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.