embryonic and fetal periods -...
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Embryonic and Fetal Periods of the Human Embryo Development
1. Embryonic period (organogenesis): derivatives of ectodermal germ layer (neurulation)
derivatives of endodermal germ layer
derivatives of mesodermal germ layer
2. Fetal period
3. Fetal membranes: yolk sac and allantois
amnion and amniotic fluid
connecting stalk and umbilical cord
4. Placenta and embryonic circulation
Prof. Dr. Nikolai Lazarov
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Embryonic period 3rd-8th week
(period of organogenesis)
Development of the germ layers
Primary axial rudiments
Germ layers Pander, von Baer:
ectoderm (ectoblast)
endoderm (endoblast)
mesoderm (mesoblast)
mesenchyme lateral mesoderm
Prof. Dr. Nikolai Lazarov
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Ectodermal derivatives: nerve tissue, incl. retina and neurohypophysis
epidermis of the skin and melanocytes hairs and nails
sweat and sebaceous glands
mammary glands
epithelium of the oral cavity and anus
enamel of the teeth
epithelium of the nasal cavity and paranasal sinuses
some endocrine glands: adenohypophysis
pinealocytes of the epiphysis
adrenal medulla
chromaffin cells
lens and cornea of the eye, ciliary muscle
epithelium of the external auditory canal
myoepithelial cells of the sweat glands
Ectodermal germ layer:
neuroectoderm skin ectoderm
Derivatives of the ectoderm
Prof. Dr. Nikolai Lazarov
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formation of neural tube (neurulation)
begin of the process E17
neural (primary embryonic) induction signaling molecules (growth factors) from the underlying notochord:
neural plate
neural groove
neural fold
neural tube CNS
neural crest ganglion ridge PNS
transverse segmentation of neural tube: cranial neuropore 25
caudal neuropore 27
embryonic origin: neuroectoderm
Neurulation
Prof. Dr. Nikolai Lazarov
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Cranial crest cells: craniofacial skeleton
connective tissues bones of craniofacial structures neurons for cranial ganglia
V, VII, IX and X autonomic ganglia glial cells melanocytes meninges (pia and arachnoid)
of the brain
Trunk crest cells: dorsal pathway:
melanocytes in skin and hair follicles
ventral pathway: neurons for spinal ganglia sympathetic and enteric neurons Schwann cells cells of the adrenal medulla cells of the conotruncal cushions of the heart meninges (pia and arachnoid) of the spinal cord
Neural crest cells:
craniofacial mesenchyme
Neural crest derivatives
Prof. Dr. Nikolai Lazarov
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Endodermal derivatives: epithelial lining of respiratory tract epithelial lining of primitive gut and
portions of allantois and vitelline duct parenchyma of:
liver pancreas large salivary glands some endocrine glands:
thyroid parathyroid islets of Langerhans in the pancreas
reticular stroma of: tonsils thymus
epithelial lining of the urinary bladder and of the urethra
epithelial lining of the tympanic cavity and auditory tube
Derivatives of the endoderm
Endodermal germ layer:
gastrointestinal tract respiratory system some endocrine glands
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Derivatives of the mesoderm
Mesodermal germ layer development 17th day:
paraaxial mesoderm future somites
intermediate mesoderm future excretory units
lateral plate mesoderm: parietal (somatic) mesoderm layer visceral (splanchnic) mesodermal layer intraembryonic
cavity (coelom)
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Paraxial mesoderm Epimer beginning of the 3rd week of development:
segments somitomeres first appear in the cephalic region
formation in craniocaudal sequence
consist of mesodermal cells
in cranial region neuromeres
somites beginning of the 4th week
sclerotome mesenchyme
vertebral column (cartilage and bone components)
limb and body wall musculature
dura mater
myotome
epaxial musculature(muscles of the back)
skeletal and cardiac muscle tissue
dermatome
dermis of the skin
subcutaneous tissue
Prof. Dr. Nikolai Lazarov
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Intermediate mesoderm Mesomer:
urogenital structures:
kidney and nephric tubules
Wolffian duct and derivatives
Mllerian duct and derivatives
endocrine part of gonads
adrenal cortex
cervical and upper thoracic region:
nephrotomes
caudal regions:
nephrogenic cord
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Prof. Dr. Nikolai Lazarov
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Lateral plate mesoderm Hypomer:
parietal (somatic) mesodermal layer:
lateral and ventral body wall
serous (mesothelial) membranes
peritoneum
pleura
pericardium
visceral (splanchnic)
mesodermal layer:
heart
blood vessel
endothelium
spleen
part of the
bladder epithelium
Prof. Dr. Nikolai Lazarov
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mesenchyme beginning of the 3rd week:
smooth muscle tissue
connective tissue
blood
blood vessels
hemopoietic tissue
Derivatives of the mesenchyme
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External appearance during the embryonic period
beginning 20-21 days:
external features somites pharyngeal arches
age of the embryo CRL (crowm-rump length in mm)
formation of organs formation of limbs, face,
ears, nose and eyes establishment of body form
Prof. Dr. Nikolai Lazarov
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flat embryonic disc: cephalocaudal folding
head fold
tail fold
lateral folding rounded body form
head region: pharyngeal arches
somites
organogenesis up to E56
formation of limbs:
the end of 4th week
upper limbs
lower limbs
digits:
hand 50-51
foot 52-53
Establishment of body form
beginning 20-21 days:
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Development of the fetus: quantitative changes
length of the fetus crown-rump length (CRL)
crown-heel length (CHL)
weigth of the fetus
Growth of the head: 3rd month head size =~ CRL
5th month head size =~ CHL
at birth head size =~ CHL
Fetal period Fetal period 9th week to birth
maturation of tissues and organs rapid grow of the body
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Monthly changes:
3rd month: face (eyes, ears)
become more human-looking limbs rich their relative length external genitalia liver major hemopoietic organ
4th-5th month: fetus lengthens rapidly
fetal movements recognized
fine hair lanugo, vernix caseosa
eyebrows and head hair visible
fetus weight less than 500 g
6th month: organ systems are able to function
fetus weight ~800 g
7th month weight ~1300 g 8-10th months:
organ systems specialization and coordination adaptation to the environment conditions
Fetal period
Prof. Dr. Nikolai Lazarov
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Embryonic membranes Fetal membranes:
membranous structures that surround the developing fetus
provide for its nutrition, respiration, excretionand protection
yolk sack: hypoblast derivative the first element seen in the
gestational sac during pregnancy at 5 weeks gestation
soon disappears complete obliteration during the 7th week
forms the primitive gut main function hemopoiesis
(up to 7th-8th week) source of gonoblasts genital ridges
allantois, Gr. alas, alantos, sausage: a membranous sac appears at ~16th day from the
posterior wall of the yolk sac reduction at the 2nd month
rudimentary in humans forms part of the urinary bladder in the umbilical cord urachus main function helps the embryo
exchange gases handle liquid waste
Prof. Dr. Nikolai Lazarov
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amnion, Gr. amnos, , a lamb: epiblast derivative amniotic cavity amniotic sac
amniotic fluid, liquor amnii: clear, watery fluid, derived
primarily from maternal blood amount at birth 600 ml
oligohydramnios (less than 400 ml) hydramnios (1500-2000 ml
excess of amniotic fluid)
its volume is replaced every 3 h main functions:
absorbs jolts prevents adherence of the embryo helps protect the fetus from heat loss allows for fetal movements
amniocentesis diagnosis of chromosomal problems baby's genetic health
Fetal membranes:
the allantois, amnion and chorionidentify humans as amniotes
Embryonic membranes
Prof. Dr. Nikolai Lazarov
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chorion, Gr. , skin, leather:
formed by extraembryonic mesoderm
and the two layers of trophoblast
consists of two layers:
an inner formed by the somatic mesoderm
an outer formed by the primitive ectoderm trophoblast
cytotrophoblast (layer of Langhans)
syncytiotrophoblast
rapid proliferation
chorionic villi
parts:
chorion lve (non-placental part)
chorion frondosum (bushy chorion)
placenta
Fetal membranes:
membranes that exists during pregnancy between the developing fetus and mother
Embryonic membranes
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formation and development: develops from and contains
remnants of the yolk sac and allantois
primitive umbilical ring 5th week:
connecting stalk
yolk stalk (vitelline duct)
vitelline vessels
primitive umbilical cord composition:
distally
the yolk sac stalk
umbilical vessels
proximally
some intestinal loops
remnant of the allantois and its vessels
amnion
Umbilical cord birth cord or funiculus umbilicalis:
the connecting cord from the developing embryo or fetus to the placenta
Prof. Dr. Nikolai Lazarov
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definitive umbilical cord: composition:
Whartons jelly a mucous tissue that protects and insulates vessels
two umbilical arteries
one umbilical vein
amnion
functions: the umbilical vein supplies the fetus
with oxygenated, nutrient-rich blood from the placenta
the umbilical arteries return thedeoxygenated, nutrient-depleted blood
abnormalities: false knots, longer or shorter cord
missing artery 20% chance of havingcardiac and other vascular defects
umbilical cord at birth:
length 50-60 cm (10-120 cm) diameter 1.5-2.5 cm weight 100-150 g
Umbilical cord
Prof. Dr. Nikolai Lazarov
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placentation: formation, type and structure,
or arrangement of the placenta
3rd week 3rd month (~12-13 weeks)
formation and growth:
syncytiotrophoblast trophoblastic lacunae
primary villi:
cytotrophoblastic core
syncytiotrophoblast covering
secondary and tertiary (definitive) placental villi 3rd week
cotyledons total number 200
lacunar (intervillous) spaces mothers blood
Placenta placenta, Gr. , flat, slab-like; Lat. cake:
an organ that connects the developing fetus to the uterine wall
allows nutrient uptake, waste elimination andgas exchange via the mother's blood supply
Prof. Dr. Nikolai Lazarov
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fetal portion:chorion frondosum
s. villosum
bordered by thechorionic plate
covered by theamnion
maternal portion:bordered by the
decidua basalis decidual plate
decidual septa 4th-5th month
cotyledons
junctional zonetrophoblast and
decidua cells
Structure of the placenta beginning of the 4th month:
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Location of placenta in the uterine wall:
normal location:
anterior uterine wall
posterior uterine wall
fundal at the top of the uterine wall
abnormal locations:
placenta accreta too deep in uterine wall
placenta previa near the opening of the uterus
Full-term placenta: discoid shape, placenta discoidalis diameter 15-20 cm thickness 2-3 cm weight ~ 500-600 g
Placenta
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main functions:
exchange of gaseous products by simple diffusion
O2, CO2 and CO
exchange of nutrients and electrolytes (nutrition)
mediated by proteins called nutrient transporters
amino acids
free fatty acids
carbohydrates
vitamins
transfer of waste products
transmission of maternal
antibodies (passive immunity)
maternal immunoglobulin G (IgG)
hormone production
steroids and proteins
progesterone 4th month
estrogenic hormones (predominantly estriol)
human chorionic gonadotropin (hCG)
somatomammotropin (placental lactogen)
Functions of the placenta
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Thank you