Embryology examines fertilization and tracks the development of the embryo until it bears a resemblance to its progenitors. We bring you a compact timeline of the entire process of embryology to understand the concept and appreciate the scientific process.
After fertilization, cellular divisions begin at a rapid pace in the first month itself. By the end of the first week, the embryo implants itself to the uterine wall.
The next stage of development is the formation of the three germ layers.
The placenta begins to form and cell differentiation is well under way by the end of the first month.
Day 0 • Embryogenesis begins
Embryogenesis is the most primary stage in embryology.
The cells that make up our body are organized into three germ layers.
The outer ectoderm cells become structures like your outer skin layers and your inner nervous system cells.
The middle mesoderm cells form many of the structures that lie in between your outer skin and your inner body cavities. These mostly include your muscle cells, such as skeletal, smooth and cardiac muscle, but it also includes other cells like the red blood cells that run through your arteries and veins and the cells in your kidneys.
And those inner body cavities? Well, they are filled with your organs, and organs come from the inner endoderm. These include your lungs and respiratory system, your liver, your digestive system and intestinal tract and your endocrine glands.
Week 1 • Cleavage and blastocyst formation
o The zygote undergoes mitotic divisions.
Then blastomeres are formed which then converts to morula and then finally blastocyst formation begins.
o Blastocyst cavity forms two cell masses
inner cell mass = embryoblast which will become embryo
outer cell mass = trophoblast which becomes placenta
• hCG secretion
o After blastocyst implantation, syncytiotrophoblast invades uterine wall and secretes hCG
o hCG is detected in blood at day 8 and in urine at day 10 (pregnancy test)
Lower hCG signifies ectopic pregnancy or a sign for spontaneous abortion.
Higher hCG: signifies multiple pregnancy, hydatidiform mole, or gestational trophoblastic neoplasia
Week 2 • Bilaminar disk formation
o Consists of 2 germ layers: epiblast and hypoblast.
o The 2 cavities present are amniotic cavity and yolk sac.
Week 3 • Gastrulation
o The three embryonic germ layers (ectoderm, mesoderm, and endoderm) are established.
o This is initiated by formation of primitive streak.
• Formation of Notochord and neural plate
o Notochord induces its overlying ectoderm to become neuroectoderm
Neuroectoderm will form neural plate and it closes completely by week 4
Neural plate forms neural tube and neural crest cells
o Notochord transforms to adult Nucleus Pulposus in the vertebral column
Week 3-8 • Embryonic period
o All major organ systems of the fetus have begun development.
• Initiation of heart beat
• Formation of upper and lower limb buds
Week 8 • Fetal Period
o The movement of the fetus begins.
Week 10 – 12 • Sex of fetus becomes recognizable
• Phenotypic differentiation completed by week 20