8.25.11+-+Fertilization,+Implantation+and+Formation

**Introduction: Fertilization, Implantation, and the Formation of the Bilaminar Embryo**

 * 1) Be able to define the following terms...**
 * **Teratology** - The study of abnormalities in development
 * **Zona Pellucida** - The glycoprotein surrounding an oocyte. [|zona pellucida]
 * **Corona Radiata** - two to three layers of cells surrounding unfertilized oocyte. sperm must break through for fertilization to occur. [|corona radiata]
 * **Zygote** - Initial cell formed when a sperm and egg cell combine during sexual reproduction. earliest possible cell during fertilization. [|zygote]
 * **Blastomere -** any of the cells resulting from a division of a division of a fertilized ovum
 * **Blastocyst cavity** - the fluid filled region of the blastocyst
 * **Blastocyst** - A early embryonic structure composing of the embryoblast (inner cell mass), the trophoblast and the blastocyst cavity
 * **Morula** - The embryo at an early stage of development. A solid mass of cells prior to seperation into the hollow blastomere
 * **Compaction** - A tight binding of cells during development of the embryo, resulting in a compact sphere.
 * **Conceptus** - Denotes the embryo and the associated membranes. All non-maternal tissue
 * **Cytotrophoblast** - A layer of cells, derived from the trophoblast sphere, that surrounds the inner cell mass and the blastocyst cavity.
 * **Syncytiotrophoblast** - A multinucleated cell mass found in the placenta of the embryo. It is formed from the outer layer of the trophoblast, and it actively invade the uterine wall
 * **Placenta Previa** - an obstretic complication which occurs when the embryo attaches to the uterine wall so close to the cervix that the placenta blocks the os of the cervix (image).
 * **Ectopic Pregnancy** - a complication when the embryo implants outside the uterine cavity. Most occur in the Fallopian tube, called tubal pregnancy
 * **Epiblast** - A layer of cells formed from the inner cell mass that constitutes the dorsal pole of the double-bubble embryo (image).
 * **Hypoblast** - A layer of cells formed from the inner cell mass that constitute the dorsal pole of the double-bubble embryo (image). Is used to form the secondary (definitive) yolk sac.
 * **Embryonic Disc** - A bilaminar sheet of cells composed of both epiblast and hypoblast layers (see green arrow in image). It exists after implantation and prior to embryonic folding. All mature tissues and organs are derived from the embryonic disc.
 * **Amnion** - a membrane composing of the amniotic sac which holds and protects an embryo
 * **Primary and Secondary Yolk Sac** - The primary yolk sack forms from the blastocyst cavity. Around day 13 a small part of the primary yolk sac pinches off from the larger yolk sac. The primary yolk sac migrates to the ventral extremity, leaving behind a much larger secondary (definitive) yolk sack.
 * **Extraembryonic Mesoderm (EEM) -** A layer of cells that develops within the cytotrophoblast during the double-bubble phase of the embryo. It proliferates and expands to allow for creation of the chorionic cavity by the process of cavitation (image).‍‍‍‍‍
 * **Chorion** - The outer layer of the extraembryonic membrane plus cytotrophoblast. The amnion in contained within the chorion.
 * **Chorionic Cavity** - The portion of the conceptus which divides the EEM into two layers [ MORE ]


 * 2) Be able to describe the following concepts...**
 * **Meaning of "embryonic" vs. "fetal" period:** The embryo refers to the human during all stages of intrauterine development (//Before We Are Born//, pp. 2). The basic body plan is generated during the pre-fetal phase of embryonic development (occurs approximately weeks 2 to 8). Fetal development occurs from week 9 to birth and is concerned with additional growth and differentiation of the organs (//B.W.A.B.,// pp. 62-63).
 * **Concept of a "critical period" in development:** A critical period occurs when cell division is most rapid and thus the structure will be most susceptible to teratogens
 * **Transition from the zygote to the blastocyst (general sequence of early cleavage):** Ovulation of the oocyte creates the zygote. The cells in the zygote begin to divide but the overall zygote remains the same size. Once the zygote reaches 32 cells, it resembles a berry and is called a Morula. The cells then scatter to form a central cavity and then hollow ball is then called a blastocyst. [Figure 1][[image:zygote_to_blastocyst.gif caption="Figure 1: Transition from Zygote to Blastocyst"]]
 * **The difference between the trophoblast and the inner cell mass:** The trophoblast is the outer layer of the blastocyte and eventually forms the placenta. The inner cell mass (or embryoblast) is the other major collection of cells, which goes on to become the fetus. [Figure 2] [[image:Blastocyte.png width="241" height="186" caption="Figure 2: Blastocyte"]]
 * **Difference between the cytotrophoblast and the syncytiotrophoblast:** The trophoblast is the outer layer of the blastocyte. It changes names when the blastula makes contact with the endometrium lining. At this point there is the cytotrophoblast (undifferentiated from the previous trophoblast cells) and the syncytiotrophoblast (the multinucleated outer layer which actively invades the uterine wall). This layer later become the placenta. The syncytiotrophoblast greatly expands the surface area available for the nutrient exchange between mother and fetus [Figure 3]. [[image:trophoblast.png width="253" height="170" align="left" caption="Figure 3: Close up of trophoblast"]]


 * **Sequence of implantation:**
 * Initially the zona pellucida degenerates, this is caused by enlargement of the blastocyst and degeneration caused by enzymatic lysis caused by enzymes released by the sperms that partially penetrate the zona pellucida
 * the blastocyst adheres to the endometrial epithelium
 * the trophoblast differentiates into two layers - syncytiotrophoblast and cytotrophoblast
 * the syncytiotrophoblast erodes endometrial tissues and the blastocyst starts to embed in the endometrium
 * blood-filled lacunae appear in the syncytiotrophoblast
 * the blastocyst sinks beneath the endometrial epithelium and the defect is filled by a closing plug
 * lacunar networks form by fusion of adjacent lacunae
 * syncytiotrophoblast erodes endometrial blood vessels, allowing maternal blood to sep in and out of lacunar networks establishing uteroplacental circulation
 * defect in the endometrial epithelium gradually disappears as the epithelium is repaired
 * primary chorionic villi develop
 * **Clinical significance of extrauterine implantations**
 * Extrauterine implantations are when the blastocyst implants outside the uterus, which result in ectopic pregnancies. 95% of ectopic implantations occur in the uterine tube. This can cause substancial harm to both mother and fetus. MORE
 * **Meaning and significance of cavitation**
 * Cavitation occurs when the trophoblast, surrounding the morula, secretes fluids, causing a cavity to form. WHAT IS THE SIGNIFICANT OF CAVITATION?
 * **Formation of the epiblast and the hypoblast**
 * The
 * **Contributions of the trophoblast (major cell types? what functions?)**
 * **Formation of the chorionic cavity and cell layers forming the chorion**


 * Overview:**