Session 2: Human Reproduction

Question 1

What are the six main parts of the male reproductive system and what are their functions?

Answer 1

Testis: responsible for the production of sperm and testostrerone, epididymis: site where sperm matures and develops the ability to be motile - mature sperm is stored here until ejaculation, vas deferens: long tube which conducts sperm from the testes to the prostate gland during ejaculation, seminal vesicle: secretes fluid containing fructose (to nourish sperm) mucus (to protect sperm) and prostaglandin (triggers uterine contractions), prostate gland: secretes an alkaline fluid to neutralise vaginal acids, urethra: conducts sperm from the prostate gland to the outside of the body via the penis (also used to convey urine).

Question 2

List the male reproductive hormones.

Answer 2

Gonadotropin Releasing Hormone (GnRH), Luteinising Hormone (LH), Follicle Stimulating Hormone (FSH), Testosterone.

Question 3

Describe GnRH

Answer 3

Gonadotropin Releasing Hormone is produced by the hypothalamus and stimulates the anterior pituitary gland to release gonadotropins that act on the glands (testes or ovaries)

Question 4

Describe LH

Answer 4

Luteinising Hormone –> Gonadotropin stimulates the Leydig Cells in the testes to produce testosterone, or has a surge in release that causes ovulations resulting in the formation of the corpus luteum.

Question 5

Describe FSH

Answer 5

Follicle stimulating hormone – gonadotropin stimulates the Sertoli cells in the testes to facilitate sperm production, or stimulates estrogen secretion from developing follicles

Question 6

Describe testosterone

Answer 6

Male reproductive hormones - stimulates sperm production (and development of secondary sexual characteristics at puberty)

Question 7

Describe spermatogenesis

Answer 7

Spermatogenesis (sperm production) occurs in the seminiferous tubules of the testes. The basement membrane for each tubule is lined by germline epithelium which at puberty divide by mitosis to make spermatogonia (which divide by meiosis to make spermatids). The spermatids differentiate into functional spermatozoa, which are then released into the lumen of the tubule. These developing spermatozoa are nourished by Sertoli cells, which reside in the tubule lining. Outside of the tubules are blood capillaries and interstitial cells (Leydig cells) which produce testosterone.

Question 8

Describe sperm structure

Answer 8

Sperm’s sole purpose is to swim to the egg and deliver their genetic material. They are composed of three main regions: the head (contains the nucleus and acrosome), the mid-piece (has many mitochondria to provide the energy for swimming), and the tail (a long flagellum that propels the sperm in its swim to the egg). The acrosome: enzyme filled bag at the tip of the sperm

Question 9

Overall, what does the female reproductive system do?

Answer 9

The female reproductive system includes all the organs responsible for the production of an oocyte (the female gamete). It also includes the organs involved in initially developing and maintaining an embryo during the early stages of pregnancy.

Question 10

What are the six main parts of the female reproductive system (and what do they do)?

Answer 10

Ovary (Where oocytes mature prior to release (ovulation) – it also responsible for estrogen and progesterone secretion (female sex hormones), fimbria (a fringe of tissue adjacent to an ovary that sweep an oocyte into the fallopian tube/oviduct), fallopian tube/oviduct (Transports the oocyte to the uterus – it is also typically where fertilisation occurs), uterus (organ where a fertilised egg will implant and develop (becoming an embryo), endometrium (The mucous membrane lining of the uterus, it thickens in preparation for implantation or is otherwise lost (via menstruation), vagina (passage leading to the uterus by which the penis can enter (uterus protected by a muscular opening called the cervix)

Question 11

What is oogenesis? How does it occur?

Answer 11

Oogenesis refers to egg production. It occurs in the ovaries and begins during foetal development, when a large number of primordial cells are formed by mitosis. These cells (oogonia) undergo cell growth until they are large enough to undergo meiosis (becoming primary oocytes). The primary oocytes begin meiosis but are arrested in prophase I when granulosa cells surround them to form follicles. The primary oocytes remain arrested in prophase I until puberty, when menstruation begins. Each month, hormones (FSH) will trigger the continued division of some of the primary oocytes. These cells will complete the first meiotic division to form two cells of unequal size. One cell retains the entirety of the cytoplasm to form a secondary oocyte, while the other cell forms a polar body. The secondary oocyte begins the second meiotic division but is arrested in metaphase II. The secondary oocyte is released from the ovary (ovulation) and enters into the oviduct (or fallopian tube). The follicular cells surrounding the oocyte form a corona radiata and function to nourish the secondary oocyte. The follicular tissue in the ovary after ovulation forms the corpus luteum. If the oocyte is fertilised by a sperm, chemical changes will trigger the completion of meiosis II and the formation of another polar body (the first polar body may also undergo a second division to form a third polar body). Once meiosis II is complete the mature egg forms a ovum, before fusing its nucleus with the sperm nucleus to form a zygote

Question 12

What are the female reproductive hormones?

Answer 12

Gonadotropin Releasing Hormone (GnRH), Luteinising Hormone (LH), Follicle Stimulating Hormone (FSH), Estrogen, Progesterone.

Question 13

Describe estrogen.

Answer 13

Produced by follicles - thickens endometrium, inhibits FSH and LH for most of the cycle, stimulates FSH and LH release pre-ovulation

Question 14

Describe progesterone

Answer 14

Produced by corpus luteum - maintains the endometrium, inhibits FSH and LH

Question 15

What are the ovarian and menstrual cycles?

Answer 15

Cyclic changes of the endometrium and the ovary. Regulated by cyclic production of estrogens and progesterone - a combination of negative and positive feedback regulates the two cycles. FSH and LH regulate the production of estrogens and progesterone. Both cycles are approximately 28 days in length. Ovulation typically occurs midway through cycle (approximately day 14). The cycles are generally divided into three main components:
The follicular phase
Ovulation
The luteal phase

Question 16

Describe LH and FSH levels in ovarian and menstrual cycles.

Answer 16

FSH levels are initially high in the follicular phase to develop follicles before being inhibited by estrogen production. There is a spike in both LH and FSH in ovulation but LH has a much larger spike in levels which triggers ovulation (release of an ova). In the luteal phase rising levels of estrogen and progesterone inhibit both FSH and LH and hence both levels decrease. Finally, in menstruation when the corpus luteum degenerates, estrogen and progesterone levels fall and FSH levels rise.

Question 17

Describe estrogen and progesterone levels in ovarian and menstrual cycles.

Answer 17

In follicular phase: estrogen levels rise as estrogen is produced by developing follicles (progesterone not produced). Ovulation: estrogen levels drop upon ovulation (follicle is ruptured, cannot produce estrogen). Luteal phase: progesterone and estrogen levels rise with the development of the corpus luteum. Menstruation: when the corpus luteum degenerates, estrogen and progesterone levels fall.

Question 18

Hormones and the ovarian cycle info:

Answer 18

Follicular phase: FSH causes follicle to mature and produce estrogen (inhibiting development of other follicles). Ovulation: an LH surge causes ovulation (follicle ruptures and releases a secondary oocyte). Luteal phase: ruptured follicle forms a corpus luteum and secretes progesterone (and some estrogen). Menstruation: when corpus luteum degenerates (forms a corpus albicans) a new ovarian cycle can begin

Question 19

Hormones & the menstrual cycle:

Answer 19

Follicular phase: follicles release estrogen which causes the uterine wall (endometrium) to thicken. Ovulation: when the follicle ruptures and develops into a corpus luteum, progesterone is produced. Luteal phase: progesterone and estrogen are released from the corpus luteum and thicken the endometrium. Menstruation: the endometrium is sloughed away when the corpus luteum degenerates (a new cycle begins)

Question 20

What is fertilisation?

Answer 20

Fertilisation occurs when a single sperm penetrates an egg cell. The sperm and egg unite to form a zygote. Fertilisation has several stages (capacitation, acrosome reaction, fusion of sperm, cortical reaction, fusion of nuclei). After fertilization, the zygote begins its development into a multicellular organism.

Question 21

What is capacitation

Answer 21

The surface of the sperm undergoes functional changes while in the female reproductive tract that allow it to fertilize the egg cell. Its cholesterol coat is dissolved which improves motility, and the acrosome cap is destablised.

Question 22

What is the acrosome reaction?

Answer 22

Enzymes from the acrosome are released and digest a pathway through the follicle cells and zona pellucida surrounding the secondary oocyte. The action of many sperm are required for just one to enter the egg cell.

Question 23

What is the fusion of membranes?

Answer 23

The membranes of the sperm and egg fuse, and the nucleus of the sperm enters the egg cytoplasm which causes a sudden membrane depolarization that acts as a fast block to further sperm entry. This also triggers the completion of meiosis II in the egg cell and induces the next stage of the fertilization sequence.

Question 24

What is the cortical reaction?

Answer 24

Cortical granules in the egg cytoplasm release their contents into the space between the plasma membrane and the vitelline layer. Substances released from the granules raise and harden the vitelline layer to form a fast (permanent) block to further sperm entry.

Question 25

What is the fusion of nuclei?

Answer 25

The membranes of both gametes disappear so that the chromosomes can fuse together, returning to a diploid state. The diploid fertilised egg is now referred to as a zygote.

Question 26

What is cleavage and morula?

Answer 26

Rapid cell division takes place Immediately after fertilization (cleavage), increasing the number of cells, but not the size of the zygote. The first cleavage is completed after 36 hours, and each succeeding division takes less time. After three days, successive cleavages have produced a solid mass of cells called a morula, which is still about the same size as the original zygote.

Question 27

What are blastocyst and implantation?

Answer 27

After several days in the uterus, the morula develops into the blastocyst. It makes contact with the uterine lining and pushes deeply into it, ensuring a close maternal-fetal contact. The embryo produces HCG (human chorionic gonadotropin), which prevents degeneration of the corpus luteum and signals that the woman is pregnant.

Question 28

Describe sexual development.

Answer 28

The reproductive system develops from common tissue found in the developing embryo. The paired mesonephric ducts (Wolffian ducts) and paramesonephric ducts (Müllerian ducts) contribute the majority of male and female internal genital tract respectively. Development of this system commences in the embryo, continues through the foetal period then with key changes around birth, only completes functional development postnatally at puberty. The mesonephric/paramesonephric duct changes are one of the first male/female differences that occur in development, while external genitalia remain indeterminate in appearance for quite a while. The male and female gametes produce different hormones to promote further development of sex characteristics:
The testes produce testosterone to promote the further development of male sex characteristics. The ovaries will produce estrogen and progesterone to promote the development of female sex characteristics

Question 29

Describe the Y chromosome & sexual development.

Answer 29

The Y chromosome includes a gene called the SRY gene (Sex Determining Region Y), which leads to male development. The SRY gene codes for a testis-determining factor (TDF) that causes embryonic gonads to form into testes (male gonads). In the absence of the TDF protein (i.e. no Y chromosome), the embryonic gonads will develop into ovaries (female gonads)

Question 30

What is the placenta?

Answer 30

The placenta is the specialized organ that enables the exchange of nutrients from the mother to the fetus and wastes from the fetus to the mother. The placenta also has an endocrine role, producing hormones that enable the pregnancy to be maintained. It develops when finger-like projections of the fetal chorion (the chorionic villi) grow into the endometrium of the uterus containing the numerous capillaries connecting the fetal arteries and vein. The close proximity of the fetal and maternal blood vessels allows for the exchange of oxygen, nutrients, and wastes between the maternal and fetal blood.

Question 31

How does progesterone change in pregnancy?

Answer 31

During the first 12 weeks of pregnancy, the corpus luteum secretes enough progesterone to maintain the uterine lining and sustain the developing embryo. After this, the placenta takes over as the primary endocrine organ of pregnancy. It also prepares the mammary glands for lactation and inhibits labour.

Question 32

How do estrogen levels change in pregnancy?

Answer 32

Estrogen levels increase to maintain the uterine lining and prepare the mammary glands for lactation. It also helps to induce labour at the end of pregnancy.

Question 33

Describe: labour and oxytocin

Answer 33

At the end of pregnancy, the placenta loses competency as a result of falling progesterone levels. High estrogen levels then trigger the onset of labour. A estrogen peak coincides with an increase in oxytocin, which stimulates uterine contractions. Labour progresses as a result of positive feedback. The increasing pressure on the cervix stimulates the release of more oxytocin, which increases the force of the contractions. After birth, the secretion of prolactin increases. Prolactin maintains lactation during the period of infant nursing. Prolactin secretion is inhibited by high progesterone during pregnancy.