Skeletal Homeostasis and Hindlimb Suspension Lecture Flashcards

A set of vocabulary flashcards covering the biological mechanisms of bone remodeling, key signaling pathways, and the hindlimb suspension model for studying disuse-driven bone loss.

Skeletal homeostasis

The process by which bone is constantly renewed to remain strong, repair microdamage, regulate minerals, and adapt to loading through a balance of resorption and formation.

Osteoclasts

Large multinucleated cells derived from the macrophage/monocyte lineage responsible for breaking down bone mineral and collagen.

Osteoblasts

Bone-forming cells derived from mesenchymal stem cells that produce osteoid, which is primarily composed of type I collagen.

Osteocytes

Former osteoblasts trapped in mineralized bone that act as a mechanosensing control center by detecting loading, fluid flow, and microdamage.

Bone lining cells

Quiet cells located on bone surfaces that participate in the regulation of remodeling activation.

Coupled remodeling

A remodeling cycle where bone resorption and bone formation are linked through a sequence of activation, resorption, reversal, formation, and termination.

RANKL

A signal produced by osteoblast-lineage cells and osteocytes that binds to RANK on osteoclast precursors to promote osteoclast formation.

OPG

A decoy receptor that blocks the binding of RANKL to RANK, thereby inhibiting bone resorption.

Wnt/$\beta$-catenin signaling

A signaling pathway that promotes osteoblast activity and the formation of new bone.

Sclerostin

A protein produced mainly by osteocytes that blocks Wnt signaling, leading to decreased osteoblast formation and bone formation.

Hindlimb suspension

A rodent model used to simulate microgravity or disuse by suspending the animal's tail so the hindlimbs do not bear weight.

Trabecular bone

Spongy bone found near joints and inside vertebrae that remodels quickly and is significantly affected by unloading.

Cortical bone

The dense outer layer of bone which changes more slowly than trabecular bone but can experience thinning or increased porosity during disuse.

Bone mineral density (BMD)

A measurement of bone mineral content; typically decreases in the femur and tibia during hindlimb unloading.

microCT

A 3D imaging technique used to measure bone structures such as $BV/TV$, trabecular thickness, and cortical thickness.

Histomorphometry

A microscope-based analysis of bone used to measure surfaces of osteoblasts and osteoclasts as well as bone formation rates.

TRAP staining

A staining technique specifically used to identify and visualize osteoclasts.

P1NP

A serum marker used to measure bone formation activity.

CTX

A serum marker used to measure bone resorption activity.

Bone modeling

The process of changing the shape or structure of bone, often during growth or adaptation to loads.

Uncoupling

A state where the balance between bone resorption and bone formation is disrupted, leading to an imbalance in bone mass.

Disuse osteoporosis

Bone loss resulting from a lack of mechanical loading on the skeletal system.

Osteopenia

A condition characterized by low bone mass that is less severe than osteoporosis.

$$BV/TV$$

Bone volume divided by total volume; a major metric used to evaluate trabecular bone structure via microCT.