Embryology
Review
Embryology


Embryology – study of origin and
development in utero—prenatal formation,
growth and differentiation
Prenatal period
Embryonic period – first 8 weeks
 Fetal period – remaining 30 weeks

Embryonic Period
Fetal Period
The Embryonic Period
Week 1 – from zygote to blastocyst
 Fertilization (Conception) – in lateral third
of uterine tube
 Zygote (fertilized oocyte) moves toward the
uterus
 Blastomeres – daughter cells formed from
zygote through mitotic division called
cleavage
 Morula – solid cluster of 12–16 blastomeres
 Blastocyst
– fluid-filled embryonic
stage– ~ 60 cells
The Embryonic Period

Events in first week
 Zygote
 4-cell
 Morula
 Early blastocyst
 Late blastocyst— implantations occur at
this stage
Fertilization and the Events of the
First 6 Days of Development
Week 2 – Blastulation


Two-layered embryo formation
Bilaminar embryonic disc – inner cell mass
divided into two sheets
 Epiblast and the hypoblast
 Together
they make up the bilaminar
embryonic disc
Week 2 – Blastulation
 Amniotic
sac – formed by an
extension of epiblast
 Outer membrane forms the amnion
 Inner membrane forms the amniotic
sac cavity
 The cavity is filled with amniotic
fluid
Week 2 – Blastulation
 Yolk
sac – formed by an lateral
extension of hypoblast
 Digestive
tube forms from yolk sac
 NOT a major source of nutrients for
embryo
 Tissues around yolk sac gives rise to
earliest blood cells and blood vessels
Implantation of the Blastocyst
Implantation of the Blastocyst
Implantation of the Blastocyst
Disorders of implantation

Ectopic pregnancy
Tubal— 95%
 Peritoneal
 Douglas pouch


Pracental previa
-Placenta forms at the inner cervical os
-Characterized with uterine bleeding
Week 3 – Trilaminar embryo
 Primitive
streak – raised groove
on the dorsal surface of the
epiblast
 Gastrulation – a process of
invagination of epiblast cells
Begins at the primitive streak
Forms the three primary germ
layers
Week 3 – Tri-laminar embryo
 Three
Germ Layers
Endoderm
– formed from
migrating cells that replace the
hypoblast
Mesoderm – formed between
epiblast and endoderm
Ectoderm – formed from epiblast
cells that stay on dorsal surface
Note that all layers derive from
epiblast cells
Week 3: The Primitive Streak
Week 3 – Tri-laminar embryo
Week 3: Notochord formation
 Primitive
node – a swelling at one
end of primitive streak
 Notochord forms from primitive
node and endoderm
 Notochord – defines body axis
Is the site of the future vertebral
column
Appears on day 16
Week 3: Notochord and Mesoderm
Week 3: Notochord and Mesoderm
Week 3: Neurulation
 Neurulation
– ectoderm starts
forming brain and spinal cord
 Neural
plate – ectoderm in the
dorsal midline thickens
 Neural groove – ectoderm
folds inward
Week 3: Neurulation
 Neurulation (continued)
 Neural
tube – a hollow tube
pinches off into the body
Cranial
part of the neural tube
becomes the brain
Folic acid deficiency at this
stage causes neural tube defects
Week 3: Neurulation
 Neural
crest
Cells originate from ectodermal
cells
Forms sensory nerve cells
 Induction
Ability of one group of cells to
influence developmental direction
of other cells
Week 3: Mesodermal differentiation
 Somites
– Body segementation
Paraxial mesoderm
Intermediate mesoderm –
begins as a continuous strip of
tissue just lateral to the paraxial
mesoderm
Week 3: Mesodermal differentiation

Lateral plate – most lateral part of the
mesoderm
 Coelom
– becomes serous body cavities
 Somatic
mesoderm – apposed to
the ectoderm
 Splanchnic mesoderm – apposed
to the endoderm
Parts of the mesoderm
Neurulation and notocord
Neuralation and notocord
Week 4 – Embryonic folding
 Folding
of embryo laterally and at the
head and tail
 Embryonic disc bulges; growing faster
than yolk sac
 Primitive gut – encloses tubular part
of the yolk sac
 Site of future digestive tube and
respiratory structures
Week 4 – The Body Takes Shape
Week 4 – Folding

Derivatives of the germ layers
 Ectoderm forms
Brain, spinal cord, and epidermis
 Endoderm forms
Inner epithelial lining of the gut tube
Respiratory tubes, digestive organs, and
urinary bladder
Notochord – gives rise to nucleus
pulposus within intervertebral discs
Week 4 – Folding and systemic
development

Mesoderm – forms
Muscle
 Bone
 Dermis
 Connective tissues


Mesoderm differentiates further and is more
complex than the other two layers
Week 4 – System development

Mesoderm (continued)

Somites divides into
Sclerotome
 Dermatome
 Myotome


Intermediate mesoderm forms

Kidneys and gonads
Week 4 – System development

Mesoderm (continued)
 Splanchnic mesoderm
Forms musculature, connective tissues,
and serosa of the digestive and respiratory
structures
Forms heart and most blood vessels
 Somatic mesoderm – forms
Dermis of skin
Bones
Ligaments
Derivatives of Germ Layers
Figure 3.10
The Germ Layers in Week Four
Week 5-8 – Organogenesis
 Limb
buds form
 Embryo first looks recognizably human
(week 8)
 Head is disproportionately large
 All major organs are in place
Fetal period- summary
Fetal period- summary
Fetal period- summary