Development+of+the+Bilaminar+Embryo

//**Note**- It seems to be well known that Dr. Yeasting's lectures rarely correspond well to his objectives, so if these notes do not seem to answer the objective questions, that is why. Objectives will be added retroactively for this lecture with the appropriate information.****//

****Embryonic Development:****
Fetus 8-36 wks after conception.
 * Embryo 1-8 weeks after conception.

Fetus is surrounded by amnion, which is in turn surrounded by the chorion. (Water break= amnion+chorion rupture). Amnion serves to dilate cervical canal. Chorion= bulky tissue, including placenta. One per fetus.****


 * Endometrium******- side of placenta facing away from fetus. Contains maternal tissue and embryonic tissue. Functional segments called**** Cotyledons. ****Have villi made of embryonic tissues, and intervillous space made of maternal blood. Nutrient and waste exchange here, because as embryonic tissue invades maternal tissue it breaks open blood vessels, allowing blood to fill intervillous space.****


 * Zone Pellucida******: Corrals fertilized material and helps implantation. Degenerates 5-6 days after fertilization (egg should be in uterine cavity).

First week of life spent traveling/scouting uterine cavity. Start of second week: Conceptus begins to invade endometrium. End of second week, implantation completed.

After implantation, conceptus must signal its presence to prevent menses. -Extraembryonic(chorionic) tissues release**** **Chorionic Gonadotropin** into maternal circulation, where it circulates to the ovary and maintains corpus luteum. CL continues to make high levels of progesterone, and menses does not occur.**
 * -Home pregnancy tests (3-4 wks) are designed to detect Chorionic Gonadotropin.**
 * -Some early pregnancy factors may also be released by conceptus as it travels down the tubes. This is the basis for early pregnancy tests (Early Pregnancy Factor).**

__**Fertilization:**__ **Spermatazoa can make it from the vagina to the tubes in about an hour. Corona radiata cells surrounding the oocyte make it larger and easier for the ciliary activity of the tubes to handle. Oocytes releasing chemical attractants that guide spermatazoa.**
 * -Spermatazoa make contact with corona radiata and have an acrosomal interaction (acrosome ruptures).**
 * -Acrosomal enzymes break down corona radiata and allow access to the zona pellucida. ZP has docking sites that will hold a spermatazoa in position.**
 * -Spermatazoa that break down CR are NOT the same that make contact with ZP, it's a second wave.**
 * -When sperm makes contact with the oocyte through ZP, the plasmalemma of the sperm and egg fuse. Sperm ruptures and the nucleus, mito, and parts of tail enter the plasma of the oocyte.**

__**Cortical reaction/Cellular Retraction(Zona reaction):**__
 * -Oocyte undergoes a reaction where it allows release of chemicals from vesicles between oocyte and ZP. This chemical change takes 15-20min and it renders the ZP material undigestable by other sperm.**


 * -The Oocyte also shrinks when a sperm enters..creating a gap between oocyte and ZP, which most sperm cant jump across.**


 * Conceptus is carried down uterine tube by ciliary action. Corona Radiata breaks down 4-6 days after fertilization.**
 * At implantation, conceptus is a ball of cells. Shortly after (day 6 ish), a small cavity forms. Cells begin to differentiate/specialize. Which cells end up where is random.**


 * Blastocele- cavity. Blastocyst- cell grouping. These cells are totipotential (can become anything)**
 * Compaction- cells divide and get smaller, and more compact, increasing cell to cell contact, junctions etc**
 * Blastula: dividing cells of conceptus.**
 * Blastocystic cavity is the same as Blastocele.**


 * Inner set of cells= embryoblast, gives rise to embryo.**
 * Outer set of cells= Trophoblast, gives rise to (Tropho=eating/nourishing)**


 * Exchange of chemical messengers happens even before the blastocyst makes contact with the endometrium.**
 * Cells undergoing rapid mitoses at the point of contact between the trophoblast and endometrium are blending and fusing together-- they become a large multinucleated cytoplasmic mass, This is called a …synsicium? Cynsishium? Syncytium.**


 * Mass is called Syncytiotrophoblast. This part becomes the differentiated tissue that produces chemicals and messengers.**


 * Nonfused tropho becomes Cytotrophoblast-- retain their individual identity, and is a reserve population of cells for ST.**


 * Syncytiotrophoblast injects itself into the endometrium and surrounds conceptus.**
 * Membrane bound spaces form within both ST and cytotrophoblast, called lacunae. Lacunar network has contact with outside materials, and serves as an avenue for endometrial secretions to obtain access to the conceptual mass, getting closer to the inner cell mass.**


 * Syncytiotrophoblast also taps into maternal vascular system through the endometrium. Maternal blood then begins to circulate through the lacunar network by arterial pressure. ("extravascular shunt") Maternal blood pressure eventually forces maternal blood back into maternal veins.**


 * Cytotrophoblastic cells grow into Syncytiotrophoblast, forming villi. Embryonic mesoderm begins to be found withing villar projections, forming a secondary villus. Developing Villus projections begin to have embryonic blood vessels..tertiary villus?**
 * A small delicate barrier is formed.**


 * Embryo needs trophoblast, but trophoblast does not need embryo. TB can survive without conceptus, but not vice versa.**

__**Decidual Reaction:**__ **Cells in stroma of endometrium undergo functional changes based on secretions from the Syncytiotrophoblast. New endometrium is called the Deciduum(sp?)**


 * -This reaction also downregulates maternal lymphocytes in the endometrium.**


 * -Embryo has "stealth technology", does not express normal antigens. In addition, it changes its microenvironment-- sum result is maternal immune system does not recognize it as foreign.**


 * Lacunar cells form intervillous space.**
 * Placental barrier not very selective. Also called Diffusion Membrane.**
 * Synctytiotrophoblast cells create the walll of the intervillous space.**


 * Smooth chorion: becomes devascularized, stretched out and smoothed out.**
 * Tissues towards uterine cavity away from myometrium are compressed, undergo tension, and create smooth connective tissue component.**


 * Villus Chorion becomes the placenta (no compression).**
 * Decidua Basalis: found between the implantation site and endometrium-- is basal to the conceptus.**
 * Decidua Capsularis- between conceptus and uterine cavity, ultimately degenerates.**
 * Decidua Parietalis- all other decidua (altered endometrium).**


 * Implantation can occur anywhere with vascularization. Will break through whatever internal lining it needs to get to blood vessels.**