Week 3: Pregnancy 1 Flashcards
What are the key events of early pregnancy
- Fertilization
- Implantation
- Secretion of HCG and HCG rescue of the corpus luteum
What is embryonic age?
calculated from the date of conception
What is gestational age?
calculated from the LMP (menstrual gestational age) or by ultrasound (sonographic gestational age)
Describe the events from fertilization to implantation
Describe the process of fertilization of the ovum
- Require mature spermatozzon and a mature oocyte
- Capacitation of the sperm occurs during their ascent
- Usually takes place within 24-hours of ovulation in the ampulla of the fallopian tube
- The acrosome reaction occurs with dissolution of the spermatic membrane and the zona pellucida of the ovum
- With impregnation of the oocyte by a spermatozoon polyspermy block follows
When & where does fertilization of the ovum usually occur?
usually takes place within 24-hours of ovulation in the ampulla of the fallopian tube
What is the acrosome reaction?
- During the acrosome reaction the contents of the acrosome are released outwardly. The cell membrane of the spermatozoon fuses with the outer membrane of the acrosome. The contents of the acrosome flow out through the resulting pores.
- A prerequisite for the success of the acrosome reaction is the previous binding of the spermatozoon to the pellucid zone.
- The enzymes that are released in the immediate vicinity of the pellucid zone by the acrosome reaction dissolve it locally and thus create a way through it for the sperm cells. A number of enzymes that have been released are involved. The best known are the already mentioned hyaluronidase and acrosin, whereby the acrosin makes it possible for the spermatozoa to get through the pellucid zone.
What stage of division occurs in fertilization of the ovum?
Metaphase II is completed
What events occur in fertilization of the ovum
- acrosome reaction
- polyspermy block
- metaphase II is completed
- the second polar body is extruded and the ovum begins to cleave and move toward the uterus
- Paternal and maternal pronuclei are formed
- At about 22 hours post-fertilization the zygote arises
- First cell division takes place in the mitotic spindle
- With the creation of 2 daughter cells the fertilization is complete
- @ day 4 it is called a morula (latin for mulberry) which is the 16-32 cell stage
- @ day 4-5: Na+/K+ ATPase pumps deliver Na+ into the interior of the blastocyst, crating an osmotic gradient/fluid filled cavity in the morula
- @ the end of day 4 the blastocyst, approximately 100 cells arives in the uterine cavity
When does implantation occur?
~ day 5-6 after fertilization it implants at the wall of the uterus
What does the blastocyst need to implant into the uterus?
- Needs a receptive endometrium which is dependent upon a low estrogen/high progesterone ratio
The blastocyst layers
The blastocyst divides into two cell layers:
- Embryoblast
- Trophoblast
What is the embryoblast?
- one of the cell layers of a blastocyst
- embryoblast cells are in a cluster as the inner cell mass and become the embryo
What is the trophoblast?
- one of the cell layers a blastocyst divides into
- The outer rim of cells (trophoblast) invades the endometrium and contributes to the fetal portion of the placenta
What is the decidua?
The maternal part of the placenta
Rule of Twos - Events of pregnancy in the 2nd week
“Rule of twos”
- The trophoblast has two layers
- cytotrophoblast
- syncytiotrophoblast
- The embryoblast differentiates into two layers
- hypoblast
- epiblast
- There are two cavities
- amniotic cavity
- yolk sac
Trophoblast in the second week
The trophoblast has two layers
- cytotrophoblast
- syncytiotrophoblast
embryoblast in the 2nd week
The embryoblast differentiates into two layers
- hypoblast
- epiblast
Cavities in the 2nd week after fertilization
- Amniotic cavity
- Yolk sac
Key events in week 2 after fertilization
“Rule of twos”
- The trophoblast has two layers
- cytotrophoblast
- syncytiotrophoblast
- The embryoblast differentiates into two layers
- hypoblast
- epiblast
- There are two cavities
- amniotic cavity
- yolk sac
- Day 8: The syncytiotrophoblast starts secreting hCG
- Day 10: The hCG tells the corpus luteum that fertilization has occurred, hCG rescues the corpus luteum from regression so that it can function to secrete progesterone until 12 weeks
What prevents the corpus luteum from degenerating and when does this occur?
The syncytiotrophoblast layer of the trophoblast starts secreting hCG in the 2nd week (~day 8) which “rescues” the corpus luteum from regression so that it can continue to secrete progesterone until 12 weeks when the placenta takes over
What do pregnancy tests detect?
hCG
When is hCG detectable?
hCG is detectable in maternal urine or blood as early as 9 days after ovulation which is even before the expected menses
How can hCG be used to evaluate pregnancy?
Can be quantified in the blood “beta” to assistant in evaluation of pregnancy viability and/or ectopic pregnancy (approximately doubles every 24 hours)
What is β-hCG?
β-hCG is quantitative test for hCG which can be used in assisting evaluation of pregnancy viability and/or ectopic pregnancy
How much does hCG increase as the pregnancy continues?
approximately doubles every 48 hours
What is the decidua basalis?
the maternal portion of the placenta
Describe formation of the decidua basalis
At the implantation site, the endometrium under stimulation of progesterone forms the decidual layer to support the conceptus causing these changes:
- increased vascularity and thickness
- tortuous, secretion-filled glands
The decidua envelops the inner-mass that becomes the forming embryo
Fertilization occurs how long after ovulation
1 day
Entrance of the blastocyst into the uterine cavity occurs how long after ovulation?
4 days
Implantation occurs how long after ovulation?
5 days
Formation of the trophoblast and attachment to the endometrium occurs how long after ovulation?
6 days
Onset of trophoblast secretion of hCG occurs how long after ovulation?
8 days
hCG “rescue” of the corpus luteum occurs how long after ovulation?
10 days
Third week after fertilization: Rule of Threes
-
3 germ layers after gastrulation
- ectoderm
- mesoderm
- endoderm
- Further three subtypes for ectoderm and mesoderm
What process dominates the 3rd week after fertilization?
Gastrulation
Key events in the 3rd week after fertilization
- Days 14-15: Formation of the primitive streak (midline invagination)
- Day 18: Epiblast cells migrate inward through the primitive streak
- Top layer (doesn’t migrate): Ectoderm
- Middle layer: Mesoderm
- Bottom layer: Endoderm
Which epiblast layer doesn’t migrate through the primitive streak?
Top layer (Ectoderm)
3rd week ectoderm types
- Surface ectoderm
- Neuroectoderm
- Neural crest cells
Week 3 after fertilization: surface ectoderm
forms epidermis skin, mouth & olfactory, Rathke pouch, mammary, sweat and salivary glands
Week 3 after fertilization: Neuroectoderm
Forms CNS and retina
Week 3 after fertilization: Neural crest cells
form peripheral nervous system, parafollicular (C) cells thyroid, chromaffin cells adrenal medulla, role conotruncal endocardial cushion formation, facial structures
Week 3 after fertilization: endoderm types
- The gut
- gut-associated organs (eg pancreas)
- Some endocrine glands (thyroid follicles, parathyroid)
Week 3 after fertilization: mesoderm types
- Sclerotome (bone)
- Myotome (muscle)
- Dermatome (skin structures)
Rule of three summary for week 3
4th week after fertilization: Rule of four
- 4 limb buds begin to grow
- 4 chamber heart w/ heartbeat
When does the neural tube close?
~ day 28
Key events for the 4th week after fertilization
- 4 limb buds begin to grow
- 4 chamber heart with a heart beat
- Neural tube closure
Abnormal neural tube closure associations
heavily associated with folic acid metabolism
Normal closure of the neural tube gives off
The spinal cord
Forms of neural tube defects
- Anencephaly
- Exencephaly
- Spina bifida
- meningocele
- Meningomyelocele
- Spina bifida occulta
What is exencephaly?
Exencephaly, is a type of cephalic disorder wherein the brain is located outside of the skull. This condition is usually found in embryos as an early stage of anencephaly. As an exencephalic pregnancy progresses, the neural tissue gradually degenerates. The prognosis for infants born with exencephaly is extremely poor.
What is anencephaly?
Anencephaly is the absence of a major portion of the brain, skull, and scalp that occurs during embryonic development. It is a cephalic disorder that results from a neural tube defect that occurs when the rostral (head) end of the neural tube fails to close, usually between the 23rd and 26th day following conception.
What is spina bifida meningocele?
In a form of spina bifida called meningocele, the protective membranes around the spinal cord (meninges) push out through the opening in the vertebrae, forming a sac filled with fluid. But this sac doesn’t include the spinal cord, so nerve damage is less likely, though later complications are possible.
What is spina bifida meningomyelocele?
In individuals with myelomeningocele, the unfused portion of the spinal column allows the spinal cord to protrude through an opening.
What is spina bifida occulta?
Spina bifida occulta is when a baby’s backbone (spine) does not fully form during pregnancy. The baby is born with a small gap in the bones of the spine
Describe the process of neurulation
Formation of the primitive streak occurs how long after ovulation?
14-15 days
Epiblast cells migrate inward through the primitive streak occurs how long after ovulation?
16 days
Neural tube closure occurs how long after ovulation?
28 days
Types of abnormal fertilization and implantation
- Multiple gestation
- Ectopic pregnancy
Multiple gestation is a problem with
abnormal fertilization
Ectopic pregnancy is a problem with?
Abnormal implantation
Describe Multiple gestation
Basically .. . . . . twins or triplets etc.
Types of twins
- Identical
- Fraternal
Identical twins AKA
Monozygotic twins
Fraternal twins AKA
Dizygotic twins
Describe how monozygotic twins arise
Describe how dizygotic twins arise
What is chorionicity?
of placentas
What is Amnionicity?
of cavities
The # of cavities in twins is referred to as?
Amnionicity
The # of placentas in twins is referred to as?
Chorionicity
Dizygotic twins amnionicity and chorionicity
- Dizyogtic twins will virtually always be Di/Di (Dichorionic/Diamniotic)
Monozygotic twins amnionicity and chorionicity
4 listed
Possible combinations:
- Di/Di (Dichorionic/Diamniotic)
- Mo/Di (Monochorionic/Diamniotic)
- Mo/Mo (Monochorionic/Monoamniotic)
- Conjoined
Describe Di/Di
Describe Mo/Di
Describe Mo/Mo
Describe conjoined
When& how is chorionicity/amnionicity determined?
It is most reliably determined between 6-10 weeks by counting the number of gestational sacs and evaluate the thickness/structure of the dividing membrane
Why is determining Chorionicity/Amnionicity important?
Classifying multiple gestation as monochorionic or dichorionic is essential for management
Di/Di twins cleavage
days 1-3
Di/Di twins percentage monozygotic
25-30%
Di/Di twins gestational sacs
2
Di/Di twins yolk sacs
2
Di/Di twins amniotic cavities
2
Di/Di twins ultrasound
Di/Di twins placentation
Mo/Di twins placentation
Mo/Di twins ultrasound
Mo/Di twins percentage monozygotic
70-75%
Mo/Di twins gestational sacs
1
Mo/Di twins yolk sacs
2
Mo/Di twins amniotic cavities
2
Mo/Di twins cleavage
days 4-8
Mo/Mo twins cleavage
Day 8-13
Mo/Mo twins placentation
Mo/Mo twins ultrasound
Mo/Mo twins yolk sacs
1
*although this is nearly always true, there are case reports of two yolk sacs in early pregnancy in twins later confirmed as monoamniotic*
Mo/Mo twins amniotic cavities
1
Describe Ectopic pregnancy
Any pregnancy in an abnormal location (ie not implanted in the uterus on the endometrium)
Ectopic pregnancy etiology
Caused by delayed transport of the blastocyst
Ectopic pregnancy clinical presentation
Clinical triad:
- Amenorrhea
- vaginal spotting
- abdominal pain +/- adnexal mass
Risk factors for Ectopic pregnancy
8 listed
Diagnosis of Ectopic pregnancy
Work up for Ectopic pregnancy
- (+) hCG
- Abdominal/pelvic exam
- Ultrasound
- localize pregnancy
- if hCG > 1500 should see intrauterine gestational sac and yolk sac means IUP
- evaluate for free fluid
- evaluate for adnexal mass
Ectopic pregnancy Ultrasound
Ultrasound
localize pregnancy
if hCG > 1500 should see intrauterine gestational sac and yolk sac means IUP
evaluate for free fluid
evaluate for adnexal mass
Tx of Ectopic Pregnancy
- Medication
- Surgery
Medication Tx of ectopic pregnancy
Methotrexate (MTX)
MTX AKA
Methotrexate
Methotrexate MOA
- Folic acid antagonist
- deactivates dihydrofolate reductase
- leads to decreased tetrahydrofolate levels (which is needed for DNA/RNA synthesis)
- Disrupts rapidly dividing trophoblastic cells
- deactivates dihydrofolate reductase
Surgical Tx of Ectopic Pregnancy
*
When is Surgery for ectopic pregnancy necessary?
Surgery is necessary if:
- Ruptured or unstable
- Advanced gestation
- Cardiac activity
- less likely to respond to MTX)
What is a Salpingectomy?
Salpingectomy is the surgical removal of one (unilateral) or both (bilateral) fallopian tubes. Fallopian tubes allow eggs to travel from the ovaries to the uterus.
What is a salpingostomy?
surgical unblocking of a blocked fallopian tube.
Question 1
D. Blastocyst
Question 2
C. Di/DI
Question 3
A. Mo/Di
Adnexae
The adnexae is ovary and tube (you shouldn’t see a normal fallopian tube)
this is an ectopic pregnancy, swollen tube and non-viable embryo
What is the diagnosis?
Abdominal pregnancy
