embryo intro Flashcards
Understand the primary axes of the central nervous system, and the designations dorsal, ventral, rostral, and caudal. Understand the planes of section used to view the nervous system (coronal, horizontal and sagittal)
Before the cephalic flexure (btw diencephalon and mesencephalon), dorsal is towards the top of the head, ventral is towards the chin, rostral is towards nose and caudal is towards back of head. Below the flexure, ventral is towards front, dorsal towards the back, rostral towards the top and caudal towards the bottom
Which are the primary cerebral vesicles and where do they develop from
Primary ceebral ventricles develop at the rostral end of the neural tube. They are colled the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain).
Name the secondary cerebral vesicles and where they come from
From cranial to caudal: 2 telencephalic vesicles (pros) > diencephalic (pros) > mesencephalic (mes) > metencephalon (Rhomb) > myelencephalon (Rhomb)
Which components of the adult nervous system do the secndary cerebral vesicles develop into, including ventricles
Telencephalic: cerebral hemispheres (each has its own lateral ventricle). Diencephalic: thalamus, hypothalamus, subthalamus, epithalamus, and third ventricle. Mesencephalic: midbrain and aqueduct of Sylvius. Metencephalon: pons, cerebellum, upper part of fourth ventricle. Myelencephalon: medulla and lower part of fourth ventricle. The rest of the neural tube forms the spinal cord
Stages in development where axes are developed
Sperm entry creates embryonic/abembryonic axis > inner cell mass forms on embryonic pole > the side with ICM implants in uterine wall and the edge leading implantation becomes caudal end > Nodal signaling becomes high caudally, low rostrally forming a gradient > The primitive node is made at high nodal areas.
Describe development of two layered germ disc, primitive node, streak and gastrulation
inner cell mass becomes epiblast (dorsal/amniotic cavity side) and hypoblast (ventral/yolk sac side) > primitive streak develops on caudal half of epiblast > cranial end of streak forms primitive node > primitive groove forms in middle of streak
Describe process of gastrulation
epiblast cells migrate into groove then form mesoblast or intraembryonic mesoderm or embryonc endoderm. The rest of the epiblast becomes embryonic ectoderm > Gradients of signalling molecules determine anterior/posterior and dorsal/ventral axes > Cells from node migrate towards buccopharyngeal membrane forming notochordal process then fusing with endoderm and forming the notochord
Describe notochord development into neural plate and the organization of the neural plate
The notochord releases Sonic Hedgehog which induces the overlying ectoderm to develop into the neural plate. The neural plate has a wide end rostrally (brain) and narrow end caudally (spinal cord), with bilateral symmetry
Describe primary neurulation
The neural folds first close at caudal rhombencephalon at 22 days, then closure proceeds caudally from rhombencephalon and rostrally from future optic chiasm over 4-6 days, keeping pace with somites. Final closure sites are at the site of the futur anterior comminssure and the S2 level
failure of neural tube closure
anterior closure failure causes anencephaly and posterior closure failure results in myelomeningocele
Describe secondary neurulation and what structures will eventually form
Caudal cell mass (caudal to the neural tube) develop vacuoles and enlarge, finally making contact with central canal of neural tube. This will become the conus medullaris and filum terminale
morphogens that promote caudal neural tube differentiation
Wnts, FGFs and retinoic acif from the primitive node create a gradient that is higher caudally
Morphogens that promote forebrain specialization
cerebrus and dickkopf from anterior visceral endoderm creates gradient that is higher rostrally
Rhombencephalon segmentation
Early on, the rhombencephalon consists of 8 rhombomeres which vary in the specific combo of Hox genes expressed along the AP axis. Retinoic acid plays a role in this by binding its receptor in the cytoplasm then binding regions of Hox genes.
All of the cellular elements of CNS develop from what structure? And what is the exception?
The neuroepithelial layer (ectoderm)- exception: microglia come from cells of the reticuloendothelial system
Describe cell migration in the neural tube at the cortex
intense cellular proliferation occurs, and the new neural cells migrate away from the inner part of the tube outwards, causing the tube to thicken selectively in certain areas (ie. The cerebral hemispheres get large while spinal cord stays small). Cells travel along radial glia from inside out, each trip getting longer and longer.
Describe cell migration in the neural tube at the spinal cord
cells proliferate around the ventricular
zone, then send their axons out towards the periphery, resulting in a gray matter centrally and white matter peripherally (as opposed to CNS)
Describe dorsal ventral axis development
notochord secretes sonic hedgehog which induces formation of floor plate (ventrally). Floor plate then secretes SHH forming a ventral to dorsal gradient. BMPs are secreted by ectoderm and concentrate dorsally and give rise to roof plate (dorsally) and dorsalizing signal. This leads to a dorsal-ventral gradeint of Pax gene expression called the alar plate (dorsal) and basal plate (ventral), separated by the sulcus limitans
Describe the alar and basal plates in the spinal cord
The basal plate neurons will become motor neurons and extend their axons out the ventral root to targets in their respective myotomes, while neural crest cells that make up the dorsal root ganglia at each segment send processes inwards to innervate neurons that develop from the alar plate.
Which structure looks like mickey mouse on cross section
midbrain
Understand the basic scheme of dorsoventral patterning of the prosencephalon, and how this relates to the adult three-dimensional structure.
Ventral structures in rostral end of tube produce sonic hedgehog creating a gradient. Three zones develop in the telencephalon (cortex, lateral ganglionic eminence and medial ganglionic eminence). The lateral surface of telencephalon folds over itself, forming sylvian fissure and distorting the vesicle, plus the rest of the telencephalon, into a C shape.
What do the lateral/medial ganglionic emininences become
Basal ganglia: putamen and globus pallidus laterally, caudate nucleus medially. They are split by the internal capsule (fiber pathway from thalamus to cortex)
