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For clarity erectile dysfunction icd 9 2014 cheap 25 mg fildena visa, vasa recta are shown only on the left and peritubular capillaries only on the plasma erectile dysfunction treatment by ayurveda fildena 50mg with visa, and a long coiled renal tubule, which converts the filtrate to urine. The parietal (outer) layer is a simple squamous epithelium, and the visceral (inner) layer consists of elaborate cells called podocytes7 wrapped around the capillaries of the glomerulus. The two layers are separated 7 podo = foot; cyte = cell by a filtrate-collecting capsular space. In tissue sections, this space appears as an empty circular or C-shaped space around the glomerulus. At the vascular pole, the afferent arteriole enters the capsule, bringing blood to the glomerulus, and the efferent arteriole leaves the capsule and carries blood away. Thus, the glomerulus has a large inlet and a small outlet-a point whose functional significance will become apparent later. At the urinary pole, the parietal wall of the capsule turns away from the corpuscle and gives rise to the renal tubule. The simple squamous epithelium of the capsule becomes simple cuboidal in the tubule. It is about 3 cm long and divided into four regions: the proximal convoluted tubule, nephron loop, distal convoluted tubule, and collecting duct (fig. The first three of these are parts of one nephron; the collecting duct receives fluid from many nephrons. Each region has unique physiological properties and roles in the production of urine. It is the longest and most coiled of the four regions and therefore dominates histological sections of renal cortex. It has a simple cuboidal epithelium with prominent microvilli (a brush border), which attests to the great deal of absorption that occurs here. The nephron loop (formerly called the loop of Henle) is a long U-shaped portion of the renal tubule found mostly in the medulla. They form the initial part of the descending limb and part or all of the ascending limb. The cells here are heavily engaged in active transport of salts, so they have very high metabolic activity and are loaded with mitochondria, accounting for their thickness. It forms most of the descending limb, and in some nephrons, it rounds the bend and continues partway up the ascending limb. The cells here have low metabolic activity, but the thin segment of the descending limb is very permeable to water. It is shorter and less coiled than the proximal convoluted tubule, so fewer sections of it are seen in histological sections. It has a cuboidal epithelium with smooth-surfaced cells nearly devoid of microvilli. Numerous collecting ducts converge toward the tip of a medullary pyramid, and near the papilla, they merge to form a larger papillary duct. The flow of fluid from the point where the glomerular filtrate is formed to the point where urine leaves the body is: glomerular capsule proximal convoluted tubule nephron loop distal convoluted tubule collecting duct papillary duct minor calyx major calyx renal pelvis ureter urinary bladder urethra. Their proximal and distal convoluted tubules are commingled in a single tangled mass in each nephron. They have very long nephron loops that extend as far as the apex of the renal pyramid. They have relatively short nephron loops that dip only slightly into the outer medulla before turning back (see fig. As you will see later, nephron loops are responsible for maintaining an osmotic gradient in the medulla that helps the body conserve water. Although only 15% of the nephrons are juxtamedullary, they are almost solely responsible for maintaining this gradient. Explain the forces that promote and oppose filtration, and calculate the filtration pressure if given the magnitude of these forces; and c. The plexus follows branches of the renal artery into the parenchyma of the kidney, issuing nerve fibers to the blood vessels and convoluted tubules of the nephrons.

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The vagina has no glands erectile dysfunction drugs generic names generic fildena 150mg mastercard, but it is lubricated by the seepage of serous fluid through its walls (transudation) and by mucus from the cervical glands above it erectile dysfunction medications generic discount fildena 100mg on-line. The vagina extends slightly beyond the cervix as a blind pouch called the fornix15 (see figs. The vaginal, uterine, and ovarian arteries are exaggerated in length by the perspective of the drawing, moving the aorta away from the uterus for clarity. Ed Reschke the lower end of the vagina has transverse friction ridges, or vaginal rugae, which contribute to both male and female stimulation during intercourse. At the vaginal orifice, the mucosa folds inward and forms a membrane, the hymen, which stretches across the opening. It is often torn during childhood or adolescence by ordinary physical activity, the use of tampons, or medical examination with a speculum, or it may tear at the first act of sexual intercourse. The vaginal epithelium is simple cuboidal in childhood, but the estrogens of puberty transform it into a stratified squamous epithelium. This is an example of metaplasia, the transformation of one tissue type to another. These structures lie within the same anatomical boundaries as described for the male perineum (see fig. Muscles of the pelvic floor in this region are especially important in pregnancy and obstetrics and in supporting the internal reproductive organs (see section 10. The mons18 pubis consists mainly of an anterior mound of adipose tissue overlying the pubic symphysis, bearing most of the pubic hair (see fig. The labia majora19 (singular, labium majus) are a pair of thick folds of skin and adipose tissue inferior to the mons; the fissure between them is the pudendal cleft. Pubic hair grows on the lateral surfaces of the labia majora at puberty, but the medial surfaces remain hairless. Medial to the labia majora are the much thinner, entirely hairless labia minora20 (singular, labium minus). The area enclosed by them, called the vestibule,21 contains the urinary and vaginal orifices. Its function is entirely sensory, serving as the primary center of sexual stimulation. On each side of the vagina is a pea-size greater vestibular (Bartholin23) gland with a short duct opening into the vestibule or lower vagina (fig. They keep the vulva moist, and during sexual excitement they provide most of the lubrication for intercourse. A pair of mucous paraurethral glands opens into the vestibule near the external urethral orifice. These are homologous to the male prostate and are also called the female prostate or, formerly, the Skene24 glands. A discussion of the physiology of intercourse later in this chapter details the functions of these organs. The circulation and innervation of the clitoris are much like those of the penis (see section 27. It enlarges at puberty and remains so for life, but most of this time it contains very little mammary gland. The mammary gland develops within the breast during pregnancy, remains active in the lactating breast, and atrophies when a woman ceases to nurse. The breast has two principal regions: the conical to pendulous body, with the nipple at its apex, and an extension toward the armpit called the axillary tail. Lymphatics of the axillary tail are especially important as a route of breast cancer metastasis. The nipple is surrounded by a circular zone, the areola, usually darker than the rest of the breast. Dermal blood capillaries and nerves come closer to the surface here than in the surrounding skin, accentuating the color and sensitivity of the areola. Pregnancy increases melanin deposition in the areola and nipple, making them a more visible "target" to the indistinct vision of a nursing infant. Sensory nerve fibers of the areola are important in triggering a milk ejection reflex when an infant nurses.

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In females erectile dysfunction caused by steroids buy generic fildena on-line, the external urethral sphincter may be the only mechanism for urine retention erectile dysfunction penile injections purchase discount fildena online. Retention therefore depends on the strength of the voluntary pelvic floor muscles, which can be compromised by a history of pregnancy or obesity. If the urge to urinate arises at an inconvenient time and one must suppress it, the stretch receptors fatigue and stop firing. As bladder tension increases, however, the signals return with increasing frequency and persistence. Compare and contrast the functions of the internal and external urethral sphincters. In males, the sympathetic nervous system triggers ejaculation and, at the same time, stimulates constriction of the internal urethral sphincter. These are absorbed into the blood from inhaled fumes or by skin contact and then filtered by the glomeruli. Even when some of the nephrons are irreversibly destroyed, others hypertrophy and compensate for their lost function. When 75% of the nephrons are lost, however, urine output may be as low as 30 mL/h compared with the normal rate of 50 to 60 mL/h. This is insufficient to maintain homeostasis and is accompanied by azotemia and acidosis. Blood is pumped from the radial artery to a dialysis machine (artificial kidney) and returned to the patient by way of a vein. In the dialysis machine, the blood flows through a semipermeable cellophane tube surrounded by dialysis fluid. Urea, potassium, and other solutes that are more concentrated in the blood than in the dialysis fluid diffuse through the membrane into the fluid, which is discarded. Glucose, electrolytes, and drugs can be administered by adding them to the dialysis fluid so they will diffuse through the membrane into the blood. People with renal insufficiency accumulate substantial amounts of excess body water between treatments, and dialysis serves also to remove it. Hemodialysis patients typically have three sessions per week for 4 to 8 hours per session. In addition to inconvenience, hemodialysis carries risks of infection and thrombosis. Blood tends to clot when exposed to foreign surfaces, so an anticoagulant such as heparin is added during dialysis. It can be carried out at home by the patient, who is provided with plastic bags of dialysis fluid. The fluid is left in the body cavity for 15 to 60 minutes to allow the blood to equilibrate with it; then it is drained, discarded, and replaced with fresh dialysis fluid. The patient is not limited by a stationary dialysis machine and can go about most normal activities. Blood is Blood pump pumped into a dialysis chamber, where it flows through a selectively permeable membrane surrounded by dialysis fluid. Structure of the glomerular filtration membrane; roles of the capillary fenestrations, basement membrane, and filtration slits in producing glomerular filtrate; the difference between the filtrate and blood plasma 3. Structure and function of the juxtaglomerular apparatus and how tubuloglomerular feedback works 9. Effects of antidiuretic hormone and aldosterone on the kidney and how they reduce water loss from the body 5. Effects of aldosterone, natriuretic peptides, parathyroid hormone, calcitonin, and calcitriol on nephron function Assess Your Learning Outcomes To test your knowledge, discuss the following topics with a study partner or in writing, ideally from memory. Four main nitrogenous wastes and their metabolic sources; how metabolic wastes differ from other wastes 4.

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As ovulation approaches impotence at 43 buy 50 mg fildena with amex, the cumulus oophorus swells and its attachment to the follicle wall constricts until the oocyte is attached to the wall by only a narrow stalk erectile dysfunction 27 discount fildena 50 mg fast delivery. In the last day or so, the oocyte and cumulus break away and float freely in the antrum, ready for that momentous event in its life-ovulation. The cycle averages 28 days in length, so we will use that as the basis for the timetable described in the following discussion. It commonly varies from 20 to 45 days, however, so be aware that the timetable given in this discussion may differ from person to person and month to month. That is, the basic hierarchy of control can be represented: hypothalamus pituitary ovaries uterus. However, the ovaries also exert positive and negative feedback controls over the hypothalamus and pituitary, so these hormonal relationships are more like a communication loop than a straight-line chain of command. The first 3 to 5 days of this are marked by menstruation, the vaginal discharge of blood and endometrial tissue. During this endometrial reconstruction, a cohort of tertiary follicles grows until the dominant follicle ovulates around day 14. After ovulation, the remainder of that follicle becomes a body called the corpus luteum. Over the next 2 weeks, called the luteal phase, the corpus luteum stimulates endometrial secretion, making the endometrium thicken still more, up to about day 26. As loose tissue and blood accumulate, menstruation begins and the cycle starts over. The follicular phase of the cycle extends from the beginning of menstruation until ovulation-that is, from day 1 to day 14 in an average cycle. The portion from the end of menstruation until ovulation is also called the preovulatory phase. The follicular phase is the most variable part of the cycle, so unfortunately for family planning or pregnancy avoidance, it is seldom possible to reliably predict the date of ovulation. The ovary, at this stage, also exhibits follicles in many other stages, belonging to other cohorts trailing behind the lead cohort like freshmen to juniors trailing behind the senior class. The primary oocyte completes meiosis I, producing a haploid secondary oocyte and the first polar body. Follicular fluid builds rapidly; the follicle swells to as much as 25 or 30 mm in diameter, contains up to 7 mL of fluid, and bulges from the ovary like a blister. Macrophages and leukocytes are attracted to the area and secrete enzymes that weaken the follicular wall and adjacent ovarian tissue. With mounting internal pressure and a weakening wall, the mature follicle approaches rupture. These are normally swept up by the ciliary current and taken into the uterine tube, although many oocytes fall into the pelvic cavity and die. Days 15 to 28, from just after ovulation to the onset of menstruation, are called the luteal (postovulatory) phase. The two hormone levels in part (a) are drawn to the same scale, but those in part (b) are not. The peak progesterone concentration is about 17 times as high as the peak estradiol concentration. As the clotted blood is slowly absorbed, granulosa and theca interna cells multiply and fill the antrum, and a dense bed of blood capillaries grows amid them. The ovulated follicle has now become a structure called the corpus luteum,36 named for a yellow lipid that accumulates in the theca interna cells. This is because the high levels of estradiol and progesterone, along with inhibin from the corpus luteum, have a negative feedback effect on the pituitary. This is best measured first thing in the morning, before rising from bed; the change can be detected if basal temperatures are recorded for several days before ovulation in order to see the difference. Finally, some women experience twinges of ovarian pain known by the German name, mittelschmerz,38 which lasts from a few hours to a day or so at the time of ovulation.

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Alveolar macrophages remove debris from the lungs; pulmonary lymphatic vessels are especially abundant and are needed to prevent fluid accumulation in the lungs yohimbine treatment erectile dysfunction order fildena 150mg online. Immune hypersensitivity has effects ranging from respiratory congestion to asthma erectile dysfunction pills wiki purchase fildena 25mg online. Lymphatics absorb fluid and proteins in the kidneys, which is essential to enabling kidneys to concentrate the urine and conserve water. Immunity to cells that are genetically different from other body cells requires the testes and ovaries to have barriers that protect sperm and eggs from immune destruction. Location, gross anatomy, and histology of the spleen; the difference between the red and white pulp; and functions associated with each type of pulp 17. How hyperemia, bradykinin, and other factors account for the four cardinal signs of inflammation 19. Mechanisms of tissue repair carried out after a pathogen is defeated Assess Your Learning Outcomes To test your knowledge, discuss the following topics with a study partner or in writing, ideally from memory. How lymph is produced; characteristics of lymphatic capillaries that allow cells and other large particles to enter the lymph 4. Lymphatic collecting vessels, trunks, and collecting ducts; the similarity of lymphatic vessels to some blood vessels; and their relationship to the lymph nodes 5. Names of the 11 lymphatic trunks and two collecting ducts; the body regions drained by them; and the two points at which lymph empties into the bloodstream 6. How lymphatic nodules differ from diffuse lymphatic tissue; the name of the clusters of lymphatic nodules found in the distal small intestine 10. How lymphatic organs differ from diffuse lymphatic tissue and lymphatic nodules; the two primary lymphatic organs and three secondary lymphatic organs, and why they are called this 11. The location, gross anatomy, and histology of the thymus; the functional difference between its cortex and medulla; the functions of its epithelial cells; and the necessity of the thymus to immunity 13. Structure and function of lymph nodes; the significance of lymph nodes having both afferent and efferent lymphatic vessels, unlike any other lymphatic organs; the approximate number of lymph nodes and seven regions in which they are especially concentrated; and the meaning of lymphadenitis and lymphadenopathy 14. Types of tonsils, where they are located, and their structure and function; the most common cause of tonsillitis 21. Differences between innate and adaptive immunity; defining characteristics of the former 4. Three properties of the skin that make it an effective barrier to pathogens; the roles of organic acids and antimicrobial peptides in its barrier function 5. Three pathways of complement activation; how each is initiated; which pathways function in innate and adaptive immunity; and four mechanisms of pathogen destruction aided by complement 12. The actions of natural killer cells and the roles of perforins and granzymes in defense 13. The neutrophil actions of margination, diapedesis, chemotaxis, phagocytosis, the respiratory burst, and cytokine secretion 16. Two basic forms of adaptive immunity, the differences between them, and the types of pathogens each one attacks 3. How adaptive immunity is classified as active or passive and as natural or artificial; which types result in immune memory and lasting protection, and which do not 4. The life history of T cells including their origin; migration to the thymus; the sites, processes, and purposes of positive and negative selection; and dispersal of the naive lymphocyte pool 11. The meanings of immunocompetence and self-tolerance, how these are attained, and their importance to immunity 12. The life history of B cells including their origin, positive and negative selection, and dispersal 13. Four classes of T lymphocytes involved in cellular immunity, and the function of each 2. The basic cause of autoimmune diseases; what normally prevents them; and three reasons why an autoimmune disease may appear, with an example of each 4. Characteristics of immune memory and the T cell recall response in cellular immunity 21. Hypersensitivity; names and characteristics of its four types, and examples of disorders of each type 21. Similarities and differences between humoral and cellular immunity Testing Your Recall 1. Which of the following cells are involved in innate immunity but not in adaptive immunity Let 1 = antigen display, 2 = antibody secretion, 3 = secretion of interleukin, 4 = clonal selection, and 5 = endocytosis of an antigen.

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Its head (fundus) usually projects slightly beyond the inferior margin of the liver erectile dysfunction doctor exam buy 150 mg fildena fast delivery. Its neck (cervix) leads into the cystic duct erectile dysfunction treatment jaipur purchase fildena with paypal, which leads in turn to the bile duct. Bile is a light yellow-green color when secreted by the liver, but becomes a deep, intense green when concentrated in the gallbladder. It is a watery solution of minerals, cholesterol, neutral fats, phospholipids, bile pigments, bile acids, and lipidtransport vesicles called micelles (explained in section 25. The principal bile pigment is bilirubin, derived from the decomposition of hemoglobin. Bacteria of the intestine metabolize bilirubin to a colorless product called urobilinogen. About half of this is reabsorbed in the small intestine and excreted by the kidneys. Urobilinogen remaining in the intestine is converted to stercobilin, from which feces get their brown color. In the absence of bile secretion, the feces are grayish white and marked with streaks of undigested fat (acholic feces). Bile acids, micelles, and lecithin, a phospholipid, aid in fat digestion and absorption, as discussed later. When these waste products become excessively concentrated, they may form gallstones (see Deeper Insight 25. Bile gets into the gallbladder by first filling the bile duct, then overflowing into the gallbladder. Between meals, the gallbladder absorbs water and electrolytes from the bile and concentrates it by a factor of 5 to 20 times. About 80% of the bile acids are reabsorbed in the ileum, the last portion of the small intestine, and returned to the liver, where the hepatocytes absorb and resecrete them. Cholelithiasis,17 the formation or presence of gallstones, is most common in obese women over the age of 40, but certainly not limited to them; it occurs also in men and sometimes even children. It can occur when cholesterol becomes too concentrated to stay in solution and begins to precipitate out as crystals that steadily grow in size. The gallbladder may contain dozens of gallstones or even more, some over 1 cm in diameter. Gallstones can cause excruciating pain when they obstruct the bile ducts or when the gallbladder or bile ducts contract. When they block the flow of bile into the duodenum, they cause jaundice (yellowing of the skin due to bile pigment accumulation), poor fat digestion, inflammation of the pancreas (pancreatitis), and impaired absorption of fat-soluble vitamins. Only the gallbladder can concentrate bile enough to cause gallstones, so it is often removed along with the gallstones to prevent recurrence. Biophoto Associates/Science Source reuses the bile acids two or more times during the digestion of an average meal. The liver synthesizes new bile acids from cholesterol to replace the quantity lost in the feces. It has a globose head encircled by the duodenum, a midportion called the body, and a blunt, tapered tail on the left. Its endocrine part is the pancreatic islets, which secrete insulin and glucagon (see section 17. About 99% of the pancreas is exocrine tissue, which secretes 1,200 to 1,500 mL of pancreatic juice per day. Pancreatic islets are relatively concentrated in the tail of the pancreas, whereas the head is more exocrine. Over 90% of pancreatic cancers arise from the ducts of the exocrine portion (ductal carcinomas), so cancer is most common in the head of the gland. The acini open into a system of branched ducts that eventually converge on the main pancreatic duct. This duct runs lengthwise through the middle of the gland and joins the bile duct at the hepatopancreatic ampulla.

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The problem erectile dysfunction images cheap fildena 50mg otc, however erectile dysfunction new drug buy fildena with a mastercard, is that a person who has taken drugs may commit suicide in the bathtub by intentionally inhaling water or may just wait until reaching a state of reduced consciousness when the drowning will be a more or less passive process. The latter may also be true for unintentional drownings among intoxicated persons. Because there is as yet no published compilation of drug levels found in bathtub victims that could be used as a reference, it seems reasonable to use particular caution when evaluating such cases. Positional asphyxia implies that a person has assumed an unnatural position which compromises their respiration. In some cases, the person is trapped in such a position from which it is difficult to escape, but more commonly the victim has sustained injuries during a fall, or is intoxicated by alcohol or drugs, reducing their ability to change position. In general, alcohol intake is associated with increased risk for accidental deaths, including those caused by fatal asphyxia. In experimental settings, even moderate amounts of alcohol have been shown to reduce the tension of the genioglossal muscle complex in awake subjects, increasing the risk that the tongue will fall back and block the pharynx as the person fails to maintain their alertness [19]. These results were confirmed in other studies, in which the number of sleep apnoea events exceeded the normal limit after similar moderate alcohol intake [24,39]. Thus, there is ample evidence to support the notion that alcohol promotes asphyxiation in cases where the victim has adopted an abnormal position compromising the process of respiration. In addition, the general respiratory depressant effect of alcohol, as well as the reduced strength, and in many cases, the effect of alcohol on judgement and coordination, may contribute to a fatal outcome. It is therefore often justified to consider alcohol as a significant contributory factor in positional asphyxia deaths. Pharmaceutical drugs may also be important in positional asphyxia by attenuating the genioglossal tone and/or by affecting other systems important for airway patency. The pedunculopontine tegmental nucleus has been identified as an important control centre for the regulation of upper airway muscle tone. In addition to these electromyogram recordings, they showed that the same treatments to some extent also modulated the breathing pattern. In addition, certain drugs, such as the antipsychotic drug clozapine, can also cause hypersalivation [31]. A person who has adopted an abnormal, yet supine position, and is either comatose or immobilized, may aspirate saliva, and if a drug-induced hypersalivation is present, the amounts may be sufficient to compromise the airway flow and contribute to the asphyxia. However, it is a challenge to estimate the respiratory depressant effect only by reviewing the postmortem blood drug concentrations. In order to be considered significant, the drug concentrations should clearly exceed those that can be seen in deaths where the victim obviously was not incapacitated by drugs [10]. Hence, a humble attitude is warranted when assessing the impact on blood drug levels for the fatal outcome in cases of positional asphyxia. A particular form of positional asphyxia is when the body is lying head-down, or even hanging upside down. A number of case reports have described such cases and the circumstances are typically unique to each case. In more recent publications, the pattern is similar; about half of the victims were seemingly influenced by alcohol or drugs at the time of the incident. Given the importance of diaphragm exhaustion for the fatal outcome, drugs that reduce the neural signalling to the diaphragm are likely to facilitate the process. Fatal asphyxiation caused by chest compression, often referred to as traumatic asphyxia, will in most cases lead to death fairly rapidly. The petechiae typically appearing on the head, neck and shoulders are not a proof of a long agony but may be produced within minutes. The possible influence of alcohol or drugs will therefore usually not make much difference to the fatal outcome. However, the presence of alcohol or drugs in such deaths may rather be associated with an increased risk for the event as such by contributing to , for example, incautious operation of a heavy machine, or poor driving ability, regarding traumatic asphyxia due to motor vehicle accidents. In many cases, positional asphyxia has also been suggested to have played an important role in these deaths. A large number of violent persons resisting arrest or emergency care are under the influence of alcohol, yet sudden deaths during apprehension and subdual of drunk subjects is rare. In parallel, many reports dealt with positional asphyxia as a competing cause for these in-custody deaths, and a number of experimental studies on human subjects were performed to evaluate the effects of various restraint measures and body positions on respiratory and circulatory parameters [7,8,33].

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Along with the declining testosterone level low testosterone causes erectile dysfunction order fildena in india, the sperm count and libido diminish erectile dysfunction cleveland clinic buy line fildena, as do the volume and force of ejaculation. Older men may become less interested in sex, but nevertheless remain capable of fathering a child throughout old age. As testosterone and inhibin levels decline, so does feedback inhibition of the pituitary. These gonadotropins may cause mood changes, hot flashes, or even illusions of suffocation- symptoms similar to those in perimenopausal women. About 20% of men in their 60s and 50% of men in their 80s experience erectile dysfunction (impotence), the frequent inability to produce or maintain an erection sufficient for intercourse (see table 27. It is essentially the splitting of a cell with a distribution of chromosomes that results in two genetically identical daughter cells. In mitosis, each double-stranded chromosome divides into two single-stranded ones, but each daughter cell still has 46 chromosomes (23 pairs). The parent cell is diploid (2n), meaning it has 46 chromosomes in 23 homologous pairs, whereas the daughter cells are haploid (n), with 23 unpaired chromosomes. If these gametes combined, the zygote and the next generation would have 92 chromosomes per cell, the generation after that would have 184, and so forth. To prevent the chromosome number from doubling in every generation, the number is reduced by half during gametogenesis. In prophase I, pairs of homologous chromosomes line up side by side and form tetrads (tetra denoting the four chromatids). This ensures genetic variety in the offspring, further accentuated by the independent assortment of chromosomes into different sperm, and the randomness of fertilization. The genetic recombination and reduction in chromosome number are achieved through a form of cell division called meiosis, which produces four daughter cells that subsequently differentiate into sperm. Centrioles Mid- to late prophase I Homologous chromosomes form pairs called tetrads. Crossing-over Spindle fibers Metaphase I Tetrads align on equatorial plane of cell with centromeres attached to spindle fibers. Anaphase I Homologous chromosomes separate and migrate to opposite poles of the cell. Telophase I New nuclear envelopes form around chromosomes; cell undergoes cytoplasmic division (cytokinesis). Although we pass the same genes to our offspring as we inherit from our parents, we do not pass on the same chromosomes. This creates new combinations of genes and thus contributes to genetic variety in the offspring. After crossing-over, the chromosomes line up at the midline of the cell in metaphase I, they separate at anaphase I, and the cell divides in two at telophase I. Therefore, at the conclusion of meiosis I, each chromosome is still double-stranded, but each daughter cell has only 23 chromosomes- it has become haploid. Fertilization combines 23 chromosomes from the father with 23 chromosomes from the mother and reestablishes the diploid number of 46 in the zygote. The first stem cells specifically destined to become sperm are primordial germ cells. Like the first blood cells, these form in the yolk sac, a membrane associated with the developing embryo. In the fifth to sixth week of development, they crawl into the embryo itself and colonize the gonadal ridges. At puberty, testosterone secretion rises, reactivates the spermatogonia, and brings on spermatogenesis. The essential steps of spermatogenesis are as follows, numbered to match figure 27. One daughter cell from each division remains near the tubule wall as a stem cell called a type A spermatogonium. Type A spermatogonia serve as a lifetime supply of stem cells, so men normally remain fertile even in old age.

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