Embryology Ch 2 and 3 9-2 Flashcards
Embryo week 2
Week of 2s. Lots of cavities, layers, becoming a bilaminar disc. Implantation finished. Trophoblast differentiates into two layers, embryoblast forms 2 layers (hypoblast and then some), extraembryonic mesoderm splits into 2 layers, 2 cavities form -amniotic and choreic cavity.
Embryo week 3
Focus on beginning to form body axis of embryo (notochord, primitive streak, become trilaminar disc).
Trophoblast differentiated
Day 7 or day 8 - cytotrophoblast and syncitiotrophoblast. form.
Embryoblast differentiates
Day 8 - hypoblast and epiblast. Flat cells along interior base form hypoblast (primary endoderm). Epiblast stay where it is (near cytotrophoblast).
Epiblast
Hugely important - all 3 of primary tissue layers originate from this layer. Endoderm, ectoderm, mesoderm.
Hypoblast
Flat cells along interior base (primary endoderm).
Amniotic cavity
Within epiblast region. This forms day 8. Amnioblasts are epiblast cells adjacent to cytotrophoblast. This occurs at the same time as the creation of the difference between epiblast and hypoblast layers.
Amnioblast
Epiblast cells that push outwards to form amniotic cavity. Just adjacent to cytotrophoblast.
Day 9
Fully implanted (little fibrin coagulum plugs the cell in - sealing it essentially).
Exocoelomic (heuser) membrane
Hypoblast cells are displaced along cytotrophoblasts, forms a lining of exocoelomic cavity (primitive yolk sac, primary umbilical vessicle).
Fibrin coagulum
Fills in tiny space in endometrial wall. Day 9 Happens in syncitial layer. Fibrin coagulum (connective tissue/scar), within external syncitial layer there are tiny decayed spots that forms Lacunae.
Vacuoles
Appear in syncytium of embryonic pole.
Lacunae
Small holes/pits, important for developing blood flow.
Lacunar stage
Begin to form vacuoles (see slides)
Day 11 and 12
Trophoblast characterized by lacunar spaces in the syncytium, Forms intercommunicating network.
Cells of syncytiotrophoblast erode endothelial lining of maternal capillaries. Setting up distribution for nutrients.
Syncytial Lacunae
become continuous with sinusoids. Establishes uteroplacental circulation.
Sinusoids
**
Extraembryonic mesoderm
Form extra-embryonic structures, New cells appear between inner surface of cytotrophoblast and outer surface of exocoelomic cavity. As you form this, you get tiny fluid filled spaces. Known as Extraembryonic fluid filled spaces which come together to form a new cavity.
Extraembryonic cavity
Coreonic cavity - tiny spaces form giant corionic cavity. As this forms, there is distinct mesoderm that surrounds lining of cytotrophoblasts and amnion, this is the extraembryonic somatic mesoderm. Day 11-12
Extraembryonic somatic mesoderm
Lines inner lining of cytotrophoblasts and amnion.
Extraembryonic splanchnic mesoderm
Lines yolk sac, is exposed to extraembryonic cavity.
Connecting stalk
Connects yolk sac and amniotic cavity to endometrium.
Day 13 **listen to this slide
Cytotrophoblasts are still out there, most of the syncitial layer is gone, and primary villi … Hypoblast produces cells that form secondary yolk sac (definitive yolk sac)
Secondary Yolk Sac
Definitive yolk sac. Exocoelemic cysts in the extraembryonic coelom (chorionic cavity).
Chorionic cavity forms
Day 13 - extraembryonic mesoderm lining the inside of cytotrophoblast. This is known as the chorionic plate
Chorionic plate
extraembryonic mesoderm lining the inside of cytotrophoblast. Important in the extablishment of the connecting stalk and develops into extraembryonic structures. *does not include stalk
Connecting stalk
Will develop into the umbilical cord with development of blood vessels.
Prechordal plate
If you look at what will become the embryo on cross-section, you see tight fusion between hypoblast and epiblast. There are columnar cells, and these cells along with the general area will become the oral cavity.
Recap of week 2
Implantation finished, uteroplacental ciruclation established via lacunae blood vessels. Embryoblast forms bilaminar disc, amniotic and chorionic cavities form. Extraembryonic mesoderm forms - somatic (amniotic sac) and splanctic (yolk sac), primary and secondary yolk sacs.
Week 3
Formation of primitive streak and node (leads to gastrulation). Gastrulation occurs- which forms 3 primary tissue layers. All 3 originate from epiblast. Formation of notochord occurs (solid rod that runs dorsal to embryo that is important for nervous system - provides axis). Embryo also grows significantly.
Gastrulation
Formation of 3 layers - ectoderm, mesoderm, endoderm. All layers formed from epiblast. Begins with formation of primitive streak. Starts with swellings in epiblast.
Primitive streak
Raised ridge towards posterior grows towards cranial end. This ridge is the streak. Two ridges with a groove. Occurs in epiblast as cells start to move. This will continue to grow cranially (to prechordal plate/oropharyngeal membrane). Will ahve distinct cephalic end (primitive node) which surrounds the primitive pit.
Oropharyngeal membrane
Will become mouth
Primitive pit
Where cells will move their way under the surface towards the hypoblast. Moves. Detach cells from epiblast, move down into pit or groove.
Invagination
Epiblast cells that go down into hypoblast that displace and replace hypoblast. This forms definitive endoderm. Those that lie between epiblast and endoderm will become mesoderm. Cells that do not detach and migrate remain in epiblast to form ectoderm.
Gastrulation
Cells continue to move laterally and cranially. In cephalic diretion, cells pass on each side of prechordal plate. Now a trilaminar disc.
Prechordal plate
Formed between tip of notochord and oropharyngeal membrane.
Notochord formation
Occurs w/gastrulation. Acts as rod that creates the backbone to the formation of the nervous system. Grows until prechordal plate, then once it hits that plate, the structure starts to acquire a lumen.
Prenotochordal cells
invaginate in the primitive pit. Instead of forming primary tissue layers, forms notochordal process. This process pushes forward to prechordal plate. This also helps push everything to prechordal plate.
Notochordal canal
Lumen in notochord that occurs once it hits the prechordal plate.
Notochordal process
floor fuses with endoderm below and it degernates. Notochordal process is close to developing endoderm. Once it fuses, it degerates, leading to opening between amnionic cavity and umbilical vesicle.
Neurenteric canal
Canal between amnionic cavity and umbilical vesicle - very brief. You need to look at images for this.
Notochordal plate
Remains of notochordal process after formation of neurenteric canal. Rises back up off of endoderm. As it does this it separates to form notochord.
Notochord
Initially a single rod - not hollow. Forms all of intervertebral discs.
Cloacal Membrane
Formed at caudal end of embryonic disc. Similar to oropharyngeal membrane. Yolk sac forms diverticulum that extends into connecting stalk. Allantois forms around day 16. *Will eventually form anus.
Allantois
Not any real function in developing humans (have more of a play in orgs. that develop in eggs). Forms around day 16 caudally.
Due to primitive streak, we’ve established
Body axis. (primitive streak forms caudally to cranially, you also have a right and left side, and on a sagittal section you can figure this out as well).
Beginning of week 3 trophoblast
Further develops - characterized by primary villi. Primary villi nothing more than cytotrophoblastic core covered by syncytial layer. Once blood comes here, mesoderm penetrates core of villi.
Secondary villus
Has extra-embryonic mesodermal core (different from others), covered by cytotrophoblast and then syncytio…. End of week 3, these mesodermal c ells differentiate into bloodcells and small blood vessels. These form…
Tertiary villus
Core of mesoderm, blood islands within it, cytotrophoblast, syncytio, etc. Will form blood connection in and will have a play in umbilical cord development. Does this through vasculogenesis
Vasculogenesis
Blood vessels arise from blood islands. Hemangioblasts accumulate and form vessels.
Angiogenesis
Blood vessels sprouting from existing vessels.
Further trophoblast development
Capillaries in tertiary villi make contact with capillaries developing in extra-embryonic mesoderm of chorionic plate and in connecting stalk THIS IS OUTSIDE OF THE EMBRYO. Choreonic plate = extra-embryonic mesoderm. Choreonic cavity enlarges, embryo attaches to trophoblastic cell by connecting stalk.
Intraembryonic circulatory system
Connecting the placenta and the embryo
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