Gametogenesis And Fertilization (wk 2) Flashcards
Sex cells are called
Gametes
Spermatogenesis creates what
Sperm
Oogenesis creates
Oocyte in females
Haploid is
The number of chromosomes (23)
22+either x or y
Usual number is 46
Meiosis is
Process of cell division that occurs in the production of gametes
2 cell divisions (meiosis 1 & 2)
Results in 4 gametes - each contain 1/2 chromosomes
Meiosis step 1
Reduction division
- reduced from 46 to 23 chromosomes
- 22 homologues and one sex (x or y) chromosome
Secondary gamatocytes are produced
Meiosis step 2
Each secondary gamatocyte divides
Skips nml interphase (no DNA replication)
Forms 2 haploid cells
Spermatogonium
Primordial sperm cells (stem cells)
How long are spermatogonium sperm cells dormant
Until puberty
Inb the seminiferous tubles
During spermatogenesis, what occurs
1 spermatogonium becomes 1 primary spermatocyte (46 diploid)
What occurs to the primary spermatocyte during the first meiotic division
It becomes 2 secondary spermatocytes
The cytoplasm division is equal ***
What occurs to the 2 secondary spermatocytes during the second meiotic division?
They form 4 spermatids (23 haploid)
No DNA replication in the second meiotic division
Spermiogenesis is the process that
Transforms spermatids into mature sperm
Where are the spermitids stored
In the epididymis until they become functionally mature
2 months
What are the charateristics of mature sperm
Head
Tail
Majority of the sperm is the head - contains the nucleus w/ the 23 chromosomes
Free swimming and motile
Head of the sperm characteristics
Anerior 2/3 covered by the acrosome
What is an acrosome
A sac like organelle containing several enzymes that allow the sperm to penetrate the oocyte during fertilization
- corona radiata and ZONA PELLUCIDA
Tail of the sperm characteristics
3 segments
Middle piece
Principal piece
End piece
What does the middle piece of the sperm contain
Mitochondria - provides ATP required for motility
Oogenesis is the process where
Oogonia (primordial oocytes) become mature oocytes
When does oogenesis occur/stop
Begins prior to birth early in the fetal period
Stops at birth
Resumes at puberty
Complete cessation at menopause
During prenatal maturation of oocytes the oogonia become what
During what period
Primary oocytes
Prenatal period
Are oogonia present in the post natal period
No
What forms as the primary oocyte forms
A connective tissue surrounds the oocyte producing a single layer of follicular cells
This is called the primordial follicle
During puberty what occurs to the primordial follicle
It enlarges to become a primary ovarian follicle
What is the primary oocyte surrounded by
A glycoprotein material called the zona pellucida
When does the first meiotic division begin
When the primary oocyte is surrounded by a glycoprotein material called the zona pellucida
But it is not completed
When is prophase completed
Not until puberty
What occurs between the time of the first meiotic division and puberty
No further development
When does a primary oocyte complete the first meiotic division?
(Postnatal maturation of oocytes)
Shortly before ovulation
First meiotic division what occurs
Postnatal maturation of oocytes
Cytoplasm division is unequal
Secondary oocyte is formed w/ most of the cytoplasm
First polar body is formed with little cytoplasm-degenerates
What happens to the secondary oocyte at ovulation
Postnatal maturation of oocytes
The nucleus of the secondary oocyte begins the second meiotic division
When does the second meiotic division stop
At metaphase
If fertilization occurs at ovulation, what happens
The second meiotic division is completed
Second polar body is also formed
What happens if no fertilization occurs at ovulation?
No fertilization - the oocyte is discarded in menses
Polar bodies
The left over chromosomes from meiosis
23 total
Ovarian vs menstrual cycle
Both approx 28 days
Ovarian - development of the primary oocyte and supporting tissues in the ovary
Menstrual - preparation of the uterus for implantation of the fertilized oocyte
Hypothalamus produces what
Gonadotropin-releasing hormone
Transported to the anterior lobe of the pituitary gland of the pituitary gland
What does the anterior lobe of the pituitary gland release
Follicle stimulating hormone (FSH)
Luteinizing hormone (LH)
FSH
Follicle stimulating hormone
Stimulates the development of ovarian follicles and the production of estrogen by the follicular cells
LH
Luteinizing hormone
Serves as the “trigger” for ovulation (release of the secondary oocyte) and stimulates the follicular cells and corpus luteum to produce progesterone
What does LH and FSH do during the ovarian cycle
They cause changes in the ovaries resulting in:
- development of follicles (FSH) that produce estrogen
- ovulation (LH)
- corpus luteum formation (LH) produces progesterone
FSH stimulates primary follicular growth causing what
Connective tissue surrounding the follicle forms a capsule called the theca folliculi *
Theca folliculi forms 2 layers
- theca externa-outer capsule
- theca interna-vascular layer
Follicle cells form a layer around the primary oocyte and what happens
It is pushed aside
The adjacent cavity is filled with a follicular fluid and is called there antrum*
Once the antrum is formed the ovarian follicle is called the secondary follicle*
Characteristics of the secondary follicle
Becomes oval shaped
The primary oocyte is at the concentric end
The follicular cells surrounding the primary oocytes are called the cumulus oophorus
Follicular cells produce estrogen
What occurs to the follicle mid ovarian cycle
It has a growth spurt
What occurs during the follicles growth spurt
It produces a bulge on the surface of the ovary called a stigma*
Caused by both FSH and LH
Prior to ovulation the primary oocyte completes what
Its first meiotic division
Primary oocyte and first polar body
A surge of LH production is triggered by what and causes what?
Triggered by high levels of estrogen in the blood
Forms the growth of follicular cells
High levels of LH cause the stigma to bulge out forming a vesicle and eventually ruptures, what is this called
Ovulation
12-24 hours after the LH surge
When does the second meiotic division start and stop
Starts just after ovulation
Stops at metaphase
Some follicular cells surround the primary oocyte and for what
The corona radiate
Covers the zona pelluicida - covers the oocyte
After ovulation, the ovarian follicle and the theca folliculi collapse and form grandular tissue called
Corpus luteum
Secretes progesterone and small amounts of estrogen
If fertilization occurs, the corpus luteum becomes
The corpus luteum of pregnancy and produces progesterone and estrogen until 20 weeks when it is replaced by the placenta
If fertilization does not occur, the corpus luteum becomes
The corpus luteum of menstruation and degenerates
Corpus albicans
White scar tissue on the ovary from the corpus luteum
What does the menstrual cycle mirror
The ovarian cycle
What are the 4 phases of the menstrual cycle
Menstrual : 1-5
Proliferative : 5-14
Luteal : 14-27 (secretory)
Ischemia : 27-28
What occurs during the menstrual phase
The functional layer of the uterine wall is sloughed off and discarded with menstrual flow or menses 4-5 days
Compact and spongy layers
What occurs during the proliferative phase
Lasts 9 days
Coincides with the growth of ovarian follicles and is controlled by estrogen secreted by the follicles
There is two- to three-fold increase in the thickness of the endometrium and in its water content during this phase of repair and proliferation
What occurs during the luteal phase
Aka secretory phase
Ovulation is at the beginning of this phase
Lasts 13 days
Growth of the corpus luteum (from follicular cells)
Spiral arteries grow into the compact layer
The venous network grows and large lacunae (venous spaces) form
Whats lacunae
Venous spaces
If fertilization does not occur what happens and what phase comes up
The corpora lutea degenerate, menstration occurs
Estrogen and progesterone levels fall and the secretory endometrium enters the ischemic phase
What occurs during the ischemic phase
Spiral arteries constrict
Corpora lutea degenerates dec progesterone levels
Uterine gland secretions stop
Endometrium shrinks
Necrosis occurs in functional tissues
Blood vessels rupture resulting in bleeding into the uterus
20-80 ml of blood is lost
Compact and most spongy layers are shed in menses
If fertilization does occur what happens?
Cleavage of the zygote and formation of a blastocyst begin
- beginning of the embryo
The blastocyst begins to implant in the endometrium on 6th day of luteal phase
HCG keeps the corpora lutea secreating estrogens and progesterone - produced in the outer layer of the blastocyst (synchytiotrophoblast)
The luteal phase continues and menstration does not occur
Zygote
Fertilized ovum
Blastocyst
Pre-implantation embryo consisting of a thin-walled hollow sphere of 16-40 cells
Trophoblast
The outer wall cells of the blastocyst which attach the zygote to the uterus
Becomes the placenta
Syncytiotrophoblast
Outter layer of the trophoblast
Produces HCG
Second Oocyte is expelled when and from where
At ovulation from the ovarian follicle
Fimbriae move back and forth over the overy
They sweep the secondary oocyte into the infundibulum of the uterine tube
Oocyte passes into ampulla of the uterine tube and on into the uterine body of the peristaltic action of the uterine tube
Fertilization occurs ampulla
Ejaculation of semen - 2 phases
Emission - semen passes to the prostatic part of the urethra through the ejaculatory ducts by the peristaltic movements of the ductus deferens
Ejaculation - semen is expelled from the urethra through the external urethral orifice
- the prostates change shape, closing off the bladder and opening the ejaculatory duct into the urethra
Semen is composed of how many sperm
400-600 million
2-6 ml
About 200 sperm reach the fertilization site in the ampulla
Before fertilization can occur, sperm must undergo a maturation process called
Capacitation
Takes 7ish hours
Exposing the acrosome
Once the sperm has matured, what happens
They pass through the corona radiata
They penetrate the zona pellucida
Membranes fuse, the sperm enters
Once the sperm enters the zona pellucida what happens
It transforms into a zona reaction and becomes impermeable to other sperms
What happens when the second meiotic division completes
Forms a second polar body and a zygote is formed
Enzymes in the acrosome are called ____ and ___and do what?
Hyaluronidase to help the sperm transverse the coronat radiata
Acrosin to Help the sperm transverse the zona pellucida
Results of fertilization
Stimulates the secondary oocyte to complete the second meiotic division
Restores the nml diploid number of chromosomes (46) in the zygote
Results in variation!
Determines sex of embryo
X = female
Y = Male
Causes metabolic activation of the oocyte which initiates cleavage of the zygote
Cleavage of the zygote consists of
Begins when
Undergoes what
Consists of repeated mitotic divisions of the zygote resulting in a rapid increase in the number of cells - blastomeres
Begins 30 hrs after fertilization
Zygote still surrounded by the zona reaction
Undergoes compaction after the 8 cell stage - allowing greater cell to cell interaction
Morula
12-32 blastomeres
2 layers
- inner cells : embryoblast or inner cell mass
- outer layer : layer of flattened blastomeres that form the trophoblast
Trophoblast secrete what
An immunosuppressant protein called early pregnancy factor
- appers in the maternal serum w/in 24-48 hrs after implantation
Formation of the blastocyst
The morula enters the uterus
- 4 days after fertilization
Uterine fluid passes through the zona rxn to form a fluid filled space
- the blastocystic cavity aka blastocoele
As fluid increases in the cavity, the blastomeres are seperated into two parts
- trophoblast - thin outer cells that give rise to the embryonic part of the placenta
- embryoblast - primordium of the embryo
Implantation occurs when
Approx 6 days after fertilization of the blastocyst attaches to the epithelium of endometrium
Embryonic pole - area where the embryoblast attaches to the trophoblast and then to the endometrial epithelium
The trophoblast of the blastocyst being differentiating into 2 layers
Cytotrophoblast - inner layer of cells
Synchtiotrophoblast - outer layer
Begins at end of week 1
Ctyotrophoblast
A layer of mononucleated cells
It forms new trophoblastic cells that migrateinto the increasing mass of the synchytiotrophoblast
Synchtiotrophoblast
A rapidly expanding multinucleated mass in which no cell boundaries are discernible
Fingerlike processes of synchytiotrophoblast extend through what to do what
Through the endometrial epithelium
To invade the endmetrial connective tissue
By the end of the first week the blastocyst is superficially implanted in the compact layer of the endometrium
Synchtiotrophoblasts produce what
Proteolytic enzymes that cause adjacent endometrial cells to ungergo apoptosis (programmed cell death) to allow for implantation
The degenerated cells (decidual cells) contain lipids and glycoproteins that do what
Provide an immunology privileged site for the conceptus
Provide nutrition for the embryo
Produce hCG
When the syncytiotrophoblast produces proteolytic enzymes that erode the endometrium the blastocyst does what
Burrows into the endometrium
What occurs during the hypoblast formation
A cuboidal layer of cells on the surface of the embryoblast, facing the blastocystic cavity
Appears at end of first week
This begins the formation of the embryonic disc
What does hCG do
Maintains the development of the spiral arteries
Continues the development of the synchtiotrophoblast
- self maintaining
How does the embryonic disc form
Embryoblast (inner cell mass) seperates into two layers forming the bilaminar embryonic disc
Epiblast - thicker layer, high columnar cells adjacent to the amniotic cabity
Hypoblast - thinner layer, small cuboidal cells adjacent to the exocoelomic cavity
Happens spontaneously w/ implantation
How does the formation of the amniotic cavity occur
Simultaneously with implantation
A small space developos in the embryoblast - amniotic cavity
- lined with aminoblasts which form a thin membrane called the amnion - lines the amniotic cavity
Epiblast is the floor
Exocoelomic cavity
When the amniotic cavity and embryonic disc from the exocoelomic membrane forms lining the blastocystic cavity
- lines the cytotrophoblast
Forms the exocoelomic cavity
Roof is the hypoblast
The exocoelomic membrane and cavity from the primary umbilical vesicle
- aka primary yolk sac
Primary umbilical vesicle is
Aka yolk sac
Seperated from the amnion and embryonic disc
The outer layer of cells from extraembryonic mesoderm
- connective tissue
- premordium of multiple structures
Lacunae formation
Takes place as the amnion, embryonic disc, and primary umbilical vesicle form
Isolated cavities filled with a mixture of maternal blood and cellular debris provide embryonic nutrition
Fluid in the lacunae called embryotroph
Lacunae formation is the beginning of
Uteroplacental circulation
What happens at the 10 day mark
Implantation is complete
The implantation site does not completely close until the 12th day
Covered by a closing plug - fibrous coagulum of blood
Day 12 the lacunae have fused to form large lacunar networks - spongelike
Extraembryonic mesoderm enlarges after implantation is complete to form what
Extraembryonic coelomic spaces
Spaces rapidly fuse to form a lg cavity called the extraembryonic coelom
Extraembryonic coelom surrounds the amnion and the umbilical vesicle, except for where they are attached to the chorion by the connecting stalk
As the extraembryonic coelom forms, the primary umbilical vesicle (yolk sac) does what
Decreases in size
Forms a larger secondary umbilical vesicle
Day 14
Part of the primary vesicle gets pinched off
The extraembryonic coelom splits the extraembryonic mesoderm into two layers
1) extraembryonic somatic mesoderm - lines the trophoblast and covers the amnion
2) extraembryonic splanchnic mesoderm - surrounds the umbilical vesicle
A new membrane surrounds the embryo called ____ that has what three layers
Called the Chorion
1) extraembryonic somatic mesoderm
2) cytotrophoblast
3) synchtiotrophoblast
The internal wall of the chorionic sac surrounds the chorionic cavity
- extraembryonic somatic mesoderm
What is the role of the connecting stalk
Suspends the embryo in the chorionic cavity
This is the future site of the placenta
When do primary chorionic villi appear
At the end of the second week
These are cellular extentions formed from cytotrophoblastic cells that grow into the overlying syncytiotrophoblast forming the primary chorionic villi
This is all caused by extraembryonic somatic mesoderm
Prechordial plate forms when
Day 14
Endodermal cells in a localized are from a thickened circular area
This is the site of the mouth and is an important organizer of the head rgn
