Detailed Notes on Pregnancy and Human Development

Chapter 28: Pregnancy and Human Development

Fertilization to Embryo to Fetus

• Human Gestation Period: 280 days from the last menstrual period.
• Stages of Development:
  • Fertilization: Conceptus in the first week.
  • Embryonic Development:
  • 3 weeks: embryo (3 mm)
  • 5 weeks: embryo (10 mm)
  • 8 weeks: embryo (30 mm)
  • Fetal Development:
  • 12 weeks: fetus (90 mm)
• Embryonic Period: First 8 weeks post-fertilization.
• Fetal Period: Weeks 9-38 or until birth.

Fertilization Process

• Definition: Fusion of sperm and oocyte chromosomes forming a zygote. Successful fertilization usually involves monospermy (only one sperm fertilizing the egg).
• Sperm's Journey:
  • Delivered to the vagina, capacitated in the female reproductive tract.
  • Moves toward secondary oocyte through cumulus oophorus and corona radiata.

Steps to Fertilization

1. Approach: Sperm weaves through granulosa cells aided by enzymes.
2. Acrosomal Reaction:
   • Sperm binds to zona pellucida receptors, increasing Ca²⁺ levels, releasing acrosomal enzymes that break down the zona pellucida, clear path to oocyte.
3. Fusion of Membranes: Sperm binds to oocyte membrane and they fuse together.
   • Blocks to Polyspermy:
     • Oocyte membrane block: Additional sperm-binding receptors shed.
     • Cortical reaction: Triggered Ca²⁺ surge, causing granules to release enzymes that harden the zona pellucida.

Zygote Formation

• Two Pronuclei Fusion: Male and female pronuclei merge to form a zygote (2n chromosomes).

Cleavage and Early Development

• Cleavage Stages:
  • 2 days: 4-cell stage.
  • 3 days: Morula (solid ball).
  • 4 days: Early blastocyst (hollow ball).
  • 7 days: Implanting blastocyst (trophoblast and embryoblast).
• Implantation: Occurs in the functional layer of the endometrium, forming a two-layered embryonic disc (epiblast and hypoblast).

Gastrulation

• Converts the two-layered embryonic disc into three germ layers:
  • Ectoderm (outer layer)
  • Mesoderm (middle layer)
  • Endoderm (inner layer)

Neurulation

• The formation of the neural tube which will develop into the Central Nervous System (CNS).
• The neural plate forms as a thickening of the ectoderm over the notochord, later folding to create the neural groove and ultimately fuses to form the neural tube.
  • Neural Crest: Will develop into peripheral nervous system (PNS) structures including nerves and ganglia.
  • Somites: Form skeletal muscle, vertebrae, and ribs.

Hormonal Regulation of Pregnancy

• hCG: Human chorionic gonadotropin produced by trophoblast cells; maintains endometrium.
• Role of Placenta:
  • Produces estrogen and progesterone, maintains pregnancy, and acts as a selective permeable barrier for nutrients and waste exchange.

Umbilical Cord and Placenta

• Cord Composition: 2 umbilical arteries (fetus to placenta) and 1 umbilical vein (placenta to fetus).
• Placental Function: Gas exchange, nutrient transfer, and waste removal between maternal and fetal blood without direct blood exchange.

Fetal Heart Adaptations

• Shunts:
  • Foramen Ovale: Directs blood from right atrium to left atrium, bypassing the lungs.
  • Ductus Arteriosus: Carries blood from the pulmonary trunk to the aorta, allows most blood flow to bypass the lungs.
  • Ductus Venosus: Carries blood from the umbilical vein to the inferior vena cava, bypassing liver capillaries.

Teratogens

• Substances capable of causing congenital abnormalities if they cross the placental barrier (e.g., alcohol, retinoic acid).

Hormonal Induction of Labor

• Increased fetal cortisol leads to changes in hormone levels that prepare the uterus for labor, leading to contractions and cervical changes.

Stages of Labor

1. Dilation: Preparation and positioning of the fetus for birth.
2. Expulsion: Actual delivery of the baby.
3. Placental Stage: Detachment and delivery of the placenta after the baby exits.