Sexual Reproduction (11.4)

Question 1

Describe the Soil and Seed theory.

Answer 1

The soil and seed theory proposed by Aristotle described how animals were thought to reproduce sexually. The male produced a seed which formed an egg when mixed with menstrual blood (the soil). The egg would then develop into a fetus inside the mother.

Question 2

Who debunked the Soil and Seed theory and how?

Answer 2

William Harvey debunked the soil and seed theory by studying the sexual organs of female deer after mating. He was unable to detect a growing embryo until 6-7 weeks after mating had occurred. So he concluded that Aristotle’s soil and seed theory was incorrect and menstrual blood did not contribute to the development of a fetus. However, Harvey was unable to identify the correct mechanism of sexual reproduction and incorrectly stated that the fetus did not develop by a mixture of male and female ‘seeds’.

Question 3

How was the modern theory of sexual reproduction developed?

Answer 3

Our current understanding of sexual reproduction is based on evidence discovered with the use of light microscopes (invented after William Harvey’s death).

Question 4

What is the purpose of the female reproductive system?

Answer 4

To produce eggs, receive the sperm, develop the embryo in the uterus and to give birth.

Question 5

Give the function of the following structures:
Ova
Ovary
Fallopian tube
Uterus
Cervix
Vagina

Answer 5

Ova: The female sex cell.
Ovary: Where oocytes mature prior to ovulation. Responsible for the secretion of oestrogen and progesterone.
Fallopian tube: The tube through which the ova travels to reach the uterus. Fertilisation occurs here.
Uterus: Where a fertilised egg will implant and develop into an embryo.
Cervix: The passage between the uterus and the vagina.
Vagina: The passage through which the semen enters the female reproductive system. The baby passes through here.

Question 6

What are the female and male sex hormones?

Answer 6

Female sex hormones:
Oestrogen, progesterone.

Male sex hormones:
Testosterone.

Question 7

What determines sex in humans?

Answer 7

Females possess two X chromosomes (XX) while males possess one X chromosome and one Y chromosome (XY).

The Y chromosome contains a gene called the SRY gene (Sex determining Region Y) which codes for TDF (Testis Determining Factor). The TDF causes the embryonic gonads to develop into testes.

In the absence of the TDF protein (i.e. no Y chromosome), the embryonic gonads will develop into ovaries.

Question 8

What is the role of oestrogen and progesterone in female sex development?

Answer 8

Oestrogen and progesterone are secreted by the ovaries. The pituitary gland produces Follicle Stimulating Hormones (FSH) which stimulate the ovaries to produce oestrogen. It also produces the Lutenising Hormone (LH) which stimulates the follicles to become mature and release their egg and then develop into the corpus luteum.

Oestrogen and progesterone promote the pre-natal development of the female reproductive organs (primary sex characteristic) and are responsible for the development of secondary sex characteristics (body hair and breast development). They are also involved in monthly preparation of egg release following puberty.

Question 9

What are the two key groups of hormones that control the menstrual cycle?

Answer 9

Pituitary hormones (FSH, LH)
- Act on the ovaries to develop follicles.
Ovarian hormones (oestrogen, progesterone)
- Act on the uterus to prepare for pregnancy.

Question 10

Describe the follicular phase of the menstrual cycle.

Answer 10

The pituitary gland releases the follicle stimulating hormone (FSH) which stimulates the production of follicles on the ovary. The ovarian follicles secrete oestrogen. When oestrogen peaks there is a surge in the lutenising hormone which intiates ovulation.

Question 11

Describe the ovulation phase of the menstrual cycle.

Answer 11

The ovary releases an egg. The egg travels down the fallopian tube where it waits to be fertilised by sperm.

Question 12

Describe the luteal phase of the menstrual cycle.

Answer 12

The dominant follicle that released the egg in ovulation changes into a structure called the corpus luteum. This produces oestrogen and progesterone which help to thicken the lining of the uterus.

If fertilisation does not occur the corpus luteum breaks down. Progesterone production decreases and the endometrial lining is shed.

Question 13

What is IVF?

Answer 13

Fertilisation that occurs outside of the body (in vitro). It occurs via the usage of drugs to stop the normal menstrual cycle and hormone treatments to harvest multiple eggs (super ovulation).

Question 14

Give a summary of the key stages of IVF.

Answer 14

SHE’S FIT

Stop normal menstrual cycle via drugs
Hormone treatments for superovulation
Extract multiple eggs from the ovaries
Sperm collected

Fertilisation occurs externally (in vitro)
Implantation of multiple embryos into the uterus
Test for pregnancy

Question 15

What are the primary and secondary sex characteristics of females?

Answer 15

Primary sex characteristics: The prenatal development of the vagina, uterus, fallopian tubes, clitoris and cervix and the ability to menstruate after maturity and give birth.

Secondary sex characteristics: Breast development, wider hips and fat deposition, body hair and external genitalia development.

Question 16

What are the primary and secondary sex characteristics of males?

Answer 16

Primary sex characteristics: The prenatal development of male genitalia, including a penis.

Secondary sex characteristics: An enlarged penis, growth of skeletal muscle, pubic hair, facial hair, sperm production, larynx enlargement.

Question 17

What is the purpose of the male reproductive system?

Answer 17

To produce sperm in the testes and to deliver the sperm into the female reproductive system.

Question 18

Give the function of the following structures:
Testes
Scrotum
Epididymis
Vas deferens
Prostate gland
Seminal vesicle
Urethra
Penis

Answer 18

Testes: Where sperm is produced.
Scrotum: Where the testes are located. Keeps the testes at a slighter lower temperature than the body.
Epididymis: Where sperm mature and develop motility.
Vas deferens: The tube through which the sperm cells travel from the testes to the penis.
Prostate gland: Secretes an alkaline liquid that neutralises vaginal acid (needed to maintain sperm viability).
Seminal vesicle: Secretes a fluid containing enzymes, proteins, sugar and vitamins.
Urethra: Tube inside the penis through which semen leaves the body. Also used for urine.
Penis: Semen is ejected from this into the vagina.

Question 19

Define gametogenesis.

Answer 19

The process by which diploid (2n) cells undergo meiotic division to become haploid (n) gametes (sex cells). In males this is called spermatogenesis and in females this is oogenesis.

Question 20

Explain the function of the following structures in the sperm:
- Head
- Mid-piece
- Tail

Answer 20

The head contains the acrosome, nucleus and centrioles. The acrosome cap contains enzymes which help the sperm to penetrate the jelly coat of the egg. The nucleus contains the paternal DNA. The centrioles are needed by a zygote in order to divide.

The mid-piece contains mitochondria which provides the energy (ATP) needed for the tail to move.

The tail (flagellum) is composed of a microtubule structure called the axoneme which bends to facilitate movement.

Question 21

Explain the function of the following structures in the egg:
- Corona radiata
- Zona pellucida
- Cortical granule
- Nucleus

Answer 21

The corona radiata is an external layer of follicular cells which provide support and nourishment to the egg cell.

The zona pellucida (jelly coat) is a glycoprotein matrix which acts as a barrier to sperm entry.

The cortical granules release their contents upon fertilisation to prevent polyspermy from occurring.

The nucleus contains the maternal DNA.

Question 22

Explain the process of oogenesis.

Answer 22

During foetal development a large number of primordial cells are formed by mitosis. These cells undergo a period of cell growth and become primary oocytes. The primary oocytes undergo meiosis but the process stops at prophase I. It resumes during puberty. Each month FSH triggers some of the primary oocytes to continue meiosis and form secondary oocytes and polar bodies. After meiosis I, the secondary oocyte undergoes meiosis II which stops at metaphase II until fertilisation occurs. When this happens, meiosis II continues and an ootid and a polar body are created. The polar bodies disintegrate and the ootid undergoes maturity, eventually forming an ovum.

Question 23

Explain the process of spermatogenesis.

Answer 23

During puberty, the germline epithelium of the seminiferous tubules of the testes divide by mitosis forming spermatogonia. The spermatogonia undergo a period of cell growth and form spermatocytes. The spermatocytes undergo meiosis and form four haploid daughter cells (spermatids). The spermatids differentiate and become functional sperm cells. They are released into the tubule and transported to the epididymis.

Question 24

What are the hormones involved in spermatogenesis and what is their role?

Answer 24

FSH: Produced in the pituitary gland. Stimulates primary spermatocytes to undergo the first division of meiosis and form secondary spermatocytes.
LH: Produced in the pituitary gland. Stimulates the secretion of testosterone by the testis.
Testosterone: Produced by the interstitial (Leydig) cells in the testis. Stimulates the development of secondary spermatocytes into mature sperm.

Question 25

How do sperm mature?

Answer 25

Sperm from the testis arrive in the epididymis and are unable to swim. There they undergo a maturing process. The fluid from the seminal vesicles contains nutrients which provide energy to the sperm so it can swim. The fluid from the prostate gland is alkaline and protects the sperm from the acidic conditions of the vagina (protects its viability).

Question 26

Compare oogenesis and spermatogenesis.

Answer 26

Oogenesis begins in foetal development and stops at menopause. Only one egg is produced every 28 days and it is released on day 14 of the menstrual cycle. It occurs in the ovaries.

Spermatogenesis begins in puberty and continues throughout the adult life. Four sperm are produced by meiosis and millions are produced daily. They are released during ejaculation. It occurs in the testes.

Question 27

Define external fertilisation.

Answer 27

The fusion of gametes (egg and sperm) outside the body of the parent. Common in aquatic animals as water is a good medium for the gametes to travel in.

Question 28

Define internal fertilisation.

Answer 28

The union of the gametes (egg and sperm) inside the body of a parent. Requires a method by which the gamete of one parent can be introduced to the other (eg. copulation).

Question 29

Describe the steps of human fertilisation.

Answer 29

1) Capacitation
Post ejaculation, chemicals released by the uterus dissolve the sperm’s cholesterol coat which improves the sperm’s motility. It also destabilises the acrosome cap which is necessary for the next reaction.

2) Acrosome reaction
When the sperm reaches the egg, the contents of the acrosome are released by exocytosis. This releases enzymes that help the sperm to penetrate the zona pellucida.

3) Cortical reaction
Once the sperm has penetrated the egg, cortical granules inside the egg release enzymes via exocytosis into the zona pellucida. These enzymes destroy the sperm binding sites and harden the zona pellucida, preventing polyspermy from occurring.

Question 30

Describe the steps of pregnancy.

Answer 30

Day 1: Fertilisation occurs to make the zygote.
Day 3: Zygote divides by mitosis many times to become a ball of 16 cells (called a morula).
Day 4: Cells continue dividing by mitosis to become a blastocyst.
Day 7: The cells implant into the wall of the uterus. The growing embryo will gain oxygen and nutrients from the endometrial tissue fluid, ensuring its continued development.

*Note: The entire process from fertilisation to implantation takes roughly 6-8 days.

Day 8 to week 8: Cells continue to divide to form an embryo.
Week 8 to birth: Cells continue to divide to form a foetus.

Question 31

What are signs of pregnancy?

Answer 31

• Presence of human choronic gonadotropin (hCG) in blood/urine.
• Missed menstrual period.
• Increased body temperature.

Question 32

What is the role of hCG in pregnancy?

Answer 32

When the blastocyst becomes implanted in the uterus wall, it secretes hCG. Its role is to prevent the disintegration of the corpus luteum (consequently maintaining progesterone/oestrogen production).

• Oestrogen inhibits FSH and LH production by the pituitary gland, preventing the release of more eggs from the ovaries.
• Progesterone maintains the endometrium (which is nourishing the embryo) and thickens the cervix.

Levels of hCG are for around 8-10 weeks while the placenta is being developed. After that, the placenta becomes responsible for the production of progesterone and oestrogen. The corpus luteum is no longer required and degenerates.

Question 33

What is the function of the placenta?

Answer 33

To facilitate the exchange of materials between the mother and child and to secrete hormones to maintain the pregnancy after the corpus luteum has degenerated. It acts as a life support system.

Question 34

What is the hormonal role of the placenta?

Answer 34

The placenta produces oestrogen and progesterone.
- Oestrogen stimulates the growth of uterine muscles and the development of the mammary glands.
- Progesterone maintains the endometrium, reduces uterine contractions and potential maternal immune responses.

Question 35

Describe the material exchange in the placenta.

Answer 35

The chorionic villi exchanges materials between the mother and the foetus. Foetal capillaries are within the chorionic villi.

• Useful materials such as oxygen and nutrients diffuse from the lacunae (the intervillous space) into the foetal capillaries where they diffuse into the umbilical cord and are delivered to the foetus.
• Foetal waste such as carbon dioxide diffuses from the lacunae into the maternal blood vessels.

Question 36

What is the structure of the placenta?

Answer 36

The placenta is a disk-shaped structure which is connected to the foetus via an umbilical cord. Exchanged materials are transported from the villi to the foetus via the umbilical cord.

Question 37

What signals the end of the pregnancy?

Answer 37

The end of the pregnancy is signalled by a fall in progesterone and oestrogen levels. The baby will push against the cervix, causing it to stretch. The stretching of the cervix causes nerve impulses to be sent to the brain. The brain stimulates the posterior pituitary to release oxytocin. Oxytocin stimulates the contraction of the uterus. Uterine contractions stimulate the secretion of more oxytocin, therefore the uterine contractions become stronger and stronger in a cycle.

Question 38

Explain the process of birth.

Answer 38

Oxytocin is secreted and stimulates the contraction of the uterus. The cervix relaxes and becomes wider. The amniotic sac bursts and the amniotic fluid is released. This is what is known as the water breaking. After many hours of contractions, the baby is pushed through the cervix and the vagina. The umbilical cord is cut and the baby begins its independent life. Contractions continue until the placenta is expelled.

Question 39

What factors contribute to the length of the gestation period?

Answer 39

• The organism’s size/mass
• The level of development at birth (atricial or precocial)

Question 40

Describe atricial and precocial mammals.

Answer 40

Atricial mammals give birth to helpless, undeveloped offspring that need extended care (eg. rodents). Atricial mammals have a shorter gestation period than precocial mammals.

Precocial mammals give birth to more developed offspring that are mobile and independent (eg. cows).