The while the collagen prevents provides the skin with

The skin is made
up of two different layers known as the epidermis and dermis. The epidermis is
the outermost layer and is designed to provide protection to the body. The
dermis is located beneath the epidermis and contains nerve endings that deal
with touch and temperature, blood vessels that help nourish the skin, sweat and
oil glands that produce secretions and hair follicles that allow the body to
produce hair (Page). The dermis is
comprised of the papillary and reticular layers. The papillary is the most
superficial and is made up of areolar connective tissue, which allow bacteria
eating cells to regulate the skin and protect it. (Marieb 154-156). The
papillary layer extends to the epidermis of the skin, known as dermal papillae.
These papillae play different roles depending on what they contain. Some
contain blood vessels that carry blood while others have nerves that deal with
the detection of pain and touch (A.D.A.M).
The feet and palms contain five layers of the epidermis and are known as
thick skin because they have an extra layer known as the stratum lucidum. In
these areas the papillae are found on top of the dermal ridges which leads to
the creation of epidermal ridges in the epidermis. These ridges deal with
traction and surface area and are the reason why fingerprints and footprints
are existent. The reticular is composed of dense irregular connective tissue and
contains elastin and collagen fibers. The elastin allows the skin to be able to
stretch while the collagen prevents provides the skin with strength. Collagen
fibers are arranged in bundles and line the body. Cleavage lines can be found
in areas that contain spaces between the collagen. These cleavage lines run in
the same direction as the collagen (Marieb 155-156). The direction of the
cleavage line dictates the healing of the skin, parallel incisions to the
cleavage line heal quicker than incisions that are made perpendicularly
(Martini).

            Hair is made of dead, keratinized cells. The
shaft and the root are the main regions of hair. The shaft projects from the
skin and is the part of the hair that can be seen. It is made up of three
layers known as the medulla, cortex and cuticle. The medulla is the central
core, 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 the
skin’s dermis layer (Ivey Rose). A
hair follicle goes through the epidermal surface to the dermis. At the bottom
of the hair follicle are melanocytes that move to cortical cells. Different
colors and amounts of melanin come together to create different hair colors. A
high production of melanin leads to dark hair and air bubbles taking over the
place of melanin leads to gray and white hair (Marieb 159). There are two types
of hair known as terminal and vellus. Terminal hair is active and continues to
grow a little bit every day. It is the hair that is visible on the outside of
the skin, such as hair on the arms, legs and head. Vellus hair is in a dormant
phase and cannot be seen because it is not long enough. An example of vellus
hair is peach fuzz located on a person’s face. Hormones are a factor that cause
hair to grow. Hormones such as androgens stimulate hair to grow in certain
areas depending on if the gender of the person. Approximately ninety hairs a
day are converted from the terminal form to the vellus form. Hair is capable of
going from the vellus form to the terminal form, but not all ninety hairs are
gained back this way, which leads to hair thinning. True baldness is something
that occurs in men because it is a sex influenced condition. The hormone that
is thought to induce male patterned balding is known as DHT. This is a type of
hormone that binds to a receptor and causes balding to occur (Marieb 159).

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            Bones are made out of two different
layers known as compact and spongy. Compact is the continuous outermost layer
that serves the main role of protection. It is organized into units called
osteons, which are different layers composed of collagen that has been
mineralized. The collagen is laid down first and provides shape and support
while the mineralization makes the structure firm. Each osteon contains
different lamellae, which are the different layers. Each lamellae contains
collagen fibers that go in different directions from lamellae layer beside it.
The purpose of this is to help with flexibility, stability, and resist twisting
motions. Compact bone also contains the Haversian and Volkmann Canals. They
contain blood vessels and nerves that give the bone the nutrients it needs. The
Haversian canals run parallel through the osteon while the Volkmann canals run
perpendicularly. The Haversian canal serves as a connector, connecting the
blood 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 it
is organized. Spongy bone is made up of trabeculae, which is fibrous and
contains many pores that are filled with red or yellow bone marrow. Trabeculae
weighs less than compact bone and is therefore easier to move. It helps with
shifting stress from one point to another (OpenStaxCollege).

            Endochondral ossification is the process
of replacing cartilage with bone and it occurs in the primary ossification
center in the diaphysis. During this type of ossification, the original hyaline
cartilage model is broken down and swapped with bone. In the first step of
endochondral ossification, osteoblast create a bone collar around the
diaphysis, which helps with maintaining shape (Marieb 184). In the next step of
the process, cartilage cells expand and block nutrients from coming in. This
causes the chondrocytes to decay so the calcification occurs and cavities form.
Next, periosteal buds which contain blood vessels and nerves go into the
cavities (Briers). In this step, trabeculae are formed within. In the next step
of the process, a medullary cavity is formed and secondary ossification centers
can be found in the epiphyses. This is another place where bone formation
occurs and it leaves hyaline cartilage limited in the bone (Mananatomy). The
last step of this process deals with the epiphyses turning into bone. The
growth of length in a long bone depends on the epiphyses and takes place in the
epiphyseal plate. The resting zone can be found on the side of the epiphyseal
plate that is closest to the epiphysis. It deals with the creation of
cartilage. On the side that is closest to the diaphysis, there are four
different zones that are basically stacked upon on another known as the
proliferation, hypertrophic, calcification and ossification zone. They deal
with 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 cartilage
cells that go through mitosis. These cells push the epiphysis and diaphysis
away from one another. The hypertrophic zone is located under the proliferation
zone and involves the expansion of cartilage cells that have been around for a
while. In the calcification zone, the matrix is calcified and the expanding
cartilage cells decay. In the ossification zone, cartilage is cleaved and new
bone is formed. (Marieb 186). Endochondral ossification is a process that happens
before long bone growth. Both processes involve lengthening of the diaphysis
and similar steps like the decay of cells followed by formation of the bone.