Embryology Flashcards
what does the embryonic period encompass
all of
organogenesis
what period of development is most susceptible to disruption and disorder?
organogenesis
development of the CNS entails several coordinated _________ and ________ ______
molecular and anatomical changes
what entails several molecular and anatomical changes
development of the CNS
embryology definition
study of initial development of an embryo
study of initial development of an embryo is
embryology
all organ systems are developed by how many weeks?
8 weeks
what happens by week 8 of development?
all organ systems are developed
after 8 weeks the remainder of gestation is referred to as
the fetal period
what is the fetal period?
after 8 weeks of development
Describe the first 5 initial events
fertilisation - zygote is formed
division of the early embryo - cleavage
after around 3 days embryo is now MORULA
by 4 days - BLASTOCOEL develops and the embryo is now a blastocyte
5 Days - EMBRYOBLAST/inner cell mass is distinct
describe events 6-10
hatching occurs day 6/7 - from ZONA PELLUCIDA
day 8-9 blastocyte IMPLANTATION
HYPOPLAST and EPIBLAST cells divide into 2 growing lumens (amniotic cavity and YOLK SAC)
Interface of 2 cell tyes is called - BILAMINAR GERMINAL DISC and becomes the fetus
GERMIAL DISC is where GASTRULATION OCCURS
where does gastrulation occur
germinal disc
what is the interface between 2 cell types called?
bilaminar germinal disc
what is the bilaminar germinal disc
interface between 2 cell types
what is gastrulation
generation of the 3 germ layers which derive all embryonic structures
what is the generation of the 3 germ layers that derive all embryonic structures called?
gastrulation
describe initial events 11-14
Gastrulation occurs along PRIMITIVE STREAK in a caudal to cranial orientation
cells from epiblast invaginate into primitive streak to form ENDODERM and MESODERM - 3 layers form
Led by primitive node
Primitive node also allows for formation of Notochord that orchestrates formation of neural tube
what does primitive node determine
anterior-posterior access
where does gastrulation occur
along primitive streak in caudal to cranial direction
What does notochord orchestrate
formation of neural tube
what orchestrates formation of the neural tube
notochord
what are the 3 germ layers
ectoderm, mesoderm and endoderm
what does ectoderm form
epidermis, epithelial lining of mouth and anus, cornea and lens of eye, nervous system, adrenal medulla
what does meso derm form
notochord, skeletal system, muscular system, excretory system, reproductive system, dermis of skin
what does endoderm form
epithelial lining of digestive tract and respiratory system, lining of urethra urinary bladder and reproductive system, liver, pancreas, thymus, thyroid
what foes notochord form in line with
primitive node
what is the notochord in vertebrates
transient but essential
what does notochord do
organises surrounding tissues and provides structural support
what does notochord secrete
factors which control CNS development from overlying ectoderm
what does notochord regulate
left/right asymmetry
arterial/venous identity in major vessels
aids in the specification of forming somites
notochord secretes:
signalling factors which induce specification of the overlying ectoderm to form the neural plate (starts day 18)
uneven proliferation causes the formation of:
neural groove and neural folds
_____ converge at _____ to form the neural tube
neural folds at the neural plate border
neural folds converge at neural plate border to form
neural tube
final closures of the neural tube at the ________
caudal and cranial neuropores
when does the cranial neuropore close
day 24 of development
when does the caudal neuropore close
day 28 of development
what gives sheets of cells the ability to specify and develop multiple organ systems, many cell types and all in the correct orientation and order
morphogens and also via a cell’s competence for that morphogen signal (external and internal control)
what do morphogens do
specifying and developing multiple organ systems, many cell types and all in the correct orientation and order
what does notochord induce and via what
induces the floor plate via Sonic HedgeHog
what does induction of notochord by SHH allow for
a switch in cadherin expression (E to N) and initiation of inward folding
what signals are required for notochord to switch in cadherin expression
Chordin, Noggin and SHH
where are the morphogens Chordin and Noggin expressed
in the node at the cranial end of the primitive streak
what are Chordin and Noggin morphogens key for
setting the midline of the embryo - determining dorsal/ventral AND left/right axes both of which are required for future brain segmentation
after initiation of initial folding, what does the surrounding ectoderm express
opposing morphogens BMPs (4 and 7) and WNTs
what doe the opposing morphogens after initial folding allow for
orientation of the roof plate and closure of the neural tube. aid in specification of the now overlying ectoderm - epithelium
what population of cells are specified after the closure of the neural tube
neural crest cells
what happens after closure of the neural tube
neural crest cells detach from the neural folds and sit dorsal to the roof plate of the neural tube
neural crest cells migrate and differentiate into what key structures
Neurons and glia of the PNS
Adrenal Medulla
Melanocytes
Facial cartilage and bone
Corneal epithelium
Endothelial lining of the aortic arches, endocardial cushion and aorticopulmonary septum
as the neural tube starts to close, distinct structures are specified at the _____ end
cranial
as the neural tube starts to close, what distinct structures are specified at the cranial end
primary vesicles (week 4)
what are the primary vesicles
prosencephalon
mesencephalon
rhomboencephalon
primary vesicles after further specification form:
secondary vesicles
what are the secondary vesicles
telencephalon
diencephalon
mesencephalon
metencephalon
myelencephalon
describe initial formation of vesicles
via tightly orchestrated proliferation events
cranial end of the neural tube proliferates before closure even occurs to crease the 3 primary vesicles
proliferation of neural tube occurs in restricted space creates 3 bends of neural tube to allow for further specification
3 bends in the neural tube called:
Cephalic flexure (ventral) - midway through the mesencephalon
Pontine flexure (dorsal) - at the base of metencephalon
Cervical flexure (ventral) - separates the myelencephalon from the spinal core
when do the flexures in neural tube develop
between primary and secondary vesicle formation - present by week 5
what does the telencephalon become
cerebrum
neurogenesis meaning
process by which neurons and glia proliferate, migrate and are specified to form the mature brain
what is synaptogenesis
synapse formation and organisation occurs
what is neural pruning
orchestrated apoptosis
what is orchestrated apoptosis known as
neural pruning
where is myelination initiated
starts in the cerebellum and the brain stem
what is deemed a multipotent neuroepithelial population
neural stem cells
what initiates proliferation in the embryonic brain
neural stem cells
what doe neural stem cells produce and what do they develop
radial glia that develop into both glia and neurons
what are radial glia essential for
development of the embryonic neural network - comprise the ventricular zone
what do NSCs divide by to continue what
NSCs divide by both asymmetric and symmetric division to continue self-propagation during development
what do NSCs primarily divide into
neurons, astrocytes and oligodendrocytes
what is cell specification determined by
spatial patterning (dorsal/ventral)
time/chronological
local signalling (microenvironment/morphogens)
what is a key requirement of the developing brain
migration
what are the 2 types of migration
glial mediated - use of radial glia framework
somal translocation 0- self propelled
what are the 2 directions of migration
radial - radiates out from the centre
tangential - moves around or along the neural tube
when does formation of synapses start
during fetal life (mid-second trimester) and is intensely built in the first few years of life
when does axonal myelination start
late in gestation and is continued throughout adolescence and into adulthood
at 12 weeks gestation
brain structures similar to the adolescent/adult brain
lack of gyri and sulci in the cerebral cortex
what increases cortical volume
sulcation and gyrification
what is sulcation and gyrification essential for
increasing cortical volume and allowing for increased cognitive fucntion/processing
when are primary sulci initiated and formed
week 12- 28
vast majority of gyri and sulci are formed and defined in second and third trimester
how much does grey matter increase per week from week 29
15ml/week
how are preterm infants kept
in conditions which prevent overstimulation/ can impact neural development as a result
what is one of the most essential pieces of embryology
closure of the neural tube
what occurs if neural tube is not closed
neural tube defects
what is anencephaly
failure of the cranial neuropore to close
what occurs when cranial neuropore fails to close
anencephaly
what is telencephalon
absence of major aspects of the brain and skull
what is the most common neural tube defect
spinal bifida
how does spina bifida occur
when the caudal neuropore fails to close
what are neural tube defects linked to
folic acid metabolism
mechanism of folic acid preventing neural tube defects?
not fully understood
possible role in epigenetic reprogramming which coincides with the time of neural tube formation
what is the most severe neural tube defect
craniorachischisis
what is Craniorachischisis
both brain and spinal cord are exposed
what causes craniorachischisis
multi factorial cause - genetic and environmental disruption.
linked with genetic disruption of the folate metabolism pathway
seen in cases of trisomy 18
describe microcephaly
smaller than normal head circumference due to lack of brain development
associated with many exposures to teratogens (ZIKA)
genetic links - trisomies
usually lack of cerebral cortex development
poor neurogeneis, loss of neural stem cells or uncontrolled apoptosis
describe holoprocencephaly (HPE)
defect whereby the procencephalon fails segment at the midline
failure of segmentation at week 5-6
detected by ultrasound
poor prognosis
describe variances of holoproencephaly
classifications - lobar, semi-lobar, alobar
reflected in facial patterning of fetus- most severe is cyclopia
less severe- cleft lip
