Early Embryonic Development 2Dr. JohnsonWeek 6, Friday 9/19/14

Lesson Objectives1. Describe fate of epiblast and hypoblast of bilaminar disc embryo2. Describe role of primitive streak in defining future embryonic axes3. Describe cell movements of gastrulation4.Describe origin of notochord5. List fates of ectoderm, mesoderm, and endoderm6. Using esophagus, learn germ layers sources of various tissues7. Describe saccrococcygeal teratoma

PART 1: Fertilization

*Summary: We have a bunch of follicles developing, one of those follicles ovulates and the remnants of the unovulated part of the follicle form the corpus luteum. The 2 oocyte is dumped out into the female reproductive tract and it begins to be transported. Meanwhile spermatozoa come from the opposite direction, meet up and fertilize. At the end of about 2 days or 30 hours, were at 2 cells, 3 days were at the morula, 4 days a blastocyst that hasnt hatched yet and at around 5 days blastocysts hatch out of the zona pellucida and they attach to the endometrium and begin to implant. So were about 6 days in at the beginning of implantation

A. Human Chorionic Gonadotropin (hCG)1. Secreted by syncytiotrophoblasts (STB)2. hCG has alpha and beta subunits: alpha subunit is common to several hormones however the beta subunit is unique to the hCG hormone. hCG is a glycoprotein. The alpha subunit is also found in human placental glycogen and other hormones. 3. Pregnancy tests accurate within 7 days of fertilization via sensitive RIA(radioimmunoassay) for blood beta-hCG4. Home test kits less sensitive colorimetric assays-soon after missed period, can detect beta-hCG in urine. The assay has hCG antibodies that will recognize the hCG antigen and will produce a color change that shows up on the stick. By the time you miss a period, you are about 2 weeks out after fertilization.

B. If Fertilization Occurs1. STB becomes source of hCG2. hCG stimulates the pituitary to continue progesterone secretion by corpus luteum3. No menses4. First missed period and pregnancy

5. This is an implanted embryo and we will look at what happens in the bilaminar disc: there are two cell layers now consisting of the epiblast and the hypoblast. C. Bilaminar Disc1. Epiblast is floor of amniotic cavity2. Hypoblast is roof of yolk sac3. Two epithelial layers sandwiched4. Primitive streak forms as mid-line invagination depression in epiblast

*The image on the left shows 4.1A which is the implantation site at the end of the second week. You can see the syncytiotrophoblast and the cytotrophoblast. You can see the two layers of cells that help to delineate between the amniotic cavity and the definitive yolk sac. Both layers epiblast and hypoblast are shown in 4.1B in a cross sectional view. The hypoblast faces the inner yolk sac and the epiblast faces the inside of the amniotic cavity. So the amniotic cavity is shaped like an oval. The epiblast is one layer of epithelial cells. Down the center is an invagination site(indentation) that is called the primitive streak. This is the site where gastrulation starts. The epiblast cells which are tightly coupled, will begin to migrate from the lateral ends medially toward the primitive streak and will travel through the streak and migrate laterally back toward the outside of the amniotic cavity. The result is that the cells will displace the hypoblast and will create another layer of cells. SO in the end, the top layer of cells will be called the ectoderm, the lower layer is called the endoderm and the group of cells at the invagination site will become the mesoderm. * Now the image on the right. In the center of the amniotic cavity epiblast cell layer is the primitive node which touches the tip of the primitive streak. So the cells will start at the head of the primitive streak(which is touching the outer layer of amniotic cavity) and will migrate toward the primitive node and will make a U-turn and travel back to the head of the primitive streak. What does this do? This is creating the notochord. By the end of gastrulation there will be the ectoderm, the notochord, two layers of mesoderm and the endoderm underneath.

*This pics are very detailedjust focus on the three layers of ectoderm-mesoderm-endoderm, with the notochord in between the ectoderm and endoderm. Also note that there is intraembryonic mesoderm between the endo and ecto and there is extraembryonic mesoderm which is on the sides

D. Consequences of Primitive Node/streak1. Establishes axis of bilateral symmetry2.Establishes head and tail end3. Serves as site for invagination, migration, and differentiation of epiblast cells to establish three primary germ layers

PART 2: Cell Layer Derivatives

A. Ectodermal Derivatives1. Forms everything you can see in the mirror with eyes and mouth open-epidermis and appendages, cornea, mammary glands, enamel2. Forms brain, spinal cord, and neural crestB. Mesodermal Derivatives1. Forms most of what you are working with in gross lab-connective tissues, muscles, most of urogenital system, all of cardiovascular system and its contentsC. Endodermal Derivatives1. Forms epithelium(only) lining the GI tract and its associated glands-posterior salivary glands, liver, gallbladder and pancreas, auditory tube, palatine tonsils, thyroid, parathyroids, thymus2. Walls of GI tract contain mesodermally derived CT and muscle and intrinsic autonomic innervation from neural crestD. Lets Build an Organ1. Many organs have derivatives from more than one primary germ layer2. Esophagus has luminal lining from endoderm, mural smooth muscle from splanchnic mesoderm, and innervation from ectoderm(via neural crest).

*All GI organs have the lumen through which food passes, epithelial lining and a mucosa. The mucosa is composed of a thin layer of smooth muscle, the muscularis mucosa, loose irregular connective tissue domain called lamina propria. The mucosa consists of muscularis mucosa, lamina propira, and mucosal epithelium. Deep to the muscularis there is a loose CT layer called submucosa. On the outer sections you have muscularis externa, there is also an outer edge of CT called adventitia. So we have Mucosa, Submucosa, muscularis externa and adventitia, those are the layers of any GI organ. *For the esophagus specifically, there is the mucosal epithelium which consists of stratified squamous unkerratinized epithelium, then there is lamina propira, deep to the lamina propria is a thin layer of smooth muscle, the muscularis mucosa. The muscularis mucosa has two layers: the inner layer is circularly arranged with respect to the lumen and the outer layer is longitudinally arranged and runs parallel to axis of lumen. In between the two layers there is a bunch of parasympathetic innervation there. There are neurons and nerve fibers that innervate the layers of smooth muscle here that cause them to contract. They are the motor innervation of that smooth muscle. * So we have the basic structure of this GI organ. What comes from the mesoderm, ectoderm endoderm. These GI organs represent all three. The lumen is stratified squamous unkerratinized epithelium which is endorderm derivative. Everything from the muscularis externa, lamina propria, submucosa, muscularis mucosa and adventitia is mesoderm. So the rest of the wall of the esophagus is mesodermally derived except for the intrinsic innervation, except for the parasympathetic ganglia that are between the two muscle layers. Those cells come from the neural crest, which is an ectoderm derivative. So we have endoderm, mesoderm and ectoderm making up the esophagus.

PART 3: Medical Conditions of Early DevelopmentA. Sarcococcygeal Teratoma1. Persistent Remnants of Primitive Streak form masses of undifferentiated, abnormally arranged tissues.2. Contain Pluripotent cells3. Forms disorganized mass of different tissues from several primary germ layers, ex hair, fat, cartilage, hematopoietic tissue etc4. Most common tumor in newborns(1/30,000 newborns), 3-4:1::F:M ratio5. The cells do not receive proper signals to determine what they should be so they form random tissues

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