The First Three Weeks of Development Flashcards
time frame of embryo
week 1-8
time frame of fetus
week 8-36
What causes most deaths during the first year post-natal (infancy)?
developmental anomalies cause the most deaths during infancy
morphogenesis
activities & processes that take place during embryology
induction
tissue or cell tells another cell to do something (factor acts as intermediary)
coelom
empty space inside organism not connected with outside
zona pellucida
proteoglycan shell that keeps the egg substance together
corona radiata
accumulation of cells around outside of egg
Describe the formation of a zygote
- fusion of pronuclei –> first time in which there is the full complement of chromosomes in human
- happens in Fallopian tube
blastomere formation
- zygote splits into two cells where the mass of the two cells equals the mass of the original cell (the daughter cells are half the size of the first)
- protoplasm being reduced
morula formation
- zona pellucida still exists
- at 16 cell phase –> no longer dividing in sequence
- individual cells, especially cells around edge, develop tight junctions
- ring of cells that are no longer free to float around (morula)
blastocyst formation
- cells on outside are pumping fluid into cavity and pushing inside cells to one end (compaction –> process developing inner cell mass)
- all cells are identical (have same potential) except for their position
- usually in the uterus, ready to embed into wall of uterus
What is a possible result of blastocyst formation in the Fallopian tube?
- may try to embed in Fallopian tube leading to an ectopic pregnancy
hatching
- zona pellucida starts to degenerate
- free to expand
- cells no longer cuboidal –> flatten and stretch out due to one way pumping by exterior cell group
- inner cell mass becomes embryo and exterior cell group supports embryo (trophoblast)
blastocyst attachment to endometrium
- blastocyst rolls around slowly on wall of uterus inducing increased concentration of glands and blood vessels
- when end with inner cell mass gets close, trophoblast attaches to endometrium
syncytiotrophoblasts
- produced by cytotrophoblasts just outside of attachment point to endometrium
- cells are multinucleate (tissue is called syncytium)
- produce enzymes that are inverse and break up ECM of maternal tissue
- cells move into empty space (maternal tissue also building up and embracing embryoblast)
bilaminar embryo
- mass develops into upper (epiblast/ectoderm) and lower (hydroblast/endoderm) layer
8 day embryo
- cells facing fluid-filled cavity (hypoblasts) develop into simple cuboidal epithelium
- cells facing maternal tissue (epiblasts) become pseudostratified columnar, make an empty space that becomes the amniotic cavity
- endometrium proliferating and completely encapsulating embryo
What cells produce the amniotic fluid in the growing amniotic cavity?
- epiblastic derivatives
Discuss the formation of Heuser’s membrane
- from hypoblasts, cells forming edge and creating lining for cavity
- cover cytotrophoblasts
What function to the trophoblastic lacuna play in the developing embryo?
- trophoblastic lacuna are fluid filled spaces in the trophoblast
- maternal blood vessels will invade area and eventually dump blood into the lacuna but this blood does not come into contact with the embryo yet
What function does the coagulation plug have?
- coagulation plug allows doctors to endoscopically identify where the embryo has embedded in the uterine wall
What cells produce the fibrous acellular reticulum just outside Heuser’s membrane?
- cytotrophoblasts
What cells produce the extraembryonic mesoderm?
- hydroblasts produce this subset of cells that move along Heuser’s membrane and along the cytotrophoblasts
primary yolk sac
edge is continuous with edge of hypoblast (fluid of primary yolk sac touching top side of hypoblast
Where does the chorionic cavity being to form?
- forms in the extramembryonic reticulum
What layer of cells comes down and pinches primary yolk sac?
- extraembryonic mesoderm
- will cause the creation of the definitive yolk sac with the outer remnant being pushed away
What function do the primary stem villi serve?
- embryo is getting big and needs to be anchored in the endometrium
- cytotrophoblast make villi projections that extend all the way around outside of embryo to keep it fixed in endometrium
connecting stalk
- holds embryo within the huge chorionic cavity
- becomes the umbilical cord
gastrulation
- beginning is the formation of the primitive streak
- epiblast producing mesoderm
- epiblastic cells leave from primitive streak, turning into mesoderm, and moving into space between ectoderm and endoderm
- cells repopulate middle AND bottom layers –> epiblastic derivatives move into middle and push hypoblasts out to become definitive endoderm
primitive streak
- formation signals the beginning of gastrulation
- long axis along endoderm
- more toward tail end
- little dimple with edge that surrounds –> lifting up causes groove
What causes the dimples in the endoderm? What are they called?
- the dimples are caused from the fusing of embryonic ectoderm and endoderm
- head region dimple = buccopharyngeal membrane (becomes mouth, etc.)
- tail region dimple = cloacal membrane (becomes anus, etc.)
What does gastrulation give rise to?
- definitive endoderm
2. intraembryonic mesoderm
What cell layers are derived from the epiblasts?
All three (ectoderm, mesoderm, and endoderm)
lateral plate mesoderm
group of epiblasts that move laterally to the primitive streak
paraxial mesoderm
group of epiblasts that move up and out from primitive streak
notochordal process
- group of epiblasts that move from dimple and move longitudinally
- cells knock away endoderm and then they themselves leave
- amniotic cavity and definitive yolk sac in contact for a short amount of time
What are the neural crest cells?
- ectodermal derivative
What causes the mesoderm to reorganize at the end of the third week?
- caused by the folding of the neural tube
Describe the process by which the embryo develops primitive circulation
- secondary villus –> some of embryonic mesoderm
- tertiary anchoring villus –> creates circulation (the blood vessels that are found in the umbilical cord)
- maternal and embryonic circulations do not cross over at this point
