Lecture Notes on Sex Chromosome Disorders, Embryonic Development, Fetal Development and Diagnostic Testing

Nondisjunction Disorders of Sex Chromosomes

  • Nondisjunction process occurs during anaphase I or anaphase II of meiosis.
  • Presence of a Y chromosome determines male sex.
  • Additional X chromosomes or removal of X chromosomes often leads to reproductive system issues and sterility, particularly in females.
  • Adding X chromosomes tends to result in cognitive decline, though the exact cause is not fully understood.

XYY Syndrome

  • Individuals with XYY syndrome are male due to the presence of a Y chromosome.
  • Sperm production is often significantly reduced, leading to potential sterility or impotence.
  • May have issues with impulse control and potentially be more violent.
  • Tend to be taller, often exceeding six feet.
  • There is no direct link between XYY syndrome and gigantism; they are separate conditions.

Embryonic Development Review

  • Heart starts beating around day 18 of week 3.
  • Blood vessels form in week 4.
  • Blood is pumped through the heart in week 5.
  • Heartbeat can be heard on ultrasound by the end of week 8.
  • hCG regulates the corpus luteum until the placenta forms. Both structures produce progesterone and estrogen to maintain the endometrium lining and prevent miscarriage

Remaining Lectures

  • Focus on embryo-to-fetus development, trimesters, technologies for detecting problems, the labor process, breastfeeding, and factors increasing the risk of structural malformations.
  • Review and potential review on Monday, and exam on Wednesday.

Treatment for X Chromosome Abnormalities

  • Chromosomal makeup is set at fertilization and cannot be changed.
  • Treatment focuses on managing the condition and improving quality of life through therapies like speech and physical therapy.
  • Ethical and legal restrictions prevent genetic editing of egg and sperm cells, which could potentially prevent such conditions.

Extraembryonic Layers and Placenta

  • By the end of week 8, yolk sac and allantois combine and shrink to form the umbilical cord, while the amniotic sac enlarges.
  • The placenta is formed from the joining of the mother's endometrial tissue and the embryo's chorion layer.
  • The placenta facilitates the exchange of oxygen and nutrients between mother and fetus.
  • The embryo/fetus is connected to the placenta via the umbilical cord.

Placenta Details

  • The placenta is about the size of a dinner plate at birth.
  • One side contains the chorion layer (fetal side), identified by a shiny membranous surface, while the other side connects to the mother's endometrium.
  • The umbilical cord connects to the side that is facing the fetus.
  • The placenta supplies oxygen-rich blood and nutrients to the fetus and removes waste and carbon dioxide, which are processed by the mother's body.
  • The placenta has an endocrine function, producing estrogen and progesterone to maintain the endometrium lining after the tenth week.

Endometrium Lining

  • The maintenance is necessary to prevent shedding and subsequent miscarriage.
  • The umbilical cord, formed from the allantois and yolk sac, acts as a tether between the fetus and placenta, enabling movement while ensuring a secure connection for nutrient and waste exchange.

Uterine Wall

  • The placenta consists of the endometrium layer (from the mother) and the chorion layer (from the trophoblast cells of the blastocyst).
  • These layers facilitate the exchange of oxygen, carbon dioxide, and nutrients via blood vessels.
  • The amniotic sac encases the embryo within the endometrium lining, with the placenta fully formed by week 10.

Fetal Development

  • After week 9, the embryo is considered a fetus. The embryonic stage focuses on specialization and differentiation, while fetal development is about refining these structures.
  • At week 9, the fetus is about six inches long and begins to move, though the mother cannot yet feel it.

Prenatal Testing and Chorionic Villi Sampling (CVS).

  • Prenatal testing is performed typically to detect suspected disorders. CVS is usually performed at the end of the first trimester.
  • CVS is used to detect chromosomal or genetic defects by sampling the chorionic tissue of the placenta.
  • CVS is used in conjunction with ultrasound.

Types of CVS

  • Transcervical CVS: Accesses the placenta through the vagina and cervix; used when the placenta is located towards the back of the uterus.
  • Transabdominal CVS: Accesses the placenta through the abdominal wall; used when the placenta is located towards the front of the uterus.
  • Both methods use ultrasound guidance to extract cells from the fetal side of the placenta for genetic analysis.
  • At twelve weeks, external reproductive organs are distinguishable; cartilage begins to harden into bone.

Second Trimester (Weeks 13-24)

  • The fetus actively turns, swims, and develops a regular sleep cycle.
  • The body grows in proportion to the head.
  • The fetus assumes the fetal position, often due to limited space in the amniotic sac.
  • Cartilage fully develops into bone, and soft hair, eyelids, and eyelashes form.
  • No new structural development occurs; refinement of existing structures takes place.
  • By the end of the second trimester, the fetus is about 12 inches long and weighs 1.5 pounds.

Fetal Refinement Explanation

  • Embryonic cells might not realize the formation of abnormal tissue, thus continuing the growth and refinement process.
  • The mother's body may sense genetic metabolic issues in the fetus, potentially leading to miscarriage.
  • However, the growing fetus often continues to refine those abnormalities
  • Abnormalities exist on a range with symptoms having a wide range of severity.

Obstetric Ultrasound

  • From the beginning to the end of the second trimester, an obstetric ultrasound may be used.
  • Ultrasound uses sound waves to make a visual image.
  • It allows the practitioner to observe the fetal heart rate and position. Measure it's growth and see any abnormalities.
  • It is considered a safe process on the body due to no radiation, medications, or needles.

Amniocentesis

  • Amniocentesis is another prenatal testing option for checking chromosomal and/or genetic abnormalities or fetal infection during the second trimester.
  • In Amniocentesis, a transducer is used to use a needle to hit the amniotic fluid.
  • They puncture the the amniotic sac, which may lead to higher risks of miscarriage.
  • Amniocentesis can potentially check for fetal infections and the sex of the offspring and any nondisjunction disorders during the process.

Amniocentesis and High Risk Patients

  • It is typically not used for high risk patients due to the sensitivity of high risk pregnancies.
  • It is sometimes a decision that women make not to do the test if they want the pregnancy to continue naturally.

Third Trimester (Weeks 25-39)

  • Brain cells rapidly form.
  • The digestive and respiratory systems are the last to develop.
  • Around the seventh month, the lungs can sustain life out of the womb outside the mother.
  • The testicles descend into the scrotal sac on males.
  • Fatty tissue develops.
  • The fetus turns upside down into a head-down position in the uterus.