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Week 7 · Chapter 6

Osseous Tissue & Bone Structure

Bone is the most rigid connective tissue type from Chapter 4, and one of the most dynamic — despite feeling permanent, it's constantly being broken down and rebuilt. This week covers bone classification, structure at both gross and microscopic levels, and how bone regulates calcium throughout the body.

By the end of this week, you will be able to:

1List the five main functions of bone
2Classify bones by shape
3Label the gross anatomy of a long bone
4Compare compact and spongy bone structure
5Describe the osteon (Haversian system) and the three bone cell types
6Explain bone remodeling and its role in calcium homeostasis

Big Picture

Bone: connective tissue that never stops working

Bone is a form of connective tissue — the same category from Chapter 4 that also includes blood and cartilage — but with a mineralized, rigid extracellular matrix. It's easy to picture bone as static scaffolding, but it's actually one of the most metabolically active tissues in the body, constantly remodeling itself and serving as the body's mineral reservoir.

This week ties directly back to Chapter 1's homeostasis concept: bone tissue is a central player in keeping blood calcium levels stable, using the same negative feedback logic you've now seen apply to temperature, pH, and osmotic balance.

Vocabulary & Word Parts

Osseous tissue roots

oste/o-bone

Osteoporosis = porous bone. Osteoblast = bone-building cell.

-blastimmature, building cell

Osteoblast = a cell actively building new bone matrix.

-clastbreaking down

Osteoclast = a cell that breaks down bone tissue.

-cytemature cell

Osteocyte = a mature bone cell maintaining existing matrix.

peri-around

Periosteum = the membrane around the outside of a bone.

medull-middle, marrow

Medullary cavity = the central marrow-containing space in a long bone's shaft.

Structure & Function

What bone actually does

Support — the rigid framework that holds the body's shape and supports soft tissues.
Protection — bones like the skull and rib cage shield delicate organs (brain, heart, lungs).
Movement — bones act as rigid levers that muscles pull against to create motion.
Mineral storage — bone is the body's primary reservoir for calcium and phosphorus.
Blood cell production — red bone marrow (found in spongy bone) produces red blood cells, white blood cells, and platelets.

Structure & Function

Classifying bones by shape

Bone shape reflects bone function — long bones act as levers for movement, short bones provide stability with some mobility (wrists, ankles), flat bones offer broad surfaces for protection and muscle attachment, and irregular bones have complex shapes that don't fit the other three categories.

Structure & Function

Anatomy of a long bone

The diaphysis (shaft) is mostly compact bone surrounding a hollow medullary cavity. The epiphyses (rounded ends) contain more spongy bone and are where the bone meets a joint. The periosteum is a tough membrane covering the outer surface — it's where tendons and ligaments anchor, and it contains cells that repair bone after a fracture.

Structure & Function

Compact vs. spongy bone

Compact bone is dense and forms the strong outer shell of every bone. Spongy bone has an open, lattice-like structure (trabeculae) that makes it lighter while still resisting stress — and its open spaces are where red bone marrow lives.

Structure & Function

The osteon: compact bone's microscopic unit

The osteon (or Haversian system) is the basic structural unit of compact bone — concentric rings (lamellae) of mineralized matrix surrounding a central canal that carries blood vessels and nerves. Bone cells sit in small spaces between the rings, connected by tiny channels that let nutrients pass through even in this dense tissue.

Structure & Function

Three bone cell types, three jobs

This is a build/maintain/break-down cycle: osteoblasts build new bone matrix, osteocytes are the mature, settled cells that maintain it day to day, and osteoclasts break matrix down when calcium needs to be released or bone needs to be remodeled. All three types work together continuously, even in healthy adult bone.

Structure & Function

Bone remodeling: a continuous balanced cycle

Bone is never truly "finished" — osteoclasts continuously remove old or damaged bone matrix while osteoblasts lay down new matrix to replace it. In healthy adults these two processes stay balanced; an imbalance in either direction is the root cause of several major bone diseases.

Structure & Function

Calcium homeostasis: bone as the body's mineral bank

This is another negative feedback loop, just like Chapter 1's temperature example. When blood calcium drops, the parathyroid gland releases PTH, which activates osteoclasts to break down bone matrix and release stored calcium into the blood — restoring the normal range and shutting the loop back off.

Your Turn — Guided Practice

Apply it before the quiz

A patient's bloodwork shows unusually low blood calcium levels.

Question 1 — Which bone cell type would the body activate to help correct this?

Low blood calcium triggers PTH release, which activates osteoclasts to break down bone matrix and release its stored calcium into the blood. Activating osteoblasts would do the opposite — pull MORE calcium out of the blood to build new matrix, worsening the problem.

Question 2 — Is this response an example of negative or positive feedback?

The response works to reverse the original change (low calcium), pushing the level back toward normal — the defining signature of negative feedback, the same pattern from Chapter 1's temperature regulation example.

Clinical Correlation

Osteoporosis: when remodeling falls out of balance

This is a direct, high-stakes consequence of the osteoblast/osteoclast balance covered this week.

🩺 In practice

Osteoporosis occurs when osteoclast activity outpaces osteoblast activity over time, causing bone to become progressively less dense and more fragile. It's especially common after menopause, when declining estrogen (which normally helps restrain osteoclast activity) shifts the balance toward bone breakdown — which is exactly why bone density scans and calcium/vitamin D management are standard preventive care for this population.

Clinical Correlation

Why the periosteum matters for fracture healing

The periosteum's living cells are a direct application of this week's structural anatomy.

🩺 In practice

When a bone fractures, the periosteum is one of the first tissues to respond — its inner layer contains cells capable of becoming osteoblasts, which begin laying down new bone matrix (initially as a soft callus) to bridge the break. This is part of why an open fracture that strips away periosteum is a more serious injury than a clean break where the periosteum stays intact — the healing machinery itself has been damaged.

Mini Knowledge Test

Show what you know — 7 questions

Question 1

Which is NOT one of bone's five main functions?

Bone's five functions are support, protection, movement, mineral storage, and blood cell production. Digestion is not one of them — that's the job of the digestive system's organs, not the skeleton.

Question 2

The femur (thigh bone) is classified as which bone shape?

The femur is a classic long bone — longer than it is wide, acting as a lever for movement. Flat bones (like the skull) and irregular bones (like vertebrae) have very different shapes suited to different jobs.

Question 3

The medullary cavity is located:

The medullary cavity is the hollow space running through the center of the diaphysis (shaft), containing marrow. The periosteum covers the outer surface instead, and the medullary cavity is a diaphysis feature, not a spongy-bone-only structure.

Question 4

Which type of bone tissue has an open, lattice-like structure?

Spongy bone has an open trabecular lattice, making it lighter while housing marrow. Compact bone (also called cortical bone — the same thing, not a third type) is dense with few open spaces.

Question 5

In an osteon, blood vessels and nerves travel through the:

The central canal at the core of each osteon carries blood vessels and nerves into compact bone. The lamellae are the surrounding mineralized rings, not passageways; the periosteum is a separate outer membrane, not part of the osteon's internal structure.

Question 6

Which bone cell type is responsible for breaking down bone matrix?

Osteoclasts break down bone matrix, releasing stored minerals. Osteoblasts build NEW matrix instead, and osteocytes are mature cells that maintain existing matrix — neither breaks bone down.

Question 7

When blood calcium drops, the parathyroid gland's hormone (PTH) ultimately causes:

PTH activates osteoclasts, which break down bone matrix and release its stored calcium into the blood — restoring the normal range. Increasing osteoblast activity would pull calcium OUT of the blood instead, the opposite of what's needed.

Score: 0 / 7

Debrief

Week 7 Quick-Reference Recap

One page to glance at before Exam 2 — everything from this week, condensed.

Bone Functions

Support, protection, movement, mineral storage, blood cell production.

Bone Shapes

Long — femur, humerus
Short — carpals, tarsals
Flat — skull, ribs
Irregular — vertebrae

Long Bone Anatomy

Epiphysis (ends) + Diaphysis (shaft) + Medullary cavity + Periosteum (outer membrane).

Bone Tissue Types

Compact — dense outer shell
Spongy — open lattice, holds marrow

Bone Cells

Osteoblast — builds
Osteocyte — maintains
Osteoclast — breaks down

Calcium Homeostasis

Low blood Ca²⁺ → PTH released → osteoclasts release bone calcium → blood Ca²⁺ restored.

Key Terms This Week

OsteoblastOsteocyteOsteoclastDiaphysisEpiphysisPeriosteumMedullary CavityOsteonCompact BoneSpongy BoneCalcium Homeostasis

Next — Week 8: Exam 2 Synthesis. You'll pull Chapters 4–6 together, connect them to Lab 3 and Lab Exam 1, and build your final study sheet before the real exam.

Next: Week 8 →