“ Make No Bones About It!” Bones can tell us a lot about a person’s physical and medical history. Bone structure and connective tissue account for about 25% of the body’s weight. Bone Formation and Growth Early embryo - bones start out as cartilage or fibrous structures. The cartilage is gradually replaced by calcified bone matrix. Ossification - the process where mineral matter starts to replace previously formed cartilage, thus creating bone. (Usually starts during the 8th week of embryonic development.) Infant bones are soft because ossification is not complete. Continues through childhood.

You are born with about 300 bones bones but by the time you are an adult, you have about 206 bones. Many bones fuse together as you grow. Soft Spot (Fused) The 206 bones are divided into 4 types based on their form. 1. Long Bones - constructed for weight- bearing and movement. Examples are the femur and the humerus 2. Short Bones - make flexible movement possible. Examples are wrist bones (carpals) and foot bones (tarsals). 3. Flat Bones - these bones help provide organ protection and are important sites for hemopoiesis (blood cell formation.) Examples are skull bones, sternum, and ribs. 4. Irregular Bones - odd shapes that do not fit into the other three categories. They have unique shapes related to their function. Examples are the vertebrae and the ear bones.

There are two major types of bone based on their histological (tissue) structure: 1. Compact or Cortical Bone - mostly solid bone matrix and cells with few spaces. 2. Spongy or Cancellous bone - has many spaces within a lacy network of bone. Epiphyses Long Bones Long bones are hollow and shaped like rods or shafts with rounded ends Parts of the long bone are: 1. Diaphysis  Also called the “shaft.”  Mostly compact or cortical bone.  Strong yet light enough in weight to permit easy movement. 2. Epiphyses  Mostly spongy or cancellous bone.  Has many spaces filled with red bone marrow which produces red and white blood cells. Epiphyseal Plate Epiphyseal Plate Diaphysis 3. Epiphyseal Plate  Cartilage between the epiphyses and the diaphysis.  Site of growth in bone length.  Growth ceases when all epiphyseal cartilage is transformed into bone.

Periosteum Endosteum 4. Periosteum  “Peri” means around and “osteum” means bone  Tough, white, vascular, fibrous membrane on the outside of a bone  Contains blood vessels, lymph vessels, and nerves  Responsible for bone growth, bone repair, and nutrition.  If periosteum is removed, bone will die. 5. Endosteum  “Endo” means within and “osteum” means bones  Membrane within the bone that lines the medullary cavity.  Osteoclasts (tear down bones) are located in this cavity. They dissolve bone to keep the cavity. 6. Medullary Cavity  Hollow space or cavity inside the diaphyses of the bone that contains the yellow bone marrow.  Yellow marrow started out as red marrow, but gradually turns to fat cells in long bones.  These fat cells can be converted to energy when needed and some WBC’s are made here. Medullary Cavity

7. Articular Cartilage  Thin layer of cartilage covering each epiphysis  Acts like a shock absorber between bones 8. Haversian System (HC)  Circulatory system within bone  Contains blood vessels that run parallel to the long axis of the bone  Provides nutrients to the osteocytes (OC) and removes waste..

Our bones are constantly renewing themselves. Up to 10% of your bones is eaten away and replaced each year. There are 3 basic types of bone cells: How Bones Grow Osteoblasts  Are the “bone-builders” and are responsible for building new bone  These cells secrete bone matrix Osteocytes  Living bone cell located in the lacunae (gaps) in the bone matrix.  These are mature bone cells  These cells live for about 20 years Osteoclasts  Are the “bone breakers”  Are large phagocytic cells  Tear down bones by excavating channels within the bone  Eat away bone in the medullary cavity, preventing bone from becoming too thick

 Bone mass peaks at about age 35, after which there is a universal, gradual loss of bone.  Decreased activities and metabolic changes (such as menopause and decreased estrogen levels) contribute to bone loss.  Women lose more bone mass than men.  Bones also change in shape and have reduced strength - leads to increased change of fractures.  Fractures are slower to heal because new fibrous tissue develops more slowly in the elderly.  The ability of the collagen structure to absorb energy is reduced - leads to osteoarthritis.  The articular cartilage decreases in weight-bearing areas and has reduced ability to heal.

I. Axial Skeleton  80 bones of the head and trunk  Many of these bones protect the major organs of the body  Consists of the bones which run down the middle of the body (it’s axis!) II. Appendicular Skeleton  Appendicular means “to hang” and these bones are attached to or “hang” from the bones of the axial skeleton.  Consists of the 126 bones of the arms (upper appendages), the legs (lower appendages), as well as the bones of the hips and shoulders.

1. Support Provides the framework to support the body’s fat, muscles, and skin. Gives shape to the body. 2. Protection Protect delicate structures within them (heart, lungs, brain) as well as protecting bone marrow which is responsible for forming blood cells. 3. Movement Serves as a point of attachment for skeletal muscles. As muscles contract and shorten, they pull on bones and move them. 4. Storage Stores most of the body’s calcium supply. 98% of the body’s extracellular calcium is stored in bones. Excess calcium in the blood, calcium goes into bone for storage. Lack of calcium in blood and it goes from bone to blood.

5. Hemiopoiesis (hee-mo-poy-EE-sis)  “Hemo” means blood and “poiesis” means to make.  Blood cell formation takes place in the red bone marrow.  The average life of a RBC is 120 days so your body must produce 3 million new RBC’s every second!  If your body’s need for red blood cells is greater than what the body can supply, some of the fatty yellow matter can be converted to red marrow.