 This video will cover an introduction to the skeletal system part 2. As we go, we will cover the following study objectives. Describe the structure of osseous tissue, including the major types of cells found in bone. Describe the structure and locations of compact and spongy bone. Identify and describe the gross anatomical areas of a long bone, apophysis, articular cartilage, epiphyseal plate, epiphyseal line, diaphysis, medullary cavity, periosteum, and endosteum. Define the following terms used to describe bone features, process, trochanter, tubercle, crest, spine, head, condyle, facet, fossa, sulcus, mietis, fissure, foramen, sinus, and neck. Here we can see an illustration showing the major types of cells found in bone. The osteocytes are the mature cells of bone that maintain bone tissue. However, osteocytes are a mature form that differentiates from the osteoblast. An osteoblast is an immature bone cell that forms the matrix of bone. The matrix of bone is known as an osteoid, which is mostly collagen fibers that are covered with hydroxyapatite. The collagen itself provides flexibility and tensile strength to the bone, and collagen resists stretching. Hydroxyapatite, which is a layer of calcium phosphate hydroxide salts, gives hardness to bone and resists compression. The osteoblasts, in turn, come from a stem cell known as an osteogenic cell, sometimes called an osteoprogenitor cell. Osteogenic cells divide and differentiate into osteoblasts that produce the matrix of bone, the osteoid, and once the osteoblasts surround themselves with osteoid, they differentiate into osteocytes. Osteoclasts are another type of cell found in bone. Osteoclasts break down bone, that is, they reabsorb bone. They break down the matrix of bone in order to release calcium from the bone into the blood as a mechanism to maintain blood calcium concentration. The activity of osteoclasts is regulated by the endocrine system. Parathyroid hormone stimulates osteoclasts activity in order to release calcium from bone, maintaining blood calcium homeostasis. Compact bone is a superficial layer in our bone organs. Compact bone is a tissue that has a dense, smooth, solid appearance, but under the microscope, compact bone has a repeating structural and functional unit called an osteon. An osteon consists of a central canal surrounded by plates of bone that form concentric circular plates, which are called concentric lamellae. So an osteon is a central canal surrounded with concentric lamellae. The central canal contains blood vessels and nerves that are traveling through the bone. The central canals run parallel to the long axis of a long bone. In order to provide the greatest strength to the diaphysis of the long bone, the central canals run parallel to that length of the long bone. The lamellae are plates of oseous tissue, and concentric lamellae surround the central canal within an osteon. There are also interstitial lamellae that are plates of oseous tissue wedged in between the osteons, and the entire outer layer and inner layer of compact bone is formed from plates of oseous tissue called circumferential lamellae. There's outer circumferential lamellae, forming the most superficial outer layer of the compact bone, and inner circumferential lamellae lining the deep surface of the compact bone. In addition to the central canals that run parallel to the long axis of a long bone, there are perforating canals that run perpendicular to the central canals. These perforating canals are also known as Volkman's canals, and they contain blood vessels and nerves as well, connecting from the surface deep into the bone and into the medullary cavity. Both the canals running through compact bone are lined with an inner layer and inner membrane known as endosteum, and endosteum contains the osteoclast cells that break down the matrix of bone, and there's another type of membrane surrounding the superficial layer of the bone known as the periosteum. Endosteum is found deep inside of the bone, lining central canals and perforating canals, and deeper into the bone there's a medullary cavity, which is also lined with endosteum. The spongy bone is found deep within the bone surrounding the medullary cavity, and so spongy bone, in contrast to compact bone, has lots of small holes in it called the trabecular cavities that are in between irregularly arranged struts of bone known as trabecue. Then the superficial outermost layer of the compact bone is covered with the periosteal membrane, which is a membrane of connective tissue that also contains osteogenic cells that are the stem cells that can differentiate into osteoblasts, and the osteoblasts will build bone and eventually differentiate into osteocytes, the mature cells of bone. We see an illustration showing spongy bone, which is also known as trabecular bone. Spongy bone has lots of spaces, lots of small spaces known as trabecular cavities, and lots of irregularly arranged branching struts or plates of bone known as trabeculi. So trabecular bone gets its name from the trabeculi, the irregularly arranged struts of bone, and the trabecular cavities, the spaces in between the struts of bone. Spongy bone is found deep within the bone, the deep layer of long bones is spongy bone. Spongy bone surrounds a medullary cavity within the long bone and also fills the expanded ends, the epithesis of the long bone, and there is endosteum lining all of the trabecular cavities through the spongy bone. Bone marrow is found inside of those trabecular cavities, and blood vessels and nerves also run through the trabecular cavities. So here in the illustration you can see there are osteoclasts found in the outer layer of the trabeculi. Those osteoclasts are part of the endosteum, the layer of cells surrounding the trabeculi and lining the trabecular cavities. And there are also osteoblasts found in the endosteum. And remember osteoblasts build bone and osteoclasts break down bone. Here we can see the structure of a flat bone where there are outer layers of compact bone surrounding an inner layer of spongy bone. And the overall surface of the flat bone is a broad flat surface, and you can see that it's slightly curved. So the outer surface of the cranium is a convex curvature, but it's formed out of flat bone. Here we can see the structure of a long bone. The epiphysis is the expanded end. There's a proximal and distal epiphysis. The epiphysis is mostly composed of spongy bone. And articular cartilage covers the surface where the joint is formed, known as the articular surface. The diaphysis is the narrow shaft of the long bone, and the outer layer of the diaphysis is composed of compact bone. Then there's a medullary cavity, a space inside of the diaphysis filled with bone marrow. In an adult, that's mostly yellow bone marrow within the medullary cavity. And the medullary cavity is lined with spongy bone. At the border between the diaphysis and the epiphysis is a region known as the metaphysis where a epiphyseal line is found. The epiphyseal line is a remnant of the epiphyseal growth plate. In adolescence, the epiphyseal growth plate is where bones grow longer through a mechanism known as interstitial growth. But as we mature later in life, the epiphyseal growth plate converts into the epiphyseal line. And so the epiphyseal line is composed of compact bone at the border between the epiphysis and the diaphysis. But it's a remnant of the epiphyseal growth plate, which was a layer of cartilage where the bones grow longer during adolescence. The bone is surrounded with an outer membrane known as periosteum. The periosteum has a rich blood supply known as periosteal vessels, and then blood vessels branch from the periosteal blood vessels deeper into the bone. The cells of the periosteum include osteogenic cells that are stem cells that divide and then differentiate into osteoblast cells, the cells that build up the bone matrix, and eventually osteoblasts differentiate into the mature bone cells called osteocytes. The endosteum is a membrane lining the medullary cavity inside of long bones and also the trabecular cavities throughout spongy bone and the canals, the central canal, and the volcans canals running through the compact bone. Endosteum contains osteoclast cells, and these osteoclast cells are the cells that break down the matrix of bone, releasing calcium into the blood to maintain calcium homeostasis. Here we can see an illustration of the blood supply to a bone. So the nutrient artery carries blood into the diaphysis and branches throughout the medullary cavity, and the nutrient vein carries blood out of the diaphysis. There are also metaphysial arteries that carry blood into the metaphysis and branch up through the epiphysis as epiphysial arteries, and then metaphysial veins carry blood out through the metaphysis. Bone features include major projections or elevations that can provide points of attachment for muscles. So we will learn the major features of the bones, and then these will be useful later as we study the muscles to learn the origins and insertions where the tendons attach to the bones. A process is a bump or projection of bone. For example, there's a temporal process, which is part of the zygomatic bone. The zygomatic bone found in the lateral and inferior portion of the orbit. The zygomatic bone has this frontal process that extends up towards the frontal bone at the lateral portion of the orbit. And the frontal bone has a zygomatic process that extends inferior along the lateral part of the orbit. And these two processes articulate, they form a joint. The frontal process of the zygomatic bone forms a joint with the zygomatic process of the frontal bone. On the lateral portion of the orbit. A trochanter is a large rough projection of bone. For example, the femur has a greater trochanter and lesser trochanter at the proximal end. A tubercle is a small rounded projection of bone. For example, the humerus has a lesser and greater tubercle at the proximal end. Then a sulcus is a groove. We can see the sulcus on the humerus right next to the tubercle, and that's known as the inter tubercular sulcus, which is found in between the greater and lesser tubercles of the humerus. A crest is a prominent ridge. For example, the coxal bone, the superior portion of the coxal bone known as the ilium forms a crest where if you put your hand on your hip, you're resting your hand on the crest of the ilium, the iliac crest. A spine is a sharp slender or pointed process. For example, the anterior of the iliac crest forms the anterior superior iliac spine. The term head refers to the expanded end of a long bone at the proximal end of the femur. The head of the femur articulates with the coxal bone to form the hip joint, and at the proximal end of the humerus, the head of the humerus articulates with the scapula to form the shoulder joint. We will also see the head of the radius, which is found at the proximal end of the radius, and the head of the ulna, which is found at the distal end of the ulna. A facet is a small, flat, articular surface. For example, the patella, the kneecap, articulates with the anterior surface of the femur in a flat surface on the anterior and distal end of the femur, known as the patella facet of the femur. A fossa is a shallow depression. For example, the coronoid fossa on the anterior of the humerus, or the iliac fossa on the anterior and superior region of the coxal bone. A sulcus is a groove. The example I explained earlier is what's shown here, the sulcus in between the greater and lesser tubercle on the humerus and intertubercular sulcus. A myatus is a canal or tube, a passageway through bone. For example, the external auditory myatus is the ear canal, which is an opening of the temporal bone on the lateral part of the cranium. The external auditory myatus, also known as external acoustic myatus, allows the vibrations of air to be conducted into the eardrum, the tympanic membrane inside of the temporal bone. A fissure is a slit-like opening or a crack. For example, the superior orbital fissure is shown here in the illustration. In the posterior of the orbit, the superior orbital fissure is a pathway for nerves that are traveling in to control the eye muscles. A foramen is a rounded opening through a bone and is commonly a passage for nerves and blood vessels to travel through. For example, there's an infraorbital foramen found just inferior to the orbit in the maxillary bone, the upper jaw bone, and there is a branch of a cranial nerve, an axillary branch of the trigeminal nerve that travels through the infraorbital foramen. A sinus is a cavity inside of a bone. For example, there are paranasal sinuses that connect into the nasal cavity, are found inside of the frontal bones. We see here the illustration of the sinus inside of the frontal bone is the one that's labeled in the illustration, but all of the paranasal sinuses are shown in green. So there are cavities inside of the maxillary bones, maxillary sinuses. There are also ethmoid sinuses inside the ethmoid bone and sphenoid sinuses inside of the sphenoid bone. A neck is a narrowing of a bone just below the head. So if we look at the head of the humerus or the head of the femur, the neck is a narrow region that catches to the head.