Pregnancy and Human Development Overview

Overview of Development Topics

• Development: Period from fertilization to maturity.
• Embryology: Study of events in the first two months after fertilization.
• Fetal Development: Development from two months until birth.
• Differentiation: Creation of different types of cells.
• Genetics: Study of inheritance mechanisms.

Fertilization

• Usually happens in the distal one third of the uterine tube.
• This timing allows the zygote to divide before implantation.
• Haploid (1n) gametes (egg and sperm) form a diploid (2n) zygote.
• Sperm become motile due to seminal vesicle secretions.
• Sperm require activation in the female reproductive tract.
• Attrition rate: High sperm loss; only ~10,000 of 2 billion sperm enter the uterine tube, <100 reach the isthmus.
• Sperm count less than 20 million/ml is considered functionally sterile.
• Oocyte is protected by the corona radiata.

Fertilization Steps

1. Sperm try to penetrate the corona radiata.
2. Acrosomal enzymes help sperm pass through the zona pellucida to reach the oocyte cell membrane.
   • Fast block: First sperm binding causes rapid depolarization, repelling other sperm and preventing polyspermy (fertilization by multiple sperm).
     • Polyspermy leads to incorrect chromosome number and is incompatible with life.
   • Slow block: Cortical granules release enzymes converting the zona pellucida into an impenetrable fertilization membrane.
3. Egg and sperm nuclei fuse to form the zygote which begins to divide.
   • Cleavage: Cell division without cell growth, resulting in smaller, more numerous cells.

Stages of Prenatal Development

• Three aspects of development:
  1. Differentiation: Genetic activity changes in some cells, leading to different cell types.
  2. Growth: Increase in size.
  3. Morphogenesis: Taking shape.
• Gestation: Prenatal (before birth) development time.
  • Divided into three trimesters (3 months each):
    1. First trimester: Rudiments of major organ systems appear (differentiation).
    2. Second trimester: Organ and organ system development; body shape changes; fetus appears human by month 6 (morphogenesis).
    3. Third trimester: Rapid fetal growth; organs become fully functional (growth and maturation).
• Sonogram/ultrasound uses sound waves for fetal imaging.

Embryonic Development

• Zygote: Single-celled organism from fertilization; travels to the uterus.
• Cleavage: Cell division without growth.
• Morula: Solid ball of ~32 identical cells, barely larger than the zygote.
• Blastula: Morula accumulates fluid, cells move to the periphery, forming a fluid-filled cavity (blastocoele).
• Implantation: Blastula attaches to the uterine lining around day 7; placenta development begins (fetal and maternal portions).
  • Placenta facilitates gas, nutrient, and waste exchange.
• Gastrula and gastrulation: Blastocyst invaginates, forming a two-layered sac.
  • A third cell layer forms by day 12, creating three germ layers.

Germ Layers

• Three layers of cells that differentiate:
  1. Ectoderm: Outer layer; becomes the nervous system, epidermis (including hair, nails, and glands), and epithelial lining of the oral cavity and rectum.
  2. Mesoderm: Middle layer; becomes muscular and skeletal systems.
  3. Endoderm: Innermost layer; becomes the epithelial lining of the digestive tract and urinary bladder.

Extraembryonic Membranes

• Membranes surround the embryo/fetus for protection and nourishment:
  1. Yolk sac: Early blood cell formation site.
  2. Amnion: Outer membrane filled with amniotic fluid for cushioning.
  3. Allantois: Collects nitrogenous waste; base forms the urinary bladder; forms umbilical blood vessels for waste disposal.
  4. Chorion: Forms the fetal portion of the placenta; chorionic villi extend into the maternal endometrium.

Placenta

• Joint fetal and maternal structure.
• Umbilical arteries carry blood to the placenta, and the umbilical vein returns blood.
• Chorionic villi facilitate gas, nutrient, and waste exchange between fetal and maternal blood (no direct mixing).
• Placenta acts as an endocrine organ:
  • Human Chorionic Gonadotropin (HCG):
    • Appears in maternal bloodstream soon after implantation.
    • Measured by pregnancy tests.
    • Sustains the corpus luteum to continue progesterone production until the placenta develops enough to take over.
    • Once the placenta takes over, corpus luteum becomes the corpus albicans.
  • Human Placental Lactogen and Placental Prolactin:
    • Prepare mammary glands for milk production.
  • Relaxin:
    • Increases flexibility of the pubic symphysis for pelvic expansion during delivery.
    • Causes dilation of the cervix.
    • Suppresses oxytocin release to delay labor onset.

Pregnancy and Maternal Systems Changes

• Maternal respiratory rate and depth increase.
• Maternal blood volume increases.
• Maternal nutrient and vitamin requirements increase.
• Maternal glomerular filtration rate increases.
• Uterus size increases substantially.
• Mammary glands increase in size and secretory activity begins.

Labor and Delivery

• Goal of labor: Parturition (forcible expulsion of the fetus).
• Stages of Labor:
  1. Dilation Stage:
     • Begins with true labor onset.
     • Amniotic membranes rupture, releasing amniotic fluid ("water breaking").
     • Cervix dilates to 10 cm.
  2. Expulsion Stage:
     • Contractions reach maximum intensity until fetal emergence.
     • Episiotomy (surgical cut) may be performed to enlarge the vaginal opening.
     • Caesarian section (C-section) may be required due to complications.
     • Normal fetal orientation: head first.
     • Breech presentation (feet or butt first) increases risks.
  3. Placental Stage:
     • Uterine contractions continue, decreasing uterine size.
     • Placenta separates from the endometrium and is delivered.
• Multiple Births:
  • Twins:
    • Dizygotic (fraternal): Two different eggs and sperm.
    • Monozygotic (identical): One fertilized egg splits into two embryos before gastrulation.
  • Other multiple pregnancies are less common and increase maternal and fetal risks.

Postnatal Development

• Development after birth - five life stages:
  • Neonatal period: Birth to one month.
  • Infancy: One month to two years.
  • Childhood: Two years to adolescence (puberty).
  • Adolescence: Puberty (sexual maturity) to completion of growth (epiphyseal plates fuse around age 25).
  • Maturity: Active growth stops, aging (senescence) begins, ends in death.