Reproductive Physiology, Fertilization, & Pregnancy

Orgasm

• Under Sympathetic NS control

• M and F: Reproductive muscles contract (bulbospongiosus m.), 

  • psychological feelings of extreme pleasure and general sympathetic NS response

• Ejaculation – typically only occurs in males

  • Movement of sperm in vas deferens initiated, accessory glands secrete and all combines to make semen

  • Constriction of internal urethral sphincter –prevents semen entering bladder

  • Bulbospingiosus mm  contracts to propel semen out of penis

• After orgasm, males have latent period – cannot have additional orgasm.  

  • Females do not – multiple orgasms occur in single sexual experience.

Sperm to Egg: Copulation window

• Copulation window: 2-3 days before ovulation; 24 hours after ovulation; 4 days where fertilization is possible

• Sperm propelled into vagina by ejaculation

  • Only about 5 inches from oocyte 

    • may take as long as 1-2 hours to get there

  • Many sperm never make it near the secondary oocyte

    • Leak out of vagina, blocked by cervical mucus, destroyed by vaginal acid, phagocytized in uterus

    • Only 200-1000 sperm actually make it to the oocyte

  • Sperm are helped by some favorable currents (reverse peristaltic contractions) in uterus, 

    • there are also currents in other direction (to move oocyte toward uterus)

Sperm to Egg: Capacitation & How find the oocyte

1. Fresh sperm can’t penetrate egg  🡪  need to be capacitated

   1. 6-8 hour process

      • Mobility enhanced, membrane becomes fragile so acrosome can release hydrolytic enzymes

2. They sniff for ‘em

   1. Contain “olfactory receptors” that detect chemicals released by the oocyte

   2. Given a choice or left or right 🡪  most sperm correctly choose the fallopian containing the oocyte

Fertilization: Corona radiata cells

• Once at secondary oocyte, sperm encounters barriers

  • Corona radiata cells – easily passed by dissolving intercellular cement

    • Granulosa cells then fall away from the oocyte

  • Zona pellucida – membrane barrier that must be destroyed

    • Sperm heads bind to receptors on surface

      • Acrosome of sperm breaks open and releases lysozymes to “eat” holes thru the zona pellucida

      • 100’s of sperm will do so and never get close to breaching barrier

        • Need lots of holes before the barrier is breachable

      • A later arriving sperm will bind to & actually breach the zona pellucida

Fertilization: Zona pellucida (up to DNA being pulled)

• membrane barrier that must be destroyed (hole created)

  • Sperm heads bind to receptors on surface

    • Acrosome of sperm breaks open and releases lysozymes to “eat” holes thru the zona pellucida

    • 100’s of sperm will do so and never get close to breaching barrier

      • Need lots of holes before the barrier is breachable

    • A later arriving sperm will bind to & actually breach the zona pellucida

    • Then that sperm will bind to the oocyte’s membrane receptors

    • It’s DNA will be “pulled” into the oocyte’s cytoplasm (but NOT it’s midpiece or tail)

Feralization: Zona pellucida (up to completion)

• NEXT, oocyte depolarizes and prevents any other sperm entry by destroying the oocyte’s membrane receptor site for sperm

  • Also causes any sperm currently attached to membrane to fall off

  • Also stimulates completion of meiosis II

• Secondary oocyte completes meiosis II 

  • the ovum nuclei and sperm nuclei merge 

  • Fertilization

Ca2+ shock wave

Ca2+ shock wave → kills sperm center → becomes impenetrable

In Vitro Fertilization

• When there are problems with the sperm reaching &/or penetrating the oocyte

• Give drugs to overstimulate the ovaries  -  Extract oocytes  -  either mix washed sperm with oocyte or insert sperm with a needle - wait till Morula or Blatocyst Stage  -  Then implant into uterus

Sexually transmitted diseases (STDs)

• Bacterial STDs – easily treatable, symptoms vary – death or infertility are worst cases

  • Gonorrhea, Chlamydia

• Viral STDs – difficult to treat, often not curable

  • Syphilis, HPV, HIV, Genital herpes

Zygote vs Cleavage

1. fertilized egg; immediately upon fertilization, new zygote begins mitosis

2. early stgaes of mitosis—2 layers of cells—provides a lot of cells that can be used as building blocks for the embryo—fallopian tube

Morula vs Blastocyst

1. lump of ~16 cells 3 days after fertilization - 2 layers - fallopian tube

2. lump of ~100 cells that has divided into 3 layers – still in the fallopian tube 

   • Next it falls into the uterine cavity  

     • blastocyst continues to multiple cells & is fed by uterine secretions → floats for 2-3 days

Blastocyst

• Begins secreting human chorionic gonadotropin (hCG) which keeps the corpus luteum secreting progesterone

  • What pregnancy tests detect

• implants into endometrium by day 21 (7 days post-ovulation)

  • erodes an area, feeding off the blood & nutrients  -  Then placenta forms

• Part of this structure will become the chorion which then later becomes the embryonic component of the placents

• ~ 2/3 of blastocysts fail to implant;  30% of implanted embryos miscarry

• Day 28 = uterine mucosa covers buried embryo

Embryonic & Placental Development: after implantation

• Embryo = implanted firmly & chorionic villi begin to form

• Chorion:  embryonic cells that secrete hCH

  • Chorionic villi: embryonic cells stretch out & form projections  -  

    • with help of mother’s tissues, these will combine to form the placenta

Placentation

• Placenta (afterbirth): highly vascularized structure that originates from both maternal & embryonic tissues. 

  • The umbilical cord projects from the placenta

    • where nutrients, oxygen, waste, etc are exchanged between the mother and fetus

  • Partially Functional ~ 3 weeks after ovulation (5 weeks from 1^st day of last period)

    • Fully function 🡪  end of 1^st trimester

  • End of 2^nd month  🡪  secreting estrogen, progesterones, + other hormones  🡪  now corpus luteum becomes inactive

Amniotic sac (cowl)

• extraembryonic membrane that surrounds & contains the fetus  --  amniotic fluid is also within the sac 

  • Protects fetus, makes it lighter, maintains homeostatic temperature, prevents embryonic tissues from sticking together & fusing

Yolk sac vs Allantios

1. forms from cells of primitive gut that hangs from ventral side of embryo

   • Forms part of the gut, produces earliest blood cells & blood vessels

2. out-pocket of tissue at caudal end of yolk sac

   • structural base for umbilical cord & becomes part of the urinary bladder

8th week after ovulation: Fetus & birth

• all organ system have been sketched out; End of embryonic period

• Fetus

  • 9th Week (3^rd month)

    • Organ development/maturation, ossification of bones, 

    • Months 6 & 7 🡪 24weeks gestations = earliest viability w/ modern medical equipment

      • Wt gain (but very little fat), myelination of spinal cord, eyes open, distal bones begin to ossify, 

  • Months 8 & 9

    • Fat deposited

• Birth = ~ 270 days after fertilization, ~ 280 days after 1^st day of last period,  or end of 10th lunar month

Ectopic pregnancy (signs)

• Fertilized eggs implants somewhere outside the uterus

  • Fallopian tubes (most common), ovaries, abdomen, or cervix

• Eventually ruptures where it is growing 🡪  lots of bleeding

• Signs

  • Early signs same as normal pregnancy

  • Problems 🡪  pain is usually 1^st indicators followed by blood loss or breakthrough bleeding, low blood pressure (due to blood loss), dizziness, faintain

    • Lower back pain, sharp pain in the pelvis or abdomen

Ectopic Pregnancy (treatment)

• Methotrexate injection to “dissolve” fertilized egg (stop division of rapidly growing cells which results in spontaneous abortion)

• Laproscopy, surgery

Pregnancy stats

• Only 1/3 of fertilized eggs result in pregnancy

• Fertility peaks at 25 // have 25% chance per month of getting pregnant

• Fertility begins to noticeably decline at 27

• At 20 = 9% miscarriage // at 35 = 18% miscarriage

• After 42, women have only 10% chance of pregnancy with their own eggs.

• At 40, ½ of eggs are chromosomally abnormal

• At 42, 90% of eggs are abnormal

Maternal effects

• 16 Weeks = fetus occupies entire pelvic cavity

• Uterus enlarges gradually = pushing gut back & up against the diaphragm

• Ribs spread a little so there is more room for the gut to move upward

• Center of gravity changes = lumbar curvature & waddling gait

• Relaxin hormone is secreted by Placenta  

  • causes ligaments to relax & stretch more -                                       also contributes to waddling gait

• Caloric Need = 300 additional calories daily

Childbirth & Labor (OT receptors, progesterone, Braxton Hick contraction)

1. Aka Parturition; Happens within ~15 days of due date  -  Due date is 280 days since 1st day of last menstrual cycle or about 270 days since fertilization

2. Labor - Last few weeks estrogen levels peak  

   • causes myometrium to make Oxytocin receptors (up-regulation)

   • Antagonizes (reduces) progesterone’s ability to control uterine muscles

   • Braxton Hick Contractions = weak, irregular contractions signaling high estrogen levels  🡪  “exercises” the uterus

     • Will have exercising “mini-contractions” throughout preg.

Labor: Fetal cells & Stage 1

• Fetal Cells produce oxytocin = stimulates more frequent & stronger contractions

  • stimulates mother’s hypothalamus to release more oxytocin from posterior pituitary

  • Stimulates placenta to secrete prostaglandins which is also a contraction stimulator

• Stage 1: Dilation (6-12 hours)

  • when true contractions start until cervix is effaced (thinned) & dilated ~10 cm

  • contractions increase in frequency & strength - eventually rupturing the amniotic sac

Labor: Stage 2-3

2. Expulsion Stage (50 minutes to 2 hours = 1st birth)

   • From full dilation until delivery

   • Contractions every 2-3 minutes

3. Placental Stage

   • about 15 minutes after birth

   • delivery or expulsion of the placenta (afterbirth)

   • Strong contractions not only expel the placenta but also compress uterine blood vessels reducing risks of hemorrhaging