Embryology Flashcards
Process of Fertilisation
Capacitation occurs- removes inhibitory molecules so sperm swims faster towards egg-locates egg by chemotaxis.
Acrosomal enzymes disperse the cells of the corona radiata and bind to the fibrous zona pellucida to borrow into the oocyte.
Upon fertilisation, meiosis II completes and fuses during syngamy to form the zygote.
Blastocyst formation
Ball of cells move down the oviduct towards uterus.
24hrs AF (after fertilisation): 6 hour long mitosis to form 2 blastomeres.
48hrs AF: 4 cell stage.
4 days AF: Formation of morula-loose cell ball.
5 days AF: Uterine milk enters morula to form a blastocoel to allow movement of cells and increase cell adhesion. Blastocyst now formed.
Blastocyst structure
Wall of blastocyst: Forms trophoblasts.
Inner cell mass: Forms embryo.
Implantation
Day 6: Zona pellucida digested by enzymes.
Blastocyst sticks to uterus lining.
Day 8: Trophoblasts differentiates to syncytiotrophoblast (burrows into uterus-poly-nuclear mass of cell with no boundaries) and cytotrophoblasts (makes more syncytiotrophoblasts)
Secretes hCG to maintain corpus luteum and increase endometrium vascularity.
Day 9-12: Blastocyst burrowed into uterus. More lacunae formed and join up with uterine sinusoids to form placenta precursor which secretes substances to inhibit lymphocytes.
Inner Cell Mass/Bilaminar disc Development
Splits into epiblast and hypoblast.
Epiblast- precursor to amnion and ectoderm
Hypoblast- yolk sac precursor
During implantation:
Squamous epithelial cells form a dome over the epiblast layer-amnion. Cavity formed will eventually surround embryo and become amniotic sac. Provides mechanical protection.
Hypoblast migrates around blastocyst to form a yolk sac.
Extraembryonic mesoderm forms between trophoblast and bilaminar disc-forms extraembryonic coelom and eventually the chorion.
Gastrulation
Day 16: Primitive streak forms on the epiblast away from connecting stalk. Mass of epiblast forms at the end.
Epiblasts start to move down the slope of the groove in a process called invagination. Cells that detach from the epiblast layer in the process become the endoderm and displace the hypoblast, while the others connected to the epiblast layer between the epiblast and endoderm form mesoderm. Remaining epiblast form ectoderm.
Notochordal Process
Day 16
Cell mass at the primitive node moves downwards into the mesoderm, then migrate towards the head end to leave behind a hollow mesodermal tube. This solidifies with mesodermal cells by day 22-24.
Acts as ‘inducer’-tells what nearby cells to specialise into. Epiblasts above the tube will elongate and form neural plate.
Neurulation
As the notochord forms and the epiblast is specialised into the neural plate, the lateral edges of the neural plate rises up to form the neural groove, which grows towards each other until they meet to form the neural tube.
Cells at the edge of the neural plate that end up at the top of the tube are called the neural crest cells, and they are precursors to the peripheral nervous system.
Notochord instructs paraxial mesoderm tissue to specialise into somites. (Counting from ectoderm)
Dermatome: Connective tissue
Myotome: Muscles in back/neck
Sclerotome: Ribs and vertebrae
Embryonic Folding
Occurs because the ectoderm grows faster than the endoderm, so it folds inwards to form a C-shape bend.
Day 26: Ectoderm/neural tube bends around endoderm to form a tube/ primitive gut in the lateral view. Transverse view shows amnion folding completely around the tube.
Gut specialises into the hindgut (cloacal membrane forms), midgut and foregut (oropharyngeal membrane)
Pharyngeal Structure Formation
Small pharyngeal arches form under the ‘brain’ region to form different areas in the head. Pharyngeal pouches inside the arches connect to the esophagus and form the pharynx.
Optic and otic placodes formed.
Ectoderm develops into (6)
Integumentary, skeletal, nervous, endocrine, respiratory, digestive
Mesoderm develops into (8)
Integumentary, skeletal, muscular, endocrine, cardiovascular, lymphatic, urinary, reproductive
~~~~CONNECTIVE TISSUE~~~~
Endoderm develops into (5)
Endocrine, respiratory, digestive, urinary, reproductive
