MSCT Week 3: Embryology Basic Body Plan Flashcards
First Trimester AKA
Embryonic Period
2nd and 3rd Trimesters AKA
Fetal Period
Embryonic Period Timeline
Fertilization to 8 weeks
Organogenesis is?
Formation of Organs
The basic body plan is set up during _______________.
Weeks 1-3 of the embryonic period
Fetal Period Timeline
9 weeks until birth (38-weeks post-fertilization)
Clinicians describe pregnancy by trimesters, three-month periods starting with the date of the onset of? and ending with?
Onset of the last menstrual period and ending with birth
landmark events during the first week
4 listed
- Fertilization
- Cleavage
- Blastocyst formation
- Implantation
describe how oocytes are selected
4 steps
- with each ovarian cycle, 5-15 ovarian follicles with surrounding epithelial cells begin to grow
- only one reaches maturity (called an Ovum)
- ovum is ovulated (released) from the follicle in response to a surge in luteinizing hormone from the pituitary gland
- The fimbrae, (fingerlike projections of the uterine or oviduct sweep the ovum into the uterine tube or oviduct
Ovum are ovulated in response to?
A Surge of luteinizing hormone (LH) from the pituitary gland
Fimbrae are?
Finger-like projections of the uterine tube or oviduct that sweep the ovum into the uterine tube
uterine tube AKA
Oviduct
Oviduct AKA
Uterine Tube
How does fertilization occur?
4 steps
- when sperm swim through the uterus and oviduct to meet the ovum in a wide part of the oviduct called the ampulla
- As soon as a sperm enters the ovum, the ovum completes the second meiotic division and the female pronucleus forms
- The head of the sperm swells and becomes the male pronucleus
- This diploid cell is now called a zygote
Zygote is?
Diploid cell that is the progenitor of the embryo
Cleavage is characterized by?
Mitotic divisions without growth
Cleavage begins…
as the one-celled zygote moves through the oviduct toward the uterus
The cells of the cleaving embryo are called?
Blastomeres
Blastomere potency
Blastomeres are totipotent until the 4-8 cell stage
Blastomeres give rise to…
The embryo and the fetal part of the placenta
When an embryo has 16 cells it is called a…
Morula (mulberry)
Morula means?
Mulberry
As the cells of the morula divide, they _________ and ______________.
As the cells of the morula divide, they compact (move closer together) and secrete fluid to form a cavity (cavitate)
After the morula divides and cavitates it is now a?
Blastocyst
The blastocyst arrives in the uterine cavity by
4-5 days after fertilization
4-5 days after fertilization?
The blastocyst arrives in the uterine cavity
When does the blastocyst attach to the uterine wall (implant)?
By day 6
By day 6, the blastocyst…
begins to invade the uterine wall (Implantation)
Implantation in the uterus timeline
day 6 - first 2 weeks
Identify
+
What time in development is pictured?
Week 1 of embryonic development
blastomere cell types
blastomeres of the 6-day embryo segregate into two distinct cell types
Inner cell mass (embryoblast)
Outer Cell Mass (Trophoblast)
Inner cell mass AKA
Embryoblast
Outer Cell Mass AKA
Trophoblast
Identify
Embryoblast gives rise to?
these cells form the embryo and are pluripotent stem cells
The Trophoblast gives rise to?
These cells surround the embryoblast and blastocyst cavity and will form the fetal part of the placenta
During week 2, the embryoblast and trophoblast each differentiate into?
2 layers and 2 cavities form giving rise to the bilaminar embryo
Bilaminar Embryo is?
embryoblast with 2 layers and 2 cavities formed by the embryoblast and trophoblast
Name the layers of the embryoblast
- Epiblast
- Hypoblast
The trophoblast differentiates into the?
- Syncytiotrophoblast
- Cytotrophoblast
The Epiblast is?
Columnar cells that secrete fluid to form the amniotic cavity above them
The Hypoblast is?
Cuboidal cells below the epiblast that migrate to line the blastocyst cavity converting it to the primitive yolk sac
Identify
Syncytiotrophoblast
- Outer layer
- one cytoplasm with many nuclei; a syncytium
Cytotrophoblast
- Inner Layer
- Actively proliferating cells
describe embryoblast development
By 12 days, the blastocyst is?
Completely embedded in the uterine wall
Lacunae means
Lakes
spaces in the syncytiotrophoblast called Lacunae serve what function?
Connect to capillaries in the uterine wall to establish the placental blood supply
The hypoblast cells lining the primitive yolk sac proliferate to form?
Loose connective tissue called extraembryonic mesoderm
Extraembryonic mesoderm arises from
The hypoblast cells lining the primitive yolk sac
How is the chorionic cavity formed?
spaces appear in the extraembryonic mesoderm and coalesce to form the chorionic cavity by 13 days
The connecting stalk function
suspends the bilaminar embryo within the chorionic cavity
The connecting stalk is composed of?
Extraembryonic mesoderm
The secondary or definitive yolk sac is characterized by?
The suspension of the bilaminar embryo in the chorionic cavity by the connecting stalk
Identify
+
what is this called?
Primary Yolk Sac 12 days
Identify
+
What is this called?
+
When does this occur?
13 days Secondary Yolk Sac
Explain the primary to secondary yolk sac transition
Dizygotic or Fraternal Twins arise from?
+
Dizogitic twins represent what percentage of twins?
- Two oocytes fertilized by two sperm and represent
- 70% of all twins
The zygotes in dizygotic twins are genetically?
Distinct
Implantation of dizygotic twins?
Implant separately
Dizygotic twins fetal structures
Have separate
- fetal membranes
- placentas
- amnions
- chorions
% of twins that are monozygotic?
30% of twins
Monozygotic twins develop from?
One oocyte fertilized by one sperm
Monozygotic twins mechanism
the zygote splitting at different stages of development so the twins are genetically identical
Monozygotic twins are genetically?
Identical
Fetal structures of monozygotic twins
Depending on when the splitting occurs, the placentas, amnions and chorions can be separate or shared
If the zygote splits at the two-cell stage…
The embryos implant separately and develop similarly to dizygotic twins, each with its own placenta, amnion and chorion
If the inner cell mass splits at the early blastocyst stage…
The embryos occupy the same chorion, but have separate amnions and one placenta
If splitting occurs in the bilaminar embryo…
The embryos share an amnion, chorion and placenta
Conjoined twins result from?
Very rarely when the embryos do not separate fully
Describe monozygotic and dizygotic twinning and structures shared, splitting stages…
Hydatidiform mole properties and prevalence
- In 0.1% to 0.5% of pregnancies the fetal part of the placenta forms but the embryo does not.
- This is called a molar pregnancy or hydatidiform mole
- Hydatidiform moles can be complete or partial
Hydatidiform mole means
- Greek
- hydatidos meaning water
- mole means spot
A complete mole characterization
- complete lack of an embryo
- the placental or chorionic villi become swollen as fluid is taken up from the maternal circulation and has nowhere to go and become swollen and resemble bunches of grapes or drops of water
Do Complete moles abort?
Yes, usually early
Clinical signs of a complete mole
5 listed
- preeclampsia (maternal high-blood pressure) during the 1st trimester
- vaginal bleeding
- elevated chorionic gonadotropin (hCG)
- High hCG secreted by the trophoblast result in excessive nausea and vomiting
- 3-5% of complete moles develop into malingnat disease called gestational trophoblastic neoplasia, a malignant tumor of the trophoblast (one histologic type is carcinoma)
Gestational Trophoblastic Neoplasia
- A malignant tumor of the trophoblast (one histologic type is choriocarcinoma)
- 3-5% of complete moles develop into gestational trophoblastic neoplasia
High levels of hCG result in?
Excessive Nausea and vomiting
Preeclampsia during the first trimester, vaginal bleeding and elevated human chorionic gonadotropin indicate what?
Complete mole
Diagnosis method of a complete molar pregnancy
Cytogenetic analysis
A cytogenetic analysis of a complete molar pregnancy will show?
normal diploid karyotype but all chromosomes are parentally derived
Possible karyotypes of a complete molar pregnancy
3 listed
- 46 XX
- 46 XY
- 46 YY
Dispermic fertilization mechanism and possible karyotypes
two sperm fertilizing an oocyte that lacks or loses its own nucleus with a karyotype of…
- 46 XX
- 46 XY
Monospermic Fertilization mechanism and possible karyotypes
One sperm that fertilizes an oocyte lacking a nucleus, the sperm undergoes mitosis without cytokinesis, doubling its DNA with a karyotype of…
46 XX
46 YY Zygote
doesn’t develop as it lacks X chromosome genes essential for development
Partial moles result from?
dispermic fertilization of a normal oocyte or fertilization by a single abnormal diploid sperm
PArtial moles result in?
fetal tissue is present and spontaneous abortion usually occurs in the second trimester
Possible karyotypes of partial moles
Usually triploid
- 69, XXX
- 69, XXY
- 69, XYY
Week 3 of development
The major event of the third week is converting the 2-layer embryo into a 3-layer embryo (trilaminar embryo)
blastomeres are totipotent until the?
4-8 cell stage
Trilaminar Embryo AKA
3-layer embryo
Gastrulation establishes
the 3 layers that will give rise to all adult tissues and organs
When is the primitive streak developed?
15 days
Where is the primitive groove?
In the center of the primitive streak
Where is the primitive node?
along the cranial end of the primitive groove
Where is the primitive pit?
In the center of the primitive node
What defines the major body axes?
Formation of the primitive streak (cranial-caudal, medial-lateral) (primitive streak is the median or midline
During gastrulation, TGFβ and Wnt signaling causes?
FGF8 production
FGF8 stimulates
epiblast cell migration (epithelial to mesenchymal transition)
During gastrulation, epiblast cells migrate where?
Toward the center of the bilaminar disc-shaped embryo forming the primitive streak and dive down or invaginate through the primitive groove to replace hypoblast cells beneath them
When the hypoblast is replaced by epiblast cells this layer is now called?
Endoderm (inside layer)
Epiblast cells then migrate between the newly formed endoderm and epiblast layers form the?
Intra-embryonic mesoderm (middle layer)
The remaining epiblast cells that did not migrate from the upper layer become the?
Ectoderm (outside layer)
Identify structures and stage
Trilaminar Embryo
Cells migrate which direction first in gastrulation?
Cranially first and then caudally last so that cranial or head mesoderm forms first
Ectoderm forms?
- Central and Peripheral Nervous Systems
- Some head skeletal and connective tissue
- Epidermis, hair, nails
- Sensory epithelium of nose, ear and eye
Mesoderm forms?
- Skeletal, smooth, and cardiac muscle
- cartilage, bone, connective tissue, blood
- components of the kidneys and gonads (urogenital system)
Endoderm forms?
- Epithelium of gut and its derivatives (live, gallbladder, pancreas)
- Epithelium of the respiratory system
gastrulation 17-22 days
Tube-within-a-tube body plan
The neural tube is not considered one of the tubes because it is not formed by body folding
Caudal Regression Syndrome AKA
Caudal Dysplasia
Caudal Regression Syndrome is?
- A disorder that impairs the development of the lower (caudal) half of the body
- this can include the lower limbs, lower back, and the lower gastrointestinal (gut) and urogenital (urinary and genital) tracts
The underlying mechanism of Caudal Regression Syndrome
- abnormal growth and migration during gastrulation resulting in abnormal development of the caudal mesoderm
- Which is the last mesoderm to form
The last mesoderm to form is?
Caudal Mesoderm
Caudal Dysplasia Risk Factors
Multiple genetic and environmental factors are implicated in caudal dysplasia including:
- maternal diabetes
- Wnt gene defects
- Vascular Abnormalities
- Teratogens
Sirenomelia AKA
Mermaid Syndrome
Sirenomelia is?
an extreme and rare form of caudal dysplasia but it may be a distinct disorder
Sirenomelia example
Embryonic Induction
- the stimulation of a specific developmental pathway in one group of cells (the responding tissue) by a closely approximated second group of cells (the inducing tissue).
- It is also referred to as instructive interactions
- In other words, a signal passes from the inducing to responding tissue, changing the fate of the responding tissue
Embryonic Induction AKA
Instructive Interactions
Embryonic Induction mechanisms
3 listed
- Diffusion of the inducing molecule from one cell to another
- Contact between the extracellular matrix of one cell to another
- Direct contact between the inducing and responding cells
Examples of embryonic Induction
4 listed
Primitive Streak Formation Embryonic induction
Mesoderm Formation Embryonic induction
Neural Induction Embryonic induction
Lens Induction Embryonic induction
Ectopic Induction
The period of susceptibility AKA
period of maximal sensitivity
The period of susceptibility
- is when the embryo is especially vulnerable to environmental factors causing abnormal development
- this occurs during weeks 4-8 of the embryonic period when organs are forming during organogenesis
- Insults during the first 3 weeks generally result in death of the embryo and spontaneous abortion
Congenital Anomaly
- something that is different or unusual at birth
- AKA birth defect
*
Congenital anomalies account for more than _____ of infant deaths
20%
Etiology of congenital anomailes
- 50% of etiology of congenital anomalies is unknown
- the other 50% is due to environmental factors, chromosomal defects, mutant genes, or are multifactorial
Possible mechanisms of congenital anomalies
4 listed
- Faulty embryonic induction (mutant signaling molecule genes)
- defects in apoptosis
- defects in migration
- developmental arrest
Period of susceptibility timeline
Causes of Congenital Anomalies
5 listed
Anomaly definition
means a marked deviation from normal
Association definition
nonrandom appearance of 2 or more anomalies together; cause is not known
Congenital definition
Present at birth
Deformations definition
result from mechanical forces that mold a part of the fetus over a prolonged period
Malformation definition
a structural defect in the body due to abnormal embryonic or fetal development
Syndrome Definition
Is a group of anomalies occurring together that have a common cause
Sequence Definition
is when a primary anomaly itself determines additional defects
Teratogen Definition
Is any agent or factor that can disturb the development of an embryo
