October 21, 2018
1. The dermis is made up of two different layers the papillary dermis, top layer, and the reticular dermis, bottom layer. The papillary is superficial to the reticular dermis and consists of areolar connective tissue that contain both elastic and collagen fibers. This layer is loose in a sense allowing defensive cells to move throughout its matrix freely. Within the papillary dermis are dermal papillae which are structures that are small projections from the surface of this layer and lie on dermal ridges. These projections can contain capillary loops and/or nerve endings that help with detecting vibrations in our sense of touch. The reticular dermis makes up most of the dermis, as dense irregular connective tissue. This layer contains dermal vascular plexus, which house blood vessels that supply nutrients to this layer. An abundance of collagen fibers are housed here, the network and directions of these collagen fibers create cleavage lines, which are used by surgeons to make sure that wounds heal faster and properly. These collagen fibers also allow for extra strength and protection from abrasions and scraps. The elastic fibers in tis layer allow for the skin to stretch and then return back to its original state.
2. In the compact bone there are several cells that contribute to its composition. The first being an osteon, which is the structural unit of the bone itself. The osteons are cylinder shaped and are weight bearing units of the bone. The compact bone also contain lamella which are hollow tube-like structures one surrounding the other, containing collagen fibers all parallel to one another in each layer, but alternate directions at each layer of tubing. This structure that is created by multiple layers resist stress placed on the bone. Within each osteon is the central canal that contains nerves and blood vessels, these connect to perpendicular canals called perforating canals that connect the central canals to medullary canals. Lacunae are hollow structures that contain osteocytes and are connected to canaliculi which connect the lacunae to the central canals. Osteoblasts within the compact bone have the function of secreting bone matrix. When the bone matrix is hardened osteocytes are created. Osteocytes are responsible for relaying nutrients throughout the osteon using gap junctions and the canaliculi. However, some osteons that are incomplete have interstitial lamellae lying in between them allow them to fill the gaps that could be there due to remodeling or new forming osteons. Last, circumferential lamella covers all of the diaphysis allowing extra support against twisting of the bone. In contrast, the spongy bone appears to be much less organized than that of compact bone. Within spongy bone are trabeculae that are aligned along the stress lines and are precisely placed to resist as much stress as possible. Within the trabeculae are both lamellae and osteocytes that are linked using canaliculi. Osteons are missing in the composition of the spongy bone, unlike the histology of the compact bone. Spongy bone is also different about retaining nutrients in which capillaries supply nutrients to spongy bone.
3. Endochondral ossification is a slightly longer process than that of plate growth of a long bone. Endochondral ossification is the process by which hyaline cartilage is broken down and replaced by bone in early stages of life. During endochondral ossification there are five steps that occur. First, the hyaline cartilage acts as a blue print for a bone collar in the primary ossification center. At the sight of this mesenchymal cells become osteoblasts and secrete osteoid that gets trapped within the collar of the bone. Second, the hyaline cartilage begins to calcify in the diaphysis, and while doing so cavities develop within the bone. Once calcified, chondrocytes die and leads to matrixes to begin to die leaving behind cavities throughout the bone, but other cartilage remains nutrient and still grows allowing for the bone to keep elongating. Third, this is when spongy bone is developed. The cavities that were formed in the second step are consumed with periosteal buds. These buds contain the following, an artery, vein, nerve fibers, red marrow elements, osteoclasts, and osteoprogenitor cells. Both the osteoclasts and osteoprogenitor cells begin the process of the early stages of new spongy bone. In the fourth step, the medullary cavity is formed while the diaphysis of the bone elongates. Osteoclasts are responsible for breaking down some spongy bone that has formed creating a space for the medullary cavity. The hyaline cartilage continues growing elongating the model that ossification has to continue following. During this step is also when secondary ossification centers are produced. In the last step is when the cartilaginous epiphyses are ossified. Secondary ossification leaves behind spongy bone and no medullary cavity is formed, but once complete the only cartilage that remains on the epiphyseal surfaces and the epiphyseal plates. Like endochondral ossification bone growth at the epiphyseal plate are very similar process’s. At the epiphyseal plate on side is the resting zone where cartilage is almost inactive, but on the other side column shaped cartilage cells allow for fast growth. The first step in the growth process at the epiphyseal plate occurs at the proliferation zone. In this zone, the top zone closest to the epiphyseal plate, cells rapidly divide which causes the epiphysis to be pushed away from the diaphysis, creating length. In the hypertrophic zone, older chondrocytes are enlarged and the lacunae inside is deteriorated leaving behind large connecting spaces. In the calcification zone, chondrocytes die as well as matrixes which gives space for blood vessels and leaves calcified cartilage at the epiphysis-diaphysis junction. In the ossification zone, osteoclasts erode the cartilage that has been calcified then osteoblasts cover the leftovers in new bone, leading to medullary cavity elongation. The plate remains at the same thickness throughout each process, and during the elongation of the bone remodeling is continuous. These process’s in each zone usually cease after adolescence. After adolescence the epiphyseal plate thins out until it is completely replaced by bone tissue, and the bone no longer can grow longitudinally.
4. Myofibrils are a unit of muscle fibers, they are rod like that run the whole length of the fiber. They are responsible for about eighty percent of cellular volume. Myofibril contain striations that alternated light, I band, and dark, A bands. These myofibrils are composed of chains of sarcomeres. In the myofibrils the sarcomeres are attached at each z-disc. The sarcomere is the smallest functioning unit of the muscle fiber. Myofilaments are a smaller structure with in the sarcomere. There are two types, thick and thin, thick filaments contain myosin which stretch the whole length of A bands and connected at the M line, thin filaments contain actin and stretch length of the I band and partially into the A band. Sarcoplasmic reticulum monitors levels of ionic calcium in the muscle. I have the ability to store and releases it when muscles contract. Tubules of sarcoplasmic reticulum run longitudinally and surround myofibrils. Mitochondria and glycogen granules, work with the sarcoplasmic reticulum to produce energy for when the muscle contracts. T tubules are sarcolemma protrusions into the cell interior which form long tube. These tubules increase the surface area for the muscle fibers and runs through the terminal cisterns of the sarcoplasmic reticulum creating triads. These T tubules allow for nerve signals to reach the deepest parts of the muscle fiber, releasing calcium in every area of the cells to allow for the muscle to successfully contract.