Fertilization and Pregnancy Flashcards
How many eggs are released in a typical menstrual cycle
Only 1!
How many eggs do you have at birth? At puberty? In a typical menstrual cycle?
Birth = 2 million
Puberty = 400,000
Typical menstrual cycle = 1
What are the products of gametogenesis and where does it occur
Occurs in the gonads
- sperm and eggs are released by the gonads
- sperm and secondary oocytes
- leads to the release of many sperm and one secondary oocyte
How many sperm are released in gametogenesis
MANY
How many eggs are released in gametogenesis
Just one!
Compare the size of sperm to eggs
Eggs are REALLY BIG in comparison to the miniature sperm cells
What is fertilization
Fusions of two haploid gametes, which produce a zygote
- successful fertilization involves the combined actions of many many sperm in order for one sperm to fuse with the oocyte
When does successful fertilization occur
Within a day of ovulation
- not long after the occyte enters the uterine tube
How long is ejaculated sperm viable in the uterine tube? How long is a secondary oocyte viable after ovulation?
At least some of the ejaculated sperm can reach the uterine tube within a few hours of ejaculation
- can remain viable for 5-7 days in the female reproductive tract
A secondary oocyte remains viable for 24 hours after ovulation
How many sperm actually reach the egg after penetrating the oocyte’s membrane
Many sperm are required to break through the oocyte’s membrane, but only ONE sperm can penetrate the oocyte
Which part of the sperm is used to break down the layers of follicular cells surrounding the occyte
The enzymes in the ACROSOME (tip) of the sperm are used to break down the layers of follicular cells that make up the corona radiata
What is the corona radiata
The outer layer of follicular cells surrounding the oocyte
How does an oocyte get activated during fertilization
When one sperm breaks the oocyte membrane, the oocyte is activated
- must break down the outer layer to penetrate the ovum and fertilize it
What is the zona pellucida
A thick membrane inside of the corona radiata that protects the ovum from having more than one sperm penetrate it
- biochemical reactions in the zona pellucida prevent more than one sperm from entering the ovum by releasing enzymes
Determine if fertilization will or will not occur in the following scenario:
Sperm reach the uterine tube days before ovulation.
FERTILIZATION MIGHT OCCUR
Sperm can remain viable for days, as long as they are in the area where the ovum will end up after ovulation (Fallopian tubes) fertilization might occur
- especially if the sperm is still viable in that window
Determine if fertilization will or will not occur in the following scenario:
Ovulation occurs more than a day before sperm enter the female reproductive tract.
FERTILIZATION MIGHT NOT OCCUR
The ovum is only viable within 24 hours of ovulation, so if sperm enters the female reproductive tract more than a day after ovulation, fertilization will be difficult because the egg is not viable anymore
Determine if fertilization will or will not occur in the following scenario:
The ejaculate contains low numbers of sperm, so too few reach the oocyte.
FERTILIZATION MIGHT NOT OCCUR
Penetration requires many sperm together to break through the ovum, so too few sperm would not be enough to reach the ovum for fertilization
- there is a threshold number of sperm
What happens after a sperm enters the ovum and how does a zygote form
After a sperm has entered the ovum, two haploid pronuclei develop and mingle their chromosomes
- this is what develops a zygote
- as soon as the chromosomes mingle, the first mitotic cell division begins -> around 24 hours after the point of fertilization
Where does fertilization occur
In the Fallopian/uterine tubes of the female reproductive system
Describe step 1 of fertilization:
Oocyte at ovulation
Secondary oocyte is released at ovulation (occurs on day 14 of the uterine cycle)
Describe step 2 of fertilization:
Fertilization and oocyte activation
Sperm penetrates the corona radiata and fertilizes ovum
- oocyte is activated
- zona pellucida releases enzymes to prevent more than one sperm from entering the ovum
Describe step 3 of fertilization:
Pronuclei develop and DNA
Once the sperm has entered the ovum, the nuclei from the sperm and egg develop into two haploid pronuclei and mingle their chromosomes
- a zygote is formed
Describe step 4 of fertilization:
Spindle formation begins
- male and female pronuclei are developing
- spindle formation also begins
Describe step 5 of fertilization:
Amphimixis occurs and cleavage begins
- mitosis begins and metaphase of the first cleavage division occurs
- starts with 46 chromosomes and undergoes mitotic division
- size of the zygote itself does not change, the cells within the zygote just get smaller
Describe step 6 of fertilization:
First cleavage forms two blastomeres
- first cleavage division begins and eventually forms two blastomeres
- the zygote itself is the same size, but the blastomeres are smaller
When does cleavage begin
Begins immediately after the zygote is formed
What is a pre-embryo
During the period of cleavage, the zygote starts a series of repeated mitotic cell divisions, increasing its cell numbers without growing in size
- it is now considered a pre-embryo
What is a(n advanced) morula and how does it differ from blastomeres
A solid ball of cells surrounded by zona pellucida
- blastommeres are just two cells inside the zona pellucida
What is a plastomere
Daughter cells produced during cleavage
What happens on day 1-10 after the zygote is formed
First cleavage division
- cells get smaller but the size of the zygote remains the same
- Two-cell stage
- AKA blastomeres - Four-cell stage
- AKA more blastomeres - Early morula
- solid ball of cells surrounded by zona pellucida - Advanced morula
- solid ball of cells surrounded by zona pellucida - Shedding of zona pellucida and loss of zona pellucida allows for transport of zygote to uterus
6-9. Implantation of zygote into uterine wall
- Yolk sac formation
Where is the morula typically found
Found in the uterine tube
What is a blastocyst
- develops from the morula
- hatches from zona pellucida
- around the time the morula enters the uterus, it sheds the zona pellucida and undergoes cell rearrangements, creating a blastocyst
- hollow ball of cells with a fluid-filled inner cavity and an inner mass of cells that will become the embryo
What is the blastocoele
- blastocyst cavity
- hollow cavity
What is the embryoblast
- inner cell mass of a blastocyst
- will become the embryo
What is implantation
- the process by which the blastocyst fuses with the functional layer of the endometrium
What is the endometrium
- layer lining the lumen of the uterus
- has a functional layer
- functional layer gets shedded during menstrual cycle
What are trophoblasts
Outer cell layer of the blastocyst which later becomes part of the placenta
- drive erosion of the endometrial epithelium, lodging the blastocyst within the endometrium
- becomes implanted
List the parts of a blastocyst
BLASTOCYST
- blastocoele (cavity)
- trophoblast
- inner cell mass (embryoblast)
- shedded zona pellucida
- trophoblast
How does implantation work
- multi-day process
- involves changes to both the trophoblast and inner cell mass
- trophoblast cells divide rapidly and separate into two layers which surround the inner cell mass
The trophoblast cells divide rapidly and separate into two layers which surround the inner cell mass,
What are the two layers
Cytrotrophoblast
Syncytiotrophoblast
What are cytotrophoblasts
Layer that has individual cells with plasma membranes
What are syncytiotrophoblasts
Many nuclei sharing one cytoplasm
The inner cell mass forms two layers of cells sandwiched between two new cavities,
What are the two new cavities
Amnion
Yolk sac
- these two layers of cells are surrounded by the cytotrophoblast and syncytiotrophoblast
What is the amnion
- a membrane forming a fluid-filled cavity (the amniotic sac) that encloses the embryo
What is the yolk sac
- supplies nutrients to developing embryo
What happens if implantation occurs in an area other than the uterus = ECTOPIC PREGNANCY
AKA Why must implantation occur in the uterus
- pregnancy cannot be carried out full term
- myometrium / smooth muscles / uterus is designed to support a growing fetus
- can expand, vascularized, has nutrients, blood supply, oxygen, ability to remove waste
- same structural support is not available in the
Where does fertilization occur
Where does implantation occur
Fertilization = fallopian tubes
Implantation = uterus
What is gastrulation
Creation of three distinct germ layers for the developing embryo following implantation
- during gastrulation, cells from one layer of the inner cell mass migrate into the space between the two cell layers
Order the following events correctly:
- Gastrulation
- Fertilization
- Implantation
- Fertilization
- Implantation
- Gastrulation
Describe where each of the following are in relation to the inner cell mass:
- cytotrophoblast
- syncytiotrophoblast
- amnion
- yolk sac
- syncytiotrophoblast
- cytrotrophoblast
- yolk sac (or amnion)
inner cell mass - amnion (or yolk sac)
- cytrotrophoblast
- syncytiotrophoblast
What are the three germ layers that form during gastrulation
- Ectoderm
- Mesoderm
- Endoderm
What is the ectoderm
layer of cells in contact with the amniotic cavity
What is the mesoderm
middle layer between the endoderm and ectoderm
What is the endoderm
layer of cells in contact with the yolk sac
The three germ layers of the gastrula have distinct properties, and each will eventually be responsible for forming different _________
ORGANS
- the three germ layers of the gastrula will eventually give rise to different organs
Briefly describe what the ectoderm contributes to the body
- epidermis
- nervous tissue
- pituitary gland and adrenal medullae
Briefly describe what the mesoderm contributes to the body
- dermis
- subcutaneous layer (hypodermis)
- all structures of the muscular system
- all structures of the cardiovascular system
- all structures of the lymphatic system
- gonads
Briefly describe what the endoderm contributes to the body
- thymus
- thyroid gland
- pancreas
- urinary bladder
What are the four extra-embryonic membranes that form around the developing embryo
- Amnion
- Yolk sac
- Chorion
- Allantois
What is the amnion
layer surrounding amniotic cavity where the embryo develops within
What is the yolk sac
creates a pouch involved in storing, collecting and distributing nutrients to growing embryo
What is the chorion
separates space in blastocoele from trophoblast (layer of cells as embryo is developing)
What is the allantois
eventually becomes the umbilical stock/cord
- will form part of the umbilical cord
How long is the first trimester
weeks 1-12
How long is the second trimester
weeks 13-26
How long is the third trimester
weeks 27 to end of pregnancy
What is chorionic villous sampling (CVC)
- testing on the chorion
- retrieving cell samples
- occurs in the first trimester
What is amniocentesis
- testing on the amnion
- retrieving amniotic fluid samples
- usually performed after 15 weeks
What are the risks of CVC and amniocentesis
potential risk of termination of pregnancy
- you are inserting a syringe into the area where the embryo is developing
- can be dangerous for the baby
What is the placenta and what are its three functions
- fetal organ that acts as an interface between the fetus and the pregnant mother
- way of passing materials to/from baby
FUNCTIONS:
1. Nutrient and oxygen exchange between fetal and maternal blood stream
2. Removal of CO2 and other wastes from fetal blood
3. Secretes hormones into both bloodstreams
What connects the fetus to the placenta
Umbilical cord
What is the umbilical cord
Connection from the placenta to the fetus and vice versa
Describe the relationship between the placenta and embryonic/maternal issues
CHORION = major component of the placenta, forms chorionic villi
CHORIONIC VILLI = invade the endometrium and break down its blood vessels
- forms area of pooled blood which exchanges with fetal blood
TROPHOBLAST = layers surround each villus and are responsible for hormone secretion by the placenta
Which hormones maintains the production of ovarian progesterone after formation of embryo
hCG = gonadotropin (peptide hormone that causes the release of FSH and LH)
What is the role of hCG in the ovaries
- closely related to LH
- acts on the same tissues within the ovary
- signals the corpus luteum to survive and keep secreting progesterone
What happens during pregnancy tests, what is it looking for
Looking for hCG in the maternal blood/urine
= detectable sign of pregnancy
Not long after the embryo forms, which layer of the embryo begins to secrete the human chorionic gonadotropin hormone
SYNCTIOTROPHOBLAST
Before the end of the first trimester, ___________ cells take over the secretion of progesterone and estrogen
PLACENTA takes over the secretion of progesterone and estrogen
Trace what part of the female body is in charge of synthesizing progesterone
- Uterine cycle = corpus luteum
- First trimester of pregnancy = corpus luteum
- Corpus luteum degenerates after first two months of pregnancy
- Placental trophoblast tissues begin to synthesize larger amounts of progesterone
Why is the maintenance of high levels of progesterone necessary for pregnancy
Maintenance of endometrium
What is the role of trophoblast after the first trimester of pregnancy
- produce larger amounts of progesterone to maintain the integrity of the endometrium
- begins to synthesize estrogens (especially estriol) as pregnancy progresses
What is an abortion
A procedure to terminate a pregnancy
What are the two types of abortions
Refers to the method of termination used
- Medical
- Surgical
What is a medical abortion
Involves taking a combination of medicines to end the pregnancy by causing the uterine lining to be shed
- administration or intake of meds that cause endometrium to shed
- no structures to support developing embryo
- administration of progesterone antagonist
What is a surgical abortion
Involves physical disruption to the lining of the uterus to end the pregnancy
- typically by suction
True or False:
The endometrium becomes part of the placenta
TRUE!
What is mifepristone
- progesterone antagonist
- antagonist = prevents binding of a certain hormone from its receptor to prevent further signalling
- synthetic steroid derivative that is able to prevent progesterone from activating the progesterone receptors
How do progesterone antagonists terminate a pregnancy
Blocking progesterone leads to the softening of the cervix
- smooth muscle relaxing
- disintegration of the endometrium
- detaches the placenta and embryo
What is misoprostol
Medication that is combined with mifepristone
- prostaglandin which induces the uterus to contract, aiding expulsion of the embryo
Breakdown the roles of mifepristone and misoprostol in abortions
Mifepristone = causes progesterone blockade
= decidual necrosis (death of endometrial tissue)
= cervical ripening (disintegration of endometrium)
Misoprostol = causes uterine cramping and expulsion
