Embryology Flashcards
Where abouts is the oocyte fertilised?
Ampulla
What is a fertilised oocyte called?
Zygote
When is the pre embryonic period from and til?
Fertilisation to the end of week 2 of pregnancy
When is the embryonic period from and til?
Start of week 3 to the end of week 8
How long is pregnancy?
38 weeks, plus 2 weeks since the last menstrual period
What does cleavage of the zygote form?
Morulla
What does compaction of the morulla form?
Blastocyst
What is implantation and when does it occur?
The process by which the pre embryo gains access to the maternal tissue and associated blood supply (day 6)
Where is the ideal site of implantation?
The posterior uterine wall
Define: zygote
The cell produce when an ovum is fertilised by a spermatozoan, contains all the genetic information for a new individual
Define: cleavage
Successive divisions of the zygote to form a morulla. The first division into two masses produces blastomeres, 30 hours after fertilisation
Define: zona pellucida
The glycoprotein membrane surrounding the plasma membrane of an oocyte to prevent polyspermy, present before implantation
Define: morula
The result of cleavage, a ball of totipotent identical cells
Define: Ovary
Female reproductive organ that produces oocytes and steroid hormones in a regular cycle, in response to anterior pituitary hormones (FSH/LH)
Define: fallopian tube
tube which eggs travel along from the ovaries to the uterus. Eggs are fertilised in the ampulla
Define: uterus
Female reproductive organ where the blastocyst embres and growth of the embryo occurs.
Define: blastocyst
forms after compaction, as the inner and outer cell masses form, and the blastocyst cavity develops
Define: trophoblast
the outer cell mass of the blastocyst, which will later form support structures for the embryo
Define: embryoblast
The innter cell mass, will later become the bilaminar disk
Define: implantation
The attachment of the blastocyst to the wall of the uterus at day 6/7
Define:Cytotrophoblast
an inner layer of mononucleated cells in the trophiblast, placental membrane around the yolk sac
Define: syncytiotrophoblast
An outer mononucleated zone of the trophoblast,cells that invate maternal sinusoids (irregular blood vessels) resulting in uteroplacental circulation
When does cleavage occur?
30 hours after fertilisation
what does the first cleavage result in?
2 blastomeres
What day does the morula form in?
Day 3
What happens on day 4?
Compaction
Compaction makes the cells pluripotent, what does this mean?
The cells have the capacity to become one of many cell types, they are somewhat differentiated
What happens on day 5?
Hatching
What is hatching?
The blastocyst hatches from the zona pellucida, it is no longer constrained and is free to enlarge and implant on the uterine surface
When does implantation begin?
Day 6 to 7
What type of epithelium is the endometrial surface?
Simple columnar epithelium
How many cells at the implantation stage? How many will make the embryo and how many will make the foetal membrane?
107 cells. 8 will make the embryo, 99 will make the foetal membrane
How to remember week 2
The week of 2s
What has priority out of the embryo and placenta at the earlier stages?
The placenta
What forms from the embryoblast?
Epiblast (inner) and hypoblast (outer)
What forms from the trophoblast?
Syncytiotrophoblast and cytotrophoblast
What does the epiblast form?
THe amniotic cavity
What does the hypoblast become?
The bilaminar disk
What type of epithelium is the hypoblast?
Cuboidal
What type of epithelium is the epiblast?
columnar
What are two conditions concerned with implantation at inappropriate sites?
Ectopic pregnancy and placenta praevia
What is ectopic pregnancy?
Implantation at sites other than the uterine body. Can quickly become a life threatening emergency. Can be peritoneal or ovarian
What is placenta praevia?
Implantation is lower uterine segment, placenta grows across cervical opening, can haermorrage. Needs C section delivery.
Describe the endometrial stroma
Highly vascular, lots of serous fluid, glands secrete glycogen/mucus
What happens on day 9?
Implantation complete. fibrin plug, development of syncytiotrophoblast, primitive yolk sac formation
What two types of support foes implantation provide?
Histiotrophic (tissue, diffusion), haemotrophic (blood support, circulation_
Where does the primitive yolk sac develop?
Abembryonic pole. from the blastocyst cavity
What happens on day 11?
The primitive yolk sca membrane is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum, later converted to extraembryonic mesoderm.
What happens on day 12?
Uteroplacental circulation begins
Describe the invasion of maternal sinusoids
Maternal sinusoid invaded by syncytiotrophoblast, lacunae become continuous with it. Maternal blood enters the lacunar system.
What is a sinusoid?
Maternal capillaries which are congested and dilated.
When and how does the secondary yolk sac form?
Day 13, by pinching off from the primary yolk sac.
What happens on day 14?
Spaces in extraembryonic mesoderm merge to form the chorionic cavity.
What does the connecting stalk do?
What is it made of?
What will it become?
Suspends the embryo and its cavities in the chorionic cavity.
A column of mesoderm
The umbilical chord.
Why do the gut, heart and lungs become eveloped by serosal membranes?
They develop next to a bag like cavity in which they invaginate
Define: Bilaminar disk
The epiblast and the hypoblast. Derived from the embryoblast. Will later become the trilaminar disk with the formation of the primitive streak
Define: Germ layer
A layer from which other developed tissues originated from
3 primary tissues of the trilaminar disk: ectoderm, mesoderm and endoderm.
Arise through gastrulation and give rise to all tissues of the body
Define: ectoderm
The outer most layer of cells in the trilaminar disk.
Gives rise to the epidermis and nerve tissue - all with contact to the outside world
Define: endoderm
Inner most germ layer of the trilaminar disk
Gives rise to the gut lining
Define: dorsal surface
Outermost layer during early development
Dorsal means towards the back
Define: ventral surface
The innermost surface during early development
Ventral means towards the front - belly, abdomen
Define: mesoderm
Middle germ layer of the trilaminar disk
Giving rise to muscle bone and more
Define: primitive streak
A narrow groove that develops on the dorsal surface of the epiblast
Important role in orientation of the embryo, determining the front and back
Begins as a thickened region of the epiblast at the caudal end of the bilaminar disk
Is the site from which gastrulation proceeds
Define gastrulation
The process establishing the 3 germ layers and hence the origin of all the tissues of the body
Define: trilaminar disk
The three germ layers prodcued by gastrulation
Ectoderm
Mesoderm
Endoderm
What are the most important ectodermal derivatives?
Organs and structure that maintain contact with the outside world
Skin
Skin derivates (hair, nails, lens and cornea of eye, lining of mouth and anus)
Brain and spinal cord
Peripheral nerves
Retina and iris of eye
What are the most important mesodermal derivatives?
Supporting tissues (greatest variety)
Smooth, skeletal and cardiac muscle
Connective tissue
Bone
Cartilage
Blood and blood vessels
Urinary system
What are the most important endodermal derivatives?
Internal structures
Lining of the GI tract
Glands associated with digestion (pancreas, liver, parenchyma)
Other gut derivatives such as the lungs
What has happened by the end of week 2?
Oocyte has been fertilised
Zygotes have undergone cleavage and compaction to produce a blastocyst
Implantation and has started to differentiate
Bilaminar disk hsas formed inside the ICM
Extraembryonic spaces started to organise (amniotic sac, yolk sac)
Suspended connecting stalk
Supporting sac: chorionic cavity
Generally, what happens in weeks 3 to 8?
Period of greatest change
All major structures and systems are formed
Most perilous time for the developing child
Woman may not realise she is pregnant
Generally, what happens in the third week after fertilisation?
The week of 3s
3 cavities: amniotic cavity, yolk sac, chorionic cavity
3 germ layers: ectoderm, mesoderm, endoderm. Rudimentary lineages
Gastrulation: establishes 3 germ layers and sets axes observed in an adult
What axes are set in gastrulation?
Anterior/posterior (head and toe of embryo/front and back of adult)
Dorsal/ventral (front and back of embryo)
Right/left
How does the primitive streak form?
Primitive streak starts to vaguely form on the surface of the epiblast on days 13 to 14 DORSAL SURFACE
Day 15 to 16: becomes clearly visible a nararow groove with bulges outside
Primitve node is located at the cranial end of the streak, molecular signals from here
Primitive pit is at the centre of the node
Development proceeds in a cranial/rostral to caudal direction
As gastrulation proeeds and the three germ layers are established, the primitive streak regresses
Describe how mesoderm migrates through the primitive streak and 3 layers are formed
Cells on the epiblast migrate to the primitive streak
They invaginate into the epiblast and displace the hypoblast, with endoderm
creating a 3rd layer, the mesoderm
Cells remaining in the epiblast form the ectoderm
ALL BODY CELLS FROM THE EPIBLAST
Now is the trilaminar disk
As more cells migreate through the streak they spread laterally and cephalad (towards head)
Mesoderm spreads between the ectoderm and endoderm like sandwich filing, leaving two gaps/hole for future mouth and anus
How is the fate of the invaginating epiblast decided?
ie which layer in trilaminar disk
Depends on WHERE in they streak or node they invaginate
How is the notochord formed?
What does it do?
Defines the phylum chordata and is the basis for the axial skeleton
Drives formation ofthe nervous system: neuralation
Prenotochordal cells migrate through the cranial part of the primitive pit
Form a solid rod of cells running through the midline with an important signalling role - defines the midline
Axial skeleton forms around it
It regresses afterwards
What does the notochord become in adults?
Nucleus pulposus
Of the intervertebral disk
How do the primitive streak and node define the axes?
Set in the early embryo
Streak appears at one end of the bilaminar disk, defines the front and back: anterior/rostral and posterior/caudal
Molecular signals from the primitive node ensure corret dorsal ventral and left right development
Cilitated cells beat signalling molecules left, side specific cascade intiated. Right: liver and lung with 3 lobes. Left: stomach and spleen, lung with 2 lobes
What causes mirror dispostion of organs?
Usually immobile cilia
SITUS INVERSUS
Organs wrong way round
No associated morbidity
Describe how twinning can arise
Monozygotic: 1 fertilised oocyte, 2 identical infants
Splitting:
After first cleavage, 2 embryos and 2 placentas
Inner cell mass duplicates: 1 placenta
2 inner cell masses touching, duplication of primitive streak, same placenta and same amniotic sac, sometimes conjoined twins
What is teratogenesis?
Explain what happens when it occurs at particular time intervals
Process by which normal embryonic development is disrupted
If in first 2 weeks, abortionwe
eks 3 to 8, most sensitive
Each organ has a sensitive window for abnormalities
Early: major congenital defects
Later: minor functional defects
Agents such as thalidominde, rubella, alcohol and some therapeutic drugs can cause it.
What has occured by the end of week 3 in the embryonic period?
Embryo has gastrulated and the bilaminar disk has become a trilaminar disk
Axes have been set
Three germ layer, ectoderm, mesoderm and endoderm
What is the notochord?
What does the notochord signal?
Solid rod of cells running down midline
Directs the conversion of ectoderm to neuroectoderm
Signals overlying ectoderm to thicken, key hole shaped neural plate
Edges elevate out of the plane of the disc and curl towards each other, creating a neural tube
Neural tube drops below ectoderm proper
How are somites formed?
Segments of paraxial mesoderm
1st pair appears at day 20
3 pairs per day in a craniocaudal sequence until there are 42-44 pairs by the end of week 5
31 pairs left in total, 31 pairs of spinal nerves
What do somites give rise to?
Repeating structures
Vertebrae
Ribs
Intercostal muscles
This guides innervation
Explain the differentiation of somites into dermatome, myotome and sclerotome
Somites appear as regular blocks of mesoderm cells aorund a small cavity
Followed by organised degeneration, leadign to the formation of the scleortome, ventral wall breaks down
Dorsal portion forms dermomyotome
Myotome proliferates and migrates
Dermotome disperses just below ectoderm
What do the following structures become?
Dermatome
Myotome
Sclerotome
Skin dermis
Muscle
Hard tissues like bone
Explain how dermatome and myotome are used as terms to describe adult innervation of skin and muscles
Each myotome and dermatome retains innervation from its segment of origin
Dermatome is a stip of skin supplied by a single spinal nerve
The myotome is a muscle or group of muscles supplied by a single spinal nerve
Can be examined clinically
The neural tube produces the nerves that innovate the dermomyotome
What is an epimere?
What is a hypomere?
Innervated by a dorsal branch of spinal nerve
, epimere
Innervate by ventral branch of spinal nerve, hypomere
What is formed from the mesoderm?
Notochord
Paraxial: axial skeleton (vertebral column and ribs), dermis, muscles of axial body wall, some limb muscles
Somatic (lateral plate): connective tissue of the limbs, contributes to the axial body wall
Splanchnic (lateral plate): smooth musculature, connective tissue and vasculature of gut
Immediate mesoderm: urogenital system, kidneys, ureters, gonads
Describe cephalocaudal folding
Seen by saggital section
Heart lies at the cranial end, moves to future chest
Yolk sac is drawn up and extends out of the embryo to form a connection between the yolk sac and gut
Describe lateral folding
Driven by somite growth
Seen in transverse section
Pulls amniotic membrane around disc so it is suspended
Forms gut tube from yolk sac
Surrounded by intra embryonic coelem, lateral branches of mesoderm connect
What does folding achieve?
3D structure
Creates ventral body wall
Pulls amnion around disc so embryo is suspended
Pulls connecting stalk ventrally
Creation of new cavity within the embryo, the coelom
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