Revision on General Embryology 1

Weeks of Development

Dr.Sherif Fahmy

First Week1- Fertilization.2- Cleavage.3- Migration.4- Formation of morula and blastocyst.

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Second Week1- Implantation of the blastocyst.2- Changes in blastocyst to form chorionic vesicle.

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Third Week1- Formation of 3 types of chorionic velli from the chorion.2- Gastrulation which is formation of trilaminar disc.

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Organogenesis (4th -8th Week)

1- Development of ectoderm, mesoderm and endoderm.2- Folding.

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Formation of Fetal Membranes

1- Placenta.2- Amniotic cavity.3- Yolk sac.4- Umbilical cord.5- Chorion.

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First Week of Development

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FERTILIZATION

• It is the process by which a sperm units with the ovum.

• Site: It occurs at ampullary part of uterine tube (outer 1/3 of uterine tube)

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Sperm & Oocyte

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Corona radiata

Zona pellucida

Cell membrane

Secondary oocyte arrested in 2nd meiotic division

Head

Neck

Middle piece

Tail

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FERTILIZATION

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Dr.Sherif Fahmy

Results of Fertilization• Formation of zygot.• Restoration of diploid number (46

chromosomes).• Determination of sex.• Cleavage (segmentation) starts, during

which the zygote travels through uterine tube by help of cilia and contraction of uterine tube.

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Dr.Sherif Fahmy

Cleavage & Migration

(Page 14)Dr.Sherif Fahmy

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Blastocele

Inner cell mass (Embryoblast) Outer cell mass

(Trophoblast)

Embryonic pole

Abembryonic pole Blastocyst

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Second Week of PregnancyDr.Sherif Fahmy

Implantation• Dif.: It is the process by which

blastocyst is embedded in the endometrium.

• Timing: Starts at 7th day and completed at 11th day.

• Site: usually at upper part of posterior wall of uterus near fundus.

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Steps of Implantation• Blastocyst fixes its embryonic pole to

site of implantation.• Trophoblast cells at embryonic pole

ptoliferate to form outer layer of syncytiotrophoblast.

• Syncytiotrophoblast form proteolytic enzymes that erodes endometrium to form implantation cavity.

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-Blastocyst enters the implantation cavity.-Endometrium after implantation is called decidua.-Site of penetration is closed by fibrin clot (coagulum) at 9th day.-Surface epithelium overgrows the fibrin clot at 11th day.

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Outer cell mass (trophoblast)

Endometrial arteriol

Endometrial gland

Blastocyst

Fixation of embryonic pole to implantation site

Endometrium

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Endometrium

Syncytiotrophoblast

Blastocyst

Cytotrophoblast

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Syncytiotrophoblast

Cytotrophoblast

Trophoblast

Amniotic cavity

Primary yolk sac

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Dr.Sherif Fahmy

Abnormal Sites of Implantation• 1- Ectopic pregnany: They are tubal,

ovarian or omental (peritoneum).• 2- Placenta praevia: Parietalis,

marginalis and centralis.

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Ectopic Pregnancy

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Placenta Praevia

Placenta praevia parietalis

Normal implantation site

Placenta praevia marginalis

Placenta praevia centralis

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Decidua• It is endometrium after implantation which

is sheded after birth of fetus.• Character of endometrium:• Increased secretory function of uterine glands.• Decidual cells are stromal cells filled with

glycogen.• Arteries become more spiral with arterio-

venous anastomosis.

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Parts of decidua:• Decidua basalis: It is the part of decidua

between blastocyst and myometrium. It forms the fetal part of placenta.

• Decidua capsularis: It covers the blastocyst except embryonic pole and separates it from uterine cavity.

• Decidua parietalis: It is the rest of endometrium that lines the rest of uterine cavity.

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Decidua basalisDecidua capsularis

Decidua parietalis

Uterine cavity

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Fate of decidua:• Decidua basalis shares in the

formation of placenta.• Decidua capsularis and parietalis

fuse together and shedded with placenta after delivery.

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Decidua basalis

Decidua parietalis

Decidua capsularis

Uterine cavity

Fused decidua parietalis and capsularis

Decidua basalis

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Changes of Blastocyst in the Second Week of

PregnancyDr.Sherif Fahmy

Blastocele

Inner cell mass (Embryoblast) Outer cell mass

(Trophoblast)

Embryonic pole

Abembryonic pole Blastocyst

6th day

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7th day:

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8TH Day of PregnancyEndometrium

Syncytiotrophoblast

Cytotrophoblasts Hypoblasts

Amniotic cavity

Amnioblasts

Epiblast

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9th & 10th days

Fibrin clotPrimary yolk sac

Heuser’s membrane

Lacunar spacesSyncytio-trophoblast

Endometrial arteriol

Hypoblast

Amniotic cavity

Epiblast

Amnioblast

Cyto-trophoblast

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11th & 12th daysBlood inside lacunae

Endometrial sinusoid

Syncytio-trophoblast

Endometrial epithelium

Extraembryonic mesoderm

Primary yolk sac

Large spaces

Amniotic cavity

Cytotrophoblast

Bilaminar embryonic disc

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11th & 12th days

Chorionic Vesicle

Extra- embryonic mesoderm

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13th day

Endodermal cells

Secondary yolk sac

Exocoelomic cyst

Somatic mesoderm

Connecting stalk

Somatic mesoderem

Splanchnic mesoderm

1ry chorionic villi

Extra-embryonic coelom Chorionic cavity)

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Cyto-trophoblast

Amniotic cavity

Chorionic Vesicle

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3rd Week of PregnancyA- Changes in the chorion.B- Changes in the embryonic disc.

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13th day

Somatic mesoderm

1ry chorionic villiDr.Sherif Fahmy

Cyto-trophoblastChorionic

Vesicle

Intervillous space filled with maternal blood

Syncytiotrophoblast

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Chorionic Vesicle (at the end of 3rd week)

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Changes in the chorionFormation of chorionic velli:1- Primary velli.2- Secondary velli.3- Tertiary velli

A-Chorion frondosum.B-Chorion leave.

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Dr. Sherif Fahmy

Syncytiotrophoblast

Cytotrophoblast

Primary chorionic velli

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Dr. Sherif Fahmy

Secondary chorionic velli

Syncytiotrophoblas

Cytotrophoblast

Somatic mesoderm

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Dr. Sherif Fahmy

Fetal blood vessels

Tertiary velli (chorion frondosum)

Tertiary velli (chorion leave)

Cytotrophoblastic shell

Tertiary chorionic velli

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Decidua basalis

Chorion frondosum

Chorionic plate

Chorion leave

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Development of Trilaminar Embryonic

Disc

GastrulationDr.Sherif Fahmy

GastrulationIt begins by formation of:1- Primitive streak.2- Primitive node.3- Invagination.3- Bucco-pharyngeal membrane.4- Cloacal membrane.

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Amniotic cavity

Buccopharyngeal membrane

Primitive node & pit

Primitive streak

Cloacal membrane

Yolk sac

Hypoblast

Epiblast

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Invagination- Epiblast cells migrate to primitive streak.-Then they pass beneath epiblast to become flask-shaped and separated from the epiblast and form:1- Endoderm that replaces the hypoblast.2- Third layer between epiblast and endoderm which consists of intra-embryonic mesoderm with notochord in the median region..3- Remaining epiblast cells after formation of notchord and intraembryonic mesoderm will be named ectoderm.Dr.Sherif Fahmy

Primitive streakPrimitive node and pit

Epiblast

Hypoblast

Invaginating cells from epiblast lyaer

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Formation of Notochord

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Notochordal process

Extension of primitive pit into

prenotochordal process

Primitive streak

Connecting stalk

Allantois

LS

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2-Notochordal

canal

Cloacal membrane

Allantois


LS

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Extraembryonic mesodermNotochorda

l canal

Intraembryonic mesoderm

Epiblast (Ectoderm)

Endoderm

TSDr.Sherif Fahmy

Amniotic cavity

Yolk sac

Degenerating endoderm and floor of notochordal canal

Roof of notochordal canal

LS

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3-Neurenteric canal


Cloacal membrane

LS

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4-Notochordal plate

Notochordal plate (roof of the canal) which intercalate (fused) to endodermal layer.TS

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TS

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5-Definitive Notochord

TS

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Development of Notochord• 1- Pre-notochordal process: Proliferation of cells from

primitive pit forms a cord of cells in median plane till prochordal plate.

• 2- Notochordal canal: Canalization of the process forms notochordal canal.

• 3- Notochordal-ectodermal fusion: fusion between floor of the canal and endoderm.

• 4- Neur-enteric canal: Temporary communication between amniotic cavity and yolk sac due to degeneration of floor of notochordal canal and underlying endoderm.

• 5- Notochordal plate Persistence roof of notochordal canal.• 6- Defenitive notochord: Regeneration of endoderm only.

Persistent roof of notochordal canal becomes folded upon itself to form defenitive notochord. Dr.Sherif Fahmy

Importance of Notochord

1- Induction of vertebral column development.2- Temporary axial skeleton.

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Fate of Notochord• It is the primitive axial skeleton around

which the vertebral column is formed.• It remains in intervertebral disc as

nucleus pulposus.

Dr.Sherif Fahmy

Dr.Sherif Fahmy

Annulus fibrosus

Nucleus pulposus

Parts of Intervertebral disc

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Intra-embryonic Mesoderm• It is formed from proliferating cells from sides

of primitive node and streak.• It fills the space between ectoderm and

endoderm except at buccopharyngeal membrane, cloacal membrane (fusion between ectoderm and endoderm caudal to primitive streak) and median region which is occupied by notochord.

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Primitive streak

Primitive node and pit Buccopharyngeal

membrane

Epiblast (ectoderm)

EndodermIntra-embryonic mesoderm

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Epiblast (ectoderm)

Hypoblast

Endoderm

Flask-shaped cells

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Amniotic cavity

Amnio-ectodermal junction

Intra-emryonic mesoderm

Secondary yolk sac

Endoderm

Epiblast (ectoderm)

Bucco-pharyngeal membrane

Cloacal membrane

Primitive streak

Primitive node

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17 th DayDr.Sherif Fahmy

Differentiation of Intra-embryonic Mesoderm

• Intra-embryonic mesoderm on each side of notochord, divides into:1- Paraxial Mesoderm: on both sides of

notochord.2- Intermediate Mesoderm: Middle part of the

mesoderm.3- Lateral plate Mesoderm: Lateral part which

communicates with that of the opposite side infront prochordal plate.

Dr.Sherif Fahmy


Cloacal membrane

Notochord

Intermediate mesoderme

Paraxial mesoderm

Lateral plate mesoderm

Dr.Sherif Fahmy

1-Paraxial Mesoderm• It is the most medial mesoderm.• It is divided into cubical masses called

somites (42 – 44 pairs).• Formation of somites starts At the 20th

day by formation of one pair at cranial region.

• Somites are classified into: 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8 – 10 coccygeal. Dr.Sherif Fahmy

Fate of Somites• 1- Sclerotome: It is ventro-medial part

that form vertebral column and intervertebral discs.

• 2- Dermo-myotome: It is the dorso-lateral part which subdivided into:A- Dermatome: Forms dermis of skin.B- Myotome: Forms skeletal muscles of

trunk and limbs.Dr.Sherif Fahmy

NotochordSomites of Paraxial mesoderm

Intermediate mesoderm

Intraembryonic coelom

Pericardium

Pleura

Peritoneal canal

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Dr.Sherif Fahmy

Notochord

Neural tube

Somite

Sclerotome

Myotome

Dermatome

Neural tubeVertebra

Skeletal muscles

Dermis

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Muscles of back

Muscles of anterolateral aspect of body

Muscles of limb

Dorsal ramus of spinal nerve

Ventral ramus of spinal nerve

Dermo-myotomes of brachial plexus

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2-Intermediate Mesoderm• Narrow strip between paraxial

and lateral plate mesoderm.• It is divided into many

segments.• It forms uro-genital system.

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Lateral Plate Mesoderm• Flat plate of mesoderm between intermediate

mesoderm and margin of embryonic disc.• It is continuous with that of other side infront

prochordal plate.• Intra-embryonic coelom: It is formed from fused

small cavities. This coelom forms serous membranes of the body (pericardium, pleura and peritoneum).

• Lateral plate mesoderm is split by the coelom into:-Somatic mesoderm & Splanchnic mesoderm.

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Cloacal membrane

Notochord

Intermediate mesoderme

Paraxial mesoderm


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NotochordSomites of Paraxial mesoderm


Intr

aem

bryo

nic

coel

omPericardium

Pleura

Peritoneal canal

Cardiogenic area

Septum transversum

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Neural groove

Amnion

Somatic mesoderm

Intraembryonic coelom

Notochord

Splanchnic mesoderm

Paraaxial mesoderm



T.S.

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Amnion

Somatic mesoderm

Intraembryonic coelom Splanchnic

mesoderm


T.S.

Dr.Sherif Fahmy

Dr.Sherif Fahmy

Organogenesis

Embryonic Period (4th – 8th week)

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Fate of Ectoderm

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DEVELOPMENT OF NEURAL TUBE• Neural plate is median thickened area between

primitive node and prochordal membrane. Two strips separate neural plate from the rest of ectoderm which are called neural crest.

• Neural folds are raised margins of neural plate while depressed median region is called neural groove.

• Neural tube is formed by fusion between two neural folds in its middle and extends cranio-caudally. Cranial and caudal ends (neuropores) are the last to be closed.

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Neural groove

Neural fold

Notochord

Fusing neural folds to form neural tube

Neural crest

EctodermEndoderm

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Dr.Sherif Fahmy

Fate of the neural tube• The tube grows in the median region leading to

elongation of the embryonic disc in cranio-caudal direction.

• The cranial part of the tube dilates to form the brain vesicle while the caudal part forms the spinal cord.

• The brain vesicle divides by 2 constrictions into:– Forebrain: forms cerebral hemispheres and

diencephalone.– Midbrain: forms the midbrain (upper part of brain stem).– Hindbrain: forms medulla, pones and cerebellum.

Dr.Sherif Fahmy

Dr.Sherif Fahmy

Fate of neural crest• Ganglia: Sensory (of cranial and spinal

nerves), sympathetic and parasympathetic.• Cells: Chromaffin cells of supra-renal

medulla, Schwann cells and melanoblasts.• Others: Pia mater, arachnoid mater, enamel

of teeth, septa of the heart and some bones of the skull.

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Other derivatives of ectoderm- Otic placodes form internal ear.- Lens placodes form lens of the eye.- Peripheral nerves.- Sensory epithelium in ear, nose, eye and

epidermis of skin.- Pituitary gland.- Anterior part of oral cavity and lower ½ of

anal canal.Dr.Sherif Fahmy

Development of Endoderm(Page 30)

-Epithelium of digestive system, respiratory tract, most of urinary bladder and urethera, tympanic cavity and Eustachian tube.-Parenchyma of liver, pancreas, thymus, thyroid, parathyroid and palatine tonsils. Dr.Sherif Fahmy

FoldingDr.Sherif Fahmy

FOLDING OF THE EMBRYO• It is the process by which the embryo becomes folded upon

itself.Time of folding: • At the end of 3rd week and completed at the end of 4th

week.Causes of folding:• Rapid increase of cranio-caudal length due to rapid growth

of neural tube and somites.• Rapid expansion of amniotic cavity.Types of folding:• Head and tail folds are folding of cranial and caudal parts of

the disc.• Lateral folds are folding of lateral parts of the disc.

Dr.Sherif Fahmy

Dr.Sherif Fahmy

Results of Folding

Dr.Sherif Fahmy

Dr.Sherif Fahmy

Embryonic disc with removed ectoderm

Cloacal membrane

Notochord

Paraxial mesoderm (somites)


Cardiogenic area

Septum transversum

Peritoneal canal

Pericardium

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Ectoderm

Mesoderm

Endoderm


Cloacal membrane

Hindgut

Midgut

Foregut

Forebrain

Forebrain bulge

Pericardial bulge Vitelline duct AllantoisDefinitive yolk sac

Stomodeum

L.S. in folded embryoHeart

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Peritoneal canals

Gut

Ventral mersentry

Dorsal mesentry

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RESULTS OF FOLDING1-Cylindrical appearance: Transformation of emryonic disc to cylindrical shape.

2- Amniotic cavity: Before folding it lies dorsal to embryonic disc, after folding, it surrounds all aspects of the embryo.

3- Formation of definitive yolk sac: It is the part of yolk sac outside the embryo in the umbilical cord.4- Formation of primitive umbilical ring: It is a ventral defect in anterior abdominal wall that contains connecting stalk, allantois and vitello-intestinal duct

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5-Formation of the gut: •It is formed from endodermal layer together with part of yolk sac. Foregut is formed in head fold with bucco-pharyngeal membrane closing its cranial end. Hindgut: is formed in tail fold and closed caudally by cloacal membrane. The caudal part is dilated and called cloaca which is connected ventrally to allantois. Midgut: is formed by lateral folds and present between foregut and hindgut. It is connected with defenitive yolk sac by vitelline duct.

6- Formation of stomodeum: Ectodermal depression between forebrain bulge and cardiac bulge.

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7- Formation of mesenteries: Ventral and dorsal mesenteries are formed around gut.

8- Reversal of positions:-Heart and pericardium become cranial to septum transversum (before folding septum transversum is most cranial).

-Connecting stalk becomes ventral and more cranial inspite of being most caudal.

Dr.Sherif Fahmy

Revision on General Embryology 1

Education

Transcript of Revision on General Embryology 1