test 2 Flashcards
Fertilization occurs where
- ampullary region of the fallopian tube
Sperm + Oocyte =
Zygote
Reaches 2 cell stage about how long after fertilization
- 30 hours post fertilization
- happens in fallopian tube
Reaches 4 cell stage about how long after fertilization
- 40 hours
- happens in Fallopian tube
Reaches 12-16 cell stage about how long after fertilization
- 72 hours (3 days)
- happens in Fallopian tube
mitotic divisions
Series of mitotic divisions occurs.
Cells increase in number
Become smaller with each division
Cells are called BLASTOMERES
8 cell stage
Cells are loosely arranged until 8 cell stage.
After the 3rd cleavage, cells have maximized contact with each other.
Start to undergo compaction
Cells separate into inner and outer cells.
16 cell stage
called a MORULA
Had defined inner and outer cells.
Inner cells = inner cell mass (will become the embryo)
Outer cells = outer cell mass (will become the trophoblasts, which will be the placenta)
What happens when the morula makes wits way into the uterus
Fluid penetrates into the intercellular spaces of Inner Cell Mass.
Spaces become a single cavity
BLASTOCELE
Enlarges and becomes a BLASTOCYST
Embryo is called a BlastocystEarly Embryonic Development process (6 steps)
- fertilization (12-24 hours after ovulation)
- cleavages (first cleavage completed about 30 hours after fertilization)
- Morula (3-4 days after fertilization)
- Blastocyst (4.5-5 days after fertilization)
- Implantation (6 days after fertilization)
Blastocyst is comprised of:
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
Trophoblast and part of inner cell mass will become and the rest of the inner cell mass will become?
- the placenta
Rest of inner cell mass will become the embryo
Around day 6 after fertilization
implantation occurs
Trophoblastic cells over the embryoblast pole penetrate between the epithelial cells of the uterine wall.
L-Selectin (Trophoblasts) and carbohydrate receptor on uterine epithelium mediate attachment and capture of the capture and holding of the leukocytes from the blood onto the endothelial cells
Blastocyst remains free in the uterus for 2 days
Blastocyst attaches to
uterine epithelium, and subsequently the endometrium
Oriented so the inner cell mass is near the endometrium
At day 8
- the Blastocyst is partially embedded
The Trophoblast cells over the embryoblast differentiate into 2 layers:
Syncytiotrophoblast – multi-nucleated, outer zone
no cell boundries
Cytotrophoblast – mononucleated cells, inner layer (look like normal cells)
distinct cells
Cells from cytotrophoblast divide and migrate into the syncytiotrophoblast
Fuse and lose individual cell membrane – multi-nucleated
Both layers become part of the chorion (one of the fetal membranes)
Inner cell mass –embryoblast, differentiates too
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
Forms a flat disc
Small cavity appears within the epiblast
Enlarges to become amniotic cavity
Day 11-12
Completely embedded
Cells of the Synctiotrophoblast penetrate deeper into endothelial lining of maternal capillaries
Capillaries are congested and dilated
Called SINUSOIDS
Causes blood to flow through the trophoblastic system
Beginning of the uteroplacental circulation (primitive placenta)
Cytotrophoblast proliferates to form:
Amnion
Thin Protective membrane that surrounds the developing embryo.
Amniotic Cavity
Space, eventually filled with fluid.
What starts at day 8
GASTRULATION: Process for establishing 3 germ layers
Inner cell mass differentiates into:
Ectoderm
Endoderm
Mesoderm
These are the major embryonic tissues from which all tissues and organs of the body develop.
Gastrulation: Cells of the epiblast migrate to the primitive streak
Regulated by fibroblast growth factor 8 (FGF8)
Made by streak cells
Down-regulated with E-Cadherin (causing things to be loose)
Protein normally holds epiblasts together
Loss of E-Cadherin allows cells to detach and move
Arrive and become “flask-like”
Detach from epiblast and slip beneath it
Epiblast is the source of the germ layers.
Inward movement / cause invagination
Cells displace the hypoblast
Creates Endoderm
Others lie between the epiblast and the new endoderm
Mesoderm
Cells remaining in the epiblast
Ectoderm
Around 14 days after fertilization after gastrulation
Inner Cell Mass now called an Embryonic Disc as amniotic cavity starts to form.
Ectoderm – Layer of cells of the inner cell mass closest to the amniotic cavity.
Endoderm – layers of inner cell mass that borders the blastocele.
Mesoderm – Lies between the ectoderm and endoderm.
Ectoderm Becomes:
Skin, teeth, mouth glands, nervous system, some endocrine glands.
Endoderm Becomes:
Epithelium of digestive tract, respiratory system, bladder, reproductive organs, urethra
Mesoderm Becomes:
All connective tissue, the muscular, skeletal, lymphatic, and circulatory systems.
Cardiovascular system development
Cardiovascular System appears in the middle of the 3rd week.
At this point the embryo is no longer able to survive via diffusion alone.
The Cardiovascular System reaches a functional state long before any of the other systems
Vascular system develops from a simple symmetrical plexus, into an asymmetrical complex system
The size of the embryonic heart in relation to the size of the embryo is enormous compared to an adult heart in an adult size body.
Progenitor Heart Cells
Day 16-18
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.
Form a horse-shoe shaped cluster of cells called the Primitive Heart Field (PHF)
Will form blood islands and cardiac myoblasts
These will form blood cells and blood vessels.
Blood islands unite and form a horse-shoe shaped tube
Endothelial cell lined
Surrounded by myoblasts
First parts of the heart to show up (from primary heart field cells) (set up the sturcture)
- Specified on both sides
• Atria
• LV
• RV (part of it)
Day 20-21 secondary heart field cells develop and add to (part of the growth and development of the heart tube and heart itself)
• RV
• Conus Cordis
• Truncus Arteriosus
• Lengthen outflow tract
• Also have laterality
• Those on right side add to left side of heart tube
• Those on left side add to the right side of heart
tube.Serotonin (5HT)
Serotonin (5HT) initiates the R/L patterning
Concentrated on the LEFT side
Degrading enzyme (Monoamine Oxidase) is in high concentrations on the RIGHT side
Leads to expression of the PITX2 gene (master gene for the LEFT side)
All occurs about day 16-18 – critical for heart development
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
Eventually becomes the pericardial cavity
Blood Islands
Heart development starts at the end of the 3rd week of gestation.
Occurs in the ventral region of the embryo, inferior to the foregut.
Scattered masses of angiogenic cells appear in the mesenchyme derived from PHF
Occurs ventral (in front of) the intraembryonic celom.
Anterior part of the celom will develop the pericardial cavity
Endocardial Tube Formation
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.
The Endocardial tubes unite to form a common
tube
Primitive Heart Tube
Day 21-22
the single heart tube starts to beat
Formation of the Heart Tube
Heart tube continues to elongate
Cells are added from the secondary heart field to the cranial end of the tube
Process is essential for normal formation of part of the RV, the Conus Cordis (RV outflow) and the Truncus Arteriosus (LV outflow)
Also essential for looping
If lengthening doesn’t occur, where outflow tract defects come from
Dorsal Aortas
Meanwhile, Other blood islands appear inferior (below) to the endocardial tubes
Eventually give rise to dorsal aortas.
Will connect with the endocardial tubes
Establish the arterial end of the heart
Other end of the future endocardial tubes will make contact with the vitelline veins (via the sinus venosus) and establish a venous pole.
Day 23 after the tube starting to beat
Newly formed heart tube starts to bend
Cephalic part bends ventrally, caudally, and to the right.
Caudal part bends dorsocranially, and to the left.
Bulges into the pericardial cavity
Attached to dorsal wall by mesoderm
Mesoderm eventually ruptures
Leaves heart tube suspended in pericardial cavity
Dorsal Aorta and Aortic
Sac Becomes
- common outflow tract
Bulbus Cordis becomes
- Primitive RV
Primitive Ventricle becomes
- Primitive LV
Atrioventricular Sulcus becomes
- Divides atria from
ventricles
Paired Primitive Atria becomes
- Will form common atria
Sinus Venosus becomes
- Eventually forms the coronary sinus and oblique vein of the LA
Heart Folding
Day 23-28 (TAKES 5 DAYS)
Heart undergoes a series of folding that leads to the formation of the bulboventricular loop.
Heart Tube grows rapidly in length, especially the midsection (Bulbus Cordis and Ventricle)
Because the two ends are fixed, heart tube is forced to bend in order to adapt itself to the pericardial space causing:
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
What happens to the comparments after heart folding?
atria
Atrioventricular juntion
bulbus cordis
Atria initially paired – forms common atrium
Atrioventricular junction remains narrow
Forms AV Canal – connects common atrium and ventricle.
Bulbus Cordis is narrow, except proximal 1/3
Will form trabeculated part of RV
Junction between the ventricle and the bulbus cordis externally is the?
bulboventricular sulcus
Remains narrow
Called the primary interventricular foramen
Heart Folding – Atrium
Atrial portions of the Heart Tube dilate
Form Common Atrium
Atrium “climbs up” the dorsal pericardial wall
Takes Atrioventricular junction with it
Atrioventricular junction remains narrow
Now called AV Canal
Forms the communication between the atrium and the primitive ventricle.
Heart Folding - Ventricle
Ventricle also dilates
Acquires a larger lumen
Junction of the ventricle and the bulbus cordis remains narrow
Junction is now called the intraventricular foramen
Heart Folding – Bulbus
Cordis
Further dilation of the bulbus cordis
Lies on top of and in between the atrium
