The skin is madeup of two different layers known as the epidermis and dermis. The epidermis isthe outermost layer and is designed to provide protection to the body. Thedermis is located beneath the epidermis and contains nerve endings that dealwith touch and temperature, blood vessels that help nourish the skin, sweat andoil glands that produce secretions and hair follicles that allow the body toproduce hair (Page). The dermis iscomprised of the papillary and reticular layers.
The papillary is the mostsuperficial and is made up of areolar connective tissue, which allow bacteriaeating cells to regulate the skin and protect it. (Marieb 154-156). Thepapillary layer extends to the epidermis of the skin, known as dermal papillae.These papillae play different roles depending on what they contain. Somecontain blood vessels that carry blood while others have nerves that deal withthe detection of pain and touch (A.D.A.M).
The feet and palms contain five layers of the epidermis and are known asthick skin because they have an extra layer known as the stratum lucidum. Inthese areas the papillae are found on top of the dermal ridges which leads tothe creation of epidermal ridges in the epidermis. These ridges deal withtraction and surface area and are the reason why fingerprints and footprintsare existent. The reticular is composed of dense irregular connective tissue andcontains elastin and collagen fibers. The elastin allows the skin to be able tostretch while the collagen prevents provides the skin with strength.
Collagenfibers are arranged in bundles and line the body. Cleavage lines can be foundin areas that contain spaces between the collagen. These cleavage lines run inthe same direction as the collagen (Marieb 155-156). The direction of thecleavage line dictates the healing of the skin, parallel incisions to thecleavage line heal quicker than incisions that are made perpendicularly(Martini). Hair is made of dead, keratinized cells. Theshaft and the root are the main regions of hair.
The shaft projects from theskin and is the part of the hair that can be seen. It is made up of threelayers known as the medulla, cortex and cuticle. The medulla is the centralcore, the cortex covers the medulla and the cuticle helps with giving the hair strength(Marieb 158-159).
The root is the region of hair that is inserted into theskin’s dermis layer (Ivey Rose). Ahair follicle goes through the epidermal surface to the dermis. At the bottomof the hair follicle are melanocytes that move to cortical cells.
Differentcolors and amounts of melanin come together to create different hair colors. Ahigh production of melanin leads to dark hair and air bubbles taking over theplace of melanin leads to gray and white hair (Marieb 159). There are two typesof hair known as terminal and vellus. Terminal hair is active and continues togrow a little bit every day.
It is the hair that is visible on the outside ofthe skin, such as hair on the arms, legs and head. Vellus hair is in a dormantphase and cannot be seen because it is not long enough. An example of vellushair is peach fuzz located on a person’s face. Hormones are a factor that causehair to grow.
Hormones such as androgens stimulate hair to grow in certainareas depending on if the gender of the person. Approximately ninety hairs aday are converted from the terminal form to the vellus form. Hair is capable ofgoing from the vellus form to the terminal form, but not all ninety hairs aregained back this way, which leads to hair thinning. True baldness is somethingthat occurs in men because it is a sex influenced condition.
The hormone thatis thought to induce male patterned balding is known as DHT. This is a type ofhormone that binds to a receptor and causes balding to occur (Marieb 159). Bones are made out of two differentlayers known as compact and spongy. Compact is the continuous outermost layerthat serves the main role of protection.
It is organized into units calledosteons, which are different layers composed of collagen that has beenmineralized. The collagen is laid down first and provides shape and supportwhile the mineralization makes the structure firm. Each osteon containsdifferent lamellae, which are the different layers. Each lamellae containscollagen fibers that go in different directions from lamellae layer beside it.The purpose of this is to help with flexibility, stability, and resist twistingmotions.
Compact bone also contains the Haversian and Volkmann Canals. Theycontain blood vessels and nerves that give the bone the nutrients it needs. TheHaversian canals run parallel through the osteon while the Volkmann canals runperpendicularly. The Haversian canal serves as a connector, connecting theblood and blood vessels of the bone’s outermost membrane to the Haversian canal(Marieb 177-179). The main difference between compact and spongy bone is how itis organized.
Spongy bone is made up of trabeculae, which is fibrous andcontains many pores that are filled with red or yellow bone marrow. Trabeculaeweighs less than compact bone and is therefore easier to move. It helps withshifting stress from one point to another (OpenStaxCollege). Endochondral ossification is the processof replacing cartilage with bone and it occurs in the primary ossificationcenter in the diaphysis. During this type of ossification, the original hyalinecartilage model is broken down and swapped with bone. In the first step ofendochondral ossification, osteoblast create a bone collar around thediaphysis, which helps with maintaining shape (Marieb 184). In the next step ofthe process, cartilage cells expand and block nutrients from coming in. Thiscauses the chondrocytes to decay so the calcification occurs and cavities form.
Next, periosteal buds which contain blood vessels and nerves go into thecavities (Briers). In this step, trabeculae are formed within. In the next stepof the process, a medullary cavity is formed and secondary ossification centerscan be found in the epiphyses. This is another place where bone formationoccurs and it leaves hyaline cartilage limited in the bone (Mananatomy). Thelast step of this process deals with the epiphyses turning into bone. Thegrowth of length in a long bone depends on the epiphyses and takes place in theepiphyseal plate. The resting zone can be found on the side of the epiphysealplate that is closest to the epiphysis.
It deals with the creation ofcartilage. On the side that is closest to the diaphysis, there are fourdifferent zones that are basically stacked upon on another known as theproliferation, hypertrophic, calcification and ossification zone. They dealwith the calcification of cartilage and the growth in length of the long bone.
(OpenStax).The proliferation zone is located closest to the plate and contains cartilagecells that go through mitosis. These cells push the epiphysis and diaphysisaway from one another. The hypertrophic zone is located under the proliferationzone and involves the expansion of cartilage cells that have been around for awhile. In the calcification zone, the matrix is calcified and the expandingcartilage cells decay. In the ossification zone, cartilage is cleaved and newbone is formed.
(Marieb 186). Endochondral ossification is a process that happensbefore long bone growth. Both processes involve lengthening of the diaphysisand similar steps like the decay of cells followed by formation of the bone.
