Embryology Timeline


Embryology examines fertilization and tracks the development of the embryo until it bears a resemblance to its progenitors. To young aspirants and embryologists alike, Embryology Training Courses involve very judicious mixture of various teaching modules, lectures, actual hands-on-training and demonstrations to ensure quality training.
A very important aspect of the Embryology Training Courses is to understand the primary facts so as to have a complete understanding of the subject matter and take apt and relevant decisions. To have a thorough understanding, it is of paramount importance to have a clear idea about the embryology timeline. Although the intricate details shall be elaborated upon in the actual Embryology Training Courses, we shall try to give you a rough idea about it in a succinct yet accurate way here.

The Embryology Timeline is outlined in the following steps.

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Week 0-4

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 the outer skin and the inner body cavities. These mostly include muscle cells, such as skeletal, smooth and cardiac muscle, but it also includes other cells like the red blood cells that run through arteries and veins and the cells in kidneys.

The inner body cavities are filled with organs, and organs come from the inner endoderm. These include lungs and respiratory system,liver,digestive system and intestinal tract and 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

o 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.

o 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.

Week 4 • Initiation of heart beat

o 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

o Phenotypic differentiation completed by week 20

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