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Human Reproduction & Development. Meiosis. An Overview of Meiosis
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Meiosis • An Overview of Meiosis • In human beings, nearly all cells have paired sets of chromosomes, meaning these cells are diploid. Meiosis is the process by which a single diploid cell divides to produce four haploid cells—that contain a single set of chromosomes. • The haploid cells produced through meiosis are called gametes. Female gametes are eggs; male gametes are sperm. They are the reproductive cells of human beings and many other organisms. • When the haploid sperm and haploid egg fuse, a diploid fertilized egg (or zygote) is produced, setting into development a new generation of organism.
Meiosis • There are two stages of meiosis, namely, meiosis I and meiosis II. The parent cell or the dividing cell undergoes a preparatory phase, known as interphase, before entering the two stages of meiosis. • In the interphase, the parent cell synthesizes more DNA and proteins, increasing the overall size and mass of the cell. As a part of the preparatory phase, the dividing cell duplicates or doubles its chromosomes. With these major changes, the parent cell enters the first stage of meiosis. The following is a brief description about the two stages and their phases: Meiosis I: • Prophase I: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing-over can occur during the latter part of this stage. • Metaphase I: Homologous chromosomes align at the equatorial plate. • Anaphase I: Homologous pairs separate with sister chromatids remaining together. • Telophase I: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.
Meiosis Meiosis II: • Prophase II: DNA does not replicate. • Metaphase II: Chromosomes align at the equatorial plate. • Anaphase II: Centromeres divide and sister chromatids migrate separately to each pole. • Telophase II: Cell division is complete. Four haploid daughter cells or gametes are formed. • The resulting four gametes are haploid; meaning they contain half the number of chromosomes. This is the reason as to why meiosis cell division is also referred to as reduction division. During fertilization, two gametes, one from the mother and another from the father, fuse, thus resulting in doubling of chromosome number. The fusion of gametes leads to the production of a zygote thus has the same chromosome number of the parents. Variation occurs in the resulting zygote due to the process of meiosis and fertilization of gametes. Zygote after attaining maturity, is capable of dividing into daughter cells.
Figure 1: Meiosis Overview: • 1st Division is very important as it reduces the numbers of chromosomes by half. • Prophase I has the homologous chromosomes (chromosomes that appear identical to one another because they carry the same genes, one comes from our mom, and the other comes from our dad) crossing over to exchange material. • Metaphase I have the homologous pairs line up at the equator of the cell and in Anaphase I, the homologous chromosomes separate and thus reducing the chromosome number in each cell. • 2nd Division of Meiosis is exactly like mitosis, except that 2 cells start off in prophase 2 and you end up with 4 daughter cells that are completely different from each other.
Gametogenesis • Gametogenesis is the development and production of the male and female germ cells required to form a new individual. • The male and female germ cells are called gametes. • In males, the gametes (sperm) are produced by the testes. • In females, the gametes (eggs or ova) are produced by the ovaries. • During sexual intercourse, an ejaculated sperm cell penetrates an egg and unites with it (fertilizes it). The fertilized egg is called the zygote. • Figure 2 : Gametogenesis
Oogenesis • Oogenesis is the formation of the ovum (female sex cells), which begin as hundreds of thousands of oogonia (stem cells) in the fetal ovaries. • In the ovaries of human females, primary oocytes with 46 chromosomes divide meiotically to form two cells, each with 23 duplicated chromosomes. • One of the cells, a secondary oocyte, receives most cytoplasm; the other cell, a polar body, disintegrates or divides again. • A secondary oocyte begins meiosis II and then stops at metaphase II. • At ovulation, the secondary oocyte leaves the ovary and enters an oviduct where it may meet a sperm. • If a sperm enters secondary oocyte, the oocyte is activated to continue meiosis II to completion; the result is a mature egg and another polar body, each with 23 daughter chromosomes. • Meiosis produces one egg and three polar bodies; polar bodies serve to discard unnecessary chromosomes and retain most of the cytoplasm in the egg. • The cytoplasm serves as a source of nutrients for the developing embryo.
Spermatogenesis • Spermatogenesis is the production of mature sperm cells. • In the testes of males, primary spermatocytes with 46 chromosomes divide meiotically to form two secondary spermatocytes, each with 23 duplicated chromosomes. • Secondary spermatocytes divide to produce four spermatids, also with 23 daughter chromosomes. • Spermatids then differentiate into sperm. • Meiotic cell division in males always results in four cells that become sperm.
Female Reproductive Anatomy • The female reproductive system is designed to carry out several functions as follows: • It produces the female egg cells necessary for reproduction, called the ova or oocytes. • The system is designed to transport the ova to the site of fertilization. • Conception, the fertilization of an egg by a sperm, normally occurs in the fallopian tubes. • After conception, the uterus offers a safe and favorable environment for a baby to develop before it is time for it to make its way into the outside world. • If fertilization does not take place, the system is designed to menstruate (the monthly shedding of the uterine lining). • In addition, the female reproductive system produces female sex hormones that maintain the reproductive cycle. • During menopause the female reproductive system gradually stops making the female hormones necessary for the reproductive cycle to work. When the body no longer produces these hormones a woman is considered to be menopausal.
Internal Female Reproductive Anatomy • The female reproductive anatomy includes internal and external structures. • The internal reproductive organs include: • Vagina: The vagina is a canal that joins the cervix (the lower part of uterus) to the outside of the body. It also is known as the birth canal. • Uterus (womb): The uterus is a hollow, pear-shaped organ that is the home to a developing fetus. The uterus is divided into two parts: the cervix, which is the lower part that opens into the vagina, and the main body of the uterus, called the corpus. The corpus can easily expand to hold a developing baby. A channel through the cervix allows sperm to enter and menstrual blood to exit. • Ovaries: The ovaries are small, oval-shaped glands that are located on either side of the uterus. The ovaries produce eggs and hormones. • Oviducts : Also known as fallopian tubes, are narrow tubes that are attached to the upper part of the uterus and serve as tunnels for the ova (egg cells) to travel from the ovaries to the uterus. Conception, the fertilization of an egg by a sperm, normally occurs in the fallopian tubes. The fertilized egg then moves to the uterus, where it implants to the uterine wall.
External Female Reproductive Anatomy • The function of the external female reproductive structures (the genital) is twofold: To enable sperm to enter the body and to protect the internal genital organs from infectious organisms. The main external structures of the female reproductive system include: • Labia majora: The labia majora enclose and protect the other external reproductive organs. Literally translated as "large lips," the labia majora are relatively large and fleshy, and are comparable to the scrotum in males. The labia majora contain sweat and oil-secreting glands. After puberty, the labia majora are covered with hair. • Labia minora: Literally translated as "small lips," the labia minora can be very small or up to 2 inches wide. They lie just inside the labia majora, and surround the openings to the vagina (the canal that joins the lower part of the uterus to the outside of the body) and urethra (the tube that carries urine from the bladder to the outside of the body). • Bartholin’s glands: These glands are located next to the vaginal opening and produce a fluid (mucus) secretion. • Clitoris: The two labia minora meet at the clitoris, a small, sensitive protrusion that is comparable to the penis in males. The clitoris is covered by a fold of skin, called the prepuce, which is similar to the foreskin at the end of the penis. Like the penis, the clitoris is very sensitive to stimulation and can become erect.
Female Reproductive Anatomy (Continued) • Mammary Glands • They are not part of the female reproductive tract but are important secondary reproductive organs. • The mammary glands develop in the tissue underneath the skin but on top of the muscles of the chest. • Both males and females start with the same tissues, but normally only females generate the correct hormonal signals to promote development of the mammary glands at puberty. • The full ability of mammary glands to synthesize and secrete milk does not occur unless a woman is exposed to the hormonal changes of pregnancy.
Male Reproductive Anatomy • The purpose of the organs of the male reproductive system is to perform the following functions: • to produce, maintain, and transport sperm (the male reproductive cells) and protective fluid (semen); • to discharge sperm within the female reproductive tract during sex; and • to produce and secrete male sex hormones responsible for maintaining the male reproductive system. • The entire male reproductive system is dependent on hormones, which are chemicals that regulate the activity of many different types of cells or organs. The primary hormones involved in the male reproductive system are follicle-stimulating hormone, luteinizing hormone, and testosterone. • Follicle-stimulating hormone is necessary for sperm production (spermatogenesis) and luteinizing hormone stimulates the production of testosterone, which is also needed to make sperm. Testosterone is responsible for the development of male characteristics, including muscle mass and strength, fat distribution, bone mass, facial hair growth, voice change, and sex drive.
External Male Reproductive Anatomy(Continued) • Most of the male reproductive system is located outside of the body. These external structures include the following: • Penis: This male organ is used in sexual intercourse. It has three parts: the root, which attaches to the wall of the abdomen; the body, or shaft; and the glans, which is the cone-shaped part at the end of the penis. The glans, also called the head of the penis, is covered with a loose layer of skin called foreskin. This skin is sometimes removed in a procedure called circumcision. The opening of the urethra, the tube that transports semen and urine, is at the tip of the penis. The penis also contains a number of sensitive nerve endings. • Scrotum: The scrotum is a thin sac of skin and thin muscle in which lie the testicles. The scrotum acts as a climate control system, allowing the testicles to be slightly away from the rest of the body and keeping them slightly cooler than normal body temperature for optimal sperm development.
External Male Reproductive Anatomy (Continued) • Testicles (testes): The testes (or testicles) are two olive-sized oval bodies, one on the right side and one on the left side. The testes have two main functions: to produce sperm (the male sex cell) and to produce testosterone (the male sex hormone).
Internal Male Reproductive Anatomy • The internal organs of the male reproductive system, also called accessory organs, include the following: • Epididymis: The epididymis is a long, coiled tube that rests on the backside of each testicle. It functions in the transport and storage of the sperm cells that are produced in the testes. It also is the job of the epididymis to bring the sperm to maturity, since the sperm that emerge from the testes are immature and incapable of fertilization. During sexual arousal, contractions force the sperm into the vas deferens. • Vas deferens: The vas deferens is a long, muscular tube that travels from the epididymis into the pelvic cavity, to just behind the bladder. The vas deferens transports mature sperm to the urethra in preparation for ejaculation. • Ejaculatory ducts: These are formed by the fusion of the vas deferens and the seminal vesicles. The ejaculatory ducts empty into the urethra. • Urethra: The urethra is the tube that carries urine from the bladder to outside of the body. It has the additional function of expelling (ejaculating) semen when the man reaches orgasm. When the penis is erect during sex, the flow of urine is blocked from the urethra, allowing only semen to be ejaculated at orgasm.
Internal Male Reproductive Anatomy (Continued) • Seminal vesicles: The seminal vesicles are sac-like pouches that attach to the vas deferens near the base of the bladder. The seminal vesicles produce semen, a fluid that activates and protects the sperm after it has left the penis during ejaculation. • Prostate gland: The prostate gland is a walnut-sized structure that is located below the urinary bladder in front of the rectum. The prostate gland contributes additional fluid to the ejaculate. Prostate fluids also help to nourish the sperm. The urethra, which carries the ejaculate to be expelled during orgasm, runs through the center of the prostate gland. • Bulbourethral glands: The bulbourethral glands, or Cowper’s glands, are pea-sized structures located on the sides of the urethra just below the prostate gland. These glands produce a clear, slippery fluid that empties directly into the urethra. This fluid serves to lubricate the urethra and to neutralize any acidity that may be present due to residual drops of urine in the urethra.
Kinds of Physiological Reactions • Defined as increased muscle tension • During this kind of response, muscles can contract rather quickly or can slow down • Is what usually happens with an orgasm Myotonia Vascongestion •Happens when tissue in the body is filled with blood •Blood flows very fast through the arteries of that tissue
Phases of the Sexual Response Cycle • 1) Excitement • The first part is really mental. Once this occurs, the brain send signals that make blood pressure and the heart rate go up, which increases blood flow to the genitals, called engorgement. Vaginal lubrication happens. Here, the inner part of the vagina expands, the labia lips open and get bigger, then the clitoris swells and pulls back against the pubic bone. For males, the penis becomes erected. Here, it is enlarged and stiffened with blood rushing within it (vasocongestion).Tight muscles at the base of the penis hold the blood in.
Phases of the Sexual Response Cycle (Continued) • 2) Plateau • Muscle tensions and the heart rate increase. The outer part of the vagina swells up and gets tighter and the inner part expands. When this occurs, the uterus is in a certain position where it can get the sperm that has passed through the vagina, assuming there is sperm. It tends to form a dent-like shape, that sperm can easily fall into.
Phases of the Sexual Response Cycle (Continued) • 3) Orgasm • The ultimate goal of sexual intercourse, also called coitus, is the orgasm. Muscle tensions reach their highest point and are released, usually in a series of involuntary muscle contractions from the genitals. For males, first the glands and ducts contract (emission), forcing semen to go into the urethra, where sperm comes out through ejaculation
Phases of the Sexual Response Cycle (Continued) • 4) Resolution • The end of the cycle where everything slows down and goes back to normal within 5 minutes. Muscles relax, making the genitals return to their usual shape, size, and color. Major sex organs such as the penis and clitoris can take longer than 5 minutes to return to their normal being.
Hormonal Control of the Male Reproductive System • Testosterone and other androgens • Are directly responsible for the primary and secondary sex characteristics of the male • Androgen secretion and sperm production • Are both controlled by hypothalamic and pituitary hormones
Stimuli from other areas in the brain Hypothalamus GnRH from the hypothalamus reg- ulates FSH and LH release from the anterior pituitary. Anterior pituitary Negative feedback FSH acts on the Sertoli cells of the seminiferous tubules, promoting spermatogenesis. LH stimulates the Leydig cells to make testosterone, which in turn stimulates sperm production. Leydig cells make testosterone Primary and secondary sex characteristics Sertoli cells Spermatogenesis Testis Figure 46.14 Figure 8 & 9: Hormonal Control of the Male Reproductive System
Important Male Sex Hormones Key hormones are androgens, such as testosterone. Testosterone is a steroid that is made in the testes by Leydig cells. • It makes up many male characteristics such as the making of sperm and the genitals, hair growth and deeper voices • It controls sexual behavior/sex drive • In addition to testosterone, hormones and glands such as the anterior pituitary and hypothalamus control the amount of androgen being released and sperm production in the testes
Reproductive Cycles of the Female • The Ovarian Cycle • The hormone GnRH (gonadotropin releasing hormone) is released, which gets the pituitary gland to release FSH (follicle-stimulating hormone) and LH (luteinizing hormone) • FSH works with LH to stimulate follicle growth to make estrogen. As estrogen is secreted, it goes through the follicular phase. Out of the many follicles being made, one follicle survives and grows. FSH and LH levels are low due to low estrogen levels • Estrogen levels increase as the follicle rises, also increasing LH and FSH levels and GnRH levels that were secreted, stimulated by the hypothalamus. • The follicle responds more to LH because it has receptors for it. Increasing LH levels are caused by increasing estrogen secretions from the follicle (positive feedback)
The Ovarian Cycle (Continued) • The follicle continues to grow, forming barriers around itself. It then moves towards an ovary and eventually attaches itself to it, causing ovary rupture and releases a secondary oocyte (a haploid cell resulting from meiosis). From the time this cell moves from the ovary to its discharge unless fertilized is ovulation • After ovulation, the luteal phase occurs. Here, LH stimulates the changing of the rest of the follicular tissue that was left behind after the secondary oocyte was released. It forms the corpus luteum. Once this is stimulated, it secretes estrogen and progesterone, leading to the secretion of LH ad FSH. If the corpus luteum disintegrates, levels of estrogen and progesterone decrease. This alerts the pituitary gland of what just happened, allowing it to make new follicles for the next time, and the cycle starts all over again.
Figure 11: Estriol (a form of estrogen) Important Hormones Related To the Female Reproductive System • Cyclic secretion of GnRH from the hypothalamus • And of FSH and LH from the anterior pituitary orchestrates the • female reproductive cycle • Five kinds of hormones • Participate in an elaborate scheme involving both positive and negative feedback
The Uterine/Menstrual Cycle • After ovulation, estrogen and progesterone given off by the corpus luteum still continue to stimulate the endometrium, which is the inner lining of the uterus that has many blood vessels. Glands within it release nutrients that are for an embryo, even if there is no embryo to nurture. As time goes on, the secretory phase is initiated. Here, blood lining becomes thicker. • The corpus luteum disintegrates and arteries in the uterine lining are not getting enough blood. When this happens, about 2/3 of the endometrium disintegrates as well, resulting in menstruation. Menstruation can last an average of 5 days and occurs an average of every 28 days, where the cycle starts all over again.
Menopause • Human females can have an average of 450 menstruation cycles in their lifetime • Menopause is the stage of a female’s life where she stops producing hormones such as LH and FSH in the ovaries. This means that without the proper hormones, a female can no longer have children. This happens to women between the ages of 46 and 54, and sometimes earlier or later
Pregnancy • Humans are placental (eutherian) mammals. • Gestation or pregnancy takes place in humans • It is the condition of carrying one or more embryos in the uterus. • Human pregnancy averages from 266 days (38 weeks) from fertilization of the egg, or 40 weeks from the start of the last menstrual cycle. • A pregnancy includes a first, second, and third trimester. • Human gestation can be divided for convenience into three trimester of about three months each.
Fertilization • Once the sperm meets the egg, fertilization occurs and the egg begins to divide. • Immediately after fertilization, the gender is determined. • The fertilized egg begins to divide repeatedly and the zygote grows in size. • The inner cells will become the embryo while the outer group of cells will become the placenta • The mother is beginning to produce progesterone and estrogen, the pregnancy hormones that will be responsible for a host of bodily changes (and symptoms like morning sickness) to come. • It makes its way down the fallopian tube to the uterus where it will implant itself to the uterine wall. • This generally takes a week before the zygote enters the uterus and 72 hours before it is implanted in the uterine wall when it does reach the uterus.
Figure 13: Formation of the zygote and early postfertilization events
First Trimester • Is the main period of organogenesis • Development of body organs • During the first trimester: • Fertilized egg implants in uterine lining • Placenta and umbilical cord develop • Spinal cord and embryo’s three layers that will eventually become organs form • Heart begins to beat and blood is pumped • At the end of the third month, fetus is completely formed • At week 11 Baby is now officially described as a fetus. • The fifth week of pregnancy, or the third week after conception, marks the beginning of the embryonic period. This is when the baby's brain, spinal cord, heart and other organs begin to form.
Second Trimester • During this trimester, the fetus grows to about 30 cm and is very active. • The mother may feel movements during the early part of the second trimester. • The uterus grows enough for pregnancy to be obvious. • Fetal activity may be visible through the abdominal wall by the middle of this time period. • There are some changes with hormones during this time • Hormone levels stabilize as HCG declines. • The corpus luteum deteriorates. • The placenta completely takes over the production of progesterone which maintains the pregnancy
Third Trimester • During this trimester the growth of the fetus is about 3-4 kg and 50 cm. • Fetal activity may decrease because there is less room for movement. • As the fetus grows and the uterus expands around it the mother’s abdominnal organs become compressed and displaced, leading to frequent urination, digestive blockages, and strain in the back muscles. • A complex interplay of local regulators (prostaglandins) and hormones (mostly estrogen and oxytocin) induces and regulates labor. • Labor- the process by which childbirth occurs. • Estrogen- is thought to induce oxutocin receptors on the uterus, it reaches its highest level in the mother’s blood during the last weeks of pregnancy. • Oxytocin- it is produced by the fetus and the mother’s posterior pituitary. • It stimulates the placenta to secrete prostaglandins, which enhance powerful contractions. • Physical and emotional stresses associated with the contractions stimulate the release of more oxytocin and prostaglandins, a positive feedback system, that underlies the process of labor.
Estrogen Oxytocin from fetus and mother's posterior pituitary from ovaries Positive feedback Induces oxytocin receptors on uterus Stimulates uterus to contract Stimulates placenta to make Prostaglandins Stimulate more contractions of uterus Figure 46.18 Figure 14: Induction of Labor
Parturition & Lactation • Parturition- or birth is brought about by a series of strong, rhythmic uterine contractions. • The process of labor has three stages: • 1. The opening up and thinning of the cervix, ending with complete dilation. • 2. The expulsion, or delivery of the baby. • Continuous strong contractions force the fetus down and out of the uterus and vagina. • 3. The delivery of the placenta, which normally follows the baby. • Lactation- release of milk from the mammary glands that is controlled by oxytocin, • This is unique to mammals. • After birth decreasing levels of progesterone free the anterior pituitary from negative feedback and allow prolactin secretion. • Prolactin stimulates milk production after a delay of 2 or 3 days.
Placenta Umbilical cord Uterus Cervix Dilation of the cervix 1 Expulsion: delivery of the infant 2 Uterus Placenta (detaching) Umbilical cord Delivery of the placenta 3 Figure 15: The Three Stages of Labor
