EMBYOLOGY

SUPERVISED BY DR DILABAZ KHAN,DR IMDADULLAH,DR JAMEEL KIFAYATULLAH

PRESENTER HIRA NISARKHYBER COLLEGE OF DENTISTRY

• Bilaminar germ disc • Trilaminar germ disc• Pharangeal apparatus

Presented to : Dr.ImdadDr. Jameel Dr. Dilabaz

Presented by:Hira NisarRoll # 24BDS 2nd year

IN THE NAME OF ALLAH ,THE MOST MERCIFUL ,

THE MOST BENEFICIENT!

CONTENTS:

•Bilaminar Germ disc•Trilaminar Germ disc•Pharangeal Apparatus

Pharangeal Arches Pharangeal Pouches

Pharangeal Clefts Pharangeal Membrane

Bilaminar Germ Disc

Definition:

Formation of epiblast and hypoblast layers in second week of development forms Bilaminar Germ disc.

HOW???

Development Of Trophoblast

1.The trophoblast differentiates into cytotrophoblast & syncytiotrophoblast.

2.Cells of the cytotrophoblast divide and migrate externally.

3.They lose their cell membranes to form the syncytiotrophoblast

Inner cell mass differentiates into two layers

1 :Epiblast layer

2 :Hypoblast layer

Clinical correlates *The syncytiotrophoblast secretes human

chorionic gonadotropin hormone which prevents the degeneration of the corpus luteum.

By the end of the 2nd week, the amount of this hormone will be sufficient to be detected in the maternal blood and urine. This is the basis of pregnancy test.

*occasionally implantation takes place outside the uterus resulting in ectopic or

extra uterine pregnancy.

*In some cases trophoblast develops and forms placental membrane although little or no embryo tissue is present this is known as hydatiform mole.

1.Secondary yolk sac2. Exocoelomic cyst3. Aminotic cavity4. Extra-embryonic coelom5. Epiblast6. Connecting stalk 7. Hypoblast8. Primary villi9. Trophoblastic lacunae10. Extraembryonic somatic mesoderm11. Extraembryonic splanchnic mesoderm

Trilaminar Germ Disc

Definition

Formation of three germinal layers form the trilaminar germ disc

The three layers are1.Ectoderm

2.Endoderm 3.Mesoderm

HOW???

Formation of Trilaminar Germ Disc

1.. Gastrulation- most characteristic feature during 3rd week

2.3 germ layers are established- ectoderm, endoderm and mesoderm

3.Start with formation of PRIMITIVE STREAK

PRIMITIVE STREAK1.Midline groove at epiblast

2.Cephalic end-primitive node

3.Cells of epiblast migrate towards primitive streak

4.This inward movement is known as invagination

5.Invaginated cells displace hypoblast forming endoderm

6 .Some other invaginated cells come to lie between epiblast and newly formed endoderm forming mesoderm

7 .Remaining cells in epiblast form ectoderm

8.These 3 layers form trilaminar germ disc

Clinical Significance*Teratogenesis

For example large doses of alcohol at this stage result in holoprosencephally.

*Caudal dysgenesis(Sirenomelia)Insufficint mesoderm is formed in caudal most region of embryo.

*Situs inversusTransposition of viscera in the thorax and abdomen occurs.

Pharangeal apparatus

Pharangeal apparatus•The head and neck region of The head and neck region of four week human embryo four week human embryo somewhat resemble these somewhat resemble these regions of a fish embryo of regions of a fish embryo of comparable stagecomparable stage

•This explains the former use of This explains the former use of designation branchial apparatusdesignation branchial apparatus

Pharyngeal apparatus consists ofPharyngeal apparatus consists of::

Pharyngeal archesPharyngeal archesPharyngeal pouchesPharyngeal pouchesPharyngeal grooves/cleftsPharyngeal grooves/cleftsPharyngeal membranePharyngeal membrane

Pharyngeal pouchPharyngeal groove

Pharyngeal membrane

Pharyngeal arch

Pharyngeal ArchesPharyngeal Arches

Mesenchymal condensation on either side of the pharynx are known as pharangeal arches.

Definition

•Pharyngeal Pharyngeal arches begin to arches begin to develop early in develop early in the fourth week as the fourth week as neural crest cells neural crest cells migrate into the migrate into the head and neck head and neck regionregion

Pharyngeal ArchesPharyngeal Arches• By the end of the fourth week, four pairs By the end of the fourth week, four pairs

of pharyngeal arches are visible of pharyngeal arches are visible externallyexternally

• The fifth and sixth arches are rudimentary The fifth and sixth arches are rudimentary and are not visible on the surface of the and are not visible on the surface of the embryoembryo

• The pharyngeal arches are separated The pharyngeal arches are separated from each other by fissures called from each other by fissures called pharyngeal groovespharyngeal grooves

• They are numbered in craniocaudal sequenceThey are numbered in craniocaudal sequence

•Each pharyngeal arch consists of a core of Each pharyngeal arch consists of a core of mesenchymemesenchyme

•Is covered externally by ectoderm and Is covered externally by ectoderm and internally by endoderminternally by endoderm

Pharyngeal Arch ComponentsPharyngeal Arch Components

Ist Pharangeal Arch Derivatives

Ist pharangeal arch consists of•Maxillary process•Mandibular process

Skeletal component

Ist pharangeal arch is known as mandibular arch

Maxillaryprocess

Mandibular process

Maxillary process

Maxillary process gives rise to•Maxilla•Premaxilla•Zygomatic bone•Part of temporal bone

Mandibular process

Mandibular process give rise to• Meckle’s cartilage•Mandible•Malleus •Incus•Anterior ligament of malleus•Sphenomandibular ligament

Musculature of ist arch

Musculature of ist pharangeal arch includes•Muscles of mastication•Anterior belly of digastric muscle •mylohyoid•Tensor tympani•Tensor palatini

Nerve supply to the ist arch

•Nerve supply to ist arch is provided by Mandibular branch of trigeminal nerve

Note•Ist arch also contribute to dermis of the face which is innervated by opthalmic ,maxillary and mandibular branch of trigeminal nerve.

as2nd Pharangeal Arch

skeletal component

2nd pharangeal arch gives rise to•Stapes•Styloid process•Stylohyoid ligament•Lesser horn•Upper portion of the body of hyoid bone

2nd pharangeal arch is also known a hyoid arch or rechiert’s cartilage

Musculature of 2nd arch

Musculature of 2nd arch includes•Stapedius muscle•Stylohyoid muscle•Posterior belly of digastric muscle•Auricular muscle•Muscles of facial expression

Nerve supplyNerve supply•2nd arch is innervated by facial nerve

3rd Pharangeal Arch

Skeletal component

3rd arch includes•Lower part of the body•And greater horn of the hyoid bone

MusculatureMusculature is limited to stylopharangeus muscle

Nerve supply

It is innervated by glossopharangeal nerve

4th and 6th Pharangeal Arch

Skeletal component

Cartilage of 4th and 6th arch fuse together to form•Thyroid•Cricoid•Aretenoid• Corniculate• And Cuniform cartilage of larynx

Musculature of 4th Arch

It includes•Cricothyroid•Levator platini•Constrictors of pharynx

Nerve supply of 4th Arch

Innervation is provided bySuperior larangeal branch of vagus

Musculature of 6th arch

•It includes intrinsic muscles of larynx

Nerve supply

•Innervation is by recurrent larangeal branch of vagus

Pharangeal pouches

•The pharyngeal pouches are balloon like diverticula that formed on the endodermal side between the pharyngeal pharyngeal archesarches•The pairs of pouches develop in a craniocaudal sequence between the arches

• There are four well defined pairs of pharyngeal pouches

•The fifth pair is absent or rudimentary

• The first pair of pouches lies between the first and second pharyngeal arches

• The first pharyngeal pouch forms stalk like diverticulum called tubotympanic recess

•The distal portion of diverticulum widens into sac like structure primitive tympanic Or middle ear cavity

•The lining of tympanic cavity later aids in the formation of tympanic membrane or ear drum

Derivatives of Ist Pharangeal Pouch

•While proximal portion forms auditory tube

•The second pharyngeal pouch is largely obliterated ( disappear ) as the palatine tonsils develop

• Part of the cavity of this pouch remains as the tonsillar sinus or fossa

Derivatives of Second Pharyngeal Pouch

Derivatives of 2nd pharangeal pouch

Derivatives of Third Pharyngeal Pouch

Epithelium of dorsal region of 3rd pouch differntiates into inferior thyroid gland

While ventral region differntiates into thymus.

The primordia of thymus and inferior parathyroid glands lose their connections with the pharynx and migrate into the neck Later the inferior parathyroid glands separate from the thymus and lie on the dorsal surface of the thyroid gland

Derivatives of Fourth Pharyngeal PouchDerivatives of Fourth Pharyngeal Pouch

Each dorsal part develops into a Each dorsal part develops into a superior parathyroid glandsuperior parathyroid gland

It lies on the dorsal surface of the thyroid glandIt lies on the dorsal surface of the thyroid gland

Ventral region of 4th pouch forms ultimobranchial body

Ultimobranchial body is later on incorporated into thyroid gland.

Cells of Ultimobranchial body give rise to parafollicular cells Or C cells of thyroid gland which secretes calcitonin.

Derivatives of Fifth Pharyngeal PouchDerivatives of Fifth Pharyngeal Pouch

When this develops, this rudimentary pouch When this develops, this rudimentary pouch becomes part of the fourth pharyngeal becomes part of the fourth pharyngeal pouch and helps to form the pouch and helps to form the ultimopharyngeal bodyultimopharyngeal body

Pharyngeal GroovesPharyngeal Grooves

During the fourth and fifth weeks, head and During the fourth and fifth weeks, head and neck region of the human embryo exhibit neck region of the human embryo exhibit four pharyngeal grooves or clefts on each four pharyngeal grooves or clefts on each side side

These grooves separate the pharyngeal These grooves separate the pharyngeal arches externallyarches externally

Only first pair persists as the external Only first pair persists as the external acoustic meatusacoustic meatus

The other grooves normally obliterated with The other grooves normally obliterated with the cervical sinus as the neck developsthe cervical sinus as the neck develops

Pharyngeal MembranesPharyngeal Membranes

• The connection between the endoderm of the pharyngeal pouchespharyngeal pouches and the ectoderm of the pharyngeal groovespharyngeal grooves form a double layered membrane called the pharyngeal membraneThese membranes form where the epithelia These membranes form where the epithelia of the grooves and pouches approach each of the grooves and pouches approach each otherother

The endoderm of the pouches and ectoderm of The endoderm of the pouches and ectoderm of the grooves are soon separated by the grooves are soon separated by mesenchymemesenchyme

Only first pharyngeal membrane becomes the Only first pharyngeal membrane becomes the tympanic membrane, others obliteratetympanic membrane, others obliterate