Embryo Implantation
Summary
-pregnancy is the state of carrying a developing fetus, or unborn offspring within the body (womb) -the inner lining of the uterus is the endometrium, and during a narrow window of the menstrual cycle it becomes receptive to an implanting blastocyst. -for four days, the endometrial lining will rapidly absorb fluid causing the embryo to travel out of the Fallopian tube and become pressed against the uterine wall -both the blastocyst and the endometrium begin producing adhesion molecules on their cell surface -cells of the trophectoderm begin invading the endometrium and differentiate into syncytiotrophoblast, which invade the vasculature -the uterine endometrium differentiate into the decidua which secrete an extracellular protein matrix and completely embed the implanted embryo -monozygotic twins result from the partition of the inner cell mass into two separate embryos. conjoined monozygotic twins remain attached.
Implantation Blastocyst
-they will grab hold and remodel -will find blood vessels and rearrange and then tear apart so they leak into a pool -the placenta then has finger like projections which will search for these pools and allows the embryo to bathe in mother's blood
Implantation
Blastocyst becomes completely buried in the endometrial lining which is now referred to as the decidua. -set up a whole different phase, of developing blood vessels for long term
Trophectoderm Specialization through Differentiation
Cytotrophoblast Syncytiotrophoblast -makes the placenta by using all resources to form
Part of 4th step to 5th
Extravillous trophoblast grow out from the placenta and penetrate into the uterine endometrium. This process is essential not only for physically attaching the placenta to the mother, but also for altering vasculature in the uterus to allow it to provide adequate blood supply to the growing fetus as pregnancy progresses. Some of these trophoblast replace the endothelial cells in the uterine spiral arteries as they remodel these vessels into wide diameter conduits that are independent of maternal vasoconstriction. this ensures the fetus receives a steady supply of blood and the placenta is not subjected to fluctuations in oxygen that could cause it damage. -push small muscle out of blood vessel and has no control over vasocontriction -check for fetal compartment if in a car crash (may bleed out) -critical for normal healthy pregnancy to occur or may get fetal growth restriction
Extraembryonic Endoderm
Hypoblast Yolk sac
Blastocyst
Inner Cell Mass- destined to become fetus Trophoblast- accomplishes implantation and develops into fetal portions of placenta morula stage ends in the uterus and then can begin process of implantation.
3rd phase
Projections from the invading synctiotrophoblast burrow into the endometrial layer and envelop maternal blood vessels. The syncytotrophoblast will erode the walls of the maternal blood vessels and cause blood to fill isolated lacunae (vascular spaces). Leakage of blood from this process can appear as spotting and can be mis-interpreted as a menstrual period (aka breakthrough bleeding, or spotting)
Syngamy
The fusion of gamete pronuclei resulting in the formation of a zygote, which develops into a new organism
Implantation of the Mammalian Blastocyst
The trophectoderm and uterine endometrium both express adhesion molecules During the implantation window, the absorption and movement of uterine fluid presses the blastocyst up against the uterine wall The blastocyst will attach to the uterine wall at the area immediately above the inner cell mass. Cells of the trophectoderm will then begin invading the endometrium. -the movement of fluid towards the proximity and allow for communication and attachments. Then begin process of implantation
Blastocoel Formation
Trophectoderm transports of Na and Cl through this layer into the blastocoel. This generates an osmotic gradient driving fluid across this epithelium facilitates transepithelial Na and fluid transport for blastocoel formation. Transport is driven by Na, K-ATPase within basolateral membranes of the trophectoderm
Twinning
Twins may arise through the splitting of a two cell embryo, however most arise through the subdivision of the inner cell mass. You can determine the plane of division in monozygotic twins by looking at the distribution and arrangements of the extra-embryonic membranes (placental tissues). Among many sets of twins one member does not survive to birth and it has been estimated that one in eight live births is actually the sole survivor of a set of twins. -identical will have one implantation site with two umbilical cords compared to fraternal which have two implantation sites
Specialization through Differentiation
When ready, the blastocyst dissolves and hatches out of the zona pellucida. The trophoblast cells of the trophectoderm begins to express adhesion molecules on their cell surface. the apical (outer) surface of the uterine endometrium will express adhesion molecules during the narrow window (between day 20 to 24 of the menstrual cycle) referred to as the implantation window. -blastocyst- produces enzymes which dissolves the zona pellucida and squeezes out. -do not make adhesions al the time because the uterus would not have the surface area other times (only do this window so the blastocyst does not fold up in the implantation window)
Integrins
are cellular receptors that mediate attachment between cells and also participate in cell signaling, regulating cellular shape, motility, and the proliferation. -integrins will grab hold of intracellular matrix -allows for communication
Blastocyst
attaches to the endometrial epithelium through these binding ligands E-cadherin and Integrins
Implantation Window
between day 20 to 24 of the menstrual cycle this refers to the phase of the menstrual cycle in which the endometrium of the uterus is able to receive the blastocyst stage embryo and initiate the developmental events leading to pregnancy. this phase is termed the "implantation window" and lasts about 4 days.
Endometrium pregnancy
blood vessels in the endometrium increase in size and number, fuse and become interconnected, forming the maternal portion of the placenta, which will supply oxygen and nutrients to the embryo/fetus -which allows for it to implant in the maternal myometrium -very dynamic and will rearrange itself depending on situation -implantation begins here and is progressive
Epiblast
differentiates to form all three germ layers of the early embryo
Inner cell mass
epiblast embryonic ectoderm primitive streak
implantation window
follows 6 days after the luteinizing hormone (LH) surge and is characterized by slight changes tot he endometrium cells, which aid in the absorption and movement of uterine fluid. This brings the blastocyst in close proximity to the endometrium and "presses" it against the uterine wall.
endometrium normal
functions as a lining for the uterus, preventing adhesions between the opposed walls and maintains the uterine cavity. during the menstrual cycle the endometrium grows to a thick, blood vessel-rich, glandular tissue layer and is optimal environment for implantation. -will rise and fall with the menstrual cycle hormones
yolk sac
functions as the early circulatory system before internal circulation begins. Blood is conveyed to the wall of the sac by the primitive aorta and after circulating through a wide-meshed capillary plexus, is returned by the vitelline veins to the tubular heart of the embryo, and delivers nutritive material from the yolk sac to the embryo
Villous trophoblasts
give rise to a sub-population known as extravillous trophoblasts which invade the uterine wall and target blood vessels, particularly spiral arteries. -invade blood supply and rearrange
Zygote
goes through mitosis to give us a two cell organism
Preimplantation Development
goes through multiple stages of mitosis until reaches the blastula. The blastocoel is comprised of blastocoel and the inner cell mass.
How does an embryo attach and communicate with the mother?
implantation- placental interface to establish between mother and embryo for gas exchange put all resources into developing this attachment so can communicate with mom
E-cadherin
is a calcium-dependent cell-cell adhesion glycoprotein composed of five extracellular repeats in a transmembrane region -will create a permanent structure to attach -allows for communication
Preeclampsia
is a medical condition characterized by high-blood pressure or hypertension and high amounts of protein in the urine.
Serosa
is a smooth membrane which secretes fluid and a thin connective tissue layer. serosa membranes line and enclose body cavities
Next Phase of Implantation
is penetration of the uterine epithelium. In the area where the trophoblast touches the endometrium, the outer layer of trophoblasts will begin to fuse and differentiate into the syncytiotrophoblast. This layer will not proliferate itself but is constantly fed by the cytotrophoblast stem cell layer. Although initially quite small, it will soon grow to surround the entire embryo. Is a highly invasive layer and quickly expands and erodes the endometrial layer.
Implantation
is the developmental stage at which an embryo adheres to the wall of the uterus and signifies the beginning of pregnancy. through subsequent differentiation events the fetus will receive oxygen and nutrients from the mother in order to grow. -must have cross talk between fetus and maternal endometrial cells to develop
Cytotrophoblast
is the inner layer of the trophectoderm and is considered to be the stem cell pool for this tissue; it differentiates into the other forms of trohpblastic tissue
endometrium
is the inner membrane of the mammalian uterus.
cervix
is the lower, narrow portion of the uterus where it joins with the top of the vaginal canal. the cervix must remain tightly closed during pregnancy.
myometrium
is the middle layer of the uterine wall, consisting mainly of uterine smooth muscle. Its main function is to induce uterine contractions
synctiotrophoblast
is the outer layer of the trophectoderm that actively invades the uterine endometrium (wall). It forms a unique tissue in that it is a multi-nucleated, terminally differentiated syncytium. It will form the outermost fetal component of the placenta and serves to massively increase the surface area available for nutrient exchange between the mother and fetus. It cannot proliferate and is maintained by fusion of underlying cytotrophoblast cells.
Decidualization
is the process of differentiation of the spindle-shape endometrial fibroblasts into swollen secretory cell (edema). These cells secrete an extracellular matrix rich in connective proteins such as fibronectin and laminin, which aids in embryo attachment and placental organization. Vascularity as well as vascular permeability is enhanced leading to increased blood flow. The dicidual layer is rich in blood and facilitates nutrient and gas exchange as well as serving to protect the embryo/fetus from the maternal immune system. The decidua secretes crucial hormones, growth factors and cytokines necessary for placental growth and differentiation. It contains receptors for estrogen, progesterone, and growth hormone -pull on a lot of fluid and produce glycoproteins (store resources) -allow for gas exchanges and protect from immune system
First Step in Implantation
is to set up a stem cell pool. set up an amnioc cavity. trophoblastic cells begin to invade endometrium and communicate to decide what will become.
Fraternal Twins
known as dizygotic twins result from the fertilization of two ovulated oocytes. These represent about two thirds of all twins
Identical twins
known as monozygotic twins arise through the fertilization and subsequent splitting of a single oocyte/embryo
Four major structures
myometrium endometrium serosa (outer) cervix -serosa exists on the outside of the uterus -cervix acts as a barrier between vagina and uterus (mucus plug made of collagen fibers, create matrix to prevent entrance/exit) -must stay rigid through pregnancy
Symptom
outwardly visible sign to communicate exam - tell the physiological basis of symptoms (not conditions)
Preeclampsia
refers to a set of symptoms rather than any causative factor, and there are many different causes for the condition and results in damage to maternal endometrium, kidneys, and liver. Thousands of women and babies die or get very sick each year from a dangerous condition called preeclampsia, a life-threatening disorder that occurs only during pregnancy and the postpartum period. -will push more blood through spiral small opening and thus increase BP and secrete protein in urine.
Extracillous trophoblast cells
remodel the maternal spiral arteries, displacing smooth muscle and endothelial cells, in order to produce a blood vessel with a larger diameter, increased blood flow and reduced resistance. -become individual autonomous units This is an essential step in establishing and maintaining a normal pregnancy and is necessary for the higher blood requirement of the fetus later in pregnancy. -will grow/degrade based on LH surge
Blastocoel Formation
the actions of the Na-K pump. push salt into interior and balloon structure out. take a small structure and balloon out to increase surface area for implantation.
4th step implantation of maternal blastocyst
the cytotrophoblast stem cells either fuse to form synctiotrophoblasts or aggregate to form anchoring villous trophoblasts.
fetus
the developing organism after eight weeks after fertilization -undergoes growth -six to seven days after fertilization, the embryo will make a firm attachment to the uterine wall and begin the process of invading the endometrium.
embryo
the developing organism until about eight weeks after fertilization -undergoes organogenesis
Pregnant
the state of carrying a developing fetus or unborn offspring within the body From the latin pre- meaning before + g- natus meaning birth = before giving birth
Getting Ready for Pregnancy
the uterine endometrium will maximize blood flow through glandular endometrium
The uterus (The womb)
the uterus is a female reproductive organ that carries the developing offspring to term It provides the necessary microenvironment for fertilization, attachment, sustenance, and growth The lower portion contains the cervix, which opens into the vagina while the opposite ends are connected by Fallopian tubes.
primitive streak
will bisect the embryo and establish symmetry
embryonic ectoderm
will differentiate into the nervous system
Hypoblast
will differentiate into the yolk sac
