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Biology - Embryology


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What is embryology?
It is the study of the development of a unicellular zygote into a complete multicellular organization
What is cleavage?
A series of rapid mitotic divisions in early embryonic development
What do these divisions lead to?
An increase in cell number without a corresponding growth in cell protoplasm
What happens to the total volume of cell cytoplasm during cleavage?
It stays constant
What does cleavage result in?
Progressively smaller cells, with an increasing ratio of nuclear-to-cytoplasmic material
What does cleavage do to the surface-to-volume ratio, and why?
It increases it for each cell, improving gas and nutrient exchange
What is an indeterminate cleavage?
One that results in cells that maintain the ability to develop into a complete organism
Are identical twins the result of an indeterminate cleavage?
What is a determinate cleavage?
It results in cells whose future differentiation pathways are determine at an early developmental stage
What is differentiation?
A specialization of cells that occurs during development
When does the first complete cleavage occur?
Approximately 32 hours after fertilization
When does the second cleavage occur?
After 60 hours
When does the third cleavage occur?
At 72 hours
After the third cleavage, the 8-celled embryo reaches where?
The uterus
As cell division continues, what is the solid ball of embryonic cells called that forms?
A morula
What is blastulation?
It is what brgins when the morula develops a fluid-filled cavity called the blastocoel
By the fourth day, what happens?
The blastocoel becomes a hollow sphere of cells called the blastula
What is the mammalian blastula called?
The blastocyst
How many cell groups does the blastocyst have, and what are they?
2, the inner cell mass and the trophoblast
What is the inner cell mass?
It protrudes into the blastocoel
What is the trophoblast?
It surrounds the blastocoel and later gives rise to the chorion
So, in a mammalian blastocyst, which of the two makes up the border?
The trophoblast
Where is the inner cell mass?
On the inside of the trophoblast
When does the embryo implant in the uterine wall?
During blastulation
When is blastulation?
Approximately 5-8 days after fertilization
What hormone prepares the uterus for implantation?
What does progesterone actually do?
It causes glandular proliferation in the endometrium
What is the endometrium?
The mucosal lining of the uterus
What do the embryonic cells secrete?
Proteolytic enzymes that enable the embryo to digest tissue and implant itself in the endometrium
At this site what eventually occurs?
Maternal and fetal blood exchange materials at this site
What does this location later become?
The placenta
Once implanted, cell migrations transform the single cell layer of the blastula into what?
The gastrula
How many layers does it have?
3 layers
In the sea urchin, what does gastrulation begin with?
The appearance of a small invagination on the surface of the blastula
What forms next?
An inpocketing forms as cells continue to move toward the invagination, eventually eliminating the blastocoel
What is the result?
A two-layered cup, with a differentiation between an outer cellular layer and an inner cellular layer
What is the outer cellular layer called?
The ectoderm
What is the inner cell layer called?
The endoderm
What is the newly formed cavity of the two-layered gastrula called?
The archenteron
What does this later transform into?
The gut
What is the opening of the archenteron called?
The blastopore
In deuterostomes such as humans, the blastopore is the site of what?
The future anus
For protosomes on the other hand, the blastopore is the site of what?
The future mouth
Proliferation and migration of cells into the space between the ecroderm and the endoderm gives rise to a third cell layer called what?
The mesoderm
These are known as what?
The three primary germ layers
What does the ectoderm differentiate into?
Integument, the lens of the eye, and the nervous system
What does the endoderm differentiate into?
The epithelial linings of the digestive and respiratory tracts, and parts of the liver, pancreas, thyroid, and bladder
What does the mesoderm differentiate into?
The musculoskeletal system, circulatory system, excretory system ,gonads, connective tissue throughout the body, and portions of digestive and respiratory organs
How is most of differentiation accomplished?
Through selective transcription of the genoma
What is induction?
The influence of a specific group of cells sometimes called the organizer on the differentiation of another group of cells
Induction is most often mediated by what?
Chemical substances called inducers
How are they passed?
They are passed from the organizer to adjacent cells
In the development of the eyes, how does this work?
Lateral outpocketings from the brain (optic vessels) grow out and touch the overlying ectoderm
What does this mean?
The optic vesicle induces the ectoderm to thicken and form the lends placode
What does the lens placode do?
It then induces the optic vesicle to flatten and invaginate inward, forming the optic cup
Then what?
The optic cup then induces the lens placode to invaginate and form the cornea and lens
Experiments with frog embryos show what?
If this ectoderm is transplanted to the trunk, a lens will develop in the trunk
What will happen is the ectoderm is transplanted before the outgrowth of the optic vesicles?
It will not form a lens
By the end of gastrulation, regions of the germ layers begin to develop into what?
A rudimentary nervous system
What is this process known as?
What is the rod of mesodermal cells called that develops along the longitudinal axis just under the dorsal layer of the ectoderm?
The notochord
Does the notochord have an inductive effect on anything?
Yes, it does on the overlying ectoderm
What does this cause?
It causes a decrease in pressure in the right atrium
What does the dorsal ectoderm then do?
It folds on either side of the groove
What are these foldings called?
The neural folds
What do the neural folds do?
They grow upward and finally fuse, forming a closed tube
What is this tube called?
The neural tube
What does the neural tube give rise to?
It gives rise to the brain and spinal cord (central nervous system)
What happens once the neural tube is formed?
It detaches from the surface ectoderm
What are the cells at the tip of each neural fold called?
Neural crest cells
What do these cells do?
They migrate laterally and give rise to many components of the peripheral nervous system
What are some components?
The sensory ganglia, autonomic ganglia, adrenal medulla, and Schwann cells
What is a fetus?
The embryo after eight weeks of gestation
How does the fetus receive oxygen?
Directly from its mother through a specialized circulatory system
What does this system do besides supply oxygen and nutrients to the fetus?
It also removes carbon dioxide and metabolic wastes
What are the two components of this system?
The placenta and the umbilical cord
When do those develop?
In the first few weeks following fertilization
The placenta and the umbilical cord are outgrowths of what?
The four extra-embryonic membranes formed during development
What are they called?
The amnion, chorion, allantois, and yolk sac
What is the amnion?
A thin, tough membrane containing a watery fluid called amniotic fluid
What is amniotic fluid?
It acts as a shock absorber of external and localized pressure from uterine contractions during labor
When does placent formation begin?
It begins with the chorion
What is the chorion?
A membrane that completely surrounds the amnion
What happens to the chorion after two weeks of fertilization?
It extends villi into the uterine wall
These chorionic villi become closely associated with what?
Endometrial cells, developing into the spongy tissue of the placenta
What is the allantois?
It develops as an outpocketing of the gut
The blood vessels of the allantoic wall do what?
They enlarge
What do they become?
The umbilical vessels
What are umbilical vessels
They are the vessels that will connect the fetus to the developing placenta
What is the yolk sac?
It is the site of early development of blood vessels
It becomes associated with what?
Umbilical vessels
At some point the allantois and yolk sac are enveloped by what?
The amnion
What does this form?
The primitive umbilical cord
What is the umbilical cord?
The initial connection between the fetus and the placenta
What does the mature umbilical cord consist of?
The umbilical vessels, which developed from the allantoic vessels, surrounded by a jelly-like matrix
What happens to the placenta as the embryo grows?
It remains attached to the embryo by the umbilical cord
What does this permit?
It permits it to float freely in the amniotic fluid
What is the placenta the site of?
It is the site of nutrition, respiration, and waste disposal for the fetus
What diffuses across maternal capillaries into fetal blood?
Water, glucose, amino acids, vitamins, and inorganic salts
Does fetal hemoglobin have a higher or lower affinity for oxygen than does adult hemoglobin?
It has a higher affinity for oxygen than adult hemoglobin
What does this imply?
This means that in the fetus, the pulmonary arteries carry partially oxygenated blood to the lungs
During this time, metabolic wastes and carbon dioxide diffuse where?
From fetal blood into maternal blood
Are the circulatory systems of the mother and fetus directly connected?
Does maternal and fetal blood mix?
What else does the placenta offer the fetus?
It offers some immunological protection by preventing the diffusion of foreign matter like bacteria into fetal blood
The placenta is permeable to what however?
Viruses, alcohol, and many drugs and toxins
What functions as the endocrine gland for the fetus?
The placenta
What does it produce?
Progesterone, estrogen, and Human chorionic gonadotropin
What are these essential for?
Maintaining pregnancy
The presence of what in urine is the simplest test for pregnancy?
What is the major difference between fetal circulation and adult circulation?
In fetal circulation, blood is oxygenated in the placenta, while in adult circulation, blood is oxygenated in the lungs
How many shunts do the fetal circulatory contain and what do they do?
How many shunts do the fetal circulatory contain and what do they do?
It has three shunts, and they divert flow away from the developing fetal liver and lungs
What is the umbilical vein?
It carries oxygenated blood from the placenta to the fetus
The blood bypasses the fetal liver by way of what?
The shunt called the ductus venosus
After the blood passes through the shunt, it converges with the inferior vena cava
What do the inferior and superior venae cavae return?
They return deoxygenated blood to the right atrium
What happens because oxygenated blood from the umbilical vein mixes with the deoxygenated blood of the venae cavae?
The blood entering the right atrium is only partially oxygenated
How does most of this blood bypass the pulmonary circulation?
By way of the foramen ovale
What does this allow?
It allows blood to enter the left atrium directly from the right atrium
What is the formane ovale?
It is a shunt that diverts blood away form the pulmonary arteries
What happens to the remaining blood in the right atrium?
It empties into the right ventricle
Where is it pumped from there?
The pulmonary artery
Most of this blood is shunted directly from the pulmonary artery to the aorta via what?
The ductus arteriosus
The blood that does reach the lungs does what?
It is further deoxygenated as the blood unloads its oxygen to the developing lungs
Where does gas exhcnage occur?
In the placenta
What happens to the deoxygenated blood from here?
It returns to the left atrium via the pulmonary veins
Even though this blood mixes with the partially oxygenated blood that crossed over from the right atrium before being pumped into the system circulation by the left ventricle, the blood delivered via the aorta has what?
An even lower partial pressure oxygen than the blood that was delivered to the lungs
How is this deoxygenated blood returned to the placenta?
Via the umbilical arteries
After birth what changes occur in the circulatory system?
The lungs expand with air and rhythmic breathing begins
What else occurs?
Resistance in the pulmonary blood vessels decreases
What happens when umbilical blood flow stops?
The blood pressure in the inferior vena cava decreases
What happens to left atrial pressure?
It increases due to increased blood flow from the lungs
What does increased left atrial pressure coupled with decreased right atrial pressure cause?
It causes the foramen ovale to close
What happens to the ductus arteriosus?
It constricts and later closes permanently
What happens to the ductus venosus?
It degenerates over time, completely closing in most infants by three months after birth
What does the infant begin to produce?
Adult hemoglobin
And what is detectible as a result by the end of the first year of life in the blood?
Little fetal hemoglobin
How long is human pregnancy?
9 months or 266 days
How many trimesters are there?
3 trimesters
During the first weeks, what develops?
The major organs
When does the hart begin to beat?
At 22 days
When do the eyes gonads limbs and liver start to form?
Right after the heart begins to beat
By 5 weeks how long is the embryo?
By 6 weeks the embryo has grown to how long?
When does the cartilaginous skeleton begin to turn into bone?
By the seventh week
By the end of how long have most of the organs formed?
8 weeks
At the end of the third month, the fetus is how long?
9cm long
What is the major thing that happens in the second trimester?
The fetus does a tremendous amount of growing
It begins to move around in what?
The amniotic fluid
Does its face appear human at this point?
What happens to the toes and fingers?
They begin to elongate
By the end of the sixth month, how long is the fetus?
30-36cm long
What are the seventh and eighth months characterized by?
Continued rapid growth and further brain development
During the ninth month what occurs?
Antibodies are transported by highly selective active transport from the mother to the fetus for protection against foreign matter
What happens to the growth rate?
It slows and the fetus becomes less active, as it has less room to move about
How many stages are there in labor?
3 Stages
In the first stage, what occurs?
The cervix thins out and dilates, and the amniotic sac ruptures, releasing its fluids
How are the contractions during this time?
Relatively mild
The second stage is characterized by what?
Rapid contractions, resulting in the birth of the baby, followed by the cutting of the umbilical cord
What happens during the final stage?
The uterus contracts, expelling the placenta and the umbilical cord

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