MCM 2-32 Embryogenesis Flashcards
the most sensitive period for inducing birth defects
weeks 3-8 during embryogenesis
fertilization activates….
cleavage divisions. The cell number increases but the size does not change due to the zona pellucida membrane
what is compaction and when does it occur?
segregates inner from outer cells.
from the 8 cell stage to the morula. the contacts expand and smooth over
follow progression to the blastocyst
zygote undergoes cleavage divisions up until 8 cells (blastomere)
8 cell blastomere undergoes compaction, which segregates inner from outer cells
compacted embryo divides to form 16 cell morula, and then undergoes cavitation - outer “trophoblast” cells secrete fluid into morula to create pocket called blastocoel
ICM positioned to one side of the ring of trophoblast cells
outer cells flatten to form a wall - embryo is now a blastocyst
ICM vs trophoblast cells
ICM - composed of pluripotent cells which will give rise to embyronic tissues
trophoblast cells will form extraembryonic tissues such as placenta
what happens when the blastocyst reaches the uterus?
it must hatch from zona pellucida (egg-cell ECM) so it can implant into the uterine wall.
how does the blastocyst implant into the uterus?
The uterine epithelium (endometrium) catches the trophoblast cells of the blastocyst on its ECM
L-selectins (carbohydrate binding proteins) on trophoblast cells interact with carbohydrate receptors on the endometrium.
when does implantation occur?
at the end of the first week
following implantation, what do the ICM and trophoblast do?
the trophoblast differentiates into two layers
- the cytotrophoblast - inner layer or cells, involved in initial adherence to uterine tissue
- syncytiotrophoblast digests uterine tissue, further moving the embryo into the uterine wall
____
ICM developes into the bilaminar germ disk of two layers
-hypoblast - inward cell layer, will form ventral side
-epiblast - outward, closer to trophoblast layer. will form dorsal side. Amniotic cavity forms within this layer.
when is implantation complete?
what does the trophoblast do at this point?
after day 9 - embyro is fully embedded and amniotic cavity expands
it invades maternal capillaries to establish extraembyronic tissues and uteroplacental circulation. trophoblastic lacuna vacuoles open within the syncytiotrophoblast and contact maternal capillaries. Proliferation of hypoblast cells gives rise to Heuser’s membrane (the membrane of the primary yolk sac). Cells farther outward (the extraembryonic mesoderm) surround the primary yolk sac. The extraembryonic mesoderm then splits into two layers, forming the chorionic cavity between them. Hypoblast cells within Heuser’s membrane move inward towards each other, pinching off segments of the yolk sac. What remains forms the definitive yolk sac. By the end of the second week the bilaminar embryonic disc with amnion and yolk sac is suspended in the chorionic cavity, connected to the extraembryonic mesoderm by the connecting stalk.
gastrulation. what and when?
third week of development, transforms the bilaminar desk into three germ layers (ecto, endo, meso)
begins with formation of primitive streak on epiblast surface. epiblast cells undergo ingression where they move towards the primitive streak - detatch from the epiblast - and either displace hypoblast to form endodoer, lie between the epiblast and newly formed endoderm to become mesoderm, or reaim in the epiblast to form the ectoderm.
What does the anterioir visceral endoderm (AVE) express?
expresses genes that direct head formation. These genes establish the anterior end of the embryo prior to gastrulation.
neurolation?
process of forming the neural tube from the neural plate - process by which the ectoderm gets divided\
- neural plate formed and folded
- neural crest elevated
- neural folds converge
- neural tube is closed by bringing the folds into contact with each other
- neural crest cells migrate away to contribute to several tissues
the events of neurulation do what?
The events of neurulation divide the ectoderm into three major domains:
1) The internally positioned neural tube, which will form the brain and spinal cord
2) Externally positioned surface ectoderm that will primarily form skin (epidermis)
3) The neural crest cells that migrate to new locations to give rise to many cell types depending upon where they migrate to
what are teratogens?
are environmental factors that cause birth defects. The embryo is most susceptible to birth defects during embryogenesis (weeks three through eight of development), but no stage is unsusceptible to birth defects.
two infectious birth defects
Congenital rubella syndrome happens when an individual is exposed to rubella in utero. It can lead to several birth defects including deafness and congenital heart malformations.
Additionally, hyperthermia (due to many possible infections) can disrupt neurulation and result in neural tube defects.
pharmacological birth defect
In utero exposure to thalidomide (a morning sickness drug) leads to limb malformation via an unclear mechanism.
Alcoholic birth defects
Fetal alcohol spectrum disorder (FASD) refers to all alcohol-related birth defects. It is the leading cause of congenital mental retardation. It is unclear how much alcohol is required for onset of defects, but there is probably no safe level.
embryology
study of development between fertilization and birth
weeks 1-8 are known as
embryogenesis, when fetus is most susceptible to defects.
the embryonic period is followed by the
fetal period of continued differentiation and growth
describe the first week of development
ooctye fertilized, zygote undergoes cleavage divisions to form morula then blastocyst. by the end of the first week, blastocyst begins implantation into the uterine wall
compaction
segregates inner from outer, 3 days after fertilization the compacted embyro forms 16 cell morula
how does the blastocyst hatch from the zona pellucida?
trypsin like protease excreted from the trophoblast cell membrane
trophoblast cells express…
once in contact with endometrium..
integrins which interact and bind uterine collagen, fibronectin, and laminin.
trophoblast secretes proteases to digest ECM of uterine tissue enabling blastocyst to bury itself
second week of development leads to the
formation of the bilaminar germ disk.
week of twos
trophoblast gives rise to two tissues - epilblast and hypoblast forming the bilaminar disk
epiblast - dorsal, hypoblast - ventral
two yolk sacs (primary and secondary) form
two new cavities (amniotic and chorionic) form
how is uteroplacental circulation established?
vacuoles called trophoblastic lacunae open within the synctiotrophoblast and contact expanded maternal capillaries caleld “maternal sinusoids”
chorionic step villi
protrusions that grow into blood filled lacunae. by end of week 3, blood vessels develop in the villi that connect with the embyro
gastrulation begins with?
establishes what?
begins wtih formation of primitive streak on the epiblast surface. epiblast cells move towards primitive streak, detach from epiblast and move through streak, slip beneath and migrate away as individual cells (ingression)
establishes three germ layers
ecto, meso, endo
____________ is the source of all three germ layers of the trilaminar embryonic disk
epiblast
caudal dysplasia
defect due to abnormal growth and migration during gastrulation
- flexion/inversion of lower extremities
- lumbar/sacral anomolies
- imperforate anus
- agenesis of kidneys/urinary tract
sacrococcygeal teratomas
rare tumors at base of spine by tailbone, arise due to persistence of primitive germ cells that can give rise to cells of three majort tissue layers. contains mature tissue that looks like any itssue of body or immature tissue resembling embyronic tissues (arm growing out of back)
neuration. when and what?
steps?
4th week
neural plate forms neural tube
- formation/folding of neural plate
- elevation of the neural crest
- convergence of neural folds
- closure of neural tube when neural folds are brought into contact
neural crest cells migrate away to contribute to other tissues, leaving the neural tube separate from the epidermis
neural crest cells
arise from?
where do they go?
multipotent migrateory stem cell population, only exist in vertebrates.
arise in ectoderm at margins of the neural tube
migrate away to many different locations and differentiate into many different cell types
many different systems ahve a contribution from neural crest cells
spina bifida
failure of closure in different regions of neural tube results in different types of neural tube defects
failue of posterior neuropore closure results in spina bifida
anacephaly
forebrain remains in contact with amniotic fluid and degenerates. failure of anterior (Cranial)_ region to close
craniorachischisis
complete failure of closing along neural tube
after 3 days
compaction segregates inner cells from outer cells and the compacted embyro forms a morula
the only pluripotent cell lineage in the blastocyst.
what does it form?
the ICM.
forms the epiblast and hypoblast layers, which develop into the embryonic tissues and extraembryonic tissues, respectively.
large cystic mass found on sacrococcygeal region of fetus. lesion was resected and coccygectomy performed.
most likely cause of this congenital defect?
histaologic exam of the mass would reveal derivates of which germ layers?
abnormal persistence of primitive streak tissue after completion of gastrulation
endoderm, mesoderm, ectoderm.
initiates and maintains the primitive streak.
The Nodal gene
BMP4 gene?
The BMP4 gene specifies ventral structures. The node (organized) expresses BMP4 antagonists, and generates dorsal structures.
