SKELETAL SYSTEM Flashcards
ARTICLUAR CARTILAGE
The cartilage covers the articulating surfaces of the bones with a smooth, slippery surface but does not bind them together. Articular cartilage reduces friction between bones in the joint during movement and helps to absorb shock.
The articular cartilage is a thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation (joint) with another bone. Articular cartilage reduces friction and absorbs shock at freely movable joints. Because articular cartilage lacks a perichondrium and lacks blood vessels, repair of damage is limited.
SYNARTHROSES
STILL - joint with no movement
AMPHIARTHROSES
A BIT - Joints with some movement.
DIARTHROSES
DANCE - Joints that move freely
SIX MAIN FUNCTIONS OF THE SKELETAL SYSTEM
Support. The skeleton serves as the structural framework for the body by supporting soft tissues and providing attachment points for the tendons of most skeletal muscles.
Protection. The skeleton protects the most important internal organs from injury. For example, cranial bones protect the brain, and the rib cage protects the heart and lungs.
Assistance in movement. Most skeletal muscles attach to bones; when they contract, they pull on bones to produce movement. This function is discussed in detail in chapter 10.
Mineral homeostasis (storage and release). Bone tissue makes up about 18% of the weight of the human body. It stores several minerals, especially calcium and phosphorus, which contribute to the strength of bone. Bone tissue stores about 99% of the body’s calcium. On demand, bone releases minerals into the blood to maintain critical mineral balances (homeostasis) and to distribute the minerals to other parts of the body.
Blood cell production. Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called haemopoiesis (hēm-ō-poy-ē-sis; haemo- = blood; -poiesis = making). Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibres. It is present in developing bones of the foetus and in some adult bones, such as the hip (pelvic) bones, ribs, sternum (breastbone), vertebrae (backbones), skull, and ends of the bones of the humerus (arm bone) and femur (thigh bone). In a newborn, all bone marrow is red and is involved in haemopoiesis. With increasing age, much of the bone marrow changes from red to yellow. Blood cell production is considered in detail in section 19.2.
Triglyceride storage. Yellow bone marrow consists mainly of adipose cells, which store triglycerides. The stored triglycerides are a potential chemical energy reserve.
HAEMOPOISIS
Blood cell production. Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called haemopoiesis (hēm-ō-poy-ē-sis; haemo- = blood; -poiesis = making).
Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibres. It is present in developing bones of the foetus and in some adult bones, such as the hip (pelvic) bones, ribs, sternum (breastbone), vertebrae (backbones), skull, and ends of the bones of the humerus (arm bone) and femur (thigh bone). In a newborn, all bone marrow is red and is involved in haemopoiesis. With increasing age, much of the bone marrow changes from red to yellow. Blood cell production is considered in detail in section 19.2.
PERIOSTEUM
In the long bone
The periosteum (per-ē-OS-tē-um; peri- = around) is a tough connective tissue sheath and its associated blood supply that surrounds the bone surface wherever it is not covered by articular cartilage. It is composed of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer that consists of cells. Some of the cells enable bone to grow in thickness, but not in length. The periosteum also protects the bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons. The periosteum is attached to the underlying bone by perforating fibres or Sharpey’s fibres, thick bundles of collagen that extend from the periosteum into the bone extracellular matrix.
MEDULLARY CAVITY
In the long bone
MED-ul-er-ē; medulla- = marrow, pith), or marrow cavity, is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels in adults. This cavity minimises the weight of the bone by reducing the dense bony material where it is least needed. The long bones’ tubular design provides maximum strength with minimum weight.
ENDOSTEUM
end-OS-tē-um; endo- = within) is a thin membrane that lines the medullary cavity. It contains a single layer of bone-forming cells and a small amount of connective tissue.
DIAPHYSIS
in the long bone
dī-AF-i-sis = growing between) is the bone’s shaft or body— the long, cylindrical, main portion of the bone.
EPIPHYSIS
(e-PIF-i-sēz = growing over; singular is epiphysis) are the proximal and distal ends of the bone
SKELETAL SYSTEM FUNCTIONS
- Support–formsframeworkofbody
- Protection – e.g. brain, spinal cord, heart, lungs etc.
- Movement – with muscular system
- Mineralhomeostasis–storesmostlycalciumand phosphorus that can be released or removed from blood
- Bloodcellproduction(Haemopoiesis)–RBCs,WBCs and platelets in red bone marrow within the epiphyses
- Fatstorage–inyellowbonemarrowwithinthe diaphysis
METAPHYSIS
In the long bone
(me-TAF-i-sēz; meta- = between; singular is metaphysis) are the regions between the diaphysis and the epiphyses. In a growing bone, each metaphysis contains an epiphyseal (growth) plate (ep′-i-FIZ-ē-al), a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length (described later in the chapter). When a bone ceases to grow in length at about ages 14–24, the cartilage in the epiphyseal plate is replaced by bone; the resulting bony structure is known as the epiphyseal line.
COMPACT BONE
Found in the diaphysis and outer covering of epiphysis
The structural unit of compact bone is an Osteon
The central canal of each osteon contains blood vessels and nerve fibres
Collagen fibres run in the directions along which greatest force is applied.
contains few spaces (figure6.3a) and is the strongest form of bone tissue. It is found beneath the periosteum of all bones and makes up the bulk of the diaphyses of long bones. Compact bone tissue provides protection and support and resists the stresses produced by weight and movement.
SPONGY BONE
Found in the interior of the epiphysis
has a honeycomb structure of flat pieces of bone known as Trabeculae
Between the trabeculae is red bone marrow
BONE CELLS
Osteoprogenitor(osteogenic) cells form Osteoblasts
Osteoblasts are young bone cells that form new bone (e.g.ingrowth)
Found in the Periosteum & Endosteum
c) Osteocytes are mature bone cells found in lacunae surrounded by matrix
d) Osteoclasts destroy bone matrix using lysosomes and are involved in bone remodelling. Found in the Periosteum & Endosteum
LONG BONE
A long bone is one that has greater length than width.
Parts of a long bone. The spongy bone tissue of the epiphyses and metaphyses contains red bone marrow, and the medullary cavity of the diaphysis contains yellow bone marrow (in adults).
A long bone is covered by articular cartilage at the articular surfaces of its proximal and distal epiphyses and by periosteum around all other parts of the bone.
Osteoprogenitor cells
os′-tē-ō-prō-JEN-i-tor; -genic = producing) are unspecialised bone stem cells derived from mesenchyme, the tissue from which almost all connective tissues are formed. They are the only bone cells to undergo cell division; the resulting cells develop into osteoblasts. Osteoprogenitor cells are found along the inner portion of the periosteum, in the endosteum, and in the canals within bone that contain blood vessels
intramembranous ossification
in′-tra-MEM-bra-nus; intra- = within; -membran- = membrane), bone forms directly within mesenchyme, which is arranged in sheetlike layers that resemble membranes.
Intramembranous ossification is the simpler of the two methods of bone formation. The flat bones of the skull, most of the facial bones, mandible (lower jawbone), and the medial part of the clavicle (collar bone) are formed in this way. Also, the ‘soft spots’ that help the foetal skull pass through the birth canal later harden as they undergo intramembranous ossification, which occurs as follows (figure6.5).
1 Development of the ossification centre. At the site where the bone will develop, specific chemical messages cause the cells of the mesenchyme to cluster together and differentiate, first into osteoprogenitor cells and then into osteoblasts. The site of such a cluster is called an ossification centre. Osteoblasts secrete the organic extracellular matrix of bone until they are surrounded by it.
2 Calcification. Next, the secretion of extracellular matrix stops, and the cells, now called osteocytes, lie in lacunae and extend their narrow cytoplasmic processes into canaliculi that radiate in all directions. Within a few days, calcium and other mineral salts are deposited and the extracellular matrix hardens or calcifies (calcification).
3 Formation of trabeculae. As the bone extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone around the network of blood vessels in the tissue. Connective tissue associated with the blood vessels in the trabeculae differentiates into red bone marrow.
4 Development of the periosteum. In conjunction with the formation of trabeculae, the mesenchyme condenses at the periphery of the bone and develops into the periosteum. Eventually, a thin layer of compact bone replaces the surface layers of the spongy bone, but spongy bone remains in the centre. Much of the newly formed bone is remodelled (destroyed and reformed) as the bone is transformed into its adult size and shape.
endochondral ossification
en′-dō-KON-dral; endo- = within; -chondral = cartilage), bone forms within hyaline cartilage that develops from mesenchyme.
The replacement of cartilage by bone is called endochondral ossification. Although most bones of the body are formed in this way, the process is best observed in a long bone.
Development of the cartilage model. At the site where the bone is going to form, specific chemical messages cause the cells in mesenchyme to crowd together in the general shape of the future bone, and then develop into chondroblasts. The chondroblasts secrete cartilage extracellular matrix, producing a cartilage model consisting of hyaline cartilage. A covering called the perichondrium (per′-i-KON-drē-um) develops around the cartilage model.
2 Growth of the cartilage model. Once chondroblasts become deeply buried in the cartilage extracellular matrix, they are called chondrocytes. The cartilage model grows in length by continual cell division of chondrocytes, accompanied by further secretion of the cartilage extracellular matrix. This type of cartilaginous growth, called interstitial (endogenous) growth (growth from within), results in an increase in length. In contrast, growth of the cartilage in thickness is due mainly to the deposition of extracellular matrix material on the cartilage surface of the model by new chondroblasts that develop from the perichondrium. This process is called appositional (exogenous) growth (a-pō-ZISH-o-nal), meaning growth at the outer surface. Interstitial growth and appositional growth of cartilage were described in more detail in section 4.5.
As the cartilage model continues to grow, chondrocytes in its midregion hypertrophy (increase in size) and the surrounding cartilage extracellular matrix begins to calcify. Other chondrocytes within the calcifying cartilage die because nutrients can no longer diffuse quickly enough through the extracellular matrix. As these chondrocytes die, the spaces left behind by dead chondrocytes merge into small cavities called lacunae.
3 Development of the primary ossification centre. Primary ossification proceeds inward from the external surface of the bone. A nutrient artery penetrates the perichondrium and the calcifying cartilage model through a nutrient foramen in the midregion of the cartilage model, stimulating osteoprogenitor cells in the perichondrium to differentiate into osteoblasts. Once the perichondrium starts to form bone, it is known as the periosteum. Near the middle of the model, periosteal capillaries grow into the disintegrating calcified cartilage, inducing growth of a primary ossification centre, a region where bone tissue will replace most of the cartilage. Osteoblasts then begin to deposit bone extracellular matrix over the remnants of calcified cartilage, forming spongy bone trabeculae. Primary ossification spreads from this central location towards both ends of the cartilage model.
4 Development of the medullary (marrow) cavity. As the primary ossification centre grows towards the ends of the bone, osteoclasts break down some of the newly formed spongy bone trabeculae. This activity leaves a cavity, the medullary (marrow) cavity, in the diaphysis (shaft). Eventually, most of the wall of the diaphysis is replaced by compact bone.
5 Development of the secondary ossification centres. When branches of the epiphyseal artery enter the epiphyses, secondary ossification centres develop, usually around the time of birth. Bone formation is similar to what occurs in primary ossification centres. However, in the secondary ossification centres spongy bone remains in the interior of the epiphyses (no medullary cavities are formed here). In contrast to primary ossification, secondary ossification proceeds outward from the centre of the epiphysis towards the outer surface of the bone.
6 Formation of articular cartilage and the epiphyseal (growth) plate. The hyaline cartilage that covers the epiphyses becomes the articular cartilage. Prior to adulthood, hyaline cartilage remains between the diaphysis and epiphysis as the epiphyseal (growth) plate, the region responsible for the lengthwise growth of long bones that you will learn about next.
