4.1 - Sexual Reproduction in Humans

Question 1

What does the male reproductive system consist of?

Answer 1

• a pair of testes contained in an external sac, the scrotum
• the penis
• ducts connecting the testes with the penis
• accessory glands (eg a pair of seminal vesicles and the prostate gland, these glands secrete fluids that mix with the sperm to make semen)

Question 2

What do the secretions of the male accessory glands do?

Answer 2

• maintain sperm mobility
• provide nutrients for the sperm, including fructose, amino acids and zinc ions
• neutralise the acidity of any urine remaining in the urethra
• neutralise the acidity of the vaginal tract

Question 3

What does the female reproductive system consist of?

Answer 3

• there are 2 ovaries
• oocytes mature in follicles, which develop from cells in the germinal epithelium, around the periphery of the ovary
• mature follicles migrate to the surface of the ovary, from where a secondary oocyte is released at ovulation
• the ovaries alternate each month in releasing the oocyte
• cilia at the entrance of the oviductal funnel sweep the secondary oocyte into the oviduct or fallopian tube
• the ciliated epithelial cells lining the oviduct convey the secondary oocyte to the uterus

Question 4

What are the 3 walls of the uterus?

Answer 4

• the perimetrium is a thin layer around the outside
• the myometrium is the muscle layer
• the endometrium is the innermost layer, it is a mucous membrane, which is well supplied with blood. It is a layer that builds and sheds in a monthly cycle, unless an oocyte is fertilised, in which case, the embryo implants in the endometrium, establishing a pregnancy

Question 5

How does the uterus open into the vagina?

Answer 5

The uterus opens into the vagina through a narrow ring of connective tissue and muscle, the cervix. The walls of the vagina are muscular and open at the vulva.

Question 6

What is gametogenesis?

Answer 6

• the production of gametes in the sex organs
• spermatogenesis is the formation of sperm in the testes
• oogenesis is the formation of secondary oocytes in the ovary
• the cells of the germinal epithelium of both the testis and the ovary undergo a sequence of mitotic and meiotic divisions to form haploid gametes
  -it is important that the gametes are haploid so that at fertilisation, the diploid number is restored and the chromosome number does not double every generation

Question 7

Describe the stages of spermatogenesis.

Answer 7

1 - cells of the germinal epithelium are diploid, they divide by mitosis to make diploid spermatogonia and more germinal epithelium cells
- the spermatogonia divide many times by mitosis, making more spermatogonia, some enlarge to make diploid primary spermatocytes
- primary spermatocytes undergo meiosis 1, making secondary spermatocytes which are haploid
- secondary spermatocytes undergo meiosis II, making haploid spermatids
- spermatids mature into spermatozoa or sperm

Question 8

Describe the structure of sperm.

Answer 8

• the head contains a haploid nucleus, covered at the anterior end by a lysosome, called the acrosome, which contains enzymes used at fertilisation
• the middle piece is packed with mitochondria, which provides ATP for movement, they spiral around the microtubules which extends from the centrioles into the axial filament in the tail
• the tail, or the flagellum, makes lashing movements that move the sperm, although the sperm are not motive until they have been modified in the epidymis

Question 9

Describe the (very long) process of oogenesis.

Answer 9

• before birth, in the developing foetus, cells of the germinal epithelium of the ovary, which are diploid, divide by mitosis to make diploid oogonia, and more germinal epithelim cells
• the oogonia divide many time by mitosis and enlarge, making diploid primary oocytes and more oogonia
• the primary oocytes begin meisosis I but stop, a girl is born with milliions of primary oocytes at prophase I in her ovaries
• germinal epithelium cells divide to form diploid follicle cells, which surround the primary oocytes, making primary follicles
• from puberty onwards hormones stimulate the primary follicles to develop further. Just before ovulation, a primary oocyte completes meisosis II, making a secondary oocyte, which contains most of the cytoplasm. It extrudes from the end of the secondary oocyte, so it is called the first polar body. It usually disintergrates immediately, but occasionally divides first. The secondary oocyte and polar body are both haploid
• the primary follicle develops into a secondary follicle, which is called a Graaffian follicle when it is mature. It migrates to the surface of the ovary where it bursts and releases the secondary oocyte, in a process called ovulation. Each month, several primary follicles start to develop but normally, only one develops into a fully developed graafian follicle
• the secondary oocyte begins meiosis II but stops at metaphase II unless fertilisation takes place
• after fertilisation, meiosis II is completed, making an ovum containing most of the cytoplasm. The other product of meiosis is the second polar body
• after ovulation, the graafian follicle becomes the corpus luteum (yellow body). If fertilisation occurs, it produces hormones, but otherwise, it regresses

Question 10

What is the structure of a secondary oocyte?

Answer 10

• a glycoprotein layer called the zona pellucida surrounds the cell membrane of the secondary oocyte
• the chromosomes of the secondary oocyte are at metaphase II, they are at the equator, attached to microtubules that make the spindle apparatus
• the periphery of the cytoplasm contains cortical granules, which are secretory organelles that prevent the entry of more than one one sperm
• corona radiata cells surround the secondary oocyte and provide nutrients

Question 11

Describe the process of fertilisation using key words for each stage.

Answer 11

• sperm reaches the secondary oocyte
• acrosome reaction
• sperm head entry
• cortical reaction
• meiosis II
• zygote formation
• embryo developement

Question 12

What is ‘capacitation’?

Answer 12

Sperm can only fertilise a secondary oocyte after a process called capacitation
This is the removal of cholestral and glycoproteins from the cell membrane over the acrosome in the sperm head. Over several hours, the membrane becomes more fluid and permeable to calcium ions. Meanwhile, the tail motion changes to a whipping movement, increasing sperm motility

Question 13

Describe the ‘sperm reaching the oocyte’ stage of fertilisation.

Answer 13

• it takes around 5 minutes from being deposited for the sperm to respond to the oocytes chemoattractants and swim through the cervix through the uterus to the oviduct
• sperm usually remain viable for 2-5 days, but are at their most fertile 12-24 hours after their ejaculation
• if ovulation has recently taken place, there will be a secondary oocyte in the oviduct, although it only remains viable for about 24hrs unless fertilised
• despite millions being deposited, only about 200 reach the secondary oocyte in the oviduct

Question 14

What is the acrosome reaction in fertilsation?

Answer 14

The acrosome releases several enzymes, which digest the cells of the corona radiata.
Then, on contact with the zona pellucida, the acrosome membrane ruptures and releases more enzymes including protease, acrosin, which hydrolyses the zona pellucida around the secondary oocyte

Question 15

What is the ‘sperm head entry’ stage of fertilsation?

Answer 15

The cell membranes of the secondary oocyte and sperm fuse and the head sinks into the cytoplasm of the secondary oocyte. Following the entry of the sperm head, the secondary oocyte is called an ovum.

Question 16

What is the cortical reaction stage in fertilisation?

Answer 16

This is the reaction of the oocyte that produces the fertilisation membrane, preventing polyspermy via the entry of additional sperm.
- when the sperm reaches the secondary oocye, the oocytes smooth endoplasmic recticulum releases calcium ions into the cytoplasm
- they make the cortical granules fuse with the cell membrane and release their contents of enzymes, by exocytosis
- the zona pellucida is chemically modified and expands and hardens, making a fertlisation membrane, which is impossible for more sperm to try and penetrate

Question 17

What happens in the ‘meiosis II’ phase of fertlisation?

Answer 17

• entry of the sperm also stimulates the completion of the second meiotic division of the ovum nucleus
• it proceeds through anaphase II and telophase II, divides and expels the second polar body

Question 18

What is the developement of a ‘zygote’ phase of fertilisation?

Answer 18

• within about 24hrs, the first mitosis combines the genetic material of the parents to make diploid cells in the embryo
• the sperm chromosomes join the ovum chromosomes on the cells equator, the cell is now a zygote, as the chromosomes have combined

Question 19

What is the development of the ‘embryo’ stage in fertilisation?

Answer 19

The first mitotic division produces 2 cells, and until organs have developed at around 10 weeks, this collection of cells is called an embryo

Question 20

Describe the process of impantation.

Answer 20

• as the embryo moves down the oviduct, it divides many time by mitosis, in a sequence called cleavage
• a soild ball of 16 cells move in relation to each other, by 7 days the ball of cells becomes hollow and is called a blastocyst
• the cells outside the blastocyst are called trophoblasts, the divide to make an inner cell mass on one side
• the blastocyst moves from the oviduct into the uterus
• after ovulation, the endometrium thickens and gets an increased blood supply to prepare it to recieve an embryo
• there is an implantation window when the endometrium is receptive, between 6 and 10 days after ovulation
• after 9 days, protusions from the blastocyst called trophoblastic villi, penetrate the endometrium
• the villi increase the surface area for reabsorption of nutrients from the endometrium
• the embedding of the blastocyst into the endometrium is implantation and 80% of blastocysts implant within 8-10 days

Question 21

What is the placenta and how is it formed?

Answer 21

• it connects the embryo and then the foetus to the uterus wall. It is made of tissues derived from the embryo and the mother
• the trophoblast develops into the chorion, an outermembrane surrounding the embryo
• cells of the chorion move into the trophoblastic villi and form much larger chorionic villi
• they acquire blood capillaries which are connected to the imbilical arteries and vein
• these are the blood vessels that connect the embryo to the uterus wall through the imbilical cord
• projections from the endometrium between chorionic villi are the maternal tissues of the placenta

Question 22

What are the major roles of the placenta?

Answer 22

• as an endocrine organ, producing hormones to support the pregnancy
• exchange between the mother and foetus’ blood, including nutrients, waste products and other respiratory gases
• a physical barrier between the foetal and maternal circulation
• providing passive immunity to the foetus; maternal anitbodies across the placenta and attack pathogens but do not attack foetal cell, even though they carry the fathers antigens which are different from the mothers
• protection from the mothers immune system, but the placenta doesn’t always provide complete immunological protection

Question 23

How does the placenta act as a physical barrier between the foetal and maternal circulation?

Answer 23

• protects the fragile, foetal capillaries from damage by the higher blood pressure of the mother
• protects the developing foetus from changes in maternal blood pressure

Question 24

Why do placenta’s not always provide complete immunological protection?

Answer 24

• some spontaneous abortions are equivalent to the rejection of a transplanted organ
• Rhesus disease in a foetus is the destruction of its blood cells by antibodies made by a Rhesus negative mother against the blood cells of a Rhesus positive foetus. It is worse for each successive rhesus positive foetus
• in the 2nd trimester, some women develop pre-eclampsia, when they have very high blood pressure, one cause is an abnormal immune response from the placenta

Question 25

What are the 4 significant hormones in the menstrual cycle?

Answer 25

• follicle stimulating hormone (FSH)
• luteinising hormone (LH)
• oestrogen
• pregesterone

Question 26

What is the balance of mentrual hormones like at the start of a cycle?

Answer 26

• the start of the cycle, day 0, is the first day of a period. The concentrations of all the relevant hormones in the plasma are low
• gonadotrophic releasing hormone (GnRH) is secreted by the hypothalamus and stimulates the anterior pituitary gland to secrete FSH and LH

Question 27

What is FSH?

Answer 27

• stimulates the development of primary follicles in the ovary, only one matures
• it forms a fibrous outer layer, the theca, and secretes fluid into a cavity, the antrium
• a mature graafian follicle is about 10mm in diameter
• FSH simulates the thecal cells to produce oestrogen

Question 28

What is LH?

Answer 28

• reaches maximum concentration just before ovulation on about day 12
• the major role of LH is to induce ovulation, on day 14 its high concentration causes the graafian follicle at the surface of the ovary to release a secondary oocyte
• it has a postive feedback effect on LH

Question 29

How does the graafian follicle affect hormones?

Answer 29

The remains of the graafian follicle convert into the corpus luteum (yellow body) which secretes oestrogen and progesterone
- these inhibit further secretion of FSH and LH and so their concentration decrease

Question 30

What happens when oestrogen becomes more concentrated in the plasma?

Answer 30

• triggers the rebuilding of the endometrium, which was shed during menstruation
• inhibits FSH secretion by negative feedback, which brings the concentration down again
• stimulates LH production by positive feedback

Question 31

What role does the corpus luteum have on hormones?

Answer 31

• the corpus luteum secretes oestrogen and pregesterone
• the pregesterone maintains the newly built endometrium, so that if a secondary oocyte is fertilised, there will be suitable tissue in which the embryo can implant
• but if there is no fertilisation or implantation, the falling concnetrations of FSH and LH cause the corpus luteum to degenerate so oestrogen and progesterone production declines
• the endometrium is no longer being rebuilt by the oestrogen or maintained by progesterone, so it is shed

Question 32

What happens when oestrogen and progesterone levels are low?

Answer 32

• they no longer inhibit FSH production, so the menstrual cycle restarts
• this is the normal situation in modern humans
• a secondary oocyte may be fertilised but fail to divide, implant in the wrong place, or fail to impant
• the data suggests that those that are fertilised, 20-70% fail to establish pregnancy
• in most of these cases, it is because the blastocyst fails to implant

Question 33

What is the amnion?

Answer 33

The embryo develops and grows in the uterus, enclosed in the amnion, a membrane that is derived from the inner cell mass of the bastocyst
- initially the amnion is in contact with the embryo but in weeks 5-6, ammniotic fluid accumulates and increases in volume for 6-7 months
- the fluid is made from the mother initially, although from 4 months, the foetus contributes urine to it
- the fluid pushes the amnion out, eventually as for the chorion, the inner layer of the placenta

Question 34

What does amniotic fluid do?

Answer 34

• maintains the foetus temperature
• provides lubrication, in some cases the webbing of fingers and toes if too little amniotic fluid circulates between them
• contributes to lung developement
• allows movement so muscles and bones function before birth
• acts as a shock absorber, protecting the foetus from injury outside the uterus

Question 35

Describe the different stages in pregnancy.

Answer 35

• pregnancy is classically divided into 3 trimesters
• the first trimester includes conception, implantation and embryogenesis, all major organs are laid down
• 1st trimester has the highest risk of miscarrige (15%), after week 10 the baby is much less prone to miscarrige
• by the 3rd trimester, weeks 26-39, all major structures are complete, this is a period of growth
• fat is laid down, the foetuses mass increases 3 fold and the length doubles
• development is such that at 28 weeks , more than 90% of premature babies survive

Question 36

What does hCG do?

Answer 36

• about 6 days after fertilisation, the embryo, at the blastocyst stage, begins to secrete human chorionic gonadrtrophin
• following implantation, hCG is made in chorion, the inner layers of the placenta
• its a glycoprotein and maintains the corpus luteum in its secretion of progesterone for the first 16 weeks of pregnancy
• progesterone maintains the endometrium which is essential for embyronic development, as it contributes to the structure of the placenta

Question 37

What does increased oestrogen and progesterone inhibit during pregnancy?

Answer 37

• FSH, so no more follicles mature
• LH, so ovulation is not possible
• prolactin, so no milk is made

Question 38

What does oestrogen stimulate during pregnancy?

Answer 38

• the growth of the uterus to acccomodate the foetus
• oestrogen stimulates the growth of mammary glands and increases their blood supply

Question 39

What happens when progesterone levels decrease during pregnancy?

Answer 39

• oxytocin is secreted by the posterior pituitary gland, it causes contractions of the myometrium in the uterus wall, the contractions are mild initially but they stimulate the secretion of more oxytocin, by PF and so the oxytocin conc in the blood increases
• contractions therefore become stronger and more frequent, the myometrium contracts from top to bottom so that the foetus can be pushed out through the cervix
• prolactin is also secreted by the anterior pituitary gland and stimulates the glandular tissue in the mammary glands to synthesise milk
• milk is released when oxytocin causes the muscles around the milk to contract, prolactin secretion continues after the birth, as long as the milk is needed

Question 40

What does FSH do?

Answer 40

• anterior pitutiary
• stimulate developement of graafian follicle
• ## positive feedback on oestrogen in early cycle

Question 41

What does LH do?

Answer 41

• anterior pituitary
• stimulates ovulation
• positive feedback on FSH
• stimulate the conversion of graafian follcile into the corpus luteum

Question 42

What does oestrogen do?

Answer 42

• theca, corpus luteum
• negative feedback in FSH
• positive feedback on LH
  -secondary sexual characteristics
• inhibits prolactin synthesis

Question 43

What does progesterone do?

Answer 43

• corpus luteum, placenta
• maintains endometrium
• negative feedback on FSH
• negative feedback on LH
• inhibits oxytocin in pregnancy

Question 44

What does oxytocin do?

Answer 44

• hypothalamus, but stored in posterior pituitary
• contraction of smooth muscle in myometrium
• contraction of smooth muscle in milk ducts

Question 45

What does prolactin do?

Answer 45

• anterior pituitary
• milk synthesis

Question 46

What does Hcg do?

Answer 46

• blastocyst, placenta
• maintains corpus luteum