Anatomy & Physiology Chapter 6

Appendicular Skeleton

Includes all bones of the upper and lower limbs, plus the bones that attach each limb to the axial skeleton

Compact Bone

Microscopic structural unit of compact bone is called an OSTEON or HAVERSIAN SYSTEM. Composed of concentric rings of calcified matrix called LAMELLAE. -Bad at dealing with pressure from the side, most fractures going through the bone, the stress is going through the shaft. -Injuries in the hips of humerus or femur (SPONGY) occur because they are fine dealing with that pressure.

How Bones Grow in Length

The epiphyseal plate is the area of growth in a long bone. On the epiphyseal side of the epiphyseal plate, cartilage is formed. On the diaphyseal side, cartilage is ossified, and the diaphysis grows in length.

2. Osteoblasts Blast means GENERATOR

(forms bone matrix): responsible for forming new bone. Found in the growing portions of bone, including the periosteum and endosteum. Do not divide but synthesize and secrete the collagen matrix and calcium salts.

3. Osteocytes

(maintains bone tissue): primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in space called a lacuna surrounded by bone tissue. Osteocytes maintain the mineral composition of the matrix via the secretion of enzymes. Can communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi (singular = canaliculus), channels within the bone matrix Osteocytes receive nutrients and eliminate wastes through blood vessels in the compact bone. Blood vessels in the periosteum and endosteum supply blood to blood vessels in the central canals. Nutrients leave the blood vessels of the central canals and diffuse to the osteocytes through the canaliculi.

4. Osteoclasts

(resorbs bone): responsible for bone resorption, or breakdown. Continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone.

1. Osteogenic Genic means MOTHER

(stem cell): undifferentiated with high mitotic activity cells. The only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. They differentiate and develop into osteoblasts.

Intramembranous Ossification

-Fibrous connective tissue = membrane bones. Are the frame (template) used to develop bones. -Bone cells develop directly from sheets of mesenchymal (undifferentiated) connective tissue. -The flat bones of the face, most of the cranial bones, and the clavicles (collarbones) are formed this way. -Process begins when mesenychmal cells in the embryonic skeleton gather together and begin to differentiate into OSTEOBLASTS in a cluster called the OSSIFICATION CENTER.

Basic Differences/Locations of these Bone Cells

Osteogenic: develop into osteoblasts, found in deep layers of the periosteum and the marrow. Osteoblasts: bone formation, found in growing portions of bone, including periosteum and endosteum. Osteocytes: maintain mineral concentration of matrix. Entrapped in matrix. Osteoclasts: bone resorption. Found in bone surfaces and at sites of old, injured, or unneeded bone.

3. Membrane (periosteum)

Outer surface of the bone is covered with a fibrous membrane called the periosteum (peri=around or surrounding). Periosteum contains blood vessels, nerves, and lymphatic vessels that nourish compact bone. Tendons and ligaments also attach to bones at the periosteum.

Body Response to low Ca levels

Parathyroid gland release parathyroid hormone (PTH) in response to low Ca levels. PTH activates osteoclast cells and cause them to dissolve bone and release calcium back.

How do blood cells get their oxygen supplies if the trabeculae of the spongy bone do not have blood vessels?

Red bone marrow is found between the trabuculae. Blood vessels within this tissue deliver nutrients to osteocytes and remove waste. The red bone marrow of the femur and the interior of other large bones, such as the ileum, forms blood cells.

Haversial Canal or Central Canal

Runs down the center of each osteon. Contains blood vessels, nerves, and lymphatic vessels. These vessels and nerves branch off at right angles through a perforating canal also known as VOLKMANN'S canals, to extend to the periosteum and endosteum.

Functions of Bones

Supports the body, facilitates movement, protect internal organs, produces blood cells, and stores and releases minerals and fat. 206 bones in human skeleton. Divided into 2 groups based on location: Axial/Appendicular

Hormones and Bone Tissue

The pituitary gland secretes growth hormone (GH), which, as its name implies, controls bone growth in several way. Thyroxine, a hormone secreted by the thyroid gland promotes osteoblastic activity and the synthesis of bone matrix. During puberty, the sex hormones (estrogen in girls, testosterone in boys), promote osteoblastic activity and production of bone matrix, and responsible for the growth and spurt that often occurs during adolescence.

Bone Remodeling

The process in which matrix is resorbed on one surface of a bone and deposited on another is known as bone modeling. Primarily takes place during a bone's growth. However, in adult life, bone undergoes remodeling, in which resorption of old or damaged bone takes place on the same surface where osteoblasts lay new bone to replace that which is resorbed. Injury, exercise, and other activities lead to remodeling.

1. A shaft (diaphysis)

Tubular shaft that runs b/w proximal and distal ends of the bone. Hollow region = medullary cavity filled with yellow marrow. Medullary cavity has a delicate membranous lining called the endosteum (end= Inside, oste= bone), where bone growth, repair, and remodeling occur. Walls of diaphysis are composed of dense and hard compact bone.

Nutrition and Bone Tissue

Vitamin D is important for calcium reabsorption, it also plays a role, though not clearly understood, in bone remodeling.

Bone Cells and Tissues Osteo means BONE

Bone cells made of connective tissue (3 main components: cells, extracellular matrix, fibers) Intramembranous ossification broken down: -Bone cells originate from mesenchymal cells (are embryonic cells that regenerate connective tissue), which give osteogenic cells (most bones come from these cells), are highly divided, constantly undergo mitosis. -They then differentiate into osteoblasts, where some can divide but main role is to produce osteoid. Osteoid is a gel like material that gives the extracellular matrix of bone. -Osteoid turns into bone upon contact with Ca+2, where the calcium comes from blood. Calcium + osteoid = hard bone. -When osteoid becomes hard bone, osteoblasts become osteocytes. Osteocytes maintain the bone. -Canaliculi are used for communication (small canals) between osteocytes.

Endochondral Ossification

Bone develops by replacing hyaline cartilage. Cartilage serves as a template to be completely replaced by new bone. Starts about 6 to 8 weeks after conception. All the bones in the body except the skull and clavicle.

Cartilage Templates

Bone is a replacement tissue; that is, it uses a model cartilage on which to law down its mineral matrix. Therefore, these are cartilage bones. During fetal development, a framework is laid down that determines where bones will form.

Hormones that Influence Osteoclasts

Bone modeling and remodeling require osteoclasts to resorb unneeded, damaged, or old bone, and osteoblasts to lay down new bone. 2 hormones that affect the osteoclasts are PARATHYROID HORMONE (PTH) and CALCITONIN PTH: four small glands of the endocrine system which regulate the calcium in our bodies. Parathyroid glands are located in the neck behind the thyroid where they continuously monitor and regulate blood calcium levels. (increase levels) Calcitonin: involved in helping to regulate levels of calcium and phosphate in the blood, opposing the action of parathyroid hormone. (decrease levels)

Flat Bone

Consist of a layer of diploe (spongy bone), lined on either side by a layer of compact bone. Filled with red bone marrow (Ex: bones in the cranium)

Long Bone

Contain 3 Parts: a shaft (diaphysis), bone ends (epiphyses), and membrane (periosteum)

Spongy Bone

Contains osteocytes housed in lacunae, but they are not arranged in concentric circles. The lacunae and osteocytes are found in a lattice-like network of matrix spikes called trabeculae (singular = trabecula). Trabecula forms along lines of stress to provide strength to the bone. Very similar to compact but there are no osteon. It is just the lamellae. Made out of trabeculae = arrangement of lamellae. -Trabeculae run in a random direction, not parallel. Absorb pressure in a direction.

Axial Skeleton

Forms the vertical, central axis of the body and includes all bones of the head, neck, chest, and back

Bone Markings

General classes of bone markings: -Articulations: where 2 bone surfaces come together (articulus = "joint") -Projections: an area of a bone that projects above the surface of the bone. These are the attachment points for tendons and ligaments. -Holes: an opening or groove in the bone that allows blood vessels and nerves to enter the bone.

Growth in Width (Thickness)

Growth of width is a balance of osteoblast and osteoclast activities. Osteoblast cells deposit new bone layers on the outer side of the bone. Osteoclast cells resorb old bone layers from the inner side of the medullary cavity. This is a very tuned process and well balanced process.

Bone Disorders

Hypocalcemia: a condition characterized by abnormally low levels of calcium. Bones may become brittle. The causes can range from hormonal imbalances to an improper diet. Treatments vary according to the cause, but prognoses are generally good. Osteoporosis: 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. Women lose bone mass more quickly than men starting at about 50 years of age the approximate age at which women go through menopause. Current treatments for managing osteoporosis include Biphosphonates, calcitonin, and estrogen (women only)

Bone Formation/Development

In early stages of embryonic development, the embryo's skeleton consists of fibrous membranes and hyaline cartilage. By the 6th or 7th week of embryonic life, the actual process of bone development, ossification (osteogenesis) begins. 2 osteogenic pathways: Intramembranous ossification and Endochondral ossification. Bone is same regardless of the pathway that produces it.

Hormone Life Cycle

Infant-adolescent: growth/pituitary gland Adult: sex hormones, growth spurt Adulthood: thyroid gland (produces Calcitonin hormone), parathyroid hormone/gland (Ca+2 levels)

Key Osteon Notes

-In the middle = central canal that contains blood vessels/nerves -Surrounding central canal: layers of extracellular matrix. Layers = lamellae (osteoid + calcium + collagen fibers together, is the thickness of layers around central canal). -Around lamellae on their borders = Osteocytes. There is Canaliculi stretching from osteocytes which all together make up osteon. -Compact bone = group of osteons with same structure. They run parallel to the direction of the bone, along it not crossing it. -Empty spaces between them filled with lamellae = Interstitial lamellae. Surrounding outside = circumferential lamellae.

Classification of Bones

Long: cylinder-like shape, longer than it is wide. Provide leverage. Ex: femur, tibia, fibula, metatarsals, humerus, ulna, radius, metacarpals, phalanges. Short: cube-like shape, approximately equal in length, width, and thickness. Provide stability, support, while allowing for some motion. Ex: carpals, tarsals. Flat: thin and curved, points of attachment for muscles; protectors for internal organs. Ex: sternum, ribs, scapulae, cranial bones. Irregular: complex shape. Protect internal organs. Ex: vertebrae, facial bones. Sesamoid: small and round; embedded in tendons. Protect tendons from compressive forces. Ex: patellae.

Chemical Composition of Bone

Organic components: the product of cells like fibers and blood. Bone contains a relatively small number of cells entrenched in a matrix of collagen fibers that provide a surface for inorganic salt and crystals to adhere. Inorganic components: Hydroxyapatites: form when calcium phosphate and calcium carbonate combine. Incorporate other inorganic salts like magnesium hydroxide, fluoride, and sulfate as it crystallizes or calcifies on the collagen fibers. Crystals give bones their hardness and strength, while collagen fibers give them tensile strength and make them less stress resistant.

Intramembranous Ossification cont.

Osteoblasts secrete osteoid (uncalcified matrix), which calcifies (hardens) as mineral salts are deposited on it, thereby entrapping the osteoblasts within. Once entrapped, the OSTEOBLASTS BECOME OSTEOCYTES. The Osteogenic in the surrounding connective tissue differentiate into new osteoblasts. Osteoid secreted around the capillaries results in a trabecular matrix, while osteoblasts on the surface of the spongy bone become the periosteum. The periosteum then creates a protective layer of compact bone superficial to the trabecular bone. The trabecular bone crowds nearby blood vessels, which eventually condense into red marrow.

Osteocytes + Lacunae + Canaliculi

Osteocytes are located in spaces called LACUNAE (singular=lacuna), found at the borders of adjacent lamellae. Canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This allows nutrients to be transported to the osteocytes and wastes to be removed from them.

2. Bone ends (Epiphyses)

Wider section at each end of the bone. Filled with spongy bone. Red marrow fills the spaces in the spongy bone. Covered w/ articular cartilage (Hyaline) to form articular (joint) surfaces and reduce friction. Each epiphysis meets the diaphysis at the metaphysis, the narrow area that contains the epiphyseal plate (growth plate). When the bone stops growing in early adulthood (approx. 18-21 yrs) cartilage is replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal line.