Embryology: The Embryonic Period Lecture 3 Flashcards
How many days does it take to get to the uterine lining?
About 7 days
What is the purpose of the synctiotrophohlast?
Used to access glands
What happens during Implantation?
- Trophoblast cells differentiate
- Embryoblast cells differentiate
- Two cavities form
What structures make up the Bilaminar germinal disc?
The hypoblast and epiblast
What layers are in the embryo after implantation?
Epiblast and hypoblast
What layers are in the trophoblast after implantation?
Syncytiotrophoblast and cytotrophoblast
What are the two cavities formed above and below the Bilaminar disc?
Amniotic sac cavity (above)
Yolk sac cavity (below)
Where is the Extraembryonic (XE) Mesoderm found?
Found between inner lining of cytotrophoblast and yolk sac (add drawing pic)
What is the Extraembryonic Mesoderm?
It is a new layer of cells derived from the epiblast and yolk sac. It continues to separate the embryo from surrounding uterine tissue. It gives mechanical and trophic support
What are some reasons a pregnancy ultrasound might be done in the first trimester?
- confirm a normal pregnancy
- Determine age
- Detect problems (ectopic pregnancies, etc)
- Look for twins, etc
- Identify problems with placenta, uterus, ovaries
Why isn’t the fertilized egg susceptible to teratogens during the first two weeks?
The first two weeks aren’t really affected by teratogens because nothing that the mom really ingests can affect it at this stage. It’s not implanted into the mother. Not fully connected yet.
What are the major events to occur during the Embryonic Period?
- All major body systems develop
- 2D disk to 3D cylinder
- Folding of the embryo
- Craniocaudal folding (CNS)
- Lateral folding - amnion/body wall
Why does the folding of the embryo occur?
It happens because the brain grows so quickly that it bends over and folds.
What is gastrulation?
It is the beginning of morphogenesis (developing of body form)
- Forms a trilaminar embryonic disk
- Process that establishes the 3 primary germ layers
What are the 3 primary germ layers and their colors?
They give rise to all tissues and organs
Endoderm - yellow
Mesoderm - red
Ectoderm - blue
What do we get when we cut through a layer of the blastula?
The primitive streak
What is the buccopharyngeal (oropharyngeal) membrane?
It is the future site of the mouth add picture
What is the primitive streak?
It is the future axis of the embryo. If you see the streak, then it means that gastrulation has started.
Match the dorsal and ventral sides of the epiblast and hypoblast
Epiblast is dorsal and hypoblast is ventral
What is the possible connection between conjoined twins and the primitive steak?
So another way to get conjoined twins might be that we get two primitive streaks that don’t separate
How does the primitive streak form?
It forms from the proliferation of epiblast cells
How/which direction does the primitive streak elongate?
The Streak elongates with cells added to the caudal (back) end.
Where is the heart located in the blastula/primitive streak?
The heart is actually at the very top. When the brain starts to bend everything over because of it’s big size, the heart rotates down to where it is now
Where do all the germ layers come from? Give order
ALL of them come from the epiblast
Epiblast - Primitive ectoderm (for ecto) - Primitive Streak (for meso and endo)
What should happen to the primitive streak and what replaces it?
The primitive streak should eventually regress and is replaced by the notochord
What problematic mass can arise if primitive streak does not regress?
A sacrococcygeal teratoma can form
What is a sacrococcygeal teratoma?
- It is a giant non-malignant tumor that forms from remnants of streak.
- Has derivatives and tissue types of all 3 germ layers
- Technically “common” tumor
Why is the sacrococcygeal teratoma bad for the newborn?
Even though it is not malignant – if it gets too big, then it’ll strain the heart too much
What is the surgery process for fixing sacrococcygeal teratoma?
They do a surgery where they take it out, remove the bulge, insert them back in, and sew everything back up
What is Caudal Dysplasia?
It is a germ layer disorder. Usually when the mesoderm migration is disturbed
- Involves the total or partial failure of development of the lower vertebrae, including the sacrum, which results in associated abnormalities of the lower extremities, spine, kidneys, gastrointestinal and genitourinary tracts.
- Might be associated with maternal diabetes
- Has many other names add picture????
Why aren’t there many germ layer disorders?
Because most of them simply kill the fetus. Not many of them allow the fetus to survive
What are the ethical implications of primitive streak?
Generally speaking, experimentation with human embryos is only allowed before the primitive streak develops (14 days)
In what layer does the notochord form?
In the mesoderm. Add picture
What is the notochord?
It is a flexible rod shaped body composed of cells derived from the mesoderm and defines the primitive axis of the embryo
What are some of the functions of the Notochord?
Structure - acts as a rigid access around which the embryo develops
Skeletal - Foundation upon which the vertebral column (vertebral bodes) will form
- Forms part of intervertebral discs
Induction - it will bring about the formation of the neural tube (future nervous system). It will send out the signals to the ectoderm to start the nervous system formation
What is a chordoma?
It is a primary malignant bone cancer that develops from remnants of the embryonic notochord. It happens when additional notochord cells are enclosed by the developing bones
Explain how the ectoderm differentiates after the notochord forms.
It changes into two different ectoderms. The middle ectoderm area, is called the neural plate(neural ectoderm). It will eventually become the nervous system
The outer ectoderm will eventually become the outer skin.
Need to be careful with separation of these two because we can’t have the nervous system and skin together add pic
How is the neural plate separated from the rest of the ectoderm?
- Specific signaling molecules are produced by cells of the notochord that send a response in the overlying ectoderm to begin the process of neurulation (formation of a neural tube)
- Uses the BMP and neural crest cells. We get the folding to happen, then pinch it off with the crest cells.
What do neural crest cells do?
They eventually go into the peripheral nervous system. They break free and remove any connection to the skin
What is the definition of induction?
It is when one group of cells/tissues causes another set of cells/tissues to change their fate. They influence it’s development
What are some problems that arise while signaling/communication between the inducer and responder groups of cells?
If you send the signal too soon, they might not respond. You not only need to send a signal, but you need a signal back that confirms that the signal was received.
What molecules can be used for cell-to-cell signaling?
- Most signaling molecules are proteins synthesized by one cell that diffuses over short distances to contact other cells
- Growth and differentiation factors (GDF’s)
- Transcription factors and signaling molecules
What is structure is pretty much thought to be the 4th germ layer?
The Neural Crest. Since it goes everywhere. It migrates extensively to form a variety of structures throughout the body
Derivatives of Ectoderm:
Epithelial or Surface ectoderm
Epidermis hair, nails, tooth enamel, cutaneous glands (sweat, oil, ceruminous), mammary glands, anterior pituitary, lens of eye, inner ear, sensory nasal epithelium
Derivatives of Ectoderm:
Neuroectoderm (derived from neural plate and neural folds
Nerual Tube
CNS (brain and spinal cord), retina, pineal body, posterior pituitary
Derivatives of Ectoderm:
Neuroectoderm (derived from neural plate and neural folds
Neural Crest
Sensory ganglia and nerves of PNS (cranial and spinal nn) autonomic ganglia and and postganglionic fibers, Schwann cells, adrenal medulla, pigment cells, pharyngeal arch cartilages. Components of the eye, skull, teeth and skin. Also involved in heart formation
What are Ectodermal dysplasia (ED) syndromes?
- They arise from the surface ectoderm
- It is a group of about 150 heritable disorders that affect the ectoderm. Some problems that can arise are malformed teeth, deficiency in hair, nails, and sweat glands. Not having sweat glands is very problematic because that means that they can’t regulate body temp
How do you get diagnosed with an Ectodermal dysplasia (ED) syndrome?
- You get diagnosed if you have two types of abnormal features. They vary in severity and very few of them involve learning difficulties
What are pigmentary disorders and what layer do they arise from?
They are a neural crest issue.
Pigmentary disorders are diseases of melanocyte development, function, and survival
What is Piebaldism?
It is the characterized by a congenital white forelock and multiple symmetrical hypo pigmented or depigmented areas
What is albinism?
It’s a neural crest issue
Most autosomal recessive
It is the global reduction or absence of pigment in skin, hair and eyes
What is nystagmus?
It’s the condition of involuntary movement of the eye
Why does albinism cause nystagmus?
- The presence of melanin during ocular development is important. The fovea fails to develop properly if melanin is absent during development. Other area of the retina develop normally regardless of the presence of melanin
- Neural connections between the retina and the brain are altered if melanin in the retina is absent during development (so if there’s not enough melanin, it causes problems for eye/brain)
What the amount of pigment necessary for appropriate ocular development?
Amount of pigment needed is currently not known.
Pigmentary Disorders:
Vitiligo
- The loss of melanocytes
- Autoimmune disorder
- Can cause repigmentation by transplanting melanocytes to vitiligo affected areas
Explain the brain segmentation of an embryo (forebrain, midbrain, hindbrain)
Forebrain - Prosencephalon
Midbrain - Mesencephalon
Hindbrain - Rhonbencephalon
Look at Table 15.1 to see how the brain differentiates
Insert table Slide 68
What are some disorders that can arise if the Cranial neuropore is not closed at day 24?
Anencephaly (craniorachischisis), Iniencephaly, Encephalocele,
What are some disorders than can arise if the Caudal neuropore is not closed at day 27?
Spina bifida, S. b. occulta, S. b. meningocele, S. b. meningomyelocele
What is Anencephaly (craniorachischisis)?
A cranial neuropore NTD
It happens if the forebrain is not developed, then the skull will not form correctly.
Add pic slide 71
What is Iniencephaly?
A cranial neuropore NTD - Extreme retroflexion of the head - Short and almost absent neck - Hyperextended spine Facial skin is connected directly to the skin of the chest; the scalp is directly connected to the skin of the back
What is Encephalocele?
A cranial neuropore NTD
- It’s a cranium bidia. A defect with the herniation of intracranial contents, most common in the occipital region
There are other variations (meningoencephalocele and meningohydroencephalocele). The longer the word, the worse
What is the Arnold-Chiari malformation?
It is the herniation of cerebellar vermis or tonsils through the foramen magnum blocking the flow of CSF
What is Spina bifida occulta?
The mildest form of the three. The vertebral arches fail to unite
- Tufts of hair grow in the spots
- Spinal cord is normal
Compare the three types of Spina bifida NTD’s:
Spinal bifida - failure of neural arches to form
S. b. occulta - arches abest, tube is normal
S. b. meningocele - dura and arachnoid also protrude
S. b. meningomyelocele - neural tissue also protrubes, also called spina bifida cystica.
The longer the name, the worse it is
How does surgery fix a fetus with myelomeningocele?
Insert pic Slide 83
How does folic acid affect the occurrence of NTDs?
- Daily intake of 0.4mg of folic acid will decrease the occurrence of NTDs
- Prevents 70% of human NTDs but its mode of action is unclear
- Folic acid may also reduce the rate of occurrence of congenital heart disease
- *Doesn’t help everyone**
What has some research said about how NTDs arise?
- NTD linked to the transformation of homocysteine to methionine
- Increased homocysteine levels appear to prevent the closure of the neural tube
- Decreased B12 and increased total choline or homocysteine in maternal blood have been shown to be associated with increased NTD’s risk
?????
What is the Triple Marker Screen test and how does it help with NTDs?
The Triple Marker Screen test measures the levels of alpha-fetoprotein (AFP), hCG, and estriol.
- AFP is produced in the fetal liver. In a fetus with an open NTD, AFP leaks across the defect into the amniotic fluid and across the placenta into the maternal serum.
- AFP levels in maternal blood can be measured as an indicator of open NTDs.
