Lecture 2: embryogeneses Flashcards
time it takes for a oocyte to degenerate
24 hours
time it takes for a sperm to degenerate
48 hours
structures released during ovulation
- secondary oocyte
- corona radiata
- zona pellicida
sequence of events involved in fertilization (5)
- penetration of corona radiata
- penetration of zona pellicida
- fusion of plasma membranes
- zona reaction: doesn’t allow another sperm to fertilize the oocyte - meiosis 2
- nuclei fuse –> zygote forms (~12 hours later)
cleavage division
mitotic division of the zygote
daughter cells called blastomers
2 systems for dating pregnancies
- fertilization age
- menstrual age
fetilization age
dates pregnancy from the time of fertilization, thus a six week embryo is six weeks (42 days) from the day of fertilization
menstrual age
Used by clinicians and dates the pregnancy from the woman’s last normal menstrual period (menstrual age). (divided in trimesters) The menstrual age of a human embryo is two weeks greater than the fertilization age because usually two weeks elapse between the start of the last menstrual period and fertilization
embryonic stages of development
- embryonic stage
- fetal stage
- postnatal
embryonic stage
first 8 weeks
- zygote formation
- cell division
- implantation
- organ formation
- teratogen sensitivity
fetal stage
week 9 to birth
- rapid fetal growth
- sex organ formation
- organ system function
postnatal
emotional, physical, and social growth
outer cell mass/ trophoblast
gives rise to the extra-embryonic tissues that form the placenta
inner cell mass/ embryoblast
gives rise to the embryonic cells and layers that form all of the tissues and organs of the body (intra-embryonic)
stages in human implantation
5-maturation of blastocyst
5-loss of zone pellucida
6-attachment of blastocyst to uterine epithelium
6-7- epithelial penetration
7-9-trophoblastic plate formation and invasion of uterine storm by blastocyst
what is the usual site of implantation
the posterior, superior wall of the uterus
ectopic pregnancy ?
“out of place”
happens ~ 0.25 to 1% of the time
where do most ectopic pregnancies occur?
ampulla of the uterine tube
rick factors for ectopic pregnancies
pelvic inflammatory disease, endometriosis
clinical signs for ectopic pregnancies
abnormal bleeding, abdominal pain, positive pregnancy test, intraperitoneal blood
decidua
name for endometrium after implantation
after implantation trophoblast proliferates into 2 layers
cytotropoblast layer
syncytiotropoblast
cytotropoblast layer
mitotically active inner layer of cells
syncytiotropoblast
produces human chorionic gonadotropin (hCG)- feedback to the ovary to the corpus luteum (hormones maintain the pregnancy until the placenta is fully functioning)
after implantation the cells of the embryoblast reorganize into 2 epithelial layers
epiblast layer
hypoblast layer
epiblast layer
- form a membrane that lines the amniotic cavity
- dorsal aspect of the disc
- gives rise to the embryo
hypoblast layer
- form a membrane that lines the yolk sac
- also called the primitive endoderm
- does NOT contribute to the cells developing embryo
- ventral aspect of the disc
2 cavities that form
- amniotic cavity
- yolk sac/ umbilical vesicle
amniotic cavity
• The amniotic membrane surrounds the embryo
following body folding
• Prevents mechanical injury to the fetus
• Allows normal fetal movements
• Cavity expands until fluid content reaches nearly one L by 33 weeks
• Sources of amniotic fluid:
o Amnion cells
o Maternal tissue (diffusion across chorionic plate)
o Fetal urine
extraembryonic mesoderm
forms between the yolk sac and cytotrophoblast
chorionic cavity divides the extraemrbyonic mesoderm into the
- extraembryonic somatic mesoderm
- extraembryonic splanchnic mesoderm
extraembryonic somatic mesoderm
lining cytotrophoblast and amnion
extraembryonic splanchnic mesoderm
lines the yolk sac
the chorion forms the wall of the chorionic cavity and has 3 layers
- extraembryonic somatic mesoderm
- cytotrophoblast
- syncytiotrophoblast
connecting stalk
suspends the amniotic cavity and the yolk sac in the chorionic cavity
-future umbilical cord
contact to the maternal blood
ALL urfaces of chorion and villi in contact with maternal blood are lined with syncytiotrophoblast**
villi
numerous , minute, elongated projections
primary villi
made up of syncytiotrophoblast and cytotrophoblast
secondary villi
made up of syncytiotrophoblast, cytotrophoblast, mesoderm core
tertiary villi
made up of syncytiotrophoblast, cytotrophoblast, mesoderm core, and villous capillary
the placenta
is formed by both maternal and fetal contributions:
Fetal -the structures of the chorion
Maternal - the decidua
Week two = “rule of two’s”:
- Embryoblast splits into two layers (epiblast, hypoblast)
- Trophoblast gives rise to two tissues (cytotrophoblast, syncytiotrophoblast)
- Two yolk sacs form (primary, secondary)
- Two new cavities form (amnionic, chorionic)
- Extraembryonic mesoderm splits into two layers (somatic, splanchnic)
week three
(15-16 days post fertilization) gastrulation occurs producing 3 main germ layers
primitive streak
represents site of cell migration
CAUDAL to CRANIAL
disappears at the end of the 4 week
primitive node
elevated area at the cephalic end of the primitive streak
epiblast cells along the primitive streak
epiblast cells proliferate, lose their adhesion molecules(E-cadherin), detach from their neighboring cells and migrate toward the median plane of the embryonic disc
epiblast layer gives rise to
all 3 germ layers
endoderm
forms by epiblast cells that replace the entire hypoblast
mesoderm forms..
by epiblast cells that migrate between the existing layers
ectoderm forms..
by cells that remain in the epiblast after gastrulation is complete
Sacrococcygeal teratoma
remnants of the primitive streak remain in the sacral region and cells proliferate and form a tumor
most common newborn tumor
notochord
axial mesoderm that forms a long chord along the cranial- caudal axis
Ectoderm and endoderm are adhered together at the
oropharyngeal and cloacal membranes
-but two membranes will break down completely as development continues
prechordal plate
- mesoderm structure
- may act as a signaling center to stimulate the development of the forebrain and contribute to head and neck mesenchyme
importance of the notochord
-vertebral column and base of skull develop around it
-inductive signals (induction-when one population of cells influences the development of another population of cells) stimulate:
o the conversion of overlying surface ectoderm into neural tissue.
o the transformation of mesodermal cells of the somites into vertebral bodies.
the adult remnant: nucleus pulposus of the intervertebral disc.
neural plate
thickened region composed of columnar epithelial cells–> neuroectoderm
develops into the primordial central nervous system
bending of the neural plate
- the edges thicken and move up to form the neural folds
- U shaped neural groove forms in the center
closure of the neural tube
- neural folds migrate to the midline and fuse
- neural crest dissociates to become the neural crest cells
- the remaining ectoderm differentiates into the epidermis layer (of skin)
day 22
lateral edges of the neural folds first begin to fuse in the occipitocervical region
day 24
cranial neuropore of the neural tube closes
day 26
caudal neuropore of the neural tube closes
Spina bifida with meningocele
-sac contains meninges and CSF, may be spinal anomalies if nerve roots extend into sac
Spina bifida with meningomyelocele
-severe with neurological defects below the level of the lesion
Spina bifida with myeloschisis (rachischisis)
-neurofolds fail to fuse
Meroanencephaly
• Failure of the rostral neuropore to close during the fourth week • Forebrain development is abnormal • Some, or all of the brain stem is intact • Overlying bone is defective (calvaria)
neural crest cells
- neural crest cells give rise to the cells and tissues of the peripheral nervous system as well as many other critically important structures
- derived from ectoderm related to the neural tube
- often called fourth germ layer
- must migrate from their site of origin undergo ectodermal to mesenchymal differentiation (migrate into the mesenchyme on each side of the tube)
- very vulnerable cells
- ectomesenchyme
derivatives of surface ectoderm
- epidermis
- nails
- hair
- subcutaneous glands
- mammary glands
- anterior pituitary
- enamel
- lens of eye
derivatives of neural ectoderm
- central nervous system
- retina
- posterior pituitary gland
- pineal body
regions of mesoderm
paraxial, intermediate, lateral mesoderm
Paraxial mesoderm
• Bilateral- next to (para) axial mesoderm
• Organizes into segments called somitomeres in
the head region
• From occipital region caudally somitomeres organize into somites
• The age of the embryo can be correlated to the number of somites
each somite gives rise to…
its own sclerotome, myotome and dermatome
sclerotome
segmental bone (axial skeleton)
myotome
skeletal muscle
dermatome
dermis/ connective tissue of the back
each myotome and dermatome…
has its own segmental nerve component that migrates with the cells
intermediate mesoderm
• Gives rise to the urogenital system
• Functionally divided into urinary
system and genital system
lateral mesoderm
the lateral (plate) mesoderm will develop cavities that coalesce and separate the lateral mesoderm into the:
- somatic (parietal) mesoderm layer
- splanchnic (visceral) mesoderm
The somatic mesoderm and overlying ectoderm will form
the body wall
The splanchnic mesoderm and underlying endoderm form
the gut wall
the body cavity is formed by…
splitting of the lateral plate mesoderm
mesothelium
differentiated specialized simple squamous epithelium of cells lining the parietal and visceral layers
-lines the intraembryonic cavities
The space between the two layers of lateral plate mesoderm is the primitive body cavity which gives rise to the:
- Peritioneal cavity
- Pleural cavities
- Pericardial cavity
Folding occurs in simultaneously in two planes:
- longitudinal or median plane(cranial to caudal) occurs due to brain development
- horizontal plane (lateral body fold) occurs due to growth of the somite
cranial fold/ head fold
involves:
- septum transversum (the primordial diaphragm)
- primordial heart
- pericardial cavity
- oropharyngeal membrane
endoderm is incorporated into the embryo and forms the foregut
tail fold
involves:
- primitive streak
- cloacal membrane
- connecting stalk
endoderm lining the yolk sac is incorporated as the hindgut
lateral folding
growth of somites forms the body wall
endoderm is incorporated as the midgut
endoderm gives rise to
- epithelium of the gut tube
- liver, gallbladder, pancreas
- epithelium ofrespiratory system
- epithelial lining of the urinary bladder and urethra
- epithelium of some of the glands and structures of the head and neck
Omphalocele
o Failure of the intestines to return to the body cavity
o Covered by amnion
o Often associated with other malformations
Gastroschisis
o Protrusion of the viscera into the amniotic cavity due to abnormal closure of the body wall
o Viscera are not covered by amnion
