Chapter 3 OB Voice Recording

Etilization and Embryonic Development Study Notes

Fertilization Process

  • Fertilized Ovum: The fertilized ovum is commonly referred to as a zygote, which is the product of the union of a sperm and an ovum.

  • Location of Fertilization: Fertilization occurs in the outer third of the fallopian tube.

  • Definition of Fertilization: It is defined as the meeting of the sperm and egg.

Chromosome Count in Human Cells

  • Chromosomes in Body Cells: Each body cell contains 46 chromosomes, located within the nucleus.

  • Chromosomal Composition: This includes 23 pairs of chromosomes consisting of 22 pairs of autosomes and 1 pair of sex chromosomes.

    • In females: XX

    • In males: XY

Teratogens and Their Effects

  • Teratogens: Agents that cause birth defects.

    • Examples of teratogens include:

    • Smoking

    • Alcohol

    • Infections

Spermatogenesis and Oogenesis

  • Spermatogenesis: The process through which one spermatogonium can produce four spermatids, which subsequently become mature sperm.

  • Oogenesis: In contrast, one oocyte produces only one ovum; the others degenerate (called polar bodies).

Implantation and Development

  • Site of Implantation: Implantation of the blastocyst occurs in the uterus, specifically in the endometrium, known as decidua during pregnancy.

  • Development Timeline: From fertilization to implantation takes about 7 to 10 days.

Determining Fetal Sex

  • Sex Determination: The sex of the baby is determined by the sperm's chromosome:

    • A sperm carrying a Y chromosome will produce a male fetus.

    • Sperm carries either X or Y chromosomes.

  • Survival of Sperm: The survival rate of X and Y sperm can be influenced by the pH of the female reproductive tract, with studies suggesting the acidic environment favors X sperm.

Cleavage and Early Embryo Formation

  • Cleavage: After fertilization, the zygote undergoes mitotic divisions, known as cleavage, forming a morula and then a blastocyst.

    • The zygote size does not increase during cleavage, individual cells become smaller as they divide.

    • Cleavage continues as the zygote moves toward the uterus.

Amniotic Fluid Characteristics

  • Amniotic Fluid: Acts as a protective cushion, facilitating fetal movement and regulating temperature.

    • Normal volume ranges:

    • 10 weeks: ~30 ml

    • 20 weeks: ~350 ml

    • 37 weeks: ~1000 ml (1 liter)

    • Presence of vernix and lanugo (fine hair) in the fluid is normal.

  • Conditions:

    • Polyhydramnios: Excessive amniotic fluid (>2 liters), can indicate fetal abnormalities.

    • Oligohydramnios: Reduced amniotic fluid (<300 ml), may signify renal issues in the fetus.

Development of Structures During Pregnancy

  • Yolk Sac: Functions during the early weeks for hematopoiesis, until the fetal liver takes over production of red blood cells.

    • Lasts from week 5 to week 12 of gestation.

  • Germ Layers: Three germ layers develop during embryogenesis:

    • Ectoderm: Outermost layer (skin, hair, nervous system).

    • Mesoderm: Middle layer (muscle, bone, cardiovascular system).

    • Endoderm: Innermost layer (lining of the digestive tract, liver).

Stages of Prenatal Development

  • Zygote: From fertilization to implantation.

  • Embryo: From the second to the eighth week of development post-fertilization.

  • Fetus: Begins from the ninth week of gestation.

  • Age of Viability: Defined as 20 weeks gestation; indicates potential for survival outside the womb.

Development of the Fetal Circulation

  • Fetal Blood Circulation: Includes shunts that bypass the non-functioning fetal lungs and liver:

    • Foramen Ovale: Connects right and left atria, bypassing pulmonary circulation.

    • Ductus Arteriosus: Connection between pulmonary artery and aorta, allowing blood to bypass the lungs.

    • Ductus Venosus: Bypasses the liver, allowing oxygenated blood from the placenta to enter systemic circulation directly.

Characteristics of Twins

  • Types of Twins:

    • Monozygotic: Identical twins, one zygote divides into two.

    • Dizygotic: Fraternal twins, two separate zygotes develop from two eggs fertilized by two sperm.

Accessory Structures of Pregnancy

  • Placenta: Connects mother and fetus for nutrient and gas exchange, comprising maternal and fetal sides.

    • Hormones secreted by the placenta include:

    • Progesterone: Maintains the uterine lining, relaxes the uterus.

    • Estrogen: Prepares the breasts for lactation.

    • hCG: Detected in pregnancy tests, supports progestational changes.

    • hPL: Provides glucose to the fetus, decreases maternal insulin sensitivity.

Conclusion

  • Understanding the complexities of prenatal development, from fertilization through stages of growth, is crucial for fields like genetics, obstetrics, and maternal-fetal medicine.