Embryology 1 - Fertilization to Gastrulation (Week 8) Flashcards
True or False: We start out as one totipotent cell
True
An organism that breathes no air, does not use the digestive tract, and lives in a sterile environment is referred to as what?
a fetus
An air-breathing organism with a functional GI tract that quickly becomes colonized by microbial flora is referred to as what?
a newborn
Note: we undergo these massive changes within minutes after birth
True or False: Learning embryology helps us to understand developmental/congenital disorders (e.g., DiGeorge Syndrome, ectopic pregnancy, neural tube defects, etc.), infertility, and referred pain
True
Where are oocytes (female haploid gametes) produce?
the ovaries
Production of which hormones occurs in the ovaries?
progesterone and estrogens
Which structure receives the oocyte from the ovaries?
the uterine tube/fallopian tube
Where does the sperm fertilize the oocyte?
the uterine tube/fallopian tube
Where do the embryo, placenta, and membranes develop?
the uterus
The site where the maternal and embryonic vasculature exchange substances
the placenta
A __________ (haploid/diploid) germ cell undergoes meiosis to produce a unique __________ (haploid/diploid) gamete
diploid, haploid
How do haploid gametes become “unique”?
crossing over (mixing of maternal and paternal chromatids during prophase I to end up with “mixed” chromatids)
True or False: Meiosis is completed before the sperm penetrates the oocyte
False.
Meiosis is not completed in an oocyte UNTIL the sperm penetrates it
We can refer to the fusion of the spermatic pronucleus and the oocyte pronucleus as?
fertilization
A single, fertilized, diploid cell (combination of spermatic and oocyte genetic material = diploid cell) is known as?
a zygote
Note: a zygote has NOT divided yet
Females are born with a certain number of diploid oocytes that have been “paused” at what stage?
first stage of meiosis
Note: meiosis does not resume until AFTER puberty
After puberty, the ovaries release an _______ each cycle into the uterine tubes
ovum
The oocyte will not complete meiosis II unless ____________ occurs
fertilization
In what part of the uterine tube specifically does fertilization usually occur?
the ampulla
Before ovulation, we call the oocyte what?
primary oocyte
After ovulation, we call the oocyte what?
secondary oocyte
The release of a secondary oocyte from the ovarian follicle is known as?
ovulation
The ovulated secondary oocyte + the zona pellucida is covered externally with granulosa cells known as what?
cumulus oophorus
When the cumulus oophorus rearranges, it forms the what?
corona radiata
A successful sperm must penetrate the _____________ and inject its genetic material into the oocyte
zona pellucida
True or False: Zona pellucida becomes impenetrable after first sperm penetrates it
True
Note: this prevents “double fertilization”
How long after fertilization does the oocyte complete meiosis II + zygote completes first cell division?
24 hours
A morula consists of how many cells?
12-20 cells
A ___________ is what implants into the uterine wall
Blastocyst
(formed around day 5)
When sperm comes into contact with the zona pellucida, it triggers an acrosome reaction, whereby the sperm secretes ___________________, that will break down the glycoprotein membrane of the zona pellucida
digestive enzymes
Note: only 1 sperm will succeed
a haploid germ cell is also known as
a gamete
(oocyte = gamete from ovaries; sperm = gamete from testes)
the protein coat that surrounds an oocyte and eventually the early embryo
zona pellucida
a multicellular organism, prior to the fetal stage (~ from fertilization to end of week 7) is known as?
an embryo
a multicellular organism, from the end of the embryonic stage (week 8) to birth is also known as?
a fetus
___________ is another name for newborn
neonate
cells formed during development that do not become part of the neonatal organism (but contribute to the fetal membranes)
extraembryonic cells
note: eventually contributes to the placenta
16 cell stage of an embryo
morula
note: no blastocoel in this stage
a spherical mass of cells that is composed of a trophoblast that surrounds a fluid cavity (blastocoel) and an inner cell mass (embryoblast)
blastocyst
What do we call the cell division in the early embryo, whereby each division does not increase the size of the embryo but instead results in smaller and smaller cells?
cleavage
a cell that is totipotent and is present during very early development (e.g., first week) is called?
blastomere (every single cell that makes the embryo in the first week)
a cell that can become any cell
totipotent cell
when the embryo contacts and becomes surrounded by the endometrium of the uterus
implantation
Days 1-3 of development
- embryo develops from zygote to a sphere-shaped cluster of cells surrounded by the zona pellucida
- 12-32 cell stage (morula)
- cell divisions known as cleavage
- cells known as blastomeres (which are all genetically identical = embryonic stem cells)
Days 4-5 of development
- a fluid-filled cavity develops within the embryo
- four structures can be noted:
1) trophoblast (layer of cells on the outside of the sphere, covered by the 2) zona pellucida); many of these cells go on to develop into membranes of placenta
3) embryoblast (inner cell mass); surrounded by the trophoblast; these cells develop into the embryo
4) blastocoel (the fluid-filled cavity within the sphere)
The epithelial cells of the uterine tube are equipped with _________ that “wave” in a single direction
cilia
Note: ciliary movement increases as progesterone levels increase (secretion peaks shortly after ovulation)
Where does the blastocyst arrive after about 5 days?
the superior aspect of the uterus (the fundus)
What are some of the roles of the zona pellucida during early development?
- barrier that ensures only one sperm fertilizes an oocyte
- porous (allows communication between the embryo and the maternal reproductive structures
- protects the embryo from immunologic defenses
- acts as a signal to help with differentiation of trophoblast cells
- prevents premature implantation of the embryo
- prevents the blastomeres from dissociating (helps them stay together)
When does the embryo “hatch” out of the zona pellucida?
day 6
Note: the zona pellucida prevents implantation of the embryo, but once it hatches, it is ready to come into contact with the endometrium and implant itself
What is a complication that can happen when the embryo implants too early?
ectopic pregnancy
What part of the endometrium binds to the trophoblast of the blastocyst?
small apical processes known as pinopods
Note: adhesion is mediated by selectin (which binds to mucins expressed on the endometrial epithelium) and integrin binding
.. results in blastocyst invading into the endometrium
Once the trophoblast invades the endometrial epithelium, it forms two layers: _____________ and ________________
This occurs during ~ day 5-6
cytotrophoblast (inner layer),
syncytiotrophoblast (outer layer)
The syncytiotrophoblast develops into a ___________ (mononuclear/multinuclear) cell mass where the borders between cells are indistinct (looks more like a blob)
multinuclear
What does the syncytiotrophoblast secrete?
human chorionic gonadotropin (hCG)
Note: hCG prevents the shedding of the endometrium (and loss of the embryo) by maintaining ovarian secretion of steroid hormones (e.g., progesterone)
Note: hCG is the hormone detected in pregnancy tests
As the syncytiotrophoblast invades the stroma (just below the epithelium), the _______________ (aka embryoblast) differentiates into two distinct layers: ____________ and ______________
This happens @ ~ day 6-7 (end of week 1)
inner cell mass,
epiblast (becomes the actual embryo/”embryo proper”),
hypoblast (aka “primary endoderm”; lines the blastocystic cavity and forms the primary yolk sac)
a fluid-filled cavity
(week 2 & 3)
coelom
the process of forming three embryonic germ layers
gastrulation
What are the 3 embryonic germ layers?
1) ectoderm (exterior)
2) endoderm (interior)
3) mesoderm (between ectoderm and endoderm)
True or False: the endoderm and mesoderm can also be extra-embryonic. The formation of extra-embryonic endoderm and mesoderm is NOT classified as gastrulation.
True
towards the head region
cephalad
towards the “tail” region
caudad
When is implantation complete (approximately)?
day 10
umbilical vesicle is synonymous with what?
yolk sac
Stromal cells undergo decidualization and become known as decidual cells.
Decidual cells accumulate _________ and __________ throughout the uterus
glycogen, lipids
Any time the syncytiotrophoblast contacts a decidual cell, the decidual cell is programmed to undergo ___________
apoptosis
Note: this releases the stored nutrients (glucose and lipids) so that the embryo has nutrients to grow
The epiblast enlarges and gives rise to _______________
amnioblasts (cells that surround the developing amniotic cavity)
Fluid accumulating between the extraembryonic mesodermal cells to form another cavity is known as the what?
extraembryonic coelom
three fluid-filled cavities that develop in the embryo:
1) umbilical vesicle (yolk sac)
2) amniotic cavity
3) extraembryonic coelom (develops into the chorionic cavity)
The connecting stalk eventually develops into what?
the umbilical cord
What are the two layers of the extraembryonic mesoderm?
1) extraembryonic splanchnic mesoderm (surrounds the umbilical vesicle)
2) extraembryonic somatic mesoderm (just underneath the cytotrophoblast, inner lining of the chorionic sac)
What is the chorion/wall of the chorionic sac comprised of?
- extraembryonic somatic mesoderm
- trophoblast (syncytiotrophoblast + cytotrophoblast)
Note: the chorionic SAC is everything inside of this
The chorionic sac encloses the ___________ and its cavities
embryo
The extraembryonic coelom becomes the ____________ by day 14
chorionic cavity
When syncytiotrophoblasts contact endometrial blood vessels, the blood vessel deteriorates and blood pools
These “little lakes” of blood are known as what?
lacunar networks
Note: these allow for more exchange of nutrients
How do nutrients from the oxygenated maternal blood and deteriorating decidual cells get to the embryo?
simple diffusion
Note: there is no circulation yet
thickened area of columnar cells, found in the cephalad region of the hypoblast, that acts as an embryonic organizing centre and is responsible for the induction of other structures
prechordal plate
Note: this forms by the end of week 2 (day 14)
True or False: the hypoblast is an organizer of the caudal aspect of the embryo
False.
The hypoblast organizes the head and mouth region and helps to induce the formation of structures found at the cephalad pole of the embryo
the process by which the three germ layers of the embryo are established
gastrulation
During gastrulation, the bilaminar embryonic disc becomes the _____________
trilaminar embryonic disc
During gastrulation, the embryo may be referred to as the _________
gastrula
thickened linear band in the median plane of the dorsal aspect of the embryonic disc that forms at the beginning of the 3rd week
primitive streak
True or False: The primitive streak initiates the caudal region of the epiblast
True
Cells at the cephalad end of the primitive streak proliferate to form what?
primitive node
A narrow groove that develops in the primitive streak
primitive groove
Note: primitive groove is continuous with a small depression in the primitive node, known as the primitive pit
As the cells leave the deep surface of the streak, they form what?
the mesenchyme
What is mesenchyme?
embryonic connective tissue which forms the supporting tissues of the embryo (i.e., embryonic connective tissue)
Note: mesenchyme –> forms mesoblastic cells –> which form intraembryonic or embryonic mesoderm
When the hypoblast is displaced by cells from the epiblast, it forms what?
embryonic endoderm
Cells remaining in the epiblast form what?
embryonic ectoderm
___________ cells derived from the primitive streak migrate widely and differentiate into diverse types of cells (e.g., fibroblasts, chondroblasts, osteoblasts, etc.)
Mesenchymal
True or False: Eventually, the primitive streak diminishes in size and becomes an insignificant structure in the sacrococcygeal region, and disappears by the 4th week
True
