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43 Cards in this Set

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Describe fertilization and the path of the zygote to the uterus.
An ovum is fertilized by a sperm several hours after ovulation, usually in the ampulla of the uterine tube. As the fertilized cell (the zygote) progresses down the tube toward the uterus, sequential cell divisions occur until there are 16 to 32 daughter cells called blastomeres.
What is a blastomere?
What is a morula?
The 16-32 cells that developed from dividing of the zygote, the cells have not increased in size.

Morula is the name for this collection of cells because looks like a blackberry/mulberry. Develops around day 4.
What is a blastocyst?
Morula that entered uterus area, and developed a fluid-filled area called blastocyst cavity. Develops around day 5.
What cell populations are in a blastocyst?
The trophoblast is the outer cell layer which gives rise to the placenta eventually and the central blastomeres are the inner cell mass or embroblast which give rise to the embryo eventually.
How is the embryonic pole created?
the embryoblast moves to one side of the blastocyst.
What does the blastocyst lose in the uterine cavity?
Its outer protective layer called the zona pellucia, losing it allows rapid growth of the blastocyst to occur
What happens at the end of the first week with the blastocyst and the uterus?
It attaches to the uterine lining, the endometrial epithelium, usually at its embryonic pole. The trophoblast layer differentiates into two layers, the outer syncytiotrophoblast and the inner cytotrophoblast. They syncytiotrophoblast invades the uterine wall and the cytotrophoblast provides new cells.
What do the cells of the syncytiotrophoblast produce?
Human Chorionic gonadotropin (HCG) which can be detected in the urine and is the basis for a pregnancy test.
What is an ectopic pregnancy?
When the blastocyst implans in abnormal sites besides the uterus lining, usually in the uterine tube, abdominal cavity, or on the ovary.
What happens towards the end of the first week/beginning of the second week?
The blastocyst implants further into the uterine wall and the placenta develops to provide nutrition for the embryo (to be discussed in CTO). The inner cell mass becomes two distinct layers, the epiblast and the hypoblast, which are in contact and assume a disc-like shape with the hypoblast facing the blastocystic cavity. This is now called the bilaminar germ (embryonic) disc.
What is the bilaminar germ (embryonic) disk?
When the inner cell mass (cytotrophoblast) becomes two distinct laters, the epiblast and hypoblast.
What develops after the bilaminar germ disk?
The epiblast develops the amniotic cavity which is filled with amniotic fluid and lined with special cells call amnioblasts. The bilaminar disk now has fluid on both sides.
What is derived from the hypoblast?
The primary yolk sac, the cavity is called the exocoelomic cavity.
Coelom = body cavity.
What do the cells of the primary yolk sac produce?
A layer of loose connective tissue outside the sac, called the extraembryonic mesoderm. It surrounds the yolk sac, amnion, and bilaminar germ disk.
What happens to the extraembryonic mesoderm after it develops?
Vesicles appear and form an extraembryonic coelom which then seperates the extraembryonic mesoderm into two layers.
What are the two layers of the extraembryonic mesoderm which are created by the extraembryonic coelom? What gets renamed once these layers form?
The outer extraembryonic SOMATIC mesoderm and the inner extraembryonic SPLANCHNIC mesoderm

The extraembryonic coelom gets renamed chorionic cavity.

splanchnic means inside, somatic means outside.
What does the growth of the chorionic cavity cause?
It pinches off the primary yolk sac, creating a smaller secondary yolk sac that beaomes the Gastrointestical tract and its derivatives eventually.
What happens at one end of the bilaminar disk involving attachment?
A small circular area of the hypoblast cells become attached to the overlying epiblast. This is the future site of the mouth (the oropharyngeal membrane)on the top and the other end of the side a similar circular are develops, the cloacal membrane, which is the future site of the anus. .
What structures are used in the bilaminar disk to orient and what is the orientation?
The primitive streak defines the caudal aspect of the disc, the epiblast the dorsal aspect. The primitive node is towards the cranial aspect. Both node and streak elongate/move towards the cranial end of the disc and depressions develop creating the goove and pit.
What is gastrulation? How do the layers change and what are they now named?
When the epiblast cells migrate through the groove and pit ventrally, some cells displace the existing hypoblast and create a new hypoblast layer, the embryonic endoderm. Other cells move inbetween the epiblast and hypoblast creating the intraembryonic mesoderm. The cells staying in the epiblast are now called the embryonic ectoderm.
What are the three definative germ layers and what were they derived from?
Ectoderm, mesoderm and endoderm, have now been established, and they all were derived from the epiblast of the bilaminar embryonic disc, which has now become a trilaminar embryonic disc.
Where in the trilaminar disc is there only two layers?
At the oropharyngeal membrane cranial or anterior (future mouth) and the cloacal membrane caudal or posterior (future anus) there remain only two layers, specifically ectoderm and endoderm.
What is the notochord and what does it induce the formation of?
Some cells migrate through the groove cranially in the midline of the mesodermal layer until stopped by the oropharyngeal membrane. These cells form into the notochord, which in turn will induce the overlying ectoderm to thicken and form the neural plate.
What is neurulation?
First, the ectoderm over the notochord thickens and forms the neural plate and is now called the neuroectoderm. Then the plate folds into the neural folds, and then the folds close to form the neural tube from which the central nervous system (CNS) arises.
When does neurulation start and finish? What does it form?
Neurulation commences in the third week and is completed by the end of the fourth week.
The process called neurulation comprises the formation of the neural plate, the folding of that plate into the neural folds, and then the closure of those folds to form the neural tube from which the central nervous system (CNS) arises.
What is successful neurulation dependent upon?
Adequate levels of folate. Through neurulation may occur before a woman realizes she is pregnant.
What does the mesodermal layer differentiate into?
What do these layers give rise to?
Three columns, the paraxial column, intermediate column, and later plate.
What do the columns of the mesoderm give rise to?
The intermediate mesoderm gives rise to the urogenital system, and the lateral plate mesoderm is involved in body folding.The two columns of paraxial mesoderm, one on either side of the notochord and the developing neural tube, differentiate and segment into blocks of tissue called somites.
What is the basic sequence of events between zygote and the start of and a blastocyst losing its protective layer?
Zygote divides, making blastomeres, turns into morula.
Morula enters uterine cavity, changes to blastocyst w/ cavity.
Blastocyst has trophoblast and embryoblast (outer and inner cell masses).
Embryoblast moves to one side, making embryonic pole. Blastocyst loses zona pellucia, so its able to grow rapidly.
What events occur between the loss of the zona pellucia and the formation of the hypoblast?
Blastocyst attaches to endometrial epithelium (uterine lining).
Trophoblast splits into two layers: syncytiotrophoblast and cytotrophoblast.
The synctyiotrophoblast invades the uterine wall and make human chorionic gonadotropin (HCG).
The embryoblast has a new layer of cells form facing the blastocyst called the hypoblast.
What events occur between formation of the hypoblast and
formation of the extraembryonic mesoderm.
The inner cell mass (the embryoblast) becomes two distinct layers, the epiblast and hypoblast.
These are incontact and form the bilaminar disc.
A new cavity forms in the epiblast, the amniotic cavity, which is lined with special epiblast cells called amnioblasts. Bottom cavity now called primary yolk sac (exocoelomic cavity).
The cells of yolk sac produce extraembryonic mesoderm (loose CT) outside sac.
What events occur
formation of the extraembryonic mesoderm and the formation of the cloacal membrane?
Yolk sac cells produce extraembryonic mesoderm (loose CT) outside sac, amnion, disc.
Vesicles appear in this new mesoderm, which come together to form the extraembryonic coelom.
This new cavity breaks up the extraembryonic mesoderm into two layers: the ex.em. somatic mesoderm (applied to trophoblast) and the ex.em. splanchnic mesoderm (applied to yolk sac).
The chorionic cavity(extraembryonic coelom) grows and pinches off part of the primary yolk sac, creating definitive yolk sac that will be GI tract and derivatives eventually.
At one end of bilaminar disc, oropharyngeal membrane forms from attached epiblast and hypoblast cells. The cloacal membrane appears at the other end (anus)
What happens from the development of the cloacal membrane to development of trilaminar disc.
The prim. streak develops from thickened line at caudal end of disk. The primitive node forms more cranially, both move and elongate towards cranial end of disk, depress and become the prim. groove and pit.
gastrulation: epiblast cells migrate, and replace hypoblast with new hypoblast called embryonic endoderm. Other cells create intraembryonic mesoderm, the epiblast cells that stayed are embryonic ectoderm.
Now have trilaminar embryonic disc.
What happens from the start of neurulation to closure of pores?
Some migrating epiblast cells move through pit cranially in the midline of mesodermal layer making notochord.
The ectoderm overlying notochord thickens to make neural plate. The neural plate begins to make neural folds, which close up to form neural tube. Neural crest cells migrate to other places.
At the same time, the mesodermal layer differentiates into 3 regions, the paraxial columns, intermediate columns, and later plates.
Neurulation continues as neural folds approach midline, making neural tube, starting in the middle and proceeding out toward head and tail. Cranial neuropore clases earlier than caudal neuropore.
What does the intermediate mesoderm give rise to?
What do the paraxial columns become?
What is the sclerotome?
What is the dermomyotome?
urogenital sys.

somites (blocks of tissue)

the part of somites that becomes bone and cartilage to form vertebrae and ribs.

Part of somite that gives rise to skeletal muscle and connective tissue.
What does spinabifida result from?
Failure of the caudal neuropore to close properly.
What happens once/as neural folds are closing until ?
Trilaminar disk begins to fold laterally and longitudinally, creating a cylindrical body form. The 2' yolk sac gets pinched in from all sides and gets smaller, creating the umbilical area. Lateral folding, due to amniotic cavity growth, creates gut tube that runs from mouth to anus and internal body cavities.
how do the internal body cavities form in the embryo?
The vesicles in teh lateral plate mesoderm converge, dividing it into two layers, one lining the intide of the future body wall (parietal/somatic layer) and one the outside of the yolk sac (splanchnic/visceral layer). The meeting of the amniotic cavity ends creates the internal body cavity called the intraembryonic coelom.
What will the intraembryonic coelom eventually become?
pleural, pericardial, and peritoneal cavities.
What derives from the ectoderm?
CNS, epidermis, hair, nails, mammary glands, pituitary gland, subcutaneous glands, teeth enamel.
What derives from the mesoderm?
Connective tissue, cartilage, bone, striated and smooth muscle, heart, blood, lymphatic vessels, kidney, ovaries, testes, genital ducts, serous membranes (pericardial, pleural, peritoneal), spleen and adrenal cortex.
What derives from the endoderm?
Epithelial lining of GI and respiratory tracts, parenchyma of thyroid and parathyroid, tonsils, liver, pancreas, thymus, epithelial lining of urinary bladder, most of urethra, epi lining of tympanic cavity, tympanic antrum and auditory tube
What derives from neural crest cells (which were from neuroectoderm)?
Spinal, cranial, and autonomic ganglia, schwann cells of PNS, pigment of epidermis, adrenal medulla, arachnoid and pia maters, etc.