Structures of Bones

Structures of Bones

Bones are considered organs because they contain various types of tissue. They are not only dominated by osseous (bony) tissue, but also contain nervous tissue, cartilage, fibrous connective tissue, and both muscle and epithelial tissues. Nervous tissue is found in bone nerves, whereas cartilage is found in articular cartilages. Fibrous connective tissue lines bone cavi-ties, and both muscle and epithelial tissues are found in the blood vessels of bones.

Gross Anatomy

The external layer of bones is called compact bone, which contains spongy bone that is made up of small flat or needle-like pieces called trabeculae (­FIGURE 7-3). It appears smooth and solid to the naked eye. Open spaces between trabeculae are filled with red and yellow bone marrow. Spongy bone is made up of open struts and plates, covered by a thin cortex of compact bone. This covering is also referred to as cortical bone.

Bones described as short, irregular, or flat are all made up of thin plates of the spongy bone covered by the compact bone. The plates are covered by connec-tive tissue membranes (the periosteum outside and the endosteum inside). These bones have no shaft or epiphyses because they are not cylindrical. Inside, they have bone marrow between their trabeculae, although there is not a well-formed marrow cavity. Hyaline car-tilage covers the surfaces of these bones where they form movable joints with nearby bones. The spongy bone in flat bones is called the diploe.

Nearly all long bones have the structure of a shaft, bone ends, and membranes. The shaft is known as the diaphysis. It is a tubular structure forming the long bone axis, made of a thick collar of compact bone sur-rounding a central medullary cavity. This cavity is called the yellow marrow cavity in adults because it contains fat (yellow marrow).

The bone ends are called epiphyses, which are usually broader than the diaphysis. Inside, they have spongy bone, whereas their outer shell is made of compact bone. The joint surface of each epiphysis is covered by a thin layer of hyaline cartilage. This layer cushions opposing bone ends as they move and absorbs stress. Between the diaphysis and each epiph-ysis of long bones is an epiphyseal line, which is a left-over remnant of the epiphyseal plate. There is also a disc of hyaline cartilage that develops during child-hood, lengthening the bone. At the point where the diaphysis and epiphysis meet is a flared portion that is sometimes referred to as the metaphysis.

The external surface of a bone, except for the joint surfaces, is covered by a shiny, white, and double-­ layered periosteum, a membrane that is richly ­supplied with blood vessels and nerve fibers. Dense irregular connective tissue makes up the outer fibrous layer. Inside, an osteogenic layer touches the bone sur-face and is made up mostly of primitive stem cells known as osteogenic cells. These cells form all bone cells, except for those that function in bone destruc-tion. The blood vessels and nerve fibers of the peri-osteum pass through the bone shaft, entering the marrow cavity through openings known as nutrient foramina. Groups of collagen fibers, called perforat-ing fibers or Sharpey’s fibers, extend from the fibrous layer into the bone matrix and bind the periosteum to the underlying bone. The periosteum also serves to anchor tendons and ligaments in areas that are extremely dense. Metaphyseal vessels supply blood to the diaphyseal surface of the epiphyseal cartilages, where they are replacing bone. Periosteal vessels provide blood to the superficial osteons of the bone shafts. During endochondral bone formation, they branch to enter the epiphyses and provide blood to the secondary ossification centers.

The endosteum is a delicate connective tissue membrane covering internal bone surfaces. In most trabecular cavities of spongy bone (in long bones) and in the diploe of flat bones are found hematopoietic red marrow. As a result, both these cavities are referred to as red marrow cavities. The endosteum covers the trabec-ulae of spongy bone and lines canals that pass through compact bone. The endosteum also contains osteogenic cells, which may differentiate into various bone cells.

In the newborn, red bone marrow is found in the medullary cavity of the diaphysis as well as all areas of spongy bone. In adults, most long bones have a medullary­ cavity that contains fat, which extends a good length into the epiphysis. Very little red marrow is found in an adult’s spongy bone cavities. As a result, red blood cell production in adult long bones usually occurs only in the heads of the femur and humerus. There is a greater amount of hematopoietic activity in the red marrow of the diploe of flat bones (such as the sternum) and certain irregular bones (such as the hip bones). Yellow marrow in the medullary cavity can convert back to red marrow if there is significant ane-mia, requiring more red blood cells.

Bone Markings

External surfaces of bones usually have depressions, projections, and openings. These bone markings are where ligaments, muscles, and tendons attach or they may occur at joint surfaces. They also may serve as conduits for nerves and blood vessels. Pro-jections bulge outward from bone surfaces, and include heads, spines, trochanters, and others. Each of these has its own unique features. Most bone pro-jections show stresses caused by attached muscles or are modified surfaces where the bones meet and form articulations.

Depressions and openings in bones allow for pas-sage of nerves and blood vessels and include:

■■ Fissures: Narrow, slit-like openings

■■ Foramina: Oval or round openings through bones

■■ Grooves: Shallow depressions

■■ Notches: Indentations at the edges of structures

■■ Fossae: Shallow depressions in bones that often serve as articular surfaces

■■ Meatuses: Passageways that resemble canals

■■ Sinuses: Cavities inside bones that are filled with air and lined with mucous membranes

Bone projections that are the sites where muscles and ligaments attach are:

■■ Crests: Narrow, usually prominent ridges of bone

■■ Epicondyles: Raised areas on or above condyles

■■ Lines: Narrow ridges of bone that are not as prominent as crests

■■ Processes: Bony prominences

■■ Spines: Pointed, sharp, or slender projections

■■ Trochanters: Extremely large, blunt, ­irregular shaped processes that only occur on the femurs

■■ Tubercles: Small rounded projections or processes

■■ Tuberosities: Large rounded projections that may be rough

Bone projections that help to form joints include:

■■ Condyles: Rounded articular projections

■■ Facets: Smooth, almost flat articular surfaces

■■ Heads: Bony expansions that are carried on narrow necks

■■ Rami: Arm-like bars of bone

1. Which membranes cover the external and internal long bones?

2. What are the classifications of bones, according to their shape?

3. Identify grooves, fossae, crests, and tuberosities.