Embryology Flashcards
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.
