Fetal Development Flashcards
Lasts for 8 weeks, organogenesis takes place
Embryonic period
Chorionic sac 1cm
True intervillous space formed
Fetal BV in chorionic villi appear
3rd week after ovulation
Chorionic sac 2-3cm
Embryo 4-5cm
Arm and leg buds present
Formed: PUC
Primitive heart
Umbilical cord
Cardiovascular system
4th week
Embryo 22-24mm
Head larger than trunk
Finger toes present
Complete:
Heart
Upper lip
6th week
External ears form definitive elevations on either side of head
6th wk
Earliest synapses in the spinal cord develop
6th
Consist of growth and maturation of structures that were formed during the embryonic period
Fetal period
8 weeks after fertilization or 10 weeks after onset of last menses
Fetal period
Embryo-fetys is nearly
4 cm long
UCC
Uterus is palpable (above symphysis pubis)
Crown-rump length 6-7cm
Centers for ossification appear
12 weeks
Fingers toes have become differentiated
Skin and nails develop
12 weeks
External genitalia start to show definitive signs of gender
Fetus begin to make spontaneous movements
12th
CRL (crown rump length) 12cm
Weight: 110g
16th week
Eye movement begin at
16-18 weeks with midbrain maturation
Weight >300g
Fetal skin = less transparent
Downy lanugo covers entire body
Developed scalp hairs
20 weeks
Weight 630g
Skin is wrinkled
Fat deposition
Eyebrows and eyelashes recognize
24th week
Canalicular period of lung development
24th week
CRL: 25cm
Weight: 1,100g
Thin skin is red and covered w vernix caseosa
90% chance of survival
28 weeks
CRL: 28cm
Weight: 1,800g
32 weeks
CRL: 32cm
Weight: 2,500g
Body more rotund bcs of deposition of subcutaneous fat
36 weeks
CRL: 36cm
Weight: 3,400g
Fully developed
40 weeks
Thickened disk-shaped growing to a diameter of 20cm
Placenta
Fetal portion is derived from
Chorion
One of the surrounding extraembryonic membranes of conceptus
Chorion
Derived from the region of endometrium that underlies the implantation site (the deciduas basalis)
Maternal portion
Development of the placenta: the EARLY blastocyst consists of a single layer of
Ectoderm
What consists of an inner cell mass that gives rise to the embryo and outer, single layer of trophoblast cells that encloses the blastocyst cabity
Late blastocyst
Blastocyst implants high up on the
Posterior wall of uterus
Following implantation, these become highly invasive and erode the endometrium
Trophoblast cells
As the blastocyst buries itself in the endometrium, it is covered by both
Trophoblast and decidua (the endometrium of the preg uterus)
One pole of the buried blastocyst extends towards the endometrial cavity and is covered by
Chorion frondosum and deciduas capsularis
The innermost pole ultimately forms the
Placenta
Contiguous with maternal decidua (and later maternal blood)
Syncytiotrophoblast
Outer layer
Syncytiotrophoblast
Syncytiotrophoblast has no individual cells instead it had an
Amorphous cytoplasm
without cell borders, and nuclei that are multiple and diverse
Innermost layer (embryonic side)
Cytotrophoblast
Cells nearest the intervillous space; well-demarcated cell border and a single nucleus
Cytotrophoblast
After implantation is complete, trophoblast further differentiates into:
- Villous trophoblasts
- Extravillous trophoblasts
• Interstitial
• Endovascular
Give rise to chorionic villi which transport oxygen and nutrients between fetus and mother
Villous trophoblasts
Invades deciduas and penetrates myometrium to form placental bed giant cells
Interstitial trophoblasts
Surrounds spiral arteries and prepare these vessels for endovascular trophoblast invasion
Interstitial trophoblasts
Surrounds spiral arteries and prepare these vessels for endovascular trophoblast invasion
Interstitial trophoblasts
Penetrates lumen of spiral arteries by intially forming cellular plugs then destroying vascular endothelium via an apoptosis mechanism then invading vascular media
Endovascular trophoblasts
Trophoblasts only invade
Spiral arteries
NOT VEINS
Endovascular invasion proceeds into 2 waves/stages
First wave (BEFORE 12 WKS POSTFERTILIZATION)
Second wave (BETWEEN 12 AND 16 WEEKS)
Consists of invasion and modification of spiral arteries up to the border between decidua and myometrium
First wave
Involves invasion of intramyometrial segments of spiral arteries
Second wave
Endovascular invasion of trophoblasts converts narrow-lumen muscular spiral into
Dilated, low resistance uteroplacental vessels (also important in pathogenesis of pre eclampsia and fetal growth restriction
Invades most superficial portion of endometrium, spiral arteries and arterioles are invaded and destroyed
Cytotrophoblasts
As endometrial invasion proceeds, maternal BV are trapped to form
Lacunae (lakes) - which soon fill w maternal bld
Invades the endometrium and become surrounded by lacunae
Irregular protrusions of syncytiotrophoblast
Finger-like extensions of the cytotrophoblast grow into these protrusions forming structures called
Primary placental villi
Distinguished on about 12th day after fertilization
Chorionic villi
When the solid trophoblastic column is invaded by a mesenchymal core, these are formed
Secondary villi
After angiogenesis occurs in situ from mesenchymal core, the villi are
Tertiary
Maternal arterial blood enters the intervillous space by day
14 or 15 postfertilization
Fetal and maternal BV of the embryo are fxnal and placental circulation is est
By about 17th day
The feto-placental circ is completed when BV of embryo are connected w the chorionic vessels wc are formed in situ from
cytotrophoblasts
Chorionic plate
Roof of intervillous spaces
Basal plate
Floor of intervillous spaces
Are villi of the chorion frondosum extending from the chorionic plate to the deciduas
Anchoring villi
most villi arbores and end freely in the
intervillous spaces
supplied w a chorionic artery and vein
cotyledon
consist of decidual tissue w trophoblasts and are of fetal and maternal origin
placental septa
through the 2 umbilical arteries, then into the capillaries of the villi and finally back thru a single umbilical vein into the fetus
fetal blood flow
branches of the umbilical vessels that traverses along the surface of the placenta
placental surface or chorionic vessels
Chorionic ___ always cross over chorionic _____
Chorionic artery always cross over chorionic veins
what are perforating branches of the surface arteries that pass thru the chorionic plate
truncal arteries
placental circulation
intervillous spaces –> chorionic plate –> basal plate –> uterine veins –> placenta
maternal blood enters intervillous spaces in spurts propelled by
maternal blood pressure
this force blood in discrete streams towards the chorionic plate until the head of pressure is
reduced then lateral spread then occurs
arterial bld exits basal plate and drains thru
uterine veins
TO the uterine wall, what are the arteries and veins
arteries are perpendicular to uterine wall
veins are parallel to uterine wall
6 components of placental barrier lying between the maternal and fetal circulations are:
- syncytiotrophoblasts
- cytotrophoblast
- trophoblastic basement membrane
- fetal loose CT
- endothelium of fetal capillaries
- its surrounding basement membrane
at full term, the cytotrophoblast is represented only as
fragments (hence the placental barriers consists only 5 fetal compo)
early months of pregnancy, placental mem is still
thick and not fully dev
therefore, permeability is low
surface area is slight
later pregnancy
increase permeability
thinning mem
SA expands
increase placental diffusion
dissolved O2 in bld of large maternal sinuses passes into fetal bld by
simple diffusion, driven by an O2 pressure gradient
3 reasons why fetus is able to obtain sufficient O2 to maternal blood
- HbF carry more O2 (20-50%)
- Hb conc of fetal is 50% greater than mom’s (enhancing the amount of O2 transported to fetal tissues)
- Bohr effect (Hb can carry more oxygen at a low pCO2 than it can at a high pCO2)
loss CO2 makes FETAL blood
more alkaline
increased CO2 in MATERNAL blood makes it:
more acidic
diffusion of CO2
thru simple diffusion
diffusion of food stuff
thru facilitated diffusion
before 5 weeks, HCG is produced in
both syncytio and cytotrophoblasts
later, whne maternal serum is at its peak
HCG prod in syncytiotrophoblast onlee hehehehehekapoyaperolabanlaaaaaaang
HCG detected in plasma of preggy
7-9 dayz (after the midcycle surge of LH that precedes ovulation
HCG maximal lvls attained at abt 8-10 weeks
(peak 100,000 mIU/mL between 60th and 80th days after menses)
levels begin to decline by
10-12 wks and nadir by 16 wks
if corpus luteum is removed before approx 7th week of preg (sometimes 12th wk)
spontaneous abortion
the corpus luteum involutes slowly after
13th-17th week
has GH-like bioactivity; demonstrable in placenta within 5-10 days after conception in maternal serum as early as 3 wks after fertilization
HPLactogen or HCS
It is demonstrated in cytotrophoblasts and later concnted in syncytiotrophoblast
Before 6 weeks
Rate of secretion is proportional to placental mass
Therefore lvls rise steadily until 34-36 weeks
HPL actions
- Lipolysis
- Anti insulin
- Potent angiogenic hormone
Demonstrated in CL, decidua and placenta
Relaxin
Rise jn maternal circulating relaxin
Early pregnancy
May act on myometrium to promote relaxation and quiescence of early pregnancy
Relaxin
Enhancement of GFR
Relaxin
Large amnts secreted by syncytiotrophoblast during pregnancy
Estrogen
Serve as estrogen precursors
C19 steroids (DHEA-S dehydroepiadrosterone and its sulfate)
Quantitatively the most important source of placental estrogen precursors in preg
Fetal adrenal gland
Effects of estrogen
1 enlarge maternal uterus, breast and ductal structures, external genitalia
2 relax the pelvic ligaments (prep for delivery)
3 may affect fetal dev (rate of cell reprod in early embryo)
Principal precursor of progesterone biosynthesis
Maternal plasma CHOL
Progesterone effects
1 causes decidual cells to develop (embryo nutrition)
2 decrease uterus contractility
3 dev and conceptus even b4 implantation by increasing secretions of fallopian tubes and uterus to provide appropriate nutritive matter for developing morula and blastocyst
4 helps estrogen prepare mom’$ breasts for lactation
By _______, amniotic fluid consists largely of extracellular fluid that diffuses thru fetal skin and thus reflects compo of fetal plasma
2nd trimester
After _____, composed largely of fetal urine ew ans smol proportion of pulmonary fluid
20wwwks
Volume increases from 10mL/week at 8 weeks to
60mL/week at 21 weeks ans peaks at 34 weeks
