bones notes.docx

Objective 1: List and discuss the six functions of bone tissue. What are the components of skeletal system? Bones – dense connective tissues - majority of skeletal system – mainly providing hardness and strength Cartilage – semi-rigid connective tissues – mainly provide flexibility or smooth surf...

Objective 1: List and discuss the six functions of bone tissue. What are the components of skeletal system? Bones – dense connective tissues - majority of skeletal system – mainly providing hardness and strength Cartilage – semi-rigid connective tissues – mainly provide flexibility or smooth surfaces for bones to move against each other, reduce friction, and absorb shock What are the functions of skeletal system? Support: structural framework for body by and support posture and position, support soft organs, provide attachment sites for muscles Protect: protect internal organs Hemopoiesis/ red blood cell production: the red marrow within the bones is the site of the blood cell formation Mineral homeostasis: Bones store minerals such as calcium and phosphorus within their matrix and these minerals are important for cardiac, muscles, nervous, enzyme and blood physiology so by storing and releasing them only when needed, bone contributes to maintenance of homeostasis. Fat/triglyceride storage: the yellow marrow of adult bones contains adipose tissues and adipocytes that store fat/triglyceride as a source of energy for the body. Assistance in generating movement: bones not only act as levers and provide attachment surfaces for skeletal muscles and support them, they also transmit the force created by contraction of skeletal muscles and assist them to generate movements. Objective 2: Classify bones on the basis of their shape and location. How are bones classified to 5 groups? Based on their shape They could also be classified based on their function: Axial: provide protection to vital organs Appendicular: enable and engage movements – limbs Type of the bone Characteristics Function Example Long bones - longer than their wide - long cylindrical shaft extending from one end to the other Act as levers for muscles and are moved when muscles contract Humerus, femur Short bones Cube shaped, all sides same length Support Stability small range of movement Tarsals, carpals Flat bones Thin and curved Attachment surface for muscles protection of internal organs Cranial bones of skull, ribs Irregular bones Complex shaped -not long, short or flat Protect- ion of internal organs Facial bones, vertebrae Sesamoid Small and round embedded in tendons Protect and Support tendons to overcome compressive forces Patellae Objective 3: List and discuss the major features of a long bone, including the following: diaphysis, epiphyses, metaphyses, epiphyseal line, periosteum, endosteum, medullary cavity, yellow marrow, red marrow, and articular cartilage What are the major features/ structures of long bones? Diaphysis: the long cylindrical tube like shaft that runs from proximal to distal end of the bone is called diaphysis. The hollow internal region of this part of the long bones is filled with yellow marrow and their walls are made of hard compact osseous tissues Medullary cavity: the hollow region within the diaphysis that is filled with yellow marrow is called medullary cavity Epiphysis: the wide regions at proximal and distal ends of the long bone are called epiphysis. They are filled with spongy osseous tissues and red marrow fills the holes in the spongy bone Metaphysis: the narrow region that epiphysis and diaphysis meet. This region differs if it is a growing bone or not In a growing bone, there is a layer made of hyaline cartilage in the metaphysis that allows the longitudinal growth of the bone and it is called growth or epiphyseal plate In mature bone, the hyaline cartilage of epiphyseal plate has been replaced by osseous tissue and there is only a remnant left of the growth plate which is now called epiphyseal line Endosteum: lining the medullary cavity is endosteum and it consists of a flattened layer of osteogenic cells . This layer is active meaning that it is a site of growth, repair and replacement. Endosteum also covers the trabeculae of spongy bones and also lines the central canal of osteons in compact bones Periosteum: fibrous connective tissue membrane covering most of the outer surface of bones except within the joint cavities and separate them from surrounding tissues. They contain blood and lymph vessels as well as nerves to provide nutrients to osteogenic cells, it protects the bone, provide an attachment surface for ligaments and tendons as their fibrous layer intertwine with those of tendons, and also assists in bone growth and fracture repair. Marrow cavity: space within the diaphysis Red bone marrow: fills the spaces in the spongy bone of epiphyses and it is the site of hematopoiesis Yellow bone marrow: fills the medullary cavity of the diaphysis and it is where bones store fat as a source of energy Articular cartilage: a thin layer of hyaline cartilage that covers the surfaces of epiphyses of bones that meet to form joints to absorb shock and reduce friction. Periosteum and Endosteum. The periosteum forms the outer surface of bone, and the endosteum lines the medullary cavity. Name Characteristics Diaphysis Tubular shaft surrounds medullary cavity filled with yellow marrow walls are made of compact bone Epiphyses Wide ends filled with spongy bone and red marrow Metaphysis Connecting region between epiphysis and diaphysis contains the epiphyseal plate/ line Endosteum membranous lining of the medullary cavity consist of flattened layer of osteogenic cells sites of growth, repair, and replacement Also covers trabeculae of spongy bone and central canal of osteons Periosteum Fibrous connective tissue membrane covering surface of bone except at joint cavities. Protect attachment for tendons and ligaments blood, lymph, and nerves to nourish osteogenic cells. involve in repair and growth Yellow marrow Adipose tissue filling the medullary cavity and stores fat Red marrow Filling the spaces of spongy bone and blood cell formation Articular cartilage Hyaline cartilage covering the epiphysis to reduce friction and absorb shock Objective 4: Describe the microscopic structure of bone. Bones - connective tissue support Cells are separated from each other by large amounts of matrix that is made of water collagen fibers: provide structural framework for inorganic salts to deposit on and attach to. Their function is to give tensile strength and flexibility so bones are not brittle inorganic salt crystals that provide hardness to the bone. The main salts are hydroxyapatite, as well as some calcium carbonate and calcium phosphate other minerals such as fluoride, magnesium hydroxide and sulfate calcification also known as mineralization is the process during which these salts are deposited on a collagen framework and this process only occurs when there collagen fiber present. What types of cells are found embedded within the matrix? Osteoprogenitor/ osteogenic cells: derived from mesenchymal cells, undifferentiated, stem cells, mitotically active, form osteoblasts, found in deeper layers of periosteum and marrow. Osteoblasts: Derived from osteogenic cells, these form the bones through osteogenesis. These cells synthesize and secrete the collagen framework and mineral/calcium salts that undergo calcification and make the calcified matrix of bones. Therefore, these are found in growing regions of bone including both endosteum and periosteum. Osteocytes: mature bone cells. These are derived from osteoblasts that get trapped within the calcified matrix and underwent structural and functional changes. These are responsible in maintaining the bone matrix and mineral concentrations by releasing enzymes. These are found all over the matrix in pockets called lacunae. Osteoclasts: derived from monocytes and macrophages and are multinucleated. These are responsible for bone resorption. They are found on bone surfaces as well as the damaged, old and unneeded regions of bone. They breakdown the osseous tissues. Four types of cells are found in bone tissue. Osteogenic cells are undifferentiated and develop into osteoblasts. When osteoblasts get trapped within the calcified matrix, their structure and function changes, and they become osteocytes. Osteoclasts develop from monocytes and macrophages and differ in appearance from other bOne cells. Bones have a dynamic nature meaning that new bone tissues are formed while older and damaged bone tissues get dissolved for repair or to release calcium - this explains how the shapes of the bones are adjusted to accommodate stress and also how they contribute to mineral homeostasis - so they undergo constant but subtle reshaping Calcification/ mineralization: process during which inorganic salt crystals hold onto and get deposited on to the framework composed of ONLY collagen fibers Osteocytes located in the lacunae communicate with each other and receive nutrients using long cytoplasmic processes that extend through the channels within the bone matrix that are called canaliculi . Most bones have both spongy and compact osseous tissues but depending on their function, their distribution and concentrations differ. Type of osseous tissue Characteristics Location Major functions Compact Denser and stronger Arranged in units called osteons/ haversian system Under periosteum diaphysis of long bones Tolerate compressive forces. Tolerate force of weight Provide support, protection, and strength Spongy Less dense and heavy Softer and weaker but more flexible Made of trabeculae Epiphyses of long bones (where harder compact bone surrounds it) Structure of most short, flat, irregular bones Reduce the weight of overall bone so they can be moved by muscles. Handle forces from multiple direction and provide strength Surround and protect red marrow so they can make blood cells Objective 5: Discuss the Haversian system as the structural unit of compact bone by using the following terms: osteocytes, lacunae, lamellae, Haversian canal, blood vessels, bone matrix, and canaliculi. Compact bone is arranged in units called osteons or Haversian systems. Osteons contain blood vessels, lymphatic vessels, nerves, and osteocytes, along with the calcified matrix. Osteons are aligned in the same direction along the lines of stress. These lines can change as the stresses on the bone change. Haversian System: Structural units of compact bones Run parallel to long axis of the bone which is oriented along the lines of stress - resist bending and fracturing - so found more in areas where stresses are applied in only a few directions Contain blood vessels, lymph vessels, nerves, osteocytes and calcified matrix 4 main component: Lamellae: concentric rings of calcified matrix that surround a central canal Central/haversian canal: canals surrounded by lamellae running down the center of each osteon unit containing nerves, blood vessels and lymph vessels. Osteocytes and lacunae: located on edges of adjacent lamellae are tiny spaces that mature osteocytes reside in them. The osteocytes have long cytoplasmic processes extending out from them. Canaliculi: tiny passageways between the lamellae that branch out of the central canal and connect to each lacuna. The cytoplasmic processes of osteocytes extend through these channels and connect them to each other as well as to the central canal and these channels provide a pathway for exchange of nutrients, wastes and also allow communication between the osteocytes. Perforating/ volkmann’s canal - perpendicular to long axis of bone Interconnect haversian units Connect them to the surface of the bone – periosteum Carry blood vessels and nerves for exchange of nutrients, wastes and signaling molecules Objective 6: Discuss spongy bone in terms of trabeculae. Spongy (cancellous) bone does not contain osteons. It consists of trabeculae (columns or beams of bone), which surround many red-marrow-filled spaces. It forms most of the structure of short, flat, and irregular bones; and the epiphyses of long bones. Spongy bone tissue is light, and supports and protects the red bone marrow. Calcified matrix, osteocytes and lacunae create bony spicules or spikes called trabeculae and these make a lattice and porous like network that give them a spongy appearance and house the red marrow These trabeculae are formed along the lines of stress and allow them to handle forces from multiple direction and give them strength. Blood vessels travel through compact bone and reach to the spongy bone to provide supplies needed for creating blood cells The osteocytes that located close to blood vessels get nutrients and release waste through canaliculi which are tiny interconnected channel found on the surface of trabeculae Spongy bone can be converted to compact bone by the actions of osteoblasts Spongy bone is composed of trabeculae that contain the osteocytes. Red marrow fills the spaces in some bones. Objective 7: Define the term ossification. Ossification/ osteogenesis: process of bone formation and it initiation is with mesenchymal cells forming a template for later ossification begins in embryo where initially an embryonic skeleton is made as a template This embryonic skeleton is made of fibrous connective tissue membranes and hyaline cartilage Template then undergoes either endochondral or membranous ossification to form bony tissues Objective 8: Distinguish between intramembranous and endochondral ossification, and denote which parts of the skeleton are formed by each. Intramembranous ossification : Flat bones of skull, mandible and clavicle During this process, bones are formed directly from or within a fibrous connective tissue membrane. This happens as undifferentiated mesenchymal cells within the fibrous membrane directly transform into osseous bone tissue without forming an intermediate template. Endochondral ossification – most bones – bones form and replace a hyaline cartilage model (so bone is a replacement tissue Objective 9: Discuss the steps involved in intramembranous ossification What are the steps of intramembranous ossification? i. clustering of mesenchymal cells and formation of ossification center: an ossification center forms within the fibrous connective tissue membrane when groups of mesenchymal cells cluster together and differentiate into osteoblasts. ii. secretion of osteoid by osteoblasts: osteoblasts begin secreting an unmineralized matrix known as osteoid, composed of collagen fibers. mineralization occurs as calcium and other mineral salts are deposited onto the osteoid, causing the matrix to harden and become calcified. osteoblasts trapped within this matrix undergo structural and functional changes, eventually maturing into osteocytes. iii. calcification process and formation of trabeculae: during the calcification process, osteoblasts become trapped within the osteoid matrix, undergo structural and functional transformations, and mature into osteocytes. simultaneously, the osteoid is secreted around blood capillaries, forming a random mesh-like network of bony spicules or trabeculae. meanwhile, osteoblasts derived from mesenchymal cells outside of the ossification centers form a fibrous periosteum. iv. formation of compact bone and condensation of blood vessels into red marrow: a bone collar of compact bone develops beneath the periosteum as the trabeculae beneath it thicken and eventually get replaced by organized lamellar bone. this process creates a protective layer over the internal porous trabecular network of spongy bone. finally, the crowded capillaries and blood vessels condense to form red bone marrow. At birth, skull and clavicles are not fully ossified - to allow some deformation during passage through the birth canal Last bones to ossify by intramembranous ossification are the flat bones of face when they reach their adult size at the end of adolescent. Summarized steps of Intramembranous Ossification. Intramembranous ossification follows four steps. (a) Mesenchymal cells group into clusters, and ossification centres form. (b) Secreted osteoid traps osteoblasts, which then become osteocytes. (c) Trabecular matrix and periosteum form. (d) Compact bone develops superficial to the trabecular bone, and crowded blood vessels condense into red marrow. Objective 10: Discuss the steps involved in endochondral ossification most bones (below the face except the clavicles) - mesenchymal cells first form the hyaline cartilage model that eventually gets replaces by bone and the replacement begins in the middle (diaphysis) and then follows in ends (epiphyses) Cartilage Templates: Made as a template to determine where bones should form Flexible network of hyaline cartilage Sami-rigid matrix composed by chondroblasts and is made of hyaluronic acid, water, chondroitin sulfate and collagen fibers By the time of birth, osseous tissues have replaced them completely BONE IS THE REPLACEMENT TISSUE Endochondral ossification – replacement of the hyaline cartilage template by bones - hyaline cartilage grow, gets destroyed and then replaced with bone tissue. Bone grows in length due to growth at the epiphyseal plate and grows in thickness by appositional growth during which its diameter increases. When the epiphyseal plate closes, it is replaced by bone; the epiphyseal line appears, which indicates that the bone has completed its growth in length. Bones get wider due to two types of growth: one happening on the outside and the other on the inside. Osteoblasts add new bone material on the outer surface, while osteoclasts, though not as much, break down bone from the inside increasing the diameter of the medullary cavity. Steps of endochondral ossification: 1. Differentiation of Mesenchymal Cells into Chondrocytes: Mesenchymal cells undergo differentiation, transforming into chondrocytes. 2. Formation of Hyaline Cartilage Model and Development of the Perichondrium: A hyaline cartilage template for the future skeleton forms. This template is covered by a membrane known as the perichondrium. 3. Enlargement of Central Chondrocytes and Disintegration of Cartilage Matrix as Mineralization Continues: In the central region of the growing cartilaginous model, chondrocytes enlarge and eventually die due to the loss of nutrient pathways caused by the calcification of the surrounding cartilage matrix. This process leads to the formation of cavities within the cartilage matrix. 4. Penetration of Capillaries into the Cartilage, Replacement of Perichondrium with Periosteum, Formation of the Periosteal Collar, and Establishment of the Primary Ossification Center: Blood vessels extend around the edges of the cartilage model, causing the perichondrium to transform into osteoblast-producing tissue and become the periosteum. Subsequently, a periosteal collar, composed of compact bone, forms around the shaft of the cartilage model. Capillaries and blood vessels penetrate the calcified cartilage matrix from the periosteum, bringing osteogenic cells with them. These cells differentiate into osteoblasts, replacing the disintegrated cartilage matrix with osseous tissue. This process is referred to as the establishment of the primary ossification center in the bone. 5. Continued Growth of Cartilage and Chondrocytes at the Ends of the Bone: Bone formation and ossification progress from the primary ossification center toward both ends of the cartilage model. Simultaneously, the cavities within the cartilage combine to form the medullary cavity. 6. Formation of Secondary Ossification Centers: As cartilage is replaced by bone in the diaphysis (shaft), capillaries and osteoblasts penetrate into the epiphyses (ends) of the bone, forming secondary ossification centers which lay down spongy bone as they get infiltrated by blood vessels and eventually fill the epiphyses with spongy bone. 7. Epiphyseal Plate and Articular Cartilage: A narrow cartilaginous region remains between the epiphyses and diaphysis at the metaphysis, known as the epiphyseal plate. On the diaphyseal side of the growth plate, osteoblasts continuously replace cartilage with bone, while on the epiphyseal side, new cartilage is produced at a similar rate. Another layer of cartilage, known as articular cartilage, covers the bone surfaces at joint areas. 8. Epiphyseal Closure During Puberty: At the time of puberty, the rate of epiphyseal plate formation decreases due to a decrease in the proliferation of chondrocytes, while the rate of osteoblast activity to replace cartilage tissue increases. This results in the gradual narrowing and eventual disappearance of the epiphyseal plate, known as epiphyseal closure. By the end of this process, only small remnants of the plate, called the epiphyseal line, remain. Summarized steps of Endochondral Ossification. Endochondral ossification follows five steps. (a) Mesenchymal cells differentiate into chondrocytes. (b) The cartilage model of the future bony skeleton and the perichondrium form. (c) Capillaries penetrate cartilage. Perichondrium transforms into periosteum. Periosteal collar develops. Primary ossification centre develops. (d) Cartilage and chondrocytes continue to grow at ends of the bone. (e) Secondary ossification centres develop. (f) Cartilage remains at epiphyseal (growth) plate and at joint surface as articular cartilage. Objective 11: Discuss bone growth and remodeling. Bone growth: Longitudinal: growth at the epiphyseal plate of immature bones where they undergo ossification and a epiphyseal line remains when bones become mature Epiphyseal plate: a layer of hyaline cartilage within the metaphysis. It has two sides: Epiphyseal side: still cartilage tissue is being formed Diaphyseal side: cartilage tissues all replaced by osseous tissues The 4 zones of epiphyseal plate: Reserve zone - matrix production Closest to epiphyses Not involved in growth Small chondrocytes within matrix – responsible for securing epiphyseal plate to osseous tissues of epiphyses Proliferative zone - mitosis zone Stacks of larger chondrocytes Continuously undergo mitosis to replace the chondrocytes that disintegrate at the diaphysis end – continuously grow the template to be ossified and grow in length Maturation and hypertrophy zone - accumulation of minerals and matrix calcification Chondrocytes continue growing in size and become older as they get pushed out towards the diaphysis This is the stage where there is accumulation of minerals and matrix continues to calcify So the template is constantly formed by the cellular division at proliferative zone and these templates as they get pushed towards the diaphysis, become more mature and larger and ready to get ossified and be added to the diaphysis. So the action of these two layers result in the growth of bone. Calcified matrix – cell death Closest to diaphysis Most calcified out of all zones so majority of chondrocytes have been disintegrated due to calcification Capillaries and blood vessels carrying osteoblasts from the diaphysis penetrate this zone The osteoblasts replace the remaining cartilage template with osseous tissues that are added to diaphysis and lead to their longitudinal growth. Calcified matrix - connecting layer between the epiphyseal plate and the diaphysis Final stages: Bone growth continues till early adulthood and at that time the longitudinal growth stops because the chondrocytes in the growth plate reduce and their proliferation ceases so new templates for ossification do not constantly get made anymore. The last remaining cartilage tissue is instead replaced by osseous tissues of bone and only small residues of the plate remain, forming the epiphyseal line in mature and adult bones. Longitudinal Bone Growth The epiphyseal plate is responsible for longitudinal bone growth. Appositional growth/ modeling – growth in width that occurs at the perichondrium.Growth in diameter is called appositional growth as well as modeling of bones and happens even after longitudinal growth has stopped but primarily happens during bone growth. The old bone tissue lining the medullary cavity are broken down and resorbed by osteoclast cells which leads to increase in the diameter of the medullary cavity. At the same time, underneath the periosteum, new bone tissues are produced and deposited via intramembranous ossification by osteoblasts which increases the diameter of the osseous tissue of diaphysis The combined action of these cells leads to increase in the width of the bone and it is called modelling. What is bone remodeling? Bone is a dynamic tissue constantly being broken down and replaced by a process called remodeling. In bone remodeling, cells work on the same surface instead of two different surfaces like bone modeling in which bone matrix lining the medullary cavity is resorbed by osteoclasts, while new bone is deposited beneath the periosteum by osteoblasts. Bone Remodeling which happens in adults life is a process in which old, damaged bones are resorbed by osteoclasts and osteoblasts form replacement bones and deposit them on that exact surface. So it is just a process of replacing an old and damaged tissue by a new tissue. Annually 5-10% of old bones are replaced with new bones. But remodeling could also happen due to injury, exercise and et.c Objective 12: Discuss the nutrients and hormones involved in bone growth and remodeling Large concentrations of calcium, phosphorus, and magnesium, and smaller amounts of other minerals, are needed for bone growth. Vitamins A, B12, C, D, and K each have a role in bone growth. During childhood, the most important hormones for the stimulation of bone growth are the insulin-like growth factors (IGFs), which are stimulated by the human growth hormone (HGH). Thyroid hormones, insulin, and calcitonin also are necessary hormones for bone growth. At puberty, the sex hormones, estrogen and testosterone, stimulate sudden growth and modifications of the skeleton to create the male and female forms. Factors Affecting Bone Growth and remodeling: Nutrients: Nutrient Function Calcium in large concentration Needed to form calcium phosphate and calcium carbonate which then combine and form the hydroxyapatite crystals which are the major crystals of bones and give them hardness. Calcium should be picked up from diet. Fluoride in large concentration Major structural component. It moves and replaces the hydroxyl group from the hydroxyapatite crystals and forms fluorapatite which help stabilize and strengthen the bone minerals. Fluoride can also enter the spaces between hydroxyapatite crystals, increasing their density. Magnesium in large concentration Major structural component Omega 3 fatty acids Reduce inflammations which may interfere with osteoblast functions Vitamin D Is needed for calcium absorption Vitamin K Makes proteins that help bone mineralization and also may have cooperative effects with Vitamin D. Vitamin A, B12, C Assist in bone growth Osteoporosis is a disease characterized by a decrease in bone mass that occurs when the rate of bone resorption exceeds the rate of bone formation, a common occurrence as the body ages Hormones: chemical messengers secreted by endocrine system also interact with the skeletal system and control bone growth, maintenance of created bones, and remodeling them. They are divided into to groups based on the type of cells they interact with and the results they aim for. Hormones that influence osteoblasts - bone growth and maintenance of bone matrix Growth hormones from pituitary glands: Increase chondrocyte proliferation at epiphyseal plate - increase length of long bones Increase osteoblast activities - increase and improve bone density Increase calcium retention -- enhance mineralization Insulin like growth factors - stimulated by growth hormones - stimulation of bone growth during childhood Thyroxine from thyroid glands: Stimulate osteoblast activity - bone growth and matrix synthesis Sex hormones - during puberty from gonads Stimulate osteoblasts and cause sudden growth Stimulate the conversion of epiphyseal plate to epiphyseal line Modifications of skeleton to promote male and female forms Calcitriol – kindeys Stimulate intestinal absorption of calcium and phosphate Hormones That Influence Osteoclasts - Bone modeling and remodeling Parathyroid hormones – parathyroid glands Stimulate osteoclasts activity and proliferation – breakdown bones and release calcium into blood Stimulate calcium reabsorption in kidney tubules Indirectly increase intestinal calcium absorption by increasing vitamin D synthesis Calcitonin – thyroid glands Inhibit osteoclasts Stimulate uptake of calcium by bones – blood level calcium drops Objective 13: Fully discuss the negative feedback mechanisms involved in blood calcium (Ca++) homeostasis, and explain how this is related to bone remodeling. Bone is the major reservoir for calcium ions (Ca2+) in the body; the blood level calcium ions (Ca2+) are closely regulated due to the importance of calcium in cardiac, nerve, enzyme, and blood physiology. The parathyroid hormone (PTH) is an important hormone that regulates the Ca2 + exchange between bone and blood. It is secreted by the parathyroid gland, and increases blood calcium ion levels. Another hormone that contributes to the homeostasis of blood Ca2+ is calcitonin (CT). It is secreted by the thyroid gland and decreases blood Ca2 + levels. Appropriate blood level calcium ions is critical for many physiological processes including muscle contraction, nerve system, enzymatic activities, cardiac system and also blood physiology. Parathyroid hormone (PTH) secreted from parathyroid gland is an important hormone that controls the exchange of calcium ions between bones and blood. It increases blood calcium ion levels by: Stimulate osteoclasts activity and proliferation – breakdown bones and release calcium into blood Stimulate calcium reabsorption from urine in kidney tubules Indirectly increase intestinal calcium absorption by increasing vitamin D synthesis Calcitonin (CT) secreted from thyroid glands also contributes to the homeostasis of blood calcium ion levels by: Inhibit osteoclasts Stimulate uptake of calcium ions by bones – which decreases the blood calcium levels Calcium Homeostasis The body regulates calcium homeostasis with two pathways; one is signaled to turn on when blood calcium levels drop below normal, and one is the pathway that is signaled to turn on when blood calcium levels are elevated. one activates when calcium in blood is below the normal range – calcium do not bind to their receptors in the parathyroid gland so the cells release the PTH hormone which in turn : activates osteoclasts to breakdown bone stimulate calcium reabsorption from urine in kidneys stimulate intestinal absorption of calcium when there is enough calcium to bind to the receptors on the cells of the PT glands, the system turns off one activates when calcium in blood is above normal range – thyroid gland secretes calcitonin which in turn: inhibit osteoclast activities increase calcium uptake by bones decrease calcium reabsorption in kidneys when blood levels of calcium are too high – stored in bones when blood levels are low – bone resorption blood level calcium is regulated by calcitonin, parathyroid hormone, vitamin D Objective 14: Classify the bones of the body into the axial and appendicular divisions. The two divisions of the skeletal system are: Axial – bones that form the vertical central axis of the body including the bones of the head, neck, chest and back - skull, vertebral column and thoracic cage provide protection for brain, spinal cord, heart and lungs attachment site for muscles that move the head, neck and back attachment site for muscles that act across the hip and shoulder joints to move the limbs Appendicular – bones of upper and lower limbs as well as the bones that attach each limb to the axial skeleton Axial and Appendicular Skeleton. The axial skeleton supports the head, neck, back, and chest and thus forms the vertical axis of the body. It consists of the skull, vertebral column (including the sacrum and coccyx), and the thoracic cage, formed by the ribs and sternum. The appendicular skeleton is made up of all bones of the upper and lower limbs.