Embryogenesis and Gastrulation

Timeline of Early Embryonic Development

  • Fertilization Event

    • Formation of the zygote

  • Development of the Zygote

    • Zygote develops into the blastocyst

    • Blastocyst enters uterine cavity around Day 4-5

    • Movement down the uterine wall until prepared to accept implantation

  • Implantation Process

    • Takes about one week after blastocyst formation

  • Development of the Placenta

    • Invading syncytiotrophoblast helps blastocyst to burrow into the endometrium

    • Establishes blood communication with mother's blood supply

    • Formation of placenta allows blood supply from uterine arteries reaching fetus

    • Blood communication relayed through the umbilical cord

  • Extraembryonic Tissues

    • First extraembryonic membrane is the chorion

    • Contributions to the placenta by the fetus

Embryogenesis and Gastrulation

  • Embryogenesis

    • Occurs alongside implantation and placentation at Week 2

    • Results in the two-layered disc

    • Gastrulation follows and leads to the three-layered disc by the end of the third week

    • Critical to note that both processes occur simultaneously

  • Two-Layered Disc Formation

    • Inner cell mass gives rise to three of the four extraembryonic membranes and the entire embryo

    • Formation of upper layer (epiblast) and lower layer (hypoblast)

  • Epiblasts and Hypoblasts

    • Epiblasts: upper layer of cells

    • Hypoblasts: bottom layer of cells

    • Trophoblast cells form chorion of placenta

    • Amnion arises from epiblasts, surrounds embryo, fills with fluid

    • Yolk Sac: formed by hypoblasts; in humans, it serves various early functions, including some blood cell production

  • Development of the Extraembryonic Membranes

    • Chorion: first extraembryonic membrane

    • Amnion: second extraembryonic membrane, protective fluid sac

    • Yolk Sac: third membrane, involved in early blood cell development, not nutrient transfer in humans

    • Allantois: formed from hypoblast cells, invades chorionic villi, connects to umbilical cord with Wharton's jelly

Gastrulation Process

  • Week 3: the week of the three-layered disc

  • Formation of the Primitive Streak

    • Epiblasts replicate and form an indentation

    • Cells migrating in through the primitive streak form new layers

  • Layer Formation

    • Endoderm: formed by cells moving down that push hypoblasts aside

    • Mesoderm: formed by remaining epiblasts filling in the space between ectoderm and endoderm

    • Ectoderm: remaining epiblasts that are left on the top layer

    • Result is a three-layered disc

  • Significance of Germ Layers

    • Ectoderm: gives rise to epidermis and nervous system

    • Mesoderm: gives rise to somites (sclerotome, dermatome, myotome), muscles, bones, dermis

    • Endoderm: forms linings of digestive and respiratory tracts, mucous membranes

  • Analogy: Humans as Donuts

    • Ectoderm: glaze on the outside

    • Mesoderm: dough inside

    • Endoderm: the hole in the donut

Summary of Developmental Stages

  • Zygote: gives rise to four extraembryonic membranes including allantois

  • Inner Cell Mass: gives rise to three of four extraembryonic membranes (amnion, yolk sac, allantois) and the entire embryo

  • Two-layer Disc: will give rise to three of the four extraembryonic membranes and the entire embryo

  • Epiblasts: give rise to entire embryo and one extraembryonic membrane

  • Hypoblasts: give rise to two extraembryonic membranes (allantois and yolk sac)

  • Understanding these developmental processes is crucial for studying organogenesis in subsequent lessons.