Week 3 Development Flashcards
Is pregnancy detectable in the 3rd week?
yes, by ultrasound
What does the 3rd week of development coincide with?
- coincides with 1st week after missed menstrual period
- embryo is already 2 weeks old
- 5 weeks after last normal menstrual period (LNMP)
Where is the epiblast?
adjacent to amniotic cavity
where is the hypoblast?
adjacent to umbilical vesicle
Where is the prechordal plate?
thickened circular area of columnar hypoblast cells
The prechordal plate will eventually form what?
- organizes development of the head region
- future site of the mouth
What is the outcome of gastrulation?
establishes axial orientation in embryo
What does morphogenesis mean?
development of the body form begins now
Where does the primitive streak appear?
thickened median linear band on the dorsal and caudal surface of the epiblast (first sign of gastrulation)
What is causing the primitive streak to form?
forms as epiblast cells proliferate and migrate to the median plane
What orientation landmarks are established in the primitive streak?
primitive node, primitive pit, primitive groove
Where is the primitive node?
Accumulated epiblast cells at the cranial end of the primitive streak
Where is the primitive pit?
small depression in the primitive node
Where is the primitive groove?
- narrow depression oriented lengthwise within the primitive streak
- the pit and groove form as epiblast cells migrate inward through the primitive streak
How is embryonic endoderm formed?
as a result of epiblasts migrating inward through the primitive streak some epiblast cells displace the original hypoblast cells
What is the embryonic mesoderm?
as a result of epiblasts migrating inward through the primitive streak some epiblast cells take up position between the epiblast and the original hypoblast
- become mesoblasts which will form the embryonic mesoderm
What is mesenchyme?
a rudimentary connective tissue
Where does mesenchyme migrate?
migrate laterally to reach extraembryonic mesoderm of umbilical vesicle & amnion
How does mesenchyme relate to extraembryonic mesoderm?
as a result of epiblasts migrating inward through the primitive streak some epiblast cells from embryonic mesoderm and others form mesenchym
Where is the cardiogenic area?
- What will develop here?
mesenchyme will migrate cranially on either side of the prechordal plate into cardiogenic area
- site where the heart begins developing
What cell types develop from mesenchyme?
highly migratory cells which become fibroblasts, chondroblasts, osteobasts
- mesenchyme can also form ectoderm and endoderm
What is embryonic ectoderm?
the remaining epiblast cells which migrated inward through the primitive streak become the embryonic ectoderm
When does mesoderm production diminish?
early week 4
When does the primitive streak degenerate?
disappears by end of week 4
When is gastrulation completed?
end of 4th week
What are the results of gastrulation?
embryo having 3 layers:
- ectoderm
- mesoderm
- endoderm
What does the ectoderm give rise to?
- epidermis and appendages
- nervous system and pituitary gland
- sensory epithelia of the eye, ear and nose
- connective tissues of the head (via neural crest cells)
What does endoderm give rise to?
- epithelial linings of respiratory and GI tracts
- epithelial lining of the urinary bladder and urethra
- glandular cells of the liver, pancreas and salivary glands
- tonsils, thymus, thyroid and parathyroid glands
What does mesoderm give rise to?
- striated muscle
- smooth muscle
- heart
- blood vessels
- blood cells
- ducts and organs in the urinary and reproductive systems
- connective tissues in the body and extremities (including bone and cartilage)
Is a sacrococcygeal teratoma benign or mallignant?
- common?
usually benign that develops from remnants of primitive streak
- most common tumor in newborns (1/35000; 80% female)
What is the notochord process?
median cellular cord of mesenchymal cells migrating cranially from primitive node
Where is the notochord located?
lengthens in the mesodermal plane until it reaches the prechordal plate
What is the prechordal plate?
columnar endodermal cells in contact with the overlying ectoderm
- no intervening mesoderm located here
What is the oropharyngeal membrane?
future site of mouth
- prechordal plate is a precursor
What is the cloacal membrane?
- whats unique about its construction?
circular area caudal to the primitive streak (future site of anus)
- endoderm and ectoderm in contact here
By middle of week 3, embryonic mesoderm separates endoderm from ectoderm everywhere except where?
- oropharyngeal membrane (cranially)
- cloacal membrane (caudally)
- along the notochord process (which is mesenchyme)
What does the notochordal process become?
a rod-lie structure called the notochord
What are the functions of the notochord?
- defines the longitudinal axis of the embryo
- provides some rigidity to the embryo
- provides signals necessary for development of the CNS
- provides signals necessary for development of axial musculoskeletal structures
- contributes to development of intervertebral discs
- adult derivative persists s nucleus pulposus of IVD
What is the process of neurolation?
neural tube formation
When does neurolation occur?
begins in week 3, completed by end of week 4
What is a neurula?
name for the embryo during this time period
What is the neural plate?
thickened midline ectodermal cells (neuroectoderm)
How does the neural plate form?
form as a response to underlying notochord induction
Where is the neural plate located?
eventually extends cranially, beyond the end of the notochord
What does the neural plate give rise to?
responsible fro giving rise to brain and spinal cord (CNS)
What are the neural groove and neural folds?
- most prominent cranially where the brain will develop
- develop from neural plate on approximately day 18
How does the neural tube form?
forms by closure of the neural folds
Where does the neural tube close initially?
- separates from ectoderm
- closure begins in cervical region and progresses from there cranially and caudally
- separates from and is covered by surface exctoderm
Where is the cranial neuropore?
-where does it close?
- cranial opening in neural tube
- closes on approximately day 25
Where is the caudal neuropore?
- when does it close?
- caudal opening in neural tube
- closes on approximately day 27
Where do neural crest cells originate?
migratory cells originating from the “crest” of the neural folds
Where do neural crest cells migrate?
initially migrate to the dorsolateral region adjacent to the neural tube
- in this position they give rise to sensory ganglia for spinal and cranial nerves
What do neural crest cells give rise to?
- sensory neurons
- Postganglionic autonomic neurons
- Schwann cells
- Melanocytes
- Suprarenal medulla cells
- CT in head
What is the basis for neural tube defects
disturbances to development in week 3 may result in severe abnormalities in the brain and spinal cord
Why do neural tube defects occur?
due to failure of the neural tube to fuse and close into a neural tube
- often attributed to a teratogenic drug - may also be linked to problems arising during embryo folding
What is meroencephaly?
partially absence of brain (most common and very severe defect)
What is anencephaly?
complete absence of brain (misnomer)
Where is paraxial mesoderm?
located immediately adjacent to notochord
How was paraxial mesoderm formed?
derived from mesoderm cells migrating through lateral part of primitive node and cranial part of primitive streak
What structures will paraxial mesoderm form?
will form somites
Where do somites first form?
- paired cuboidal structures formed from paraxial mesoderm
- form in craniocaudal sequence on either side of notochord and neural tube
Where do somites first appear?
first appear in future occipital region of embryo (late in week 3)
how many somites pairs form?
42-44 pairs will be present by end of week 5
At what rate do somites develop?
somites develop at a rate approximately 3 pair/day
Whats a criteria used in aging embryos?
number of somites present
What is a segmentation clock?
regulated pattern of somite development depends on cyclic gene expression
What somites are formed?
- 4 occipital (first on disappears)
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 8-10 coccygeal (last 5-7 disappear)
What do somites give rise to?
- give rise to most axial and appendicular musculature and axial skeleton
- give rise to adjacent dermis of skin
Where is the intermediate mesoderm located?
mid region of primitive streak
What is the intermediate mesoderm derived from?
derived from mesoderm cells migrating through mid region of primitive streak
What will intermediate mesoderm form?
will form urogenital structures
Where is the lateral plate mesoderm located?
continuous with extraembryonic mesoderm (located within chorion)
Where is the lateral plate mesoderm derived from?
derived from mesoderm cells migrating through caudal part of primitive streak
What will the lateral plate mesoderm form?
forms the body wall structures
Where does the Intraembryonic coelom for initially?
forms initially as spaces develop within the lateral plate mesoderm
What is continuous with the Intraembryonic coelom
As the spaces coalesce they form the intraembryonic coelom
- initially continuous with extraembryonic coelom (chorionic cavity)
What will the Intraembryonic coelom eventually become?
divides the lateral plate mesoderm into 2 layers:
- somatic or parietal layer
- splanchnic or visceral layer
Where is the somatic or parietal layer?
located adjacent to the ectoderm and continuous with the extraembryonic somatic mesoderm covering the amnion
What is somatopleure?
comprised of parietal lateral plate mesoderm and overlying ectoderm
What will somatopleure form?
embryonic body wall
Where is the splanchnic or visceral layer?
located adjacent to the endoderm and continuous with the extraembryonic splanchnic mesoderm surrounding the umbilical vesicle
What is the splanchnopleure?
comprised of visceral lateral plate mesoderm and underlying endoderm
What will splanchnopleure form?
embryonic gut
Through week 2, embryo nutrition is sustained via what?
diffusion
In week 3 in early cardiovascular system development, what begins forming?
blood vessels
What is vasculogenesis?
formation of new blood vessels by precursor cells in mesoderm
What is angiogenesis?
formation of new blood vessels by budding/branching of preexisting vessels
What are angioblasts?
mesenchymal cells that form blood vessels
Where do angioblasts cluster?
they cluster to form blood islands in the extraembryonic splanchnic mesoderm
- small cavities appear within these blood islands
How do endothelial cells form?
angioblasts near the cavities within blood islands flatten to form endothelial cells
How do erythrocytes form?
blood cells develop from endothelial cells in vessels adjacent to the umbilical vesicle
- embryonic hematopoiesis begins later (in week 5)
Where is the cardiogenic area?
mesenchyme around and cranial to the oropharyngeal membrane
How is a primordial heart tube formed?
paired longitudinal endocardial tubes fuse
What do the heart tube join with?
blood vessels in embryo, connecting stalk, chorion and umbilical vesicle to form a rudimentary cardiovascular system
When does the hear begin beating?
by the end of week 3
Which organ system is the first to start functioning?
cardiovascular system
