Bone Cells + Hormones

What do osteoblasts do?

Build new bone matrix

What do osteoclasts do?

Break down bone

What do osteocytes do?

Maintain bone, live in lacunae, monitor minerals.

Which bone cells work together in remodeling?

Osteoblasts + osteoclasts — balance breakdown and rebuild.

If blood Ca²⁺ (calcium) drops… what happens?

• Blood calcium levels drop, the parathyroid glands release a hormone called parathyroid hormone (PTH).

• PTH tells osteoclasts—the cells that break down bone—to start working.

• As the bone breaks down, calcium is released into the bloodstream, which raises blood calcium levels back to normal.

If blood Ca²⁺ (calcium) rises… what happens?

• When blood calcium levels get too high, the thyroid gland releases a hormone called calcitonin.

• Calcitonin tells osteoblasts—the cells that build bone—to take calcium from the blood and store it in the bone.

• This lowers the amount of calcium in the bloodstream, bringing blood calcium levels back to normal.

What gland releases calcitonin?

Thyroid gland.

What gland releases PTH?

Parathyroid glands (behind thyroid).

How do osteoblasts respond to calcitonin?

They become more active — build bone.

How do osteoclasts respond to PTH?

They become more active — break bone.

Why does bone remodeling matter?

Keeps bone strong, adjusts to stress, maintains calcium levels.

What is hematopoiesis?

Blood cell production in red bone marrow.

What gives bone its strength?

Mineral salts (like calcium phosphate) in the matrix.

What gives bone its flexibility?

Collagen fibers in the matrix.

Describe compact bone.

Dense, solid, contains osteons, found in diaphysis (shaft).

Why is bone both strong and slightly flexible?

Minerals make it hard and strong; collagen makes it flexible so it doesn’t shatter.

What are the 5 classifications of bones by shape?

Long, Short, Flat, Irregular, and Sesamoid bones.

What is a long bone and give examples.

Longer than they are wide; act as levers for movement — femur, humerus, radius, ulna, tibia, fibula.

What is a short bone and give examples.

Cube-shaped for stability and support with limited motion — carpals (wrist), tarsals (ankle).

What is a flat bone and give examples.

Thin, flat, slightly curved; protect organs and provide surface for muscle attachment — skull bones, ribs, sternum, scapula.

What is an irregular bone and give examples.

Complex shapes that don’t fit other categories — vertebrae, pelvis, some facial bones.

What is a sesamoid bone and give examples.

Small, round bones that form in tendons to reduce friction — patella (kneecap).