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.