Early embryology (from fertilisation to week 3) Flashcards
Describe the process of fertilisation
Occurring in the ampulla of uterine tube within 12 hours of ovulation. Sperm cells penetrate corona radiata and binds to the zona pellucida and the acrosome is exocytosed. The enzymes allow for the sperm to penetrate the zona pellucida and the fround end of the spem membrane fuses with the plasma membrane of the oocyte.
What prevents more sperm entering the oocyte after fertilisation?
Immediately after fusion, the negative charge of the oocyte membrane increases preventing more sperm from entering.
Describe fusion of maternal and parental DNA
After the sperm enters the the cytoplasm and the oocyte has finished its second meiotic division (forming the female pronucleus), the DNA is coated with a nuclear envelope and become the male pronucleus.
The first mitosis then occurs after the two pronuclear membranes dissolve .
What is cleavage?
Cell division in an early embryo when the number of cells increases but without the cytoplasm mass increasing
What is compaction?
This begins at 8 cells, and it when the central cells become linked via gap junctions.
What is the morula?
It is a solid ball of 16+ cells with inner and outer layers and is transported along the uterine tube 3-4 days after fertilisation.
The outer cells divide to become trophectoderm and the and the inner cells become the inner cell mass.
What occurs after 4.5 days after fertilisation? (development of blastocyte)
Extra - draw what it looks like
Cavities form in the inner cell mass and it can now be called a blastocyte as it has a blastocoel present. The cells near the inner cell mass/embryoblast are called trophoblasts.
When is the best time to remove cells for genetic testing?
5-6 days after fertilisation. A trophectoderm cell is removed and can be tested for gene disorders.
What occurs after 5.5 to 6 days after fertilisation (the binding to uterus wall)
- Binding at uterus epithelium
- Reduced production of anti-adhesion molecule MUC-1. This allows binding via selectin to glyco-components on epithelial cells
- Laminin, fibronectin and integrins involved.
What occurs at 6-7 days after fertilisation? (implantation)
- Implantation begins.
- The trophoblasts become invasive syncytiotrophoblasts and cytotrophoblasts
- Syncytiotrophoblasts invade via metalloprotease
- immunosuppression of host
What occurs at day 8 (bilayer formation)
Two layers from in the embryo, the epiblast and hypoblast.
The amniotic cavity forms in the epiblast
What occurs at day 9 (More cavity formations)
Hypoblast cells coat the blastocyte cavity to form the primary yolk sac
Spaces develop in the syncytiotrophoblasts
Extraembryonic mesoderm will soon form deep to the trophoblasts
Describe what occurs at days 11 and 12 (burrows in endometrium)
Bilateral embryonic disc.
The blastocyte completely burrows into the endometrium.
Syncytiotrophoblast cells erode through walls of maternal capillaries and bleed into the spaces
(breakthrough/implantation bleeding may occur)
New layer forms between primary yolk sac and cytotrophoblasts, spaces in extraembryonic mesoderm - the chorionic cavity will from by fusion of these spaces
What occurs in by the end of week 2?
Two layers develop in the trophoblast - syncytiotrophoblast and the cytotrophoblast
Two layers from in the inner cell mass - Epiblast (ectoderm) and the hypoblast (endoderm)
Two cavities from - amnionic and chorionic cavities
What is preeclampsia?
Cytotrophoblast invasion into spiral arteries of uterus is defective so they keep their muscular walls and act as high resistance vessels. Features of maternal high blood pressure and proteinuria after 20 weeks.
