Fertilization through Gastrulation Flashcards
What is ovulated?
- Secondary oocyte (arrested in metaphase of meiosis II)
- Zona pelucida - glycoproteins surrounding oocyte
- Corona radiata - thecal cells surrounding zona pelucida
- Remnants of cumulus oophorus
one mass per month typically
What is ejaculated?
About 0.1-10 mL of semen/seminal fluid ~3.5 mL average
70% from seminal vesicle - fructose and prostaglandins (fuel and suppress female immune response)
25% from prostate - proteolytic enzymes and zinc (stabilizes chromatin of sperm)
1% from bulbourethral gland - mucoids (increases sperm mobility by altering viscosity)
5% from testes - sperm
Define Oligospermia
Less than 15 million sperm per mL
Typically 200-500 million sperm
What are some challenges to fertilization?
- Getting laid
- Cervical secretions - Sperm must pass through ostium and cervical canal
- Distance - fertilization typically occurs in the ampulla, corona radiata releases bourgeonal which guides sperm
- Corona radiata - forms physical barrier
- Zona pellucida - physical barrier
Describe the steps in sperm capacitation.
- Protective cholesterol over acrosome membrane is stripped off by albumin in female
- Acrosome membrane can then rupture
- Increase calcium uptake (because cholesterol gone), make flagellum beat harder and faster called sperm hyperactivity - allows sperm to force through corona radiata
Describe the acrosomal reaction.
- Sperm binds ZP-3 receptor on zona pellucida
- Acrosome ruptures, releasing Acrosin which digests glycoproteins of zona pellucida
- Acrosin plus flagella beating propels through zona pellucida so sperm can bind to plasma membrane of oocyte
Describe the response to sperm contacting the plasma membrane of the egg.
- Calcium flows into egg causing exocytosis of cortical granules of oocyte
Exocytosis causes:
2a. Disruption of bonds between plasma membrane and zona pellucida
2b. Separation of plasma membrane and zona pellucida by influx of water
2c. Disruption of sperm binding sites on zona pellucida
3. Enzymatic cascade leads to alteration of sperm-binding proteins so no more can bind
Describe the process of impregnation.
- Plasma membrane of sperm fuses with oocyte at the local site
- Nucleus of sperm enters cytoplasm of oocyte with the centrosome and portions of flagellum
- Cytoplasm of oocyte degrades and absorbs all of the flagellum but centrosome
What does impregnation signal in the secondary oocyte?
The secondary oocyte had arrested at Metaphase of Meiosis II. Impregnation signals for meiosis to resume.
After meiosis II there will be 3 polar bodies and a haploid (N) now termed a mature oocyte
Describe the formation of pronuclei.
- Condensed nucleus from sperm is unpacked and surrounded by nuclear membrane formed by oocyte - paternal pronucleus
- Oocyte chromatin that exits meiosis II is surrounded by nuclear membrane - maternal pronucleus
Describe the replication occurring after the formation of the pronuclei.
Both pronuclei undergo genetic replication resulting in haploid with two copies of each chromosome (1N, 2C)
Describe the split of the zygote into two blastomeres.
- Maternal and paternal pronucleus line up in center of mature oocyte
- Centrosome (from sperm) with microtubules (from oocyte)
- Chromosomes within each pronucleus line up on metaphase plate
- Nuclear membranes dissolve and chromosomes migrate to the two poles - maternal and paternal chromosomes going to same pole
- Nuclear membranes form resulting in each diploid (2N) with 1C of paternal and 1C of maternal chromosomes
- Cytoplasmic division
Define morula.
How much time does it take for this to form?
Describe the growth.
Morula is approximately 30 blastomeres.
This forms in approximately 96 hours
zona pellucida is still intact - no absolute growth - each division results in double cells and size is reduced in about half to stay in the zona pellucida
Describe blastocyst formation.
4-5 days after insemination
still surrounded by zona pellucida
intracellular fluid accumulates in the blastocyst cavity
asymmetric - more cells on one pole over the other
Label this image of a blastocyst.
What will each of these structures become?
Trophoblast will become portion of placenta
inner cell mass/embryoblast will come embryo
Describe blastocyst hatching.
On what day of development does this occur?
What does this allow for?
- Enzymes released by trophoblast cells at the Abembryonic pole (opposite inner cell mass) weaken zona pellucida
- Rhythmic contractions and explanations resulting in zona pellucida breaking down and embryo escaping
Occurs on 5th day of development
Allows for growth of embryo in size
How is the egg moved through the female reproductive system?
Where does fertilization typically occur?
How long does it take for the embryo to reach endometrium?
The egg is moved along the uterine (fallopian) tube by muscular contractions and beating cilia
Fertilization typically occurs in the ampulla
5-6 days after ovulation the embryo will reach endometrium lining the uterine cavity (5 days for blastocyst “hatch”)
What does the inner cell mass differentiate into?
What are its parts?
What will these form?
On what day of development will this occur?
The inner cell mass differentiates into the bilaminar embryo
The hypoblast - upper layer, located closest to a embryonic pole - made up of cuboidal cells - will form extra embryonic membranes
The epiblast - lower layer - made up of columnar cells - will form the embryo
This occurs on the 7th day of development
How does the bilaminar embryo begin to differentiate further on day 8?
Hypoblast cells - migrate over inner surface of trophoblast (forming extra embryonic membranes)
Epiblasts - begin to evacuate
resulting cavity is the amniotic cavity
Deepest epiblasts, lining trophoblasts are now the amnion
At day 12 what have the hypoblast cells formed?
What is separating the outer portions of the bilaminar embryo from the trophoblast?
At day 12
hypoblasts - formed yolk sac
yolk sac and amnion (outer portions of bilaminar embryo) are isolated from trophoblasts by the extraembryonic mesoderm
combination of extraembryonic mesoderm and trophoblast is called chorion
What is the embryonic disk?
What is the primitive streak? When does this form? How does this form the primitive groove?
Adjacent hypoblast (yolk sac) and epiblast (amniotic cavity) cells forming a flattened ovoid structure
Primitive streak is formed by epiblast cells proliferating and migrating to midline to form raised structure. (Day 17)
By day 19 the leading edge (primitive node) is more pronounced and epiblast migration has opened a fissure forming the primitive groove.
Describe gastrulation.
OVERALL: gastrulation converts bilaminar to trilaminar embryo (ectoderm, mesoderm, endoderm)
- epiblast (amniotic cavity) cells proliferate and migrate through primitive groove
- Epiblast cells reach deep surface and break free (delaminates)
- The delaminating epiblast cells fill in between epiblast and hypoblast to form intraembryonic mesoderm OR move deeper to replace hypoblast cells
- Displaced hypoblast cells move into extraembryonic tissue while cells that replaced the hypoblast form the endoderm.
What does the mesoderm differentiate into?
When does this occur?
Medial to lateral
Notochord
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
Occurs in 3rd and 4th weeks
