Embryology: Formation of Body Plan- Ziermann Flashcards
Define and distinguish between embryogenesis and organogenesis.
embryogenesis: time from fertilization of egg until bilaminar disc of embryo; is week 1-2 of development; has only formed two layers in the embryo
organogenesis: third layer that will become the mesoderm develops; the three layers in total (ectoderm, endoderm, and mesoderm) they will each differentiate into the internal organs of the organism is week 3-8 of development
Describe the processes during gastrulation, neurulation, and folding (cranio-caudal, lateral)
gastrulation: formation of the 3 germ layers
neurulation: development of the neural tube that will eventually become the spinal cord
lateral folding: due to the increasing mess of mesoderm
cranio-caudal folding: due to the fast development of the cranial and caudal ends resulting in folding anteriorly and posteriorly; on the cranial side due to the increased development of the brain; and at the end because of increased development of tail; allows for the heart to go behind our head
The cephalo-caudal flexion (in the longitudinal direction)
The lateral folding (in the transversal direction, rolling up
What embryological structure is completely replaced and not present in the embryo?
hypoblast
Describe the formation of the 3 germ layers (ecto-, endo-, meso-derm) and list their derivatives.
Endoderm is the innermost lining of the inner tube (epithelial lining) and develops into the internal epithelia of GI, respiratory tract, and parenchyma of GI glands
Mesoderm is the middle layer of muscle, bone, connective tissues that develops into the muscle, bone, cardiovascular, reproduction, and urinary
Ectoderm is the outer layer of skin, brain, and spinal cord that develops into the epidermis, CNS, and PNS.
In the posterior regions, what signaling causes the formation of the primitive streak and the trunk region?
nodal signaling
How does the primitive streak form?
when the cells of the epiblast crowd toward the central primitive node and midline primitive streak, detach from surface and spread out forming three layers
cells that are not migrating are the ectoderm
What is the epithelial mesenchymal transition?
- as cells pass through the primitive streak, they lose attachments to neighbors, to become mesenchymal cells
- epiblast produce hyaluronic acid, which binds epithelium and creates a space between the epiblast and hypoblast where mesenchymal cells can grow
a process by which epithelial cells lose their cell polarity and cell-cell adhesion, and gain migratory and invasive properties to become mesenchymal stem cells, which are multipotent stromal cells that can differentiate into a variety of cell types
mesenchyme: a loose embryonic connective tissue
What germ layer did neural crest cells start out of?
ectoderm
During axis establishment, cells that migrate inward become pluripotent. What does this mean?
axis establishment is between the anterior visceral endoderm (AVE) and nodal
the can make different things while in the mesoderm like bone, blood, or muscle but CANNOT make the ectoderm or endoderm
What do cells migrating anteriorly from the primitive node become?
prechodral plate –> head and brain notochord –> vertebral column
It’s important where you migrate!
How is the left and right established?
primitive node cells have cilia that create a current toward the left side causes expression of Pitx2 gene which leads to cranial differentiation of the mesoderm (asymmetrical visceral development)
the further you go away from the left, the more you’ll be in the cranial
Nodal also starts the expression signaling for lefty 1/2 represses nodal (regulating itself)
and we thus get left and right development
How does situs inversus occur?
lateralization is disrupted by defective cilia (Kartangener syndrome) or by ingestion of selective serotonin reuptake inhibitors (SSRIs) antidepressants during pregnancy (teratogenesis)
- most organisms are dependent on the left and right symmetry
- in individuals with situs inversus you may find the heart on the right side
An ultrasound detects a large mass near the sacrum of a 28 week old female fetus. What condition may this suggest and how is it acquired?
saccrococcygeal teratoma
- due to excess mesoderm (tumor with several tissue types derived from remnant of primitive streak)
- any agent that can cause problems with the process of the ectoderm cells migrating through the primitive streak to make mesoderm (teratogenesis)
What teratogenesis is caused by insufficient mesoderm in the caudal region?
caudal dysgenesis (mermaid) or vacteral association (not as severe as the mermaid)
How is one diagnose with vacterl association?
Need to have at least 3/6
- vertebral defects
- anal atresia
- cardiovascular defects (usually ventricular-septal defects
- trachea-esophageal fistula
- renal defects
- limb defects
What signaling molecules does the notochord secrete in order to induce the formation of neural tube by inhibiting BMP?
WNT and FGF
BMP causes continued differentiation of mesoderm.
-You need to make neural tube first before you can have differentiation of mesoderm.
What does the neural crest cells become?
The neural crest cells migrate extensively to become PNS (dorsal root ganglion), melanocytes, adrenal medulla.
They are cells at the border of the neural plate towards the normal ectoderm
What is the only remnant of notochord in the newborn?
the center of the intervertebral disc
The epithelial mesenchymal transition is important be
Snail expression causes tight junction between epithelial cells to disappear.
If left unregulated, neural crest cells escape the epithelium and they develop into tumors
Describe the formation of neural tube formation using the following: Thickening and elongation Folding Convergence Fusion Neural crest cell migration
Thickening and elongation: ectoderm over the notochord thickens to form the neural plate which elongates. Adjacent regions will form a neural crest and epidermis
Folding: cells along the medial hinge undergo apical constriction causing formation of a neural groove
Convergence: two dorsal lateral hinge points undergo apical constriction to produce a tubelike structure
Fusion: the dorsal most cells on each side of the neural tube begin to fuse, epidermis also fuses dorsal to the neural tube
Neural crest cell migration: neural crest cells migrate away as fusion is completed
Neurulation refers to the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube. The closure process is highly sensitive to what? This is also recommended for women planning to become pregnant.
closure process is highly sensitive to levels of folic acid (vitamin B9)
women are recommended to take folic acid to reduce the risk of neural tube defects
What condition results if the closure of the neural tube fails?
spina bifida if the closure fails in the posterior region of the neuropore
anencephaly if the closure fails in the anterior region of the neuropore
Describe the different types of spina bifida.
spina bifida occulta: 15- 25% affected individuals; lamina is incompletely formed and spinous process is missing, may have tuft hair in the region
meningocele: dura mater is making a liquid filled bulb that has to be treated because this area is latex sensitive!!
meningomyelocele: most severe phenotype; the dura mater and parts of the spinal cord is outside of the body cavity; defects in functionality and sensory; have to undergo surgery; problems with bladder control, lower limb control, sensitivity of lower extremities, pelvis, and urogenital system; the SC is in risk of damage of anything distal to the injured SC
During the 3-4 weeks of development, while neurulation is occuring, _______differentiation is occuring simultaneously as well.
mesoderm
Mesoderm differentiates into paraxial, intermediate, and lateral plate mesoderm which form?
- paraxial mesoderm: somites
- intermediate mesoderm: genito-urinary (kidneys and gonads)
- lateral plate mesoderm (body wall and gut tube): somatic (parietal), splanchnic (visceral) mesoderm, and coelom (space in between)
What is somitogenesis and why is it important?
paraxial mesoderm separates into a series of segmented blocks called somites
somites are a source of axial skeleton, skeletal muscle, and dermis of trunk skin
Each somites is innervated by ONE spinal nerve
so if you know the somite from which your muscle/skin came from you can determine its innervation
Retinoic acid (RA) gradient decreases from anterior front and the fibroblast growth factor decreases from the posterior front to the differentiation front. What does too much RA (acne medications) in the anterior location cause? What about too little RA (dietary insufficiency)?
Too much RA may give you cervical ribs (change identity of somite)
too little RA will make your thoracic vertebrae look cervical (change of somite identity)
What genes are expressed from anterior to posterior as somite differentiation proceeds?
Hox genes
Hox 1 is anterior and the higher the number, the more posterior.
What Hox genes are responsible for the formation of the upper limb and head?
Hox 1-4 for head
Hox 9-13 for upper limb
Retinoic acid poisoning during gastrulation or the 4th week of development, may cause what abnormalities?
- craniofacial defects
- cardiovascular defects
- central nervous system defects
- thymic abnormalities
Depending on where you are in the somite, you make different parts of the body. This is induced by different genes such as sonic hedgehog and neurotrophin 3 which induces what?
Sonic hedgehog (Shh) from notochord and neural tube floor plate induces sclerotome which expresses Pax1 (transcription factor), which initiates vertebral formation.
Neurotrophin3 (NT3) and Wnt from neural tube roof plate induce dermomyotome–> (dermis) and myotome (muscles)
What expression pattern makes our true back muscles (primaxial or epaxial)?
Myf5 expression induced by Wnt expression
Epimere of the myotome become true back muscles innervated by what?
dorsal primary rami
every other muscle (hypomere or hypaxial) in our body is innervated by the ventral primary rami
Signaling proteins secreted by different regions of the skin, neural tube, and notochord induce the somites to differentiate into myotome, dermatome, and sclerotome. Where do cranial muscles derive from?
pharyngeal arch mesoderm
-limb bones and most dermis come from the somatic lateral plate mesoderm
- myotome: skeletal muscle of trunk and limbs
- dermatome: dermis of neck, back, and lateral trunk
- sclerotome: vertebrae, ribs, meninges
Within the lateral mesoderm, a cavity forms, separating two layers which each form what? What does the cavity in between form?
dorsal layer: parietal (or somatic) mesodermal layer –> will form the lateral and ventral body wall
ventral layer: visceral (or splanchnic) mesoderm layer –> will together with the endoderm
cells lining the intraembryonic cavity (coelum) will form thin serous membranes –> will line the peritoneal, pleural, and pericardial cavities
During the first month we are still considered a flat tube. True or false?
True!!
It’s only during week 4 do we begin to undergo folding (begin making tubes).
The thoracic and abdominal cavities lined with serous membranes come from which embryological structure?
intraembryonic cavity (coelom)
pleural, peritoneal, pericardial cavities
At day 21 of development of the presomite embryo, where is the heart located?
heart is in front of head at the cardiogenic area (angiogenic cell cluster)
heart > brain (cranial)–> mouth (buccopharyngeal membrane or oropharyngeal membrane)
At day 26 of development of the fourteen-somite embryo, where is the heart located?
brain–> mouth–> heart
When do developing embryos form lung and liver buds to signaling the beginning of formation of the lung and the liver?
Day 28 of development (end of the 1st month)
due to the change of position of the pericardial cavity (WHERE YOU ARE DETERMINES WHAT YOU BECOME!!!)
it was original laying in front of the buccopharyngeal membrane but now is behind
What embryological structure makes the fetal urinary bladder and what does it degrade into?
allantois which degenerates into the urachus which extends from the fetal bladder to the umbilicus (umbilical cord)
The septum transversum of mesoderm origin develops into what?
connective tissue of the diaphragm
- it cuts off our thoracic area from our abdominal area as development continues
- very important because our lungs don’t have muscle and we need the diaphragm to change the volume inside the thoracic cavity to get a passive action of inhalation and exhalation
If this were to rupture this in the developing fetus, you would get blood poisoning in the mother.
uratus (degenerates of allantois)
There’s still a connection with the mother via the umbilical cord and placenta
The ectoderm will become the skin, which surrounds us everywhere. Which folding allows for the ectoderm to surround us? At what time?
lateral folding, end of week 4 (after day 24)
the amniotic cavity extends inferiorly while the yolk sac extends superiorly eventually they both close forming a tube within a tube
we now have the embryo within the amniotic fluid
The development of what somite derviative is influenced by the development of other somite derivates?
endoderm development is influenced by the ectoderm and mesoderm development
it has no intrinsic signal and is largely dependent on signals from the outside environment especially the developing mesoderm
The umbilical cord develops through which embryological folding?
cranio-caudal folding
umbilical cord forms when the connecting stalk (containing umbilical arteries and vein) and vitelline ducts are bound together by expanding amnion (filled with amniotic fluid= fetal urine)
Lacuna are filled with maternal blood. How are lacunae formed?
proteolytic enzymes digest the endometrium of the mother but is restricted by the immune system of the mother
What is the chorion made up of and what and what does it become?
cytotrophoblast and synctiotrophoblast
becomes the embryonic portion of the placenta
How does the yolk sac become separated by the cytotrophoblast?
by the chorionic cavity
When does the chorion develop and what does it produce to detect pregnancy?
16 days (about 2.5 weeks) hCG= human chorionic gonadotropin
hCG preserves the uterine lining and forms the basis of pregnancy tests
In order to allow for exchange of nutrient and oxygen between mother and baby, what does the syncytiotrophoblast have?
- microvilli for absorption
- anti-clotting factors (want the mother’s blood freely flowing)
What is the decidual reaction?
the stromal cells of maternal placenta accumulate glycogen and lipids which causes the fetal and maternal blood centers to get close enough where there can be exchange between two different blood centers
In normal circumstances there is direct exchange between the maternal and fetal blood. True or false?
FALSE!
Maternal and embryonic blood don’t mix. There is INDIRECT exchange through diffusion. They are separated by chorionic villus.
What is the importance of the fusion process of the chorion with the uterine wall?
By the end of the 3rd month, amnion will expand and fuse with chorion and uterine wall
- there is no way of getting the baby out of the mom
- you need to have contractions to expel the bubble with the baby
The development of the placenta involves imprinting. How so?
male genes favor placental growth
female genes restrict growth
(father’s gene indirectly supports the development of his grandchild)
the MIX of maternal and paternal genes make a HEALTHY placenta
What is pre-eclampsia?
a pregnant mother with high blood pressure can lead to proteinuria and then failure of the kidney
syncytiotrophoblast normally regulates the maternal bloodstream (regulates mom’s BP) in the development of the placenta; if faulty parts of the placenta will degenerate and enter maternal blood stream leading to blood poisoning, also insufficient supply of placenta for nutrients for the baby and thus babies with low birth weight and heart disease later on in adult offspring
Name the condition in which a trophoblast develops with no embryo present, give a false pregnancy test, and also a potential cancer.
hydatidiform mole
- embryoblast does not continue to grow
- trophoblast is active enough to stimulate placenta –> chorion–> secrete high levels of hCG
- most commonly occurs when a sperm fertilizes an oocyte with no nucleus, so all genetic material is paternal, even though it is diploid
- produces hypertension, edema, and vaginal bleeding
What is the greatest vulnerability of harmful influences to the baby?
3-8 weeks
examples of some teratogens
- alcohol
- hormones
- cigarettes
- german measles
- lead mercury
What is the most common cause of retardation in the US?
fetal alcohol spectrum disorder (FASD)
-which is due to excessive alcohol consumption
Other symptoms of FASD include:
- epicanthic folds
- microcephaly (neurulatin defect)
- growth deficiencies
- distinctive facial features
Why is fetal alcohol spectrum disorder (FASD) a “spectrum” disorder?
it depends on when excessive alcohol consumption occurs:
first 3 weeks: neurulation defects, problems with nervous, cranial, facial abnormalities
After 4 weeks: kidney that do not function fully, brain damage
Prescription drugs can cause teratogenesis. True or false?
True
- Thalidomide: disrupts hox expression
- diethylstilbestrol (DES): cancer of vagina and uterus in daughters exposed to DES in womb
What is the difference between a natural abortion and miscarriage?
- natural abortion occurs before week 3; occurs BEFORE fetus can live independently outside the mother
- miscarriage is natural abortion when it occurs before 24 weeks of pregnancy
Polydactyly, cleft lip/palate are examples of birth defects. Birth defects occur in 3% of all newborns. True or false?
True!!
You should do an amniocentesis if you want to see if your unborn child may have a birth defect.
FALSE!!
- you risk a spontaneous abortion or miscarriage with the stress of a long needle going through the amnion
- only do it when you know of a real risk (like it runs in your family)
How can you diagnose birth defects in an unborn child?
- ultrasound (5 weeks)
- serum screening (alpha fetoprotein levels increased for neural tube, omphalcele and others but decreased for trisomies
- amniocentesis after 14 weeks; AFP, genetic analysis
- chorionic villus after 10 weeks, faster genetic analysis, but risk loss of fetus and limb defects
Within which extra-embryonic membrane are blood cells formed in the early stages of development?
chorion
What is the major distinguishing factor that separates the embryonic stage from the fetal stage?
All major organ systems form during the embryonic stage; the fetal stage consists mainly of rapid growth
What is the structure that implants in the endometrium?
blastocyst
When can the term fetus be correctly used?
after the first 8 weeks which are termed embryogensis
-when the body systems have been established
Which extra-embryonic membrane encloses the embryo in a fluid filled cavity?
amnion
When is a developing individual most susceptible to genetic birth defects of smoking?
first trimester
A woman with a positive pregnany test and vaginal bleeding seeks advice. Examination reveals high hCG levels, high blood pressure, and placental tissue, but no evidence of an embryo. How would you explain this?
hydratidiform mole
A 23 year old woman has a progressive increase in her serum B-human chorionic gonadotropin (hcG) concentrations during an 8-week period. A hydatridiform mole is removed, but the hCG concentration continures to increase. What is the most likely diagnosis?
choriocarcinoma
