Lect. 1: Fertilization, Cleavage, Gastrulation Flashcards
What stage of division is a primary oocyte arrested in?
Prophase of Meiosis I
When does a primary oocyte complete Meiosis I?
shortly before ovulation
Are primary oocytes “n” or “2n”?
2n
What stage of division is a secondary oocyte arrested in? When does it complete this division?
a secondary oocyte is arrested in metaphase of Meiosis II until fertilization
Explain the stages of spermatogenesis.
Spermatogonia perform mitosis and divide into primary spermatocytes. These are stuck in a prolonged prophase of meiosis I until dividing into secondary spermatocytes. After completing meiosis II, the secondary spermatocytes form HAPLOID spermatids.
List the major steps of fertilization.
1) Penetration of the corona radiata
2) Penetration of the Zone Pellucida
3) fusion of the oocyte and sperm membrane
4) Sperm head and tail enter the oocyte
What does the entrance of sperm into the oocyte stimulate?
1) Zona reaction
2) Cortical reaction
3) resumption of second meiotic division
4) Metabolic activation of the egg
What is capacitation? How long does this take?
a period of conditioning the female reproductive tract that lasts around 7 hours. It involves the removal of a protein coat from the surface of the head of the spermatozoa and is REQUIRED for fertilization to occur
What must occur for a sperm to be able to pass through the corona cells and undergo the acrosome reaction?
capacitation
What is the acrosome reaction?
A reaction that occurs at the moment that the sperm binds to the zona pellucida. It is induced by zona proteins and results in spermatozoan release of enzymes required to penetrate the zona pellucida to begin the fertilization process
What occurs if more than 1 spermatozoa enters the zona pellucida?
polyspermy
When the sperm enters the oocyte, what is “left behind”?
sperm mitochondria does NOT enter the oocyte
What is the cortical reaction?
the contact of the sperm and oocyte results in the release of lysosomal enzymes from cortical granules that line the plasma membrane of the oocyte. This results in the oocyte membrane becoming impenetrable to other spermatozoa
What is the zona reaction?
The enzymes from the cortical reaction alter the properties of the ZP to prevent sperm penetration.
What are the consequences of fertilization?
1) completion of Meiosis II (for oocytes)
2) restoration of the diploid chromosome number
3) determination of sex
4) initiation of cleavage (first mitotic division of embryo)
What does the second meiotic division of the oocyte result in?
- One daughter cell receives barely any cytoplasm= polar body
- The other daughter cell is the larger, definitive oocyte
What does the spermatozoon become as it lies close to the female pronucleus?
male pronucleus
Are the male and female pronuclei haploid or diploid?
haploid
What must occur in the male and female pronuclei before the first mitotic division?
they must replicate their DNA so that when the cell divides, the zygote has the normal amount of DNA
What are the cells of zygote called after the first mitotic division?
two blastomeres are present in the two-cell stage
When is the 2-cell stage achieved?
around 30 hours after fertilization
When is the 4-cell stage achieved?
around 40 hours after fertilization
What is different in blastomeres in the 8-cell stage?
they undergo compaction where a compact ball of cells forms (where the inner cells are held together by tight junctions)
What is the embryo at 16 cell phase called?
the morula
When is the morula typically formed?
around 3-4 days
What surrounds the early morula that disappears after the 4th day?
zona pellucida
What are the two portions of the morula?
1) inner cell mass
2) outer cell mass
What does the inner cell mass of the morula give rise to?
embryo proper
What does the outer cell mass of the morula give rise to?
trophoblast (which later contributes to the placenta)
At what stage does the developing embryo reach the uterus?
in the morula stage (WITH the zona pellucida still in tact)
What occurs once the morula enters the uterine cavity?
fluid begins to penetrate through the ZP into the intercellular spaces of the inner cell mass leading to the formation of a blastocele
What is a blastocyst?
the name for an embryo after the formation of a blastocele (cavity formed by fluid penetration)
What are the inner and outer cell masses called in a blastocyst?
inner: embryoblast
outer: trophoblast
What is the trophoblast?
outer cells of the blastocyst that flatten and form the epithelial wall of the blastocyst
After the ZP degrades, what can occur?
implantation on the endometrium of the uterus
What cells are responsible for penetration of the uterine mucosa? When does this occur?
around the 6th day of development, trophoblast cells at the embryonic pole of the blastocyst begin to invade the uterine mucosa
When is the embryo “well adhered” to the uterus?
around day 7.5 after fertilization
After invasion of the uterine mucosa, what are the two regions of the trophoblast?
1) cytotrophoblast
2) syncytiotrophoblast
What is the cytotrophoblast?
the inner layer of mononucleated cells of the trophoblast after adhering to the uterus
What is the syncytiotrophoblast?
the outer multinucleated zone of the trophoblast after adherence to the uterus (that does not have clear cell boundaries)
Which layer of the developing embryo eventually fuses with uterine glands and blood vessels?
syncytiotrophoblast
What is a bilaminar dsc?
the two layer stage of the embryoblast
What are the two layers of the bilaminar disc?
1) hypoblast
2) epiblast
What is the hypoblast?
layer of small cuboidal cells adjacent to the blastocyst cavity
What is the epiblast?
layer of high columnar cells adjacent to the amniotic cavity
What also begins to appear within the epiblast at this time?
the amniotic cavity
Epiblast cells adjacent to the cytotrophoblast are called what? What do these cells do?
amnioblasts (they line the amniotic cavity
What are the glands of the uterus filled with at this time?
glycogen
Why is implantation so important during week two?
ovulation occurs at day 14 of a 28 day cycle, 7.5 days later is day 22 of the cycle (and the corpus luteum begins to degrade at day 24). If the CL degrades, estrogen and progesterone levels fall and the menstrual cycle will occur–sloughing off the stratum functionalis so that the embryo cannot implant
What prevents the degradation of the CL?
human chorionic gonadotrophin (hCG) secretions from the syncytiotrohpoblast signals the CL to continue secreting progesterone and estrogen
Why is hCG so useful clinically?
it is the major hormone that is used to detect pregnancy in home pregnancy tests. Pregnancy (via hCG) can usually be detected around 11 days after fertilization
What is EPF? Why is it so important?
EPF (early pregnancy factor) is released from the ovary soon after fertilization and suppresses a possible maternal immune response to the fetus
Where is the most common ectopic abdominal site of implantation?
rectouterine pouch
What are the fused vacuoles that form in the syncytiotrophoblast called?
trophoblastic lacunae
What is Heuser’s membrane?
The exocoelomic membrane formed by flattened cells that line the inner surface of the cytotrophoblast. (An extension of the hypoblast)
What helps to form the lining of the primitive yolk sac?
Heuser’s membrane and hypoblast
During what day of development is the blastocyst completely embedded in the endometrial stroma?
days 11 to 12
What must occur for maternal blood to enter the lacunar system?
trophoblastic lacunae must fuse with maternal glands and sinusoids to set up a “serviceable supply” of nutrition and oxygen
Around day 12, other than establishment of the uteroplacental circulation, what occurs?
extraembryonic mesoderm cells proliferate and fill the spaces between the Heuser’s membrane and the cytotrophoblast
Where do extraembryonic mesoderm cells come from? Waht is their action?
these cells are derived from yolk sac cells and form a fine, loose connective tissue that eventually fills all of the space between the trophoblast (externally) and the amnion (internally)
What begins to form within the extraembryonic mesoderm?
extraembryonic cavities
What is the chorionic cavity?
the fusion of the extraembryonic cavities in the extraembryonic mesoderm forms this new space that surrounds the primitive yolk sac and the amniotic cavity
Does the chorionic cavity completely surround the yolk sac and amniotic cavity?
NO, it does NOT surround the yolk sac and amniotic cavity where the germ disc is connected to the trophoblast by the connecting stalk
What is the somatic extraembryonic mesoderm?
the layer of the extraembryonic mesoderm that:
1) lines the cytotrophoblast
2) lines the amnion
3) forms the connecting stalk
* also called the chorionic plate!
What is the extraembryonic splanchnic mesoderm?
portion of the extraembryonic mesoderm that covers the secondary yolk sac
Is the secondary yolk sac larger or smaller than the primitive yolk sac?
much smaller
What are the “pinched off” portions of the primitive yolk sac called that are found in the chorionic cavity?
exocoelomic cysts
What does the connecting stalk form?
umbilical cord
When does gastrulation occur?
week 3 after fertilization
What is gastrulation?
Gastrulation is the formation of a primitive streak and node and the migration of epiblast cells through the primitive streak to form the 3 primary germ layers (ectoderm, mesoderm and endoderm) from the bilaminar disk
What is the primitive streak?
dorsal, midline streak (cranial to caudal) with a narrow groove of the epiblast with bulging regions on each side, caused by movement of epiblast cells into the region
What is the buccopharyngeal (oropharyngeal membrane) membrane?
where epiblast and hypoblast are NOT separated by developing 3rd germ layer (developmental site of future mouth)
What is the primitive node? Toward what end of the embryo is it located?
Primitive node is a slightly elevated area surrounding the small primitive pit located at the cephalic end of the primitive streak
How do epiblast cells enter the space between the epiblast and hypoblast? What do they form?
surface epiblast cells move through primitive streak and down into primitive pit to fill in space between epiblast and hypoblast (MESODERM), go cranial to the buccopharyngeal membrane but do not intervene between the two layers in this area.
Describe the formation of the notochord? When does this occur?
Some cells in epiblast make rod of cells that move forward and attach to the buccopharyngeal membrane and stop (these form the notochord). The floor (endoderm) under the cells degenerates so the notochord forms the notochordal plate. This occurs at mid-3rd week.
Is endoderm formed from the hypoblast?
NO! Endoderm: cells leave the epiblast via the primitive pit and invade the hypoblast layer and displace the hypoblast cells.
Where is the notochord?
intraembryonic mesoderm has notochord running right in the center
What is the neurenteric canal?
for a time there is a communication between the amnion and the secondary yolk sac where the notochord plate has not yet pinched off to become the notochord
Describe the changes of the notochord plate?
from cranially to caudally, the notochord plate will detach from the endodermal cells to which it is originally fused and form a chord that lies in the center of the intraembryonic mesoderm between the ectoderm and endoderm
What is a sacrococcygeal teratoma?
remnant of the primitive streak causes tumors that are masses of cells that become anything (70% of these become tumors)
What 7 structures come from the general ectoderm?
1) epidermis of skin
2) enamel of teeth
3) inner ear
4) adenohypophysis
5) parotid gland
6) mammary gland
7) lower anal canal
What 6 structures come from the neural tube?
1) CNS (brain)
2) all neurons w/cell bodies in CNS
3) Retina and cranial nerve II
4) Neurohypophysis
5) Astrocytes
6) Oligodendrocytes
What 8 structures come from neural crest cells?
1) Melanocytes
2) Cartilages/bone of head
3) C cells of thyroid gland
4) aurticopulmonary septum
5) endocardial cushions
6) adrenal medulla
7) Sensory/motor ganglia
8) Schwann cells
What 9 structures come from mesoderm?
1) Muscle tissue
2) connective tissue
3) bone
4) Cardiovascular system
5) lymphatic system
6) Spleen
7) Kidney
8) Gonads
9) Reproductive Ducts
What are the major groups of structures that come from the endoderm?
1) lining epithelium (GI, respiratory, bladder)
2) Pharyngeal pouches
3) Parenchyma of liver, pancreas, etc.
4) thyroid gland
When does organogenesis occur?
last 3 and a half days of 3rd week until 8th week
