Histology Important note: this sheet includes the information that is not found in the slide and the important information only so you have to study the slide first, thanx) Notes on the last lecture: The Bone Bone as an organ: -The periosteum covers all the long bone except the articular surface that is covered by the articular type of hyaline cartilage. -The periosteum can be removed easily from all parts of the bone except at the articular margins (the points at which the articular surface meets with the bone shaft) and the attachment sites of muscles, tendons and ligaments. -The periosteum , like the perichondrium, consists of 2 layers , outer fibrous layer (collagenous with a small amount of elastic) and inner cellular layer which is ostegenic (gives rise to the concentric rings of the new bone) -The outer layer of the perisosteum contains vessels that supply the bone, but unlike the cartilage, the bone is a highly vascular tissue. -Perforating fibers of Sharpey: collagen fibers , start from the outer layer of periosteum , attach the periosteum to the underlying bone , it is abundant at the insertion sites of muscles. -The bone is very vascular and contains a lot of sensory nerve endings especially in the periosteum, that’s why severe pain occurs when something solid hits the tibia (whose medial surface is subcutaneous) as well as in the case of fractures. -On the inside of the bone(the medullary cavity and the spaces of the spongy bone) there is the endosteum, which is similar in structure to the periosteum , however the endosteum is difficult to identify being very thin. Bone as a tissue:

-Calcium phosphate (which is the most abundant calcium salt in the intercellular matrix of the bone) is present in the form of crystals termed hydroxyapatite crystals, these crystals run along with collagen fibrils in parallel rows, that’s why the hardness and rigidity of the bone are due to the crystals and the collagen together. -Decalcification of the bone: treating the bone with an acidic solution, so that the calcium salts will dissociate and the bone will become soft, enabling us to prepare histological sections of the bone. -The basic structure of bone is large number of units called lamellae which consist of matrix. That means most of the bone tissue is matrix, and that’s because it is a connective tissue. -The growth of bone occurs by adding lamellae, one over the other, formed by osteoblasts and so on. -Osteocytes communicate with each other through cytoplasmic processes running through minute canals called Canaliculi, these canaliculi serve the function of bringing nutrients to the osteocytes, and the processes communicate with each other through gap junctions. -The basic structure of Trabeculum is the lamella. -The trabeculae are randomly arranged, this arrangement gives the Spongy bone its features (randomly arranged trabeculae with large spaces between them filled with bone marrow). -The basic structural unit of the spongy bone is the Trabeculum (random), whereas the basic structural unit of the compact bone is the Haversian System or the Osteon (concentric). -The Haversian system is cylindrical in shape. -Another method of preparing histological sections of bone is by Grounding, until the bone becomes very thin to allow the light to pass through. So now we have 2 methods for preparing bone sections: Decalcification and Grounding. -We can distinguish between the 2 methods by the presence of the Canaleculae. If the are present , then the section is prepared by grounding, and if the are not, then the section is prepared by decalcification.

-Another way to distinguish between the 2 methods is by the shape of the osteocytes, if the lacunae are black, then the section is prepared by grounding (cell debris), and if the osteocytes shrink and are on the side of the lacunae , then they are prepared by decalcification. -The Interstitial Lamellae are the remaining parts of Atypical Haversian Systems that were removed in the early stages of growth. -All newly formed bones are Spongy bones, later, they may remain spongy or they may be converted into compact bone. -Primary bone is a bone with severe random trabecullae , then it should disappear and be replaced by a more organized type of bone called the Secondary bone. -Osteoprogenitor cells become active in 2 cases, during growth of the bone, and in the case of fracture, to produce the bone forming cells, the osteoblasts. -The nucleus of an osteoblast is roundedwhen it is active, similar to the nucleus of a simple cuboidal cell, but when it is less active, its nucleus becomes flattened and the osteoblasts are called Bone Lining Cells. -Osteoid is uncalcified bone matrix that is formed by osteoblasts, later on, calcium salts are precipitated and the matrix becomes calcified. (Osteoid contains organic materials only). -The cartilage dies by precipitating calcium salts in it, so diffusion no more occurs, and the chondrocytes will die, then the cartilage is Replaced by bone, by the forming of bone matrix by the osteoblasts that are present on the remaining parts of the calcified cartilage. -The function of the parathyroid hormone is to keep the proper concentration of the ionized calcium in the blood. -The concentration of the ionized calcium in the blood becomes less than normal in the case of Renal Failure, in this case, the ionized calcium will not be reabsorbed by the kidney and its concentration drops in the blood, this case is called Chronic Hpocalcimia. -The cytoplasm of an osteoclast is acidophilic due to presence of large amounts of mitochondria and lysosomes.

-Osteoporosis is the case in which decrease in the bone mass occurs, when the activity of the osteoclasts becomes more than the activity of the osteoblasts. -In females, after the age of 50, the oestrogen level in the blood decreases, then the osteoblasts become less active relative to the osteoclasts, and Osteoporosis occurs. -Malabsorption and Lactose Intolerance occurs when there is deficiency in Lactase, the hormone that digests lactose, the milk sugar. (In the end, I want to thank all my colleagues in the faculty of Medicine hoping that all of you will be remarkable doctors someday, keep up the good work, thanx)