The collection of like cells that have an identical origin which carries out a definite function together is what I am. I can only exist if there is a cellular level organization that is intermediate between cells and an organism (Starr, Beverly 67). These cells have similar functioning even though they might not be identical. When several different us are brought together, an organ is formed. I am a tissue.
We have cells with the capacity to contract causing movement of a part of a body. We are formed by contractile muscle cells. It is a soft tissue found in animals. There are three common types of us. The skeletal type that is held to the bone by tendons and its functions are in movement and posture. Smooth type is found within the epithelia. Some examples of the smooth type include arrector pili in the skin, stomach, uterus, intestines, esophagus, bronchi, and urethra among many others. The smooth type does not exhibit conscious control. The cardiac type is another involuntary one of us. It is more analogous in the constitution to skeletal type. It is found in the heart. The Cardiac type undergoes contractions and relaxations allowing the heart to pump blood around the body. Skeletal and cardiac types have sarcomeres crammed into highly even measures of packages. The cardiac type has bundles that join at uneven branching angles. These are called intercalated discs. Such types have short intervals of contractions and relaxations. The smooth type has the ability to sustain longer contractions that can sometimes be permanent. Proteins that make us a contract and relax are actin and myosin (Starr, Beverly 72). We are the tissue muscles.
The epidermis is the peripheral layer of the varies. Its thickness varies, the thinnest epidermis on human skin being about 0. 05mm. and the thickest 1. 5mm. There are five layers in the epidermis, namely stratum corneum, stratum licidum, stratum granulosum, stratum spinosum, and stratum basale from the top to the bottom (Starr, Beverly 68). The stratum basale cells are column-shaped and are actively dividing. The cells in the higher levels are flattened and dead (Starr, Beverly 68). The stratum corneum has of the dead, flat skin cells shed after every fourteen days. The epidermis has keratinized stratified squamous epithelium (Starr, Beverly 68). The cells present are of four different types: the keratinocytes produce keratin, a protein that is waterproof and toughens the skin. Melanocytes produce a pigment that offers the protection of the cells from UV radiations known as melanin. Langerhans cells have phagocytic macrophages. They interact with the white blood cells for immune response. The Merkel cells are found deep in the epidermis they serve a sensory function.
The hair is an extension of filamentous cells. It contains keratin and grows in the dermis from the follicles. There are three layers in the hair shaft, namely cortex, medulla, and cuticle (Starr, Beverly 68). The outermost sheet is the cuticle; it is a see-through tissue and makes hair shiny. The innermost layer is the medulla; it is set up by large cells. The middle layer is the cortex; it has coloring agents and keratin. The extent and the form of hair are cortex controlled. Hair contains some amount of water that helps in the maintenance of moisture and the balancing of physiological and biochemical properties. The hair follicle is a part of hair sunk below the scalp. This is where the hair shaft originates (Starr, Beverly 68).
References
Starr, Cecie, and Beverly McMillan. Human Biology. 9th ed. Stamford, Connecticut, U.S.: Cengage Learning. 2012. Print.