Embryonic and Fetal Periods - nikolai.lazarov.pronikolai.lazarov.pro/lectures/2014/medicine/embryology/03_Embryonic... · Embryonic and Fetal Periods of the Human Embryo Development

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


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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


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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


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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


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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


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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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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


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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


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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


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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


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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


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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


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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


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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


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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

Documents

Embryonic and Fetal Periods - nikolai.lazarov.pronikolai.lazarov.pro/lectures/2014/medicine/embryology/03_Embryonic... · Embryonic and Fetal Periods of the Human Embryo Development