Bone Physiology - NCLEX-PN

There are five distinct zones in regions of endochondral ossification. These are the resting zone, the zone of proliferation, the zone of maturation, the zone of calcification, and the zone of ossification.

The behavior of cells in the zones of endochondral ossification is as follows:

• The resting zone: normal resting chondrocytes within hyaline cartilage
• The zone of proliferation: rapid mitosis of chondrocytes
• The zone of maturation: hypertrophy of chondrocytes
• The zone of calcification: the death of chondrocytes due to lack of nutrients and inability to eliminate cellular wastes
• The zone of ossification: migration of osteoprogenitor cells into the cavities left behind by dead chondrocytes and mineralization of newly formed bone

Variation in color of bone marrow cells is not related to their level of activity, calcification, or the types of blood cells they produce. Rather, "red marrow" is primarily hematopoietic cells (both red and white progenitor cells) while "yellow marrow" is primarily composed of fat cells.

In adults, hematopoietic bone marrow is generally confined to the flat bones, including the sternum, the skull, the ribs, and the pelvis. Hematopoietic bone marrow also exists in the proximal end of the femur in most adults, but is not generally found in the tibia or fibula.

Muscle attaches to bone via tendons, fibrous extensions of the sheath of the muscle body that are primarily composed of tightly packed collagen fibers. In comparison, ligaments attach bones to other bones without involvement with a muscle, such as the ligaments between the metacarpals of the wrist. Sutures are fibrous joints of the cranium, and hyaline cartilage is at the point of articulation of many bones but it is neither incorporated into muscle structure, nor does it attach to the bone with which it articulates. Rather, articular cartilage primarily serves to allow bones to glide more easily over each other during movement.

Calcium is essential for many processes of the body, including but not limited to nerve conduction, muscle contraction, blood clotting, and bone mineralization.

The rigid structure of bone is created by hydroxyapatite, a calcium apatite that contains both calcium and phosphate in the formula:Ca5(PO4)3(OH). Both calcium and phosphate are necessary to create bone structure—a deficiency of either will degrade the structural integrity of the bone matrix. Last, collagen is a protein rather than a mineral, and primarily functions to add flexibility to bone in order to prevent fracture.

Bones are made up of the following three primary tissue types:

I: Cortical bone, which is the hard exterior layer (also referred to as "compact bone);

II: Cancellous bone, which is the porous bone tissue that fills the center of bones (also referred to as "spongy bone" or "trabecular" bone tissue); and

III: Bone marrow, a hematopoietic tissue that fills spaces in trabecular bone.

Osteoblasts are the cells responsible for the deposition of hydroxyapatite into bone matrix. In addition to hydroxyapatite, they synthesize collagen, osteocalcin, and osteopontin. Osteocytes are undifferentiated osteoblasts, while osteoclast cells function in opposition to osteoblasts by removal of mineralization from the bony matrix. Myelocytes and normoblasts are both hematopoietic cells of the bone marrow and do not take part in bone mineralization.

The functional unit of compact bone is the osteon. The osteon is formed by concentric layers of compact bone called lamellae. These surround a central canal called the haversian canal. Trabeculae are the functional units of cancellous (spongy) bone.

The haversian canal allows capillaries and nerves to pass through the cortical bone to nourish osteocytes, osteoclasts, and osteoblasts. Both the white and the red bone marrow are contained within trabecular bone, while the periosteum is a thin, highly innervated membrane on the outside of bone.

Growth in long bones occurs at the epiphyseal plate, a region of hyaline cartilage at the metaphysis of the bone. In adolescents and children, cartilage cells are ossified by osteoblasts to increase bone length. Once adulthood is reached, the epiphyseal plate becomes the epiphyseal line at the junction of the epiphysis and the diaphysis (the center of the bone). The diaphysis and epiphysis themselves are composed of mature ossified bone and do not proliferate (they are simply added to by transformation of cells in the epiphyseal plate). A suture is a fibrous joint of the cranium and does not contribute to the development of long bones.

Calcium is essential for many processes of the body, including but not limited to nerve conduction, muscle contraction, blood clotting, and bone mineralization.

The rigid structure of bone is created by hydroxyapatite, a calcium apatite that contains both calcium and phosphate in the formula:Ca5(PO4)3(OH). Both calcium and phosphate are necessary to create bone structure—a deficiency of either will degrade the structural integrity of the bone matrix. Last, collagen is a protein rather than a mineral, and primarily functions to add flexibility to bone in order to prevent fracture.

Bones are made up of the following three primary tissue types:

I: Cortical bone, which is the hard exterior layer (also referred to as "compact bone);

II: Cancellous bone, which is the porous bone tissue that fills the center of bones (also referred to as "spongy bone" or "trabecular" bone tissue); and

III: Bone marrow, a hematopoietic tissue that fills spaces in trabecular bone.

Osteoblasts are the cells responsible for the deposition of hydroxyapatite into bone matrix. In addition to hydroxyapatite, they synthesize collagen, osteocalcin, and osteopontin. Osteocytes are undifferentiated osteoblasts, while osteoclast cells function in opposition to osteoblasts by removal of mineralization from the bony matrix. Myelocytes and normoblasts are both hematopoietic cells of the bone marrow and do not take part in bone mineralization.

The functional unit of compact bone is the osteon. The osteon is formed by concentric layers of compact bone called lamellae. These surround a central canal called the haversian canal. Trabeculae are the functional units of cancellous (spongy) bone.

The haversian canal allows capillaries and nerves to pass through the cortical bone to nourish osteocytes, osteoclasts, and osteoblasts. Both the white and the red bone marrow are contained within trabecular bone, while the periosteum is a thin, highly innervated membrane on the outside of bone.

Growth in long bones occurs at the epiphyseal plate, a region of hyaline cartilage at the metaphysis of the bone. In adolescents and children, cartilage cells are ossified by osteoblasts to increase bone length. Once adulthood is reached, the epiphyseal plate becomes the epiphyseal line at the junction of the epiphysis and the diaphysis (the center of the bone). The diaphysis and epiphysis themselves are composed of mature ossified bone and do not proliferate (they are simply added to by transformation of cells in the epiphyseal plate). A suture is a fibrous joint of the cranium and does not contribute to the development of long bones.