Animal reproduction and fertilization

Question 1

Sexual reproduction

Answer 1

The fusion of haploid gametes forms a diploid cell, the zygote. Asexual and sexual reproduction are both common in nature.

Question 2

Zygote

Answer 2

The fusion of haploid gametes forms a diploid cell, the zygote.

Question 3

Egg

Answer 3

The female gamete is the egg, it is large and nonmotile.

Question 4

Sperm

Answer 4

The male gamete, is generally smaller and motile.

Question 5

Asexual reproduction

Answer 5

In asexual reproduction, new individuals are generated without the fusion of egg and sperm. For most animals, it relies entirely on mitosis. Asexual and sexual reproduction are both common in nature.

Question 6

Parthenogenesis

Answer 6

A particularly intriguing form of asexual reproduction is parthenogenesis, in which an egg develops without being fertilized. This is observed in Comodo dragons and Hammerhead sharks. It also occurs in certain species of wasps, bees and ants. The offspring can be either haploid or diploid. If haploid, they develop into adults that produce eggs or sperm without meiosis. Parthenogenesis is considered a rare response to low population density.

Question 7

Hermaphroditism

Answer 7

The evolutionary solution for animals with little opportunity to find a mate of the opposite sex, is hermaphroditism, where each individual has both male and female reproductive systems. Because of this, any two individuals can mate. Snails are an example. Each animal donates and receives sperm during mating.

Question 8

Gonads

Answer 8

Gonads, organs that produce gametes, are found in many but not all animals. Exceptions include the palolo worm. It and most other polychaete worms have separate sexes, but lack distinct gonads, and the eggs and sperms develop from undifferentiated cells lining the coelom (body cavity). As the gametes mature, they are released from the body wall and fill the coelom. Depending on which kind, they can be excreted with the waste, or split the cavity and spill directly into the environment.

Question 9

Cloaca

Answer 9

In many nonmammalian vertebrates, the digestive, excretory and reproductive systems have a common opening to the outside, the cloaca, a structure probably present in the ancestors of all vertebrates. Lacking a well developed penis, males of this species release sperm by turning the cloaca inside out. Chickens are an example. Mammals generally have a separate opening for the digestive tract.

Question 10

Testes

Answer 10

The male gonads, or testes, produce sperm in highly coiled tubes called seminiferous tubules. Most animals produce sperm properly only when the testes are cooler than the rest of the body.

Question 11

Seminiferous tubules

Answer 11

The male gonads, or testes, produce sperm in highly coiled tubes called seminiferous tubules. Most animals produce sperm properly only when the testes are cooler than the rest of the body.

Question 12

Scrotum

Answer 12

In humans and many other mammals, the scrotum, a fold in the body wall, maintains testis temperature about 2 degrees below the core body temperature. The testes develop in the abdominal cavity and descend into the scrotum just before birth (a testis within a scrotum is a testicle).

Question 13

Epididymis

Answer 13

From the seminiferous tubules of a testis, the sperm pass into the coiled duct of an epididymis, where they complete maturation and become motile.

Question 14

Ejaculation

Answer 14

During ejaculation, the sperm are propelled from each epididymis through a muscular duct, the vas deferens.

Question 15

Vas deferens

Answer 15

During ejaculation, the sperm are propelled from each epididymis through a muscular duct, the vas deferens. A vas deferens from each epididymis extends around and behind the urinary bladder, where it joins a duct from the seminal vesicle, forming a short ejaculatory duct.

Question 16

Ejaculatory duct

Answer 16

A vas deferens from each epididymis extends around and behind the urinary bladder, where it joins a duct from the seminal vesicle, forming a short ejaculatory duct. The ejaculatory duct opens into the urethra, the outlet tube for both the excretory system and the reproductive system.

Question 17

Urethra

Answer 17

The ejaculatory duct opens into the urethra, the outlet tube for both the excretory system and the reproductive system. The urethra runs through the penis and opens into the outside at the tip of the penis.

Question 18

Semen

Answer 18

3 sets of accessory glands, the seminal vesicle, the prostate gland and the bulbourethral glands, produce secretions that combine with the sperm to form semen, the fluid that is ejaculated.

Question 19

Seminal vesicles

Answer 19

Two seminal vesicles contribute to about 60% of the volume of semen. The fluid from the seminal vesicles is thick, yellowish and alkaline. It contains mucus, the sugar fructose (which provides most of the sperms energy), a coagulating enzyme, ascorbic acid, and a local regulators called prostaglandins.

Question 20

Prostate gland

Answer 20

The prostate gland secretes its products into the urethra through small ducts. The fluid is thin and milky; it contains anticoagulant enzymes and citrate (a sperms nutrient).

Question 21

Bulbourethral glands

Answer 21

They are a pair of small glands along the urethra below the prostate. Before ejaculation, they secrete clear mucus that neutralizes any acidic urine remaining in the urethra. Bulbourethral fluid also carries some sperm released before ejaculation, which is one reason for the high failure rate of the with-drawal method as birth control, (coitus interruptus).

Question 22

Penis

Answer 22

The penis contains the urethra as well as 3 cylinders of spongy erectile tissue. During sexual arousal, the erectile tissue, which is derived from modified veins and capillaries, fills with blood from the arteries. As the tissue fills, the increasing pressure seals off the veins, causing it to engorge with blood. Alcohol consumption, certain drugs, emotional issues and aging all can cause the inability to achieve an erection (erectile dysfunction).

Question 23

Glans

Answer 23

The main shaft of the penis is covered by relatively thick skin. The head, or glans, of the penis has much thinner covering and is consequently more sensitive to stimulation. The human glans is covered by a fold of skin called prepuce, or the foreskin, which is removed if a male is circumcised.

Question 24

Ovaries

Answer 24

The female gonads are the ovaries that flank the uterus and are held in place in the abdominal cavity by ligaments. The outer layer of each ovary is packed with follicles, each consisting of an oocyte.

Question 25

Follicles

Answer 25

The outer layer of each ovary is packed with follicles, each consisting of an oocyte. The oocyte is a partially developed egg, surrounded by support cells. The surrounding cells nourish and protect the oocyte during much of its formation and development.

Question 26

Oocyte

Answer 26

The oocyte is a partially developed egg, surrounded by support cells. The surrounding cells nourish and protect the oocyte during much of its formation and development.

Question 27

Oviduct

Answer 27

An oviduct, or fallopian tube, extends from the uterus toward a funnel-like opening at each ovary. The dimensions of this tube vary along its length, with the inside diameter near the uterus being as narrow as human hair.

Question 28

Ovulation

Answer 28

Upon ovulation, the release of a mature egg, cilia on the epithelial lining of the oviduct help collect the egg by drawing fluid from the body cavity into the oviduct. Together with wave-like contractions of the oviduct, the cilia convey the egg down the duct to the uterus, also known as the womb.

Question 29

Uterus

Answer 29

Together with wave-like contractions of the oviduct, the cilia convey the egg down the duct to the uterus, also known as the womb. The uterus is a thick, muscular organ that can expand during pregnancy to accommodate a 4kg fetus. The inner lining of the uterus, the endometrium, is richly supplied with blood vessels.

Question 30

Endometrium

Answer 30

The inner lining of the uterus, the endometrium, is richly supplied with blood vessels.

Question 31

Cervix

Answer 31

The neck of the uterus, called the cervix, opens into the vagina.

Question 32

Vagina

Answer 32

The vagina is a muscular but elastic chamber that is the site for insertion of the penis and deposition of sperm during copulation. The vagina which also serves as the birth canal through which a baby is born, opens to the outside at the vulva.

Question 33

Vulva

Answer 33

The vagina which also serves as the birth canal through which a baby is born, opens to the outside at the vulva, the collective term for the external female genitalia.

Question 34

Labia majora

Answer 34

The labia majora, a pair of thick fatty ridges, enclose and protect the rest of the vulva. The vaginal opening and the separate opening of the urethra are located within a cavity bordered by a pair of slender skin folds, the labia minora.

Question 35

Labia minora

Answer 35

The vaginal opening and the separate opening of the urethra are located within a cavity bordered by a pair of slender skin folds, the labia minora.

Question 36

Hymen

Answer 36

A thin piece of tissue called the hymen partly covers the vaginal opening in humans at birth, but becomes thinner over time and typically wears away through physical activity.

Question 37

Clitoris

Answer 37

Located at the top of the labia minora, the clitoris consists of erectile tissue supporting a rounded glans, or head, covered by a small hood of skin, the prepuce. During sexual arousal, the clitoris, vagina, and labia minora all engorge with blood an enlarge. Richly supplied with nerve endings, the clitoris is one of the most sensitive points of sexual stimulation. Sexual arousal also induces the vestibular glands near the vaginal opening to secrete mucus, thereby facilitating intercourse.

Question 38

Mammary glands

Answer 38

The mammary glands are present in both sexes, but normally only produce milk in females. Though not part of the reproductive system, the female mammary glands are important for reproduction. Within the glands, small sacs of epithelial tissue secrete milk, which drains into a series of ducts that open at the nipple. The breasts contain connective and fatty (adipose) tissue in addition to the mammary glands.

Question 39

Gametogenesis

Answer 39

It is the production of gametes. There is a close relationship between the male and female gonads, their structure and their function. They are compared in figure 36.9.

Question 40

Spermatogenesis

Answer 40

The formation and development of sperm, is continuous and prolific in adult males. To produce hundreds of millions of sperm each day, cell division and maturation occur throughout the seminiferous tubules coiled within the two testes. For a single sperm, the process takes about 7 weeks from start to finish.

Stem cells that give rise to sperm are situated near the outer edge of the seminiferous tubules. Their progeny move inward as they pass through the spermatocyte and spermatid stages, and sperm are released into the lumen (fluid filled cavity) of the tubule. The sperm travelled along the tubule into the epididymis, where they become motile.

Question 41

Spermatogonia

Answer 41

The stem cells arise from division and differentiation of primordial germ cells in the embryonic testes. In mature testes, they divide mitotically to form spermatogonia, which in turn generate spermatocytes by mitosis. Each spermatocyte gives rise to four spermatids through meiosis, reducing the chromosome number from diploid (2n = 46 in humans) to haploid (n = 23). Spermatids undergo extensive changes in differentiating into sperm.

Question 42

Acrosome

Answer 42

The structure of a sperm cell fits its function. In humans, as in most species, a head containing the haploid nucleus is tipped with a special vesicle, the acrosome, which contains enzymes that help the sperm penetrate an egg. Behind the head, many mitochondria (for one large mitochondrion in some species) provide ATP for movement of the flagellar tail.

Question 43

Oogenesis

Answer 43

The development of mature oocytes (eggs), is a prolonged process in the human female. Immature eggs form in the ovary of the female embryo but do not complete their development until years, and often decades later.

It begins in the female embryo with the production of oogonia from primordial germ cells. At birth, the ovaries together contain 1-2 million primary oocytes, of which 500 fully mature between puberty and menopause. In other animal species, the sperm may enter the oocyte at the same stage, earlier or later. Each of the two meiotic divisions involves unequal cytokinesis, with the smaller cells becoming polar bodies that eventually degenerate, (the first polar body may or may not divide again). As a result, the functional product of complete oogenesis is a single mature egg containing a sperm head.

Question 44

Oogonia

Answer 44

It begins in the female embryo with the production of oogonia from primordial germ cells. The oogonia divide by mitosis to form cells that begin meiosis, but stop the process at prophase I before birth. These developmentally arrested cells, which are primary oocytes, each reside within a small follicle, a cavity lined with protective cells. At birth, the ovaries together contain 1-2 million primary oocytes, of which 500 fully mature between puberty and menopause.

Question 45

Primary oocytes

Answer 45

The oogonia divide by mitosis to form cells that begin meiosis, but stop the process at prophase I before birth. These developmentally arrested cells, which are primary oocytes, each reside within a small follicle, a cavity lined with protective cells.

Question 46

Secondary oocytes

Answer 46

Beginning at puberty, follicle stimulating hormone FSH, periodically stimulates a small number of follicles to resume growth and development. Typically only one follicle fully matures each month, with its primary oocyte completing meiosis I. The second meiotic division begins, but stops at metaphase. Thus arrested in meiosis II, the secondary oocyte is released at ovulation, when the follicle breaks open. Only if a sperm penetrates the oocyte, does meiosis II resume.

Question 47

Corpus luteum

Answer 47

The ruptured follicle left behind after ovulation develops into the corpus luteum. The corpus luteum secretes estradiol as well as progesterone, a hormone that helps maintain the uterine lining during pregnancy. If the egg is not fertilized, the corpus luteum degenerates, and a new follicle matures during the next cycle.

Question 48

Follicle-stimulating hormone

Answer 48

Mammalian reproduction is governed by the coordinated actions of hormones from the hypothalamus, anterior pituitary, and gonads. Endocrine control of reproduction begins with the hypothalamus, which secretes gonadotropin-releasing hormone. This directs the anterior pituitary to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

Question 49

Luteinizing hormone

Answer 49

Mammalian reproduction is governed by the coordinated actions of hormones from the hypothalamus, anterior pituitary, and gonads. Endocrine control of reproduction begins with the hypothalamus, which secretes gonadotropin-releasing hormone. This directs the anterior pituitary to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

Question 50

Tropic hormones

Answer 50

Both FHS and LH are tropic hormones, which means that they act on endocrine tissues to trigger the release of other hormones. They are called gonadotropins because the endocrine tissues they act on are in their gonads. There, FSH and LH control sex hormone production.

Question 51

Testosterone

Answer 51

The gonads produce and secrete three major types of steroid sex hormones: androgens, principally testosterone; estrogens, principally estradiol; and progesterone. All three hormones are found in both males and females, but at very different concentrations. Testosterone levels in the blood are about 10 times higher in males than females. Although the gonads are the major source of sex hormones, the adrenal gland secretes sex hormones in small amounts. Two negative-feedback mechanisms control sex hormone production in males. Testosterone regulates blood levels of GnRH, FSH and LH through inhibitory effects on the hypothalamus and anterior pituitary.

Question 52

Estradiol

Answer 52

The gonads produce and secrete three major types of steroid sex hormones: androgens, principally testosterone; estrogens, principally estradiol; and progesterone. All three hormones are found in both males and females, but at very different concentrations. Estradiol levels are about 10 times higher in females than in males. Although the gonads are the major source of sex hormones, the adrenal gland secretes sex hormones in small amounts.

Question 53

Progesterone

Answer 53

The gonads produce and secrete three major types of steroid sex hormones: androgens, principally testosterone; estrogens, principally estradiol; and progesterone. All three hormones are found in both males and females, but at very different concentrations. Peak progesterone levels are much higher in females. Although the gonads are the major source of sex hormones, the adrenal gland secretes sex hormones in small amounts.

Question 54

Sertoli cells

Answer 54

In directing spermatogenesis, FSH and LH act on two types of cells in the testes. Sertoli cells, located within the seminiferous tubules, respond to FSH by nourishing developing sperm cells. Two negative-feedback mechanisms control sex hormone production in males. Inhibin, a hormone that in males is produced by Sertoli cells, acts on the anterior pituitary gland to reduce FSH secretion.

Question 55

Leydig cells

Answer 55

In directing spermatogenesis, FSH and LH act on two types of cells in the testes. Leydig cells, scattered in connective tissue between tubules, respond to LH by producing testosterone and other androgens, which promote spermatogenesis in the tubules.

Question 56

Ovarian cycle

Answer 56

Cyclic events in the ovaries define the ovarian cycle. Once per cycle a follicle matures and an oocyte is released. Changes in the uterus define the menstrual cycle, also called the uterine cycle.

Question 57

Menstrual cycle

Answer 57

Cyclic events in the ovaries define the ovarian cycle. Once per cycle a follicle matures and an oocyte is released. Changes in the uterus define the menstrual cycle, also called the uterine cycle. During each cycle, the endometrium (lining of the uterus) thickens and develops a rich blood supply. By linking the ovarian and uterine cycles, hormone activity synchronizes ovulation with the establishment of a uterine lining that can support embryo implantation and development.

Question 58

Uterine cycle

Answer 58

Cyclic events in the ovaries define the ovarian cycle. Once per cycle a follicle matures and an oocyte is released. Changes in the uterus define the menstrual cycle, also called the uterine cycle.

Question 59

Menstruation

Answer 59

If an oocyte is not fertilized and pregnancy does not occur, the uterine lining is sloughed off, and another pair of ovarian and uterine cycles begins. The cyclic shedding of the blood-rich endometrium from the uterus, a process that occurs in a flow through the cervix and vagina, is called menstruation. Menstrual cycles average 28 days, but can range from about 20 days to 40 days.

Question 60

Follicular phase

Answer 60

Follicle-stimulating hormone (as its name implies) stimulates follicle growth aided by LH and the cells of growing follicles start to make estradiol. There is a slow rise in estradiol secreted during most of the follicular phase, the part of the ovarian cycle during which follicles grow and oocytes mature. Several follicles begin to mature each month, however, usually only one matures and the other disintegrates. The low levels of estradiol inhibit secretion of the pituitary hormones, keeping the levels of FSH and LH relatively low.

Question 61

Luteal phase

Answer 61

The luteal phase of the ovarian cycle follows ovulation. LH stimulates the follicular tissue left behind in the ovary to transform into corpus luteum, a glandular structure. Under continued stimulation by LH, the corpus luteum secretes progesterone and estradiol, which in combination exert negative feedback on the hypothalamus and pituitary. This feedback reduces the secretion of FSH and LH to very low levels, preventing another egg from maturing when a pregnancy may already be underway. Near the end of the luteal phase, low gonadotropin levels cause the corpus luteum to disintegrate, triggering a sharp decline in estradiol and progesterone concentration. The decrease of ovarian steroid hormones liberate the hypothalamus and pituitary from the negative-feedback effect of these hormones. The pituitary can then begin to secrete enough FSH to stimulate the growth of new follicles in the ovary, initiating the next ovarian cycle.

Question 62

Proliferative phase

Answer 62

Prior to ovulation, ovarian steroid hormones stimulate the uterus to prepare for support of an embryo. Estradiol secreted in increasing amounts by growing follicles signals the endometrium to thicken. In this way, the follicular phase of the ovarian cycle is coordinated with the proliferate phase of the uterine cycle.

Question 63

Secretory phase

Answer 63

After ovulation the estradiol and progesterone secreted by the corpus luteum stimulate maintenance of the uterine lining, as well as further development, including enlargement of arteries and growth of endometrial glands. These glands secrete a nutrient fluid that can sustain an early embryo even before it implants in the uterine lining. Thus, the luteal phase of the ovarian cycle is coordinated with what is called the secretory phase of the uterine lining.

Question 64

Menstrual flow phase

Answer 64

Once the corpus luteum has disintegrated the rapid drop in ovarian hormone levels causes arteries in the endometrium to constrict. Deprived of its circulation, the uterine lining largely disintegrates, and the uterus, in response to prostaglandin secretion, contracts. Small endometrial blood vessels constrict, releasing blood that is shed along with endometrial tissue and fluid. The result is menstruation, the menstrual flow phase of the uterine lining. During this phase, which usually lasts a few days, a new set of ovarian follicles begin to grow. By convention, the first day of flow is designated day 1 of the new uterine (and ovarian) cycle.

Question 65

Menopause

Answer 65

After about 500 cycles, a woman undergoes menopause, the cessation of ovulation and menstruation. Menopause usually occurs between the ages of 46 and 54. During this interval, the ovaries lose their responsiveness to FSH and LH, resulting in a decline in estradiol production. A hypothesis for why women go through menopause is that after having several children and undergoing menopause, it allows the mother to care better for her children and grandchildren, increasing the survival of individuals who share much of her genetic makeup.

Question 66

Estrus cycle

Answer 66

In all female mammals, the endometrium thickens before ovulation, but only humans and some other primates have menstrual cycles. Other animals have estrous cycles, in which in the absence of a pregnancy, the uterus reabsorbs the endometrium and no excessive fluid flow occurs. The period called the estrus, is the only time the female is receptive to mating. The estrus cycle varies from one long per year in bears and wolves, to elephants having multiple that spans 14-16 weeks. Rats have multiple throughout the year each lasting 5 days. The household cat ovulates only upon mating.