2. Coitus, Fertilisation and Preimplantation

Question 1

Describe the human sexual response? and justify

Answer 1

• Human ovula/on however is ‘hidden’.
• Human females are recep/ve to males throughout their
menstrual cycle.
• It has been suggested that human sexual behaviour has developed as a way of strengthening pair bonding to support the long development of human children.

Question 2

What are the 4 phases of the human sexual response?

Answer 2

EPOR: Excitement, Plateau. Organism, Resolution

Question 3

What is the excitement phase of the human sexual response?

Answer 3

Sexual arousal – psychological and physical stimula/on of erogenous zones.
Tumesence and erec/on of penis and clitoris, engorgement of female tract
• Parasympathetic - erection
• Sympathetic - ejaculation

Question 4

What is the plateau phase of the human sexual response?

Answer 4

Intensification of arousal

Question 5

What is the orgasm phase of the human sexual response?

Answer 5

Series of involuntary muscular contractions in both sexes with intense pleasurable sensations

Question 6

What is the resolution phase of the human sexual response?

Answer 6

Detumescence and time during which re-arousal is impossible (may not be true in women)

Question 7

Variation of ovarian cycle?

Answer 7

Luteal fixed at 14 days

Follicular and menstrual vary

Question 8

How long are sperm and oocytes viable for?

Answer 8

Sperm: 24-72 hours
Oocytes: 12-24 hours

Question 9

For fertilisation to occur, coitus must occur….

Answer 9

Between:

• 3 days before ovulation
• 1 day after ovulation

Question 10

For pregnancy to occur…

Answer 10

Sperm introduction should be between 5 days before and one day after ovulation

Question 11

After fertilisation will switch from maternal to …

Answer 11

foetal

Question 12

What are the time spans of the different trimesters of pregnancy?

Answer 12

First trimester weeks 1-12 (most miscarriages occur)
Second trimester weeks 13-28 (At 24 weeks 50% survival rate for early prematurity)
Third trimester weeks 29-40

Question 13

What does parturition mean?

Answer 13

Birth

Question 14

How likely are women to conceive?

Answer 14

50% pregnant after 2 cycles
85% pregnant after 6 months
Leaves 5% subfertile
10–15% of all pregnancy miscarry

Question 15

Abnormal development?

Answer 15

Only 20% of unprotected intercourse results in development to blastocyst stage
Sponanteous abortion (pre- and post-implanation failure) occurs frequently
8-20% of blastocysts fail to implant
Clinically detected pregnancies 15-20% will fail in the first 12 weeks
Possible that 15-20% of human conceptions survive to successful birth

Question 16

Why do sperm show high rate of mutation?

Answer 16

Lack DNA repair system

Question 17

What are the two periods of foetal development?

Answer 17

Embryonic period 0-8 weeks

• Preimplantation embryo
• Implantation
• Differentiation and development of the organ system

Foetal period 8-40 weeks

• Differentiation continues
• Growth

Question 18

How is the egg/oocyte transported at ovulation?

Answer 18

1. Egg extruded onto surface of ovary
2. Smooth muscle of fimbriae cause them to pass over ovary while cilia beat in waves toward interior duct
3. Cumulus cells from the surface of the ovary aid transport. They cling to the ciliated surHface of the fimbriae.
4. Transport by a ciliary current (+ peristalsis) to the ampulla.

Question 19

How is sperm transported to the egg from ejaculate?

Answer 19

1. Ejaculation deposits semen into vagina then movement into cervix
2. Passage into cervical mucus is dependent on oestrogen-induced changes in mucus consistency
3. Sperm can reach uterus minutes/hours after ejaculation. Can survive 1-2 days within cervical mucus before release to enter uterus
4. Movement through uterus and fallopian tubes is via sperm’s own propulsions and uterine contractions
5. Sperm mortality from vagina to fallopian tubes is large.

Question 20

Sperms movement through uterus and fallopian tubes is via….

Answer 20

Sperm’s own propulsions and uterine contractions

Question 21

Why is Sperm mortality from vagina to fallopian tubes large?

Answer 21

1. Vaginal environment is acidic

2. Length and energy requirements of trip

Question 22

What is capacitation?

Answer 22

Capacitation is a functional maturation of the spermatozoon. The changes take place via the sperm cell membrane in which it may be that receptors are made available through the removal of a glycoprotein layer. The area of the acrosomal cap is also so altered thereby that the acrosome reaction becomes possible.
Due to female tract secretions

Question 23

What is the result of capacitation?

Answer 23

1. Change from wavelike beats of sperm tail to whip-like action to propel sperm forward
2. Sperm’s plasma membrane is altered so it is capable of fusing with surface membrane of egg (acrosome reaction)
   - Destabilises surface membrane to enhance fusion capability

Question 24

What are the 4 changes in the sperm membrane during capacitation that enhance its fusion abilities with oocytes?

Answer 24

1. Increase in Ca2+ permeability = Rise in intracellular Ca2+
2. Removal of membrane proteins (glycoprotein)
3. Change in surface charge
4. Depletion of cholesterol

Question 25

Where is the normal site of fertilisation occurring?

Answer 25

The ampulla of the uterine tube

Question 26

Stages of fertilisation: | From fusion of sperm to egg --> Contents of sperm-cell entering oocyte

Answer 26

1. Fusion of sperm to egg 2. The sperm cell weaves past follicular cells and binds to the zona pellucida 3. A rise in [Ca2+]i inside the cell triggers the exocytosis of the acrosome (acrosome reaction) which contains hydrolytic enzymes 4. Hydrolytic enzymes contained in the acrosomal cap are released. These enzymes locally dissolve the ZP. The whip-like action of the tail pushes the sperm head towards the oocyte membrane 5. With the head of the sperm now lying sideways, microvilli on the oocyte surround the sperm head. The two membranes fuse. The contents of the sperm-cell enter the oocyte, the sperm-cell membrane remains behind.

Question 27

What is the acrosome reaction?

Answer 27

Induced by the sperm head contacting the zona pellucida and binding to glycoproteins ZP2 and ZP3 1. Fusion of acrosome plasma membranes 2. Releases contents of the acrosome 3. Causes entry of more Ca++ 4. Sperm then digests a path through the ZP via proteolytic enzymes i.e. exocytosis of acrosomes hydrolytic enzymes to dissolve ZP

Question 28

What are the stages of fertilsation: | From the contents of sperm-cell entering oocyte --> male and female pronuclei fusion

Answer 28

6. A rise in [Ca2+]i inside the oocyte triggers the cortical reactions. The enzymes released result in ZP protein changes, causing the ZP to harden which prevented other sperm entry. 7. The rise in [Ca2+]i inside the oocyte induces the completion of the oocytes 2nd meiotic division + formation of 2nd polar body 8. The sperm head enlarges to become the male pronucleus 9. The male and female pronuclei fuse.

Question 29

What is the cortical reaction?

Answer 29

Exocytosis of granules that previously lay immediately beneath the plasma membrane.

Question 30

What is the male and sperm contribution during pronuclei fusion in fertilisation?

Answer 30

Sperm: Nuclear material and centrioles Oocyte cytoplasm: All other organelles Mitochondrial DNA inherited exclusively via maternal route

Question 31

2 results of rise of [Ca2+]i in oocyte

Answer 31

1. ZP hardening via cortical reaction | 2. Completion on 2nd meiotic division of oocyte + formation of 2nd polar body

Question 32

For the following stages of ovulation-fertilisation-implanation, what is the day after LH peak and location: 1. Ovulation 2. Fertilisation 3. Cell division to 32 cells 4. Blastocyst enters uterine cavity 5. Implantation 6. Human chorionic gonadotropin (hCG) from implanted blastocyst (trophoblast cells) rescues CL

Answer 32

1. Ovulation - 1 day after LH peak - In ovary 2. Fertilisation - 2 days after LH peak - In Fallopian tube, ampulla 3. Cell division to 32 cells - 2-4 days after LH peak - In fallopian tube 4. Blastocyst enters uterine cavity - 5 days after LH peak - In uterus 5. Implantation - 6-7 days after LH peak - Uterus 6. Human chorionic gonadotropin (hCG) from implanted blastocyst (trophoblast cells) rescues CL - 9-10 days after LH peak - In Trophoblast then maternal ovary

Question 33

What is conceptus?

Answer 33

The embryo in the uterus

Question 34

Features of conceptus? Why in fallopian tube, formation of new structures, cell type?

Answer 34

Cenceptus is held in the fallopian tube as oestrogen maintains contraction of smooth muscle near where the fallopian tube enters wall of uterus Conceptus undergoes mitotic cell divisions i.e. cleavage and morula is formed Cells are totipotent

Question 35

Why is the conceptus the same size as the fertilized egg?

Answer 35

As no cell growth has occurred before each division

Question 36

What are totipotent cells?

Answer 36

Totipotent cells can form all the cell types in a body (plus the extraembryonic or placental cells) Embryonic cells within the first couple of cell divisions after fertilization are the only cells that are totipotent.

Question 37

What are pluripotent cells?

Answer 37

Pluripotent cells can give rise to all of the cell types that make up the body. E.g. Embryonic stem cells are considered pluripotent.

Question 38

What is multipotent cells?

Answer 38

Multipotent cells can develop into more than one cell type, but are more limited than pluripotent cells E.g. Adult stem cells and cord blood stem cells are considered mul/potent.

Question 39

Why can blastocyst cells be removed whilst damage to embryo?

Answer 39

As the cells are pluripotent

Question 40

What are the events that occur when the conceptus/blastocyst enters uterine cavity?

Answer 40

1. Plasma progesterone levels rise 3-4 days after fertilisation, smooth muscle relaxes and conceptus passes into uterus 2. Approx 4-5 days after fertilisation, cavities develop between the cells 3. For approx 3 days, conceptus/blastocyst lies free in the uterine cavity supported by uterine secretions, received nutrients from it. 4. Day 6 the blastocyst attaches to the endometrium adjacent to the inner cell mass (embryonic pole) 5. Trophoblast differentiates into: Inner cytotrophoblast and outer syncytiotrophoblast - Fingers of syncytiotrophoblast invade the endometrium

Question 41

What is the structure of the blastocyst

Answer 41

Inner cell mass: Forms entire body Fluid filled cavity i.e. blastocoele Trophoblast: Outer layer of blastocyst, gives rise to placenta Zona pellucida= Egg shell