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
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
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
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 Outline
Formation 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 mesoderm
HeartHematopoietic system
Pharyngeal archesConnective tissue
Bones of most of the body (everything except the head)
Muscles of the body and headPharyngeal archesConnective tissue
Endoderm
Lining of GI tract
Surface ectoderm
Epidermis
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
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
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
Formation 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
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 digastricTensors tympani and veli palatini
Meckel’s cartilage (malleus, incus)
2 VII (facial)
Facial expression musclesPosterior digastric
StylohyoidStapedius
Reichert’s cartilage(stapes, styloid,
lesser hyoid)
3 IX (glossopharyngeal) Stylopharyngeus Greater hyoid
4-6 X (vagus)
Larynx musclesCricothyroid
Levator veli palatiniConstrictors of pharynx
Laryngeal cartilages
Important!Meckel’s cartilage indicates wherethe mandible will develop – but itdoes 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?
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
DeliveryWe 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.