Microscopic Structure and Function of the Main Types of Human Cells - Assignment Example

Human Cells Question A. Outline the microscopic structure and function of the main types of cells found in human bone. (500 words) Bones are organs that consist of tough connecting tissue. They form the endoskeleton, which defines the supporting framework of the body. Bones work to mobilise the body by providing muscle attachment points, and protect the internal organs of the body. In addition they serve as factories for the production of the red and white blood cells. Bones store various vital minerals, and are a reservoir of calcium, phosphorous, sodium, magnesium and carbonate in the body. (Bone, 2010). Bone cells do not multiply, by the process of cell division. They adopt a mechanism, where the individual bone cells form a matrix, break the matrix and maintain it. A bone gets continuously rebuilt in this process and the areas under the greatest become the densest regions. This process promotes the healing of bone fractures and maintains calcium levels in the body (bone, 2009). Human bone tissue consists mainly of 5 different types of cells namely Osteogenic cells, Osteoblasts, Osteocytes, Osteoclasts and bone-lining cells. Osteogenic cells are capable of forming other bone cells. They become active in situations, such as fractures and form bone generating and bone destroying cells to heal the damaged bone (Bone Cells). The Osteogenic cells give rise to Osteoblasts, which are the actual bone-forming cells. Furthermore, these cells synthesise and regulate the deposition and mineralisation of the extracellular matrix of the bone. These cells have a single nucleus, and they work in unison to form bones. This new bone is termed osteoid, and it consists of bone collagen and other protein. Osteoblasts regulate the deposition of minerals and calcium, and they are located on the surface of the new bone. These cells synthesise and secrete collagen and glycoproteins around them, and they are found in the highly metabolic areas of the bone (Bone Cells). Osteocytes develop from the osteoblasts. The osteoblasts that have generated bone tissue around them mature, and are termed osteocytes. The amalgamation of osteoblasts into the bone matrix, results in osteocytes. These cells depict phosphatase alkaline behaviour and they are highly differentiated. They behave as mechanosensory cells and act as PTH receptors. Modifications of the bone structure and mass can be achieved by the mechanical stimuli of these cells. The biochemical mechanisms, which initiate the mechanotransduction of the stimuli, concentrate on these cells (Cells). Osteocytes exhibit lacuno-canalicular organisation in the bone porosity that facilitates mechanosensing. The interstitial flow of fluid in the bones is brought about by strain, and this could stimulate the osteocytes. The stimulation of osteocytes, involves the processing of intracellular signalling molecules. Moreover, osteocyte network in the bone cause the cellular organisation of the bone to respond to mechanical demands. This response could either augment or reduce bone apposition (Cells). In some receptors of the osteoblast, the phenotype of osteocytes is seen to be deficient. Despite this feature, the osteocytes are well suited to their function in bone homeostasis. Furthermore, these cells maintain intracellular signalling, so as to respond to the exclusive demands of its location (Cells). The term Osteoclast is derived from the Greek words “octo” and “clast”. Octo denotes bone, while clast means broken. They originate in the bone marrow and are related to the white blood cells. These cells are formed when the fusion of two or more cells transpires. Consequently, these cells display more than one nucleus; and they destroy the bone tissue by getting rid of the bone matrix. In this process minerals, like calcium are released to the blood from the bone. This process is termed bone resorption. Osteoclasts are located on the surface of bone mineral, adjacent to the dissolving bone (Cells). Bone lining cells are inactive osteoblast cells. These are generated on the surface of the bones of adults. Bone lining cells function in the transfer of minerals and a few ions in and out of the bone (Bone Cells). Question B. Describe the process of mitosis and outline how cell aging and death affects normal cell replication. (1000 words) Mitosis is a process where a division of the cell’s nucleus takes place. in this process the chromosomes that carry the hereditary information are replicated exactly. Moreover, each of the daughter nucleie formed during this process, receives a chromosome identical to that of the other. The resultant daughter cells are identical to each other and to the parent cell. Mitosis is almost always accompanied by the process of cell division known as cytokinesis. The latter is sometimes considered as a stage in the mitosis process (mitosis, 2008). Mitosis is a four stage process, and these stages are the prophase, the metaphase, the anaphase and the telophase. These stages transpire in an uninterrupted manner, and mitosis takes on an average, about an hour for completion. The time taken for successive cell divisions or interkinesis varies, but is considerably longer in duration (mitosis, 2008). (mitosis, 2003) Prophase The initial phase in the mitosis process is the prophase. In general, the genetic material in the nucleus is dispersed in the form of long networked filaments, which are denoted by the term chromatin. In this phase, the chromatin diffuses and condenses into chromosomes. Each of these chromosomes replicates itself and consists of two sister chromatids (mitosis, 2008). These two chromatids are attached to one another at the region, termed the centromere. Thereafter, each chromatid contracts into a tightly coiled body, known as the nucleolus. In most cases, the nuclear envelope disappears at this stage, and the spindles are formed (mitosis, 2008). Metaphase This phase marks the alignment of the chromosomes at the equatorial plate, midway between the ends that the spindles taper to. This equatorial plane marks the point where the cell divides into two. The chromosomes are held in place by the microtubules that are attached to the mitotic spindle and to a portion of the centromere. The ends of the spindles are the place where the chromosomes migrate to at the end of this process (mitosis, 2008).  Anaphase In this phase, the chromatids move apart to the opposite poles, just as if they had been pulled along the spindle fibres by the centromeres. Thereafter, the centromeres divide into two and the daughter chromatids separate and move towards the corresponding sides. Hence, this phase is also defined as the separation of the chromatids (mitosis, 2008). Telophase In this final phase of mitosis, the daughter chromosomes move towards the poles and the microtubules disappear. Two new nuclear envelopes begin to reappear around the newly formed daughter chromosomes. The condensed chromatin now begins to expand, and finally the cytoplasm of the cell divides. Lastly, the cell membrane separates and forms two distinct daughter cells (mitosis, 2008). Over the course of a cell’s life, its activity progressively deteriorates. After a certain number of replications, cells stop growing and undertaking further replication. This phenomenon is termed as cell senescence. After this happens, the cells will eventually die (apoptosis, 2009). All cells contain genetic information that makes them self-destruct in response to certain external conditions such as injuries and disease. This also transpires due to the natural process of maintaining the required number of cells in an organism. Cell deaths are of two kinds – apoptosis and necrosis. Apoptosis is a carefully planned process, whereby the cell dies to maintain the appropriate number of cells in the organism. Hence it can be considered as the regular process that opposes mitosis. However necrosis is different, as it is unplanned. It occurs spontaneously in response to external injury, lack of bacteria or due to some toxins (apoptosis, 2009). In the beginning of apoptosis, a cell first produces the required enzymes that dissolve the cell contents. Then the cell transforms into a spherical shape with balloon like bumps on the outer surface of the cell membrane. The enzymes then break down the cell contents into small entities, which are then absorbed by the neighbouring cells. The cancerous cells have mutations in the genetic information that provide instructions for apoptosis, which disables the latter process and renders the malignant cells “immortal”. Thus, cancerous cells disrupt the balance of creation and destruction of cells (apoptosis, 2009). Mitosis and apoptosis are two inherently opposing life processes; one creates cells, whilst the other destroys them. These processes go hand in hand to enable the body to carry out its metabolic activities, in an effective manner. Cells are created, destroyed and renewed on account of these processes. Furthermore, tissues remain healthy, and the injuries and damage sustained by the tissues are healed, due to the newly formed cells, which replace the older or unhealthy cells (apoptosis, 2009). A loss of this delicate balance between mitosis and apoptosis engenders dire consequences. If the rate of apoptosis increases or is triggered without a valid reason, a large number of important cells are lost, and this often proves fatal. For example, scientists believe that Alzheimers, Parkinsons, and Lou Gehrigs diseases are due to an increase of apoptosis. Moreover, an increase in the rate of mitosis results in cancer (Saltsman, 2005). Hence mitosis and apoptosis are two complementary processes that work in tandem and are quintessential for the healthy functioning of the body. An increase in the rate of either of these processes, results in abnormal situations, with unwanted results. List of References mitosis. (2003). Retrieved October 30, 2010, from The Macmillan Encyclopedia: http://www.credoreference.com/entry/move/mitosis mitosis. (2008). Retrieved October 30, 2010, from The Columbia Encyclopedia: http://www.credoreference.com/entry/columency/mitosis apoptosis. (2009). Retrieved October 30, 2010, from The Hutchison Unabridged Encyclopedia : http://www.credoreference.com/entry/heliconhe/apoptosis bone. (2009). Retrieved October 30, 2010, from Britannica Concise Encyclopedia: http://www.credoreference.com/entry/ebconcise/bone Bone. (2010). Retrieved October 30, 2010, from Blacks Medical Dictionary: http://www.credoreference.com/entry/blackmed/bone Bone Cells. (n.d.). Retrieved October 30, 2010, from http://www.mnsu.edu/emuseum/biology/humananatomy/skeletal/cells.html Cells. (n.d.). Retrieved November 9, 2010, from http://www.btec.cmu.edu/tutorial/cells/cells.htm Saltsman, K. (2005). The Last Chapter: Cell Aging and Death. Retrieved October 30, 2010, from National Institute of General Medicine: http://publications.nigms.nih.gov/insidethecell/chapter5.html Read More