Video-Overview of Human Prenatal Development

Sep 2, 2024

Human Prenatal Development

Stages of Development

  1. Pre-embryonic Stage
  2. Embryonic Stage
  3. Fetal Development

Fertilization Process

  • Occurs in the ampulla of the fallopian tube.
  • Fertilized egg, or conceptus, travels to the uterus.
    • Undergoes mitotic divisions to form blastomeres (cleavage).

Conceptus Development

  • Day 3: Conceptus reaches the uterus (16 cells, called a morula).
    • Cells are totipotent (capable of differentiating into all cell types).
  • Morula continues dividing, nourished by egg cytoplasm and "uterine milk."

Formation of Blastocyst

  • 100 cells: Form a blastocyst with:
    • Embryoblast: Inner cell mass, becomes the embryo.
    • Trophoblast: Outer shell, nourishes the embryo, becomes chorion (part of placenta).
  • Trophoblast secretes enzyme to hatch blastocyst for implantation.

Implantation

  • At the end of the first week, the blastocyst attaches to the endometrium.
  • Syncytiotrophoblast forms, growing into the endometrium for nutrition.
  • Secretes Human Chorionic Gonadotropin (HCG) to maintain progesterone production by corpus luteum.

Formation of the Embryonic Disc (Week 2)

  • Embryoblast forms a two-layered disc (hypoblast and epiblast).
    • Sets dorsal/ventral axis (epiblast is dorsal).
  • Formation of yolk sac and amnion:
    • Yolk Sac: Provides nutrients and gas exchange, first site of blood cell production.
    • Amnion: Protects embryo, allows movement.

Gastrulation (Week 3)

  • Transformation into 3 germ layers: endoderm, mesoderm, ectoderm.
    • Initiated by the primitive streak formation.
    • Determines cranial/caudal axis.
    • Cells migrate through the primitive streak:
      • First cells form endoderm.
      • Second cells form mesoderm.
      • Remaining cells form ectoderm.

Formation of Organs and Tissues

  • By end of embryonic period, primitive forms of all organ systems are present.
  • Yolk Sac gives rise to allantois (primitive urinary bladder).
  • Amnion surrounds embryo with protective amniotic fluid.

Embryonic Membranes

  • Yolk Sac
  • Amnion
  • Allantois
  • Chorion