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Early EmbryologyKristine Krafts, M.D.

• In general, what happens in the first and second phases of the embryonic period?

• What happens during week 1 and 2?

• Describe the main events in gastrulation.

• How does the neural tube form, and what happens to it?

Embryology Lecture Objectives

• What is the fate of the endoderm, mesoderm and ectoderm?

• What are the pharyngeal arches? What structures arise from each arch, groove, and cleft?

• What structures do neural crest cells form? major stuff forms from pouches?

Embryology Lecture Objectives

General overview of prenatal development

Embryonic period phase 1• Formation of bilaminar disk• Formation of trilaminar disk (gastrulation)

Embryonic period phase 2• Formation of neural tube• Differentiation of mesoderm• Folding of embryo• Formation of pharyngeal arches

Embryology Lecture Outline

General overview of prenatal development

Embryology Lecture Outline Prenatal Development

Fertilization

1 2 3 4

Phase 1Cellular

proliferation and migration

5 6 7 8

Phase 2Differentiation of

internal & external structures

40

Phase 3Growth and

maturation

Embryo Fetus

0

Delivery

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This YouTube video is awesome at explaining early embryonic development:

http://www.youtube.com/watch?v=rN3lep6roRI General overview of prenatal development

Embryonic period phase 1

Embryology Lecture Outline

Prenatal Development

Fertilization

1 2 3 4

Phase 1Cellular

proliferation and migration

5 6 7 8

Phase 2Differentiation of

internal & external structures

40

Phase 3Growth and

maturation

Embryo Fetus

0

Delivery

General overview of prenatal development

Embryonic period phase 1• Formation of bilaminar disk

Embryology Lecture Outline

Week 1: Differentiation of Morula into Blastocyst

Morula Blastocyst

Week 2: Formation of Bilaminar Germ Disk

epiblast

hypoblast

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General overview of prenatal development

Embryonic period phase 1• Formation of bilaminar disk• Formation of trilaminar disk (gastrulation)

Embryology Lecture Outline

"It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life.”

- Lewis Wolpert (1986)

Gastrulation: formation of primitive streak

primitive streak

primitive node

epiblast

Gastrulation: movement and differentiation of epiblast cells

Epiblast cells give rise to all three germ cell layers!(the hypoblast does NOT turn into endoderm)

Bilaminar germ disk

EpiblastHypoblast

Endoderm EndodermMesoderm

Primitive streak

Ectoderm

Gastrulation: formation of notochord

The notochord is super important because it tells the three layers what to do next.

General overview of prenatal development

Embryonic period phase 1• Formation of bilaminar disk• Formation of trilaminar disk (gastrulation)

Embryonic period phase 2

Embryology Lecture Outline Prenatal Development

Fertilization

1 2 3 4

Phase 1Cellular

proliferation and migration

5 6 7 8

Phase 2Differentiation of

internal & external structures

40

Phase 3Growth and

maturation

Embryo Fetus

0

Delivery

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General overview of prenatal development

Embryonic period phase 1• Formation of bilaminar disk• Formation of trilaminar disk (gastrulation)

Embryonic period phase 2• Formation of neural tube

Embryology Lecture OutlineFormation of Neural Tube

Formation of the Neural Tube

General overview of prenatal development

Embryonic period phase 1• Formation of bilaminar disk• Formation of trilaminar disk (gastrulation)

Embryonic period phase 2• Formation of neural tube• Differentiation of mesoderm

Embryology Lecture Outline

Differentiation of mesoderm

Paraxial mesoderm forms bones and muscles of most of the body (except head), pharyngeal arches, and connective tissue

Intermediate mesoderm forms urogenital system

Lateral plate mesoderm forms hematopoietic system, heart, pharyngeal arches, and connective tissue

Know this!

Cranial nervesBones and connective tissue of head

Pharyngeal arches

Neural crest

NeuroectodermCentral nervous system

Intermediate plate mesoderm

Urogenital system

Lateral plate mesodermParaxial mesodermHeart

Hematopoietic systemPharyngeal archesConnective tissue

Bones and muscles of body (except head)

Pharyngeal archesConnective tissue

EndodermLining of GI tract

Surface ectoderm

Epidermis

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Clinical Correlation: Treacher Collins Syndrome

Neural crest cells don’t migrate properly to the facial region. Structures derived from 1st and 2nd pharyngeal arches don’t develop properly.

General overview of prenatal development

Embryonic period phase 1• Formation of bilaminar disk• Formation of trilaminar disk (gastrulation)

Embryonic period phase 2• Formation of neural tube• Differentiation of mesoderm• Folding of embryo

Embryology Lecture Outline

Folding of the Embryo

Lateral plate mesoderm splits in two.

One part remains near the ectoderm.

The other part follows the endoderm.

Lateral Folding of the Embryo

Endodermal layer (lined by mesoderm) bends, the edges reaching towards each other, meeting in front to form the gut.

Ectodermal layer (lined by mesoderm) grows forward, reaches around the gut, and zips up the front to form the anterior body wall.

Lateral Folding of the Embryo Lateral Folding of the Embryo

Amnion (and amniotic cavity) comes along for the ride, eventually surrounding entire embryo.

Gut (lined by endoderm, surrounded by mesoderm)

Anterior thoracic wall (mesoderm covered with ectoderm)

Back, with ectoderm overlying neural tube

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Head-Tail Folding of the Embryo

Before folding Day 24

Day 26 Day 28

Stomatodeum

Head-Tail Folding of the Embryo

What happens to the neural tube?

Hey, what’s that mesoderm doing there? It’s forming:• Somatomeres (which turn into

muscles of the head and neck)• Somites (which turn into the

bones and muscles of the back).

It turns into the brain(forebrain, midbrain and hindbrain)

and the spinal cord.

Embryo, day 23-26

Neural tube closed

Neural tube still open

Neural tube still open

Somites

Embryo, day 26-30

Neural tube still a little bit open

Neural tube closed

Somites

Embryo, day 26-30

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General overview of prenatal development

Embryonic period phase 1• Formation of bilaminar disk• Formation of trilaminar disk (gastrulation)

Embryonic period phase 2• Formation of neural tube• Differentiation of mesoderm• Folding of embryo• Formation of pharyngeal arches

Embryology Lecture OutlineFormation of Pharyngeal Arches

Arches have a core that’s derived from lateral and paraxial mesoderm and neural crest cells.

They are covered with ectoderm on the outside and lined by endoderm on the inside.

Each arch has its own cartilage, artery, and nerve.

Pharyngeal Arch Anatomy Formation of Pharyngeal Arches

25-day-old embryo 35-day-old embryoNo arches yet

Buccopharyngeal membrane intactArches and pouches nicely formed

Mouth now open to esophagus

26-day-old embryo showing stomatodeum and first two arches

Embryo, day 26-30

1

Somites

Frontal prominence

23

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First four arches in a 32-day-old embryo

Somites

Heart

1

2

34

Mandibular

StomatodeumMaxillary

Optic placodeNasal

placode

Pharyngeal arches, grooves, pouches

Fish have branchial (“gill”) arches, with slits in between for water flow.Humans don’t have gills. So our arches are called pharyngeal.

Wait, are they branchial or pharyngeal arches?

Pharyngeal arches, grooves/clefts and pouches

ArchGroove/cleft

Pouch

Arch Nerve Muscles Skeleton

1 V (trigeminal)

Mastication muscles

Mylohyoid, anterior digastric

Tensors tympani and veli palatini

Meckel’s cartilage

(malleus, incus)

2 VII (facial)

Facial expression muscles

Posterior digastric

Stylohyoid

Stapedius

Reichert’s cartilage

(stapes, styloid,

lesser hyoid)

3IX

(glossopharyngeal)Stylopharyngeus Greater hyoid

4-6 X (vagus)

Larynx muscles

Cricothyroid

Levator veli palatini

Constrictors of pharynx

Laryngeal cartilages

Important!Meckel’s cartilage indicates where

the mandible will develop – but it

does not turn into the mandible!

Know the stuff in red!

• 1st groove and pouch -> ear stuff• Rest of grooves disappear• 2nd pouch obliterated by tonsil• 3rd pouch -> inferior parathyroid, thymus • 4th and 5th pouches -> superior parathyroid, ultimobranchial body

What happens to the pouches and grooves?

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Pharyngeal sinuses and cysts Auricular sinuses and cysts

Head and Neck Anomalies From Improper Groove Closure

Prenatal Development

Fertilization

1 2 3 4

Phase 1Cellular

proliferation and migration

5 6 7 8

Phase 2Differentiation of

internal & external structures

40

Phase 3Growth and

maturation

Embryo Fetus

0

DeliveryWe covered this in this lecture.

Prenatal Development

Fertilization

1 2 3 4

Phase 1Cellular

proliferation and migration

5 6 7 8

Phase 2Differentiation of

internal & external structures

40

Phase 3Growth and

maturation

Embryo Fetus

0

Delivery

We covered some of this in this lecture.

Prenatal Development

Fertilization

1 2 3 4

Phase 1Cellular

proliferation and migration

5 6 7 8

Phase 2Differentiation of

internal & external structures

40

Phase 3Growth and maturation

Embryo Fetus

0

Delivery

You’ll learn more details about head and neck development (e.g., tooth development) in oral histology next year.

Prenatal Development

Fertilization

1 2 3 4

Phase 1Cellular

proliferation and migration

5 6 7 8

Phase 2Differentiation of

internal & external structures

40

Phase 3Growth and

maturation

Embryo Fetus

0

Delivery

We’ll talk about the development of other, non-oral organs as we go through this course.