Topic 11.4 - Sexual reporduction

Question 1

Oogenesis

Answer 1

Production of egg cells in the ovaries.
Germ cells in the fetal ovaries divide by mitosis and distribute themselves to the cortex of the ovary.
By 4/5 months, the cells begin meiosis.
By 7 months they are still in the first stage of meiosis and a layer of follicle cells surround the cells.
No further progress occurs until puberty

Question 2

Spermatogenesis

Answer 2

Production of sperm cells in the testes which are composed of narrow tubules called seminiferous tubules.

There are small gaps between tubules - called interstices which are filled with interstitial/Leydig cells.

Outer layer of seminiferous tubules is germinal epithelium. Germinal epithelium contains sperm being produced, most mature ones are nearer to liquid in seminiferous tubules.

Sperms with tails - spermatozoa

Tubule wall contains Sertoli (nurse) cells

Question 3

Germ cells

Answer 3

Founder cells of all sexually reproducing organisms. During development, they are set aside from all somatic cells of the embryo.
In many species, germ cells form at the fringe of the embryo proper and then traverse through several developing somatic tissues on their journey to the emerging gonad.
Once in the gonad, they acquire sex-specific morphologies and the ability to undergo meiosis to generate egg and sperm.

Question 4

Primary follicles

Answer 4

The first cell to divide by meiosis with surrounding follicle cells.
Around 400,000 produced at birth, no more produced.
During the start of each menstruation cycle, several are stimulated to develop by FSH but only one usually matures and developed a secondary oocyte.

Question 5

Sertoli cells

Answer 5

Supply essential nutrients to sperm cells and do phagocytosis to get rid of excess spermatid cytoplasm.

Question 6

Gonad

Answer 6

Something producing gametes (testes or ovaries)

Question 7

Epidermis

Answer 7

Outermost layer - skin

Question 8

Epithelium

Answer 8

Inner lining that doesn’t connect to the outside (vascular)

Question 9

Endothelium

Answer 9

Inner lining that also connects to the outside (seminiferous tubules / pulmonary / digestive system)

Question 10

Sperm and egg cells

Answer 10

(page 500-501 in biobook)

Question 11

Differences between spermatogenesis and oogenesis

Answer 11

Sperm cells -> once differentiated, cytoplasm mostly removed - only haploid nucleus, mechanisms for movement and proteins/enzymes for entering egg cell remain.
Meiosis leads to 4 identical sperm cells.
Production is constant once puberty starts and millions may be in some development stage at any time.

Egg cells -> once differentiated, haploid nucleus and all requirements for embryo growth are in the egg.
Meiosis 1 produces large cell and small polar body (degenerates and dies) and then the larger cell in meiosis 2 produces another large cell ready for fertilisation and another small polar body (ded agen :() ~ this allows the egg to be so large.
Production occurs once per menstruation cycle and from puberty to menopause, only a few hundred gametes are produced

Question 12

Polyspermy

Answer 12

Multiple sperm cells fusing with the egg cell during fertilisation.

Bad as this would cause infertility due to incorrect chromosome numbers or possibly even death.

Because of this, there are methods to prevent polyspermy occuring.

Question 13

Methods preventing polyspermy

Answer 13

Acrosome reaction ->
zona pellucida (protective egg layer containing glycoproteins) is bound to by sperm cells. The acrosome is a large membrane-bound sac of enzymes that are released to break down the zp 

Penetration of egg membrane ->
The first cell to enter the zp has its membrane fuse with the membrane of the egg - fertilisation occurs.

Cortical reaction ->
After fertilisation, the egg becomes activated. Cortical granules (located near the egg membrane) have their contents released by exocytosis. Enzymes are released that digest binding proteins - sperms can no longer bind.

Question 14

Internal fertilisation

Answer 14

To prevent drying out, terrestrial animals do int fert.

Allow for close proximity for gametes and protection.

Question 15

External fertilisation

Answer 15

Drying out is not an issue so aquatic animals release the gametes directly into the water.

There are several risks with ext fert: pH levels changing, predation, temperature variations, and pollution etc

Question 16

Why blastocyst implantation occurs

Answer 16

48 hours in - zygote has 4 cells, after this mitosis may lead to uneven division and cells may migrate.

Once a hollow ball of cells is created, the zygote has become a blastocyst

7 days in - blastocyst has 125 cells and has reached the uterus due to cilia in oviduct wall. At this point, the zp has broken down and the reserves from the ova (egg cell) have been used up - external sources of food needed.

Because of this, the blastocyst implants in the endometrium (lining of the uterus)

Question 17

How blastocyst implantation occurs

Answer 17

Blastocyst develops finger-like projections (villi) that sink into the endometrium.

After this, materials can be exchanged between embryo and the mother’s blood, including oxygen and food.

8 weeks in, bone tissue developed, fetus now

Question 18

Role of hCG in pregnancy

Answer 18

Pregnancy - depends on the maintenance of the endometrium which is maintained by oestrogen and progesterone preventing its breakdown.

hCG produced by the embryo, signalling corpus luteum in the embryo to continue to secrete oe & p

Question 19

Materials exchanged by the placenta

Answer 19

Because body surface area to volume ratio increases as size increases, a placenta is used to supply necessary materials to the developing fetus.

Amniotic sac -> produced by the fetus which produces amniotic fluid that protects the fetus and regulates temp.

Placental villi -> More as the placenta develops (to support fetus growth), have mother’s blood and fetal blood very close together (< 5μm), the placental barrier separates the bloods and is selectively permeable. Microvilli are on placental villi to further increase SA.

The materials exchanged across the placenta are: glucose, oxygen, water, amino acids, carbon dioxide, urea, waste materials, antibodies

Question 20

Types of mammal

Answer 20

Placental -> mammals with a placenta which is used to supply their young with necessary materials for life. Young birthed when body functions independently.
Includes - most mammals tbh

Monotremes -> mammals that produce eggs
Includes - Platypus and Echidna

Marsupials -> mammals that birth their young early and have them enter a pouch on the mother and continue development
Includes - kangaroos, koalas, wallabys

Question 21

Release of hormones during pregnancy

Answer 21

Ninth week of pregnancy - placenta takes the job of the corpus luteum and secretes oe & p in order to supply sufficient amounts of these hormones.

If this switch over fails, a miscarriage may occur.

Question 22

Role of hormones in parturition

Answer 22

Birth is mediated by positive feedback of oe and oxytocin

During pregnancy, progesterone inhibits the release of oxytocin from the pituitary gland and the contractions of the muscular wall of the uterus (myometrium)

Progesterone is inhibited by hormones produced by the fetus near the end of the pregnancy.

Question 23

Hormones on contractions during parturition

second part

Answer 23

Endometrium muscle fibre contractions stimulated by oxytocin, these are then detected by stretch receptors, which signal for the increase in oxytocin secretion.
More oxytocin - more vigorous and frequent contractions.

This positive feedback loop allows for a gradual increase in myometrial contractions, allowing minimal contraction intensity for fetus birth.

Relaxation of muscle fibres in the cervix cause dilation, uterine contractions then cause the amniotic sac to burst and the fluid to be released.

The next few hours contain uterine contractions which push the baby out. Once the baby is out, the umbilical cord is broken off and the baby starts breathing itself and has physiological independence from its mother.

Question 24

Open circulatory system

Answer 24

Blood not confined to blood vessels, found in insects.

Question 25

Closed circulatory system

Answer 25

Blood confined to blood vessels, found in humans.

Question 26

Haemoglobin (adults)

Haemoglobin (fetuses)

Answer 26

HbA - contains two alpha and two betas (α₂β₂)
HbF - contains two alpha and two gammas (α₂γ₂)

Fetus has gammas as it needs a higher affinity for oxygen absorbtion.

Question 27

Glucose across the placental barrier

Answer 27

Occurs by facilitated diffusion

Question 28

Water across the placental barrier

Answer 28

Occurs by osmosis

Question 29

Oxygen across the placental barrier

Answer 29

Occurs by diffusion

Question 30

Carbon dioxide across the placental barrier

Answer 30

Occurs by diffusion

Question 31

Antibodies across the placental barrier

Answer 31

Occurs by endocytosis

Question 32

Urea across the placental barrier

Answer 32

Occurs by diffusion

Question 33

Amino acids across the placental barrier

Answer 33

Occurs by facilitated diffusion