Development+of+Bilaminar+Embryo

=**Objectives**=


 * 1. Define and describe: hypoblast, epiblast, primitive streak, primitive node, notocordal process, prochrodal plate, cloacal plate, notocord, ectoderm, mesoderm, and endoderm.**

__Hypoblast__ Constists of small cuboidal cells adjacent to the exocoelomic cavity

__Epiblast__ The thicker layer consisting of high columnar cells related to the amniotic cavity

__Primative Streak__ A thickened linear band of epiblast appearing caudally in the median plane of the dorsal aspect of the embryonic disk.

__Primitive Node__ Proliferating cells at the cranial end of the primitive streak.

__Notocordal Process__ Mesenchymal cells that migrate cranially from the primitive node and pit, forming a median cellular rod.

__Prochrodal Plate__ A small circular area of columnar endodermal cells where the ectoderm and endoderm contact, preventing further extension of the notochordal process cranially. This is the future site for deveopment of the mouth.

__Cloacal Plate__ Circular area caudal to the primitive streak where the future side of the anus will develop. Remains bilaminar here, preventing the migration fo mesenchymal cells between them.

__Notocord__ Cellular rod that develops by tranformation of hte notochordal processes. Instructive signals from the primitive streak region induce notochrodal precursors cells tof or the notocord.

__Ectoderm (Embryonic)__ Cells that remain in the epiblast after mesenchymal cells begin migrating to from the embrynoic mesoderm.

__Mesoderm (Embryonic)__ Formed from migrating mesenchymal cells that come from the deep surface of the primitive streak. These fells form mesoblast whcih eventually forms the mesoderm.

__Endoderm (Embryonic)__ Cells from the epiblast displace the hypoblast during msenchymal cell migration and from the endoderm.


 * 2. Describe the formation of the bilaminar embryonic disk from the inner cell mass.**

By the blastocyst stage, the embryo has formed a hollow ball of cells with the inner cell mass (embryoblast) off to one side and the blastocystic (exocoelomic) cavity filling up the rest of the sphere. As implantation progresses, a small space appears in the embryoblast and forms the amniotic cavity. Concurrently, morphological changes ocur in the embryoblast that result in the formation of a flat, almost circular bilaminar plate of cells, the embryonic disk, which incldues the epiblast and the hypoblast. The epiblast formsthe floor of the amniotic cavity and is continusuous with the amnion. The hypoblast forms the roof of the exocoelomic cavity and is continuous with the thin exocoelomic membrane.


 * 3. Describe the formation of the extraemrbyonic mesoderm.**

The exocoelomic membrane along with the hypoblast forms the primary yolk sac. Cells from the yolk sac form a layer of connective tissue, the extraembryonic mesoderm which surrounds the amnion and the yolk sac. Later, etraembryonic coelomic spaces form within the extraembryonic mesoderm and fuse, forming the extraembryonic coelom.


 * 4. Describe the formation of the primary and secondary yolk sacs.**

The exocoelomic membrane along with the hypoblast forms the primary yolk sac. As the extraembryonic coelom forms, the primary yolk sac decreases in size and a secondary yolk sac forms as epidermalcells migrate down and displace the hypoblastic cells. Eventually, the primary yolk sac is pinched off.


 * 5. Describe the formation of the body (connecting) stalk.**

As the extraembryonic coelom forms, it surrounds the amnion and the yolk sac except where they are attached to the chorion, where the connecting stalk now forms.


 * 6. List three functions of the secondary yolk sac.**

The yolk sac contains no yolk. It has a role in the //transfer of nutrients// to the embryo during the second and third weeks when the uteroplacental circulation is being established. //Blood development// first occures in the well vascularized extraembryonic mesoderm covering the wall of the yolk sac beginnig in the third week and continues to form there until hemopoietic activity begins in the liver during the sixth week. During the fourth week, the endoderm of the yolk sac is incorporatedinot the embryo as the //primordial gut//. Its endoderm, derived from epiblast, gives rise to the epithelium of the trachea, bronchi, lungs, and digestive tract. //Primordial germ cells// appear in the endodermal lining of the wall of the yolk sac in the third week and subsequently migrate to the developing sex glands. They differentiate into the germ cells (spermatogonia in males and oogonia in females).


 * 7. Define and describe: lacuna, intervillous space, decidual basalis, decidual capsularis, decidual parietalis, amnion, and chorion.**

__Lacuna__ Isolated cavities in the syncytiotrophoblast that appear as the amnion,embryonic disk, and primary yolk sac form. Lacunae soon fill with a mixture of maternal blood from ruptured endometrial capillaries and cellular debris from eroded uterine glands. Materinal blood in the lacunae also contain bCG produced by the syncytiotrophoblast, which maintains the corpus luteium. Fluid in teh lacunar spaces, embryotrophe, passes to the embryonic disk by diffusion and provides nutritive material to the embryo.

__Intervillous Space__ Contains maternal blood derived from the lacunae that develop in the syncytiotrophoblast during the seocnd week of development. This large blood-filled space results from coalescence and enlargement of the lacunar networks. The intravillous space is divided into compartments by the placental septa. There is free communication between compartments because the septa do not reach the chorionic plate.

__Decidual Basalis__ Part of the decidua deep to the conceptus that forms the maternal part of the placenta.

__Decidual Capsularis__ Superficial part of the decidua overlying the comceptus.

__Decidual Parietalis__ All the remaining parts of the decidua.

__Amnion__ The amnion encloses the amniotic cavity.

__Chorion__ The extraembryonic somatic mesoderm and the two layers of trophoblast form the chorion. The chorion forms the wall of the chorion sac, within which the embryo and its amniotic and yolk sacs are suspended by the connecting stalk.


 * 8. Describe the formation of the amnion and the chorion.**

Amnion forms after inplantation as a small space appears in the embryoblast, forming the amniotic cavity. Amniogenic cells called amnioblasts separate from teh epiblast and linet ha mnion, enclosing the amniotic cavity.

Chorion forms after extraembryonic mesoderm forms and develops extraembryonic coelomic spaces. These spaces fuse and produce the extraembryonic coelom with extraembryonc mesoderm on the inside lining. The extraembryonic mesoderm plus the surrounding cytotorphoblast and syncytiotrophoblast make up the chorion.


 * 9. List the functions of the placenta.**

The placenta serves functions in metabolism (synthesis of glycogen), transportation of gases and nutrients, and endocrine secretion (hCG). Placenta synthesizes glycogen, cholesterol, and fatty acids particularly during early pregnancy for sources of nutrients and energy for the embryo/fetus. Transport of substances in both directions between the placenta and maternal blood is faciliated by the great surface area of the placental membrane. Protein hormones secreted by the placenta incude human chorionic gonadotrophin, human chorionic somatomammotrophin or human placental lactogen, human chorionic thyrotrophin, and human chorionic corticotrophin.


 * 10. Describe the morphological changes that occur during the development of the placental villi.**

Cytotrophoblast invades into the syncytiotrophoblast and forms the primary villi projections with the interlacunar network. When extraembryonic mesoderm coat over the primary villi, the villi, cytotrophoblast, syncytiotrophoblast, and extraembryonic mesoderm collectively become the secondary villi. When fetal blood vessesl develop within the secondary villi, it becomes a tertiary villi. Eventuallythe cytotrophoblast will thin down and allow material exchange to occur through the barrier.


 * 11. Describe the components of the placental membrane or barrier.**

The placental membrane consists of the syncytiotrophoblast, cytotrophoblast, connective tissue of villus, and the endothelium of fetal capillaries.


 * 12. Describe maternal and fetal circulation in the placenta.**

Poorly oxygenated blood leaves the fetus and passes through the umbilical arteries to the placenta. At the site of attachment of the cord to the placenta, these arteries divide into several radially disposed chorionic arteries that branch freely in the chorionic plate before entering the chorionic villi. The blood vessels form an extensive arterio-capillary-vernus system within the chorionic villi which brings fetal blood extremely close to the maternal blood. This system provides a large surface area for the exchange of metabilic and gaseous products between maternal and fetal blood streams. //There is normally no intermingling of fetal and maternal blood//. However, small amounts of fetal blood may enter the maternal circulation when minute defects develop in the placental membrane. The well oxygenated fetal blood inteh fetal capillaries passes into thin walled veins that follow the chorionic arteries to the site of attachment of the umbilical cord. THey converge here to form the umbilical vein. Tihs arge vessel carries oxygen-rich blood to the fetus.

Maternal blood in the intravillous space is temproary outside the maternal circulatory system. It enters the intervillous space through 80-100 spiral endometrial arteries in the decidua basalis. These vessles discharge into the intervillous space through gaps in the cytotrophoblastic shell. The blood flowed from the spiral arteries is pulsatile and is propelled in jetlike fountains by maternal blood pressure. The entering blood is at a considerably higher pressure than that in the intervillous space and spurts toward the chorionic plate formign the "roof" of the intervillous space. As the pressure dissipates, the blood flows slowly over the branch villi, allowing an exhcnage of metabolic and gaseous products with the fetal blood. THe blood eventually returns through teh endometrial veins to the maternal circulation.


 * 13. Describe the formation of the amniontic fluid.**

Some amniotic fluid may be screted by amniotic cells. However most fluid is dervied from //maternal tissue// by diffusion across the amniochorionic membrane from the decidual parietalis. Later there is diffusion of fluid through the chorionic plate from blood in the intervillous space of the placenta. Fluid is also secreted by the fetal respiratory tract and enters the amniotic calvity. By the 11th week, the feuts contributes to the amniotic fluid by execreting urine into the amniotic calvity.


 * 14. List several functions of the amniotic fluid.**

Amniotic fluid allows symmetrical external growth of the embryo and fetus, acts as a barrier to infection, permits normal fetal lung development, prevents adherence of the amnion to the embryo and fetus, cushions the embryo and fetus by distributing impacts the mother recieves, helps controlthe embryo's body temperature by maintaining a relatively constant temperature, enables the fetus to move freely to aid muscular development in the limbs, and is involved in maintaining homeostasis of fluid and electrolytes.


 * 15. Define amniocentesis and list several indications for the procedure.**

Acommon invasive prenatal diagnostic procedure. Amniotic fluid si sampled by inserting a hollow needle through the mother's anterior abdomenal and uterine walls into the amniotic cavity by piercing the chorion and amnion. A syringe is attached and amniotic fluid is withdrawn. Indications for amniocentesis are:


 * Advanced maternal age
 * Previous birth ofa trisomic child
 * Chromosomal abnormality of eitherparents
 * Women who are carriers of x-linked recessive disorders
 * History of neural tube defects in the family
 * Carriers of inborn errors of metabolism