Human Chorionic Gonadotropin

The syncytiotrophoblast is responsible for producing a hormone called human chorionic gonadotropin (hCG). This hormone signals other parts of the female reproductive system that fertilization and implantation have occurred, so the uterine lining should continue to grow and develop (rather than being shed as menstruation). By the end of the second week of development, sufficient quantities of hCG are produced to be detected in a woman’s urine. The presence of hCG in urine indicates a woman is pregnant, and thus hCG is the basis for modern-day pregnancy tests. For the first 3 months of pregnancy, hCG levels remain high, but after that they decline. By this time, hCG is no longer needed because the placenta is producing its own hormones to maintain the pregnancy.

source :human anatomy

Chromosomal Abnormalities and Their Effect on the Blastocyst

Abnormalities in chromosome number, shape, or form occur regularly. These abnormalities can occur during gametogenesis, fertilization, or cleavage. If the chromosomal abnormalities are severe enough, they result in the spontaneous abortion (miscarriage) of the blastocyst or embryo. Many of these spontaneous abortions occur early in pregnancy (within 2 to 3 weeks after fertilization), so a woman often spontaneously aborts without realizing she was ever pregnant.



Some estimates propose that approximately 50% of all pregnancies terminate as a result of spontaneous abortion; perhaps half of these are caused by chromosomal abnormalities in the developing organism. As a consequence, fewer organisms are stillborn or born with severe congenital malformations (birth defects).
Thus, while 2–3% of all infants are born with some type of birth defect, this percentage would be much higher if not for the high frequency of spontaneous abortions very early in pregnancy.

source : human anatomy

Cystic Fibrosis and Chloride Channels

The inherited disease cystic fibrosis (CF) involves defective plasma membrane proteins that affect chloride ion (Cl − ) channels in the membrane. These channels are transport proteins that use facilitated diffusion to move chloride ions across the plasma membrane. The genetic defect that causes CF results in the formation of abnormal chloride channel proteins in the membranes of cells lining the respiratory passageways and ducts in glands, such as the pancreas. The primary defect in these chloride channels results in an abnormal flow of chloride ions across the membrane, causing salt to be trapped within the cytoplasm of affected cells.
Ultimately, the normal osmotic flow of water across the plasma membrane breaks down. The concentration of salt within the cytoplasm of these cells causes an increase in the osmotic flow of water into the cell, thereby resulting in thickening of the mucus in the respiratory passageways and the pancreatic ducts. The aggregation of thickened mucus plugs the airways of the lungs, leading to breathing problems and increasing the risk of infection.
Therefore, a single genetic and biochemical defect in a transport protein produces significant health problems.

source : human anatomy

Hypertension: The “Silent Killer”

Hypertension is chronically elevated blood pressure, defined as a systolic pressure greater than 140 mm Hg and/or a diastolic pressure greater than 90 mm Hg. About 90–95% of all hypertension cases are essential hypertension, in which the cause is idiopathic (unknown).
Secondary hypertension accounts for the other 5–10% of cases, meaning that the high blood pressure is caused by another condition, usually renal disease or an adrenal gland tumor.


Hypertension has many serious effects on the body. It causes changes in the blood vessel walls, making them prone to further injury.
Increased damage makes the blood vessels more likely to develop atherosclerosis (see Clinical View: In Depth on page 708). In addition, hypertension causes undue stress on arterioles, resulting in thickening of the arteriole walls and reduction in luminal diameter, a condition called arteriolosclerosis. Furthermore, hypertension causes thickening of the renal arteries, leading to renal failure, and it can greatly damage the cerebral arteries, making them prone to rupture, which results in a fatal brain hemorrhage or stroke. Finally, hypertension is a major cause of heart failure owing to the extra workload placed on the heart.


Since hypertension is initially asymptomatic (meaning no noticeable symptoms are present), it has been dubbed the “silent killer.”
Everyone is encouraged to have their blood pressure checked early in life, and regularly, to make sure they are not suffering from hypertension. Mild hypertension may be controlled by losing weight, eating a healthy diet, exercising regularly, and not smoking. Stress is also associated with hypertension; thus, reducing stress, or learning to manage it, is important in treatment. In many instances, however, medication may be needed to control hypertension. Diuretics increase urine output, thereby reducing salt and water retention and consequently lowering blood volume. Beta-blockers slow the heart rate and lower heart output, while ACE (angiotensin-converting enzyme) inhibitors block angiotensin II (a protein that constricts arterioles), thereby increasing vasodilation.

source : human anatomy

Deep Vein Thrombosis

Deep vein thrombosis (throm-bō ′sis; a clotting) (DVT) refers to a thrombus,
which is a blood clot in a vein. The most common site for the thrombus is a vein in the calf (sural) region; the femoral region is another common site. The blood clot partially or completely blocks the flow of blood in the vein. DVT typically occurs in individuals with heart disease or those who are inactive or immobile for a long period of time, such as bedridden patients or those who have been immobilized in a cast. Even healthy individuals who have been on a long airline trip may develop DVT. In fact, DVT is sometimes called “economy class syndrome” in reference to the reduced amount of leg room in economy class seating on airlines.

DVT may also be a complication in pregnancy, where fluid accumulation in the legs and impingement of the fetus on the inferior vena cava may prevent efficient blood flow back to the heart. For inactive individuals, the leg muscles (e.g., gastrocnemius and soleus) do not contract as often and can’t help propel blood through the deep veins, thus allowing the blood to pool and potentially to clot. Initial signs of DVT include fever, tenderness and redness in the affected area, severe pain and swelling in the areas drained by the affected vein, and rapid heartbeat. A person experiencing these symptoms should seek immediate medical attention. The most serious complication of DVT is a pulmonary embolus (em′bō -lū s; a plug), in which a blood clot breaks free within the vein and travels through vessels to the lung, eventually blocking a branch of the pulmonary artery and potentially causing respiratory failure and death. If a DVT is diagnosed, the patient is given anticoagulation medication (such as low-molecular-weight heparin) to help prevent further clotting and break up the existing clot.


To reduce the risk for DVT, a person should maintain a healthy weight, stay active, and treat medical conditions that may increase the risk for DVT. On a long airline flight or car trip, stretching the legs and moving the feet frequently assist venous circulation in the legs.
Bedridden individuals may wear full-length compression stockings to assist circulation in the lower limbs.



Source : human anatomy

varicose veins

Varicose (var′i-kōs; varix = dilated vein) veins are dilated, tortuous (having many curves or twists) veins. The valves in these veins have become nonfunctional, causing blood to pool in one area and the vein to swell and bulge. Varicose veins are most common in the superficial veins of the lower limbs. They may be a result of genetic predisposition, aging, or some form of stress on the venous system that inhibits venous return (such as standing for long periods of time, obesity, or pregnancy). Varicose veins may become inflamed and painful, especially if fluid leaks from them into the tissues.

Symptoms of varicose veins may be alleviated by elevating the affected body part or wearing compression stockings (to promote blood movement in the lower limbs). In a procedure called sclerotherapy, an irritant is injected into small varicose veins to make them scar and seal off. Typically, a patient needs multiple sclerotherapy sessions before optimal results are seen. For larger varicose veins, an outpatient surgical procedure called stripping or vein removal (phlebectomy) is necessary.

 These veins can be removed without affecting the circulation, since the blood may be shunted to other veins that are not varicose. Even after treatment, it is possible for varicose veins to recur.
Varicose veins in the anorectal region are called hemorrhoids (hem′ō -royd). Hemorrhoids occur due to increased intra-abdominal pressure, as when a person strains to have a bowel movement or is in labor during childbirth. Hemorrhoids may need to be surgically excised if they become too painful or bleed excessively.


source : human anatomy

Cardiac Arrhythmia

Cardiac arrhythmia (ā-rith ́mē -ā ; a = not, rhythmos = rhythm), also called dysrhythmia, is any abnormality in the rate, regularity, or sequence of the cardiac cycle. Several common arrhythmias have been described:

• Atrial flutter occurs when the atria attempt to beat at a rate of 200 to 400 times per minute, and as a consequence literally bombard the AV node with muscle impulses. Abnormal muscle impulses flow continuously through the atrial conduction system, thus stimulating the atrial musculature and AV node over and over. This condition may persist for years, and frequently degenerates into atrial fibrillation.
• Atrial fibrillation (fı¯-bri-lā ́shūn) differs from atrial flutter in that the muscle impulses are significantly more chaotic, leading to an irregular heart rate. The ventricles respond by increasing and decreasing contraction activities, which may lead to serious disturbances in the cardiac rhythm.
• Premature ventricular contractions (PVCs) often result from stress, stimulants such as caffeine, or sleep deprivation. They occur either singly or in rapid bursts due to abnormal impulses initiated within the AV node or the ventricular conduction system. All of us experience an occasional PVC, and they are not detrimental unless they occur in great numbers. Most PVCs go unnoticed, although occasionally one is perceived as the heart “skipping a beat” and then “jumping” in the chest.

A more serious arrhythmia is ventricular fibrillation, a rapid, repetitious movement of the ventricular muscle that replaces normal contraction. This is a life-threatening condition caused by scattered impulses originating at different times and places throughout the entire myocardium. Because the contractions of a heart in fibrillation are uncoordinated, the heart does not pump blood, and blood circulation stops. This cessation of cardiac activity is called cardiac arrest.
Fibrillation almost certainly results in death unless the normal rhythmic contractions of the heart are promptly restored. To restore normal heart contractions, medical personnel apply a strong electrical shock to the skin of the chest using paddle electrodes. The electrical current passes through the chest wall to completely and immediately depolarize the entire myocardium. This procedure is analogous to pushing the reset button on a computer—and as in rebooting the computer, the hope is that when the heart begins to function again, it will work as intended.


Source : human anatomy

Valve Defects and Their Effects on Circulation

Structural damage to the heart valves can impair blood circulation and lead to serious health problems. Damage may result from developmental abnormalities, infection, hypertension, or other cardiovascular problems.


Valvular insufficiency, also termed valvular incompetence, occurs when one or more of the cardiac valves leaks (called “regurgitant flow”) because the valve cusps do not close tightly enough. Inflammation or disease may cause the free edges of the valve cusps to become scarred and constricted, allowing blood to regurgitate back through the valve. As the heart works to overcome the effect of the backflow, blood must be forced through the valve openings and this effort may cause heart enlargement. As a result, the heart must work harder to circulate the normal amount of blood.




Valvular stenosis (ste-nō ́sis; narrowing) is scarring of the valve cusps so that they become rigid or partially fused and cannot open completely. A stenotic valve is narrowed and presents resistance to the flow of blood, so that output from the affected chamber decreases.
Often the affected chamber hypertrophies and dilates—both conditions that may have deleterious consequences. Heart function may become so reduced that the rest of the body cannot receive adequate blood flow. A primary cause of valvular stenosis is rheumatic heart disease.




Rheumatic (roo-mat ́ik) heart disease may follow a streptococcal infection of the throat. It results when antibodies produced to kill the bacteria cross-react with the body’s own connective tissue, thereby initiating an autoimmune disease. All parts of the heart are subject to injury, but the endocardium, the valve cusps, and the left AV valve are typically most affected. Significantly scarred and narrow valves must be surgically repaired or replaced. Patients with a history of rheumatic heart disease must take antibiotics before undergoing dental or medical procedures that are likely to introduce bacteria into the bloodstream.

source : human anatomy

Angina Pectoris and Myocardial Infarction

The most common cause of death in the United States is coronary atherosclerosis ( ath ér-ō-skler-ō ́sis; athere = gruel, sclerosis = hardness), or coronary heart disease (see Clinical View, “Atherosclerosis,” in chapter 23).
This condition is characterized by narrowing of the coronary arteries that reduces blood flow to the myocardium and gives rise to chest pain.
Coronary atherosclerosis can lead to either angina pectoris or myocardial infarction.


Angina pectoris (an ́jı¯-nā , an-ji ́na) is not a disease, it is actually a symptom of coronary artery disease caused by narrowing or blockage of coronary arteries. Generally it results from strenuous activity, when workload demands on the heart exceed the ability of the narrowed coronary vessels to supply blood. The pain from angina is typically referred along the sympathetic pathways (T1–T5 spinal cord segments), so an individual may experience pain in the chest region or down the left arm, where the T1
dermatome is located. The pain diminishes shortly after the person stops the exertion, and normal blood flow to the heart is restored. Although many people are successfully treated for years with medications that cause temporary vascular dilation, such as nitroglycerine, the prognosis and longterm therapy for angina depend on the severity of the vascular narrowing.

Myocardial infarction (in-fark ́shūn) (MI), commonly called a heart attack, is a potentially fatal condition resulting from sudden and complete occlusion (blockage) of a coronary artery. A region of the myocardium is deprived of oxygen, and some of this tissue may die (necrose). The symptoms of MI are often different for men and women. Most men experiencing an MI report a sudden, excruciating, and crushing substernal chest pain, and marked sweating, Many women, however, have relatively little chest pain. When they experience pain, they describe it as an “aching,” “tightness,” or “pressure,” rather than pain, and the main locations are in the back and high chest. In addition, women more often experience shortness of breath, but because they do not exhibit t he traditional or classic symptoms that some doctors expect, it is likely for a female to be sent home from the ER with an incorrect diagnosis of heartburn or anxiety, rather than MI.

source : human anatomy

pericarditis - inflammation of the pericardium

Pericarditis (per ́i-kar-dı¯ ́tis; peri = around, kardia = heart, ite s = inflammation) is an inflammation of the pericardium typically caused by viruses, bacteria, or fungi. Whatever the cause of pericarditis, the pericardium is inflamed. The inflammation causes an increase in capillary permeability. Thus, the capillaries become more “leaky,” resulting in fluid accumulation in the pericardial cavity. At this point, the potential space of the pericardial cavity becomes a real space as it fills with fluid and pus. In severe cases, the excess fluid accumulation limits the heart’s movement and keeps it from filling with an adequate amount of blood. The heart is unable to pump blood, leading to a medical emergency called cardiac tamponade and resulting in heart failure and death.

Pericarditis typically occurs between the ages of 20 and 50. Fever and chest pain are frequent symptoms. Pericarditis pain is located over the center or left side of the chest, and may extend to the neck or left shoulder. Patients often describe the pain as piercing or “knifelike,” and say that breathing worsens it. In contrast, pain from a myocardial infarction typically is described as crushing. But although the two conditions are different, the diagnosis of myocardial infarction and pericarditis often may be confused, especially by the patients experiencing the symptoms. A helpful diagnostic finding in pericarditis is friction rub, a crackling or scraping sound heard with a stethoscope that is caused by the movement of the inflamed pericardial layers against each other. The inflammation results in the loss of the lubricating action of the serous membranes.

source : human anatomy

Tonsillitis and Tonsillectomy

Because the tonsils are designed to protect the pharynx from infection, they frequently become inflamed and infected, a condition called acute tonsillitis (ton ́si-lı¯ ́tis). The palatine tonsils are most commonly affected. The tonsils redden and enlarge—in severe cases, to the point that they partially obstruct the pharynx and may cause respiratory distress.

Tonsils may be infected by viruses (such as adenoviruses) or bacteria (most commonly Streptococcus). Streptococcal tonsillitis often results in very red tonsils that have whitish specks (called whitish exudate). The symptoms of tonsillitis include fever, chills, sore throat, and difficulty swallowing. Bacterial tonsillitis (e.g., “strep throat”) is successfully treated with antibiotics such as penicillin or amoxicillin. If tonsillitis is caused by a virus, measures to relieve the inflammation (such as pain medication and/or gargling) are advised, since standard antibiotics are not effective against viruses.

Persistent or recurrent infections can lead to permanent enlargement of the tonsils and a condition known as chronic tonsillitis.

If medical treatment does not help the chronic tonsillitis, surgical removal of the tonsils (tonsillectomy) may be indicated. Typically, medical guidelines suggest performing a tonsillectomy only if the person has had six to seven tonsillar infections in 1 year, or two to three infections per year for several years running. Research indicates that tonsillectomy does not significantly affect the body’s response to new infections.

source : Human anatomy

Diabetes Mellitus

Diabetes (dı¯-ā -bē ́tez) mellitus (me-lı¯ ́tū s; sweetened with honey) is a metabolic condition marked by inadequate uptake of glucose from the blood. The name “diabetes mellitus” is derived from the phrase “sweet urine” because some of the excess glucose is expelled into the urine, resulting in glycosuria (glucose in the urine). Chronically elevated blood glucose levels damage blood vessels, especially the smaller arterioles.
Because of its damaging effects on the vascular system, diabetes is the leading cause of retinal blindness, kidney failure, and nontraumatic leg amputations in the United States. Diabetes is also associated with increased incidence of heart disease and stroke. Three categories of diabetes mellitus are type 1 diabetes, type 2 diabetes, and gestational diabetes.


Type 1 diabetes, also referred to as insulin-dependent diabetes mellitus (IDDM), is characterized by absent or diminished production and release of insulin by the pancreatic islet cells. This type tends to occur in children and younger individuals, and is not directly associated with obesity. Type 1 diabetes develops in a person who harbors a genetic predisposition, although some kind of triggering event is required to start the process. Often, the trigger is a viral infection; then the process continues as an autoimmune condition in which the beta cells of the pancreatic islets are the primary focus of destruction. When the beta cells are destroyed, no insulin is produced, so blood glucose cannot be taken up and utilized by the body tissues. Treatment of type 1 diabetes requires daily injections of insulin. Newer monitoring instruments allow rapid monitoring of blood glucose, allowing better management of food and insulin. Insulin pumps provide the ability to program insulin delivery as well as manual dosage, greatly improving the treatment and lifestyle of people with IDDM.


Type 2 diabetes, also known as insulin-independent diabetes mellitus
(IIDM), results from either decreased insulin release by the pancreatic beta cells or decreased insulin effectiveness at peripheral tissues. This type of diabetes was previously referred to as adult-onset diabetes because it tended to occur in people over the age of 30. However, type 2 diabetes is now rampant in adolescents and young adults. Obesity plays a major role in the development of type 2 diabetes, and more young people today are considered overweight than ever before. Weight reduction supports the prevention of type 2 diabetes and appears to decrease the symptoms of type 2 diabetes that are already presenting. Most type 2 diabetes patients can be successfully treated with a combination of diet, exercise, and medications that enhance insulin release or increase its sensitivity at the tissue level. In more severe cases, a person with type 2 diabetes must take insulin injections.









Gestational diabetes is seen in some pregnant women, typically in the latter half of the pregnancy. If untreated, gestational diabetes can pose a risk to the fetus as well as increase delivery complications. Most at risk for developing this condition are women who are overweight, African American, Native American, or Hispanic, or those who have a family history of diabetes. While gestational diabetes usually resolves after giving birth, a woman who presents with the condition has a 20–50% chance of developing type 2 diabetes within 10 years.


Until recently, there was no cure for diabetes, but in the past few years pancreas transplants have helped individuals with severe cases of diabetes. Pancreas transplants have several drawbacks, however:
They require major surgery, there is a long donor waiting list, and many complications can arise due to the surgery, either from potential rejection of the transplanted organ or the toxic effects of the necessary immunosuppressant antirejection drugs. Recently, a less invasive surgery, called an islet cell transplant, has been developed.
In this procedure, the islet cells are removed from a donor pancreas and purified. Then the cells are injected into a vein that enters the liver. Once in the liver, the islet cells embolize (form big clots) and start producing insulin almost immediately. Islet cell transplants are still very new and have many complications. The process of extracting and purifying the islet cells is complicated and can have a high failure rate; bleeding or major blood clots may occur in the vein where the islet cells are transplanted; and the immunosuppressant drugs that must be taken have serious side effects. Furthermore, recent studies have shown that the efficacy of islet cell transplants is only temporary. Most patients need to resume insulin shots within 2 years of transplantation. Thus, islet cell transplant surgery is reserved for patients who have severe forms of diabetes.

source : Human anatomy

Cataract

A cataract is a clouding of the lens inside the eye which leads to a decrease in vision. It is the most common cause of blindness and is conventionally treated with surgery. Visual loss occurs because opacification of the lens obstructs light from passing and being focused on to the retina at the back of the eye. 

It is most commonly due to biological aging, but there are a wide variety of other causes. Over time, yellow-brown pigment is deposited within the lens and this, together with disruption of the normal architecture of the lens fibers, leads to reduced transmission of light, which in turn leads to visual problems.

Those with cataracts commonly experience diffculty in appreciating colors and changes in contrast, driving, reading, recognizing faces, and coping with glare from bright lights.

Sign and symptoms

Signs and symptoms vary depending on the type of cataract, though there is considerable overlap. People with nuclear sclerotic or brunescent cataracts often notice a reduction of vision. Those with posterior subcapsular cataracts usually complain of glare as their major symptom.

The severity of cataract formation, assuming that no other eye disease is present, is judged primarily by visual acuity test. The appropriateness of surgery depends on a patient's particular functional and visual needs and other risk factors, all of which may vary widely.

Cause

1. Age

Age is the most common cause.  Lens proteins denature and degrade over time and this process is accelerated by diseases such as diabetes and hypertension. With the passage of time, environmental factors including toxins, radiation and UV light have a cumulative effect. These effects are worsened by the loss of protective and restorative mechanisms due to alterations in gene expression and chemical processes within the eye. 

2. Trauma

Blunt trauma causes swelling, thickening and whitening of the lens fibers.

While the swelling normally resolves with time, the white color may remain. In severe blunt trauma, or injuries which penetrate the eye, the capsule in which the lens sits can be damaged. This allows water from other parts of the eye to rapidly enter the lens leading to swelling and then whitening, obstructing light from reaching the retina at the back of the eye. Following electrical injuries, cataracts may develop in 0.7 to 8%.

######### Autism #########

               Autism is a disorder of neural development characterized by impaired social interaction and verbal and non-verbal communication, and by restricted repetitive or stereotyped behavior. The diagnostic criteria require that symptoms become apparent before a child is three years old. Autism affects information processing in the brain by altering how nerve cells and their synapses connect and organize; how this occurs is not well understood.  It is one of three recognized disorders in the autism spectrum (ASDs), the other two being Asperger syndrome, which lacks delays in cognitive development and language, and pervasive developmental disorder, not otherwise specified (commonly abbreviated as PDD-NOS), which is diagnosed when the full set of criteria for autism or Asperger syndrome are not met.

Autism has a strong genetic basis, although the genetics of autism are complex and it is unclear whether ASD is explained more by rare mutations, or by rare combinations of common genetic variants. In rare cases, autism is strongly associated with agents that cause birth defects. Controversies surround other proposed environmental causes, such as heavy metals, pesticides or childhood vaccines;  the vaccine hypotheses are biologically implausible and lack convincing scientific evidence. The prevalence of autism is about 1–2 per 1,000 people worldwide, and it occurs about four times more often in boys than girls. The Centers for Disease Control and Prevention (CDC) report 1.5% of children in the United States (one in 68) are diagnosed with ASD as of 2014, a 30% increase from one in 88 in 2012. The number of people diagnosed with autism has been increasing dramatically since the 1980s, partly due to changes in diagnostic practice and government-subsidized financial incentives for named diagnoses; the question of whether actual prevalence has increased is unresolved.
Parents usually notice signs in the first two years of their child's life.  The signs usually develop gradually, but some autistic children first develop more normally and then regress.Early behavioral, cognitive, or speech interventions can help autistic children gain self-care, social, and communication skills. Although there is no known cure,  there have been reported cases of children who recovered.  Not many children with autism live independently after reaching adulthood, though some become successful.  An autistic culture has developed, with some individuals seeking a cure and others believing autism should be accepted as a difference and not treated as a disorder.


characteristic

Autism is a highly variable neurodevelopmental disorder that first appears during infancy or childhood, and generally follows a steady course without remission.  Overt symptoms gradually begin after the age of six months, become established by age two or three years, and tend to continue through adulthood, although often in more muted form. It is distinguished not by a single symptom, but by a characteristic triad of symptoms:
impairments in social interaction; impairments in communication; and restricted interests and repetitive behavior. Other aspects, such as atypical eating, are also common but are not essential for diagnosis. Autism's individual symptoms occur in the general population and appear not to associate highly, without a sharp line separating pathologically severe from common traits.

Causes 


It has long been presumed that there is a common cause at the genetic, cognitive, and neural levels for autism's characteristic triad of symptoms.
However, there is increasing suspicion that autism is instead a complex disorder whose core aspects have distinct causes the often co-occur.

Autism has a strong genetic basis, although the genetics of autism are complex and it is unclear whether ASD is explained more by rare mutations with major effects, or by rare multigene interactions of common genetic variants.  Complexity arises due to interactions among multiple genes, the environment, and epigenetic factors which do not change DNA but are heritable and influence gene expression. Studies of twins suggest that heritability is 0.7 for autism and as high as 0.9 for ASD, and siblings of those with autism are about 25 times more likely to be autistic than the general population. However, most of the mutations that increase autism risk have not been identified. Typically, autism cannot be traced to a Mendelian (single-gene) mutation or to a single chromosome abnormality, and none of the genetic syndromes associated with ASDs have been shown to selectively cause ASD. Numerous candidate genes have been located, with only small effects attributable to any particular gene. The large number of autistic individuals with unaffected family members may result from copy number variations—spontaneous deletions or duplications in genetic material during meiosis.  Hence, a substantial fraction of autism cases may be traceable to genetic causes that are highly heritable but not inherited: that is, the mutation that causes the autism is not present in the parental genome

The cells the basic unit of structure and function

Cells are the structural and functional units of all organisms, including humans. An adult human body contains about 75 trillion cells. Most cells are composed of characteristic parts that work together to allow them to perform specific body functions.

There are approximately 200 different types of cells in the human body, but all of them share certain common characteristics:

■ All cells perform the general housekeeping functions necessary to sustain life. Each cell must obtain nutrients and other materials essential for survival from its surrounding fluids. Recall from chapter 1 that the total of all the chemical reactions that occur in cells is called metabolism.
■ Cells must dispose of the wastes they produce. If a cell didn’t remove its waste products, this waste would build up in the cell and lead to its death.
■ The shape and integrity of a cell is maintained by both its internal contents and its surrounding membrane.
■ Most cells are capable of undergoing cell division to make more cells of the same type.





The small size of cells is the greatest obstacle to determining their nature. Cells were discovered after microscopes were invented, and high-magnification microscopes are required to see the smallest human body cells. The dimensional unit often used to measure cell size is the micrometer (μm). One micrometer is equal to 1/10,000 of a centimeter (about 1/125,000 of an inch). For example, a red blood cell has a diameter of about 7–8 μm, whereas one of the largest human cells, an oocyte, has a diameter of about 120 μm.
 compares the size of the smallest unit of structure in the human body (an atom) to various cell types as well as to other macroscopic structures, such as an ostrich egg and a human.