development and inheritance1

Fertilization (Conception)

• Mature Egg (Ovum)

  • Contains 23 chromosomes (half boy, haploid count).

• Sperm

  • Head: Contains nucleus with 23 chromosomes.

  • Neck: Rich in mitochondria (for energy).

  • Flagellum: For motility (only cell in the human body with a flagellum).

Sperm Contribution

• Only the 23 chromosomes from the sperm head enter the egg during fertilization.

• The neck and tail do not become part of the baby.

Mitochondrial Inheritance

• Babies inherit mitochondria from their mothers (via the egg).

• Mitochondrial dysfunction is traced through the maternal family history.

Egg Protection

• Mature egg to immature egg ratio is one to one.

• The egg provides cytoplasm, organelles, and mitochondria.

• The egg has multiple barriers for protection:

  • Corona Radiata:

    • Comprised of granulosa cells (outer layer).

  • Zona Pellucida:

    • Inner layer plays a role in selecting the most worthy sperm.

Acrosome and Enzymes

• Sperm contains an acrosome filled with enzymes at the tip of its head.

• Enzymes reduce activation energy, speeding up reactions.

• The acrosome releases enzymes to break down the corona radiata.

Polyspermy Prevention

• Polyspermy:

  • Multiple sperm penetrating the egg.

• Only one sperm should insert 23 chromosomes into the egg.

• The zona pellucida filters sperm.

• Polyspermy leads to aneuploidy (incorrect chromosome count), resulting in a nonviable pregnancy and often miscarriage in the first trimester.

Capacitation

• Capacitation allows the sperm flagella to increase its lifting capacity.

• Movement in the tail enhances the likelihood of reaching the egg.

Intercourse Timing

• Intercourse should occur within two to three days of ovulation for conception.

Cervical Mucus

• Cervical mucus helps sperm live longer.

• Estrogen during the first two weeks of the menstrual cycle increases cervical mucus production.

• Cervical mucus presence indicates the best time for intercourse to conceive.

• Absence of cervical mucus indicates a time to avoid intercourse to prevent pregnancy.

Prevention Mechanisms Against Polyspermy

• Fast Block:

  • Sodium channels open, causing an influx of sodium and depolarization of the zona pellucida.

  • This electrical charge repels other sperm.

• Slow Block:

  • Involves the formation and exocytosis of secretory vesicles, requiring energy (active transport).

  • Hardens the zona pellucida to prevent further sperm penetration, acting as an insurance policy.

• Both mechanisms ensure a diploid count (46 chromosomes).

Implications of Chromosome Count

• Any chromosome count other than diploid results in a nonviable egg, preventing embryo development and implantation.

Human Chorionic Gonadotropin (hCG)

• hCG is produced when sperm meets egg.

• It is detected in pregnancy tests (urine and blood samples).

• A positive pregnancy test alone doesn't guarantee a viable pregnancy; implantation and heartbeat are also required.

Cleavage Stage

• Following fertilization, mitosis occurs rapidly.

• The cell divides frequently to ensure every cell has 46 chromosomes.

• It takes about three days for the egg to move through the fallopian tube into the uterus.

• Implantation occurs approximately 7 to 10 days post-conception.

• Implantation bleeding may occur, mistaken for a light period.

Embryo Development

• An embryo appears as a ball of cells on an ultrasound.

• Before fetal development (around weeks 9-10), it's difficult to determine pregnancy viability via ultrasound.

Morula Stage

• By day four, the embryo is at the morula stage.

• Fluid starts to fill the embryo, leading to cell arrangement.

• Cells differentiate to contribute to the baby and the placenta.

Blastocyst

• A blastocyst is seen around day five.

Trophoblasts and Inner Cell Mass

• Trophoblasts:

  • Outer cells that implant into the endometrial lining and form the baby's half of the placenta.

• Inner Cell Mass:

  • Inner cells that develop into the baby and give rise to the three germ layers.

Three Germ Layers

• Ectoderm:

  • Gives rise to nerve cells.

• Mesoderm:

  • Gives rise to muscle tissue, connective tissue (including bone, cartilage, and blood).

• Endoderm:

  • Gives rise to epithelium.

Gastrulation

• The process by which the inner cell mass is formed.

Neurulation

• Development of the primitive nervous system.

• Occurs around one month of pregnancy.

Notochord

• The mesoderm develops the notochord.

• The notochord gives rise to the central nervous

• Prenatal support, including B6 and folic acid, is essential for successful neurulation.

Fetal Heart

• Arteries carry oxygenated blood, and veins carry deoxygenated blood.

• Mom's heart does most of the work, so oxygen flow is referenced by mom's heart.

Fetal Heart Development

• The baby's heart develops but doesn't function independently.

• Screening for cyanosis (blue skin) occurs after birth.

Fetal Heart Structure

• In the fetal heart, there is only one atrium (no wall between left and right).

• After birth, a wall (fossa ovalis) needs to form to separate the atria.

• Foreamen ovale:

  *   Opening in between what would be the right and feft atrium of the fetal heart.
  

• The foramen ovale allows blood to bypass the lungs.

Post-Birth Heart Changes

• The foramen ovale closes after birth to separate oxygenated and deoxygenated blood.

• Failure to close results in a patent foramen ovale (PFO).

Chorionic Villi

• Tree-like branching formation of blood vessels in the placenta.

• Chorionic villi sampling (CVS) is a prenatal test done between weeks 10-12 of pregnancy.

Twins

• Monozygotic Twins:

  • Identical twins, one egg and one sperm.

• Dizygotic Twins:

  • Fraternal twins, two eggs and two sperm.

Postpartum Uterus

• The uterus takes up to six weeks to shrink back to its normal size after delivery.

Gonad Development

• Gonads develop around week five of pregnancy.

• They become ovaries if the genetic profile is XX and testes if it is XY.