Fertilization + implantation

• Fertilization


  The process where the sperm fertilizes the ovum
• Implantation


  The process where the blastocyst attaches to the uterine wall
• Fertilization of the ovum by the sperm

  1. C- Chemoattraction of the sperm to the ovum

  2. A- Adherence to the zona pellucida

  3. P- Penetration of the zona pellucida and the acrosome reaction

  4. A- Adherence of the sperm head to the cell membrane of the ovum, with breakdown of the area of fusion & release of the sperm nucleus into the cytoplasm of the ovum

  
• Millions of sperm are deposited in the vagina during intercourse. Eventually, 50–100 sperm reach the ovum, & many of them contact the zona pellucida.
• Fertilization and implantation


  1. Sperm bind to a receptor in the zona followed by the acrosomal reaction

  2. Various enzymes are released, including the trypsin-like protease acrosin which facilitates the penetration of the sperm through the zona pellucida

  3. When one sperm reaches the membrane of the ovum, fusion to the ovum membrane is mediated by fertilin

  4. The fusion provides the signal that initiates a reduction in the membrane potential of the ovum that prevents polyspermy

  5. This transient potential change is followed by a structural change in the zona pellucida that provides protection against polyspermy on a more long-term basis
• Implantation

  1. The developing embryo, now called a blastocyst, moves down the tube into the uterus (takes about 3 days)

  2. The blastocyst reaches the 8 or 16 cell stage

  3. The zona pellucida disintegrates and the blastocyst becomes surrounded by an outer layer of syncytiotrophoblast and an inner layer of cytotrophoblast

  4. The syncytiotrophoblast cells produce hCG, estradiol and EGF which signal implantation

  5. The implantation site is usually on the dorsal wall of the uterus

  6. A placenta then develops, and the trophoblast remains associated with it

  
• Implantation contd.

  1. The microvilli connect with the pinopodes on the uterine epithelium linking the blastocyst to the uterus

  2. The cytotrophoblast cells produce hyperglycosylated hCG which drives cytotrophoblast cell invasion of the uterus

  3. The uterine epithelium seals itself closed, burying partly the cytotrophoblast inside the uterine wall

  4. The hyperglycosylated hCG continues driving invasion to >30% of thickness of the uterus

  5. It also drives growth of cytotrophoblast cells in the blastocyst complex

  6. The columns of cytotrophoblast cells develop in the blastocyst

  7. The columns of cytotrophoblast grow into villous structures of hemochorial placentation

  
• Human chorionic gonadotropin (hCG)

  A hormone produced primarily by syncytiotrophoblastic cells of the placenta and cytotrophoblast during pregnancy.

  
  

  Smaller amounts of hCG also produced by in pituitary gland , liver, colon.
• hCG


  • Stimulates the corpus luteum to produce progesterone to maintain the pregnancy.
  • A glycoprotein composed of two subunits, the alpha and beta subunits
  • Primarily catabolized by the liver, although about 20% is excreted in the urine
  • The beta subunit is degraded in the kidney to make a core fragment which is measured by urine hCG tests.

  Trophoblastic cancers (hydatidiform mole, choriocarcinoma, and germ cell tumors) are associated with high serum levels of hCG-related molecules.
• Function of hCG


  Cytotrophoblast: Hyperglycosylated hCG

  Syncytiotrophoblast: hCG

  Both of these:

  1. P- Promotes progesterone production,
  2. U- uterine spiral artery angiogenesis
  3. F- fetal growth
  4. F- formation of hemochorial placentation
  5. U- uterine growth & myometrial muscle relaxation
  6. T- trophoblast differentiation
  7. I- Invasion as part of implantation
  8. I- immune & macrophage suppression
• Clinical significance of hCG

  Important hormone in pregnancy, clinically used in detection of early pregnancy, along with serial measurement during pregnancy and pregnancy-related complications

  
• hCG levels rise exponentially in the first trimester of pregnancy, doubling about every 24 hours during the first 8 weeks. The peak is usually around 10 weeks of gestation and then levels decrease until about the 16th week of gestation & remain fairly constant until term.

  
• Patients who have hCG levels that plateau prior to 8 weeks or that fail to double commonly have a nonviable pregnancy, whether intra-uterine or extra-uterine.

  
• Extra-uterine (ectopic) pregnancies usually have a rate-of-rise that is low without the typical doubling. However, given the large range of normal hCG levels & inconsistent rates-of-rise of this hormone, checking serum levels is typically paired with ultrasound evaluation to improve sensitivity and specificity.
• Immunological tests for the diagnosis of pregnancy

  • F- Fluoroimmunoassay (FIA)
  • A- Agglutination inhibition test
  • I- Immunoradiometric assay (IRMA)
  • R- Radioimmunoassay (RIA)
  • E- Enzyme linked immunosorbent assay (ELISA)
  • D- Direct agglutination test
• Other uses of ß HCG testing

  • Diagnosing ectopic pregnancy
  • Monitoring patients who have underwent in vitro fertilization and embryo transfer
  • Follow up of patients after treatment of hydatidiform mole and choriocarcinoma

  
• Specimen requirement and procedure for hCG detection

  Immunometric assay: Two antibodies- a fixed antibody & a radiolabeled antibody are used , which adhere to different sites on the HCG molecule, sandwiching and immobilize it to make it detectable –measurement of the amount of labeled hCG gives quantitative result
• Serum testing is much more sensitive and specific than urine testing. Urine assays detect total hCG levels greater than 20 mIU/mL.

  
  

  Advantages of urine testing: more convenient, affordable, comfortable for patients, gives fast result in 5 to 10 minutes, and does not require a medical prescription.
• Urine False Positives


  • Human error in result interpretation
  • Blood or protein in the urine
  • Ectopic production of hCG
  • Exogenous hCG
  • Drugs (aspirin, carbamazepine, methadone, high urinary pH and seminal fluid)
• Urine False Negatives: Early measurement after conception, dilute urine specimen
• Failure to reject the fetal graft

  Normally,

  The fetus & the mother are two genetically distinct individuals & the fetus is a transplant of foreign tissue in the mother. However, the transplant is tolerated, and rejection reaction does not occur. This may be because the placental trophoblast does not express the polymorphic Major Histocompatibility Genes class I & class II (MHC genes) and instead expresses Human Leukocyte Antigens HLA-G, a nonpolymorphic gene. Additionally, there is Fas ligand on the surface of the placenta, and this binds to T cells, causing them to undergo apoptosis.
• Endocrinal changes- Third month to term

  1. Placenta secretes enough progesterone and estrogen to maintain the pregnancy

  2. The function of the corpus luteum begins to decline after 8 weeks of pregnancy, but it persists throughout pregnancy

  3. Progesterone secretion of the placenta is limited only by the amount of precursor cholesterol, delivered to the placenta

  4. Progesterone maintains uterine quiescence during pregnancy

  5. The secretion of estrogen is by both- the fetus and the placenta

  6. The fetal adrenal gland secretes dehydroepiandrosterone (DHEA) which is converted to androstenedione and testosterone by the fetal liver

  7. The placenta expresses aromatase which converts the androstenedione & testosterone from the fetus into estrogens, estriol being the primary one
• Peripheral Effects of Hormonal Changes

  • Massive growth of the uterus, especially the myometrium
  • Increased growth of all components (glands, stroma, and fat) of the breasts
  • Increased prolactin secretion by the pituitary in response to elevated estrogens
• Human Chorionic Somatomammotropin (hCS)


  Secreted by the syncytiotrophoblast, is lactogenic and has very little growth-stimulating activity. Secretion markedly increased during latter half of pregnancy.

  
  

  Secretion of growth hormone from the maternal pituitary is not increased during pregnancy & may actually be decreased by hCS.

  
  

  Functions as a "maternal growth hormone of pregnancy" causing nitrogen, K, and Ca retention, increased lipolysis & ketogenesis, and decreased maternal glucose utilization seen in pregnancy.
• During the second trimester pregnancy, the mother is in a hyperinsulinemic state with peripheral resistance to the metabolic effects of insulin. This reserves glucose for fetal needs & the mother depends more heavily on fatty acids as a source of energy. Under these conditions even modest fasting can cause ketosis.
• Human chorionic somatotrophin--> accounts for gestation diabetes because of its anti insulin effects.

  hPL (hCS) is secreted in proportion to the size of the placenta and is an index of placental well-being.
• Estrogen is in oral contraceptives because it suppresses FSH and LH production preventing ovulation.

  
• Changes in hormone levels during pregnancy

  1. Estrogen levels increase 30-fold by childbirth

  2. Raised estrogen levels suppress FSH & LH production, preventing ovulation

  3. Estrogen induces growth of fetal tissues and is necessary for fetal lung & liver maturation

  4. Estrogen promotes fetal viability by regulating progesterone production and triggering fetal synthesis of cortisol

  5. Estrogen stimulates maternal tissue growth, leading to uterine enlargement, mammary duct expansion and branching

  
• Progesterone in pregnancy


  • Secreted by the ovaries, placenta (during pregnancy), and the inner layer of the adrenal cortex
  • The placenta takes over the synthesis and secretion of progesterone throughout the pregnancy as corpus luteum degenerates
  • Suppresses FSH and LH
  • Inhibits uterine contractions, protecting the fetus from preterm birth
  • Decreases in late gestation, allowing uterine contractions to intensify and eventually progress to true labor
• Other endocrine effects of pregnancy

  1. Anterior pituitary enlarges and ramps up hormone production, raising levels of thyrotropin, prolactin, and ACTH

  2. Thyrotropin, in conjunction with placental hormones, increases thyroid hormone production, raising maternal metabolic rate

  3. Prolactin stimulates enlargement of mammary glands in preparation for milk production

  4. ACTH stimulates maternal cortisol secretion, contributing to fetal protein synthesis

  5. Increased parathyroid levels mobilize calcium from maternal bones for fetal use
• Other placental hormones(atleast 14)

  • Human placental fragments probably produce:
  • Pro-opio-melanocortin (POMC)

  In culture, they release:

  • Corticotropin-releasing hormone (CRH)
  • β-endorphin
  • α-melanocyte-stimulating hormone (MSH)
  • Dynorphin A
  • GnRH and Inhibin(Regulates hCG secretion as GnRH stimulates inhibin which inhibits hCG secretion)
  • Leptin(by trophoblast cells and amnion cells)
  • Prolactin
  • hCG α subunits
• Fetoplacental unit


  1. Placenta synthesizes pregnenolone and progesterone from cholesterol

  2. Some pregnenolone enters the fetus

  3. Fetal liver has substrate for dehydroepiandrosterone sulfate (DHEAS) formation

  4. In fetal adrenal, DHEAS is converted to 16-hydroxy-dehydro-epiandro-sterone sulfate (16-OHDHEAS)

  5. Some progesterone enters fetal circulation and provides substrate for cortisol and corticosterone formation in fetal adrenal

  6. DHEAS & 16-OHDHEAS transported back to placenta, where DHEAS forms estradiol and 16-OHDHEAS forms estriol
• Maternal changes of pregnancy

  • Full term pregnancy lasts approximately 270 days (38.5 weeks) from conception
  • Due date counted from last menstrual period (284 days), assuming conception on 14th day
  • Pregnancy divided into three 13-week trimesters

  
• Cardiovascular and renal changes in pregnancy

  1. Hyperdynamic circulation: Cardiac output increases but peripheral resistance decreases, no hypertension

  2. Blood pressure declines in first trimester, gradually rises toward pre-pregnancy levels

  3. Blood volume increases

  4. GFR increases, plasma glucose increases and renal threshold decreases, leading to glucose in urine
• Endocrine changes in pregnancy

  1. Anterior pituitary enlarges by one-third due to estrogen-driven lactotroph hyperplasia

  2. Postpartum pituitary necrosis (Sheehan syndrome) can occur after obstetric hemorrhage

  3. Estrogen increases circulating steroid-binding globulins, increasing bound hormone levels but free T4 is normal

  4. Hyperthyroidism increases risk of preterm delivery, hypothyroidism is unusual

  5. Estrogen increases renin secretion, increasing renin-angiotensin-aldosterone activity, causing fluid retention and hemodilution
• Changes induced near the end of pregnancy

  1. Pubic symphysis, cervix, and vagina become more distensible, facilitated by placental relaxin

  2. Parturition in humans is normal in the absence of ovaries

  3. Estrogen increases oxytocin receptors in myometrium, increasing sensitivity to oxytocin
• Mechanism responsible for the onset of labor

  1. Increase in circulating estrogens produced by increased DHEAS

  2. Estrogen makes uterus more excitable, increases gap junctions, and causes more prostaglandin production which in turn causes uterine contractions.

  3. Increased CRH secretion by fetal hypothalamus and placenta

  4. increased fetal ACTH and cortisol(Cortisol hastens the maturation of the respiratory system)
• Intrauterine fetal death


  Toxic products from the fetus increase prostaglandin release in the uterus, initiating contractions and spontaneous abortion (miscarriage)
• Premature labor


  • Intramuscular 17α-hydroxyprogesterone decreases incidence of premature labor
  • Progesterone relaxes uterine smooth muscle, inhibits oxytocin action, and reduces gap junction formation, inhibiting onset of labor
• Development of the breasts during pregnancy

  1. Proliferation of mammary ducts stimulated by estrogens

  2. Development of lobules stimulated by progesterone

  3. During pregnancy, increased prolactin, estrogens, and progesterone produce full lobulo-alveolar development