Pregnancy and reproduction

Question 1

Discuss spermatozoon voyage and maturation

Answer 1

Voyage of the Spermatozoon

• The journey from the testes to the oviduct is a long one 9 100,000 times the length of a sperm cell (comparable to 150 km for a 1.5 m human being)
• It is also very much against all odds (< 1/10⁶ spermatozoa reach the ovum)

Within the male reproductive tract

• Most tubular fluid is reabsorbed within the rete testis and early epididymis

under the control of OESTROGEN

• Oestrogen is mainly in tubular fluid produced by Sertoli cells
• Nutrients and other molecules (e.g. glycoproteins) are secreted into the epididymal fluid under the influence of ANDROGENS to:
  
  • Provide energy for impending possible journey
  • Coat the surface of the spermatozoon (to protect them from the hostile environment)
• The fluid is reabsorbed and the secretory products are put in within the epididymis
• In the vas deferens you get a collection of spermatozoa
• In the vas deferens the concentration of spermatozoa is higher that it is later on in the reproductive tract once other fluids have been added
• When performing a vasectomy 9 you want to cut the vas deferens towards the bottom end

Ejaculation

• A lot of fluid will be added to the spermatozoa between the vas deferens and the urethra so the concentration further down the reproductive tract is lower than in the vas deferens
  
  • Spermatozoa 15-120 million/ml
  • Seminal fluid 2-5ml
  • Leucocytes
  • (potentially viruses e.g. hepatitis B, HIV)

1/100 of spermatozoa in ejaculate enter the cervix
* 1/10,000 cervix to ovum

Overall 1/million reach ovum

Seminal Fluid

• Main point: a lot of these molecules are important for providing energy, and the environment for the spermatozoa to work properly
• You get fibrinogen and fibrinolytic enzymes in the seminal fluid because after ejaculation, the semen initially clots and then must be broken down

Small contribution from:

• Epididymis/testis
• Mainly from accessory sex glands:
• Seminal vesicles
• Prostate
• Bulbourethral glands

Spermatozoon Activation

• Spermatozoa from the seminiferous tubule:
  
  • Quiescent and incapable of fertilising an ovum
• Spermatozoa from the vas deferens:
  
  • They are capable of limited movement (whiplash activity)
  • They have limited capability for fertilising the ovum
• FULL ACTIVITY and fertilising capability is only achieved once the spermatozoa are within the female reproductive tract
  
  • This is CAPACITATION

Question 2

Outline the process of capacitation and the importance of the acrosome reaction

Answer 2

Capacitation of the Spermatozoa

• 1. The glycoprotein coat has a protective function in the vagina but once it enters the uterus it is no longer necessary
• 2. Changes in the surface membrane 9 some of which lead to the acrosome reaction when in close proximity to the ovum
• 3. Once in the uterus, the spermatozoa achieve real whiplash movement of the tail
• All of these changes are OESTROGEN DEPENDENT
• All the components of capacitation are also CALCIUM DEPENDENT (present within the fluid of the female reproductive tract)

Acrosome Reaction

As the sperm approaches the ovum, you get changes appearing in the membrane

1. Spermatozoon binds to the ZP3 glycoprotein on the zona pellucida
2. Once bound to ZP3, progesterone stimulates calcium influx into the spermatozoon
3. This results in the calcium-dependent acrosome reaction
   NOTE: the binding of the spermatozoon to ZP3 is short-lived
4. This enables an exposed spermatozoon recognition site to bind to a second glycoprotein (ZP2)
5. Once ZP2 has bound, the acrosome releases its enzymes allowing penetration of the zona pellucida so that the head of the spermatozoon can enter the ovum​

Question 3

Explain the processes of fertilization and the development of the conceptus including reference to the roles of specific hormones during these processes.

Answer 3

Fertilisation

• Normally occurs within the Fallopian Tube
• Results in the expulsion of the second polar body
• Leads immediately to the zonal reaction:
  
  • Cortical granules release molecules which degrade the zona pellucida (including ZP2 and ZP3)
  • This prevents further binding of other sperm
  • This is also calcium dependent
• Once diploidy is established, the zygote will start dividing to form the initial 29 cell conceptus

Development of the Conceptus

• The conceptus continues to divide as it moves down the fallopian tube to the uterus (3-4 days)
• Until implantation, the developing conceptus receives its nutrients from uterine secretions
• As cell division continues you will have a ball of cells with the outside cells receiving the nutrients but the inner cells will be receiving less and less nutrients
• The free living phase will last about 9-10 days
• This all occurs while the woman is in the luteal phase so her oestrogen and progesterone will be high
• The conceptus first compacts to an 8916 cell morula
• Then it becomes a blastocyst with:
  
  • An inner cell mass 9 becomes the embryo
  • Trophectoderm 9 becomes the chorion (which becomes the placenta)
• This transfer to the uterus is facilitated by increasing progesterone: oestrogen ratio (in the luteal phase)
• Implantation leads to the establishment of physical and nutritional contact with maternal tissues

Summary of the journey up to implantation (image)

Question 4

Explain the processes of implantation and decidualization, including reference to the roles of specific hormones during these processes.

Answer 4

Implantation

• INVASIVE in humans
• Involves an initial ATTACHMENT phase
  
  • The outer trophoblast cells make contact with the uterine surface epithelium
• Implantation is all about establishing a system for getting the nutrients from the mother to all the cells within the embryo
• Within a few hours you get decidualisation of the underlying uterine stromal tissue
• NOTE: decidua = thick layer of modified mucous membrane which lines the uterus during pregnancy and is shed with afterbirth
• This whole process requires PROGESTERONE DOMINATION in the presence of oestrogen

Attachment

• Many molecules are involved in the process of attachment but a few are of particular importance
• TWO molecules of particular importance:

Leukaemia Inhibitory Factor (LIF)

• InterleukinK11 (ILK11)
• Leukaemia Inhibitory Factor (LIF)
  
  • From endometrial secretory glands (and blastocyst?)
  • Stimulates adhesion (attachment) of blastocyst to the endometrial cells

InterleukinK11 (ILK11)

• Also released from the endometrial cells
• Released into the uterine fluid
• Many other molecules are involved in this process:
  
  • E.g. HB9EGF (heparin binding epidermal growth factor) involved in stimulating LIF production

Decidualisation Reaction

• Invasion of the underlying uterine stromal tissue by the trophoblast cells of the blastocyst
• Within hours you get:
  
  • Increased vascular permeability in the invasion region, association with oedema of tissues
  • Localised changes in intracellular composition and progressive sprouting and growth of capillaries
  • Glandular epithelial secretion
  • Glycogen accumulation in stromal cell cytoplasm
• Factors involved in the decidualisation reaction:
  
  • Mainly ILK11
  • Histamine
  • Certain prostaglandins
  • TGFb 9 promotes angiogenesis

Question 5

Hormones of pregnancy: Explain the physiological roles of specific hormones in pregnancy, their sites of synthesis and how their circulating concentrations change throughout pregnancy.

Answer 5

Hormone Changes and Effects during Pregnancy

• hCG reaches a peak around 8 weeks
• hCG is vital for the first 6 weeks at least as its purpose is to replace LH
• In a normal menstrual cycle during the luteal phase, high oestrogen and progesterone will have a negative feedback effect on LH and FSH so their levels will be low
• So in a normal menstrual cycle, the oestrogen and progesterone levels will eventually fall and you’ll get menstruation
• The same thing will happen in pregnancy but you still need increasingly large amounts of oestrogen and progesterone
• To maintain the stimulation of oestrogen and progesterone, hCG is being produced by the trophoblast cells
• This can bind to the LH receptors on the corpus luteum hence replacing the effects of LH
• For the first few weeks of pregnancy, if you remove the ovaries then the pregnancy will be terminated because you have taken away the source of oestrogens and progestogens
• After about 5 weeks or so, the placenta will have taken over the production of hormones so the ovaries are no longer necessary
• Oestrogen and Progesterone levels increase throughout pregnancy and right until the end, progesterone remains the DOMINANT influence
• Human Placental Lactogen 9 produced by the placenta
  
  • It is a growth hormone
  • It has prolactin like effects
  • It is important for the growth and development of the foetus

Question 6

Discuss the production of oestrogens in pregnancy

Answer 6

Progesterone & Oestrogen Production during Pregnancy

First 40 days

• Produced in corpus luteum (in maternal ovary)
  
  • stimulated by hCG (produced by trophoblasts) which acts on LH receptors
• Essential for developing fetoplacental unit
• Inhibits maternal LH & FSH (-ve feedback)

From day 40

• Placenta starts to take over

There are THREE components that are important in providing steroids:

• Mother
• Foetus
• Placenta
• The mother provides the precursors which tends to be pregnenolone which leads to progesterone
• The progesterone is then going to lead to steroid production by the foetus (in the foetal adrenals)
• The maternal AND foetal adrenals produce a precursor (which is an androgen) dehydroepiandrosterone sulfate (DHEAS)
• DHEAS is then taken up by the placenta to produce OESTRADIOL
• DHEAS is also used by the placenta to produce small amounts of oestrone
• If you wanted to look at the health of the baby and you look at the levels of the different oestrogens, oestradiol and oestrone will NOT tell you much about the health of the baby
• This is because oestradiol and oestrone come from the MOTHER as well as the foetus
• So if there is a change in the oestradiol level, it wont necessarily tell you that the foetus is in distress
• OESTRIOL IS THE MAIN OESTROGEN OF PREGNANCY
• This is, molecule for molecule, weaker than oestradiol but it is produced in large amounts during pregnancy
• The source of oestriol is FOETAL
• It comes from the conjugation of DHEAS in the foetal liver to form 16aK hydroxy DHEAS
• This then goes to the placenta which deconjugates it and uses it to form OESTRIOL
• If you are worried about the foetus and you measured the oestrogens and you looked at the ratio of oestriol: oestradiol and oestrone or the ratio of oestriol: total oestrogens and you saw a change, then you have reason to be worried about the foetus

Question 7

Outline changes of maternal hormones in pregnancy

Answer 7

Changes in Maternal Hormones

• Most of these hormones increase
• During pregnancy the pituitary gland ENLARGES
• hGH (human growth hormone) decreases as the placental hGH variant increases towards term
• Patients with hypothyroidism are asked to take more T3
• High PTH leads to high Ca
• Placental hGH is similar to GH but it will inhibit the production of hGH in the mother

Question 8

Explain the hormonal regulation of parturition

Answer 8

• Parturition is all about contraction of actin and myosin filaments
• This contraction requires CALCIUM
• So to initiate contraction you need to increase the intracellular calcium concentration
• The calcium can come from stores within the cell (e.g. microsomes) or it can come from outside
• Oestrogen stimulates production of prostaglandins by the endometrial cells
• The prostaglandins stimulates the production and release of calcium into the cytoplasm from intracellular stores

·Main Point: oestrogen tends to INCREASE the chance of contraction

• In addition, oestrogen stimulates oestrogen receptors on the endometrial cells
• Progesterone has the OPPOSITE EFFECT TO OESTROGEN
  
  • It inhibits prostaglandin synthesis
  • It inhibits oestrogen receptors
• So progesterone keeps the effects of oestrogen under control
• At parturition, oxytocin will be released which will bind to its receptor and open calcium channels allowing calcium ions to move in from outside
• In addition, you need the progesterone: oestrogen ratio to change and this is driven by the foetus
• Once the foetus reaches a certain size, the production of steroids is switched from the production of progesterone to the production of oestrogen
• This means that oestrogen dominates the process
• This leads to prostaglandin production in the endometrial cells, release of calcium into the cytoplasm thus increasing cytoplasmic calcium ion concentration and the promotion of actin9myosin contraction
• Oxytocin

Uterine contraction

Cervical dilation

Milk ejection

And you will also have the effects of oxytocin

Question 9

Explain the hormonal regulation of lactation

Answer 9

• Breast feeding involved TWO hormones that are stimulated by the same neuroendocrine reflex arc:
  
  • Prolactin 9 promotes milk synthesis
  • Oxytocin 9 promotes milk ejection

Prolactin in women causes amenhoria

High prolactin loss of libido in men

Not enough leptin – switches off syseptin neuron

Marathon runners can get a low testosterone or females can get galactorrhea à has to do with the nipple stimulation