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3002 > 81 Coitus, fertilisation and preimplantation > Flashcards

81 Coitus, fertilisation and preimplantation Flashcards

1
Q

Describe the human sexual response? and justify

A

• Human ovulation is ‘hidden’
• Human females are receptive to males throughout their
menstrual cycle
• Has been suggested that human sexual behaviour has developed as a way of strengthening pair bonding to support the long development of human children

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2
Q

What are the 4 phases of the human sexual response?

A 
EPOR: 
• Excitement 
• Plateau
• Orgasm
• Resolution
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3
Q

What is the excitement phase of the human sexual response?

A

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

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4
Q

What is the plateau phase of the human sexual response?

A

Intensification of arousal

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5
Q

What is the orgasm phase of the human sexual response?

A

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

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6
Q

What is the resolution phase of the human sexual response?

A

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

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7
Q

Variation of ovarian cycle?

A
  • Luteal phase fixed at 14 days

* Follicular and menstrual phases vary

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8
Q

How long are sperm and oocytes viable for?

A
  • Sperm: 24-72 hours

* Oocytes: 12-24 hours

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9
Q

For fertilisation to occur, coitus must occur….

A

Between:
• 3 days before ovulation
• 1 day after ovulation

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10
Q

For pregnancy to occur….

A

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

  • Sperm are capable of fertilising an egg for approx 4-6 days
  • Ovulated egg remains viable for approx 24-48h
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11
Q

After fertilisation, what happens to the control of development?

A

Control of development will switch from maternal to foetal

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12
Q

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

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

How likely are women to conceive?

A
  • 50% pregnant after 2 cycles
  • 85% pregnant after 6 months
  • 1/2 the remaining couples pregnant after 1 year
  • Leaves 5% subfertile

• 10–15% of all pregnancies miscarry

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14
Q

Abnormal development?

A

• Only 20 % of unprotected intercourse results in development to the blastocyst stage (Similar success rate for IVF)
• Pre-implantation and post-implantation failure occurs frequently - spontaneous abortion
• 8-20% of blastocysts fail to implant
• Clinically detected pregnancies 15 – 20 % will fail in the first 12
weeks
• Possible that less that 15 – 20% of human conceptions survive to successful birth

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15
Q

What causes abnormal development?

A

Abnormalities are most commonly chromosomal caused by non-disjunction (aneuploidy) sperm DNA -shows high rates of mutation because they lack DNA repair system

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16
Q

What are the two periods of foetal development?

A
  1. Embryonic period 8 weeks
    • Preimplantation embryo
    • Implantation
    • Differentiation and development of the organ system
  2. Foetal period 8-40 weeks
    • Differentiation continues
    • Growth
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17
Q

How is the egg/oocyte transported at ovulation?

A
  • Egg extruded onto surface of ovary
  • Smooth muscle of fimbriae cause them to pass over ovary while cilia beat in waves toward interior of duct
  • Cumulus cells aid transport from surface of the ovary
  • Sticky cumulus cells cling to the ciliated surface of the fimbriae
  • Transported by a ciliary current (and perhaps also peristalsis) to the ampulla
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18
Q

How is sperm transported to the egg from ejaculate?

A

• Ejaculation deposits semen into vagina then movement into cervix
• Passage into cervical mucus dependent on oestrogen-induced changes in mucus consistency
• Sperm can reach uterus minutes/hours after ejaculation
– But can survive 1-2 days within cervical mucus before release to enter uterus

19
Q

Sperms movement through uterus and fallopian tubes is via….

A

Movement via:
• Sperm’s own propulsions
• Uterine contractions

20
Q

Why is sperm mortality from vagina to fallopian tubes large?

A

Several hundred million to 100-200

  • Vaginal environment is acidic
  • Length and energy requirements of trip
21
Q

What is capacitation?

A

• Action of female tract secretions on sperm over several (6-8?) hours.
– Essential for them to be capable of fertilisation

22
Q

What is the result of capacitation?

A
  • Change from wavelike beats of sperm tail to whip-like action to propel sperm forward
  • Sperm’s plasma membrane is altered so it is capable of fusing with surface membrane of egg (acrosome reaction)
23
Q

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

A

Capacitation destabilises the sperm surface membrane to enhance fusion with oocyte:

  • Increase in Ca2+ permeability - rise in intracellular Ca2+
  • Removal of membrane proteins (glycoprotein)
  • Change in surface charge
  • Depletion of cholesterol
24
Q

Where is the normal site of fertilisation occurring?

A

Ampulla of the uterine tube

25
Q

When does fertilisation begin?

A
  • Begins with fusion of sperm and egg

* Usually a few hours after ovulation and within 24-48h of ovulation

26
Q

Stages of fertilisation:

From fusion of sperm to egg –> Contents of sperm-cell entering oocyte

A
  1. Sperm cell weaves past follicular cells and binds to zona pellucida
  2. A rise in [Ca2+]i inside the sperm cell triggers the exocytosis of the acrosome, which contains hydrolytic enzymes
  3. Hydrolytic enzymes contained in the acrosomal cap are released. These enzymes locally dissolve the zona pellucida. The whip-like action of the tail pushes the sperm head towards the oocyte membrane
  4. With the head of the sperm now lying sideways, microvilli on the oocyte surround the sperm head. The 2 membranes fuse. The contents of the sperm-cell enter the oocyte; the sperm-cell membrane remains behind
27
Q

What is the acrosome reaction?

A

Induced by the sperm head contacting the zona pellucida and binding to glycoproteins ZP2 + ZP3

  • Fusion of the acrosome plasma membranes
  • Releases contents of the acrosome
  • Causes entry of more Ca++
  • Sperm then digests a path through the zona pellucida (proteolytic enzymes)
28
Q

What are the stages of fertilsation:

From the contents of sperm-cell entering oocyte –> male and female pronuclei fusion

A
  1. A rise in [Ca2+]i inside the oocyte triggers the cortical reaction, in which there is exocytosis of granules that previously lay immediately beneath the plasma membrane. The enzymes released lead to changes in the zona pellucida proteins, causing the zona pellucida to harden, preventing the entry of other sperm cells
  2. The rise in [Ca2+]i inside the oocyte induces the completion of the oocyte’s 2nd meiotic division and the formation of the 2nd polar body, which usually lies next to the 1st polar body
  3. The head of the sperm enlarges to become the male pronucleus
  4. The male and female pronuclei fuse
29
Q

What is the cortical reaction?

A
  • Exocytosis of granules in egg

* Enzymes released cause the zona pellucida to harden => preventing the entry of other sperm cells

30
Q

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

A
  1. Cortical reaction

2. Induces completion of the oocyte’s 2nd meiotic division and formation of the 2nd polar body

31
Q

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
A
  1. Ovulation = 1 day (ovary)
  2. Fertilisation = 2 days (fallopian tube)
  3. Cell division to 32 cells = 2-4 days (fallopian tube)
  4. Blastocyst enters uterine cavity = 5 days (uterus)
  5. Implantation = 6-7 days (uterus)
  6. Human chorionic gonadotropin (hCG) from implanted blastocyst (trophoblast cells) rescues CL = 9-10 days (trophoblast –> maternal ovary)
32
Q

What is conceptus?

A

Fertilised egg, embryo, fetus

early stages of pregnancy

33
Q

Features of conceptus? Why in fallopian tube, formation of new structures (cell division to ~ 32 cells), cell type?

A
  • Conceptus “held” in fallopian tube as oestrogen maintains contraction of smooth muscle near where fallopian tube enters wall of uterus
  • Conceptus undergoes a number of mitotic cell divisions ie cleavage and morula is formed
  • Cells are totipotent
34
Q

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

A

No cell growth occurs before each division

35
Q

What are totipotent cells?

A
  • 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 fertilisation are the only cells that are totipotent
36
Q

What are pluripotent cells?

A
  • Pluripotent cells can give rise to all of the cell types that make up the body
  • e.g. Embryonic stem cells
37
Q

What are multipotent cells?

A
  • 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
38
Q

Why can blastocyst cells be removed without damage to embryo in genetic screening?

A

Blastocyst is pluripotent

39
Q

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

A
  • Plasma progesterone levels rise 3-4 days after fertilisation, smooth muscle relaxes and conceptus passes into uterus
  • ~ 4-5 days after fertilisation, cavities develop between the cells
  • For ~ 3 days, conceptus/blastocyst lies free in the uterine cavity supported by uterine secretions, receiving nutrients from it
40
Q

What dose the trophoblast give rise to?

A

Placenta

41
Q

What does the inner cell mass form?

A

Form the embryo

42
Q

When does the blastocyst attach to the endometrium adjacent to the inner cell mass (embryonic pole)?

A

Day 6

43
Q

What does trophoblast differentiate into?

A
  1. Inner cytotrophoblast
  2. Syncytiotrophoblast (loses cell boundaries)
    • Fingers of syncytiotrophoblast invade the endometrium

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