Embryology Flashcards
Where does fertilization occur?
Ampulla of uterine tube
What results from fertilization?
Zygote that begins a series of cleavages to form blastomeres
A zygote is a fertilized egg. What is it called once the zygote reaches 16 cells?
Morula
What is the difference from the morula stage to the blastocyst phase to the partially embedded blastocyst stage?
The morula looses the zona pellucida becoming a blastocyst and then the blastocyst partially implants into the endometrium becoming a partially embedded blastocyst
At this point of partial implantation, cells move towards the side of the uterine wall and a fluid filled cyst cavity appears on the bottom side
What happens in days 1-5 following fertilization
Zygote is encased by zona pellucida. Cells increase in number but not in size. Once there are 16 cells, it is called a morula.
Morula: blastomeres (cells resulting from cleavage of a fertilized egg) at 16 cell stage that form a compact ball of cells
How are cells organized in a morula?
inner cell mass: forms embryo and contributes components to fetal membranes (i.e., amniotic membrane)
outer cell mass: contributes to fetal membranes (fetal component of placenta and umbilical cord)
What are the roles of the zona pellucida? What happens if it is lost too early?
The zona pellucida:
1. Serves as a barrier to prevent multiple fertilizations
2. Acts as a filter to allow uterine secretions to reach embryo
3. Serves as immunological barrier between mother and embryo
4. Prevents the blastomeres from dissociating from each other
5. Prevents premature implantation of embryo until it reaches uterine wall (would be nonviable and dangerous to mother)
What happens in days 6-7 (blastocyst stage)
Blastocyst: cells of the morula begin to secrete a fluid forcing the inner cell mass to one side (embryonic pole). Results in formation of blastocyst cavity. Embryo will begin to implant into uterine wall.
Embryoblast: (inner cell mass) at one end of the blastocyst (embryonic pole); becomes embryo proper. This end of the blastocyst is typically first to enter the uterine wall.
Day 6-7 (blastocyst stage): what happens to outer cell mass?
Trophoblast (outer cell mass) begins to line blastocyst cavity and contributes to the placenta.
Trophoblast differentiates further into:
cytotrophoblast which is the cellular inner layer lining blastocyst cavity
syncytiotrophoblast which is a mass of multnucleated cells that invades the uterine endometrium to allow to blastocyst to implant into the uterus and secretes hCG (human chorionic gonadotrophin)
What happens to the embryo blast on day 8?
Embryoblast differentiates into bilaminar disc comprised of
epiblast: cells adjacent to embryonic pole of blastocyst
hypoblast: cells adjacent to blastocyst cavity
Additionally, moving further into uterine wall. Left will become head and right will be come tail in this “lateral view.”
What happens in day 8 as the epiblast and hypoblast continue to divide?
Amniotic cavity develops from migration of epiblast cells.
What happens in day 8 as the epiblast and hypoblast continue to divide?
Amniotic cavity develops from migration of epiblast cells.
Day 8: what is the amnion?
Amnion a membrane that develops of cells lining the amniotic cavity
Day 8: what is the amnion?
Amnion a membrane that develops of cells lining the amniotic cavity
Day 8: what is happening to hypoblasts?
They are also dividing at the same time. They begin to migrate, although they remain in contact with the original hypoblast layer. A new cavity is beginning to form within the new blastocyst cavity. This will become the yolk sac.
What happens on days 9 and 10?
The blastocyst continues to invade deeper into endometrium.
Hypoblast layer continues to divide and its cells migrate along the inner surface of the cytotrophoblast resulting in loss of much of the blastocyst cavity.
Exocoelomic membrane: formed from migrating cells of the hypoblast; surrounds the exocoelomic cavity (primitive yolk sac)
What happens on days 9 and 10 with the syncytiotrophoblast and the blood filled lacunae?
Syncytiotrophoblast continues to expand, covering more of the blastocyst as it penetrates deeper into the endometrium of the uterus.
Blood filled lacunae (spaces) are appearing in the embryonic pole of the syncytiotrophoblast and will begin to connect to intact uterine vessels, forming the primitive uteroplacental circulation.
What happens in day 11?
Embryo fully embedded.
Extraembryonic mesoderm: second wave of cells (derived from exocoelomic membrane) migrate between exocoelomic membrane and cytotrophoblast layer
Extraembryonic mesoderm totally separates embryo proper with its amniotic cavity and primitive yolk sac from the overlying cytotrophoblast.
Within a few days of forming, the majority of the extraembryonic mesoderm begins to ___ forming ____ _____ (light blue areas) within it.
Die; isolated spaces
What do the isolated spaces from extraembryonic mesoderm cell death turn into?
The chorionic cavity: one large space surrounding embryo proper, amnion, and primitive yolk sac
Connecting stalk: black line that connects chorion
What will connecting stalk eventually participate in?
Formation of umbilical cord
What is the chorion?
Blue/purple space - extraembryonic mesoderm and cytotrophoblast. The chorion becomes the fetal component of the placenta.
What happens with the exocoelomic membrane?
It will begin to produce a second wave of cells which begin to wall off primitive yolk sac into two areas, forming the secondary or definitive yolk sac
Portions of the primitive yolk sac may remain as small cyst.
What is the buccopharyngeal membrane? What is the cloacal membrane?
Hypoblast and epiblast layer near head end that will turn into mouth.
Hypoblast and epiblast layer at caudal end of embryo that fuse to form anus.
Additionally, cytotrophoblast cells begin to form columns of cells called primary villi (blue cells) which will invade syncytiotrophoblast
What is the buccopharyngeal membrane? What is the cloacal membrane?
Hypoblast and epiblast layer near head end that will turn into mouth.
Hypoblast and epiblast layer at caudal end of embryo that fuse to form anus.
Additionally, cytotrophoblast cells begin to form columns of cells called primary villi (blue cells) which will invade syncytiotrophoblast
2nd week of development: week of twos
- Trophoblast (2 layers)
- Embyroblast (2 layers)
- Extraembryonic mesoderm
- Two cavities develop
- Cytotrophoblast and syncytiotrophoblast
- Epiblast and hypoblast
- Surrounds cavities of embryo and deep to the cytotrophoblast
- Amniotic cavity and yolk sac
What secretes hCG?
syncytiotrophoblast which is a mass of multnucleated cells that invades the uterine endometrium to allow to blastocyst to implant into the uterus and secretes hCG (human chorionic gonadotrophin)
How is the zona pellusida destroyed?
Via trypsin-like secretions from the trophoblast in week 1
When does blastocyst form
Week 1
What is the blastocyst made of?
- embryoblast (inner cell mass)
- trophoblast (outer cell mass)
What does the trophoblast (outer cell mass) which lines the blastocyst further differentiate into?
Cytotrophoblast: inner lining of blastocyst
Syncytiotrophoblast: mass of multinucleated cells that allow blastocyst to implant into uterine wall
What are the two layers of an embryoblast (original inner cell mass)? (Week 2)
Epiblast cells which form amniotic sac
Hypoblast cells which form the yolk sac
What do the hypo blast cells begin to form in week 2?
Exocoelomic membrane; THEN the extraembryonic membrane is formed from this and cytotrophoblast to separate embryo proper from cytotrophoblast
What day/week is the embryo completely embedded in the uterine wall?
Day 11 of week 2
Describe what chorionic cavity is
Small space from dead cells surrounding embryo proper (amnion and primitive yolk sac
Describe connecting stalk and what it will become
Portion of extraembryonic mesoderm that connects embryo to chorion; will help with formation of umbilical cord
Describe chorion
Extraembryonic mesoderm and cytotrophoblast
What will cytotrophoblast cells become?
Primary chorionic villi
What will buccopharyngel membrane become? Where is it in lateral view?
Mouth; to the left
What will cloacal membrane become?
Anus; to the right
What cells migrate to create secondary/definitive yolk sac?
Wave of cells from hypoblast (primitive pop off into cyst-like area)
Week 1 synopsis
Morula —> blastocyst
Week 2 synopsis
bilaminar disc —> amniotic & yolk sac
Generally, what happens in week 3?
gastrulation (3 layers)
Generally, what happens in week 4?
Neurulation and folding
What happens in gastrulation?
Epiblast cells begin to move inwards through the invagination created by primitive streak towards the hypoblast layer underneath it
Generally, what does the mesoderm become?
Skeletal, smooth, and cardiac muscle; most cartilage and bone; dermis of skin; portion of kidneys and gonads
Generally, what does the endoderm become?
Linings of digestive, respiratory, and urogenital systems; pharyngeal pouches of head and neck regions
Generally, what does the ectoderm become?
Nervous system and epidermis
What is neurlation?
Is the process resulting in the formation of the primitive nervous system
What is the purpose of the notochord? Where is it derived from?
It is derived from the mesoderm; cells that migrate rostrally from the primitive node into the prechordal plate (towards buccopharyngeal membrane)
Forms the basis of the vertebral column (not actual column itself; remnants found in nucleus pulposis) AND induces formation of CNS from the ectoderm
How many layers of membrane will rupture to form mouth and anus?
Two; layers of three will not rupture
What promotes the ectoderm to proliferate and thicken into the neural plate?
Notocord
What forms along the midline of the neural plate? Where are its remnants found?
Neural grove; nucleus pulposis
What happens to neural crest cells?
Form from neuroectoderm cells at the crest of the neural folds by day 21 (week 3)
Where do the neural crest cells go?
They dissociate from the ectoderm and disseminate throughout the embryo and eventually become:
Spinal, autonomic, and cranial nerve ganglia
Schwann cells
Medulla of the adrenal gland
Spinal meninges
Pigment cells of the skin
Glial cells
C-cells of the thyroid
Conotruncal region of the heart
Connective tissue, some bones, dermis of head/neck
What is a neuropore? Where are they located?
Neuropores are present at anterior and posterior at the ends of the neural tube and remain open until day 25-26
Describe what would happen if the anterior neuropore failed to close
Exencephaly; leads to anencephaly
What is a mild case of the posterior neuropore failing to close?
Spinal Bifida Occulta: absence of portion of vertebral arch; covered by skin in lumbosacral region and commonly marked by patch of hair (no paralysis; does not result in death)
Describe a serious case of spinal bifida
Spina bifida cystic; meninges and or spinal cord are protruding
Spinal cord protrusion can lead to paralysis (meningomyelocele)
Meninges only = no paralysis (meningocele)
What is cranioschisis?
failure of anterior neuropore and it’s immediately caudal somites to close leading to exposure of brain and ultimately death
What 3 levels does the mesoderm specialize into?
- paraxial (most medial)
- Intermediate
- Lateral plate
What are the longitudinal, paired columns of the paraxial called? Where do they end up at?
Somites, 44 pairs
end up on either side of the notochord/neural tube region
Describe the intermediate mesoderm
Least extensively developed of mesoderm region; differentiates into portions of the urogenital system
Describe the lateral (plate) mesoderm
Most lateral mass of mesoderm and quickly splits to form two distinct but continuous layers: somatic (parietal) and splanchnic (visceral)
Describe the lateral (plate) mesoderm layer somatic (parietal)
This portion is in contact with the ectoderm; body walls/linings
Describe the lateral (plate) mesoderm layer splanchnic (visceral)
This portion is in contact with the underlying endoderm; heart, organs
What will the somatic (parietal) mesoderm contribute to?
Serra’s lining of the body cavities, long bones (appendicular skeleton), and connective tissues of the body wall
What will the splanchnic (visceral) mesoderm contribute to?
Smooth muscle of GI and serous lining on superficial surface of organs
What is the main takeaway from transverse folding?
embryo is folding and putting things where they need to go
Ex: yolk sac become future gut; the “spaces” also known as the intraembryonic coelom becomes the future cavities of the body (heart cavity, etc.)
What happens to the forgut in longitudinal folding?
Becomes lower respiratory system, pharynx, esophagus, stomach and initial part of duodenum
What happens to the midgut in longitudinal folding?
It becomes remainder of the small intestine and much of the large intestine
What happens to the hind gut in longitudinal folding?
It enlarges to become distal part of large intestine, rectum, and anal canal
How many umbilical arteries are there (placenta)?
TWO; they carry nutrients and oxygen-poor blood FROM the fetus TO the mother
How many umbilical veins are there?
ONE; this carries oxygen-rich blood FROM the mother TO the fetus
What is the difference between secondary and tertiary villi with regard to the placenta?
tertiary have capillary networks
Secondary still just protrusions
