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Flashcards in Embryology Deck (7)
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What occurs before fertilization?

Egg is ovulated and halted at metaphase II of meiosis


What occurs 12 hrs to 1 day after fertilization?

Sperm penetrates corona radiata then zona pellucida

Acrosome reaction occurs preventing polyspermy

Male sperm becomes male pronuclei

Male pronuclei fuses w/ female pronuclei = 46 chromosomes

DNA replication occurs and each chromosome now has 2 sister chromatids

Second meiotic division occurs
- other cell dies off

First mitotic division occurs creating two cells
- known as 2 cell stage
- subsequent cell division


What is a blastomere?

after fertilization each cell is a blastomere until it divides into 16 cells
1 -> 2 -> 4 -> 8 -> 16


What occurs 1 to 4 days after fertilization?

Cell division continues to occur

Once there are 16 cells it is called a morula—“berry of cells”

It starts to become hollow and forms an inner cell mass and an outer cell mass

Now approaching the uterine cavity.


What occurs 4 to 6 days after fertilization?

Fluid penetrates zona pellucida and cavity forms

Once there is fluid in the hollow ball of cells, it is now called a blastocyst
— “cyst” think: fluid filled

A single cavity forms called the blastocyst cavity

Zona pellucida disappears

Inner cell mass becomes the embryoblast (eventually becomes embryo

Outer cell mass becomes the trophoblast (eventually becomes placenta)

Trophoblast penetrates the uterine epithelium around the 6th day.


What occurs 8-9 days after fertilization?

Week 2 = 2 Layers!

Trophoblast differentiates into cytotrophoblast and syncytiotrophoblast

Cytotrophoblast is where cells are dividing so these cells enter the area of the syncytiotrophoblast

Embryoblast flattens out and becomes bilaminar (two layered) with one layer being the hypoblast and the other layer being the epiblast

A cavity then forms in the epiblast—this is the first appearance of the amniotic cavity (the primary area where the embryo develops)

Hypoblast cavity was the blastocyst and is now called the primitive yolk sac.


What occurs 11-12 days after fertilization?

Syncytiotrophoblast penetrates endometrium even further establishing fetal-maternal circulation

Extraembryonic mesoderm develops from the primitive yolk sac

This is between the primitive yolk sac and the layer of cytotrophoblast. It forms around the bilaminar embryoblast. This starts to form lacunae too—these form the extraembryonic cavity. The bilaminar embryoblast is now suspended in the extraembryonic mesoderm.