Embryonic Period ( 3Embryonic Period ( 3rdrd to 8 to 8thth weeks) weeks)

Fetus (3Fetus (3rdrd month to Birth) month to Birth)

First week of development:First week of development:Ovulation to ImplantationOvulation to Implantation

Second week of development: Second week of development: Bilaminar germ discBilaminar germ disc

Two embryonic layers – epiblast and Two embryonic layers – epiblast and hypoblast.hypoblast.

Two cavities – Amnionic cavity and Two cavities – Amnionic cavity and exocoelomic cavity (yolk sac).exocoelomic cavity (yolk sac).

Two layers of trophoblast – Two layers of trophoblast – cytotrophoblast & syncytiotrophoblast.cytotrophoblast & syncytiotrophoblast.

Formation of extraembryonic Formation of extraembryonic mesoderm.mesoderm.

Third week of development:Third week of development:Trilaminar germ discTrilaminar germ disc

Formation of theFormation of the Three Germ Layer - Three Germ Layer - GASTRULATIONGASTRULATION

- Migration of epiblast cells to form the mesoderm and - Migration of epiblast cells to form the mesoderm and endodermendoderm

- The remaining epiblast gives rise to the ectoderm- The remaining epiblast gives rise to the ectoderm

Formation of the primitive streak, pit and nodeFormation of the primitive streak, pit and node

Formation of the notochordFormation of the notochord  

Embryonic Period/ Organogenesis (3Embryonic Period/ Organogenesis (3rdrd to 8 to 8thth weeks) weeks)

Formation & development of body organs from embryonic Formation & development of body organs from embryonic tissue is called tissue is called organogenesisorganogenesis

Each of the 3 germ layers gives rise to a number of specific Each of the 3 germ layers gives rise to a number of specific tissues & organs tissues & organs

Formation of the Neural tube (Neurulation)Formation of the Neural tube (Neurulation) The neuro-ectoderm (The neuro-ectoderm (neural plate)neural plate) is derived from the is derived from the

ectoderm overlying the notochord.ectoderm overlying the notochord. By the end of third week the lateral edges of neural plate By the end of third week the lateral edges of neural plate

elevates to form neural fold.elevates to form neural fold. Depressed mid region forms the neural groove The neural groove deepens and eventually forms a The neural groove deepens and eventually forms a neural neural

tubetube.. Two masses of ectoderm at edges of neural plate, form Two masses of ectoderm at edges of neural plate, form neural neural

crestcrest.  . 

Initially the neural crest separates neuro-ectoderm Initially the neural crest separates neuro-ectoderm from skin ectoderm.from skin ectoderm.

As folding of the neural tube occurs, the neural crest As folding of the neural tube occurs, the neural crest cells detach from the ectoderm and form clusters cells detach from the ectoderm and form clusters that migrate into the mesoderm.that migrate into the mesoderm.

Derivaties of EctodermDerivaties of Ectoderm1.1. Surface Ectoderm:Surface Ectoderm:

A.A. Lining epitheliaLining epithelia Skin epidermisSkin epidermis Mucous membrane of lips ,cheeks , gums, part of Mucous membrane of lips ,cheeks , gums, part of

floor of mouth , part of the palate, nasal cavity & floor of mouth , part of the palate, nasal cavity & sinusessinuses

Lower part of anal canal ( below pectinate line)Lower part of anal canal ( below pectinate line) Terminal parts of male urethraTerminal parts of male urethra Outer surface of labia minora & whole of labia Outer surface of labia minora & whole of labia

majoramajora Anterior epithelium of cornea, epithelium of Anterior epithelium of cornea, epithelium of

conjunctiva, epithelial layers of ciliary body & irisconjunctiva, epithelial layers of ciliary body & iris Outer layer of tympanic membrane, epithelial lining Outer layer of tympanic membrane, epithelial lining

of membranous labyrinth including the special end of membranous labyrinth including the special end organsorgans

Lacrimal canaliculi, sac, nasolacrimal duct Lacrimal canaliculi, sac, nasolacrimal duct

BB. . Glands :Glands : • Sweat glands, sebaceous glands, parotid Sweat glands, sebaceous glands, parotid gland, mammary glands, pars anterior of gland, mammary glands, pars anterior of pituitary glandpituitary gland

CC. . Other derivatives:Other derivatives:• Tooth enamelTooth enamel• Hair Hair • NailsNails

2. 2. NeuroectodermNeuroectoderm A. A. Neural tubeNeural tube

CNSCNS Retina & Optic nerve & musculature of irisRetina & Optic nerve & musculature of iris Pineal & pituitary glandPineal & pituitary gland NeuronsNeurons Neuroglia – Ependymal cells , Macroglia(Astrocytes, Neuroglia – Ependymal cells , Macroglia(Astrocytes,

oligodendrocytes)oligodendrocytes) B. B. Neural crestNeural crest

Schwann cells Schwann cells Chromaffin cells (adrenal medulla)Chromaffin cells (adrenal medulla) Dorsal root ganglia & dorsal root of spinal nerveDorsal root ganglia & dorsal root of spinal nerve Sympathetic gangliaSympathetic ganglia Sensory ganglia of V, VII, IX & X cranial nervesSensory ganglia of V, VII, IX & X cranial nerves Melanocytes of skinMelanocytes of skin LeptomeningesLeptomeninges Bones & connective tissues of cranio-fascial structuesBones & connective tissues of cranio-fascial structues Parts of heartParts of heart OdontoblastOdontoblast Adrenal medullaAdrenal medulla

3. 3. Ectodermal PlacodeEctodermal Placode Otic placode – gives rise to structures needed Otic placode – gives rise to structures needed

for hearing & balancefor hearing & balance Lens placode – forms lensLens placode – forms lens Nasal placode – nasal cavities & para nasal Nasal placode – nasal cavities & para nasal

sinussinus

Derivaties of MesodermDerivaties of MesodermMesoderm in the lateral part of the embryo is divided into Mesoderm in the lateral part of the embryo is divided into

three distinct longitudinal masses:three distinct longitudinal masses:Paraxial mesoderm Paraxial mesoderm -  a longitudinal column of cells that lies next to the -  a longitudinal column of cells that lies next to the

notochord notochord - it gives rise to the axial skeleton and skeletal muscle- it gives rise to the axial skeleton and skeletal muscleIntermediate mesodermIntermediate mesoderm -  it gives rise to the genitourinary system-  it gives rise to the genitourinary systemLateral plate mesodermLateral plate mesoderm - gives rise to body wall structures- gives rise to body wall structures -  is continuous with the extra-embryonic mesoderm-  is continuous with the extra-embryonic mesoderm - splits into two layers enclosing the intra-embryonic - splits into two layers enclosing the intra-embryonic

coelomcoelom a. Somatic layer of mesoderm – a. Somatic layer of mesoderm – mesodermal derivatives of the body wall.mesodermal derivatives of the body wall. b. Splanchnic layer of mesoderm – b. Splanchnic layer of mesoderm – wall of the gut tube & connective tissue of wall of the gut tube & connective tissue of

visceraviscera

Paraxial mesodermParaxial mesoderm

The Paraxial mesoderm further forms segmental cuboidal The Paraxial mesoderm further forms segmental cuboidal bodies by third week bodies by third week - somites- somites

Differentiates – Differentiates – sclerotome sclerotome && dermatomyotome dermatomyotome..

It gives rise to the axial skeletal ,the associated muscles It gives rise to the axial skeletal ,the associated muscles ( true back muscles) and part of the overlying ( true back muscles) and part of the overlying dermis.

There are about 42- 44 pairs of somites – but a first There are about 42- 44 pairs of somites – but a first occipital and last 5-7 coccygeal somites disappear.occipital and last 5-7 coccygeal somites disappear.

Lateral plate mesoderm & intraembryonic coelomLateral plate mesoderm & intraembryonic coelom

A cavity – intraembryonic coelom appears in the lateral plate A cavity – intraembryonic coelom appears in the lateral plate mesoderm.mesoderm.

Has Has somatic (parietal)somatic (parietal) layer and layer and splanchnic (visceral)splanchnic (visceral) layer: layer:

a. a. Somatic layer of mesodermSomatic layer of mesoderm – mesodermal derivatives of – mesodermal derivatives of the body wall.the body wall.

b. b. Splanchnic layer of mesodermSplanchnic layer of mesoderm – wall of the gut tube & – wall of the gut tube & connective tissue of viscera.connective tissue of viscera.

Development of blood vesselDevelopment of blood vessel Blood cells and blood vessels develop from the mesoderm.Blood cells and blood vessels develop from the mesoderm. Mesodermal cells differentiate into endothelial cells Mesodermal cells differentiate into endothelial cells

surrounding a central group of erythroblasts.  These are surrounding a central group of erythroblasts.  These are blood islands that coalesce to form blood vessels. Almost all blood islands that coalesce to form blood vessels. Almost all parts of the mesoderm gives rise to blood vessels.  parts of the mesoderm gives rise to blood vessels. 

Differentiation of blood vesselsDifferentiation of blood vessels Blood islands and eventually blood vessels appear:Blood islands and eventually blood vessels appear: in the in the extra-embryonic mesodermextra-embryonic mesoderm in the in the second weeksecond week in the in the intra-embryonic mesodermintra-embryonic mesoderm in the in the third weekthird week the the primitve heart tubeprimitve heart tube develops in the cardiogenic develops in the cardiogenic

mesoderm (in the transverse mesoderm) at the beginning of mesoderm (in the transverse mesoderm) at the beginning of the the fourth weekfourth week and a primitive circulation is established and a primitive circulation is established

Derivatives of EndodermDerivatives of Endoderm

Epithelium : G.I.tract, liver, pancreas, gallbladder, part of the urinary bladder & urachus.

Epithelium: trachea, bronchi & lungs.

Epithelium [in part] : pharynx, thyroid, tympanic cavity, auditory tube & tonsils.

Folding of the EmbryoFolding of the Embryo Folding occurs by differential growth of tissues.  Folding occurs by differential growth of tissues. 

Neural ectoderm grows faster than the surrounding Neural ectoderm grows faster than the surrounding skin ectoderm and consequently fold to form a neural skin ectoderm and consequently fold to form a neural tube.  Similarly, skin ectoderm grows faster than the tube.  Similarly, skin ectoderm grows faster than the underlying mesoderm and endoderm, and this underlying mesoderm and endoderm, and this differential growth causes folding of the trialminar differential growth causes folding of the trialminar disc and gives shape to the embryo.  disc and gives shape to the embryo. 

Folding occurs mainly at the edges of the embryonic Folding occurs mainly at the edges of the embryonic disc and forms three main folds: disc and forms three main folds:

Head fold Head fold Tail foldTail fold Lateral folds  - convert the embryo into a tubular Lateral folds  - convert the embryo into a tubular

structure. structure. The notochord, neural tube and somites stiffen the The notochord, neural tube and somites stiffen the

dorsal axis of the embryo.dorsal axis of the embryo.

As a result of the formation of the As a result of the formation of the head foldhead fold

The The foregutforegut is formed by folding of the endoderm is formed by folding of the endoderm The The stomadeumstomadeum  is an invagination of ectoderm,   is an invagination of ectoderm,

and has the buccopharyngeal membrane separating and has the buccopharyngeal membrane separating it from the foregut. It opens into the amniotic cavity.it from the foregut. It opens into the amniotic cavity.

The The pericardial cavitypericardial cavity and cardiogenic mesoderm and cardiogenic mesoderm are shifted to the ventral aspect of the embryo and are shifted to the ventral aspect of the embryo and lie lie ventralventral to the foregut. to the foregut.

The part of the transverse mesoderm between the The part of the transverse mesoderm between the pericardial cavity and the yolk sac is the pericardial cavity and the yolk sac is the septum septum transversumtransversum proper.  In it the liver will develop. proper.  In it the liver will develop.

The The amniotic cavityamniotic cavity extends ventral to the cranial extends ventral to the cranial end of the embryo.end of the embryo.

The The yolk sacyolk sac is constricted from the cranial aspect. is constricted from the cranial aspect.

As a result of the formation of the As a result of the formation of the tail foldtail fold

The hindgut is formedThe hindgut is formed The cloaca is an invagination of ectoderm and has the The cloaca is an invagination of ectoderm and has the

cloacal membrane separating it from the hindgut.cloacal membrane separating it from the hindgut. The connecting stalk is shifted ventrallyThe connecting stalk is shifted ventrally The allantoic diverticulum is shifted ventrally.  It is an The allantoic diverticulum is shifted ventrally.  It is an

invagination of hindgut endoderm into the connecting invagination of hindgut endoderm into the connecting stalk. stalk.

The amniotic cavity extends ventral to the caudal end The amniotic cavity extends ventral to the caudal end of the embryo.of the embryo.

The yolk sac is constricted from the caudal endThe yolk sac is constricted from the caudal end

Transverse folding of the embryoTransverse folding of the embryo Converts the endoderm into a primitive gut tubeConverts the endoderm into a primitive gut tube The intra-embryonic coelom surrounds the gut tubeThe intra-embryonic coelom surrounds the gut tube The communication between the intra- and extra- The communication between the intra- and extra-

embryonic coeloms becomes constricted and eventually embryonic coeloms becomes constricted and eventually obliteratedobliterated

Note that drastic and important changes Note that drastic and important changes occur in the embryonic cavities as a occur in the embryonic cavities as a consequence of folding:consequence of folding:

The The amniotic cavityamniotic cavity surrounds the embryo completely on surrounds the embryo completely on all aspects and becomes the predominant cavity. It enlarges all aspects and becomes the predominant cavity. It enlarges progressively.progressively.

The The yolk sacyolk sac becomes constricted on all sides, and becomes becomes constricted on all sides, and becomes a small sac connected to the midgut by a narrow vitelline a small sac connected to the midgut by a narrow vitelline duct.  It becomes progressively smaller.duct.  It becomes progressively smaller.

The The extra-embryonic coelomextra-embryonic coelom is gradually obliterated by is gradually obliterated by the expanding amnion and eventually disappears completelythe expanding amnion and eventually disappears completely

BREIF OVERVIEW –EMBRYONIC BREIF OVERVIEW –EMBRYONIC PERIODPERIOD

4th week : closure of neural tube, optic vesicles, otic placodes, 1st & 2nd branchial arches, heart tube folding.

5th week : neuropores close, upper & lower limb buds, otic vesicle, lens placode, nasal pits, 3rd & 4th branchial arches.

6th week : finger & toe rays, foot plate, auricular hillocks,fore brain vesicle prominent, pigmentation of retina.

7th week: eyelids, nipples, limbs extend ventrally, medial 7th week: eyelids, nipples, limbs extend ventrally, medial nasal swellings - upper lips , midgut herniation.nasal swellings - upper lips , midgut herniation.

8th week: fingers & toes distinct, nails, limbs bent at 8th week: fingers & toes distinct, nails, limbs bent at elbows & knees, eyelids & auricles more developed, elbows & knees, eyelids & auricles more developed, external genitalia seen.external genitalia seen.

Fetus (3Fetus (3rdrd month to Birth) month to Birth)

Fetal Period:Fetal Period: The Period from the beginning of 3The Period from the beginning of 3rdrd month to the month to the

birth is known as fetal period. birth is known as fetal period. The main character of this period is the period of The main character of this period is the period of

rapid growth and maturation of the tissue.rapid growth and maturation of the tissue. During the 4th week of the development, the age of During the 4th week of the development, the age of

the embryo is expressed in the number of somites.the embryo is expressed in the number of somites. The length of the embryo between 4th and 8th The length of the embryo between 4th and 8th

developmental weeks is indicated as the developmental weeks is indicated as the crown-crown-rump length (CRL)rump length (CRL) and is expressed in millimeters. and is expressed in millimeters.

During fetal development (9th - 40th week) the During fetal development (9th - 40th week) the length of the conceptus is indicated as CRL or as the length of the conceptus is indicated as CRL or as the crown-heel length (CHL)crown-heel length (CHL) expressed in centimeters. expressed in centimeters.

Growth in length is most intensive during the 3rd to Growth in length is most intensive during the 3rd to the 5th month, while the increase in weight is most the 5th month, while the increase in weight is most evident during the last two months of gestation.evident during the last two months of gestation.

Monthly changes: 9Monthly changes: 9thth – – 1212thth week week

The head constitute half of fetus.The head constitute half of fetus.

Erythropoiesis occurs in liver.Erythropoiesis occurs in liver.

Metanephric kidney becomes functional and urine is Metanephric kidney becomes functional and urine is formed.formed.

Physiological hernia occurs and persists till 10Physiological hernia occurs and persists till 10thth week & week & returns by 11returns by 11thth week. week.

Primary Ossification centre appears in the long bones.Primary Ossification centre appears in the long bones.

External genitalia differentiated, identification of sex External genitalia differentiated, identification of sex possible.possible.

1313thth-16-16thth weeks weeks Head growth slows down but rapid growth in length. Head growth slows down but rapid growth in length.

The not so developed lower limbs start to develop. The not so developed lower limbs start to develop.

Ossification centres becomes clearly evident in x-rays. Ossification centres becomes clearly evident in x-rays.

Erythropoiesis begins in spleen. In liver it is seized. Erythropoiesis begins in spleen. In liver it is seized.

Primary follicles in ovaries seen. Primary follicles in ovaries seen.

Scalp hair pattern determined.Scalp hair pattern determined.

Soft thin hairs ‘lanugo’ begins to develop.Soft thin hairs ‘lanugo’ begins to develop.

1717thth-20-20thth weeks weeks Growth slows down.Growth slows down.

Quickening felt by mother i.e., first sign of Quickening felt by mother i.e., first sign of locomotion.locomotion.

Development of uterus is complete.Development of uterus is complete.

Testes begin to descend.Testes begin to descend.

2121stst-25-25thth Weeks Weeks

Pneumocytes type II is differentiated and becomes Pneumocytes type II is differentiated and becomes functional. Therefore, surfactants are released. functional. Therefore, surfactants are released.

Formation of capillaries plexus and pink coloration Formation of capillaries plexus and pink coloration of the skin seen. of the skin seen.

Finger nails visible.Finger nails visible.

2626thth- 29- 29thth Weeks Weeks Bone marrow differentiated and becomes functional. Bone marrow differentiated and becomes functional.

Erythropoiesis ceases in spleen. Erythropoiesis ceases in spleen.

Lungs have matured enough to be functional. Lungs have matured enough to be functional. Rhythmic breathing is possible and the foetus is Rhythmic breathing is possible and the foetus is viable. viable.

Subcutaneous fat develops- smoothing the wrinkles. Subcutaneous fat develops- smoothing the wrinkles.

Eyes reopen. Eyes reopen.

Hairs develop on the scalp.Hairs develop on the scalp.

Toe nail visible. Toe nail visible.

3030thth to 34 to 34thth weeks weeks

Quantity of fat increases considerably.Quantity of fat increases considerably.

Pupillary light reflex develops by 30Pupillary light reflex develops by 30thth weeks. weeks.

All body systems develop by 32All body systems develop by 32ndnd weeks and weeks and foetus can be born ‘premature baby’.foetus can be born ‘premature baby’.

3535thth- 38- 38thth Weeks Weeks

Fat increase considerably.Fat increase considerably.

The skin is covered by a whitish, fatty substance The skin is covered by a whitish, fatty substance (vernix caseosa) composed of secretory products (vernix caseosa) composed of secretory products from sebaceous gland.from sebaceous gland.

Head circumference is the largest of all body Head circumference is the largest of all body partsparts

Time of birthTime of birth

280 days or 40 weeks after the onset of last 280 days or 40 weeks after the onset of last normal menstrual period normal menstrual period

More accurately, 266 days or 38 weeks after More accurately, 266 days or 38 weeks after fertilization.fertilization.

Preterm :Preterm : born before 37 weeks of gestation born before 37 weeks of gestation

Post term or post maturityPost term or post maturity : born after 42 weeks : born after 42 weeks of gestationof gestation