Skeletal system

1. Skeletal system

2. Skeletal system • Skeletal system is an organ system • Functions to: • Support the body • Enable movement • Protect various organs • Store minerals • Store fat • Produce blood cells

3. Skeletal system • Important cell types: • Osteocyte: looks like a spider • Bone building cell • Makes the structure that the minerals will be deposited into • Osteoclast: looks like a broom • Bone digesting cell • Breaks down bone minerals to send them into the blood, or to remodel the bone as needed

4. Skeletal system • General bone structure: • Very thin outer layer of connective tissue • Periosteum (“peri” -“osteum”) • Helps to repair fractures, bone growth • Hard, dense outer layer “compact bone” • Inside, “spongy bone” layer • Connected to the outer compact bone layer • Acts like a honeycomb: strength without the weight • Inner layer of connective tissue • Endosteum (“endo”-“osteum”) • Middle = medullary cavity • Where red bone marrow (blood cell production) or yellow bone marrow (fat storage) is located

7. Hemopoiesis/hematopoiesis: development of blood cells (immune cells, platelets & red blood cells).

8. As an adult, the actual hemopoetic “bones” are the bones of the axial skeleton (the central “core” bones.

9. Skeletal system • Bone development: • Skull and collarbone (clavicle) are unique bones • Develop differently than the rest of your skeleton • “dermal bones” • During embryonic development, connective tissue condenses to form the shape of the skull bones and clavicle. • Bone growth (ossification…depositing minerals to make the bone “hard”) begins from the inside of these bones

10. Note the difference in the cranial bones vs. the bones of the arm

11. At birth, the bones of the skull are “partially ossified”. They have not yet fused into an intact skull. The larger regions between the individual skull bones are known as “fontanels”. These fontanels allow the skull to compress during passage through the birth canal.

12. Skeletal system • Bone development: • All other bones develop through a process called “endochondrial ossification” • Endo = inside • Chondrial = cartilage • Bones start as cartilage, are ossified (minerals deposited to make them “bone”) starting from the outside. • Initiation of the outer “collar” of bone triggers ossification of the inner cavity. • Bones ossify from the outside first

13. Remember that all the bones (other than the skull and clavicle) form first as cartilage

14. After birth, the middle of the bone is solid, and the ends are still cartilage. These ends then begin their own ossification. The “joint” between the middle and the ends is called the “growth plate”. This is where bones will grow in length.

15. The “epiphyseal plate” is the joint between the middle of the bone, and the ends. The epiphyseal plate & growth plate are the same thing. Eventually, the bone will stop growing in length, and the growth plate will fuse or ossify.

16. Skeletal system • Throughout your life, your bones will constantly remodel themselves. • Bone is not a “solid” substance • In a constant “flux” or dynamic state • Bone building cells constantly building to accommodate different stresses • Bone digesting cells constantly digesting to provide minerals to the rest of the body and remodel • Lighten bone where it doesn’t need weight or strength • In fact…bone isn’t the most “hard” substance in your body (your teeth are much more hard than bone).

17. Calcium homeostasis • Calcium homeostasis is reliant on the balance of bone calcium deposition and re-absorption (bone remodeling), as well as, • Dietary intake • Fecal loss • Urinary loss

18. Calcium homeostasis • 3 important hormones help control calcium homeostasis: • Calcitrol: derived from Vitamin D • Must be processed in a “metabolic pathway” • Keratinocytes in SKIN use UV from sunlight to convert a steroid to Vitamin D3 • Liver receives Vitamin D3 from skin and adds OH- group (this is now called “calcidiol”) • Kidney receives calcidiol and adds final OH- group • Now called calcitrol

19. Calcitrol processing follows a path from the skin-liver-kidneys (get your sun!)

20. Calcium homeostasis • 3 important hormones help control calcium homeostasis: • Calcitrol: derived from Vitamin D • Increases dietary calcium absorption • Increases bone calcium re-absorption • Reduces urinary calcium loss • Deficiency in calcitrol will lead to decreased blood calcium (hypocalcemia), and subsequent reduced bone deposition • Rickets in children, osteomalacia in adults

21. Calcium homeostasis • 3 important hormones help control calcium homeostasis: • Calcitrol: derived from Vitamin D • Should actually be called a “steroid hormone” because it acts as a hormone • Only called a “vitamin” because it is added or fortified to milk etc. to prevent rickets/osteomalacia

22. Calcium homeostasis • 3 important hormones help control calcium homeostasis: • Calcitonin: • Secreted by “C” cells (calcitonin cells) in the thyroid gland when blood calcium too high (hypercalcemia) to lower blood calcium levels • Inhibitsosteoclasts to reduce re-absorption (within 15 minutes, osteoclasts are inhibited by 70%) • Stimulates osteoblasts to promote mineralization

23. Calcium homeostasis • 3 important hormones help control calcium homeostasis: • Calcitonin: • Important in children, less important for adults • Osteoclasts in children are VERY active, but less so in adults (due to high rate of skeletal remodeling) • In adults, calcitonin cannot change blood calcium very well

24. Calcium homeostasis • 3 important hormones help control calcium homeostasis: • Parathyoid hormone (PTH): • Secreted by the parathyroid gland (dorsal the thyroid) in response to low blood calcium (hypocalcemia) • As little as 1% drop in blood calcium = double the rate of PTH secretion • Acts on osteoblasts • Reduces urinary calcium loss • Also stimulates kidney production of calcitrol • Inhibits osteoblasts from synthesizing more collagen (when collagen is laid down, minerals are deposited)

25. Calcium homeostasis • 3 important hormones help control calcium homeostasis: • Parathyoid hormone (PTH): • Interesting note: you can actually use PTH to STIMULATE bone deposition! • If you “pulse” PTH injections (give patient a large dose at once), you will actually stimulate bone deposition • If you provide PTH gradually, you inhibit bone deposition and stimulate bone re-absorption

26. Calcium homeostasis 3 important hormones help control calcium homeostasis: Calcitrol (Vitamin D) = increase blood calcium Increase calcium absorption from foods Increase calcium digestion from bone Parathyroid hormone = increase blood calcium Increases bone digestion by osteoclasts Calcitonin = decrease blood calcium Inhibits osteoclasts from digesting bone and releasing calcium into the blood

27. Skeletal system • When the remodeling process is unbalanced (homeostasis is hampered), often suffer osteoporosis. • Bone growth is influenced by various hormones • If you need more bone growth (stronger bone, growth as a child), various hormones are produced • Testosterone, estrogen, Vitamin D • During menopause, estrogen production decreases • Estrogen controls the osteoclasts…without proper controls, the osteoclasts digest out of control

28. Osteoporosis • Osteo = bone porosis = pores (holes in bone) • Due to imbalance between bone digestion & bone building • Generally related to the 3 bone “hormones” we just covered (estrogen also plays an important role in controlling the osteoclasts…the digesting cells)

29. Skeletal fractures • A broken bone = “fracture” in clinical terms • Different types of fractures

30. Fractures & healing

31. Regions of the skeleton • Skeleton is often divided into 2 regions: • Axial (head, torso, abdomen/pelvis) • Appendicular (appendages…arms & legs) Remember which bones contain red bone marrow and which ones contain yellow bone marrow?

32. Axial skeleton: • Skull • Vertebrae • Rib cage

33. Appendicular skeleton: • “Pectoral girdle” • Collarbone (clavicle) • Shoulder blade (scapula) • Arm/brachium (humerus) • Forearm/antebrachium (radius & ulna) • Hand • “Pelvic girdle” • Pelvic bone (ossa coxae) • Thigh (femur) • Shin/leg (tibia & fibula) • Foot

34. Joints • Where 2 or more bones form a joint, those bones are held together by ligaments • Ligaments hold bones together • Tendons hold muscle to bone • “Sprain” = stretched or damaged ligament • NOT damage to tendon

35. Joints • Your knee is a very complicated joint that is under immense stresses. • Held together by at least 4 main ligaments, and at least 6 others that “help” indirectly • Also helped by muscles of the leg

36. Joints • Bones generally do not contact other bones directly • Where bones “join” = joint • Usually lined with a form of cartilage • Fibrocartilage = cartilage with long structural proteins (fibers) • Between each vertebrae (intervertebral disc) • Between each pelvic bone (pubic symphysis) • Hyaline/articular cartilage = “normal” cartilage • Generally clear (no microscopic “fibers”)

37. Joints Hyaline cartilage vs. fibrocartilage

38. Cartilage • Important facts about cartilage. • Apart from your collarbone and skull, every bone in your body will have originated from cartilage. • The “clear” area is actually various proteins. • Small enough that they’re clear (elastic cartilage has long elastic fibrous proteins) • The “lubricant” = other proteins & fluid (similar to blood plasma)

39. Cartilage • Cartilage “made” by chondrocyte cell. • “solitary” cell type • Will make enough cartilage “proteins” to surround themselves • Essentially cuts them off from blood, oxygen etc. • Have a very low metabolic rate (almost in a cell “coma”)

41. Joint pathophysiology • Since cartilage is made by chondrocytes, and these cells are “solitary” and practically cut off from blood (oxygen & nutrients), damage to cartilage is often difficult to repair • If anything, cartilage is more likely to turn into bone (like it did during development) rather than replace the damaged cartilage

42. Joint pathophysiology • Arthritis: • Two forms, both of which “damage” joints • Osteoarthritis: “wear and tear” erosion of the cartilage • Results in bone-bone contact, intense pain • Rheumatoid arthritis: autoimmune-based inflammation of the joint • Immune system attacks joint • Arthritis meds are in the top 4 $$$ pharmaceuticals prescribed in the world

43. Joint pathophysiology • Rheumatoid Arthritis: • “rheumatism” is an old term, gradually being phased out • Rheumatic disease = chronic inflammation of joint, joint cavity, ligaments, tendons, muscles.

44. Rheumatoid/inflammatory arthritis: swelling within joint capsules often results in disarticulated joints. Osteoarthritis: gradual degradation of the articular cartilage facets.