Anatomy and Physiology: Bone Tissue
Discuss the Haversian System as the structural unit of compact bone by using the following terms: osteocytes, lacunae, lamellae, Haversian canal, blood vessels, bone matrix, and canaliculi.
Compact bone is arranged in units called osteons or Haversian systems. Osteons contain blood vessels, lymphatic vessels, nerves, and osteocytes, along with the calcified matrix. Osteons are aligned in the same direction along the lines of stress. These lines can change as the stresses on the bone changes.
Classify bones on the basis of their shape and location.
Bones are classified according to their shape: Long bones consist of a shaft with two ends, for example, the thigh bone (femur) and the upper arm bone (humerus). Short bones are cube-like, for example, wrist bones (carpals) and ankle bones (tarsals). Flat bones are thin and usually curved, for example, skull bones and ribs. Irregular bones are not long, short, or flat, for example, the vertebrae.
List and discuss the major features of a long bong including the following: diaphysis, epiphyses, metaphyses, epiphyseal line, periosteum, endosteum, medullary cavity, yellow marrow, red marrow, and articular cartilage.
A typical long bone consists of numerous parts: The diaphysis is the shaft of the long bone. The epiphyses are the ends of the bone. The metaphyses are the areas between the epiphysis and diaphysis, and include the epiphyseal plate in growing bones. Hyaline cartilage (articular cartilage) at the ends of the bones reduces friction and absorbs shock at freely moveable joints. The periosteum is a connective tissue that covers the surface of the bone; it contains osteogenic cells, protects bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons. The space within the diaphysis is the marrow cavity. Red marrow is the site of blood production. Yellow marrow is the site of fat storage. The endosteum is the lining of the medullary cavity.
Describe the microscopic structures of bone.
Bone (osseous) tissue consists of widely separated cells surrounded by large amounts of matrix. The four principal types of bone cells are the following: - Osteogenics are precursor cells. - Osteoblasts are bone-building cells. - Osteocytes are mature bone cells, the principal cells of bone tissue. - Osteoclasts are derived from monocytes and serve to break down bone tissue. The matrix of bone contains inorganic salts, primarily hydroxyapatite and some calcium carbonate, and collagen fibres. These and a few other salts are deposited in a framework of collagen fibres, a process called calcification or mineralization. Mineral salts confer hardness on bone, while collagen fibres give bone its great tensile strength. The process of calcification occurs only in the presence of collagen fibres.
Define the term ossification.
Bone formation is termed osteogenesis or ossification; it begins when mesenchymal cells provide the template for subsequent ossification.
Fully discuss the negative feedback mechanisms involved in blood calcium (Ca++) homeostasis, and explain how this is related to bone remodeling.
Bone is the major reservoir for calcium ions (Ca2+) in the body; the blood level calcium ions (Ca2+) are closely regulated due to the importance of calcium in cardiac, nerve, enzyme, and blood physiology. The parathyroid hormone (PTH) is an important hormone that regulates the Ca2+ exchange between bone and blood. It is secreted by the parathyroid gland, and increases blood calcium ion levels. The parathyroid gland has receptors which sense when Ca2+ is low, triggering secretion of cAMP, which in turn increases the production of PTH (PTH gene turned "on"). When Ca2+ is normal again, the receptors in the parathyroid gland stop signalling production of cAMP. Another hormone that contributes to the homeostasis of blood Ca2+ is calcitonin (CT). It is secreted by the thyroid gland and decreases blood Ca2+ levels.
Discuss the steps involved in endochondral ossification.
Endochondral ossification involves six steps. The first step is development of the cartilage model, where cells in the mesenchyme crowd together, develop into chrondroblasts, and secrete ECM in the form of the future bone. A perichondrium then develops around the model (later to become periosteum). Chondrocytes in the mid-region grow large and kill off others, and ECM in this region begins to calcify. Lacunae form and merge into small cavities. A nutrient artery penetrates the perichondrium and into the mid-region. This stimulates cells in the perichondrium to begin forming osteoblasts from osteogenic cells. It is now known as the periosteum. As blood vessels grow into the center of the bone, a primary ossification center develops. Osteoblasts then begin to replace the remaining cartilage in the diaphysis with bone ECM. Osteoclasts break down some of the newly formed bone to create the medullary cavity. Secondary ossification centres develop in the epiphyses around the time of birth. No medullary cavity is formed. The hyaline cartilage that covers the epiphyses becomes the articular cartilage. Hyaline cartilage also remains between the diaphysis and the epiphysis as the epiphyseal growth place.
Discuss the steps involved in intramembranous ossification.
Intramembranous ossification involves the following four steps: Cells of the mesenchyme cluster together and differentiate into osteoblasts at the site where bone will develop. Osteoblasts cluster at this "centre of ossification". Osteoblasts constitutively secrete the matrix. Osteoblasts soon become trapped by their own extracellular matrix and become osteocytes. They lie in lacunae and extend their cytoplasmic processes through canaliculi that radiate in all directions. Soon the minerals deposited in the ECM harden, or calcify. Merging of bone spicules called trabeculae ultimately form spongy bone. The outer surface is covered with periosteum. This happens due to the condensation of mesenchyme at the periphery. Eventually a thin layer of compact bone replaces the surface layer of the spongy bone.
Describe intramembranous ossification. What types of bones are formed in this manner?
Intramembranous ossification is the formation of bone directly from or within the fibrous connective tissue membranes. - center of ossification formed directly in mesenchyme Intramembranous ossification forms the flat bones of the skull and the mandible. - flat, so perhaps not in need of a model so much
Discuss the nutrients and hormones involved in bone growth.
Large concentrations of calcium and phosphorus and smaller amounts of other minerals are needed for bone growth. Vitamins A, B12, C, D, and K are needed for bone growth. During childhood, the most important hormones for the stimulation of bone growth are the insulin-like growth factors (IGFs), which are stimulated by the human growth hormone (hGH). Thyroid hormones and insulin also are necessary hormones for bone growth. At puberty, the sex hormones, estrogen and testosterone, stimulate sudden growth and modifications of the skeleton to create the male and female forms. - sex hormones - IGFs stimulated by hGH - insulin
Discuss bone growth and remodeling.
Long bones grow in length and thickness. Growth in length occurs at the epiphyseal line/growth plate. Growth in width occurs at the perichondrium. When the epiphyseal plate closes, it is replaced by bone and the epiphyseal line appears, which indicates that the bone has completed its growth in length. Bone can grow in thickness or diameter only by appositional growth. Bone grows in diameter as a result of interstitial and appositional growth; osteoblasts add new bone tissue around the outer surface of the bone, and to a lesser extent, osteoclasts reabsorb the internal bone in the bone cavity. Bone is a dynamic tissue constantly being broken down and replaced by a process called remodeling, which is the ongoing replacement of old bone tissue by new bone tissue. It involves bone resorption by osteoclasts and bone deposition by osteoblasts.
Discuss spongy bone in terms of trabeculae.
Spongy (cancellous) bone does not contain osteons. It consists of trabeculae (columns or beams of bone), which surround many red- marrow-filled spaces. It forms most of the structure of short, flat, and irregular bones, and the epiphyses of long bones. Spongy bone tissue is light and supports and protects the red bone marrow.
List and discuss the six functions of bone tissue.
The functions of the skeletal system include the following: Support—Bones support the soft tissues and provide attachment sites for muscles, thereby serving as the structural framework for the body. Protection—Many of the body's internal organs are protected by bony coverings. Assist in movement—Bones assist skeletal muscles to produce movement. Mineral homeostasis—Bones store and release several minerals, especially calcium and phosphorus, to help maintain mineral homeostasis. Production of blood cells—Hemopoiesis, blood cell formation, occurs in the red marrow of bones. Triglyceride storage—The yellow marrow of adult bones serves as a site of triglyceride storage.
Describe endochondral ossification. What types of bones are formed in this manner?
The two types of ossification that can occur are as follows: Endochondrial ossification is the formation of bone from hyaline cartilage models. Endochondrial ossification involves the replacement of cartilage by bone, and forms most of the bones of the body. - requires a model
Briefly describe the hormones which maintain calcium homeostasis in the blood.
parathyroid hormone (parathyroid gland) - increases blood calcium levels by breaking down calcium stored in bone via osteoclasts, promotes calcium absorption by small intestines calcitonin (thyroid gland) - decreases calcium blood levels by storing calcium in the bones via osteoblasts
