Embryology (three)

  • Last lecture for ANHB1101 focusing on body folding and organ development.

Body Folding

  • Transforms a flat, trilaminar embryo into a 3D structure.
  • Involves folding in head-tail (craniocaudal) and lateral directions.
  • Creates thoracic and abdominopelvic cavities for organ growth.

Initial State

  • Flat trilaminar disc with endoderm facing the yolk sac.
  • Neurulation commences, leading to bending of the flat structure.

Objectives

  • Describe head and tail folding (craniocaudal).
  • Explain lateral folding and ventral body wall formation.
  • Understand germ layer contributions to organ formation.

Gastrulation Revision

  • Bilaminar embryo (epiblast and hypoblast) transitions to trilaminar (ectoderm, mesoderm, endoderm).
  • Epiblast cells migrate through the primitive streak, differentiating into endoderm and mesoderm.

Neurulation Reminder

  • Neural plate folds to form the neural tube; neural crest cells migrate away.
  • Neurulation initiates body folding.

Week 4 Development

  • Fully folded embryo with head, tail, and closed ventral body wall.
  • Ventral body wall provides space for organ development.
  • The embryo becomes a tube within a tube.

Head and Tail Folding

  • The developing heart moves from a cranial to a ventral position due to brain growth.
  • The amniotic cavity expands, while the yolk sac relatively shrinks.
  • The oropharyngeal membrane disintegrates, and the cloacal membrane remains intact temporarily.
  • Early gut development: foregut, midgut (facing yolk sac), and hindgut.
  • The vitelline duct connects the midgut to the yolk sac.

Lateral Folding

  • Simultaneous neurulation and somite formation (paraxial mesoderm).
  • Intraembryonic mesoderm differentiates and forms primitive body cavities (coelom).
  • Amnion enlarges and folds down, aiding in narrowing the yolk sac.
  • Lateral folds join to form a complete ventral body wall.

Completion of Folding

  • The ventral body wall seals, except for a small region around the vitelline duct.
  • Small channels persist between intra- and extraembryonic cavities until week 11.
  • Surface ectoderm becomes epidermis; amnion covers the umbilical cord.

Germ Layer Derivatives

  • Ectoderm: Brain, spinal cord, peripheral nervous system, epidermis, hair, nails.
  • Mesoderm: Dermis, connective tissues (bone, cartilage, ligaments, tendons, adipose, blood), muscles (skeletal, cardiac, smooth), reproductive and lower urinary tracts, kidney, spleen, limbs.
  • Endoderm: Epithelial lining of the lung, pancreas, prostate, gastrointestinal tract, liver, urinary bladder, and allantois.
  • Organs are combinations of germ layer derivatives (e.g., stomach: endodermal lining, mesodermal muscle and connective tissue, ectodermal nerve input).

Morning Sickness

  • Human Chorionic Gonadotropin (HCG) maintains the corpus luteum and prevents menstruation.
  • HCG secretion corresponds to peak organ development (organogenesis).
  • Morning sickness is possibly an evolutionary adaptation to protect against teratogens (environmental insults).
  • Teratogens can cause congenital malformations.
  • Examples: food poisoning (Listeriosis), alcohol, certain drugs, tobacco, infections, excess retinoic acid.
  • Embryonic period (weeks 3-8) is highly sensitive to teratogens.