Embryology Flashcards
When are the pre embryonic, embryonic and foetal periods of human development?
Pre embryonic = weeks 1-2
Embryonic = weeks 3-8
Foetal = weeks 9-38 (+/- 1/2 weeks is normal)
When is pregnancy calculated from?
Date of lmp so term is 40 pregnancy weeks.
How does 1 cell become a multi cellular body?
Growth:
Morphogenesis, lot of individual cells, development of form and structure.
Differentiation, cells of the right type, specialisation for function.
What is a zygote?
Fertilised ovum/oocyte
What is cleavage?
The dividing of cells into two masses known as blastomeres. First cleavage happens around thirty hours after fertilisation.
What is the Zona pellucida?
The glycoprotein shell to prevent polyspermy.
What is the morula?
The result of cleavage of the fertilised oocyte, each cell is totipotent.
What is the blastocyst?
Formed from the compaction of cells that make up the morula.
What is trophoblast?
Outer cell mass that will later form support structures for the embryo eg. Placenta
What is embryoblast?
Inner cell mass that will later become the bilaminar disk.
What is implantation?
The attachment of the blastocyst to the wall of the uterus at day 6/7
What is cytotrophoblast?
Derivative of the trophoblast, placental membrane around the the yolk sac
What is the syncitiotrophoblast?
Derivative of trophoblast, cells that invade maternal sinusoids (irregular blood vessels) resulting in uteroplacental circulation.
What are the first and second developmental processes?
1st - cleavage (first mitotic division), formation of morula, clump of identical cells.
2nd - compaction, formation of blastocyst.
When does implantation begin?
When blastocyst makes contact with the endometrium of uterus, establishing of pregnancy
What is formed during compaction at day 4?
Blastomeres making up morula compact to form the blastocyst. Form embryoblast, trophoblast and blastocyst cavity encased in Zona pellucida.
What is hatching? Day 5
When the blastocyst hatches from the Zona pellucida. No longer constrained and free to enlarge. Can now interact with the uterine surge to implant.
What are some assistive reproductive techniques?
Oocytes can be fertilised in vitro and allowed to divide until the 4/8 cell stage where the morula is then implanted into the uterus.
PGD-pre implantation genetic diagnosis. A cell can be safely removed from the morula and tested for serious heritable conditions prior to transfer of the embryo into the mother if couple have serious risk of inheritable conditions.
How is the blastocyst cavity formed during compaction?
The morula secrete a small amount of tissue fluid which collects and forms the cavity.
Are the cells pluripotent Or totipotent after compaction?
Pluripotent
What has the priority at the beginning of development?
The placenta
In week 2 distinct layers form from the inner and outer cell mass. What are they and what are their features?
From trophoblast:
Syncitiotrophoblast, multi nucliated layer/ sheet for support and good for transport.
Cytotrophoblast, seed layer for the syncitiotrophoblast
From the embryoblast:
Epiblast
Hypoblast
Both become bilaminar disk
What happens by the end of the second week?
Concept is has implanted, the embryo and its 2 cavities (amniotic cavity and yolk sac) will be suspended by a connecting stalk within a supporting sac (chorionic cavity)
How is the amniotic cavity produced?
Cells keep producing tissue fluid
What happens in the second week?
The embryo and placenta start to be formed. At the earliest stages the placenta has priority. Differentiation occurs in the layers of cells and the bilaminar disk and the amniotic cavity are formed.
What happens in implantation?
It is interstitial (one tissue within another). The uterine epithelium is breached and the conceptus implants within the uterine stoma. As it is invasive and leaves a hole in the endometrium a fibrin plug is needed to repair it. The embryo them has access to all the glands of the glandular epithelium.
What does implantation do?
Establishes maternal blood flow within the placenta. Support of embryo changes from histiotrophic (relies on simple tissue diffusion only) to haemotrophic (relying on support of maternal circulatory system)
Establishes basic structure of materno-foetal exchange.
What are implantation defects a result of?
Implantation in the wrong place
What are some examples of implantation defects?
IUGR, interuterine growth restriction Preeclampsia Both more subtle implantation processes Ectopic pregnancy Placenta praevia Both implantation in the wrong place.
What happens in an ectopic pregnancy?
Implantation at site other than the uterine body (most commonly Fallopian tube)
Can be peritoneal or ovarian
Can very quickly become life threatening emergency, due to haemorrhage. Implantation is invasive and will implant wherever it ends up, there are lots of blood vessels in the hips
What happens in placenta praevia?
Implantation in the lower uterine segment across point of exit at cervical opening, placenta at risk of bleeding during pregnancy, require c section delivery.
What is the embryonic pole and the abembryonic pole? Day 9
Embryonic- rapid development of syncytiotrophoblast.
Abembryonic- primitive yolk sac formed by hypoblast cells. Yolk sac membrane in contact with cytotrophoblast layer
What happens after the primitive yolk sac is formed? Day 11
Primitive yolk sac membrane is pushed away from cytotrophoblast layer by an acellular extra embryonic reticulum (primitive ground substance)
Reticulum later converted to extra embryonic mesoderm by cell migration (differentiates)
What role does the syncytiotrophoblast have on day 12?
Invades the maternal sinusoids which are capillaries that empty into and fill the spaces in the lacuna which are ready to be filled with blood (spaces in the syncytiotrophoblast). Lacunae become continuous with sinusoids and so uteroplacental circulation begins.
How is the secondary yolk sac formed? Day 13
Pinches off from primitive yolk sac, aka definitive yolk sac.
How is the chorionic cavity formed and what is the connecting stalk? What can bleeding around this time be confused with? Day 14
Spaces within the extra embryonic mesoderm merge to form chorionic cavity.
The connecting stalk suspends the embryo and its cavities in the chorionic cavity. The connecting stalk is a column of mesoderm and is the future umbilical cord.
Bleeding can be confused with menstrual bleeding.
What is the bilaminar disk?
The epiblast and hypoblast. Derived from the embryoblast. Will later become trilaminar disk with formation of primitive streak.
What is the germ layer?
A layer from which other develop tissues originated from
What is ectoderm?
Outermost layer of cells giving rise to epidermis and nerve tissue
What is endoderm?
The innermost layer of cells giving rise to the gut lining
What is the doral surface?
The outermost surface during early development.
What is the ventral surface?
The innermost surface during early development.
What is the mesoderm?
The middle germ layer giving rise to muscle, bone etc
What is the primitive streak?
A narrow groove that develops on the dorsal surface of the epiblast. It has an important role in the orientation of the embryo, determining the front and the back
What is gastrulation?
The reorganisation of the germ layers into the ectoderm, mesoderm and endoderm and hence establishing the origin of all tissues.
What are some ectodermal derivatives?
Organs that maintain contact with the outside world
Skin
Skin derivatives eg nails, hair, mouth and anus lining.
Brain and spinal cord
Peripheral nerves
Retina and iris of eye
What are some mesodermal derivatives?
Gives rise to biggest variety of tissues. Smooth, skeletal and cardiac muscle Connective tissue Bone Cartilage Blood and blood vessels Urinary system
What are some endodermal derivatives?
Internal structures
Lining of digestive tract
Glands associated with digestion eg liver, pancreas
Other gut derivatives eg lungs
Overall, what happens in the embryonic period?
Greatest period of change
All major structures and systems are formed
Most perilous for developing child, where congenital defects originate.
Why is week 3 the week of threes?
Three cavities are present, amniotic cavity, yolk sac, chorionic cavity
Three germ layers are formed.
Gastrulation - when, what, how, why?
When- 3rd week, marks start of embryonic period
What- establishes the three germ layers, bilaminar to trilaminar disk to achieve endoderm, mesoderm and ectoderm. Axis observed in an adult are set.
How- starts with appearance of primitive streak (primitive node with primitive pit). Forms in caudal epiblast, leading to migration (controlled and highly regulated) and invagination (interdigitate between other layers).
Why- ensure correct placement (right cells in right place) of precursor tissues to allow subsequent morphogenesis to take place.
What are the axis observed in an adult?
Anterior/posterior
Dorsal/ventral
Right/left
How is the primitive streak formed?
Appears on the dorsal surface of epiblast. Streak is a narrow groove with ridges at the edges. The primitive node (coordinated all the cell signals responsible for migration) is located at the cranial end. The primitive out is at the centre of the node. As gastrulation proceeds the primitive streak regresses. Development precedes in cranial/rostral to caudal direction.
What do the hypoblast do?
Line primitive yolk sac and create the secondary yolk sac, once they’ve done this they’ve done their job and are finished so do not form the trilaminar disk.
What is migration and invagination?
The epiblast cells have to move from where they are and then penetrate and spread out.
What happens in migration and invagination?
Cells on the epiblast migrate towards the primitive streak. The then invaginate into the epiblast and displace the hypoblast creating a third layer, the mesoderm. As more cells migrate through the streak, they spread laterally and towards the head. Mesoderm spreads between the ecto and endoderm leaving only 2 gaps/holes for the future mouth and anus.
What is the notochord and how is it formed?
Invagination in the cranial end produces the notochord which defines the phylum Chordata. It is the basis for the axial skeleton and drives the formation of the CNS. Prenotochordal cells of epiblast migrate through the the cranial part of the primitive pit. Forms a solid rod of cells (instead of getting instructions to spread out its told to form a solid rod) running in the midline with an important signalling role. Defines the midline (divides trilaminar into two equal parts) and axial skeleton forms around it. Then it regresses.
What does the primitive node (axis) ensure and why?
Defines front and back and Correct doral-ventral and left-right development as it has molecular signals that emanate from it.
What makes up the embryonic disk at the end of week three?
Three germ layers in place anteriorly. Because development proceeds head to tail, gastrulation is not complete in the caudal region until week 4. Axes are established. Primitive node has major role in this due to giving the signals for positioning.
After gastrulation, is the embryonic disk bilateral?
No, clear left and right in neonates. Eg thoracic and abdominal viscera, liver on right, stomach and spleen on left, left lung has 2 lines and right lung has 3.
What are the axis in the embryo?
Rostral/cephalic (anterior, head unlike in adults) and caudal (posterior, tail unlike in adults).
Left, right
Dorsal (fin) and ventral
What are two ways identical (monozygotic) twins are formed?
Embryo can split after first cleavage so there are 2 embryos each with their own placenta.
Or inner cell mass is duplicated so there are 2 embryos sharing a placenta and amniotic sac but each follow their own developmental processes, if separation happens too late or isn’t complete this can lead to conjoined twins.
What is teratogenesis? When is the embryo most sensitive to this?
Process through which normal embryonic development is disrupted. Most sensitive weeks 3-8 but each organ has a particular window where they are most sensitive, depends on when the most development occurs.
What are tetratogenetic agents and examples.
Large number of chemical and infectious agents that are known to cause developmental defects.
Thalidomide
Rubella
Alcohol (no known safe level in pregnancy, even low levels will affect CNS)
Certain therapeutic drugs eg warfarin can interfere.
What happens during fertilisation?
Oocyte released from ovary. Travels along fallopian (uterine) tube. Is fertilised by sperm in the ampulla. Fertilised oocyte is called the zygote. Ideal site for implantation is posterior uterine wall.
What does the notochord do in neurulation?
sets the midline and releases signalling molecule to ectoderm lying very close to it, triggering differentiation steps to form CNS.
what the notochord direct the conversion of?
overlying ectoderm to neuroderm
what does neuroectoderm form and what does this form?
neural plate. the edges of this elevate out of the plane of the disk and curl towards each other to form the neural tube
what does the neural tube eventually form?
brain and spinal cord
what do the neuroectoderm signals do to the ecotderm?
signals it to thicken (key-hole shaped neural plate)
what happens while neurulation is preceeding?
mesoderm organises itself with lots of cell division into different zones
what are somites?
segments of the paraxial mesoderm (discrete chunks of tissue)
when do somites form? how many are their in total at the end? what will they eventually form?
1st pair appears at day 20 in the future occipital region (base of skull). More appear in a craniocaudal sequence at three pairs per day until there are 42-44 pairs by the end of week5.
31 pairs left as some disappear. will become the 31 pairs of spinal nerves.
what does the organisation of the mesoderm into somites give rise to and what does guide?
repeating structures (eg ribs, intercostal muscles, vertebrae) and this guides innervation.
what are the zones the mesoderm organises itself into? where are they found?
paraxial mesoderm - either side of axis
intermediate mesoderm - not next to axis but not at the edge
lateral margins form 2 different zones:
somatic mesoderm - closer to ectoderm, will form skeletal muscle etc
splanchnic mesoderm - closer to endoderm, will form viscera (circulatory system etc)
intraembryonic coelom - new cavity inside the embryo, will form gut
when is the intraembryonic coelom formed?
during lateral folding of the embryo
what happens in the differentiation of somites?
appear as regular blocks of mesoderm arranged around a small cavity.
followed by organised degeneration where the ventral wall of the somite breaks down, leading to the formation of sclerotome.
further organisation of the dorsal portion forms the combined dermomyotme.
myotome proliferates and migrates (but remaining affiliated to its parent somite) and the dermatome disperses.
what is sclerotome?
hard tissue
what is dermomyotome?
skin tissue and muscle tissue
what is dermatome and myotome developmentally?
dermatome - part of the somite that gives rise to the dermis.
myotome - gives rise to the muscles
what is dermatome and myotome clinically?
dermatome - a strip of skin supplied by a single spinal nerve.
myotome - muscle/group of muscles supplied by a single spinal nerve.
how are the neural tube and the dermomyotome connected?
neural tube produces the nerves that innovates the dermomyotome.
what can we see from a saggital cross-section of the trilaminar disk?
cephalocaudal folding
what is cephalocaudal?
head, tail
what can you see happening in cephalocaudal folding through a saggital cross section?
The heart lies at the cranial end. The yolk sac pinches up into the embryo.
as development continues the heart moves inwards with the folding of the embryo until it lies in the future chest.
the yolk sac extends out of the embryo to form a connection between the yolk sac and the gut.
what can you see with a transverse section of the trilaminar disk?
lateral folding
what is lateral folding driven by?
size of the somites
what can you see in lateral folding through a transverse cross section?
space increasing between the somatic and splanchnic layers. the pinched off yolk sac forms a cavity inside the embryo (endodermlining) and the entire embryo is suspended in amniotic fluid.
what does the cavity formed during the folding of the trilaminar disk allow for for the future?
development and growth of the rest of the body
what does folding of the trilaminar disk achieve?
3D structure
creates ventral body wall
pulls amniotic membrane around the disk so the embryo becomes suspended in the amniotic sac.
pulls the connective stalk ventrally
creation of a new cavity within the embryo (the coelom)
what 2 main things happen in week 4?
neurulation
organisation of the mesoderm
