Extra-Embryonic Development Flashcards
Describe basic principles of post-implantation embryology Identify different types of "embryo-derived" stem cells, including TS, XEN, EpiSC. Identify different stategies and experimental systems to study this stage of development
How do the embryo and the uterine wall work together to determine where and when the embryo will implant?
There is a "cross talk" between embryo and uterine wall. The factors at play are : Hormones Growth Factors Adhesion Molecules Extracellular Matrix proteins
Describe and explain the two different types of implantation
Invasive implantation - Embryo’s TE cells break through the epithelial surface and into the stroma underneath. The uterine tissue (endometrium) reacts drastically in “decidualisation”, which forms a decidua - the maternal interface with the placenta. Happens in humans, non-human primates, mice, rabbits, cats, dogs.
Non-Invasive implantation - The attachment of the embryo to the uterus is superficial. There are many layers of cells between the maternal and fetal circulations, and no decidual response. Happens in pigs, sheep, cows, horses.
How does the maternal endocrine system recognise and alter its function in the presence of a baby?
The mechanics changes in different animals but the main point is that progesterone production is continued. In humans, the embryo’s TE cells (and consequently placenta) secrete hCG, which maintains progesterone production by acting on the ovary. This happens in the first 8 weeks of pregnancy. After this, the placenta becomes the source of progesterone.
What is embryonic diapause?
It is a process which some animals exhibit and it means that the early embryo is held in relative dormancy. The mechanisms vary, and it often takes place at the early blastocyst stage.
Describe the two basic patterns of embryonic diapause
Obligatory diapause - This pattern occurs with every pregnancy and ensures that the offspring are born at a time where the environment is favourable (photosensors linked to time of year)
Facultative diapause - Environmental conditions can trigger delayed implantation - doesn’t happen all the time. In mice, if the mother is suckling a litter, the new embryos won’t implant until the litter stops suckling, to ensure the resources are sufficient for all offspring.
Explain placental development in mice via invasive implantation
The TE cells invade the stromal cells, leading the embryo into the uterine wall. TE proliferates extensively. The TE that’s in contact with the EPI is polar TE, and forms extra-embryonic ectoderm, and ectoplacental cone. ExE ectoderm becomes important signalling centre and gives rise to chorion and labyrinth - placental exchange. TE cells not contacting EPI stop proliferating but replicate DNA - endoreduplication to form trophoblast giant cells which invade uterus and modify maternal blood vessels for placental perfusion. Humans have cytotrophoblasts not giant.
What is the role of cytotrophoblast cells in mice and humans?
They’re the main proliferative cell type in the developing placenta of mice and humans. They differentiate to form specialised cell types, also fusing together to form syncytriotrophoblast (epithelial covering). The mouse placenta contains two different types of cells with “extra” DNA - giant cells formed by endoreduplication and syncytiotrophoblast formed by cell fusion.
In humans, the extravillous cytotrophoblast cells invade the uterus’ spiral arteries, remodelling them to be able to carry large volumes of blood efficiently to the placental interface. For the first trimester they actually block them, reducing the blood flow to the placenta.
In humans, why is it important that cytotrophoblast cells block the spiral arteries?
It allows the embryo to develop in a low oxygen environment. Higher oxygen levels risk the release of free radicals which could damage the developing embryo. In the later embryo, restricting the blood flow could be a problem so the blocking needs to be carefully monitored. When blood flow is established, maternal and fetal circulations are brought close together for exchange but never touch. Placental villus allows this exchange.
What are the 5 main topics of function of the placenta?
It contacts the maternal decidua and uterine spiral arteries in mouse and human pregnancy.
It connects the fetus to the mother, retaining the pregnancy
It actively transfers nutrients from maternal blood –> fetus, and waste in opposite direction.
The two blood supplies are kept separate but close - if they mix the pregnancy could be lost.
Mother’s immune system is down regulated so that the body doesn’t reject the placenta.
Placenta synthesises many biologically active molecules which regulate placenta and affect maternal system.
Describe the development of the yolk sac in mice
The yolk sac is formed by the PE. During implantation, the PE migrates around the inside of the TE forming an internal covering. Part of PE that stays in contact with the embryo is the visceral endoderm, and the part that lines the TE is called the partial endoderm. Yolk sac cavity is enclosed by these two parts. Visceral endoderms role is patterning the developing embryo by signalling to adjacent epiblast cells
Describe the development of the yolk sac in humans
The yolk sac develops as a structure that’s attached to only the central part of the developing embryo. In the first trimester, it’s thought that the yolk sac provides nutritional support. Once this role has been taken over by the placenta, the yolk sac shrinks and may be absent by the end of the pregnancy
What are fetal membranes?
In both mice and humans, the developing embryo is surrounded by thin tissues which are genetically the same as the embryo. They develop as separate layers, fusing into a multilayered structure.
Describe the structure of the fetal membranes
Innermost layer (closest to embryo) is the amnion which is derived from the epiblast cells
The outer layer in the mouse is derived from the parietal yolk sac
The outer layer in humans is the chorion, which is mainly cytotrophoblast cells, which demonstrates that the human fetal membranes partly share their origin with the placenta.
What are the functions of the fetal membranes?
The primary function, in both humans and mice is to retain the amniotic fluid around the developing embryo and fetus as it is essential for normal development. Mouse fetal membranes are tiny so hard to investigate. Human fetal membranes are continuous with the placenta and share some of its functions. Chorion is next to the maternal decidua but doesn’t have a direct link the maternal blood supply. Fetal membranes may be paracrine/autocrine regulators of amnion, chlorion, decidua, myometrium (next to decidua)
