Cell Specialization
All organisms are composed of cells. Unicellular organisms are
composed of a single cell. Multicellular organisms are composed
of many specialized cells. Specialized cells differ in structure
(size, shape...) and function (the role they perform in the
organism). The structural modifications that occur in a
specialized cell equip it to do its job in the organism. An adult
human is composed of approximately 100 trillion cells and has
over 200 different types of specialized cells.
Examples of Specialized Cells
Sperm Cell
http://utahhealthsciences.net/customer/image_gallery/373/13SEM.J
PG
Fat Cells
www.broadinstitute.org
White Blood
Cell
http://medpediamedia.com/u/653pxSEM_Lymphocyte.jpg/653px-SEM_Lymphocyte_large.jpg
Macrophage
s
surrounding
parasite
www.eyeofscience.com
The life cycle of a cell consists of a growth phase (interphase)
followed by a division phase (either mitosis or meiosis).
Some cells cycle between interphase and division continuously
through the life of an organism. This allows the body to
produce new cells allowing for growth and maintenance of
tissues. Other cells exit the cell cycle and enter a non-dividing
phase called GO. Depending on environmental signals, they may
reenter the cell cycle or remain in GO permanently.
A cell specializes while in interphase or GO. The process in
which a cell becomes specialized is called differentiation and
occurs when the cell selectively activates or inactivates specific
genes.
The specialized cells of an organism contain the exact same
complement of genes. In humans, this means that each cell type
contains appromixately 30,000 genes. This is because each cell
is the descendent of a single cell (the fertilized egg) that
underwent mitotic cell division to form a multicellular organism
during embryonic development. Cell differences are the result
of differences in gene expression. A gene is expressed
(activated) when it is transcribed and translated into a protein.
scienceblog.com
Some genes (such as those responsible for glucose metabolism
or protein synthesis) are expressed in all cells. Other genes
are expressed in a single or select few specialized cells. The
expression of a gene within a particular specialized cell changes
over an individual’s life. For example, all human cells contain a
gene that codes for caesin, the major protein found in
milk. However, the gene is only expressed milk producing cells
of lactating females.
Melanocytes
Obtained from www1.imperial.ac.uk/
Location
Melanocytes are specialized skin cells located in the basal
(bottom) layer of the epidermis (the outermost layer of
skin).
Function
Melanocytes produce and secrete melanin, the pigment
that gives skin color and protects the DNA in skin cells
from UV radiation.
How cell structure relates to function
Melanocytes are rounded cells with long, branch-like
extensions called dendrites. The dendrites extend into the
lower layers of the epidermis.
http://photoprotection.clinuvel.com/custom/uploads/2008
0616_fig1_melanosome_v1(2).gif
Within each meloanocyte are unique organelles called
melanosomes where the production of melanin takes
place. The cell produces mass quanities of an enzyme
called tyrosinase that catalyzes the reaction that produces
melanin. A defect in the tyrosinase gene or the expression
of the gene blocks the production of melanin and results in
albinsm. The biochemical pathway that produces melanin
is common to many organisms, thus albinism occurs in
many species.
www.stormfront.org
sunde.worldpress.com
www.sharenator.com
After the melanin is produced, the dendrites of the
melanocytes transfer the melanosome (with the melanin)
to keratinocytes (the pigment storing cells of the
skin). The melanosomes are taken in by receptor
mediated endocytosis and are deposited over the nucleus
of the keratinocytes to protect the DNA from UV radiation.
Differences in skin pigmentation are not due to the activity
of the melanocytes, but the keratinocytes. In Caucasians,
lysosomes in the keratinocytes actually degrade some of
the melanin. Less pigment means lighter skin. The
melanosomes in individuals with darker skin are not
degraded.
Differentiation of Melanocytes
Melanocytes differentiate from unspecialized skin cells
during embryonic development. Changes in gene
expression result in the formation of the melanosomes
from components of the SER, RER and Golgi, the activation
of the tyrosinase gene, and modifications to the
cytoskeleton that allows for the formation of dendrites.
Melanocytes and the Cell Cycle
Melanocytes exit the cell cycle and enter a non-dividing
phase called G0 where they remain permanently. Here,
they perform their specialized role of producing and
secreting melanin. As you age, your melanocytes begin to
die. Since they don't divide, they slowly decline in
number. This is why skin becomes lighter as you age.
Red Blood Cells
http://media-2.web.britannica.com/
Location
Red blood cells (erythrocytes) are specialized cells of the
circulatory system. They are the most common component of
blood. A small droplet of blood contains about 5 million
erythrocytes. They constitute 40% of a female's blood volume and
45% of a male's blood volume. (Males have more red blood cells
than females! This is one of the reason males typically have
greater physical stamina then females.)
Function
RBCs carry oxygen from the lungs to body cells.
How cell structure relates to
function:
Red blood cells are flexible, flattened, disk shaped cells. They
resemble a ball of clay squeezed between the thumb and
forefinger. This shape is due to the fact that red blood cells don't
have a nucleus! When they differentiate, they lose their nucleus
which increases the area available inside the cell for hemoglobin, a
large, iron containing protein that acts as the binding site for the
oxygen. Each hemoglobin protein can bind four molecules of
oxygen. The flattened surface of the red blood cells increases the
area through which oxygen can diffuse through the cell
membrane. The flexibility of the red blood cell allows it to squeeze
through the tiny capillaries of the circulatory system.
Hemoglobin, oxygen binding protein in RBCs
www.daviddarling.info
Differentiation of RBC's
Red blood cells form from undifferentiated cells in the bone marrow
throughout your life. Bone marrow is the soft, interior portion of
certain bones found in the chest, upper arms, upper legs and
hips. The cells located here are undifferentiated, but limited in the
type of cell they can become. They can only differentiate into a
type of blood cell. They are capable of dividing continuously to
allow lost blood cells to be replenished. As RBC's differentiate,
changes in gene expression result in the destruction of their
nucleus, the production of hemoglobin proteins, and modifications
to the cytoskeleton allowing them to take their donut shape. Once
differentiation is complete, they are released into the blood
stream. You body replaces 2 million red blood cells every second.
blog.everyonemd.com
ml-leukemia.com
a
RBC's and the Cell Cycle
Red Blood Cells never divide. Before they are released into
circulation, they enter the non-dividing G0 phase and remain here
for their entire life span (which is only 120 days). Dead or damaged
red blood cells are removed from circulation by the spleen and
liver. The iron is recycled back to the bone marrow and used to
make more red blood cells. This is not a 100% efficient
process. Some iron is excreted and that is why you need iron in
your diet.
Sperm Cells
www.stanford.edu
Location
Sperm cells are specialized cells of the male reproductive system that form
in the testes.
Function:
The function of sperm cells is to deliver the paternal (father) contribution
of chromosomes to the ovum (egg).
How cell structure relates to function
Sperm cells have three distinct regions, the head, midpiece and tail. The
head consists of an acrosome and a nucleus. The acrosome is a
specialized lysosome containing enzymes that digest the protective outer
layers of the ovum allowing the sperm to penetrate and fertilize the
egg. The nucleus contains the father’s contribution of chromosomes for
the soon to be embryo. All cytoplasm surrounding the nucleus is
partitioned off during the maturation process to increase efficiency. The
midpiece is packed with mitochondria that break down glucose (present in
semen) producing ATP to power the movement of the flagella. The tail is
a single flagellum composed of microtubules. The flagellum is the source
of locomotion. Sperm can move about 3 mm per hour and must wave
their flagellum more than 1000 times to move half an inch.
Differentiation Process
Sperm cells differentiate from precursor cells called spermatogonia in the
testes. This begins at puberty and occurs for the rest of life. The
differentiation process begins when spermatogonia undergo meiosis, a
special form of cell division that cuts the chromosome number in
half. After meiosis, changes in gene expression result in the formation of
the acrosome, loss of excess cytoplasm, and formation of a flagellum.
Sperm Cells and the Cell Cycle
Fully differentiated sperm cells do not divide. After spermatogonia
complete meiosis, the resulting cells enter G0, a nondividing phase. Here
they complete their differentiation process. However, the spermatogonia
(precurser cells) divide daily. A normal adult male produces between 50
and 150 million sperm cells each day.
Adipocytes
http://www.biochem.arizona.edu/classes/bioc462
Location
Adipocytes are fat cells located under the skin
(subcutaneous fat), above the kidneys, and in the liver and
muscles. Men tend to concentrate their fat stores in the
chest, abdomen and buttocks producing an "apple"
shape. Women tent to carry their fat stores in the breasts,
hips, waist and buttocks, creating a "pear" shape. The
average person has between 25 and 30 million
adipocytes. This number can rise to 360 million in
seriously obese individuals.
Function
Adipocytes serve three functions. They insulate, provide
cushion, and most importantly store energy in the form of
triglycerides (1 glycerol plus 3 fatty acids).
How Structure relates to Function
Adipocytes resemble tiny plastic bags that are filled with
air (except they are filled with fat. They have a small,
flattened nucleus that sits off center along the plasma
membrane. 85% of the interior is occupied by one large
fat droplet. The surface of the plasma membrane has
receptors for insulin (the hormone that triggers the fat cell
to take up and store fat) and glucogon (the hormone that
induces the fat cell to release fat stores for energy).
Adipocytes and the Cell Cycle
As adipocytes differentiate, they exit the cell cycle and
enter the non-dividing phase (G0). When an individual
overeats, their fats store increase two ways. The
differentiated adipocytes increase in size as they take in
more triglycerides and the number of adipocytes increases
and excessive food intake stimulates the production of
more adipocytes from precursor cells.
It is currently under debate whether adipocytes remain in
G0 permanently or can reenter the cell cycle. If they
remain in G0 permanently, an individual can only
decreases fat stores by decreasing the size of the lipid
droplet within each adipocyte, not by reducing the number
of adipocytes. (Once you make a fat cell is stays with you
forever!)
However, there is evidence that adipocytes may be
capable of dedifferentiating (reverting back to their
precursor form) and reentering the cell cycle. This would
mean that people can reduce the number of adipocytes in
the body over time and decrease in fat stores occurs by
reducing the number and size of the adipocytes.