Sexual reproduction Flashcards
(69 cards)
1
Q
What are ovaries and testes called?
A
Gonads
2
Q
What determines the sex in males?
A
The SRY gene found on the Y chromosome codes for the protein TDF. TDF binds to the DNA and stimulates the expression of other genes that cause testis development
3
Q
What does testosterone do?
A
- the hormone testosterone is produced in the testes
- In the embryo it causes the development of male genitalia
- during puberty, it causes the development of male secondary sexual characteristics
- after puberty, it causes sperm production
4
Q
Where are estrogen and progesterone produced?
A
In the ovaries
5
Q
What does estrogen do?
A
- They cause development of female reproductive organs in the absence of testosterone
- causes development of female secondary sexual characteristics
6
Q
Testis
A
Produces sperm and testosterone
7
Q
Scrotum
A
Holds testes at lower than core body temperature
8
Q
Epididymis
A
Stores sperm until ejaculation; promotes sperm maturation
9
Q
Sperm duct
A
Transfers sperm during ejaculation
10
Q
Seminal vesicle and prostate gland
A
Secrete an alkaline fluid that is added to sperm to make semen. The fluid contains many substances such as fructose to nourish the sperm
11
Q
Urethra
A
Transfers semen during ejaculation
12
Q
Penis
A
Penetrates the vagina for semen ejaculation near the cervix; has erectile tissue that becomes stiff when filled with blood to allow penetration
13
Q
Ovary
A
produces eggs, estrogen and progesterone
14
Q
Oviduct
A
collects eggs at ovulation, provides site for fertilisation, then moves the embryo to the uterus
15
Q
Uterus
A
Site of embryo implantation, protects the fetus during pregnancy and provides it with food, oxygen and removal of waste products
16
Q
Cervix
A
Protects the fetus during pregnancy, then dilates to provide a birth canal
17
Q
Vagina
A
Stimulates penis to cause ejaculation and provides a birth canal
18
Q
Vulva
A
Protects internal parts of the female reproductive system
19
Q
Oogenesis
A
Production of female gametes in the ovaries
20
Q
Spermatogenesis
A
Production of male gametes in the testes
21
Q
Which steps do both oogenesis and spermatogenesis have?
A
- Mitosis to generate large number of diploid cells
- Cell growth so that cells have sufficient resources to undergo meiosis
- Two divisions of meiosis to produce haploid cells
- Differentiation so that the haploid cells develop into gametes with structures needed for fertilisation
22
Q
Describe spermatogenesis
A
- occurs in the seminiferous tubules
- it starts at puberty and goes on throughout adult life
- millions of sperm are produced each day
23
Q
Stages of spermatogenesis
A
- An outer layer of germinal epithelial cells called spermatogonia divide endlessly by mitosis to produce more diploid cells.
- The diploid cells grow into larger cells with more cytoplasm and divide by mitosis to form primary spermatocytes
- Each primary spermatocyte undergoes the first meiotic division to produce two secondary spermatocytes
- Each secondary spermatocyte undergoes the second meiotic division to produce two spermatids
- Spermatids are associated with Sertoli cells which nourish the spermatids and help them differentiate and develop into spermatozoa. The differentiation includes growth of a tail
- Sperm detach from Sertoli cells and enter the lumen of the seminiferous tubule.
24
Q
What fills the gaps between the seminiferous tubules and what is its role?
A
Leydig cells which produce testosterone
25
Describe the structure of human sperm
- the human sperm consists of three parts: head, midpiece and tail
- at the head, there is a haploid nucleus
- There is also acrosome- containing hydrolytic enzymes that digest the zona pellucida around the egg
- At the neck, there is a centriole
- At the midpiece, there are helical mitochondria which produce ATP by aerobic respiration
- The tail provides the propulsion that allows
- Microtubules in a 9+2 array make the tail beat from side to side and generate the forces that propel the sperm
- Protein fibres around the microtubules strengthen the tail
26
Describe oogenesis
- Occurs in the ovaries
- starts during fetal development
- from puberty to menopause
- one egg every 28 days
27
Stages in oogenesis
1. In the ovaries of a female fetus, germinal epithelium cells divide by mitosis to form more diploid cells
2. Diploid cells grow into larger cells called primary ooyctes
3. Primary oocytes start at the first division of meiosis but stop at prophase I. The primary oocyte and a single layer of follicle cells around it form a primary follicle
4. Every menstrual cycle a few primary follicle are stimulated to develop by FSH. Usually one goes on to become a mature follicle, containing a secondary oocyte. The primary oocyte completes the first division of meiosis forming two haploid nuclei. The cytoplasm of the primary oocyte is divided unequally forming a large secondary oocyte and a small polar body
5. The secondary oocyte starts the second division of meiosis but stops in prophase II. The follicle cells are proliferating and follicular fluid is forming
6. When the mature follicle bursts, at ovulation the egg released is actually a secondary oocyte
7. After ovulation, the follicle develops into the corpus luteum
8. After fertilisation, the secondary oocyte completes the second meiotic division to form an ovum and a second polar body.
28
What is the structure of a mature human egg?
- haploid nucleus
- cytoplasm contains droplets of fat and other nutrients during the early stages of embryo development
- two centrioles
- plasma membrane
- there is a coat of glycoproteins called the zona pellucida which protects the egg cell and restricts the entry of sperm
- there are layers of follicle cells- corona radiata
- cortical granules harden the zona pellucida to prevent polyspermy
29
Where is the mitochondria of the fetus from?
From the egg's cytoplasm
30
What are the four phases in the menstrual cycle?
Follicular phase
Ovulation
Luteal phase
Mensturation
31
Draw the hormone levels in a woman during mensuration
See notes
32
Describe follicular phase
- Towards the end of mensuration, FSH secreted by the anterior pituitary rises to a peak and causes development of follicles in the ovary
- FSH also stimulates secretion of estrogen by the follicle wall
- Estrogen stimulates an increase in FSH receptors that make the follicles more receptive to FSH, boosting estrogen production. Estrogen reaches a peak towards the end of the follicular phase.
- Estrogen stimulates the repair of the endometrium
33
Describe Ovulation
- When estrogen reaches high levels it stimulates LH secretion. It also inhibits secretion of FSH leading to a fall in estrogen secretion
- LH rises to a sudden and sharp peak. It stimulates ovulation: meiosis in the oocyte is completed and the follicle wall is partially digested allowing it to burst open and release the oocyte
34
Describe the luteal phase
- After ovulation, LH promotes the development of the follicle wall into the corpus luteum which secretes estrogen and progesterone
- Progesterone levels rise at the start of the luteal phase and promotes the thickening and maintenance of the endometrium to prepare it for implantation. It also inhibits FSH and LH secretion
- If there is fertilisation progesterone levels remain high and inhibit FSH production
35
Describe mensturation
- Progesterone and estrogen levels drop and the endometrium can no longer be maintained
- When progesterone and estrogen levels are low enough, FSH is secreted again and its level rise starting the next menstrual cycle
36
Fertilisation
Fusion of a haploid sperm with a haploid egg to produce a diploid zygote
37
External fertilisation
Females release unfertilised eggs and fertilisation happens outside the body
38
Internal fertilisation
The male passes sperm into the female body and fertilisation takes place there
39
Polyspermy
Fusion of two or more sperm with an egg
40
List the stages in human fertilisation
1. Arrival of sperm
2. Binding
3. Acrosome reaction
4. Fusion
5. Cortical reaction
6. Mitosis
41
Arrival of sperm
Sperm are attracted by a chemical signal and swim up to reach the egg. Fertilisation is only successful of many sperm reach it. Sperm try to push through the layers of follicle cells around the egg
42
Binding
The first sperm to break through the layers of follicle cells binds to the zona pellucida. This triggers the acrosome reaction
43
Acrosome reaction
The acrosomal cap separates from the sperm and the contents of the acrosome are released. Enzymes from teh acrosome digest a route through the zona pellucida allowing the sperm to reach the egg's plasma membrane
44
Fusion
The plasma membranes of the egg and the first sperm that gets through the zona pellucida fuse. The sperm nucleus enters the egg. This is the moment of fertilisation. Fusion causes the cortical reaction. The
mitochondria of the zygote are derived from the egg’s cytoplasm.
45
Cortical reaction
Small vesicles, called cortical granules, move to the plasma membrane of the egg and fuse with it, releasing their contents by exocytosis. Enzymes from
the cortical granules cause cross-linking of glycoproteins in the zona pellucida, making it hard and preventing polyspermy. In mammals, the enzymes also digest the binding proteins so that no further sperm can bind.
46
Mitosis
The secondary oocyte completes meiosis II. Both nuclei carry out mitosis, using the same centrioles and spindle of microtubules
47
Why has there been declining male fertility?
The presence of estrogen and progesterone in water since the introduction of the female contraceptive pill
48
Describe the steps of IVF
1. Down-regulation: the woman takes drugs to stop the menstrual cycle so that doctors can control the timing. The drugs inhibit FSH and LH secretion. Therefore, secretion of estrogen and progesterone also stops
2. injections of hormones: FSH and LH are injected to stimulate follicle
development. The FSH dose is high and as a result more follicles develop. This stage is called superovulation. Subsequently, the hormone hCG is also injected
3. Egg and sperm retrieval: eggs are removed from the follicles of the woman’s ovaries. Sperm is taken from the father/donor and is often processed to make it more concentrated
4. In vitro fertilization of egg and sperm: Each egg is mixed with 50,000-100,000
sperm cells in a dish, which is incubated at 37°C for a day. The eggs are examined with a microscope for successful fertilisation
5. Establishment of pregnancy: zygotes are grown in a medium and embryos are placed in the uterus when they are about 48 hours old. 2-3 embryos are transferred to improve the chances of successful implantation The woman is given extra progesterone to ensure that the endometrium is maintained.
49
What is Aristotle's seed and soil theory
The male produces a seed which forms an egg when mixed with menstrual blood
50
What is Harvey's investigation of sexual reproduction in deer?
- He examined the uterus of female deer after the mating season expecting to see the developing embryo
- He checked until approximately 6 – 7 weeks after mating, but was unable to detect an embryo
- He concluded that Aristotle’s theory was incorrect (which certainly is) and that menstrual blood does not contribute to the development of a fetus
- However, he also wrongly concluded that offspring are not the result of mating
- The problem for Harvey was that the gametes, the process of fertilization and the early stages of embryo development are too small to see without an effective microscope. Such a microscope was not invented until 17 years after his death. Harvey was also unlucky with his choice of animal, as embryos in the deer remain very small for an unusually long period.
51
Describe the early embryo development
The diploid zygote produced by fertilisation divides by mitosis until it forms a blastocyst.
52
What is a blastocyst comprised of?
The inner cell mass
Trophoblast (surrounding layer that develops into a placenta)
A fluid-filled cavity called the blastocoele
53
When does the embryo implant in the endometrium?
At about 7 days old
54
When is the embryo called a fetus?
When it is 8 weeks old and starts to develop bone tissue
55
What is the amniotic sac?
A pair of membranes and is connected to the placenta vie the umbilical cord
56
What does an embryo secrete?
hCG
57
What does hCG do?
Maintains the corpus luteum in the ovary and prevents its degeneration. So the ovary, maintains progesterone secretion for the first 12 weeks of pregnancy
58
What happens when the placenta develops?
It takes over the task of secreting progesterone so the corpus luteum degenerates as hCG levels drop. The placenta also secretes estrogen
59
What does progesterone inhibit?
Oxytocin secretion and prevents uterine contractions to prevent an abortion.
60
Describe childbirth/labour
1. The level of progesterone falls steeply
2. Allows for pituitary gland to secrete oxytocin
3. Oxytocin binds to oxytocin receptors to cause muscle contractions
4. Uterine contractions are detected by stretch receptors which signal to the pituitary gland to secrete more oxytocin
5. A rise in estrogen causes an increase in the number of oxytocin receptors
6. The uterine contractions become stronger through positive feedback
7. The cervix relaxes and dilates
8. The amniotic sac bursts and the amniotic fluid passes out
9. Baby is pushed out through the cervix and vagina
10. Umbilical cord is cut
11. Placenta is expelled as afterbirth
61
Where does the placenta grow?
Inside the endometrium and is connected to the fetus by an umbilical cord
62
Why does the mother's and fetal blood flow close together?
To facilitate the exchange of materials between maternal and fetal blood
63
What are functions of the placenta?
- gas exchange
- supply of nutrients
- supply of hormones
-supply of antibodies
- removal of waste products
- production of estrogen and progesterone
64
What passes from the maternal blood to the fetal blood and vice versa?
from the maternal blood to the fetal blood: oxygen, lipids, water, minerals, hormones, antibodies and water
From the fetal blood to the maternal blood: urea, hormones, and water
65
Describe the structure of a placenta
It has chorionic villi which increases the SA for excahnge of gases and other materials. They are formed by chorion. The fetal blood flows through the capillaries. In the inter-villous space, maternal blood flow and this is brought by the uterine arteries and carried away by uterine veins
66
What is the chorion
A layer of calls that separate maternal and fetal blood, which forms the placental barrier
67
What is carried in the umbilical vein and artery?
- Oxygenated blood flows back to the fetus from the placenta along the umbilical vein
- Deoxygenated fetal blood flows from the fetus to the placenta along umbilical arteries
68
What are structural adaptations of the villus in the placenta?
- Microvilli in the chorion increase the SA
- many mitochondria for AT of materials
- Capillaries are close to the villus surface and are one cell thick so the distance is small
-basement membrane is permeable
- cytoplasm of chorion produces estrogen adn progesterone and secretes them in the maternal blood
69
Describe the relationship between animal size and gestation period
There is a positive correlation between body mass and duration of gestation
