BIOL #21: Reproductive & Excretory Systems

Question 1

Sexual vs Asexual Reproduction

Answer 1

Sexual reproduction is the fusion of gametes (haploid cells), such as egg and sperm.
- Due to genetic recombination that occurs during meiosis plus random fertilization of gametes, sexual reproduction results in offspring that are genetically different from each other and from their parents

Asexual reproduction is the generation of new individuals without fusion of egg and sperm.
- Since this type of reproduction is based on mitosis, the resulting offspring are genetically identical to one another and to their parent.

For the vast majority of animals, reproduction is primarily or exclusively sexual reproduction.

Question 2

Fission & Budding

Answer 2

Several forms of asexual reproduction are found only among invertebrates:

Fission: the separation of a parent organism into two individuals of approximately equal size.
e.g. sea anemones

Budding: new individuals arise from outgrowths of existing individuals.
e.g. hydra

Question 3

Fragmentation & Regeneration

Answer 3

Fragmentation & Regeneration: a two step process in which the body is broken into several pieces followed by regrowth of lost body parts. If more than one piece grows and develops into a complete animal, the net effect is reproduction.

Mechanism found in sponges, flatworms, cnidarians, annelids, echinoderms.

Question 4

Parthenogenesis

Answer 4

The following form of reproduction can occur in both invertebrates (e.g. bees, wasps, ants) and some vertebrates (e.g. some lizard and fish species) but is very rare.

Parthenogenesis: is development of an egg without fertilization.

Question 5

Advantages of Sexual Reproduction

Answer 5

In a hypothetical population of asexually and sexually reproducing individuals, asexual individuals are typically expected to out-reproduce sexual individuals because they will always produce daughters that can reproduce asexually themselves.

Why is sexual reproduction so common?
- The hypotheses with the best support are those related to the variation in offspring genotypes and phenotypes that results from genetic recombination, which may be important in the face of such factors as rapidly changing environments or evolving pathogens.
+ For instance, small freshwater crustaceans, called Daphnia, reproduce via parthogenesis when conditions are favorable and reproduce sexually when conditions are poor (e.g. temperature and food stresses on the reproducing adults), which may be advantageous because, with variable genotypes, some offspring may have a better chance of survival if poor conditions persist.

Question 6

Gametogenesis

Answer 6

The mitotic cell divisions, meiotic cell divisions, and developmental events that result in the production of male and female gametes, or sperm and eggs, are collectively called gametogenesis.

• Spermatogenesis is the formation of sperm.
• Oogenesis is the formation of eggs.

In the vast majority of animals, gametogenesis occurs in a sex organ, or gonad.

• Male gonads are called testes.
• Female gonads are called ovaries.

Question 7

Spermatogenesis in Mammals

Answer 7

In the male gonad, diploid cells called spermatogonia divide by mitosis to generate the cells that undergo meiosis.

In human males, the spermatogonia divide by mitosis to produce primary spermatocytes, which undergo meiosis I and produce two secondary spermatocytes, which then undergo meiosis II.

The result is four haploid cells called spermatids, each of which matures into a sperm.

This process occurs continuously throughout a male’s adult life.

Question 8

Oogenesis in Mammals

Answer 8

In the female gonad, diploid cells called oogonia divide by mitosis to form primary oocytes.

When these cells undergo meiosis, only one of the four haploid products, known as an ovum, matures into an egg.

The other cells do not mature into eggs; these cells are called polar bodies.

The production of primary oocytes stops early in development in many mammals; in humans, it stops before birth and continues up to decades later.

Question 9

Spermatogenesis vs Oogenesis in Mammals

Answer 9

There are three main differences between human spermatogenesis and oogenesis:

Only in spermatogenesis do all four products of meiosis develop into mature gametes; in oogenesis there is unequal division of cytoplasm during cytokinesis such that only a single haploid cell is capable of survival.

Spermatogenesis occurs throughout adolescence and adulthood while the number of gametes a female will produce is currently thought to be determined before birth.

Oogenesis has long interruptions between steps while spermatogenesis occurs in a continuous sequence.

Question 10

Fertilization and Egg Development

Answer 10

Fertilization is the joining of a sperm and an egg to form a diploid zygote.

In many animal species, individuals release their gametes into their environment, and external fertilization occurs. In other animals, males deposit sperm into the reproductive tracts of females and internal fertilization occurs.

Question 11

External Fertilization

Answer 11

Most animals that rely on external fertilization live in moist or aquatic environments and tend to produce large numbers of gametes.

Gamete release for external fertilization must be coordinated:

• Gametogenesis occurs in response to environmental cues such as lengthening days and warmer water temperatures, which indicate a favorable season for breeding.
• Gametes are released in response to specific cues from individuals of the same species, typically involving some form of courtship behavior.

Research indicates that chemical messengers called pheromones might be involved in synchronized gamete release.

Question 12

Internal Fertilization

Answer 12

Internal fertilization is an adaptation that enables sperm to reach an egg efficiently, even when the external environment is dry (e.g. terrestrial habitats).

This type of fertilization typically requires cooperative behavior that leads to copulation.
- Courtship behavior may allow for mate choice

It is likely that pheromones also play an important role in coordinating mating for some animals that have internal fertilization since they can function as mate attractants.

Question 13

Egglaying vs Birth

Answer 13

Once either external or internal fertilization has occurred, the embryo is either laid as an egg outside the mother’s body or retained inside her body (with few exceptions):

Parental care by either the mother or father (or both) after birth or hatching is more common than previously recognized.

In oviparous animals, the egg is laid outside the mother’s body and the embryo develops in the external environment.
- Birds, reptiles, many fish, montremes, most insects

In viviparous species, the egg remains within the mother’s body. There the embryo develops and receives nutrition directly from the mother (placenta of mammals and some fish) or via oophagy or intra-uterine cannibalism (some sharks).

In ovoviviparity, offspring develop inside the mother’s body but are nourished by nutrient-rich yolk stored in the egg.
- Stomach brooding frogs, male seahorses.

Question 14

Anatomy of Human Male Reproductive Organs

Answer 14

The male gonads (testes) produce sperm in coiled tubes called seminiferous tubules.

• Leydig cells of the testes produce testosterone and other androgens.
• Sertoli cells of the testes provide nutrients for developing sperm cells.

The scrotum is the external sac that houses the testes because sperm development is optimal at temperatures slightly cooler than internal body temperature.

From the seminiferous tubules of the testes, the sperm pass into a coiled duct, called the epididymis, where sperm mature and become motile.

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

The vas deferens meets an ejaculatory duct behind the urinary bladder which opens to the urethra, a tube that serves both excretory and reproductive functions.

From the urethra, sperm exit the male reproductive tract via an external organ, called the penis.

Question 15

Semen

Answer 15

Semen, the ejaculatory fluid, is composed of sperm and secretions from three sets of accessory glands: the seminal vesicles, the prostate gland, and the bulbourethral gland. Content and functions are listed below:
Semen, the ejaculatory fluid, is composed of sperm and secretions from three sets of accessory glands: the seminal vesicles, the prostate gland, and the bulbourethral gland. Content and functions are listed below:

Semen, the ejaculatory fluid, is composed of sperm and secretions from three sets of accessory glands: the seminal vesicles, the prostate gland, and the bulbourethral gland. Content and functions are listed below:

Seminal Vesicles - secretes fructose which is a source of chemical energy for sperm movement. Also prostaglandins, which stiminulates smooth-muscle contractions in uterus.

Prostate Gland - secretes antibiotic compound, which prevents urinary tract infections in males. Also citric acid, with is nutrient used by sperm.

Bulbourethral Gland - secretes alkaline mucus, which lubricates tip of penis; neutralizes acids in urethra.

Question 16

The Human Female Reproductive System

Answer 16

The female gonads are a pair of ovaries
- The outer layer of each ovary is packed with follicles, each consisting of an oocyte, which is a partially developed egg.

An oviduct, or fallopian tube, extends from the uterus towards each ovary – during ovulation, an egg is released into the abdominal cavity near the funnel-like opening of the oviduct and cilia on the opening of the duct sweep the egg into the tube.

The egg moves down the oviduct to the uterus, a thick muscular organ that can accommodate a 8 lbs fetus.

The uterine lining, called the endometrium, is richly supplied with blood vessels.

The neck of the uterus is called the cervix and this structure opens up to the vagina – the muscular canal that receives the penis during copulation and serves as the birth canal.

Outside of the vagina is the vulva, which is the collective term used for the external female genitalia.

The labia majora are a pair of thick, fatty ridges that enclose and protect the rest of the vulva.

The labia minora are slender skin folds that protect both the vaginal opening and the separate urethral opening

At the top of the labia minora is the clitoris, which consists of erectile tissue, much like the penis.

The vestibular glands near the vaginal opening secrete lubricating mucus during copulation.

Question 17

Mammalian Reproduction: Sex Hormones

Answer 17

In both male and female humans, the coordinated actions of hormones from the hypothalamus, anterior pituitary, and gonads control reproduction and sexual maturation.

The hypothalamus secretes gonadotropin-releasing hormone (GnRH), which then directs the anterior pituitary to secrete the gonadotropins – follicle stimulating hormone (FSH) and lutenizing hormone (LH).

FSH and LH regulate gametogenesis directly by targeting tissues in the gonads, and indirectly by regulating sex hormone production.

The male sex hormone is testosterone, a type of androgen.
- Produced by the Leydig cells in the testes.

The main female sex hormones are estradiol and progesterone, which belongs to a class of hormones called estrogens.
- Estradiol is produced in the ovaries by cells that surround the developing egg (follicle), the corpus luteum produces both estradiol and progesterone.

Human sex hormones play a key role in:

1) Development of the reproductive tract in embryos.
- Androgens promote the appearance of external and internal reproductive structures in males.
2) Maturation of the reproductive tract and secondary sex characteristics during the transition from childhood to adulthood.
- Androgens in males and estradiol in females promote the formation of secondary sex characteristics during puberty that are not directly related to the reproductive tract (e.g. voice changes, breast development).
3) Regulation of spermatogenesis and oogenesis in adults.

Question 18

What Regulates the Gonadal Hormones?

Answer 18

Puberty is initiated when gonadotropin-releasing hormone (GnRH) is released from the hypothalamus.

GnRH triggers release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland.

LH and FSH then trigger increases in testosterone and estradiol.

Question 19

What Regulates the Regulatory Hormones?

Answer 19

There is some evidence that nutritional state is involved in triggering GnRH increases at the appropriate age:

Puberty onset in the United States has dropped from age 17 during the eighteenth and nineteenth centuries, when the general nutritional state of the population was poorer, to age 12 today.

Among girls living today, individuals with large fat stores tend to enter puberty earlier than do girls who are thin.

Question 20

The Human Female Reproductive Cycles

Answer 20

While mature human males carry out gametogenesis continuously, mature human females produce gametes in cycles.

The menstrual cycle is a monthly reproductive cycle that occurs in the uterus. It averages 28 days in humans.

In conjunction with changes in the ovary, the uterine lining (endometrium) undergoes a dramatic thickening in preparation for possible implantation of an embryo.

If pregnancy does not occur, the uterine lining is sloughed off and another cycle beings. The expulsion of the uterine lining through the vagina is the process of menstruation.

The menstrual cycle defines changes in the uterus, while the cyclic events that occur in the ovaries during this time are referred to as the ovarian cycle.

Question 21

The Ovarian Cycle: Hormonal Control

Answer 21

The ovarian cycle has two main phases:

Follicular phase: the follicle matures and ovulation occurs (lasts an average of 14 days)

Luteal phase: a structure called a corpus luteum forms from the ruptured follicle and subsequently degenerates if an embryo does not implant in the uterine lining (also lasts an average of 14 days).

Question 22

The Ovarian Cycle: Follicular Phase

Answer 22

The cycle begins when GnRH is released from the hypothalamus and stimulates the anterior pituitary to secrete small amounts of FSH and LH.
- FSH, with the aid of LH, stimulates follicle growth in the ovary and the cells of the growing follicles begin to synthesize estradiol.

The synthesis of estradiol by the growing follicles result in a slow rise of estradiol.
- Estradiol exerts negative feedback on LH and FSH at low levels: levels of FSH and LH remain relatively low during this time as well because low levels of estradiol have an inhibiting effect on secretion of pituitary hormones.

Estradiol secretion by the growing follicle begins to rise steeply approaching day 14

• This causes a spike in FSH and LH levels because estradiol exerts positive feedback on FHS and LH at high levels by increasing GnRH levels.
• This results in final maturation of the follicle, in which the follicle forms a fluid-filled bulge near the surface of the ovary.

In response to the peak in LH levels, the mature follicle and the adjacent ovary walls rupture, releasing the secondary oocyte.

• This process is called ovulation.
• A distinct abdominal pain is sometimes felt during this time in either the left or right side – corresponding to whichever ovary has a mature follicle during the cycle.

Question 23

The Ovarian Cycle: Luteal Phase

Answer 23

LH stimulates the follicular tissue left behind after ovulation to transform into a glandular structure called a corpus luteum.

• LH stimulates the corpus luteum to secrete estradiol and progesterone.
• As estradiol and progesterone levels rise, the combination exerts negative feedback on the hypothalamus and pituitary, reducing secretion of FSH and LH to low levels again.
• The negative feedback prevents another egg from maturing when a pregnancy may be beginning
• This hormone combination is mimicked by many types of birth control pills.

Near the end of the luteal phase, low gonadotropin (FSH, LH) levels cause the corpus luteum to disintegrate.
- This triggers a decline in estradiol and progesterone levels which decreases the negative feedback effect of these hormones on the hypothalamus and pituitary, allowing FSH and LH levels to begin to rise and stimulate the growth of new ovarian follicles in a new cycle.

Question 24

The Menstrual Cycle

Answer 24

The rise in estradiol before ovulation and the high levels of estradiol and progesterone after ovulation promote the thickening of the uterine walls.

The decline in estradiol and progesterone that occurs when the corpus luteum disintegrates causes the disintegration of the uterine lining.

The first day of menstruation is typically designated as day 1 for both the menstrual cycle and the ovarian cycle.

If pregnancy occurs there are mechanisms in place that override the disintegration of the uterine lining.

Question 25

Mammalian Female Reproductive Cycles

Answer 25

Menopause, the cessation of ovulation and menstruation, occurs when the ovaries lose their responsiveness to FSH and LH – resulting in a decline in estradiol production.

• Menopause typically occurs between the ages of 46 and 54.
• Menopause is an unusual phenomenon – in most other species, females retain their reproductive capacity throughout life.

Human females and several other primate species are the only animals that have menstrual cycles.

• Other mammals have estrous cycles – in the absence of pregnancy, the uterus reabsorbs rather than sheds the lining.
• Mammals with estrous cycles typically only copulate during the period surrounding ovulation, typically referred to as estrus or heat.
• The duration of both estrous and menstrual cycles are very variable but there typically are more pronounced seasonal effects on organisms with estrous cycles.

Question 26

Human Male Reproductive System: Hormonal Control

Answer 26

FSH and LH are secreted in response to GnRH for normal spermatogenesis, each acting on a distinct cell type in the testis:

• FSH promotes the activity of Sertoli cells, which are found within the seminiferous tubules and nourish the sperm.
• LH regulates testosterone and other androgen production by Leydig cells, which are located around the exterior of the seminiferous tubules – this promotes spermatogenesis in the tubules.

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.
• A hormone produced by the Sertoli cells, called inhibin, acts on the anterior pituitary to reduce FSH secretion.
• Both of these mechanisms maintain androgens at optimal levels.