implantation Flashcards
When does implantation occur
attachment of the fertilized egg to the uterine lining occurs approximately 6 or 7 days after conception
Blastocyst hatching
process when the blastocyst “escapes” from the zona pellucida, which occurs around day 6-7 after ovulation. Products of blastocysts likely activate lytic factor in uterine fluid (unfertilized eggs don’t hatch).
blastocyst hatching in IVF
done by mechanical disruption or enzymatic treatment
Which part of the blastocyst directly interacts with the uterine lining
the trophoectoderm
decidualization
Process where by the endometrial stromal cells, fibroblasts, are transformed into round decidual cells which accumulat glycogen and lipids. Depends on progesterone and cAMP. Begins in secretory phase of menstrual cycle. Also prolactin and IGFBP-1 production, activation of COX-2 increasing synthesis of PGE2.
Predecidualization
Stromal cells immediately adjacent to spiral arteries begin to transform into rounded decidual cells. Dependent on high progesterone and possibly estrogen levels of luteal phase. If fertilization takes place then this process expands and includes the remaining stromal cells
List the different decidua
The decidua basalis resides under the implanting embryo, the decidua capsularis overlies the embryo and the decidua parietalis covers the remainder of the uterine surface
When is the window of implantation and what happens in the uterus during this time
Day 20-24 of menstrual cycle. During this time frame pinopodes,small finger-like projections, form on the apical surface of the endometrial epithelium. This is dependent upon progesterone and can be suppressed by estrogen. These structures are involved with the endocytosis of macromolecules and uterine fluid. Also, the endometrium becomes more vascular and edematous. The endometrial glands have enhanced secretory activity. Proliferation decreases (decreased estrogen) and progesterone receptors are downregulated.
Where is hCG produced and what is its function
The trophectoderm of the blastocyst- maintains the corpus luteum and therefore the progesterone. Also closely resembles LH, has immunosuppressive properties, growth promoting properties and is critical for impantation.
List immunosuppressive factors produce by pre-implantation embryo
eary pregnancy factor (EPF), platelet activating factor (PAF) and hCG
List the stages of implantation
apposition, adhesion, invasion
Describe apposition
This is a loose unstable connection between the trophectoderm and the endometrial lining. The microvilli of the trophoblast interdigitate with the pinopodes.
Describe adhesion
This is a stronger connection created by ligand-receptor interactions. Likely involves integrins, heparin and its receptors. Also, L-selectin expression by trophoblast appears to play a role. These interactions cause cytoskeletal changes in the epithelial cells so they dislodge and allow access of the trophoblast to the basal lamina. The inner cell mass is on the side to embed first.
Describe invasion
After the blastocyst adheres to the epithelium, the trophoblastic cells rapidly proliferate and differentiate into syncytiotrophoblasts and cytotrophoblasts
Functions of syncytiotrophoblasts
extend long protrusions and secrete tumor necrosis factor-a (TNF-a), which interferes with the expression of cadherins and ß-catenin, thereby assisting with the dislodgement of the epithelial cells. They also secrete autocrine factors and proteases (metalloproteinases and serine proteases) that promote invasion through the basement membrane and the endometrial stroma (decidua). Other substances thought to play critical role in implantation include prostaglandins, colony-stimulating factor-1 (CSF-1), leukemia-inhibitory factor (LIF) and interleukin-1 (IL-1).
