Germ cells, fertilization, and development through gastrulation Flashcards
From where do PGCs arise?
How do PGCs get into the gonads during development?
Why is the location and migration of PGCs during development so regulated/important?
PGCs are formed in the epiblast during the second week of development.
During gastrulation, PGCs move through the primitive steal and migrate to the wall of the yolk sac. During the fourth week, these cells begin to migrate from the yolk sac toward the gonadal ridge where the primitive gonads will develop from intermediate mesoderm along either side of the mesentery of gut.
Some evidence suggests that PGCs that have strayed from their migratory paths could be responsible for some teratomas (tumors of disputed origin).
What are the differences between mitosis and meiosis?
How can chromosome abnormalities originate during mitotic or meiotic divisions and how would they affect an embryo?
During mitosis, the individual chromosomes duplicate, and while they are still attached to each other, they line up on the metaphase plate. At anaphase, the duplicated chromosomes seperate and migrate to the daughter cells during telophase. Each has 46 chromosomes.
Meiosis requires two separate rounds of cell division. During the first stage, the replicated, attached daughter chromosomes pair up (synapsis) closely on the metaphase plate and undergo crossing over, or exchange, of chromosome segments. During the second stage of division, the rearranged chromosome pairs split, and each daughter cell acquires only a haploid number of chromosomes - 26.
In meiosis - nondisjunction during the first or second meiotic division or unbalanced translocations
In mitosis - mitotic nondisjunction in an embryonic cell produce mosaicism with some cells having a normal chromosome numner ad other being normal.
How does a PGC change from arriving at the gonadal ridge to birth?
Once PGCs have arrived in the gonad (female), they undergo a number of mitotic divisions, differentiating into clusters of oogonia surrounded by a layer of follicular cells.
The majority of oogonia continue to divide by mitosis, but some of them arrest their cell division in prophase of meiosis I to form primary oocytes. Cell death begins and many oogonia and primary oocytes degenerate and become atretic.
By the seventh month almost all oogonia are atretic or transformed into primary oocytes in prophase of the first meiotic division. (Primary oocyte with surrounding flat epithelial follcular cells = primordial follicle)
Near the time of birth, all primary oocytes have started prophase of meiosis I, but instead of procedding into metaphase, they enter the diplotene stage (a resting stage during prophase that is characterized by a lazy network of chromatin). It carries 46 double-structured chromosomes.
How do hundreds of primordial follicles become one fully mature secondary follicle?
Pituitary FSH stimulates follicular development.
At puberty a pool of growing follicles is establsihed and continuously maintained from the supply of primordial follicles. In primordial follicles selected to grow, the follicular cells become cuboidal and secrete the zona pellucida.
Each month at the beginning of the reproductive cylce, pituitary FSH rescues 15-20 growing follicles to develop further; the other growing follicles become atretic. These growing follicles eventually die or begin to accumulate fluid in the atrum becoming mature vesicular follicles or graadian follicles. The “graafian” follicles are surrounded by granulosa cells and theca interna and theca externa.
How is ovulation regulated by the body and what changes need to occur for the oocyte to be ovulated?
What causes period pain in the middle of the reproductive cycle?
Where is the oocyte go immediately following ovulation?
Pituitary LH triggers ovulation, resumption of meiosis until re-arrest at metaphase of meiosis 2, and corpus luteum formation.
With the final development of the vesicular follicle, there is an abrupt increase in LH that causes the primary oocyte to complete meiosis I and the follicle to enter the preovulation mature vesicular stage. Meiosis II is also initiated but the oocyte is arrested in metaphase 3 hours prior to ovulation. In the meantime the ovary undergoes changes driven by LH to make ovulation easier such as degradation of collagen fibers surrounding the follicle. Local muscular contractions in the ovarian wall eventually extrude the oocyte.
Local muscular contractions in the ovarian wall that extrude the oocyte can also cause main during ovulation called mittelschmerz (German for “middle pain”) because it occurs in the middle of the cycle.
The rupturing follicle sheds the oocyte into the fimbriae of the uterine tube
What changes occur to allow for fertilization once the sperm reach the cervix?
Capacitated sperm digest the zona pellucida, and the first to arrive at the oocyte membrane triggers a cortical reaction.
Movement of sperm from the cervix to the uterine tube occurs by muscular contration of the uterus and the uterine tube and very little by their own propulsion (30 min to 6 days).
After reaching the isthmus, sperm become less motile and cease their migration.
At ovulation, sperm again become motile, perhaps because of chemoattractants produced by cumulus cells surrounding the egg, and swim to the ampulla.
Spermatozoa are not able to fertilize the oocyte immediately - they must first undergo (1) capacitation and (2) the acrosome reaction to acquire this capability.
How is polyspermy prevented?
Where could a mitotic error occur that would affect all the individual’s cells?
How do the commbined sperm and oocyte become a two celled zygote and eventually the morula?
The oocyte completes meiosis 2, and the pronuclei fuse to restore diploidy. Several mitotic cleavages result in a solid morula.
Consequences of fertilization in the egg:
1. Barrier to polyspermy by oocyte cortical reaction
2. Metabolic activation of the fertilized egg
3. Resumption by maternal pronucleus of 2nd
meiosis from arrested metaphase
4. Fusion of sperm and egg pronuclei mitotic
spindles
5. Completion of first mitotic division by fused
spindles to form two daughter cells.
Rapid mitosis brings the embryo to the morula stage (a solid ball of cells) in the uterine tube by 4 days after fertilization.
No G1 or G2 – no metabolic activity
What state is the morula in when it implants in the uterine mucosa?
What happens to the zona pellucida upon implantation?
How do the cells polarize upon implantation?
At what point can the cells be extracted for ESC research?
The hollow blastocyt, with an inner cell mass and trophoblast wall, implants at day 6; hCG production maintains the corpus luteum.
During the cleavage stage, the smaller cleavage divisions of the zygote, blastomeres, maximize their contact with each other forming tight junctions which allows for the segregation and for the cells of the inner cell mass to become the embryo proper and for the outer cell mass cells to become the tropholast and eventually the placenta.
By day 6, the solid morula has differentiated into a hollow blastocyst containing the inner cell mass that forms the embryo proper. These are the pleuripotent cells that can be extracted and used experimentally as “embryonic stem cells”.
How does the implanted blastocyst ensure survival?
What is the first priority of the blastocyst to ensure survival?
Syncytial trophoblast connects with maternal blood vessels.
How is the umbilicial cord formed?
Why do some women who are pregnant still have a period?
The inner cell mass forms an embryonic disc of epiblast and hypoblast between the amniotic cavity and the yolk sac.
The chorionic cavity expands around the suspended embryo and its yolk sac.
What happens to the corpus luteum after the follicle is ruptured?
After ovulation, granulosa cells remaining in the wall of the ruptured follicle, together with cells from the theca interna, are vascularlized by surrounding vessels. Under the influence of LH, these cells develop into the corpus luteum but will degenerate if fertilization and implantation do not occur.
Cells from both granulosa and theca interna will secrete progesterone in the corpus luteum; cells from the theca interna also produce estrogen precursors that are converted into estrogen by cells from the granulosa (positive feedback)
