Flashcards in Trilaminar Embryo Deck (70):
What is gastrulation?
The developmental process which establishes three embryonic germ layers: ectoderm, mesoderm, and endoderm
What is the primitive streak?
Making of primitive streak marks the beginning of gastrulation, around day 15 (start of week 3)
It is an accumulation of epiblast cells along the midline starting from the caudal end
What is the function of the primitive streak?
Definds the embryo's craniocaudal axis and right-left because the amniotic cavity is dorsal and the chorionic cavity is ventral
What is the primitive groove?
Epiblast cells reaching the primitive streak start to dive down to the ventral area, forming a groove in the midline of the primitive streak.
What is the primitive node?
Cells are added at cranial end of primitive groove, which forms the primitive node. In the center, there is a depression of ciliated cells called the primitive pit.
What is the significance of the primitive node and pit?
Major posterior signaling center of the embryo.
Cilia of primitive pit are pointed towards ventral side at 30 degrees and turn clockwise to create nodal flow to the left.
What is nodal and what induces it?
Cells in primitive node secrete fibroblast growth factor 8 which induces signalling protein Nodal. It accumulates to the left of the embryo because of primitive pit cilia
What are the properties of an epiblast cell?
They are epithelial. Have apical and basal surfaces, and are associated with a basal lamina. They express specific cell adhesion molecules (CAMs), especially E-cadherin. In order to ingress, they must undergo EMT during gastrulation
What is ingression?
When epiblast cells undergo EMT by elongating, losing basal lamina, and become bottle cells. They migrate into subepiblast territory through primtive groove.
What is a bottle cell?
The intermediate cell type between epiblast and mesenchymal cell in the EMT. When they free-float in the subepiblast territory, they are mesenchymal cells.
What changes are associated with EMT of epiblast?
Loss of E-cadherin and expression of N-cadherin, which is necessary for formation of mesoderm. EMT is associated with expression of transcription factor snail.
What forms the embryonic (definitive) endoderm?
First wave of ingressing epiblast cells through the primitive streak (around day 15). They displace the hypoblast and undergo a mesenchymal-to-epithelial transformation
How is the intraembryonic mesoderm formed?
Second wave of ingressing epiblast cells, which undergo EMT. The mesenchymal cells will have different fates based on where they ingress (four main subdivisions)
What is the fate of mesoderm ingressing just caudal to the primitive node?
They will migrate outward to the edge of the embryonic disc, and form a horse-shoe shaped mesodermal structure called the cardiac mesoderm (the fourth main subdivision)
Where does the cardiogenic mesoderm lie?
In a horseshoe formation cranial to the future position of the buccopharyngeal membrane
What is the prechordal plate?
First cells to ingress forming the mesoderm will migrate cranially between epiblast and endoderm until stopping at the prechordal plate. It stops just caudal to the future site of the buccopharyngeal membrane
What are the caudal and cranial ends where the ectoderm and endoderm adhere?
Caudal: Cloacal membrane
What is the allantois?
Structure forming from outpouching of extraembryonic endoderm from caudal wall of definitive yolk sac into the connecting stalk. It will be incorporated into the umbilical cord later, and is rudimentary in humans
How is the embryonic ectoderm formed?
After mesoderm formation, remaining epiblast cells become ectoderm.
What are the two major fates of ectoderm?
1. Neural ectoderm - formed with help from notochord
2. Surface ectoderm - rest of ectoderm
How is the prechordal plate formed?
Around day 16, the first epiblast cells ingressing through primitive pit form the precordal plate (mesodermal mass just caudal to buccopharyngeal membrane)
How is the notochordal process formed?
Next wave of ingressing cells through primitive pit following prechordal plate form tube called notochordal process. The channel within the tube is called notochordal canal and is filled with amniotic fluid since it is continuous with primitive pit
How is the nodochordal plate formed?
After growth of notochordal process is blocked by prechordal plate, it "unzips" from caudal towards cranial. The process fuses with the underlying endoderm and forms the nodochordal plate.
What is the neurenteric canal?
Transient connection from yolk sac to amniotic cavity as a result of formation of notochordal plate (Arises from lumen of notochord canal)
How is the definitive notochord made?
After the notochord plate is formed, the cells begin proliferating and separating from endoderm. This proceeds cranial (prechordal plate) to caudal until the neurenteric canal is obliterated at the end. (Once notochord reaches the primitive pit/node)
What are the main functions of the notochord?
Defines dorsal-ventral and left/right axes of embryo.
Provides midline patterning signals for surrounding tissues.
Major skeletal element of developing embryo.
Gets incorporated into the vertebral bodies during skeleton development.
How is the neuroectoderm different than the surface ectoderm?
The notochordal process releases factors as it is forming that induce the overlying ectoderm to become neural ectoderm. Surface ectoderm is unaffected by these factors
What is the neural plate?
Neural ectoderm forms this structure. It is more cranial since the notocordal process ends at the prechordal plate and runs mostly cranial
What does the neural ectoderm give rise to?
Brain, spinal cord, all CNS motor neurons, retina, neurohypophysis (posterior pituitary), epithelium and muscles of iris, epithelium of ciliary body of eye
Where is the surface ectoderm?
In cranial region, it located more laterally. However, caudal to the primitive node, all ectoderm is surface ectoderm (no notochordal process to induce neural ectoderm development)
What does the surface ectoderm give rise to?
Epidermis, epithelial lining of cornea and conjunctiva, epithelial lining of ear, mouth, and appendages. Proctodeal epithelium (anus) and terminal male urethral epithelium
What are ectodermal placode cells?
Smaller population of cells that form small aggregates of ectoderm within the surface ectoderm, lateral to neural plate
What are the first three of four main subdivisions of intraembryonic mesoderm?
Around day 16, there are three unsegmented mesoderms.
(4th is cardiogenic)
Where is the paraxial mesoderm segmented vs unsegmented?
Unsegmented when it is rostral (cranial) to otic vescles, will become head mesoderm
Segmented when it is caudal to the otic vesicles, which will form somites on both sides of the developing neural tube
What are otic vesicles?
Part of the neuroectoderm, will form the ears. Marks the line between unsegmented and segmented paraxial mesoderm
What are somites? Which ones are dorsal and which ones are ventral?
Somites are condensations of mesenchymal cells on both sides of the neural tube. They develop from segmented paraxial mesoderm.
Dorsal: Myotome, dermatome, lateral somite
What is the function of the sclerotome?
Dorsal element of the somite, it loses its compact organization first and migrates towards developing notochord and neural tube to form vertebral column later in development.
What is the function of the lateral somite?
Migrates to final destinations and gives rise to all voluntary striated muscles of body wall and limbs (hypaxial muscles)
What is the dermomyotome?
Two segments of the dorsal somite which fuse and give rise to different but related things
Dermatome - gives rise to dermis of skin over dorsal region of trunk
Myotome - gives rise to segmented muscles of axial skeleton (epaxial). I.e. intrinsic muscles of the back and ribs associated with spinal cord
What is the rate of somite development?
First pair of somites develop in the occipital region around day 20, and they develop in both directions around 3 pairs per day until end of week 5
(can be used to estimate embryo's age)
42-44 will be made, from cranial region to tail
How many somite pairs in the end?
42-44 will be made, but some caudal somites are lost so only 35-37 pairs will be counted in the end (we lose our tail)
What does the intermediate mesoderm give rise to?
It is unsegmented. Gives rise to:
Connective tissue and smooth muscle of gonads, urinary system, reproductive tracts
What forms the intraembryonic coelom? and what two layers does it create?
Lacunae form within the lateral (plate) mesoderm, which coalesce and form a continuous cavity. Forms two new layers of mesoderm: Somatic (parietal) and splanchnic (visceral)
What is somatopleure?
Dorsal surface of intraembryonic coelom, consists of somatic mesoderm + surface ectoderm
What is splanchnopleure?
Ventral surface of intraembryonic coelom, consists of splanchnic mesoderm + endoderm
How do the extraembryonic and intraembryonic coelom's meet?
Whenever the somatopleure and splanchnopleure split at the edges of the intraembryonic coelom, breaking the lateral mesoderm apart. This opens the coelom.
What does the somatic (parietal) mesoderm give rise to?
Skeleton of the limbs, connective tissues of limbs and trunk, dermis of ventral body wall and limbs, connective tissues of external genitalia, smooth muscle and connective tissues of the blood vessels
What does the splanchnic (visceral) mesoderm give rise to?
Smooth muscle and CT of GI tract, associated glands, and abdominal mesenteries. Smooth muscle and CT of respiratory tract and associated glands, cardiac muscle and the smooth muscle and connective tissue of their blood vessels.
What is hemangiogenesis and what are the two types?
Formation of blood vascular network in the embryo
Two types: Primitive (occurs in exraembryonic splanchnopleuric mesoderm in wall of secondary yolk sac)
Definitive (cardiogenic mesoderm within embryo)
When does primitive hemangiogenesis begin?
Around week 3 of development, in the walls of the secondary yolk sac
What forms blood islands?
When mesenchymal cells of the extraembryonic splanchnopleuric mesoderm of secondary yolk sac adjacent to the extraembryonic endoderm differentiate into hemangioblasts, and aggregate into blood islands (isolated cell clusters)
This is the start of primitive hemangiogenesis
What are hemangioblasts and what are they the precursor of?
They sit in blood islands and are differentiated from mesenchymal cells of extraembryonic splanchnopleuric mesoderm.
1. Angioblasts - develop into endothelial cells of blood vessels
2. Hematopoietic stem cells (HSCs) - which develop into blood cells
What is vasculogenesis?
Hemangioblasts differentiate into angioblasts and arrange themselves around cavities in blood islands to become flattened endothelial cells. When these cavities fuse into endothelial channels it is called vasculogenesis
What is angiogenesis?
When existing endothelial channels bud and form more vessels, this is angiogenesis
What forms the smooth muscles of primitive hemangiogenesis?
Mesenchymal cells surrounding the primordial blood vessels differentiate into smooth muscle and connective tissue
Where does primitive hemangiogenesis ultimately spread to?
All of the extraembryonic mesoderm, including the somatopleuric. This allows the vessels of the tertiary chorionic villi, and connects to the blood islands through the connecting stalk
When does definitive hemangiogenesis begin?
Within a short delay of primitive hemangiogenesis (about 2 days)
Where does cardiogenic mesoderm lie?
The cardiogenic mesoderm lies within the lateral mesoderm, and it sits within the embryonic splanchnic (visceral) mesoderm. During formation of intraembryonic coelom, it moves ventral and lies within the splanchnic mesoderm.
What does the rostral part of the cardiogenic mesoderm form?
Blood island in the primary heart field (definitive hemangiogenesis) frorm paired, longitudinal, endothelial-lined channels which are the endocardial heart tubes. These will later fuse to form the primordial heart tube
What do angioblasts of the cardiac mesoderm do?
They are highly invasive and migrate to their final destinations, forming the endothelial lining of blood vessels (CT and smooth muscle is formed by somatic and splanchnic mesoderm). This forms the dorsal aorta and cardinal veins, via vasculogenesis
What is the role of angiogenesis in definitive hemangiogenesis?
After the primary vascular bed has been formed via invasive vasculogenesis, there is a system of sprouting from pre-existing vessels that allows the final vasculature of the primordial cardiovascular system to form
How are definitive hematopoeitic stem cells (HSCs) different from primary HSCs?
They are derived from cardiogenic mesoderm, and will seed the embryonic liver, which is the main site of definitive hematopoiesis during embryonic development (does not shift to bone marrow until birth)
What is the septum transversum?
It is the most cranially located mesoderm, cranial to the buccopharyngeal membrane as well as the cardiac mesoderm, and is adjacent to the extraembryonic mesoderm. It is a mesenchymal cell mass
What is the fate of the septum transversum?
Becomes the majority of the mesenchyme of the developing liver, and will also give rise to epicardium, central portions of diaphragm, and esophageal mesentery. (It is located outside the buccopharyngeal membrane)
What is the fate of the embryonic endoderm?
Gives rise to the epithelial lining of the developing guts and its appendages, including foregut, midgut, hindgut, and allantois.
What is primary ciliary dyskinesia? (PCD)
Autosomal recessive genetic disorder, dynein malfunction to cilia don't work properly. Messes with leftward flow of Nodal during gastrulation and messes with left-right symmetry
What is situs inversus?
Mirror-image of normal laterality of internal organs. It is the opposite of normal (situs solitus)
What is situs ambiguous?
Atypical but not totally reversed organ placement
What is Kartagener syndrome?
Primary ciliary dyskinesia + situs inversus. happens in about 50% of PCD cases