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
