5. Embryology- Exam 2 Flashcards
where does Fertilization occurs
ampullary region of the fallopian tube
Sperm + Oocyte =
Zygote
–undergoes cell divisions
when does the zygote reach 2 cell stage
30 hours post fertilization
when does the zygote reach 4 cell stage
40 hours post fertilization
when does the zygote reach 12-16 cell stage
72 hours post fertilization
the zygote goes through a Series of mitotic divisions. describe what is happening
Cells increase in number
Become smaller with each division
Cells are called
Blastomeres
Cells are loosely arranged until what cell stage
8 cell stage
After the which cleavage do cells have maximized contact with each other
3rd cleavage
After the 3rd cleavage, they start to undergo what
compaction
–Cells separate into inner and outer cells
16 cell stage is called a
Morula
Morula=
Has defined inner and outer cells
Morula Inner cells =
inner cell mass (will become the embryo)
Morula Outer cells =
outer cell mass (will become the trophoblasts, which will be the placenta)
Morula makes its way into what
the uterus
when the Morula enters uterine cavity- Fluid penetrates into the intercellular spaces of the Inner Cell Mass. The spaces become a single cavity called what
Blastocele
After the Morula makes its way into the uterus, it Enlarges and becomes what
embryo is called a Blastocyst at this point
Blastocyst is comprised of what 3 things? describe them
- Trophoblast – outer covering. Formed from cells of the outer cell mass. Cells flatten and eventually form the epithelial wall of the blastocyst
- Blastocele – Internal fluid filled space
- Embryoblast – Inner cell mass. Located at one pole
Of the blastocyst, the Trophoblast and part of inner cell mass will become the ______ while the rest of inner cell mass will become the ______
placenta
embryo
how long does the blastocyst remain free in the uterus
2 days
when does implantation occur
Around day 6 (after fertilization)
describe what happens when implantation occurs
- Trophoblastic cells over the embryoblast pole penetrate btwn the epithelial cells of the uterine wall.
- Blastocyst attaches to uterine epithelium, the endometrium- Oriented so the inner cell mass is near the endometrium
describe the 5 steps to get to implantation
- Fertilization= 12-24 hours after ovulation
- Cleavage= completed 30 hours after fertilization
- Morula= 3-4 days after fertilization
- Blastocyst= 4.5-5 days after fertilization
- Implantation= 6 days after fertilization
at what day is the Blastocyst is partially embedded
day 8
when the Blastocyst is partially embedded- The Trophoblast cells over the embryoblast differentiate into what 2 layers? describe them
- Syncytiotrophoblast – multi-nucleated, outer zone, no cell boundries
- Cytotrophoblast – mono-nucleated cells, inner layer, distinct cells
both the Syncytiotrophoblast and the Cytotrophoblast become part of what
the chorion (one of the fetal membranes)
the inner cell mass of the blastocyst (embroblast) now differentiates in to what 2 layer? describe
- Hypoblast layer (Entoderm)= Layer of small cuboidal cells adjacent to the blastocyst cavity
- Epiblast layer (Ectoderm)= Layer of high columnar cells adjacent to amniotic cavity
when is the blastocyst completely embedded
day 11-12
what happens when the blastocyst completely embedded
Cells of the Synctiotrophoblast penetrate deeper into endothelial lining of maternal capillaries. The capillaries are congested and dilated…Called SINUSOIDS
what do the sinusoids cause? what is this the beginning of?
- Causes blood to flow through the trophoblastic system
- Beginning of the uteroplacental circulation
Cytotrophoblast proliferates to form what 2 things? describe them
- Amnion= Thin Protective membrane that surrounds the developing embryo.
- Amniotic Cavity= Space, eventually filled with fluid
what has also started to develop back at day 8
Gastrulation
define Gastrulation
- Process for establishing 3 germ layers
- These are the major embryonic tissues from which all tissues and organs of the body develop
with Gastrulation- the inner cell mass differentiates into?
Ectoderm
Endoderm
Mesoderm
Gastrulationstarts with the formation of what?
the primitive streak on the epiblast
when is the primitive streak clearly visible as a narrow groove
day 15-16
describe how the cells of the epiblast migrate to the primitive streak
Arrive and become “flask-like”
Detach from epiblast and slip beneath it
epiblast gastrulation: Epiblast is the source of the ______. Inward movement / cause _______
germ layers
invagination
- Cells displace the hypoblast=Creates ____
- Others lie between the epiblast and the new endoderm= Creates _____
- Cells remaining in the epiblast= Creates _____
- Endoderm- layers of inner cell mass that borders the blastocele.
- Mesoderm- Lies inbetween the ectoderm and endoderm.
- Ectoderm - Layer of cells of the inner cell mass closest to the amniotic cavity
now that gastrulation took place, the 3 germ layers are formed and the amniotic cavity starts to form… the inner cell mass in now called the
embryonic disc
the ectoderm becomes what?
Outer structures: Skin, teeth, mouth glands, nervous system, some endocrine glands
the mesoderm becomes what?
All connective tissue, the muscular, skeletal, lymphatic, and circulatory systems
the endoderm becomes what?
Deep structures: Epithelium of digestive tract, respiratory system, bladder, vagina, urethra
when does the Cardiovascular System appear? when does the heart start beating?
appears middle of 3rd week
beats after 21 days
when does the Cardiovascular System reach a functional state in comparison to other systems
long before any of the other systems
Vascular system develops from what?
a simple symmetrical plexus, into an asymmetrical complex system of arteries, veins and capillaries.
what is the size of the embryonic heart in relation to the size of the embryo as compared to an adult heart in an adult size body
enormous
when do Progenitor Heart Cells start to develop
day 16-18
where do Progenitor Heart Cells lie and how do they move
- Lie in the Epiblast- Adjacent to the cranial end of the primitive streak
- Move through the streak and into the splanchnic layer of the mesoderm
what do Progenitor Heart Cells form
Form a horse-shoe shaped cluster of cells called the Primitive Heart Field (PHF) [collection of cells that have a specific assignment]
Primitive Heart Field is specified on both sides and will become what
Atria, LV, RV, Conus Cordis and Truncus Arteriosus
starting with Primitive Heart Field the Conus Cordis and Truncus Arteriosus come from what? when?
come from the Secondary Heart field and show up around Day 20-21
Progenitor Heart Cells will form blood islands and cardiac myoblasts. These will form what?
blood cells and blood vessels
blood islands unite and form what?
a a horse-shoe shaped tube that is Endothelial cell lined and Surrounded by myoblasts
more blood islands appear bilaterally, parallel and close to the midline. What will these form
dorsal aortas
how is the Intraembryonic Celom (Body Cavity) formed
- by the joining of small (initially isolated) spaces which appear in the lateral mesoderm
- Bilateral cavities extend cranially and fuse with each other forming a horseshoe-shaped cavity
what eventually becomes the pericardial cavity
Intraembryonic Celom
Heart development starts at the end of the 3rd week of ____
gestation
where does heart development of the first blood island occur
Occurs in the ventral region of the embryo, inferior to the foregut [in front of the intraembryonic celom]
–Anterior part of the celom will develop the pericardial cavity
how do you know heart development of the first blood island has started
Scattered masses of angiogenic cells appear in the mesenchyme derived from PHF
how does Endocardial Tube Formation occur
- Angiogenic cells form clusters (Blood Islands) which increase in size and number.
- Acquire a lumen, unite and form a plexus of blood vessels
- From this plexus, bilateral endocardial tubes develop
bilateral endocardial tubes develop- they unite to form what?
form a common tube= Primitive Heart Tube
when does the Primitive Heart Tube (single heart tube) begin to beat
day 21-22
once the Primitive Heart Tube is formed- how does it start to elongate
Cells are added from the secondary heart field to the cranial end of the tube
Primitive Heart Tube elongation is essential for what further development
normal formation of part of the RV, the Conus Cordis, the Truncus Arteriosus and for looping
what happens if Primitive Heart Tube elongation “looping” does NOT happen know
outflow tract defects form=Abnormalities in Cardiac looping=
- DORT, VSD, TOF, PA, PS
- Ventricular inversion (corrected transposition)
- juxtaposition of the atrial appendages
- Double outlet right ventricle
while Primitive Heart Tube elongation occurs, other blood islands appear inferior to the endocardial tubes and eventually give rise to what? what will this establish?
- dorsal aortas. Will connect with the endocardial tubes
- Establish the arterial end of the heart
What will the other end of the future endocardial tubes make contact with? what does this establish?
the vitelline veins (via the sinus venosus) and establish a venous pole
when does the newly formed heart tube starts to bend
day 23
how does the Cephalic part of the newly formed heart tube bend
ventrally, caudally, and to the right
how does the Caudal part of the newly formed heart tube bend
dorsocranially, and to the left
the newly formed heart tube starts to bulge into what
the pericardial cavity
the newly formed heart tube attaches to what
to dorsal wall by mesoderm
what happens when the newly formed heart tube attaches to dorsal wall by mesoderm
Mesoderm eventually ruptures
Leaves heart tube suspended in pericardial cavity
what are the 6 regions of newly formed heart tube know
Dorsal Aorta and Aortic Sac Bulbus Cordis Primitive Ventricle Atrioventricular Sulcus Paired Primitive Atria Sinus Venosus
Dorsal Aorta and Aortic Sac= know
Becomes the common outflow tract
Bulbus Cordis= know
Primitive RV
Primitive Ventricle= know
Primitive LV
Atrioventricular Sulcus= know
Divides atria from ventricles
Paired Primitive Atria= know
Will form common atria
Sinus Venosus= know
Eventually forms the coronary sinus and oblique vein of the LA
what happens on day 23-28
Heart undergoes a series of folding that leads to the formation of the bulboventricular loop
during days 23-28, Heart Tube grows rapidly in length, especially what section?
the midsection (Bulbus Cordis and Ventricle)
during days 23-28, why is heart tube forced to bend
Because the two ends are fixed, heart tube is forced to bend in order to adapt itself to the pericardial space
during days 23-28, how exactly does the heart tube bend and how is it oriented
- Forms a U-Shape
- Then Forms an S-Shape
- Oriented so that the atrium and sinus venosus lie superior to the bulbus cordis, ventricle and dorsal aorta/aortic sac
during heart folding, the Atria initially paired and this forms what
forms common atrium
during heart folding, Atrioventricular unction remains narrow and goes on to form what
AV Canal – connects common atrium and ventricle
during heart folding, Bulbus Cordis is narrow, except what portion? what will this form?
proximal 1/3
–Will form trabeculated part of RV
during heart folding, Junction between the ventricle and the bulbus cordis externally is the?
bulboventricular sulcus
the bulboventricular sulcus remains narrow and is called the
primary interventricular foramen
during folding of the atrium, Atrial portions of the Heart Tube dilate and form the
Common Atrium
during folding of the atrium, Atrium “climbs up” the ________ wall and takes the ________ with it
dorsal pericardial
Atrioventricular junction
during folding of the atrium, the Atrioventricular junction remains narrow. This is now called the _____ and it forms communication btwn what?
AV Canal
atrium and the primitive ventricle
during folding of the ventricles, Ventricle also dilates. This acquires a?
larger lumen
during folding of the ventricles, Junction of the ventricle and the bulbus cordis remains narrow. The Junction is now called the?
intraventricular foramen
after dilation of the atrium and ventricles, we see further dilation in what?
bulbus cordis
after dilation of the atrium/ventricles.bulbus cordis… At the venous pole, growth proceeds to a point where the expanding common atrium does what?
has drawn the originally paired sinus venosus together until it consists of a midportion and right and left horns
what happens on heart folding day of 24
Diverticula appear
Diverticula appear in what two sharply defined areas
- Along front of endocardial tube
- Just proximally and distal to the primary interventricular foramen.
- -Early embryonic Ventricle - Primitive LV
- -Proximal 1/3 of bulbus cordis - Primitive RV
what is the purpose of Diverticula
expand the capacity of the heart
Give them the densely trabeculated appearance
from the diverticula, the trabeculae inside the heart leads to what?
valves, chordeae tendinae, papillary muscles, and atrioventricular valves.
heart folding starting from the top:
Aortic arches
truncus arteriosus
conus cordis
bulbus cordis
As the primitive atria grow, the truncus arteriosus is positioned in a depression between what?
the roof of the right and left primitive atria.
entrance to the common atrium=
Sinuatrial orifice
Sinuatrial orifice (entrance to the common atrium) is flanked on each side by what?
valvular folds
Sinuatrial orifice Left venous valve fuses with?
the atrial septum
Sinuatrial orifice Superior part of the right venous valve does what?
disappears
Sinuatrial orifice Inferior part of the right venous valve becomes what?
the valve of the inferior vena cava and the valve of the coronary sinus
During atrial septation, the left atrium begins to form what?
sprouts of the pulmonary veins that grow towards the lungs.
when does cardiac septation occur and how long does it last?
occurs on day 27
lasts 10 days (27-37)
during the 10 day period during cardiac septation, how much does the embryo grow in length
5mm to 16-17mm
during cardiac septation, The formation of the cardiac septa occur
simultaneously
during cardiac septation, are there any changes in external appearance
no major changes in external appearance
during cardiac septation, at day 27- describe the primitive right and left ventricles
primitive right and left ventricles are local widenings of the original heart tube -Connected to each other via narrow channel (Interventricular Foramen) [where septa start to occur]
during cardiac septation, Blood reaches primitive LV via
small opening from the primitive atria
–AV Canal
During ventricular development, what 2 important processes occur simultaneously, that result in the “chamberization” of the ventricles
- -Division of the AV Canal into a Right and Left AV orifice via the endocardial cushions
- -Ventricular Septation
where do Endocardial Cushions develop
in the Atrioventricular and Conotruncal regions
what do Endocardial Cushions assist in
Assist in the formation of the atrial and ventricular (membranous) septa, AV Canals, Valves, Aortic and Pulmonary Channels
At the end of week 4, Atrioventricular endocardial cushions appear at the AV Canal. describe them
2 lateral AV cushions appear on the right and left side of the canal
Anterior and posterior cushions push into the lumen and fuse. What does this result in?
Results in the complete division of the canal into right and left AV orifices.
–Final and the end of week 5
The role of the endocardial cushions is to:
- Form a barrier between the atria and ventricles
- Right –left division of the canal
- -The resulting canals are eventually occupied by the mitral and tricuspid valves.
what happens at the end of week 4
Ventricular Septation
The ventricular septum is formed by the outgrowth of what?
the muscular ridge at the interventricular foramen
when the muscular ridge at the interventricular foramen begins to extend- what happens
2 primative ventricles expand
–Medial walls become apposed and merge
The ventricular septum grows upward from?
the apex of the heart to the base of the heart
what happens if the ventricular septum doesnt grow all the way
muscular VSD
what happens if the ventricular septum doesnt connect to the endocusion
membranous VSD
Interventricular foramen is the space above the _______. What does it do when the conus septum is complete?
membranous septum
Shrinks
Outgrowth of tissue from the endocardial cushion along the top of the muscular interventricular septum does what?
- closes the foramen
- Fuses with the abutting part of the conus septum
when do the paired atria fuse together for form a common atrium
At day 27-28
Atrial septation occurs _______ and in _______ with ventricular septation
simultaneously
cooperation
Atrial septation also lasts approximately
10 days
during atrial septation, ______ crest grows from the roof of the common atrium
Sickle-shaped
during atrial septation, the first portion of the crest growing from the roof of the common atrium is the?
Septum Primum
during atrial septation, the first portion of the crest growing from the roof of the common atrium Extends toward the
endocardial cushions in the AV Canal
during atrial septation, the first portion of the crest growing from the roof of the common atrium does not connect to endocardial cushions- it leaves a space called the
Ostium Primum
the Ostium Primum is formed by the?
opening between the lower rim of the septum primum and the endocaridal cushions
during atrial septation, Extensions of the endocardial cushions grow along the edge of the septum primum, closing the?
Ostium Primum
Before closure of the Ostium Primum, what happens
Apoptosis (cell death) makes perforations in the upper part of the septum primum
- Forms the Ostium Secundum
- Allows for blood flow to shunt from the right to the left
After fusing with the endocardial cushion, if there is a small residual opening at the ECC, what has developed?
ostium primum ASD
the Lumen of the Right Atrium expands as a result of what?
incorporation of the sinus horns
when the Lumen of the Right Atrium expands, what appears?
New crest-shaped fold appears= Septum Secundum
describe the development of the Septum Secundum
Extends downward to the septum in the AV canal
Overlaps with the Ostium Secundum
Remaining hole is called the Foramen Ovale
recap: what is the first step to atrial septation
Septum primum grows downward from the roof of the common atrium
recap: what is the second step to atrial septation
Apoptosis of a small superior portion of septum primum occurs during continued downward growth, resulting in a foramen secundum
recap: what is the third step to atrial septation
The septum secundum then grows downward, covering the foramen secundum
recap: what is the fourth step to atrial septation
The septum secundum then grows further downward and creates a foramen ovale at the ECC
recap: what is the fifth step to atrial septation
The two septa together now act as a valve to allow blood to enter the left atrium via the foramen ovale, then the septum secundum
By the 5th week, what appears
opposing ridges appear in the truncus
ridges in the truncus cushions that lie on the right superior wall will grow…
distally and to the left.
ridges in the truncus cushions that lie on the left inferior wall will grow…
distally and to the right
both truncus cushions (right and left) will grow towards what?
the aortic sac
both truncus cushions Twist around each other and eventually fuse to form what
the aorticopulmonary septum
-Divides the truncus into aortic and pulmonary channels
Similar cushions (of the truncus cushions) appear along the?
right dorsal and left ventral walls of the conus cordis
the Similar cushions (of the truncus cushions) grow how?
Grow toward each other and distally
Unite with the truncus septum
Similar cushions (of the truncus cushions) divide what
Divides the conus into anterolateral portion (RV outflow) and posteromedial portion (LV outflow)
what happens on day 29
Partitioning of the outflow tract
-Truncus Arteriosus – Aorta
-Conus Cordis – Pulmonary Artery
(Created by a septum that forms in the outflow tract from these swellings)
how do AV valves form? 5 steps
- Each AV orifice is surrounded by local proliferations of mesenchymal tissue.
- Blood hollows out and thins the tissue on the ventricular surface
- Valves form
- Remain attached to ventricular wall by muscular cords
- Muscular tissue degenerates and is replaced by connective tissue over time
how do semilunar valves form?
Small tubercles found on main truncus swellings
Tubercles hollow out on the upper surface
when are Valves are fully developed
day 55
summary of day 1-2
Fertilization
summary of day 6-7
Implantation of blastocyst into uterine wall
summary of day 16-20
Appearance of Celom and Blood Islands
summary of day 20
Endocardial tubes are present and begin fusing
summary of day 22
Main heart tube begins to beat
summary of day 23-28
Heart Folding
summary of day 27-37
Atrial and Ventricular septation and development
summary of day 29
Outflow tract septation
summary of day 55
Complete
