Immune System

When you are asked to picture a human, what do you imagine? Someone young, strong, and healthy. When you imagine a human, you typically don't picture someone weak, frail and sick. That's courtesy of the immune system, whose function is to protect your body from the four categories of pathogens: bacteria, parasites, fungi and viruses. The immune system cells, leukocytes, can be classified into two separate categories: phagocytes and lymphocytes. Phagocytes consume invading organisms, and there are five different classes of phagocytes: basophils, neutrophils, eosinophils, monocytes and macrophages. Lymphocytes are capable of recognizing various pathogens, remembering them, and then destroying them whenever they invade. Lymphocytes can be classified into four primary groups: memory cells, killer cells, B lymphocytes and T lymphocytes. T lymphocytes can be further categorized as helper T cells and cytotoxic T cells. It is courtesy of lymphocytes that vaccines work: by injecting a weakened or dead form of a microorganism, the lymphocytes will recognize the pathogen when it attacks the body in full force, and the lymphocytes will then be able to fight them.

Types of pathogens

Types of leukocytes

Leukocytes and their functions

Key Immune System Terms:

Macrophage - A phagocyte that ingests invading organisms through phagocytosis, and destroys it by engulfing it into a lysosome.
Antigen - A foreign substance which invades the body and induces an immune response.
Helper T cell - A type of lymphocyte stored in the thymus that helps B cells destroy tagged antigens and signals phagocytes to perform their respective functions.
B cell - A type of lymphocyte stored in the bone marrow that tags invaders with antibodies to be destoryed by helper T cells.
Antibody - A type of protein manufactured by B cells and attached to antigens so that helper T cells can recognize and destroy the invading substance.
Killer cell - As their name suggests, killer cells are lymphocytes that consume invading organisms and either devour them or release chemicals which will destroy them.
Cytotoxic T cell - A type of lymphocyte that releases chemicals into a cell's plasma membrane, resulting in cytolysis. Cytotoxic T cells perform this function typically in cells infected by a virus before the viral DNA is replicated.
Memory cell - A type of lymphocyte that aids the function of antigen recognition.

Immune system cells

Immune system cells

Immune system cells

There are three primary types of immunity. The first, innate immunity, the immunity that we are born with. Some viruses or germs we are just naturally immune to, whereas other species would feel their effects. Our innate immunity also includes special immune cells, our mucus membranes and skin, which are our "first line of defense" when attacked by invading organisms. The second type of immunity is adaptive immunity, which develops as we grow, and includes lymphocytes which recognize invading organisms that we have been previously exposed to. The third type of immunity is passive immunity, is short-term immunity provided by another source. An example of passive immunity would be the antibodies passed from a mother's breast milk to her child.

Types of immunity

Two well-known immune system disorders are lupus and allergies. Lupus, made famous by the television show, House, is an autoimmune disorder. As in all autoimmune disorders, the immune system mistakes bodily tissues as foreign invaders and attacks them. Lupus is characterized by muscle soreness, joint pain and inflammation, and in some cases, attacks on the kidneys and other organs. Allergies are another abnormal immune system response, although this one is more common. Allergies occur when the immune system attacks antigens from our environment, and attack them as if they were pathogens. Allergic reactions can be characterized by a variety of responses, such as watery eyes, sneezing, itchiness, hives, swelling and anaphylaxis.

Sleep

It is what infants do all the time, and what teenagers cannot get enough of. Sleep. Approximately a third of one's life is spent sleeping, and these hours during which the body rests and the brain "recharges" is a surprisingly complex process. This process is regulated by chemical signals in the brain. These chemicals are called neurotransmitters, because they pass signals between neurons of the brain. Two primary neurotransmitters involved in sleep include serotonin and gamma-aminobutyric acid (GABA), which work as a "union of opposites," where serotinin is involved in keeping the body awake and alert, while GABA inhibits serotinin and other neurotransmitters involved in keeping one awake, promoting sleep. These neurotransmitters are signaled to be produced or inhibited by the production or lack thereof of melatonin, a hormone secreted by the pineal gland. The production of melatonin is regulated by the sleep-wake cycle, known as the circadian clock. When one is awake, light strikes the retina, which signals the suprachiasmatic nucleus to suppress the pineal gland's production of melatonin, which in turn results in the suppression of GABA production. However, at night, when less light strikes the retina, the suprachiasmatic nucleus stops suppressing the pineal gland's production of melatonin, which in turn allows for the production of GABA, thus promoting sleep.

Serotonin molecule

Gamma-aminobutyric acid molecule

Circadian Rhythm

There are five stages of sleep; Stage 1, Stage 2, Stage 3 and Stage 4 sleep are all non-Rapid Eye Movement (nREM) periods, and the fifth stage is Rapid Eye Movement (REM) sleep. The complete cycle lasts approximately 90 minutes, and the average adult experiences 4-6 sleep cycles per night. In the nREM periods, the first stage of sleep is characterized by drowsiness. The body begins to relax, although sometimes one experiences hypnic jerks, sudden muscle spasms, associated with the feeling of falling. The second stage is characterized by light sleep. In this stage, one's body temperature lowers and heart rate slows, and on an electroencephalogram (EEG), a scientist would view slowing alpha waves with occasional spindles: sudden bursts of activity.The third stage is characterized by deep sleep. During this stage, one brain waves slow to delta waves, unlike the alpha waves that appear on an EEG during awakeness and stages 1-2. Stage 4 is the deepest stage of sleep, and it is believed to be the stage during which most brain recovery occurs. Finally, there are the REM periods. During these periods, brain waves appear to be similar to those seen when one is awake. Furthermore, one's blood pressure rises, breathing becomes erratic, and the brain is very active, yet one's muscles are paralyzed (with the exception of the cardiac and respiratory muscles). It is during REM sleep when one dreams. It is believed that REM sleep is essential for learning and memory retention. When one is sleep deprived for a long time, once they go through a full REM cycle, they experience REM-rebound, which is characterized by very intense dreams or nightmares. 

Sleep Cycle

Brain waves associated with each sleep stage

Sleep deprivation not only results in REM-rebound, it also causes difficulty in concentration and feelings of drowsiness, which can be very dangerous for it can result in car accidents or accidents at work (especially when working with heavy machinery). Furthermore, sleep deprivation weakens the immune system and can hinder many brain functions. Below is a table that illustrates how many hours of sleep one needs to maintain higher brain functioning.

Sleep deprivation

Necessary amounts of sleep

However, like in any physiological process in the human body, there are many disorders associated with sleep, including insomnia, sleep apnea, narcolepsy, and restless leg syndrome. Insomnia is characterized as recurring problems in falling or staying asleep. While there are medications to treat insomnia, most doctors encourage healthier habits before going to sleep, such as abstaining from technology, alcohol and caffeine. Sleep apnea is an incredibly dangerous sleep disorder characterized by periods during which one stops breathing during sleep and suddenly awakens. This disorder typically affects overweight people, particularly men, because of the pressure of excess adipose tissue compressing the pharynx. While the disorder is difficult to diagnose because the primary symptom is snoring, once diagnosed, there are multiple routes of treatment. In mild cases, lifestyle changes such as losing weight and quitting smoking could end the problem. In severe instances, there are multiple surgeries or therapies to treat the disorder. Another major sleep disorder is narcolepsy, which continues to puzzle doctors for there is no known singular cause. Narcolepsy is characterized by uncontrollable sleep attacks. In some cases, a narcoleptic may fall directly into REM sleep. It can be managed with stimulants in an attempt to keep the brain active. An additional, common sleep disorder is restless leg syndrome, a condition in which, when trying to fall asleep, one feels intense discomfort in their legs and the necessity to move them. Patients with restless leg syndrome often experience twitching in their legs when trying to fall asleep, which can be treated with medication. In order to learn more about treatments for sleep disorders, visit the Mayo Clinic's website, which takes a comprehensive look at disorders, symptoms, causes and treatments.

Aging

Aging is accepted as a natural part of life. There is no distinct start point, it just begins in each individual person at a different time, and it affect each person uniquely. There are many lifestyle and biological factors, however, that we have learned does affect the aging process.


One such relationship is that of aging and high-density lipoprotein (HDL). In recent studies, it has been proven that there is a direct link between not only high levels of HDL and lower risk of heart disease increasing one's lifespan (Cohen, 2007), yet it is apparent that higher levels of HDL decreases the memory loss associated with aging (Nagourney, 2008). Therefore, maintaining a healthy lifestyle is essential as one ages, to ensure not only a long life, but a high quality life.

High-Density Lipoprotein

Genetics also have a drastic affect on aging. When studying the length of telomeres, the protective caps on the ends of each chromatid which prevent deterioration. As one grows, one's cells divide, and, during this process, the telomeres get shorter and shorter. It is suggested that since we are born with a specific length of our telomeres, and since siblings often have telomeres of similar lengths, that there is a gene that codes for telomere length (Moisse, 2010). Research has also been performed revealing a potential link between telomere length and heart disease, as well as the idea that unhealthy lifestyle habits such as smoking, being obese and not exercising could accelerate telomere shortening.

Telomeres

As recent as 2003, a major discovery of the link of aging and genetics was uncovered by biologist David Sinclair. Sinclair, knowing that humans and yeast cells both utilize sirtuin, decided to test the effects of the gene SIR2 in yeast, which is comparable to SIRT1 is humans. He fed one group of yeast cells a normal amount of glucose, while he fed his experimental group 30-40% fewer calories of glucose. In the control group, the yeast cells aged, losing the shape of its DNA and thus deteriorating in the same way a human's cells would with cancer. However, in the calorie-restricted diet group, the yeast cells trigged SIR2 in response to the hungry, stressful situation and, as a result, stayed young. (Chadda, 2007). Now, scientists are using Sinclair's discovery as inspiration in their own studies between SIRT1 and aging in human cells. In the meantime, pharmaceutical companies are currently looking into resveratrol, a substance that appear to mimic the affects of a calorie-restricted diet by stimulating SIRT1, yet not reducing one's calorie intake. In an experiment on mice, it was revealed that mice who were given resveratrol lived 10-20% longer than those who did not consume it. Studies have also shown that red wine also contains this precious chemical, yet, in order to obtain the desired anti-aging affects illustrated in mice, humans would have to consume approximately 1,000 glasses a day (Cohen, 2007).

Resveratrol

Sirtuin

Maintaining a healthy lifestyle is also necessary in order to prevent or control aging. Even as one ages, one must take care of and protect one's body. Provided below is a compiled list of healthy habits to extend one's quantity and quality of life (Oz, 2010):

1. Maintain a healthy diet
2. Exercise consistently
3. Don't drink or smoke
4. Take 1,000 mg of calcium
5. Take 1,000 IU of Vitamin D (or get some sun for 15 minutes)
6. Sleep at least 7 hours a day

Healthy aging

Happy elderly

Bibliography:
Cohen, C. (2007, January 9). Aging. Retrieved November 28, 2010

  from , Public Broadcasting Services: NOVA Web site:
  http://www.pbs.org/wgbh/nova/body/aging.html

Chadda, R. (2007, January 1). Healthy Old Age. Retrieved

  November 28, 2010 from , Public Broadcasting Services: NOVA Web
  site: http://www.pbs.org/wgbh/nova/body/aging-longevity.html

Moisse, K. (2010, February 8). Researchers Identify Genetic Variant

  Linked to Faster Biological Aging. Retrieved November 28, 2010
  from , Scientific American Web site:
  http://www.scientificamerican.com/article.cfm?id=aging-telomere

Nagourney, E. (2008, July 1). Aging: Good Cholesterol, Good

  Memory. Retrieved November 28, 2010 from , New York Times Web
  site:
  http://www.nytimes.com/2008/07/01/health/research/01agin.html?_r=1

Oz, M. (2010, February 11).  Living Long and Living Well.

  Retrieved November 28, 2010 from , Time Magazine Web site:
  http://www.time.com/time/specials/packages/article/0,28804,1963392_1963369_1963380,00.htm

Stress

One of the primary goals of this module is to identify areas of stress in my life. While in this blog, I typically try to maintain a third-person omniscient perspective, here is my opportunity to reveal myself as a student, for stress encompasses my life as a whole. As if taking four AP courses (one of which is notorious for the amount of time required to be devoted to outside work) weren't enough, as well as an independent study course, a slew of extra curriculars, tutoring students, beginning the college process, taking standardized exams, balancing friends, family and homework and finding "me" time...I'd say that my life is pretty stressful. Currently, in an attempt to reduce stress, I've had to prioritize. Unfortunately, as a result, this blog has not been updated in almost a month.

Stress and the Body

While my experiences with stress, illustrated by my rant above, appear to be, well, stressful, they can sometimes be beneficial. In layman's terms, there is "good" stress, formally known as eustress, and "bad" stress, known as distress. Periods of stress that cause us to be more productive are good for us. This acute stress may occur before interviews, exams, or other situations in which we want to succeed. However, once we have recovered from the situation, we feel accomplished. Yet if this period stress lasts for a long period of time, chronic stress, one may suffer from physical manifestations of this overwhelming time. These symptoms may cause migraines, upset stomach, sleep problems, elevated blood pressure and even chest pains. Stress is both a physical and emotional condition. Emotional symptoms can cause people to "act out," be more agitated, depressed, nervous,  anxious, fearful, express feelings of being under constant pressure or suffer from "emotional breakdowns." These are not good ways to deal with stress, and therefore it is necessary to turn to other outlets to cope with stress.

Stress Response

Manifestations of Stress

In order to deal with bad stress, we develop coping mechanisms. However, these aren't always healthy, such as turning to alcohol, drugs, caffeine or overeating, which, in the end, cause more harm and stress to our bodies than how they alleviate the emotional and physical pain. In order to prevent stress, it is in your best interest to prioritize, avoid procrastination, stay positive, take care of your body by eating, sleeping and exercising, maintaining balance in your life and recognize your limitations. In unavoidable situations, especially, you must recognize your limitations and don't blame yourself. Additionally, try to relax your body and mind by meditating, deep breathing, yoga, and other calming techniques.

Exercise Physiology

The role of an exercise physiologist is to study the way the body functions during intense physical activity. Typically working with athletes, exercise physiologists observe how each individual's body reacts to exercise, and then creates the most beneficial work-out plan for them. Exercise physiologists study athleticism from a cellular level up to the organism (human) level. 


Oxygen intake is an essential area of study for exercise physiologists, because the amount of oxygen taken in by the athlete can determine how effectively their cells can utilize energy. The oxygen intake is measured as VO₂, so the VO₂ max is a measure of aerobic endurance: the maximum amount of oxygen that can be taken in despite varying intensities of workload. To measure the VO₂ max, exercise physiologists calculate the VO₂ max by determining how many milliliters of oxygen are taken in per kilogram of body weight per minute. This measurement can be taken by a simple step test. The participant steps up and off a step, one foot at a time, at a steady rate for 3 minutes. Once the 3 minutes have elapsed, the pulse rate is recorded. Using these measurements, the aerobic endurance can be calculated. The higher the VO₂ max, the more "fit" one is considered. This is because when the oxygen intake is higher, more molecules of adenosine triphosphate (ATP) can be broken down into adenosine diphosphate (ADP) which releases large amounts of energy into the muscles.

VO2 Intake Graph

Glycolysis

There are three primary energy pathways, aerobic metabolism, anaerobic metabolism and CP metabolism. These pathways are not turned on and off separately, rather, they work cohesively, particularly in team sports. Aerobic metabolism utilizes aerobic respiration, which can sustain an athlete for longer periods of time because the oxygen intake is utilized to break down ATP, releasing energy. Aerobic metabolism is typically used by long distance runners. Anaerobic metabolism utilizes anaerobic respiration. This is performed through glycolysis, the process that converts glucose into pyruvate resulting in the creation of ATP. In this form of respiration, oxygen is not required. However, the utilization of anaerobic respiration for too long a time causes lactic acid build up, which results in the body becoming fatigued. Anaerobic metabolism is typically utilized by sprinters. Sprinters also take advantage of CP metabolism (also known as the ATP-CP metabolism or ATP-CP system), a process that first breaks down the ATP reserves in the body and then breaks down creatine phosphate to resynthesize ATP. Once the creatine phosphate runs out, then the sprinter returns to aerobic or anaerobic metabolism. The CP energy pathway allows for short bursts of intense energy for rapid movements.

Energy Pathway

Glucose to ATP, ADP and CP

An athlete's diet is an essential part of their success. Since ATP and CP stores in the body are relatively small, glycogen, fat and protein stores allow for quick replenishment. Exercise physiologists suggest a diet of 55% carbohydrates which supply the glycogen stores which are used for short-term energy storage, 30% fats which are used for long-term energy storage, and 15% proteins, which, although they are not typically used as energy stores, they should be consumed to maintain the health of the muscles.

Suggested Diet Graph

Heart Surgery

The history of heart surgery is rather complex. The first surgery performed literally on the heart took place in Oslo in September 1895 when a surgeon ligated the coronary artery of a patient who was stabbed. Unfortunately, the patient passed away three days later. The first successful surgery on the heart was in September 1896 on a stabbed right ventricle. In 1925, Henri Souttar performed the first successful surgical treatment for mitral valve stenosis by palpating the damaged valve. It wasn't until after World War II that this method was adopted by surgeons across the United States. The first open heart surgery was performed in September 1952 by stopping and draining the blood of the heart.

Physical symptoms of an unhealthy heart can range from sweating to intense chest pains. The physiological signs of an underlying condition include an enlarged heart, more forceful heart beats and an abnormal heart rhythm. In order to detect a heart rhythm, doctors employ the use of an electrocardiogram (EKG or ECG). Since every time the heart beats electrical signals are set off by the nodes in the heart, these signals are recorded by the electrocardiogram which utilizes several wires with electrodes attached to the body to conduct these signals and record them on the monitor. A heartbeat is defined as every time the heart contracts and relaxes. The heart typically beats 70-80 times per minute at rest. However, due to the fact that a many patients reveal no abnormalities in a typical, resting electrocardiogram, a stress test (for example, having the patient run on a treadmill) is performed with the EKG still attached to record the heart when it is performing strenuous activity. There are five major points on an EKG revealed by each heartbeat, and they are labeled the P wave, QRS complex and T wave. When blood first entered the heart, it floods into the either the right atrium  (if it is oxygen-poor) or into the left atrium (if it is oxygen-rich). The sinoatrial node fires of a signal for the atria to contract, forcing the blood up into ventricles. This the P wave on the EKG. The QRS complex appears on an EKG when the atrioventricular node fires, causing the ventricles to contract. This contraction is called systole. The blood from the right ventricle is forced into the pulmonary artery and then the lungs to become oxygenated, while the blood from the left ventricle is forced through the aorta to flow to the rest of the body. When the ventricles contract, the tricuspid valve, the barrier between the right atrium and ventricle, and the mitral valve, the barrier between the left atriuim and ventricle, close so that the contraction of the ventricles does not force blood back the way it came, making a "lub" sound. The relaxation of the ventricles, called diastole, is exhibited by the T wave on the EKG. When the ventricles relax, the pulmonary and aortic valves close to prevent blood in these two major arteries from flowing backwards, making a "dub" sound.

EKG set-up

Placement of EKG electrodes

Blood flow through the heart

The heart and an EKG

There are three primary heart surgeries performed by cardiac surgeons: coronary bypass, heart transplant and angiocardiography. A coronary bypass the replacement of a damaged artery by removing a portion of the saphenous vein from the leg and placed where the diseased artery once was. A fun way to learn (and practice!) coronary bypass surgery is in this game courtesy of the Australian Broadcasting Corporation. Another excellent learning tool is created by a company called PlayGen which creates realistic simulations for professionals. A heart transplant is another major surgery performed by cardiac surgeons, during which the patient's damaged heart is removed and an artificial heart or a heart from a donor is placed in the chest cavity, and each of the arteries, veins and vessels are reattached. Angiocardiography is another procedure performed by cardiac surgeons. First, the doctor inserts a catheter into the arm or leg of the patient. He or she threads the catheter into the coronary artery and injects a dye which will fluoresce in an x-ray. This diagnostic tool allows the doctor to see if there are blockages in the artery, for if the dye flows through without a problem, the veins will appear to light up in the x-ray, but if there are such blockages, certain parts of each artery will appear dark for there is no flow of the dye through the veins.

One famous heart patient case history is that of Eileen Saxon more commonly known as "The Blue Baby." Saxon was born with the condition Tetralogy of Fallot, a severe congenital defect that prevents the blood from flowing through the heart properly, therefore resulting in the chronic lack of oxygen in the blood. This deoxygenated blood gave Saxon a blue appearance: her lips and fingers were blue and her skin had a bluish tinge. On November 29, 1944, which Saxon was only 15 months old, she underwent the first successful corrective surgery for the Tetralogy of Fallot by pioneering the Blalock-Taussig shunt which helped reroute the blood to be oxygenated. However, only a few months later did Saxon begin exhibiting symptoms of her congenital defect, and underwent another surgery, only to die a few days later, just before her third birthday. Although the surgery was not entirely successful, after the first shunt was placed, Saxon became a happy, pink, baby within two weeks of the surgery. The third time the surgery was peformed, this time was on a six-year-old boy, he began to turn pink quite quickly, establishing the Blalock-Taussing shunt as the premier corrective surgery for the Tetralogy of Fallot. For more information, visit the website from the hospital where it all took place, Johns Hopkins University, all due to Dr. Alfred Blalock, Dr. Heather Taussing, and assistant Vivien Thomas.

Dr. Alfred Blalock and Eileen Saxon

Normal Heart v. Tetralogy of Fallot

Artificial Organs and Regenerative Medicine

Recent discoveries made by biomedical engineers, and other scientists in related areas of research, have led to a new field of medicine, and new opportunities for patients with damaged organs. This field, regenerative medicine, is based off of the concept that every tissue in the human body has the capacity to regenerate, once cells that trigger cell division send the cues to the body. Currently, scientists are utilizing extracellular matrix, a combination of proteins and connective tissue, harvested from pig bladders and manipulated into powder form. This seemingly miraculous powder works on the cellular level to activate cells that build new cells when tissues become damaged. The field of regenerative medicine has proven quite promising; scientists have been able to trigger cell growth of 22 different types of tissues at one lab at Wake Forest University alone (for a comprehensive list of all the tissues, visit the Wake Forest Institute for Regenerative Medicine website). For example, these researchers were able to construct a completely functional esophagus and bladder. What was incredibly exciting was that, since cardiac cells are the only muscle cells that can move individually, the researchers at Wake Forest were able to build a heart valve that was already beating.

A researcher at Wake Forest University uses a mold to help an artificial bladder retain its shape and continue growing correctly

The benefits of artificial organs are obvious: fewer surgeries necessary, perhaps even a reduced need for organ transplants, lower risk of rejection because the organs are made from the body's own cells, the list of advantages are numerous. However, like any new scientific discovery, it has its pitfalls. First and foremost, since this breakthrough is so recent, there is not enough research that can conclude whether or not artificial organs can cause disorders or defects. Furthermore, the ultimate success of these organs are unknown, and treating a failing organ could be difficult. Another stumbling block is, similar to the debate of whether or not stem cell research is ethical, people are concerned by the ongoing research in these labs. Those in negation of this form of research question whether or not humans have the right to "play God" and create such organs.

Stem Cells

Currently, a major focus of biomedical researchers has been the study of stem cells. Stem cells can be categorized in three major groups. The first major group is known as embryonic stem cells (ES). These stems cells are derived, as their name suggests, from embryos that were formed in vitro. These cells can be further classified into totipotent and pluripotent. The totipotent stem cells are obtained from a fertilized egg in its earliest stages, whereas pluripotent stem cells are obtained from the blastocyst. These embryonic stem cells have not yet differentiated, thus, they can be used in any part of the body and become specific to a given tissue, organ, etc. This research as been hindered by the moral dilemma regarding whether or not a fertilized embryo is considered alive.

Totipotent and pluripotent stem cells

Another major category of stem cells is induced pluripotent stem cells (iPS). These cells are adult stem cells which are essentially "reprogrammed" to exhibit behaviors of an embryonic stem cell. They behave has pluripotent stem cells and can develop all of the germ layers of a normal cell, and then differentiate.

The final major category of stem cells is adult stem cells, also known as somatic stem cells. These cells are undifferentiated and are retrieved from mature tissues, as opposed to embryonic stem cells. While they can differentiate to be any type of cell, they typically differentiate to be the same type of cell as its surrounding tissue.

Cell differentiation

These cells can be manipulated to specialize in a laboratory. After retrieving a sample of the desired stem cells, the scientists then remove the outermost layer of the blastocyst, and allow them to foster in a petri dish. Colonies of the cells begin to grow, and as they do, certain cells begin to develop into endoderm, mesoderm and ectoderm cells. Scientists can then manipulate the differentiation by adding growth factors, causing the cell to believe that its environment is that of a specific tissue type. This process results in the differentiation of a cell.



Cell development



Stem cells are invaluable to curing and treating various diseases. Currently, stem cells are being utilized to treat blood-cell disorders and in bone marrow. Ongoing research is studying the possibilities of using stem cells to treat damaged cardiac muscle in heart disease, creating skin grafts for burn victims, repairing the spinal cord, treating Parkinson's disease, Alzheimer's disease, diabetes, osteoarthritis, and rheumatoid arthritis.

Potential usage of stem cells to treat heart disease

Embryology

After the ejaculation of semen during sexual intercourse, the sperm begins its path towards fertilization. The head of the sperm is called the acrosome. Once the sperm reaches the zone pellucida of the egg, the acrosome undergoes the acrosomal reaction, during which the acrosome fuses to the sperm's plasma membrane, resulting in the release of enzymes that were once contained in the acrosome. The acrosomal reaction allows for the sperm to break through the egg's plasma membrane in order to fuse with the egg. This fusion marks the beginning of fertilization. In order to prevent the egg being fertilized by more than one sperm, the egg undergoes the cortical reaction, which depolarizes the egg's plasma membrane, thereby avoiding polyspermy.

Cortical reaction

Fertilized egg - Zygote

Once fertilized, the zygote begins cleavage. The fertilized egg undergoes mitosis and DNA synthesis of the sperm and egg, followed by the division into two separate cells. This mitotic division results in the creation of a blastula, a ball of cells surrounded by a liquid cavity, known as a blastocoel. 

The cleavage of a zygote

Through gastrulation, the blastula divides into a multi-layered embryo, creating a gastrula. The gastrula is comprised of four main parts. The outermost layer is the ectoderm, which is responsible for the creation of tissues associated with the skin, hair, sweat glands, epithelium, as well as the brain and nervous cord. The middle layer is the mesoderm, which is responsible for the formation of connective tissue, muscle, cartilage, bone, blood, kidneys and reproductive organs. The innermost layer, the endoderm, is responsible for the creation of the organs involved in the digestive system, respiratory system, as well as the liver, pancreas, gall bladder, thyroid and parathyroid glands. The inner cavity of the gastrula is the archenteron, which forms the alimentary canal responsible for carrying food through digestion.

Gastrulation

The formation of the organs in each layer of the gastrula is known as organogenesis.

Table of organs formed in each layer of the gastrula