or spongy bone?

The size and distribution of spaces between the hard components.

What proportions does the lecture give for compact and spongy bone?

Compact (dense) bone: 75%; spongy bone: 25%.

What are trabeculae, and what lies between them?

Thin plates of bone surrounding many red marrow-filled spaces in spongy bone.

Where does spongy bone form most of the structure?

Short, flat, and irregular bones, and the epiphyses of long bones.

What functions of spongy bone are listed?

Storing red marrow and providing some support.

Compare the contents and locations of yellow and red marrow.

Yellow: lipids in adipocytes, located in the medullary cavity. Red: stem cells producing mature blood cells, located in spaces of spongy bone.

How does marrow generally differ in young children and adults?

Children up to age 5 have mostly red marrow; adults have mostly yellow marrow, according to the notes.

What percentage of marrow is active red marrow at birth?

100%, according to the lecture.

Where is red marrow found in a 5-year-old child?

In spongy bone and in the medullary cavities of long bones such as the femur and tibia.

When does the major conversion from red to fatty yellow marrow begin?

Around age 5 to 7.

What proportion and distribution of adult red marrow does the lecture state?

About 50% of total marrow; absent from long-bone shafts and found strictly in spongy bone at specific sites.

Which adult red marrow sites are listed in the notes?

Hip bones/pelvis, vertebrae, ribs, sternum, skull, and the proximal epiphyses of the humerus and femur.

Which parts of the humerus and femur retain adult red marrow?

The very upper tips: proximal epiphyses, within spongy bone.

Name the seven components of a typical long bone.

Diaphysis; epiphyses; metaphysis; articular cartilage; periosteum; medullary cavity; endosteum.

What is the diaphysis?

The shaft or main portion of a long bone.

What are the epiphyses?

The proximal and distal ends of a long bone.

What is the metaphysis, and what does it contain in immature bone?

The region where the diaphysis joins the epiphysis; it contains the epiphyseal plate, made of hyaline cartilage that will be replaced by bone.

What is articular cartilage, and where is it located?

Hyaline cartilage covering the epiphysis where it forms an articulation with another bone.

Describe the periosteum: tissue, coverage, attachments, and cells.

A connective tissue membrane covering bone where articular cartilage does not; it provides ligament/tendon attachment sites and contains osteogenic cells.

What does the medullary cavity contain?

Yellow bone marrow.

Describe the endosteum: tissue, location, and cells.

A connective tissue membrane lining the medullary cavity; it contains osteogenic cells.

How do periosteum and endosteum differ, and what do they share?

Periosteum covers the outside where articular cartilage is absent; endosteum lines the medullary cavity. Both are connective tissue membranes containing osteogenic cells.

What two structural features characterize bone histology?

Widely separated cells surrounded by large amounts of matrix.

What are the four major bone cell types?

Osteogenic cells, osteoblasts, osteocytes, and osteoclasts.

Describe osteogenic cells: origin, division, product, and synonym.

Undifferentiated cells derived from mesenchyma; they may undergo mitosis and give rise to osteoblasts. Also called osteoprogenitor cells.

Do osteoblasts divide, what are their immediate precursors, and what do they do?

They do not divide; their immediate precursors are fibroblast-like preosteoblasts. They form bone by secreting collagen and other organic compounds.

What is osteoid, and which cells produce it?

The protein mixture/unmineralized organic bone matrix produced by osteoblasts; it becomes bone when mineralized.

How do osteoblasts become osteocytes, and what do osteocytes do?

Matrix surrounds and traps osteoblasts, which become mature osteocytes responsible for bone maintenance.

Give the cell sequence from mesenchymal cell to mature bone cell.

Mesenchymal cell → osteogenic cell → osteoblast → osteocyte.

Where are osteoclasts found, and what do they do?

On bone surfaces; they remove bone matrix through resorption. The notes describe them as specialized macrophages.

What size/nuclear feature distinguishes osteoclasts?

They are huge cells with 50 or more nuclei, according to the lecture.

What is the origin of osteoclasts?

They differentiate from monocytes derived from stem cells in red bone marrow.

How do multinucleated osteoclasts form, according to the notes?

Fusion of several cells or incomplete cytokinesis.

Match bone formation, maintenance, and resorption to their cell types.

Formation: osteoblasts. Maintenance: osteocytes. Resorption: osteoclasts. Osteogenic cells supply osteoblasts.

Which mineral salts are named in bone matrix?

Primarily hydroxyapatite and tricalcium phosphate, with some calcium carbonate.

What is calcification/mineralization?

Deposition of mineral salts in a framework of collagen fibers.

What provides bone hardness, and what provides tensile strength?

Mineral salts provide hardness; collagen fibers provide tensile strength.

What does tensile strength mean in the lecture?

The force required to pull something to the point where it breaks.

What is osteogenesis, and what is the matrix called before calcification?

Production of new bone by osteoblasts, which make matrix proteins and other organic compounds. Before calcification, the matrix is osteoid.

What is osteolysis/resorption, and how is it carried out?

Release of stored minerals and calcium by osteoclasts using proteolytic enzymes and acids.

What is ossification (osteogenesis), and what preexisting tissue does it replace?

Bone formation, replacing preexisting connective tissue with bone.

What cell changes initiate ossification?

Mesenchymal cells become osteogenic cells, which divide and give rise to cells differentiating into osteoblasts and osteocytes.

When does ossification begin, and how long does it continue?

During the sixth or seventh week of embryonic life; it continues throughout adulthood.

What are the two types of ossification?

Intramembranous and endochondral (intracartilaginous).

How does the clay analogy distinguish the two types of ossification?

Intramembranous: start with a core and add material, shaping as you go; no preset shape. Endochondral: first make a shaped frame, then fill it; preset shape.

Where does intramembranous ossification occur?

Within fibrous connective tissue membranes of the embryo and adult.

How does an intramembranous ossification center form?

Mesenchymal cells convert to osteogenic/osteoprogenitor cells, then osteoblasts that lay down osteoid matrix.

What happens after osteoid is deposited in intramembranous ossification?

Matrix surrounds the cells and calcifies as osteoblasts become osteocytes.

How does spongy bone arise in intramembranous ossification?

Calcifying matrix centers join into bridges of trabeculae, forming spongy bone with red marrow inside.

What happens to the periosteal collar in intramembranous ossification?

The periosteum first forms a collar of spongy bone, which is replaced by compact bone.

Give the full sequence of intramembranous ossification.

Fibrous connective tissue → ossification center and osteoblasts → osteoid → surrounding/calcifying matrix and osteocytes → trabecular bridges/spongy bone with red marrow → periosteal spongy collar replaced by compact bone.

What is heterotopic bone formation?

Bone formation where it should not occur, with osteoblasts developing at inappropriate sites.

What does the lecture say can induce heterotopic bone formation, and which tissues are susceptible?

Chemicals can induce it, with problems associated with calcium excretion and storage; most connective tissues can be affected.

What is myositis ossificans in this lecture?

An extreme case of heterotopic bone formation involving bone deposition within skeletal muscle.

What is the model used in endochondral ossification, and how does it form?

A hyaline cartilage model: mesenchyma becomes chondroblasts, which produce hyaline cartilage that is then replaced by bone.

How does the cartilage model grow in length and thickness?

Interstitial growth increases length; appositional growth increases thickness.

Where does the primary ossification center develop?

In the diaphysis of a long bone.

How does the medullary cavity form during endochondral ossification?

Cartilage degenerates, leaving cavities that merge into the medullary cavity; osteoblasts lay down bone.

Where do secondary ossification centers develop, and what cartilage is retained in the notes' description?

In the epiphyses; bone replaces cartilage except for the epiphyseal plate.

Give the full sequence of endochondral ossification.

Mesenchyma → chondroblasts → hyaline cartilage model → interstitial/appositional model growth → primary center in diaphysis → cartilage degeneration and merged medullary cavity → bone deposited by osteoblasts → secondary centers in epiphyses, with epiphyseal plate retained.

What type of growth increases bone length, and what type increases thickness?

Length: interstitial growth. Thickness: appositional growth.

What is the epiphyseal plate made of, and which transitional region faces the marrow cavity?

Hyaline cartilage with transitional zones on each side; the region facing the marrow cavity is the metaphysis.

Name the five metaphyseal zones in the order listed.

Reserve cartilage → proliferating cartilage → hypertrophic cartilage → calcified cartilage → bone deposition (ossification).

What happens in the zone of reserve cartilage?

Hyaline cartilage is present with no signs of bone.

What happens in the zone of proliferating cartilage?

Chondrocytes divide and secrete cartilage matrix.

What happens in the zone of hypertrophic cartilage?

Chondrocytes mature and no longer undergo mitosis.

What happens in the zone of calcified cartilage?

Minerals are deposited and cartilage calcifies.

What happens in the zone of bone deposition (ossification)?

Chondrocytes die because surrounding matrix has calcified; osteoclasts absorb matrix and osteoblasts invade the area.

What activity lengthens the diaphysis?

Activity of the metaphysis produces interstitial growth.

What is the epiphyseal line, and what does its formation mean for growth?

A remnant of the epiphyseal plate; its formation marks cessation of growth in length.

How do osteoblasts and osteoclasts contribute to increased bone diameter?

Osteoblasts add new bone around the outer surface (appositional growth); to a lesser extent, osteoclasts dissolve bone internally in the bone cavity.

What is bone remodeling, and which cells perform its opposing activities?

Ongoing replacement of old bone with new bone: osteoclasts destroy old tissue and osteoblasts construct new tissue.

What is Wolff's law?

Bone in a healthy person or animal adapts to the loads placed on it; bone is restructured in response to stress and strain.

Who developed Wolff's law, and when?

German anatomist and surgeon Julius Wolff (1836–1902), in the nineteenth century.

What tennis-player example illustrates adaptation of bone to loading?

More bone mineral is found in the arm used to swing the racquet than in the other arm.

How does exercise affect bone mineral accumulation, and which kind is emphasized?

Exercise promotes accumulation; weight-bearing exercise is more effective than exercise without loading, according to the notes.

How does the lecture define weight-bearing exercise, and what examples does it give?

Activities stressing bones against full body weight: walking, running, tennis, step aerobics, and actual stair climbing (not a machine).

Which gym activities does the lecture contrast with weight-bearing exercise?

Rowers, bicycles, gliders, and ski machines: the notes describe cardiovascular benefit but say they do not build or maintain bone mass.

Within limits, how does mechanical stress increase bone strength?

By increasing mineral-salt deposition and collagen-fiber production.

What happens when mechanical stress is removed?

Bone weakens through demineralization (loss of minerals) and reduced collagen.

Which activities are specifically listed as helping build and retain bone mass?

Walking and moderate weightlifting.

What astronaut bone-loss figure does the lecture give, and under what condition?

As much as 1% bone mass per week during weightlessness, according to the lecture.

Which hormones are listed as regulating bone growth before puberty?

Human growth hormone (hGH), insulin-growth factor (IGF), thyroid hormones, parathyroid hormone, and insulin.

Where is hGH produced, and where is IGF produced under its influence?

hGH: pituitary gland. IGF: bone and liver under the influence of hGH.

What effects do hGH and IGF have, and what can abnormal levels cause?

They stimulate bone deposition and growth-related changes; deviations from normal levels can lead to gigantism or dwarfism.

What does the lecture say about thyroid and parathyroid hormones in growth?

They also play important roles in bone growth.

Where is insulin produced, and what is its effect on bone growth?

The pancreas; insulin stimulates bone growth.

What do estrogen and testosterone do at puberty?

They stimulate sudden growth and skeletal modifications associated with biological female and male forms.

What explanation does the lecture give for women generally being shorter than men?

Estrogen seems to induce epiphyseal closure faster than testosterone/androgens.