Gametogenesis, Fertilisation and the Blastocyst Flashcards
What is teratology ?
Study of birth defects
Label the 4 parts of the Fallopian tube
The Fallopian tubes end in …
Fimbrae
Oogenesis
explain what happens in stages 4 and 5
most growing follicles degenerate to form corpus acreticum
fluid filled antrum made by the follicular cells
thecal layer forms around the growing follicle
follicle call secondary follicle now
How does the ovum get into the Fallopian tube?
what consequences does this have ?
Ovum is shed into abdominal cavity and then caught by the fimbrae
makes infection more likely
Female gametogenesis (oogenesis)
explain what happens in the first three stages
- Primary oocyte is surrounded by a single layer of follicular cells = primary follicle
- At beginning of each monthly cycle, 5-12 primary follicle develop to become growing follicles (only one reaches full maturity however)
- growing follicles are multiple layered; separated from ovum via the septum pellucidium
Where do the follicular cells come from?
Ovarian epithelium
What is the septum pellucidum made from ?
Acellular mucopolysaccharide(aka GAGs) layer
What two parts does the thecal layer divide into ?
- Vascular theca interna - produces oestrogen
- Avascular inactive theca external
What happens to the cells of the theca interna and stratum granulosum after the ovum is released?
They enlarge , turn yellowish and form the corpus luteum to secrete lots of progesterone and some oestrogen
Oogenesis
explain what happens in stages 6&7
Follicle now called a mature/Graafian follicle
the majority of the follicular cells form the stratum granulosum and secrete progesterone
the others form the outer layer called the cumulus oophorus ; now called secondary oocyte
mature follicle ruptures to release the ovum
ovum covered by corona radiata now
septum pellucidum expanded to become the zona pellucida
What happens to the corpus luteum if fertilisation does not occur after 12 days
Corpus luteum degenerates into the corpus alibicans which terminates hormonal output leading to menstruation
What happens to the corpus luteum if pregnancy occurs ?
Corpus luteum is sustained by HCG produced by conceptual and forms a large corpus luteum of pregnancy
eventually forms a large corpus albicans
Male gametogenesis
(Spermatogenesis)
Occurs in testes
stem cells divide by mitosis to for spermatogonium with 46 XY chromosomes
spermatogonium divide to form primary spermatocytes (still with 46 XY)
1st meiotic division to form secondary spermatocytes with 23X or 23Y chromosomes (double chromatids present at this point)
2nd meiotic division = spermatids with 23X/Y (single chromatid)
spermatids mature to form spermatozoa
Label the diagram with he terms :
Acrosomal cap
nucleus
mitochondria
axial bundle
Nucleus covered by sparse cytoplasmic layer
Purpose of acrosomal cap in spermatozoa and where is it derived from?
Derived from the Golgi apparatus
contains enzymes that aid in penetration of ovum
Describe the structure of the middle piece of the sperm
how does this relate to its function ?
Helically wound mito surrounds central axial bundle of fibril
fibrils have a 9+ 2 arrangement
How are ova moved along Fallopian tubes?
Via the action of cilia
Main difference between male and female gametogenesis
Spermatogenesis occurs continuously throughout a males life
Females are already born with their lifetime supply of oogonia ; they mature throughout their , once every month and the supply runs out between 45 and 55
Compare oogenesis and Spermatogenesis :
which process of division are involved ?
is it continuous/discontinuous ?
how long does it take ?
how many produced in each cycle ? / how many in one ejaculate ?
motile or non/motile?
cytoplasmic:nuclear ratio
does it require additional substances ?
Label the prostate gland
Label this Sagittal section of the female reproductive system
The abdominal cavity in a male is ….
Closed
Describe the events occurring before fertilisation
When the sperm reach the isthmus they become less motile and wait for chemoattractants released from the cumulus cells on ovulation tomake sperm motile again
sperm reach ampulla where fertilisation occurs
What is capacitation
Changes to spermatazoa that must occur for successful fertilisation; takes place upon ejaculation - conditioning in female reproductive tract during which acrosomal cap loses glycoprotein coat and increased motility occurs
Describe first 3 the events which occur upon fertilisation
Penetration by spermatazoon prompts completion of 2nd meiotic division
fusion of plasma membrane of both gametes ; male nucleus is injected
Zona reaction occurs ; ovum shrinks to form the perivitelline space
What is the role of the acrosomal enzymes in fertilisation
Help disperse corona radiata and aid penetration of zona pellucida
What is the zona reaction ?
Why is it important
When fertilisation occurs, the permeability of zona pellucida is altered to prevent further penetration
enzymes released by cortical granules digest sperm receptor proteins so that they can no longer bind
this prevents polyspermy (implantation of more than one sperm)
Stage 4/5 of fertilisation
Cleavage
~30h post fertilisation 2 daughter blastomeres formed by mitosis
cells continue to cleave to form progressively smaller blastomeres (DNA synthesis occurs but no new proteins)
Formation of morula
Stage 6 of fertilisation
morula refers to solid ball of cells
cytoplasmic:nuclear ratio near normal
cytoplasm differential packaged
Free within uterine cavity
zona pellucida still remains to prevent implantation
Blastocyst formation
Occurs during 2nd week
morula cells undergo compaction
tight junctions between surface cells
zona pellucida begins to breakdown (called hatching)
Formation of inner cell mass and trophoblast
blastocyst now has a cavity
Function of inner cell mass and trophoblast
Inner cell mass goes on to form the embryo
trophoblast form extra embryonic membranes and placenta
Label the inner cell mass and trophoblasts in this image
one spermatogonium divides to make _____ spermatozoa
one oogonium divides to make ______ ovum due to ______ division of the cytoplasm
4 spermatozoa
1 ovum (asymmetrical division of cytoplasm)
what are polar bodies?
small residual cells produced during the first and second meiotic divisions of oogenesis that have very little cytoplasm and no use
1 polar body produced during meiosis I
3 polar bodies formed during meisois II
give a brief summary of oogenesis
at foetal stage, female jas 400,000 primary follicles already produced
primary oocyte arrested at prophase I of meiosis I until puberty
primary oocytes completes meiosis I at beginning of menstrual cycle to form the secondary oocyte
secondary oocyte arrested at prophase II of meiosis II until fertilisation occurs
meiosis II completed upon fertilisation
Where and when does implantation occur
6 days after fertilisation
in the posterior wall of uterus nearer the fungus than the cervix
What are the 3 words that can be used to describe the trophoblast
Invasive , ingestive and digestive ; can get past the uterine epithelium
What is the decidual reaction ?
Upon implantation of the blastocyst in the uterine lining ,
Thickening of the endometrium occurs
increased secretion occurs from the endometrium (uterus lining)
Glandular epithelial cells will now contain more glycogen and lipids
What are the 3 populations of cells present upon implantation of the blastocyst?
Primitive ectoderm (epiblast)
primitive endoderm (hypoblast)
trophoblast
Events occurring in early implantation
Epiblast surrounds the amniotic cavity
hypoblast surrrounds the yolk sac cavity
wall of blastocyst is two layered now
What happens on day 10 after implantation
Blastocyst completely embedded ; epithelial continuity restored
trophoblast produces extra embryonic mesoderm which separates edges of trophoblast and epiblast/hypoblast , thus providing space for expansion of the embryo
cavities form in the extra embryonic mesoderm
Trophoblast splits into two types - syncytial trophoblast and cytotrophoblast
What does conceptus mean ?
Fertilised egg
What occurs after the extraembryonic mesoderm is produced ?
The cavities merge to form the chorionic cavity/extraembryonic coelom
derivative of trophoblast begin to form the placenta
extraembryonic mesoderm connects the conceptus to the trophoblast via the connecting stalk
Give a brief summary of what happens in the first 2 weeks after implantation
2 cell layers (Epiblast and hypoblast )
2 cavities ( amniotic and yolk sac)
2 trophoblast derivatives (cyto and syncytium trophoblast)
Describe the structure of the conceptus at the end of the 2 weeks post fertilisation
Bilaminar disc forms where the epiblast and hypoblast meet
the majority of the foetus will be produced from this
What are dichorionic - diamniotic twins
splitting of zygote at day 3 or separate fertilisation of 2 ova by 2 sperm - comprises almost all cases of dizygotic twins and 1/4 of monozygotic twins
lowest mortality risk but still higher than that of singleton
Monochorionic twins
2 types :
- share the same placenta
- Occurs in most cases of monozygotic twins
- risk of twin-twin blood transfusion
- share the same amnion
- Occurs when split is after 9th day but before 12th day
- 50% survival risk ; risk of umbilical chord wrapping around babies
- rarely occurs
How do conjoined twins occur
When division of zygote occurs later tha 12 days
highest mortality
What happens in the third week post fertilisation
process is called gastriculation
3 germ layers - Formation of intraembryonic mesoderm and definitive ecto/endoderm to form trilaminar disc
3 important structures formed :
notochord , primitive streak and neural tube
Label this cross section of the embryo
gastrulation
process of cellular rearrangement within the epiblast that leads to the formation of the 3 germ layers and a trilaminar disc
epiblast cells migrate towards primitive pit and penetrate through the epiblast layer
What happens at the prochordial plate during the 3rd week post fertilisation
Endoderm thickens and becones firmly adherent to the overlying ectoderm in the region of the prochordal plate
this is the future mouth region
Formation of the notochord (notogenesis)
what is the function of the notochord
Epiblast cells migrate from the primitive node and extend cranially to form a midline structure called the notochordal process.
Notochord then forms from the mesoderm - it is a flexible cylindrical tube
fucntions:
guide the formation of the nervous system and vertebral column
induce overlying ectoderm to form the neuroectoderm of the neural plate ; the notochord eventually underlies the neural tube
Formation of the neural tube
Neural plater form neural fold which become the neurectoderm
the neural fold fuse to form a neural tube which eventually becomes the brain and spinal chord
the neural tube loses contact with the overlying ectoderm
name the structures derived from the endoderm
Epithelial lining of digestive and respiratory tracts,
Lining of urethra, bladder and reproductive System
Liver and pancreas
name the structures derived from the mesoderm
Notochord
Musculoskeletal system
Muscular layer of stomach, intestine etc
Circulatory system
name the structures derived from the ectoderm
Epidermis of skin
Cornea and lens of eye
Nervous system
what is the purpose of the syncytiotrophoblast
syncytiotrophoblast becomes continuous with the uterus
maternal blood vessels invade spaces between the syncytiotrophoblast
this allows uterplacental circulation to begin
what is the primitive streak ?
describe its structure
a groove in the midline of the epiblast which appears in the third week post fertilisation
primitive node found at cranial end
primitive pit within primitive node
epiblast cells migrate through the primitive pit to form the notochord between the hypoblast and epiblast
how is the endoderm formed ?
formed by epiblast cells that migrate through the primitive pit and displace the hypoblast cells.
how is the mesoderm formed ?
formed by epiblast cells that migrate through the primitive pit and lie between the epiblast layer and the newly created endoderm.
how is the ectoderm formed?
formed by the epiblast cells that remain in position
purpose of notochord
essential for the development of the nervous system and primitive skeletal structures
