WSO School of Biomedical Sciences, University of Hong Kong
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WSO
School of Biomedical Sciences,
University of Hong Kong
Embryonic period –
from fertilization to
the eighth week.
Fetal period – begins at
week nine and continues
until birth.
Obstetricians divide the nine months of pregnancy
into three trimesters.
• The first trimester – the period of embryonic and
early fetal development. It is the most critical
stage of development – the rudiments of the major
organ systems appear.
• The second trimester – complete development of
organ systems
• The third trimester – rapid fetal growth. During
the early stage of this period, most of the organ
systems are fully functional.
• Describe the processes associated with
fertilization, morula formation, blastocyst
development and implantation.
• Describe the formation of the three germ
layers - ectoderm, mesoderm and
endoderm
Objectives:
Fertilization
• Gametogenesis – production of sex cells
• Gametes are haploid - ♂:sperm 22X /22Y.
♀:oocyte 22X
• Sex of embryos are determined at fertilization.
• During fertilization, the genetic materials from
a haploid sperm cell and a haploid secondary
oocyte merges into a single diploid nucleus.
• Fertilization takes place in the ampulla of the
oviduct.
Fertilization and Slow Block to Polyspermy
Prevention of polyspermy:(Polyspermy = fertilization of an ovum by more than
one sperm)
Fast block to polyspermy - binding of sperm to
egg opens sodium channels in the egg membrane.
A change in the electric charge inhibit further
binding of sperm to egg.
Slow block to polyspermy – Penetration of
sperm into the egg triggers inflow of Ca ++
calcium stimulate the release of cortical granules
on the egg surface making it impenetrable to other
sperms.
Fertilization –
At ampulla region
of oviduct
Cleavage stage – embryo
undergoes repeated division
without G1 phase
Blastocyst - reaching the
uterus around 4.5 to 5 days
post fertilization
Implantation – occurs
around 6 days post
fertilization
Fertilization and Cleavage
Maternal Paternal
Contribute maternal genome Contribute paternal genome
Restore diploidy
*** Determine sex of embryo – X/Y sperm
*** Paternal centriole contributes to embryo
fetal and adult somatic cells.
Maternal mitochondria replicate and
persist
Paternal mitochondria undergo
autophagy
Restore normal cytoplasm-nuclear ratio
Active transcription & protein synthesis ***
Changes from anaerobic to aerobic respiration
Embryo – maternal recognition of pregnancy
Cell fate determination
Activation of oocyte after fertilization
Cleavage
2-cell 30h post fertilization
4-cell 42h post fertilization
8-cell 70h post fertilization
16-cell 80h post fertilization
Morula Compaction
Blastocyst Trophectoderm
Inner cell mass
Blastocoele
Control of development – switches
from mother to conceptus
• The large volume of oocyte cytoplasm
distributes to blastomeres ribosome, protein
biosynthetic apparatus, mitochondria, Golgi
system an cytoskeletal system.
• Embryonic genome becomes active at
Mouse 2-cell
Rabbit 8-16 cell
Human 4-8 cell
Sheep 8 cell
Implantation – usually implants in the
fundus region of the uterus.
• Attachment phase: increase in vascular permeability in the area of stroma underlying the conceptus; oedema; change in extracelluar matrix composition.
• Invasion phase: cytotrophoblast cells fuse together and erode the adjacent endometrial tissues.
haemochorion
Gastrulation:
• 2nd week of development, inner cell mass
(ICM) forms bilaminar germ disc Epiblast
Hypoblast
• Bilaminar Trilaminar Ectoderm
germ disc germ disc Mesoderm
Endoderm
Cranial-caudal axis determined in gastrulation
Migration of
mesoderm
Oropharyngeal
membrane
Cloacal plate
Formation of neural crest cells
Neural tube development -1
Neural tube development -2
Neural crest cells derivatives
Fate of the ectoderm
Surface epidermal derivatives
ectoderm otic vesicles
olfactory placodes
tooth ameloblasts
anterior lobe of hypothesis lens of eyes
Ectoderm Neuroectoderm Head & pharyngeal mesenchymeNeural crest cells spinal & cranial ganglia
autonomic ganglia
melanoblast
leptomeningsadrenal medulla
tooth odontoblast
Neural tube brain
spinal cord
Paraxial m. Intermediate m.
Lateral
mesoderm
Axial and appendicular
skeletonSkeletal muscles
Diagrams to show lateral folding of the embryo.
Fate of the mesoderm
chorda mesoderm notochord
endothelioblastema
angioblastema haemoblastema
dermatome
paraxial mesoderm somites myotome
Mesoderm sclertome
reproductive system
Intermediate urinary system
mesoderm adrenal cortex
Lateral mesoderm somatopleure (Parietal)
splanchnopleure (Viceral)
Endoderm derivatives.
Fate of endoderm
Prochordal plate Pharyngeal
pouches
Foregut Respiratory
tract
Endoderm Roof of Primitive GI tract
yolk sac gut Midgut GI tract
Hindgut GI tract
cloacal plate
Allantois cloaca
References:
Netter’s Atlas of Human Embryology. Larry R.
Cochard, 2002, Icon Learning Systems, Teterboro, New
Jersey
Langman’s Medical Embryology. T.W. Sadler, 2006,
10th edition, Lippincott Williams & Wilkins, Philadelphia,
PA, USA
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