Melanie C Mann FRCOG FFFP Dip GUM

• Consultant in Contraception and Reproductive Health,
• Worcestershire PCT, Arrowside Unit, Alexandra Hospital,
• Redditch, Worcestershire

The additional vitamin D lecalciferol is to convert it to 25hydroxycholecalciferol hiv infection and. hiv disease 200 mg movfor with mastercard, which occurs in the liver antiviral purchase movfor 200 mg visa. The process is limited because the 25hydroxycholecalciferol has a feedback inhibitory effect on the conversion reactions antiviral innate immunity generic 200 mg movfor free shipping. Note that the intake of vitamin D3 can increase many times and yet the concentration of 25hydroxycholecalciferol remains nearly normal hiv infection rates per act buy generic movfor line. This high degree of feedback control pre vents excessive action of vitamin D when intake of vitamin D3 is altered over a wide range antiviral state discount 200 mg movfor with amex. Second, this controlled conversion of vitamin D3 to 25hydroxycholecalciferol conserves the vitamin D stored in the liver for future use. Once vitamin D3 is converted, the 25hydroxycholecalciferol persists in the body for only a few weeks, whereas in the vitamin D form, it can be stored in the liver for many months. Formation of 1,25-Dihydroxycholecalciferol in the Kidneys and Its Control by Parathyroid Hormone. Effect of increasing vitamin D3 intake on the plasma concentration of 25-hydroxycholecalciferol. Deficiency of activated vitamin D occurs only at very low levels of vitamin D intake. When plasma calcium concentration is already too high, formation of 1,25dihydroxycholecalciferol is greatly depressed. Lack of 1,25dihydroxycholecalciferol, in turn, decreases the absorption of calcium from the intestines, bones, and renal tubules, thus causing the calcium ion concentration to fall back toward its normal level. Vitamin D receptors are present in most cells in the body and are located mainly in the nuclei of target cells. Although the vitamin D receptor binds several forms of cholecalciferol, its affin ity for 1,25dihydroxycholecalciferol is roughly 1000 times that for 25hydroxycholecalciferol, which explains their relative biological potencies. Therefore, in the absence of the kidneys, vitamin D loses almost all its effectiveness. Calcium Ion Concentration Controls the Formation of 1,25-Dihydroxycholecalciferol. First, the calcium ion has a slight effect in pre venting the conversion of 25hydroxycholecalciferol to 1008 X ciferol functions as a type of "hormone" to promote intestinal absorption of calcium. It promotes this absorp tion principally by increasing, over a period of about 2 days, formation of calbindin, a calciumbinding protein, in the intestinal epithelial cells. This protein functions in the brush border of these cells to transport calcium into the cell cytoplasm. The calcium then moves through the basolateral membrane of the cell by facilitated diffusion. The rate of calcium absorption is directly proportional to the quantity of this calciumbinding protein. Furthermore, this protein remains in the cells for several weeks after the 1,25dihydroxycholecalciferol has been removed from the body, thus causing a prolonged effect on calcium absorption. Other effects of 1,25dihydroxycholecalciferol that might play a role in promoting calcium absorption are the Chapter 80 ParathyroidHormone,Calcitonin,CalciumandPhosphateMetabolism,VitaminD,Bone,andTeeth formation of (1) a calciumstimulated adenosine triphos phatase in the brush border of the epithelial cells and (2) an alkaline phosphatase in the epithelial cells. Although phosphate is usually absorbed Physiological Anatomy of the Parathyroid Glands. Vitamin D also increases calcium and phos phate reabsorption by the epithelial cells of the renal tubules, thereby tending to decrease excretion of these substances in the urine. However, this effect is weak and probably not of major importance in regulating the extra cellular fluid concentration of these substances. The mechanism of this action of vitamin D is not fully understood but is believed to result from the effect of 1,25dihydroxycholecalciferol to increase calcium trans port through cellular membranes. One of the ways it promotes this calcification is to increase calcium and phosphate absorption from the intestines. However, even in the absence of such an increase, it enhances the mineralization of bone. Here again, the mechanism of the effect is unclear, but it probably also results from the ability of 1,25 dihydroxycholecalciferol to cause transport of calcium ions through cell membranes-but in this instance, perhaps in the opposite direction through the osteoblastic or osteocytic cell membranes. Normally humans have four parathyroid glands, which are located immediately behind the thyroid gland-one behind each of the upper and each of the lower poles of the thyroid. Each parathyroid gland is about 6 millimeters long, 3 mil limeters wide, and 2 millimeters thick and has a macro scopic appearance of dark brown fat. The parathyroid glands are difficult to locate during thyroid operations because they often look like just another lobule of the thyroid gland. For this reason, before the importance of these glands was generally recognized, total or subtotal thyroidectomy frequently resulted in removal of the para thyroid glands as well. Removal of half the parathyroid glands usually causes no major physiological abnormalities. Removal of three of the four normal glands causes transient hypoparathyroid ism, but even a small quantity of remaining parathyroid tissue is usually capable of hypertrophying to satisfactorily perform the function of all the glands. The function of the oxyphil cells is not certain, but the cells are believed to be modified or depleted chief cells that no longer secrete hormone. Excess activity of the parathyroid gland causes rapid release of calcium salts from the bones, with resultant hypercalcemia in the extracellular fluid; conversely, hypofunction of the parathyroid glands causes hypocalcemia, often with resultant tetany. The second phase is a much slower one, requiring several days or even weeks to become fully developed; it results from proliferation of the osteoclasts, followed by greatly increased osteoclastic resorption of the bone itself, not merely release of the calcium phos phate salts from the bone. The endoplasmic reticulum and Golgi appa ratus first cleave this preprohormone to a prohormone with 90 amino acids and then to the hormone itself with 84 amino acids, and it is finally packaged in secretory gran ules in the cytoplasm of the cells. In fact, because the kidneys rapidly remove the whole 84­amino acid hormone within minutes but fail to remove many of the fragments for hours, a large share of the hormonal activity is caused by the fragments. Note that at the onset of infusion the calcium ion concentration begins to rise and reaches a plateau in about 4 hours. However, the phosphate concentration falls more rapidly than the calcium rises and reaches a depressed level within 1 or 2 hours. One is a rapid phase that begins in minutes and increases progressively for several hours. One does not usually think of either osteoblasts or osteocytes functioning to mobilize bone salt, because both these types of cells are osteoblastic in nature and are normally associated with bone deposition and its calcifi cation. In fact, long, filmy processes extend from osteocyte to osteocyte throughout the bone structure and also connect with the surface osteocytes and osteoblasts. This extensive system is called the osteo cytic membrane system, and it is believed to provide a membrane that separates the bone itself from the extra cellular fluid. Experiments suggest that the osteocytic membrane pumps calcium ions from the bone fluid into the extracellular fluid, creating a calcium ion concentration in the bone fluid only one third that in the extracellular fluid. When the osteocytic pump becomes excessively activated, the bone fluid calcium concentration falls even lower, and calcium phos phate salts are then released from the bone. When the pump is inacti vated, the bone fluid calcium concentration rises to a higher level and calcium phosphate salts are redeposited in the matrix. Then the calcium pump on the other side of the cell membrane transfers the calcium ions the rest of the way into the extracellular fluid. Slow Phase of Bone Resorption and Calcium Phosphate Release-Activation of the Osteoclasts. Instead, the activated osteoblasts and osteocytes send secondary "signals" to the osteoclasts. Activation of the osteoclastic system occurs in two stages: (1) immediate activation of the osteoclasts that are already formed and (2) formation of new osteoclasts. Therefore, the late effect is actually to enhance both osteoblastic and osteoclastic activity. For instance, the parathyroid glands become greatly enlarged in per sons with rickets, in whom the calcium level is usually depressed only a small amount. Conversely, conditions that increase the calcium ion concentration above normal cause decreased activity and reduced size of the parathyroid glands. Changes in extracellular fluid calcium ion concentra tion are detected by a calciumsensing receptor in para thyroid cell membranes. Moreover, it increases reabsorption of mag nesium ions and hydrogen ions while it decreases reab sorption of sodium, potassium, and amino acid ions in much the same way that it affects phosphate. The increased calcium reabsorption occurs mainly in the late distal tubules, the collecting tubules, the early collect ing ducts, and possibly the ascending loop of Henle to a lesser extent. This process contrasts with that in many endocrine tissues in which hormone secretion is stimulated when these pathways are activated. The solid red curve shows the acute effect when the calcium concentration is changed over a period of a few hours. Synthesis and secretion of calcitonin occur in the parafollicular cells, or C cells, lying in the interstitial fluid between the follicles of the thyroid gland. These actions 1012 nin secretion is increased extracellular fluid calcium ion concentration. For instance, in cases of diarrhea, several grams of calcium can be secreted in the intestinal juices, passed into the intestinal tract, and lost into the feces each day. Conversely, after ingestion of large quantities of calcium, particularly when there is also an excess of vitamin D activity, a person may absorb as much as 0. This figure compares with a total quantity of calcium in all the extracellular fluid of about 1 gram. However, there is a first line of defense to prevent this from occur ring even before the parathyroid and calcitonin hormonal feedback systems have a chance to act. The immediate effect is to decrease the absorptive activities of the osteoclasts and possibly the osteo lytic effect of the osteocytic membrane throughout the bone, thus shifting the balance in favor of deposition of calcium in the exchangeable bone calcium salts. This effect is especially significant in young animals because of the rapid interchange of absorbed and deposited calcium. The second and more prolonged effect of calcitonin is to decrease the formation of new osteoclasts. Also, because osteoclastic resorption of bone leads sec ondarily to osteoblastic activity, decreased numbers of osteoclasts are followed by decreased numbers of osteoblasts. Therefore, over a long period, the net result is reduced osteoclastic and osteoblastic activ ity and, consequently, little prolonged effect on plasma calcium ion concentration. That is, the effect on plasma calcium is mainly a transient one, lasting for a few hours to a few days at most. Calcitonin also has minor effects on calcium handling in the kidney tubules and the intestines. Second, in the adult human, the daily rates of absorp tion and deposition of calcium are small, and even after the rate of absorption is slowed by calcitonin, this still has only a small effect on plasma calcium ion concentration. The effect of calcitonin in children is much greater because bone remodeling occurs rapidly in children, with absorption and deposition of calcium as great as 5 grams or more per day-equal to 5 to 10 times the total calcium in all the extracellular fluid. Because of the ease of deposition of these exchangeable salts and their ease of resolubility, an increase in the concentrations of extracellular fluid calcium and phosphate ions above normal causes immediate deposition of exchangeable salt. Conversely, a decrease in these concentrations causes immediate absorption of exchangeable salt. This reaction is rapid because the amorphous bone crystals are extremely small and their total surface area exposed to the fluids of the bone is perhaps 1 acre or more. In addition, about 5 percent of all the blood flows through the bones each minute-that is, about 1 percent of all the extracellular fluid each minute. Therefore, about one half of any excess calcium that appears in the extra cellular fluid is removed by this buffer function of the bones in about 70 minutes. In addition to the buffer function of the bones, the mitochondria of many of the tissues of the body, espe cially of the liver and intestine, contain a significant amount of exchangeable calcium (a total of about 10 grams in the whole body) that provides an additional buffer system to help maintain constancy of the extracel lular fluid calcium ion concentration. As already explained, this sets into play multiple mechanisms for reducing the calcium ion concentration back toward normal. In young animals and possibly in young children (but probably to a smaller extent in adults), the calcitonin causes rapid deposition of calcium in the bones, and perhaps in some cells of other tissues. Therefore, in very young animals, excess calcitonin can cause a high calcium ion concentration to return to normal perhaps considerably more rapidly than can be achieved by the exchangeable calciumbuffering mecha nism alone. As a result, calcium release from the bones is so depressed that the level of calcium in the body fluids decreases. Yet, because calcium and phosphates are not being released from the bone, the bone usually remains strong. When the parathyroid glands are suddenly removed, the calcium level in the blood falls from the normal of 9. Among the muscles of the body especially sensitive to tetanic spasm are the laryngeal muscles. Spasm of these muscles obstructs respiration, which is the usual cause of death in persons with tetany unless appropriate treatment is provided. In most patients with hypoparathyroidism, the admin istration of extremely large quantities of vitamin D, to as high as 100,000 units per day, along with intake of 1 to 2 grams of calcium, keeps the calcium ion concentration in a normal range. At times, it might be necessary to admin ister 1,25dihydroxycholecalciferol instead of the nonacti vated form of vitamin D because of its much more potent and much more rapid action. However, administration of 1,25dihydroxycholecalciferol can also cause unwanted effects because it is sometimes difficult to prevent overac tivity by this activated form of vitamin D. The cause of primary hyperparathyroidism ordinarily is a tumor of one of the parathyroid glands; such tumors occur much more frequently in women than in men or children, mainly because pregnancy and lactation stimulate the parathyroid glands and therefore predispose to the development of such a tumor. Hyperparathyroidism causes extreme osteoclastic activ ity in the bones, which elevates the calcium ion concentra tion in the extracellular fluid while usually depressing the concentration of phosphate ions because of increased renal excretion of phosphate. Although new bone can be deposited rapidly enough to compensate for the increased osteoclastic resorption of bone in persons with mild hyperparathyroidism, in severe hyperparathy roidism, the osteoclastic absorption soon far outstrips osteoblastic deposition, and the bone may be eaten away almost entirely. Indeed, a broken bone is often the reason a person with hyperparathyroidism seeks medical atten tion. Radiographs of the bone typically show extensive decalcification and, occasionally, large punchedout cystic areas of the bone that are filled with osteoclasts in the form of socalled giant cell osteoclast "tumors. The cystic bone disease of hyperparathyroidism is called osteitis fibrosa cystica.

The autonomic nervous system is subdivided into the sympathetic division and the parasympathetic division hiv infection in nigeria 200 mg movfor for sale. The origin of their motor neurons and the organs innervated are shown in figure 8 antiviral birth control discount movfor on line. Some preganglionic sympathetic axons branch from the spinal nerves to synapse with postganglionic neurons in autonomic ganglia that are arranged in two chains antiviral 200 mg movfor for sale, one on each side of the vertebral column hiv infection sore throat movfor 200 mg cheap. Other sympathetic preganglionic axons pass through a paravertebral chain ganglion without synapsing and extend to another type of ganglion hiv infection methods purchase 200 mg movfor overnight delivery, a collateral ganglion, before synapsing with a postganglionic neuron. Preganglionic axons of the parasympathetic division arise from the brainstem and sacral segment (S2­S4) of the spinal cord. They extend through cranial or sacral nerves to synapse with postganglionic neurons within ganglia that are located very near or within visceral organs (figure 8. Most visceral organs receive postganglionic axons of both the sympathetic and the parasympathetic divisions; but a few, such as sweat glands and most blood vessels, receive only sympathetic axons. Most sympathetic postganglionic axons secrete norepinephrine, a substance similar to adrenaline, which is why they are called adrenergic axons. Parasympathetic postganglionic axons secrete acetylcholine and thus are called cholinergic axons (figure 8. Functions Both sympathetic and parasympathetic divisions stimulate some visceral organs and inhibit others. For example, the sympathetic division increases heart rate whereas the parasympathetic division decreases heart rate. The contrasting effects are due to the different neurotransmitters secreted by postganglionic sympathetic and parasympathetic axons and the receptors of the receiving organs. The sympathetic division prepares the body for physical action to meet emergencies. The parasympathetic division is dominant under the normal, nonstressful conditions of everyday life. Because its actions are usually opposite those of the sympathetic division, it is often viewed as preparing the body for resting and digesting. It not only stimulates the sympathetic division but also inhibits the parasympathetic division. In an overdose, this double-barreled action produces an erratic, uncontrollable heartbeat that may result in sudden death. The pain may be severe and often radiates inferiorly through the thigh and leg to the sole of the foot. It is caused by the reactivation of the chicken pox virus, which, until that time, has been dormant in the nerve roots. The virus causes painful blisters on the skin at the sensory nerve endings, followed by prolonged pain (figure 8. Inflammatory Disorders ¯ Meningitis (men-in-ji -tis) results from a bacterial, fungal, or viral infection of the meninges. It is associated with a loss of certain cholinergic neurons in the brain and a reduced ability of neurons to secrete acetylcholine. Cerebral palsy (ser-e -bral pawl-ze) is character¯ ¯ ized by partial paralysis and sometimes a degree of mental retardation. It may result from damage to the brain during prenatal development, often from viral infections caused by German measles or from trauma during delivery. They are a major cause of disability and the third highest cause of death in the United States. Comas are states of unconsciousness in which the patient cannot be aroused even with vigorous stimulation. Illness or trauma to the brain may alter the functioning of the reticular formation, resulting in a coma. Concussion results from a severe jarring of the brain caused by a blow to the head. Dyslexia (dis-lek -se -ah) causes the afflicted person ¯ to reverse letters or syllables in words and words within sentences. Grand mal epilepsy is the more serious form and is characterized by convulsive seizures. Petit mal epilepsy is the less serious form and is characterized by momentary loss of contact with reality without unconsciousness or convulsions. Fainting is a brief loss of consciousness due to a sudden reduction in blood supply to the brain. Headaches are triggered by various physical or psychological factors, but often result from a dilation of blood vessels within the meninges of the brain. Migraine headaches may have visual or digestive side effects and may be triggered by stress, allergies, or fatigue. Sinus headaches may result from inflammation that causes increased pressure within the paranasal sinuses. Neuroses are mild maladjustments to life situations that may produce anxiety and interfere with normal behavior. Psychoses are serious mental disorders that sometimes cause delusions, hallucinations, or withdrawal from reality. This destruction results in a short-circuiting of neural pathways and an impairment of motor functions. Neuralgia (nu-ral -je -ah) is pain arising from a nerve ¯ ¯ regardless of the cause of the pain. Parkinson disease is caused by an insufficient delivery of the neurotransmitter dopamine to neurons in certain nuclei within the cerebrum. The neurotransmitter binds to receptors on the postsynaptic neuron, causing either the formation of a nerve impulse or the inhibition of nerve impulse formation. Then, the neurotransmitter is quickly removed by reabsorption into the terminal bouton, an enzymatic reaction or diffusion out of the cleft. Schwann cells form the myelin sheath and neurilemma of peripheral myelinated axons. Each hemisphere is subdivided into five lobes: frontal, parietal, temporal, occipital, and insula. The cerebrum interprets sensations; initiates voluntary motor responses; and is involved in will, personality traits, and intellectual processes. The thalamus is formed of two lateral masses connected by the interthalamic adhesion. It is a relay station for sensory and motor nerve impulses going to and from the cerebrum and provides an uncritical awareness of sensations. The hypothalamus is located inferior to the thalamus and forms the floor of the third ventricle. It also regulates several homeostatic processes such as body temperature, mineral and water balance, appetite, digestive processes, and secretion of pituitary gland hormones. Ascending and descending axons between higher brain centers and the spinal cord pass through the brainstem. It contains reflex centers for movements associated with visual and auditory stimuli. It contains reflexive integration centers that control breathing, heart rate and force of contraction, and blood pressure. It is composed of two hemispheres separated by the vermis and coordinates skeletal muscle contractions. The 31 pairs of spinal nerves are divided into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve. Anterior rami of many spinal nerves form spinal plexuses where axons are sorted and recombined so that all axons to a particular organ are carried in the same nerve. The four pairs of spinal plexuses are cervical, brachial, lumbar, and sacral plexuses. Reflexes are rapid, involuntary, and predictable responses to internal and external stimuli. Autonomic reflexes involve smooth muscle, cardiac muscle, adipose tissue, and glands. Cranial reflexes involve the brain, while spinal reflexes involve the spinal cord. Nerves of the sympathetic division arise from the thoracic and lumbar segments of the spinal cord and prepare the body to meet emergencies. Nerves of the parasympathetic division arise from the brain and the sacral segment of the spinal cord and function mainly in nonstressful situations. Anterior horns of gray matter contain cell bodies of somatic motor neurons; posterior horns contain interneuron cell bodies that receive incoming sensory nerve impulses; lateral horns contain cell bodies of autonomic motor neurons. White matter contains ascending and descending tracts of myelinated and unmyelinated axons. Inflammatory neurological disorders include meningitis, neuritis, sciatica, and shingles. Most nerves are mixed Part 3 Integration and Control 193 Self-Review Answers are located in appendix B. Nerve impulses are carried away from the cell body of a neuron by the of the neuron. A nerve impulse is formed by the sudden flow of ions across the plasma membrane into a neuron. The area of the cerebrum is involved with decision making, conscience, and personality. The, a component of the diencephalon, regulates appetite, water balance, and body temperature. Cerebrospinal fluid fills the ventricles of the brain and the space of the meninges. The nervous system is involved in involuntary responses that maintain homeostasis. Predict the cognitive changes that will occur following physical trauma to the anterior portion of the frontal lobe. If you touch a hot stove, a reflexive withdrawal of your hand is triggered at about the same time that you feel the pain. Jeremy cannot hear warning alarms or people yelling at him when there is danger around him. He cannot hear the spoken words used for quick, easy communication between people. Because he cannot hear words, he did not develop the auditory memories needed to produce speech. He cannot verbally express his thoughts, opinions, or desires to those around him. To survive in the world and communicate with those around him, Jeremy has had to learn to use his other senses. He uses his sense of touch to identify people and objects around him and to learn about the world by reading in braille. By feeling vibrations through his skin, he can detect the rhythm in music that is playing. His sense of smell is heightened, which allows him to detect certain types of hazards and aid in the identification of people and objects. Mechanoreceptor Sensory receptor stimulated by mechanical forces such as pressure or touch. Projection the process by which the brain makes a sensation seem to come from the body part being stimulated. Proprioceptor Sensory receptor stimulated by changes in body position or movements of the body or its parts. Retina (retin = net) the internal layer of the eye, which contains the photoreceptors. Semicircular canals the portion of the internal ear containing the sensory receptors for dynamic equilibrium. Sensory adaptation the decrease in the formation of nerve impulses by a sensory receptor when repeatedly stimulated by the same stimulus. Spiral organ (Organ of Corti) the sense organ in the internal ear containing the sensory receptors for hearing. The senses may be subdivided into two broad categories: general senses and special senses. General senses include pain, touch, pressure, stretching, chemical changes, cold, and heat. Each type of sensory receptor is sensitive to a particular type of stimulus that causes the sensory receptor to form nerve impulses. The five types of sensory receptors, based on the specific stimuli to which they respond, are listed in table 9. A sensation is a conscious or subconscious awareness of a change in the internal or external environment. The conscious awareness of a sensation, or perception, results from the interpretation of nerve impulses reaching sensory areas of the cerebral cortex. The sensation that is created is determined by the area of the brain receiving the nerve impulses rather than by the type of sensory receptor forming the nerve impulses. For example, all nerve impulses reaching the visual area of the occipital lobe are interpreted as visual sensations. A strong blow to an eye or to the back of the head may produce a visual sensation (flashes of light), although the stimulus is mechanical. The perceived intensity of a sensation is dependent upon the frequency of nerve impulses reaching the cerebral cortex. The greater the frequency of nerve impulses, the greater is the intensity of the sensation.

During early embryonic development lemon antiviral purchase movfor 200 mg with mastercard, primordial germ cells from the dorsal endoderm of the yolk sac migrate along the mesentery of the hindgut to the outer surface of the ovary hiv infection stats cheapest movfor, which is covered by a germinal epithelium hiv infection by country discount movfor 200 mg without prescription, derived embryologically from the epithelium of the germinal ridges hiv infection rate statistics 200 mg movfor for sale. Once these primordial germ cells reach the germinal epithelium hiv infection to symptoms cheap movfor 200 mg visa, they migrate into the substance of the ovarian cortex and become oogonia or primordial ova. Each primordial ovum then collects around it a layer of spindle cells from the ovarian stroma (the supporting tissue of the ovary) and causes them to take on epithelioid characteristics; these epithelioid-like cells are then called granulosa cells. The ovum surrounded by a single layer of granulosa cells is called a primordial follicle. During all the reproductive years of adult life, between about 13 and 46 years of age, only 400 to 500 of the primordial follicles develop enough to expel their ova-one each month; the remainder degenerate. At the end of reproductive capability (at menopause), only a few primordial follicles remain in the ovaries, and even these follicles degenerate soon thereafter. This rhythmical pattern is called the female monthly sexual cycle (or, less accurately, the menstrual cycle). It may be as short as 20 days or as long as 45 days in some women, although abnormal cycle length is frequently associated with decreased fertility. First, only a single ovum is normally released from the ovaries each month, so normally only a single fetus will begin to grow at a time. Second, the uterine endometrium is prepared in advance for implantation of the fertilized ovum at the required time of the month. The ovarian hormones, estrogen and progesterone, which are secreted by the ovaries in response to the two female sex hormones from the anterior pituitary gland these various hormones are secreted at drastically differing rates during different parts of the female monthly sexual cycle. It is secreted in short pulses averaging once every 90 minutes, as occurs in the male. In the absence of these hormones, the ovaries remain inactive, which is the case throughout childhood, when almost no pituitary gonadotropic hormones are secreted. This period of change is called puberty, and the time of the first menstrual cycle is called menarche. These cyclical variations cause cyclical ovarian changes, which are explained in the following sections. Almost all these stimulatory effects result from activation of the cyclic adenosine monophosphate second messenger system in the cell cytoplasm, which causes the formation of protein kinase and multiple phosphorylations of key enzymes that stimulate sex hormone synthesis, as explained in Chapter 75. When a female child is born, each ovum is surrounded by a single layer of granulosa cells; the ovum, with this granulosa cell sheath, is called a primordial follicle, as shown in the figure. Throughout childhood, the granulosa cells are believed to provide nourishment for the ovum and to secrete an oocyte maturation inhibiting factor that keeps the ovum suspended in its primordial state in the prophase stage of meiotic division. The first stage of follicular growth is moderate enlargement of the ovum, which increases in diameter twofold to threefold. That stage is followed by growth of additional layers of granulosa cells in some of the follicles. The initial effect is rapid proliferation of the granulosa cells, giving rise to many more layers of these cells. In addition, spindle cells derived from the ovary interstitium collect in several layers outside the granulosa cells, giving rise to a second mass of cells called the theca. In the theca interna, the cells take on epithelioid characteristics similar to those of the granulosa cells and develop the ability to secrete additional steroid sex hormones (estrogen and progesterone). The outer layer, the theca externa, develops into a highly vascular connective tissue capsule that becomes the capsule of the developing follicle. After the early proliferative phase of growth, which lasts for a few days, the mass of granulosa cells secretes a follicular fluid that contains a high concentration of estrogen, one of the important female sex hormones (discussed later). Greatly accelerated growth then occurs, leading to still larger follicles called vesicular follicles. The ovum also enlarges in diameter another threefold to fourfold, giving a total ovum diameter increase up to 10-fold, or a mass increase of 1000-fold. As the follicle enlarges, the ovum remains embedded in a mass of granulosa cells located at one pole of the follicle. Therefore, the largest follicle continues to grow because of its intrinsic positive feedback effects, while all the other follicles stop growing and actually involute. This process of atresia is important because it normally allows only one of the follicles to grow large enough each month to ovulate, which usually prevents more than one child from developing with each pregnancy. Ovulation Ovulation in a woman who has a normal 28-day female sexual cycle occurs 14 days after the onset of menstruation. Shortly before ovulation the protruding outer wall of the follicle swells rapidly, and a small area in the center of the follicular capsule, called the stigma, protrudes like a nipple. In another 30 minutes or so, fluid begins to ooze from the follicle through the stigma, and about 2 minutes later, the stigma ruptures widely, allowing a more viscous fluid, which has occupied the central portion of the follicle, to evaginate outward. This viscous fluid carries with it the ovum surrounded by a mass of several thousand small granulosa cells, called the corona radiata. Therefore, the rate of secretion of estrogen begins to fall about 1 day before ovulation, while increasing amounts of progesterone begin to be secreted. It is in this environment of (1) rapid growth of the follicle, (2) diminishing estrogen secretion after a prolonged phase of excessive estrogen secretion, and (3) initiation of secretion of progesterone that ovulation occurs. Within a few hours, two events occur, both of which are necessary for ovulation: 1. Simultaneously there is rapid growth of new blood vessels into the follicle wall, and at the same time, prostaglandins (local hormones that cause vasodilation) are secreted into the follicular tissues. These two effects cause plasma transudation into the follicle, which contributes to follicle swelling. They enlarge in diameter two or more times and become filled with lipid inclusions that give them a yellowish appearance. The granulosa cells in the corpus luteum develop extensive intracellular smooth endoplasmic reticula that form large amounts of the female sex hormones progesterone and estrogen (with more progesterone than estrogen during the luteal phase). The theca cells form mainly the androgens androstenedione and testosterone rather than female sex hormones. However, most of these hormones are also converted by the enzyme aromatase in the granulosa cells into estrogens, the female hormones. Then the corpus luteum begins to involute and eventually loses its secretory function and its yellowish, lipid characteristic about 12 days after ovulation, becoming the corpus albicans; during the ensuing few weeks, the corpus albicans is replaced by connective tissue and over months is absorbed. In addition, the lutein cells secrete small amounts of the hormone inhibin, the same as the inhibin secreted by the Sertoli cells of the male testes. Final involution normally occurs at the end of almost exactly 12 days of corpus luteum life, which is around the 26th day of the normal female sexual cycle, 2 days before menstruation begins. The paucity of secretion of progesterone and estrogen at this time also leads to menstruation by the uterus, which will be explained later. One of these follicles finally becomes "mature" and ovulates on the 14th day of the cycle. After ovulation, the secretory cells of the ovulating follicle develop into a corpus luteum that secretes large quantities of the major female hormones, progesterone and estrogen. After another 2 weeks, the corpus luteum degenerates, whereupon the ovarian hormones estrogen and progesterone decrease greatly and menstruation begins. A yet uncharacterized local hormone in the follicular fluid, called luteinization-inhibiting factor, seems to hold the luteinization process in check until after ovulation. By far the most important of the estrogens is the hormone estradiol, and by far the most important progestin is progesterone. The progestins function mainly to prepare the uterus for pregnancy and the breasts for lactation. In the normal nonpregnant female, estrogens relative potencies, one can see that the total estrogenic effect of -estradiol is usually many times that of the other two together. For this reason, -estradiol is considered the major estrogen, although the estrogenic effects of estrone are not negligible. During pregnancy, large quantities of estrogens are also secreted by the placenta, as discussed in Chapter 83. Small amounts of estrone are also secreted, but most of this is formed in the peripheral tissues from androgens secreted by the adrenal cortices and by ovarian thecal cells. Estriol is a weak estrogen; it is an oxidative product derived from both estradiol and estrone, with the conversion occurring mainly in the liver. The estrogenic potency of -estradiol is 12 times that of estrone and 80 times that of estriol. However, small amounts of another progestin, 17-hydroxyprogesterone, are secreted along with progesterone and have essentially the same effects. Yet, for practical purposes, it is usually reasonable to consider progesterone to be the only important progestin. In the nonpregnant female, progesterone is usually secreted in significant amounts only during the latter half of each ovarian cycle, when it is secreted by the corpus luteum. As we shall see in Chapter 83, large amounts of progesterone are also secreted by the placenta during pregnancy, especially after the fourth month of gestation. During synthesis, mainly progesterone and androgens (testosterone and androstenedione) are synthesized first; then, during the follicular phase of the ovarian cycle, before these two initial hormones can leave the ovaries, almost all the androgens and much of the progesterone are converted into estrogens by the enzyme aromatase in the granulosa cells. Because the theca cells lack aromatase, they cannot convert androgens to estrogens. During the luteal phase of the cycle, far too much progesterone is formed for all of it to be converted, which accounts for the large secretion of progesterone into the circulating blood at this time. Also, about one fifteenth as much testosterone is secreted into the plasma of the female by the ovaries as is secreted into the plasma of the male by the testes. The liver conjugates the estrogens to form glucuronides and sulfates, and about one fifth of these conjugated products is excreted in the bile; most of the remainder is excreted in the urine. Also, the liver converts the potent estrogens estradiol and estrone into the almost totally impotent estrogen estriol. Therefore, diminished liver function actually increases the activity of estrogens in the body, sometimes causing hyperestrinism. Within a few minutes after secretion, almost all the progesterone is degraded to other steroids that have no progestational effect. As with the estrogens, the liver is especially important for this metabolic degradation. About 10 percent of the original progesterone is excreted in the urine in this form. Therefore, one can estimate the rate of progesterone formation in the body from the rate of this excretion. During childhood, estrogens are and progesterone are transported in the blood bound mainly with plasma albumin and with specific estrogenand progesterone-binding globulins. The binding between these hormones and the plasma proteins is loose enough that they are rapidly released to the tissues over a period of 30 minutes or so. At this time, the female sex organs change from those of a child to those of an adult. The ovaries, fallopian tubes, uterus, and vagina all increase several times in size. Also, the external genitalia enlarge, with deposition of fat in the mons pubis and labia majora and enlargement of the labia minora. In addition, estrogens change the vaginal epithelium from a cuboidal into a stratified type, which is considerably more resistant to trauma and infection than is the prepubertal cuboidal cell epithelium. Vaginal infections in children can often be cured by the administration of estrogens simply because of the resulting increased resistance of the vaginal epithelium. During the first few years after puberty, the size of the uterus increases twofold to threefold, but more important than the increase in uterus size are the changes that take place in the uterine endometrium under the influence of estrogens. Estrogens cause marked proliferation of the endometrial stroma and greatly increased development of the endometrial glands, which will later aid in providing nutrition to the implanted ovum. These effects are discussed later in the chapter in connection with the endometrial cycle. Estrogens inhibit osteoclastic activity in the bones and therefore stimulate bone growth. As discussed in Chapter 80, at least part of this effect is due to stimulation of osteoprotegerin, which is also called osteoclastogenesis inhibitory factor, a cytokine that inhibits bone resorption. At puberty, when the female enters her reproductive years, her growth in height becomes rapid for several years. However, estrogens have another potent effect on skeletal growth: They cause uniting of the epiphyses with the shafts of the long bones. This effect of estrogen in the female is much stronger than the similar effect of testosterone in the male. As a result, growth of the female usually ceases several years earlier than growth of the male. A female eunuch who is devoid of estrogen production usually grows several inches taller than a normal mature female because her epiphyses do not unite at the normal time. They cause the glandular tissues of this lining to proliferate, and especially important, they cause the number of ciliated epithelial cells that line the fallopian tubes to increase. These cilia always beat toward the uterus, which helps propel the fertilized ovum in that direction. In fact, under the influence of appropriate hormones, the masculine breast during the first 2 decades of life can develop sufficiently to produce milk in the same manner as the female breast. Estrogens cause (1) development of the stromal tissues of the breasts, (2) growth of an extensive ductile system, and (3) deposition of fat in the breasts. The lobules and alveoli of the breast develop to a slight extent under the influence of estrogens alone, but it is progesterone and prolactin that cause the ultimate determinative growth and function of these structures. In summary, the estrogens initiate growth of the breasts and of the milk-producing apparatus. They are also responsible for the characteristic growth and external appearance of the mature female breast. However, they do not complete the job of converting the breasts into milk-producing organs. This estrogen deficiency leads to (1) increased osteoclastic activity in the bones, (2) decreased bone matrix, and (3) decreased deposition of bone calcium and phosphate.

Movfor 200 mg buy overnight delivery. Dual Therapy for HIV Infection: Ready for Prime Time?.

References

• Fusaroli P, Serrani M, De Giorgio R, et al. Contrast harmonicendoscopic ultrasound is useful to identify neoplastic features of pancreatic cysts (with videos). Pancreas. 2016;45(2):265-268.
• Crombleholme TM, Adzick NS, de Lorimier AA, Longaker MT, Harrison MR, Charlton VE. Carotid Artery Reconstruction Following Extracorporeal Membrane Oxygenation. American Journal of Diseases of Children 1990;144:872-4.
• Moriya K, Higashiyama H, Tanaka H, et al: Long-term outcome of vaginoplasty with the bilateral labioscrotal flap, J Urol 182(4 Suppl):1876n1881, 2009.
• Porter A, Strasberg B, Vaturi M, et al: Correlation between electrocardiographic subtypes of anterior myocardial infarction and regional abnormalities of wall motion. Coron Artery Dis 2000;11:489-493.
• Iqbal MS, Lewis J. An overview of the management of adult ependymomas with emphasis on relapsed disease. Clin Oncol (R Coll Radiol) 2013; 25(12):726-733.
• Aguirre TV, Sampliner RE. Endoscopic surveillance of columnar-lined esophagus: frequency of intestinal metaplasia detection and impact of antireflux surgery. Am J Gastroenterol 2003;98:931.
• Mansouri A, Hadjibabaie M, Iravani M, et al. The effect of zinc sulfate in the prevention of high-dose chemotherapy-induced mucositis: a double-blind, randomized, placebo-controlled study. Hematol Oncol 2012;30(1):22-26.