Embryology Flashcards
What is the zona pellicuda?
Glycoprotein shell surrounding the blastomeres
What occurs on day 2 of pregnancy?
Cleavage - 2 blastomeres of equal size are produced.
What occurs on day 3 of pregnancy?
A morula is formed (clump of totipotent, identical cells)
What occurs on day 4 of pregnancy?
Compaction of blastomeres occurs, formation of blastocyst (pluripotent)
Describe the structure of the blastocyst
Embryoblast - inner cell mass
Trophoblast - outer cell mass
Blastocyst cavity - space
What occurs on day 5 of pregnancy?
Blastocyst hatches from zona pellicuda. Free to move around, grow and free to implant.
What occurs on days 6/7 of pregnancy?
Implantation occurs. Conceptus has 107 cells - 8 will form the embryo cells and 99 will begin development of foetal membrane
What occurs on day 8 of pregnancy?
Beginning of the week of twos. Bilaminar disk and amniotic cavity are formed. Both embryo and placenta formation begins (placenta initially has priority).
What does the embryoblast differentiate into?
Epiblast and hypoblast
What does the trophoblast differentiate into?
Syncytiotrohphoblast and cytotrophoblast.
What occurs on days 9/10 of pregnancy?
Implantation complete - endothelium is breached. Embryonic pole and abembryonic poles are formed on day 9. Maternal blood flow is established
What is the embryonic pole?
Rapid development of syncytiotrophoblast
What is the abembryonic pole?
Primitive yolk sac formed by Heuser’s membrane, spreading across the blastocyst cavity. Yolk sac membrane is in contact with the cytotrophoblast layer
What occurs on day 11 of pregnancy?
Primitive yolk sac layer is pushed away from cytotrophoblast by acellular extraembryonic reticulum. Reticulum later converted to extraembryonic mesoderm by cell migration
What occurs on day 12 of pregnancy?
Maternal sinusoids invaded by syncytiotrophoblasts, lacunae become more continuous with sinusoids, uteroplacental circulation begins, uterine stroma prepares for embryo support
What occurs on day 13 of pregnancy?
Formation of secondary yolk sac, specialisation of existing structure. Definitive yolk sac pinches off from primitive yolk sac
What occurs on day 14 of pregnancy?
Extraembryonic spaces merge to form the chorionic cavity, connecting stalk suspends the embryo and its cavities in the chorionic cavity
What is the connecting stalk?
Column of mesoderm cells and is the future umbilical cord
Briefly outline the status of the embryo after two weeks
Conceptus has been implanted, embryo and its two cavities are suspended by connecting stalk in the chorionic cavity (supporting sac)
What is placenta previa?
Implantation in lower uterine segment - placenta eventually blocks the opening of uterus and causes severe bleeding in second part of pregnancy and even during birthing. Requires C-section.
What is an ectopic pregnancy?
Implantation at a site other than uterine body
Where is the optimal site of implantation?
Posterior uterine wall
Define ovary
Female reproductive organ. Produces oocytes
Define zygote
Fertilised oocyte
Define fallopian tubes
Tube that connects ovary to uterus. Oocyte travels along this.
Where is the oocyte fertilised?
Ampulla of fallopian tubes
Define uterus
Female reproductive organ where implantation of zygote and growth of embryo occurs
Define cleavage
Dividing of cells into two masses known as blastomeres. Occurs ~30 hrs after fertilisation
Define blastocyst
Formed from compaction of cells that make up the morula
Define trophoblast
Outer cell mass which will later form supporting structures for the embryo (e.g. placenta)
Define embryoblast
Inner cell mass - will later become the bilaminar disk
Define implantation
Attachment of blastocyst to the wall of the uterus 6-7 days
Define cytotrophoblast
Placental membrane around yolk sac
Define syncytiotrophoblast
Cells that invade maternal sinusoids resulting in uteroplacental circulation
Define bilaminar
Epiblast and hypoblast. Derived from the embryoblast. This will later become the trilaminar disk with the formation of the primitive streak
Define germ layer
Layer from which other developed tissues originate
Define ectoderm
Outermost layer of cells giving rise to epidermis and nerve tissue
Define endoderm
Innermost layer of cells giving rise to gut lining
Define dorsal surface
Outermost surface during early development
Define ventral surface
Innermost surface during early development
Define mesoderm
Middle germ layer giving rise to muscle, bone and more
Define primitive streak
Narrow groove that develops on the dorsal surface of the epiblast. Important role in orientation of embryo, determining front and back
Define gastrulation
Reorganisation of germ layers into ectoderm, mesoderm and endoderm
Define trilaminar disk
Three germ layers. Ectoderm, mesoderm and endoderm
What are teratogenic compounds?
Abnormalities of physiological development. Examples include: thalidomide, rubella, alcohol
Give examples of ectodermal derivatives
skin, skin derivatives, brain and spinal cord, peripheral nerves, retina and iris of eye
Give examples of mesodermal derivatives
Smooth, skeletal and cardiac muscle, connective tissue, bone, cartilage, blood and blood vessels, urinary system
Give examples of endodermal derivatives
Lining of digestive tract, glands associated with digestion, other gut derivatives (e.g. lungs)
Which period is most perilous for developing child?
Embryonic period (weeks 3-8)
How does gastrulation start?
Appearance of primitive streak (primitive node + primitive pit). Cellular rearrangement occurs - migration/invagination
When does the primitive streak appear?
Third week
Where is the primitive node located?
Cranial end of the streak
Which direction does development proceed in?
Cranial-caudal
Describe migration and invagination
Cells on epiblast migrate towards primitive streak, invaginate into epiblast and displace hypoblast - creating mesoderm. Spread laterally and cephalad. Two gaps in mesoderm
What is the function of the notochord?
Basis of the axial skeleton and drives neurulation
What is the notochord?
Solid rod of cells running in the midline with an important signalling role - defines the midline
What is the vestigial remnant of the notochord in adults?
Nucleus pulposus of the intervertebral discs
Describe the status of the developing child by the end of week 3
Three germ layers placed anteriorly, axes are established.
How does neurulation occur?
Notochord signals overlying ectoderm to thicken - edges elevate out of plane and curve towards each other creating the neural tube
What are somites?
Segments of the paraxial mesoderm
What does folding achieve?
Creation of a ventral body wall, embryo is suspended in amniotic sac, connecting stalk pulled ventrally, creation of coelom, 3D structure
How many pairs of somites are there?
42/44 by the end of week 5, some then disappear leaving 31 pairs
How is the coelom formed?
With the differentiation of mesoderm
Give examples of derivatives of paraxial mesoderm
Axial skeleton, dermis, muscles of axial body wall, some limb muscles
Give examples of derivatives of somatic mesoderm
Connective tissue of limbs, contributes to axial body wall
Give examples of derivatives of splanchnic mesoderm
Smooth musculature, connective tissue and vasculature of gut
Give examples of derivatives of immediate mesoderm
Urogenital system e.g. kidneys, ureters, gonads
How is sclerotome formed?
Ventral wall of somite breaks down
How are dermatome and myotome formed?
Organisation of the dorsal portion forms the dermomyotome. Myotome proliferates and migrates and the dermatome disperses
What is the dermatome?
Part of somite that gives rise to the skin; strip of skin supplied by a single spinal nerve
What is the myotome?
Gives rise to muscles; muscle/group of muscles supplied by a single nerve
Briefly describe embryonic folding
Cephalocaudal folding. Heart lies at cranial end, moves inwards until it lies in chest area. Yolk sac extends to form a connection between yolk sac and gut
