Reproductive System

Question 1

How does the reproductive system differ from other body systems?

Answer 1

The rest of the body’s systems work to ensure the survival of the individual animal they are part of, whereas the reproductive system works to ensure the survival of the species of animal. It interacts with other body systems, but purely reproductive structures are not essential to the life of an animal. Another difference from other body systems is that the reproductive system requires a second animal (of the opposite sex) to fully carry out its function, which is to produce a brand new animal (offspring).

Question 2

Why is the reproductive system of an individual animal considered only half a system?

Answer 2

It is considered only half a system because it requires a second animal of the opposite sex to fully carry out its function.

Question 3

What is the difference between the diploid chromosome number and the haploid chromosome number? In which body cells is each found?

Answer 3

The total number of chromosomes in the nucleus of each of the body’s cells (except for the reproductive cells) is the same for all members of a species. This number is called the diploid chromosome number, which is always an even number because the chromosomes occur in pairs. The diploid chromosome number is sometimes expressed generically with the abbreviation 2n. The n is a mathematical expression that represents a number, and the 2 indicates that the number is doubled (diploid). The number of chromosomes in the reproductive cells is half the number in all other cells in the body. This is called haploid chromosome number. It has to be reduced to half the diploid number so that when the spermatozoon and the ovum come together, the total number of chromosomes in the fertilized ovum gets back to the diploid number.

Question 4

Which reproductive cell, the ovum or the spermatozoon, determines the genetic sex of the offspring when fertilization occurs?

Answer 4

Spermatozoon.

Question 5

How does cell division by meiosis differ from cell division by mitosis?

Answer 5

Most of the body’s cells divide through a process called mitosis. When a cell divides by mitosis, each of its chromosomes first produces a duplicate copy of itself. When the two daughter cells pull apart, half the chromosomes go to one cell and half go to the other. Each of them ends up with an identical full diploid set of chromosomes. The genetic makeup of the two daughter cells is exactly the same as each other and as the parent cell. This ensures that the genetic information in all of the body’s cells (except for the reproductive cells) stays exactly the same. Reproductive cells are produced by meiosis, in which the chromosomes do not produce duplicate copies of themselves before the daughter cells pull apart. Half of the total chromosomes (one from each diploid chromosome pair), including one sex chromosome, go to each daughter cell. Which chromosomes go to which daughter cell is entirely random. The chromosomes merely pull apart into their new cells. In this way the genetic material of the reproductive cells gets shuffled, resulting in genetically unique offspring.

Question 6

How does spermatogenesis differ from oogenesis? Why are the basic processes so different?

Answer 6

Spermatozoa are produced continuously and in very large numbers in an effort to ensure that one spermatozoon will successfully reach and fertilize the ovum when breeding occurs, so the process of spermatogenesis is designed to produce huge numbers of spermatozoa. Alternatively, ova are not produced continuously like spermatozoa. At or soon after birth, a female has a fixed number of primary oocytes (the precursor cell to ova) formed in her ovaries. That will be the total number available in her lifetime. They remain in a quiet immature state until the ovarian cycles begin. Each ovarian cycle produces one or more mature ova, depending on the species. Because spermatozoa come to the ovum to fertilize it, large numbers of ova are not needed. Therefore the process of oogenesis is designed to produce small numbers of ova at a time.

Question 7

What are the two main functions of the testes? Where in the organ does each take place?

Answer 7

Spermatogenesis takes place in seminiferous tubules, and hormone production occurs in interstitial cells between the seminiferous tubules.

Question 8

What are the three main parts of a spermatozoon? What is the main purpose or function of each?

Answer 8

The three main parts of a spermatozoon are an enlarged head, a midpiece, and a long, narrow tail. The head contains the nucleus of the cell and is covered by a caplike structure called the acrosome. The acrosome contains digestive enzymes that are released once the spermatozoon is inside the female reproductive tract, helping the spermatozoon reach the ovum and penetrate through the layers surrounding it to accomplish fertilization. The midpiece is the power plant of the cell. It contains a large conncentration of energy-producing mitochondria arranged in a spiral pattern. The long, thin tail resembles the flagellum that many singlej-celled organisms use for propulsion. It contains musclelike contractile fibrils that produce a whiplike movement of the tail and propel the cell forward once it is activated.

Question 9

Why is a bilaterally cryptorchid animal usually sterile?

Answer 9

A bilaterally cryptorchid animal is usually sterile beause spermatogenesis requires a temperature slightly lower than body temperature. The interior of the abdomen is too hot for spertozoa to be produced. Testosterone continues to be produced, however. So a bilaterally cryptorchid animal has all the characteristics of a male animal but cannot reproduce.

Question 10

Would a bilaterally cryptorchid aniumal exhibit normal male behavior? Why or why not?

Answer 10

Yes, because testosterone, which is responsible for the male libido, continues to be produced. Therefore a bilaterally cryptorchid animal has all the characteristics of a male animal but cannot reproduce.

Question 11

What is important about the scrotum’s ability to adjust the position of the testes relative to the body?

Answer 11

The testes have to be kept slightly cooler than body temperature to produce spermatozoa. A bandlike muscle, the cremaster muscle, passes down through the inguinal ring and attaches to the scrotum. It can adjust the position of the testes relative to the body. In warm conditions, the cremaster muscle relaxes, and the testes hang down away from the warm body. This position helps to reduce their temperature. In cold conditions, however, the cremaster muscle pulls the testes up tight against the body wall, which helps to warm them.

Question 12

What are the main components of the spermatic cord?

Answer 12

Spermatic cords are tubelike connective tissue structures that contain blood vessels, nerves, lymphatic vessels, and the vas deferens. Surrounding the artery is a structure called the pampiniform plexus, which is an intricate meshwork of tiny veins derived from the testicular veins.

Question 13

From what are the visceral and parietal vaginal tunics that cover the testes derived?

Answer 13

They are derived from layers of peritoneum that were pushed ahead of the testes as they descended through the inguinal rings. The very thin inner layer, the visceral vaginal tunic (a.k.a. the proper vaginal tunic) is derived from the visceral layer of peritoneum that coated the testes as they developed in the abdomen. The thick outer layer, the parietal vaginal tunic (a.k.a. the common vaginal tunic) is derived from the parietal layer of peritoneum that lines the abdominal cavity. It forms a fibrous sac around each testis and spermatic cord.

Question 14

Where are spermatozoa stored before ejaculation?

Answer 14

The epididymis.

Question 15

Will the volume or gross appearance of the semen of an animal that has had a vasectomy be significantly different from that of a normal animal? Why or why not?

Answer 15

Because sperm typically account for less than 1% of seminal fluid, their absence after a vasectomy is negligible in terms of both the amount and gross appearance of the seminal fluid.

Question 16

How does the function of the bulbourethral glands differ from that of the other accessory reproductive glands?

Answer 16

They secrete a mucinous (mucus-containing) fluid just before ejaculation that clears and lubricates the urethra for the passage of semen. The fluid from bulbourethral glands is secreted in advance of the ejaculate and does not contribute greatly to its volume, composition, or function.

Question 17

What are the three main parts of the penis?

Answer 17

Roots, body, and glans.

Question 18

How does the mechanism of erection in animals with a sigmoid flexure in their penis differ from that of other animals?

Answer 18

Erection occurs when more blood enters the penis via the arteries than leaves it via the veins. The connective-tissue-enclosed erectile tissue becomes engorged with blood causing the penis to become enlarged and rigid. Mechanically what happens is that the arteries supplying blood to the penis dilate, increasing the blood flow into the organ. At the same time, the veins carrying blood away from the penis are compressed against the brim of the pelvis by contractions of the ischiocavernosus muscles (part of the roots of the penis). This acts like a tourniquet and decreases the flow of blood out of the penis. The net effect is that more blood enters the penis than leaves it. This generates hydraulic pressure in the erectile tissue, producing the enlargement and stiffening of the penis that we call erection. Animals that have a sigmoid flexure have a higher proportion of connective tissue to erectile tissue than other species, and so the penis does not enlarge much when erection occurs. Rather, the main mechanism of erection in these species is straightening of the sigmoid flexure from internal hydraulic pressure. When blood tries to enter the penis, it causes the penis to protrude from the prepuce for breeding. A long thin cordlike muscle, the retractor penis muscle, originates up near the base of the tail and attaches to the bend of the sigmoid flexure. It functions like a small bungee cord. When erection straightens out the sigmoid flexure, the retractor penis muscle stretches. When the erection subsides, the retractor penis muscle pulls the penis back into its non-erect S-shape.

Question 19

What structures do spermatozoa pass through from their site of production to their eventual deposition in the female reproductive tract?

Answer 19

Spermatozoa pass through the seminiferous tubules, tube lumen, duct system of the rete testis, epididymis, vas deferens (including the ampulla in some species), and urethra.

Question 20

What two main types of hormones are produce in the ovary? Where is each produced?

Answer 20

The hormones produced in the ovaries are estrogens and progestins. Estrogens are produced by the cells of the developi9ng ovarian follicles. Progestins, principally progesterone, are produced by the corpus luteum that develops from the empty follicle after ovulation.

Question 21

What changes does an ovarian follicle undergo as it develops from a primordial follicle to a mature follicle?

Answer 21

The beginning stage of follicle development in the ovary is the primordial (primary) follicle. After FSH (follicle stimulating hormone) is released from the anterior pituitary, the follicle undergoes follicular recruitment or follicular activation, and is now called a growing follicle. The follicular cells become thickened into cuboidal shapes and begin to multiply. Multiple layers of follicular cells form around the developing oocyte. At this stage the follicular cells are called granulosa cells. As the granulosa cells multiply, the follicle starts to grow rapidly in size. The granulosa cells do more than just physically surround the developing oocyte. They also produce estrogen hormones that begin preparing the animal for breeding and pregnancy. The larger the follicle or follicles become, the greater the amount of estrogens produced. As the follicle continues to grow, fluid-filled spaces begin to form between the granulosa cells. The spaces gradually become confluent (join together), forming one large fluid-filled space called the antrum. When the follicle has reached its maximum size, it looks like a large, blisterlike structure on the surface of the ovary. At this stage it is called a mature follicle. Alternative names for the mature follicle include “graafian follicle” and “vesicular ovarian follicle.” At this stage, estrogen production from the follicle or follicles is at maximum level, and the animal is ready for breeding to take place.

Question 22

After ovulation has occurred, what cells in the ovary multiply to form the corpus luteum?

Answer 22

Granulosa cells.

Question 23

When ovulation occurs, what causes the ovum to enter the oviduct?

Answer 23

When ovulation takes place, the ovum is “caught” by the fimbriae of the infundibulum. The infundibulum is the enlarged opening at the ovarian end of each oviduct. At the time of ovulatioin, it more or less surrounds the area of the ovary where follicles have formed. The fimbriae are muscular, fingerlike projections that form the margin of the infundibulum. They “feel” along the surface of the ovary and position the infundibulum where the follicle or follicles are located. This helps ensure that the infundibulum is properly positioned to catch the ovum or ova when ovulation occurs. If ova miss the opening of the oviduct, they fall into the abdominal cavity, where they usually just disintegrate after a time.

Question 24

Describe the functions of the uterus relating to pregnancy and parturition.

Answer 24

The uterus is the womb, where the fertilized ovum implants and lives while it grows and develops into a new animal. When fully developed, the uterus helps push the newborn out through the birth canal into the outside world. It forms part of the placenta, which is the life-support system that keeps the fetus alive while it develops during pregnancy. The uterus has to remain quiet during the pregnancy and contract powerfully at the time of birth. After it has delivered the newborn and the placenta (the afterbirth), it has to contract quickly to stop bleeding from the site where the placenta was attached to its lining. The uterus is vital to the success of reproduction. The fertilized ovum implants in the uterus and begins development. As the offspring develops, the placenta forms around it and attaches to the lining of the uterus so that nutrients, wastes, and respiratory gases can be exchanged between the fetal bloodstream and the maternal bloodstream. When the time comes for the offspring to be delivered, the muscular uterus provides most of the force necessary to open (dilate) the cervix so that the fetus can pass through it on its way to the outside world.

Question 25

Where is the urethral opening located in the female?

Answer 25

In females, the urethra opens on the floor (ventral portion) of the vestibule, which is part of the vulva (the part that is considered the entrance to the vagina).

Question 26

What is the difference in the estrous cycle intervals of polyestrous animals, seasonally polyestrous animals, diestrous animals, and monoestrous animals?

Answer 26

Polyestrous animals, such as cattle and swine, cycle continuously throughout the year if they are not pregnant. As soon as one cycle ends, another begins. Some polyestrous animals show seasonal variations in their estrous cycles. They cycle continuously at certain times of the year and not at all at others. These animals are called seasonally polyestrous animals. Diestrous animals have two cycles per year, usually in the spring and fall. Monoestrous animals usually have only one cycle each year.

Question 27

How do the stages of the estrous cycle relate to the events of the ovarian cycle?

Answer 27

The estrous cycle stages are proestrus, estrus, metestrus, and diestrus. Anestrus occurs in some animals between breeding seasons. Proestrus is the period of follicular development in the ovary. During this stage, follicles begin developing and growing. As they increase in size, the follicles’ output of estrogen increases accordingly, causing many physical changes that prepare the rest of the reproductive tract for ovulation and breeding. These changes include thickening and development of the linings of the oviduct, uterus, and vagina. The epithelial lining of the vagina also begins cornifying, that is, forming a layer of tough keratin on its surface to help protect against the physical trauma of breeding that is about to come.
Estrus is the “heat” period, or the period of sexual receptivity in the female. It occurs when the estrogen level from the mature follicles reaches its peak. This high estrogen level causes physical and behavioral changes that signal the female’s willingness to breed to the male.
Metestrus is the period after ovulation when the corpus luteum develops. The granulosa cells left in the now empty follicle begin to multiply under continued stimulation from LH (luteinizing hormone). They soon produce a solid structure, the corpus luteum (yellow body), which is about the same size as the former mature follicle. The hormone progesterone produced by the corpus luteum temporarily inhibits follicular development in the ovary, causes the lining of the uterus to get very thick in preparation for implantation of a fertilized ovum, and causes loss of the cornified epithelial lining that developed in the vagina during proestrus and estrus.
Diestrus is the active luteal stage when the corpus luteum has reached maximum size and exerts its maximum effect. If the animal is bred and becomes pregnant, the corpus luteum receives an endocrine signal from the developing embryo and is retained well into the pregnancy. If the animal is not pregnant, the corpus luteum degenerates at the end of diestrus. The animal either goes back into proestrus or the ovary shuts down and the animal goes into anestrus. Anestrus is a period of temporary ovarian inactivity seen in seasonally polyestrus, diestrous, and monoestrous animals. It is the period between breeding cycles when the ovary essentially shuts down temporarily.