Animal Reproduction in Mammals PDF

DEV BIO Animal reproduction is a complex and fascinating process that ensures the continuity of life on our planet. From courtship rituals to ANIMAL mate selection, m...

DEV BIO Animal reproduction is a complex and fascinating process that ensures the continuity of life on our planet. From courtship rituals to ANIMAL mate selection, mating behaviors, and fertilization, animals have evolved a wide array of strategies to reproduce successfully. The amazing intricacies of animal reproduction highlight the diversity and complexity of the natural world and the incredible adaptations REPRODUCTION that have evolved over millions of years to ensure the survival of countless species. AUMENTADO HILARIO ESTOLANO REAMICO SERAFIN SEX HORMONES IN MAMMALS 50.4 The sex hormones play a key role in three events: 1. Development of the reproductive tract in embryos 2. Maturation of the reproductive tract during the transition from childhood to adulthood 3. Regulation of spermatogenesis and oogenesis in adults PUBERTY Sex hormones in mammals Puberty is the process that leads to sexual maturity in humans. In amphibians, the juvenile-to-adult transition is triggered by the hormone T3 (triiodothyronine); Vision in insects, the transition occurs in response to ecdysone (see Chapter 49). But in humans, the transition is directed by increased levels of gonadal hormones—testosterone in boys and estradiol in girls. WHAT REGULATES THE Sex hormones in mammals GONADAL HORMONES? Two advances made it possible to test this hypothesis rigorously: 1. Researchers isolated a hormone called gonadotropinreleasing hormone (GnRH) from the hypothalamus. 2. Investigators noted that boys and girls who were entering puberty experienced pulses in the concentration of two pituitary hormones, luteinizing hormone (LH) and folliclestimulating hormone (FSH). Female Gonad Male Gonad Sex hormones in mammals WHAT REGULATES THE HYPOTHALAMIC Sex hormones in mammals AND PITUITARY HORMONES? What triggers GnRH increases at the appropriate age? Although this question remains unanswered, there is some evidence that nutritional state is involved. ⦁ The current average age for the onset of menstruation in females in the United States is slightly over 12 years. This is much earlier than the average age of 17 years during the eighteenth and nineteenth centuries, when the general nutritional state of the population was poorer. ⦁ Among girls living today, individuals with large fat stores tend to enter puberty earlier than do girls who are thin. ⦁ The hormone leptin is produced by fat cells, and leptin levels are higher in people with more fat. In mice, leptin accelerates the onset of puberty, suggesting that leptin may be involved in triggering the GnRH surge WHAT REGULATES THE HYPOTHALAMIC Sex hormones in mammals AND PITUITARY HORMONES? What triggers GnRH increases at the appropriate age? Although this question remains unanswered, there is some evidence that nutritional state is involved. ⦁ The current average age for the onset of menstruation in females in the United States is slightly over 12 years. This is much earlier than the average age of 17 years during the eighteenth and nineteenth centuries, when the general nutritional state of the population was poorer. ⦁ Among girls living today, individuals with large fat stores tend to enter puberty earlier than do girls who are thin. ⦁ The hormone leptin is produced by fat cells, and leptin levels are higher in people with more fat. In mice, leptin accelerates the onset of puberty, suggesting that leptin may be involved in triggering the GnRH surge Sex hormones in mammals WHAT REGULATES THE HYPOTHALAMIC Sex hormones in mammals AND PITUITARY HORMONES? What triggers GnRH increases at the appropriate age? Although this question remains unanswered, there is some evidence that nutritional state is involved. ⦁ The current average age for the onset of menstruation in females in the United States is slightly over 12 years. This is much earlier than the average age of 17 years during the eighteenth and nineteenth centuries, when the general nutritional state of the population was poorer. ⦁ Among girls living today, individuals with large fat stores tend to enter puberty earlier than do girls who are thin. ⦁ The hormone leptin is produced by fat cells, and leptin levels are higher in people with more fat. In mice, leptin accelerates the onset of puberty, suggesting that leptin may be involved in triggering the GnRH surge MENSTRUAL CYCLE Sex hormones in mammals WHICH HORMONES CONTROL THE Sex hormones in mammals MENSTRUAL CYCLE IN MAMMALS? Day 0 in the menstrual cycle is marked by the beginning of menstruation—the expulsion of the uterine lining. The remainder of the cycle has two distinct phases: 1. Follicular phase A follicle matures during the follicular phase, which lasts an average of 14 days. Primary oocytes complete meiosis I during this phase. Ovulation occurs when the follicle is mature and releases its secondary oocyte into the oviduct. 2. Luteal phase The luteal phase begins with ovulation and averages 14 days in length. Its name was inspired by the formation and subsequent degeneration of a structure called the corpus luteum (“yellowish body”) from the ruptured follicle. WHICH HORMONES CONTROL THE Sex hormones in mammals MENSTRUAL CYCLE IN MAMMALS? The regular occurrence of ovulation throughout the year makes human females extremely unusual among mammals. Although some mammals ovulate multiple times during the year, most ovulate only during a single prescribed breeding season—often in response to environmental cues such as changing photoperiod; less often in response to cues from males. In addition, only humans and other great apes menstruate. In the vast majority of mammals, the lining of the uterus is reabsorbed if pregnancy does not occur. These females are described as having an estrous cycle and are sexually receptive only during estrus—also known as being “in heat.” Sex hormones in mammals WHICH HORMONES CONTROL THE Sex hormones in mammals MENSTRUAL CYCLE IN MAMMALS? Whether an estrous or menstrual cycle occurs, the basic sequence of events, with a follicular phase preceding ovulation and a luteal phase following ovulation, is shared among mammals. Hormonal control of the estrous and menstrual cycles is also similar. HOW DO PITUITARY AND OVARIAN Sex hormones in mammals HORMONES CHANGE DURING A MENSTRUAL CYCLE? By monitoring hormone concentrations in the blood or urine of a large number of women over the course of the menstrual cycle, researchers were able to document dramatic changes in the concentrations of estradiol and several other hormones. During each cycle, ⦁ LH and FSH are produced in the anterior pituitary gland in response to GnRH; ⦁ the steroid hormone progesterone is produced along with estrogens, including estradiol, in the ovaries. HOW DO PITUITARY AND OVARIAN Sex hormones in mammals HORMONES CHANGE DURING A MENSTRUAL CYCLE? HOW DO PITUITARY AND OVARIAN HORMONES Sex hormones in mammals CHANGE DURING A MENSTRUAL CYCLE? HOW DO PITUITARY AND OVARIAN HORMONES Sex hormones in mammals INTERACT?  Page 005 Day 0–7 Day 8–14 ⦁ As the uterus is shedding much of its lining, a ⦁ As the follicle grows, its production of estradiol gradually follicle is beginning to develop in one ovary increases. The increase in estradiol stimulates mitosis and an under the influence of FSH. increase in cell number in the uterine lining. ⦁ The enlarged follicle produces large quantities of estradiol, ⦁ The follicle produces estradiol and a small which begin to exert positive feedback on LH secretion. amount of progesterone. ⦁ Positive feedback results in a spike in LH levels, just after estradiol concentrations peak. ⦁ While its levels are still relatively low, estradiol ⦁ The LH surge triggers ovulation and ends the follicular phase suppresses LH secretion through negative feedback inhibition.  Page 005 Day 15–21 Day 22–28 ⦁ As the corpus luteum develops from the remains of the ⦁ If fertilization does not occur, the corpus luteum ruptured follicle, it secretes large amounts of degenerates. progesterone and small quantities of estradiol, in ⦁ Progesterone levels fall as the corpus luteum shrinks. response to LH. ⦁ When progesterone declines, the thickened lining of the uterus degenerates. This causes the menstrual bleeding that ⦁ The rise in progesterone lowers production of LH and marks the first day of the next cycle. FSH and activates the thickened uterine lining, creating a ⦁ GnRH, LH, and FSH are released from the inhibitory control spongy tissue with a well-developed blood supply. In this that progesterone exerts. way, progesterone fosters an environment that supports ⦁ LH and FSH levels rise, and a new menstrual cycle begins embryonic development if fertilization occurs. MANIPULATING HORMONE LEVELS TO Sex hormones in mammals PREVENT PREGNANCY Data on hormonal control of the menstrual cycle opened new avenues in birth control research. Specifically, researchers have found that manipulating levels of progesterone and estradiol can prevent ovulation and serve as a safe and effective method of contraception, or preventing unwanted pregnancies. In the United States, birth control pills are the most widely used contraceptive method. Hormone-containing pills are taken for three weeks and then stopped for one week to allow menstruation to occur. Other popular hormonal contraceptives include injections and hormone-secreting patches and implants. MANIPULATING HORMONE LEVELS TO Sex hormones in mammals PREVENT PREGNANCY PREGNANCY AND BIRTH 50.5 Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidata Presentation Design DEV BIO Pregnancy and birth are truly remarkable experiences that showcase PREGNANCY & the strength, resilience, and beauty of a woman's body. From the moment of conception to labor and delivery, every stage is a BIRTH IN testament to the incredible capabilities of the female form. The process of growing another human being inside one's own body is nothing short of miraculous, as every change and development MAMMALS serves a vital purpose in preparing for the arrival of new life. AUMENTADO HILARIO ESTOLANO REAMICO SERAFIN PREGNANCY & BIRTH IN MAMMALS 50.5 Viviparity allows the mother to provide a warm, protected environment for offspring during early development. Oviparous species that guard or incubate their eggs also provide warm, safe surroundings for their young. PREGNANCY & BIRTH IN MAMMALS 50.5 Pregnancy and lactation—providing milk that nourishes offspring after birth—represent some of the most extreme forms of parental care known in animals. And in some mammal species, parental care continues long after lactation ends. Humans, for example, are largely or completely dependent on their parents for protection and nutrition until puberty or young adulthood.  50.5 MODE OF REPRODUCTION Monotremes lay eggs and In eutherians, mothers carry Benjamin Shah incubate them until hatching offspring internally for relatively long periods of development and nourish them via a placenta. GESTATION AND 50.5 EARLY DEVELOPMENT IN MARSUPIALS In marsupials, young are ejected from the mother’s body at the end of the estrous cycle. As a result, they are far less developed than eutherian mammal offspring, which undergo a lengthier gestation—the developmental period that takes place inside the mother The jaws, gut, lungs, and forelimbs of a newly born marsupial are relatively well developed at birth. As a result, the offspring is able to climb from its mother’s vagina to a nipple, which is usually enclosed in a pouch created by a flap of skin. The offspring clamps onto the nipple and continues to develop, fed by the mother’s milk Even after growing large enough to leave the pouch and begin moving and feeding on its own, offspring will return to the pouch for protection. Marsupial mothers invest a great deal in their offspring, even though a relatively short period of development takes place inside their bodies. MAJOR EVENTS DURING HUMAN PREGNANCY 50.5 Marsupials and eutherians differ sharply in terms of how long the developing embryo is retained inside the mother’s body. Let’s consider humans as a model organism in eutherian reproduction. When a secondary oocyte is released from the human ovary, the cell is viable for less than 24 hours. Human sperm, in contrast, remain capable of fertilizing an egg for up to five days. Therefore, sexual intercourse in humans has to occur less than five days before ovulation or immediately after ovulation for pregnancy to result. Although an ejaculation may contain hundreds of millions of sperm, most die as they travel through the uterus. Only 100 to 300 actually succeed in reaching the oviduct, where fertilization takes place. You might recall that when sperm and oocyte meet, enzymes released from the head of the sperm create a path through the material surrounding the oocyte membrane. Once the membranes of the oocyte and sperm have fused, the oocyte nucleus completes meiosis II. The two nuclei then unite to form a diploid zygote. IMPLANTATION 50.5 Smooth-muscle contractions in the oviduct gradually move the zygote toward the uterus. As it travels, the cell begins to divide by mitosis. By the time it reaches the lining of the uterus, the embryo consists of a hollow ball of cells. It then undergoes implantation—meaning that it becomes embedded in the thickened, vascularized wall of the uterus. It will stay in the uterus for approximately 270 days (9 months). Once the embryo is implanted in the uterine lining, its cells begin synthesizing and secreting the hormone human chorionic gonadotropin; this hormone is later produced in larger quantities by the placenta. Human chorionic gonadotropin (hCG) is a chemical messenger that prevents the corpus luteum from degenerating. When hCG is present, the ovary continues secreting progesterone, and the menstrual cycle is arrested. Since hCG is excreted in the mother’s urine, it is the chemical used to detect pregnancy in pregnancy tests.  50.5 01 Not long after implantation is complete, mass movements of cells result in the formation of the three major embryonic tissues, called ectoderm, endoderm, and mesoderm. By 8 weeks into gestation, these tissues have differentiated into the various organs and systems of the body. Also by this time, the heart has begun pumping blood through a circulatory system. The embryo at this stage is called a fetus Early in development, the embryonic ectoderm contributes to several important membranes. One of these membranes, the amnion, completely surrounds the embryo. The amnion eventually fills with amniotic fluid, which provides the embryo with a protective cushion. The other key event in the first trimester is the formation of the placenta. This organ, which starts to form on the uterine wall a few weeks after implantation, is composed of tissues from both mother and embryo (see Chapter 35). Because the placenta contains a dense supply of blood vessels from the mother, it provides nutrition for the growing fetus. Arteries transport blood from the circulatory system of the fetus, through the umbilical cord, to an extensive capillary bed in the placenta. This capillary bed provides a large surface area for the exchange of gases, nutrients, and wastes between maternal and fetal Major Events blood, even though the maternal and fetal blood do not commingle. during Human Pregnancy  50.5 02 After the fetal organs and placenta form during the first trimester, the rest of development consists mainly of growth. During the last weeks of pregnancy, the brain and lungs undergo particularly dramatic growth and development. If a baby is born prematurely, intervention may be required to keep the baby alive until the lungs can complete their development. The machinery and level of hospital care required by premature infants 03 emphasize just how superbly adapted mothers are for nourishing a growing fetus in the uterus. It costs hundreds of thousands of dollars for health care providers to do what mothers do naturally in the last trimester. Let’s take a closer look at this critical aspect of pregnancy. Major Events during Human Pregnancy  50.5 Major Events during Human Pregnancy HOW DOES THE MOTHER 50.5 NOURISH THE FETUS? In oviparous and ovoviviparous species, mothers produce relatively large eggs that contain all of the nutrients and fluids that the embryo needs for development until hatching. But in some viviparous species, eggs are relatively small and contain almost no nutrients. In species such as humans, the developing embryo depends on the mother’s body for oxygen, chemical energy in the form of sugars, amino acids and other raw materials for growth, and waste removal. Oxygen Exchange between Mother and Fetus During pregnancy, a mother’s respiratory and circulatory systems change in ways that increase the efficiency of nutrient transfer and gas exchange with the fetus. HOW DOES THE MOTHER 50.5 NOURISH THE FETUS? In addition, important adaptations heighten the efficiency of gas exchange between the mother and the embryo, in the placenta. In many species, such as sheep, maternal and fetal blood in the placenta flow in a countercurrent fashion. Countercurrent flows maintain a concentration gradient that increases the efficiency of diffusion or other types of exchange Countercurrent flow does not occur in the human placenta. Instead, oxygen exchange between mother and fetus is efficient because of another mechanism. Maternal arteries in humans empty into a space at the junction of the maternal and fetal portions of the placenta. This space is packed with small projections called villi, which contain the fetal blood vessels.  50.5 The key message of this graph is that the data for fetal hemoglobin are shifted to the left of the data for adult hemoglobin. This means that the fetus’s blood always has a higher affinity for oxygen than does the mother’s blood. Even if the partial pressure of oxygen is the same in the mother and fetus, oxygen will move from the mother’s blood to the fetus’s blood because it is held more tightly by the fetal hemoglobin. Biologists interpret this pattern as an adaptation. The high oxygen affinity of fetal hemoglobin ensures that the fetus is always able to acquire oxygen from the mother. TOXIC CHEMICALS CAN BE TRANSFERRED FROM 50.5 MOTHER TO FETUS Mothers and embryos exchange more than nutrients and wastes— they can also exchange dangerous molecules. As an example, consider the thalidomide tragedy. During the 1950s, hundreds of children were affected by their mothers’ consumption of the tranquilizer thalidomide, which was prescribed to treat morning sickness. The molecule diffused into the fetal bloodstream and caused birth defects—often a dramatic shortening of the arms. Although thalidomide is now banned for use by pregnant women, alcohol use continues to affect newborns. Compared to children of mothers who do not drink alcohol, children of mothers who imbibe ethanol are at high risk for hyperactivity, severe learning disabilities, and depression. Collectively, these symptoms are termed fetal alcohol syndrome (FAS).  50.5 Thalidomide is a medication used to treat THALIDOMIDE certain types of cancer and inflammatory disorders. Presentation Design DEV BIO BIRTH Although the mechanisms responsible for initially triggering the birthing process are not completely understood, the posterior pituitary hormone oxytocin is important in stimulating smoothmuscle cells in the uterine wall to begin contractions. The contractions that expel the fetus from the uterus constitute labor.  50.5 As a result of sterile techniques, antibiotics, and blood transfusion technology, Sweden’s mortality rate has now declined to less than 0.007 percent. Improved nutrition, sanitation, and medical care have also reduced infant mortality rates in many countries. The huge decline in the rate of death associated with childbirth qualifies as one of the great triumphs of modern medicine. Unfortunately, because many developing nations lack sterile facilities and antibiotics, the mortality of mothers and infants remains high in those Presentation Design countries.  50.5 THANKS FOR LISTENING AUMENTADO HILARIO ESTOLANO REAMICO SERAFIN