Bones are organs. (Recall that an organ contains several different tissues.) Although bone (osseous) tissue dominates bones, they also contain nervous tissue in their nerves, cartilage in their articular cartilages, fibroils connective tissue lining their cavities, and muscle and epithelial tissues in their blood vessels. Bone anatomy has three structural levels: the gross, microscopic, and chemical levels.
Gross Anatomy
Structure of a Typical Long Bone
With few exceptions, all long bones have the same general structure.
Diaphysis: The tubular diaphysis or shaft, constitutes the long axis of the bone. It is constructed of a relatively thick collar of compact bone that surrounds a central medullary cavity or marrow cavity. In adults, the medullary cavity contains fat (yellow marrow) and is also called the yellow bone marrow cavity.
Epiphyses The epiphyses (e-piPi-sez) are the bone ends (epi == upon). In many cases, they are more expanded than the diaphysis. Compact bone forms the exterior of epiphyses; their interior contains spongy bone. The joint surface of each epiphysis is covered with a thin layer of articular (hyaline) cartilage, which cushions the opposing bone ends during joint movement and absorbs stress. Between the diaphysis and each epiphysis of an adult long bone is an epiphyseal line. This line is a remnant of the epiphyseal plate, a disclike region of hyaline cartilage that grows during childhood to lengthen the bone.
Membranes Both the internal and the external bone surfaces are associated with membranes. The outer surface of the diaphysis is covered and protected by glistening white, double-layered membrane called the periosteum (perwe-os'te-um; peri = around, osteo = bone). The outer fibrous layer of the periosteum is dense irregular connective tissue. Its inner osteogenic layer, abutting the bone surface, consists primarily of bone-forming cells, or · osteoblasts [os'te-o-blasts; “bone germinators”). And bone-destroying cells, or osteoclasts (“bone breakers”). The periosteum is richly supplied with nerve fibers, lymphatic vessels, and blood vessels, which enter the bone of the shaft via a nutrient foramen (fora'min; “opening”). It is secured to the underlying bone by Sharpey's fibers (Figure 6.3), tufts of collagen fibers that extend from the fibrous layer into the bone matrix. The periosteum also provides an insertion or anchoring point for tendons and ligaments, and at dense. . Dense. Internal bone surfaces are covered with a delicate connective tissue membrane called the endosteum (en-dos'te-um; “within the bone”). The endosteum ) covers the trabeculae of spongy bone in the marrow cavities and lines the canals that pass through compact bone. Like the periosteum, the endosteum contains both osteoblasts and osteoclasts.
Structure of Short, Irregular, and Flat Bones
Short. Irregular. And flat bones share a simple design: They consist of thin plates of periosteum-covered compact bone on the outside and endosteum-covered Spongy bone within. These bones are not cylindrical and so they have no shaft or epiphyses. They contain bone marrow (between their trabeculae), but no marrow cavity is present. In flat bones, the internal layer of spongy bone is called the mploe (dip'lo-e) and the whole arrangement resembles a stiffened sandwich.
Location of Hematopoietic Tissue in Bones
Because the hematopoietic tissue, red marrow, is typically found within the cavities of spongy bone of long bones and in the diploe of flat bones, these cavities are often referred to as red marrow cavities. In newborn infants, the medullary cavity and all areas of spongy bone contain red bone marrow. In most adult long bones, the fat-containing medullary cavity extends well into the epiphysis, and little red marrow is present in the spongy bone cavities. Hence, blood cell production in adult long bones routinely occurs only in the head of the femur and humerus (the long bone of the arm). Much more important and more aclive in hematopoiesis is the red marrow found in the diploe of flat bones (such as the sternum) and in some irregular bones (such as the hip bone). These are the sites routinely used for obtaining red marrow sampIes when problems with the blood-forming tissue are suspected. However, yellow marrow of the medullary cavity can revert to red marrow if a person becomes very anemic and needs enhanced red blood cell production.
