Skeletal System Structure, Mineral Homeostasis (3B) - Bio & Biochem

1. Which option best describes the net effect of PTH on serum $\text{Ca}^{2+}$?

   Increases serum $\text{Ca}^{2+}$. PTH elevates blood calcium by promoting bone resorption, renal reabsorption, and vitamin D activation.

2. What are lacunae in bone tissue?

   Small cavities that house osteocytes. Lacunae embed osteocytes within the mineralized matrix, allowing them to sense mechanical stress.

3. What is the endosteum and where is it located?

   Cellular lining of medullary cavity and trabecular surfaces. The endosteum contains osteoprogenitor cells essential for bone remodeling and repair.

4. What are the two main hormones that directly regulate blood calcium homeostasis?

   Parathyroid hormone (PTH) and calcitonin. PTH and calcitonin maintain calcium balance by opposing effects on bone, kidney, and intestine.

5. What is the primary inorganic mineral in bone matrix?

   Hydroxyapatite (calcium phosphate). Hydroxyapatite crystals provide compressive strength and rigidity to the bone matrix.

6. What is the difference between red marrow and yellow marrow?

   Red: hematopoiesis; yellow: fat storage (can revert to red if needed). Red marrow produces blood cells, while yellow marrow stores fat but can convert for increased hematopoiesis.

7. What structure runs perpendicular to Haversian canals to connect osteons?

   Volkmann canals (perforating canals). Volkmann canals facilitate nutrient and waste exchange by linking Haversian systems transversely.

8. What is the net renal effect of PTH on phosphate reabsorption?

   Decreases phosphate reabsorption (increases phosphate excretion). PTH promotes phosphaturia to balance calcium mobilization and prevent precipitation in tissues.

9. Identify the bone remodeling cell activated by RANKL signaling from osteoblast lineage cells.

   Osteoclast (via RANK activation on osteoclast precursors). RANKL from osteoblasts binds RANK to differentiate precursors into mature osteoclasts for resorption.

10. What is the primary organic component of bone matrix?

    Type I collagen (osteoid). Type I collagen forms the flexible organic framework that is mineralized to create bone tissue.

11. What is endochondral ossification and which bones primarily use it?

    Bone replaces cartilage model; long bones. Endochondral ossification uses a cartilage template replaced by bone, enabling longitudinal growth in long bones.

12. What is the medullary cavity and what does it typically contain in adults?

    Central cavity of long bone; usually yellow (fatty) marrow in adults. The medullary cavity stores adipose tissue, serving as an energy reserve in adult long bones.

13. What is intramembranous ossification and which bones primarily use it?

    Bone forms from mesenchyme; flat bones (skull, clavicle). Intramembranous ossification directly converts mesenchymal tissue to bone, typical for flat bones like the skull.

14. Which option best describes the net effect of mechanical loading on bone mass?

    Increases bone formation and bone density (Wolff law). Mechanical stress stimulates osteoblast activity, leading to bone adaptation and increased density per Wolff's law.

15. What is the key structural difference between compact and spongy bone?

    Compact: osteons; spongy: trabeculae with marrow spaces. Compact bone's dense osteons provide strength, while spongy bone's trabeculae offer lightness and marrow space.

16. What is the functional significance of trabeculae in spongy bone?

    Reduce weight and align along lines of stress. Trabeculae optimize bone's strength-to-weight ratio by aligning with mechanical forces per Wolff's law.

17. What are the main functions of the skeletal system tested on the MCAT?

    Support, protection, movement, mineral storage, hematopoiesis. The skeletal system provides structural support, protects organs, enables mobility, stores minerals, and produces blood cells.

18. What is the periosteum and what is its key function?

    Outer fibrous covering; anchors tendons/ligaments and supports growth/repair. The periosteum nourishes bone via blood vessels and houses progenitor cells for repair and growth.

19. What is the epiphyseal (growth) plate and what happens to it after puberty?

    Hyaline cartilage growth zone; closes to epiphyseal line. The epiphyseal plate facilitates bone lengthening through chondrocyte proliferation until fusing post-puberty.

20. What is an osteon (Haversian system) in compact bone?

    Concentric lamellae around a central (Haversian) canal. Osteons form the structural units of compact bone, providing strength and housing vascular channels.

21. What are canaliculi in compact bone?

    Channels connecting lacunae for nutrient and signal exchange. Canaliculi enable osteocytes to communicate and exchange nutrients despite being encased in bone.

22. What is the active form of vitamin D that increases intestinal calcium absorption?

    Calcitriol ($1,25$-dihydroxyvitamin D). Calcitriol, activated in the kidney, enhances intestinal uptake of calcium and phosphate for homeostasis.

23. What is the primary mineral reservoir function of bone in homeostasis?

    Storage and release of $\text{Ca}^{2+}$ and phosphate as needed. Bone acts as a dynamic reservoir, mobilizing minerals via hormonal regulation to maintain blood levels.

24. Which option best describes the net effect of calcitonin on serum $\text{Ca}^{2+}$?

    Decreases serum $\text{Ca}^{2+}$. Calcitonin lowers blood calcium by inhibiting osteoclast activity and enhancing renal excretion.

25. What are the four major bone cell types and their primary roles?

    Osteoblast build; osteoclast resorb; osteocyte maintain; progenitor form. These cells collaborate in bone formation, resorption, maintenance, and regeneration to ensure skeletal integrity and remodeling.

Study these flashcards