SPR L6 Fertilisation, Implantation and the Hormones of Pregnancy Flashcards Preview

Physiology > SPR L6 Fertilisation, Implantation and the Hormones of Pregnancy > Flashcards

Flashcards in SPR L6 Fertilisation, Implantation and the Hormones of Pregnancy Deck (14):
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Learning Outcomes

(for general perusal)

  • To elucidate the importance of Reproduction to the human
  • To discuss the formation of the diploid zygote and embryo from the haploid male and female gametes
  • To describe how the fused zygote divides to form the blastocyst which then embeds in endometrium
  • To show how the placenta functions in protecting and nourishing the growing foetus as well as producing the hormones of pregnancy.
  • Pregnacy hormones will be outlined

Concepts and Learning Objectives

  • We consider what happens when the haploid male and female gametes meet and combine to form a diploid zygote which then starts the process of cell division which eventually results in us.
  • In order for a pregnancy to be viable, the zygote divides repeatedly to form the primitive blastocyst which must implant in the endometrial wall.
  • To continue, the developing embryo develops a structure (the placenta) that is specialised to extract gases and nutrients from the endometrial blood supply and return wastes and gases to it.  As well as this delivery function, the placenta also functions in protecting the foetus and manufacturing the hormones of pregnancy.
  • Pregnancy is sustained by the high levels of oestrogen and progesterone, at first produced by the human chorionic gonadotropin (HCG) stimulated corpus luteum and then by the developing placenta.

However, there are many failed pregnancies due to failure to implant.  Equally important are the processes underlying the creation of the placenta which both nourishes the growing foetus and supplies hormones that maintain the uterine wall.  Any failure in these processes or defect in these structures may result in an unsustainable pregnancy.  Understanding is obviously required.

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When does pregnancy start?

Pregnancy starts with fertilisation and continues until birth. 

  1. How long does this take from fertilisation?
  2. How long is preganancy said to conveniently last and why is this the case?
    1. What is the starting date for this?
  3. How is the time divided up?

  1. 38 weeks
  2.  40 weeks, as it is quite difficult to pinpoint exactly when fertilisation takes place
    1. the date of the last menstrual period.
  3. into three 3 month trimesters.

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Fertilisation

What are the three conditions that must be met for successful fusion of sperm and ovum?

 

  1. The egg must be present
  2. Sperm must be present
  3. The sperm must be capacitated

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Fertilisation

  1. How long after ovulation does the egg remain viable for?
  2. How does the egg become present for fertilisation?
  3. Where does fertilisation normally take place and why?
  4. How does the sperm come to meet the egg?
    1. When is this obstacle only passible?
    2. What does this ensure?

  1. 1-2 days after ovulation
  2. egg pulled in by the fimbriae of the fallopian tube, and wafted along the tube itself by the action of cilia
  3. usually in the outer third of the fallopian tube, slow process of egg moving.
  4. sperm must swim through a cervical mucous plug
    1. normally only passable when the ratio of oestrogen to progesterone is at its highest (for a few days of the cycle)
    2. only the best, most viable motile sperm (about 5%) reach the uterus.

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Fertilisation

Capacitation of the Sperm

Although strictly speaking not fully necessary (sperm harvested from the testes may still be used for fertilisation in vitro), exposure to the environment of the female reproductive tract does enhance the ability of sperm to fertilise the egg.

  1. How many sperm are released in a single ejaculation? How many reach the ovum?
  2. What are the sperm guided by?
  3. What happens when they reach the ovum?
  4. What happens in the acrosomal reaction?
  5. What is the name of the protein that permits fusion of the sperm to the ovum membrane?
  6. What prevents polyspermy?

 

 

  1. approximately 100 to 300 million, only a couple of thousand reach the ovum. 
  2. chemoattraction of the sperm to substances produced by the ovum. 
  3. They then penetrate the corona radiata of the ovum where they burrow through the granulosa cells to reach the membranous zona pellucida, sticking to it.  At this point the  acrosomal reaction occurs
  4. the breakdown of the acrosome releasing enzymes (acrosin, related to trypsin) that break down the zona pellucida. 
  5. the protein fertilin on the surface of the sperm
  6. fusion allows the sperm access to the ovum, but also changes membrane potential in it so as to prevent polyspermy.  Very rapidly afterwards, the zona pellucida changes structurally to permanently bar access to the ovum.

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Initial Cell Division

  1. What happens onces the male pronuclus enters the ovum?
  2. What does the genetic material from the male and female coalesce to form?
  3. What does this then immediately start, and what does it become?
  4. How does the morula get transported to the uterus?
  5. When does the ball of cells develop into the blastocyst? When?
  6. What is the blastocyst made up of?
  7. What forms the foetal placenta?
  8. What becomes the embryo itself?

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  1. the secondary oocyte completes the second meiotic division to form the ovum and the second polar body. 
  2. a diploid zygote
  3. mitotic cell division to eventually become a solid ball of cells, the morula
  4. via cilia and persistaltic movements of the fallopian tube
  5. Once in the uterus. Day 5-6.
  6. an outer layer of trophoblast cells, surrounding a fluid cavity (the blastocoele) and a small mass of cells - the inner cell mass.
  7. the trophoblast
  8. the blastocoele, the yolk sac, and the inner cell mass.

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Outline the Blastocyst

  • Day 5 – 6
  • Outer shell of trophoblast
    • Foetal placenta
  • Blastocoele
    • Yolk sac
  • Inner cell mass
    • Embryo

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Implantation

  1.  Intially (7-10 days), what are the morula, and then blastocyst nourished by?
  2. After this period, what needs to happen?
    1. How does this come about?
    2. What does this process cause the release of? And what does this cause?
    3. When does the embryo embed itself fully into this structure?
  3. Why is the embryo not rejected like any other 'non-self' tissues?

  1. the glycogen from mucous secreted by the uterine sawtooth glands
  2.  the embryo must associate itself more with the uterus. 
    1. The trophoblast cells start to digest the wall of the uterus, sending finger like projections into it, releasing nutrients for the embryo. 
    2. the release of prostaglandins that make the endometrial lining more vascular - forming the decidua, into which the embryo embeds itself fully
    3. after about 12 days. 
    4. The reason may lie in that the placental trophoblast that separates mother and foetal tissues, doesn’t contain the polymorphic MHC I and MHC II genes, instead expressing HLA-G, a non polymorphic gene. This makes antibodies against the foetus difficult to develop by the mother.

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Placental Formation

a more permanent metabolic attachment between mother and foetus must develop - burrowing into endometrial tissues provides only a finite source of nutrients and gases. 

  1. What is the chorion, and what does it act to form?
  2. What do the chorionic finers have at their core?
  3. How is the maternal blood kept from coagulating?
  4. How are the chorionic fingers connected with the uterine vasculature?
  5. Describe the placenta at week 5

  1. a double cell layer derived from the foetal trophoblast. Erodes the capillaries of the decidua forming large blood filled spaces into which chorionic fingers project. 
  2. foetal capillaries
  3. by substances secreted by the chorion
  4. blood vessels start to develop in the endometrium  
  5.  the placenta is well developed with its maternal (Decidual) and foetal (chorionic) components.  In this way, maternal and foetal blood are closely associated, allowing gas and nutrient exchange, but still separated by a barrier formed by chorionic tissue and capillary walls.

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Placental Formation

What are the functions of the placenta?

  1. Nutrient and gas exchange (especially favoured by increased O2 affinity of foetal haemoglobin).
  2. Removal of foetal waste products and transfer to maternal blood.
  3. Partial protection of foetus from teratogens and pathogens in the maternal circulation
  4. Endocrine functions (see below).

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Hormones of Pregnancy 

  1. What is required to sustain a pregnancy?
    1. What provides these intially?
  2. What happens to the corpus luteum if no fertilisation occurs?
  3. What happens to it if implantation occurs?
  4. What does the corpus luetum of pregnancy secrete?
  5. When is hCG detectable in the urine?
    1. and in the blood?
  6. hCG levels increase up until when? Describe the changes then.
  7. Describe the levels of oestrogen and progestrone.
  8. What becomes the primary source of sex hormones from about 3 months on?

  1. high levels of oestrogen and progesterone
    1. the corpus luteum provides these. 
  2. it regresses after about 10 days
  3. the developing chorion secretes a hormone human chorionic gonadotropin (hCG) which prevents regression and maintains the corpus luteum of pregnancy
  4. oestrogen, progesterone and relaxin
  5. within about 14 days of conception  - used in pregnancy testing kits
    1. and in the blood by sensitive radioimmunoassay within 6 days of conception)
  6. until about 6 weeks after fertilisation as the placenta enlarges. At this time, levels fall to a lower plateau.
  7. oestrogen and progesterone continue to rise
  8. the placenta itself

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Hormones of Pregnancy

Outline the hormone levels

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When does hCG peak?

6 weeks

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Summing Up 

For general perusal 

  • A successful pregnancy starts with fertilisation of 2 haploid gametes to form a diploid zygote with divides to form an embryo
  • After approximately 6 days, the cells have developed as a fluid filled ball with a trophoblast and an inner cell mass.  This implants and forms the embryo and placenta.
  • Hormones which sustain pregnancy are manufactured by the hCG sustained corpus luteum of pregnancy initially, and then by the placenta itself.

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