Embryology and Prenatal Development

Human Development

  • Human development begins at fertilization through the union of gametes.
  • Embryology: Study of events occurring during the 38 weeks of development inside the womb (prenatal period).
  • Changes from birth through all life stages = Postnatal Period.

Blastomeres

  • Cleavage initiates ~30 hours post-fertilization: a series of rapid mitotic divisions.
  • Results in small cells, blastomeres.
  • Cell number increases without growth:
    • Day 2: 4 cells
    • Day 3: 16 cells
  • All cells differentiate into building blocks of future tissues.
  • The conceptus is called a morula at this point, still encased in zona pellucida.

Movement of the Conceptus

  • Conceptus travels from uterine tube to uterine cavity.
  • By Day 4, it changes to a blastocyst nurtured by uterine milk (glycogen-rich secretion).
  • Blastocyst hatches from the zona pellucida, forming a fluid-filled cavity.

Trophoblast Cells and Inner Cell Mass

  • In the blastocyst:
    • Trophoblast Cells: Outer layer, forms placenta, provides nutrients, suppresses maternal immune response.
    • Inner Cell Mass: Inner cluster, becomes the embryo.

Implantation

  • Implantation occurs 4-7 days post-fertilization.
  • Trophoblast invades the endometrium with digestive enzymes.

Trophoblast Division

  • Trophoblast divides into:
    • Cytotrophoblast: Cells remain intact.
    • Syncytiotrophoblast: Merged nuclei, secretes enzymes, does not trigger immune response.

Implantation of Blastocyst

  • Syncytiotrophoblast continues to invade and surrounds maternal blood vessels by Day 12.
  • By Day 16: blastocyst embedded, covered by maternal epithelial cells.

Bilaminar Embryonic Disc

  • Inner cell mass differentiates into two layers:
    • Epiblast
    • Hypoblast
  • Forms bilaminar embryonic disc, which generates all body tissues through three primary germ layers:
    • Ectoderm
    • Mesoderm
    • Endoderm
  • Amniotic cavity enlarges around the embryo.

Development of Extra Embryonic Membranes

  • Produced during the 2nd week:
    • Protects the embryo and assists in functions like nutrition and gas exchange.
  • Include:
    • Yolk Sac: Originates from hypoblast, contributes to digestive tract, first blood cells, and germ cells.
    • Amnion: Transparent membrane from epiblast, encases embryo in amniotic fluid, protects from trauma, maintains temperature, prevents adhesion during growth.
    • Allantois: Small outpocketing forming the base for the umbilical cord, later becomes part of the urinary bladder.
    • Chorion: Outermost membrane, forms chorionic villi and is principal part of the placenta by week 4.

Prenatal Development

  • Pregnancy: Events from conception until birth, includes anatomical and physiological changes of the mother.
  • Gestation period: Typically 280 days (40 weeks).

Stages of Prenatal Development

  1. Pre-Embryonic Period (Weeks 1-2): Fertilization to blastocyst formation.
  2. Embryonic Period (Weeks 3-8): Formation of rudimentary organ systems.
  3. Fetal Period (Weeks 9-38): Maturation and growth of organ systems.

The Postnatal Period

  • Rapid changes following birth, divided into 5 stages:
    • Neonatal (birth to 1 month)
    • Infancy (1 month to 2 years)
    • Childhood (2 years to puberty)
    • Adolescence (puberty to sexual maturity)
    • Adulthood (maturity to death)

The Embryonic Period (Weeks 3-8)

  • At the end of this period:
    • The blastocyst is fully implanted.
    • Inner cell mass differentiates into the bilaminar embryonic disc.
    • Organogenesis starts with the three germ layers differentiating into organs.

Gastrulation

  • Begins early in the embryonic period, where cells of the bilateral disc rearrange to form a trilaminar disc.
  • Establishes the three primary germ layers.

Folding of the Trilaminar Disc

  • Cephalocaudal Folding: Forms head and buttocks regions.
  • Transverse/Lateral Folding: Forms the trunk region and primitive gut.

Organogenesis

  • All three germ layers differentiate into organs and systems, ending at week 8.
  • Organs not yet functional.
  • Ectoderm: Forms skin epidermis, nervous system, sense organs with neurulation being the first major event.

Differentiation of Germ Layers

  • Ectoderm: Forms skin, nervous system, sense organs.
  • Mesoderm: Forms somites which later become vertebrae, dermis, and skeletal muscles. Includes notochord.
  • Endoderm: Forms internal linings of various systems and glands such as thyroid, thymus, pancreas.

Teratogens

  • Sensitive period (weeks 3-8) wherein teratogens can cause birth defects. Examples include:
    • Infectious agents (e.g., Rubella)
    • Alcohol, nicotine, various drugs (e.g., thalidomide).

The Fetal Period (Week 9 to Birth)

  • Begins with the full functionality of the placenta.
  • Characterized by rapid growth and maturation of the fetus.
  • Development features:
    • Month 3: Ossification of bones, facial changes.
    • Month 4: Rapid growth, heartbeat audible.
    • Month 5: Lanugo and vernix caseosa formation.
    • Month 6: Increased weight; lungs produce surfactant.
    • Month 7: Fat deposition, eyes open.
    • Months 8-9: Complete development of organs, preparations for birth.

Summary of Major Events

  • Events such as fertilization, cleavage, implantation, differentiation into germ layers, and organ development occur sequentially leading to a fully developed fetus ready for birth.