Download 04 Early Development - Biology Courses Server

It is not birth, marriage, or death, but
gastrulation, which is truly the most
important time in your life.
Lewis Wolpert, 1986
Early Human Development
This section will cover a series of lectures that will begin, in a serious way,
our exploration of human development. We will consider the developmental
stages critical to the first month of development as we follow the zygote along
its developmental path to its arrival at the basic vertebrate body plan. After
this lecture you should be able to:
• Outline and describe the stages associated with the first week of
development, the preimplantation period.
• Outline and describe the stages associated with the second week of
development, the implantation period.
• Outline the important third week and describe the the primitive
streak, the development of the notochord, and the differentiation of
the three germ layers.
• Describe the early differentiation of the germ layers and the folding
of the embryonic trilaminar plate.
21
First Week of Development
The result of cleavage
Zygote
Cleavage
What is it?
When does it begin?
Results – Blastomeres
Relation to zona pellucida
Timeline
Developmental stages
2-cell stage
30 hours
4-cell stage
40 hours
6 to 12-cell stage
Morula - at 12-cell stage
3 days
12 to 32-cell stage
Compaction - approximately around 19-cell stage
4 days
E m b r y o l o g y
22
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
E a r l y
Blastocyst
H u m a n
D e v e l o p m e n t
Timeline
Formation of blastocyst cavity
4 to 6
days
Shedding of the zona pellucida
Outer cell mass – trophoblast
Inner cell mass – embryoblast
Uterine contact
6 days
Embryonic pole
.
Actual size
Implantation
Differentiation of the trophoblast
Cytotrophoblast
Syncytiotrophoblast
Formation of the hypoblast (primary endoderm)
7 days
23
Second Week of Development
Timeline
Emergence of the bilaminar embryo
During this week the embryo forms two layers of important
cells and two sacs associated with each layer of cells.
It also implants into the uterine wall and initiates the
development of the placenta. The focus of this section will
be on the embryo. We will come back to the extraembryonic
membranes and implantation in a later lecture.
Overview of trophoblastic tissues
8 days
Cytotrophoblast
Syncytiotrophoblast
Bilaminar embryo stages and features
Epiblast
Hypoblast
8.5 days
Exocoelomic membrane
Exocoelomic cavity
Amnion
Primary yolk sac
9 days
Extraembryonic mesoderm
E m b r y o l o g y
24
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
E a r l y
H u m a n
D e v e l o p m e n t
Emergence of the bilaminar embryo
Closing plug
Timeline
Epiblast
Embryo proper
Amnionic sac
10 days
Actual size
Amnionic cavity
Coelomic spaces appear
12 days
Actual size
25
Hypoblast
Timeline
Primary yolk sac
Extraembryonic mesoderm
13 days
Extraembryonic coelomic space
Extraembryonic coelom
Connecting stalk
Secondary yolk sac
Prechordal plate
14 days
Actual size
E m b r y o l o g y
26
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
E a r l y
H u m a n
D e v e l o p m e n t
Bilaminar embryo
View at the end of two weeks
Superior view
Cross section
Longitudinal section
27
Third Week of Development
Emergence of the three embryonic germ layers
Gastrulation
What is it?
Timeline
Stages of gastrulation
Prechordal plate
15 days
Primitive streak
Primitive node
Primitive pit
Primitive groove
17 days
E m b r y o l o g y
28
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
E a r l y
Results of gastrulation
Formation of the mesoblast
H u m a n
D e v e l o p m e n t
Timeline
15 days
Replacement of the hypoblast
17 days
Trilaminar disc
18 days
Fate of primitive streak
29
Formation of the notochord
Notochordal process
Notochordal canal
Prechordal plate – oropharyngeal membrane
Timeline
16 days
17 days
18 days
E m b r y o l o g y
30
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
E a r l y
H u m a n
D e v e l o p m e n t
Formation of the notochord (cont.)
Fusion with underlying endoderm
Degeneration and formation of notochordal plate
Timeline
Neurenteric canal
Reclosure of notochord
18 days
18 days
19 days
31
Additional spread of the mesoblast
Lateral migrations
Cranial migration
Cardiogenic mesoblast
Prechordal mesenchyme
Caudal migration
Blocks to migration
Oropharyngeal membrane
Cloacal membrane
E m b r y o l o g y
32
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
E a r l y
H u m a n
D e v e l o p m e n t
Fate map of epiblastic plate
Notochord
Medial halves of somites
Lateral halves of somites
Lateral mesoderm
Extraembryonic mesoderm
Neural ectoderm
Body ectoderm
Primordial germ cells
Endoderm
33
Mesoderm – Differentiation of the Mesoblast
Timeline
Axial mesoderm
Notochord
21 days
Prechordal mesoderm
Cardiogenic mesoderm and septum transversum
18 days
Caudal eminence
Paraxial mesoderm
Formation of somitomeres
Formation of somites
20 days
Occipital somites
Craniocaudal progression
21 days
E m b r y o l o g y
34
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
E a r l y
H u m a n
D e v e l o p m e n t
Paraxial mesoderm (cont.)
Unsegmented paraxial head mesoderm
Intermediate mesoderm
Lateral mesoderm
Head
Postcranial
35
Ectoderm
Timeline
Neurulation
Neural plate
17 days
Neural groove
18 days
Neural folds
19 days
18 days
20 days
21 days
E m b r y o l o g y
36
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
E a r l y
H u m a n
D e v e l o p m e n t
Neurulation (cont.)
Neural tube
Neural crest
Placodes
Placodal ring
Body ectoderm
Endoerm
Allantois
37
Development of intraembryonic coelom
Timeline
Coelomic spaces
18 days
Intraembryonic coelom
Somatic mesoderm
20 days
Somatopleure
Splanchnic mesoderm
Splanchnopleure
21 days
Communication with extraembryonic coelom
E m b r y o l o g y
38
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
E a r l y
H u m a n
D e v e l o p m e n t
Fourth Week of Development
Achieving the basic vertebrate body plan
Timeline
Folding the trilaminar embryonic plate
Cranial–caudal folding
Head fold
22 days
23 days
Tail fold
26 days
28 days
Actual size
39
Side-to-side folding
Lateral folds
Midgut region
E m b r y o l o g y
40
L e c t u r e
M a n u a l
b y
M a r k
N i e l s e n
E a r l y
H u m a n
D e v e l o p m e n t
Basic vertebrate body plan
2
1
Actual size
3
Cross-section 1
4
5
6
One month embryo, lines depict levels of cross
sections below, gray depicts ectodermal ring
Cross-section 2
Cross-section 3
Cross-section 5
Cross-section 4
Cross-section 6
41
Germ Layer Derivatives
Appendicular skeleton
Dermis, subcutaneous
tissue, tendons,
ligaments, fascia, general
connective tissue
Parietal membranes
Dentine of teeth
Connective tissue of head
(including bones, fascia, tendons,
ligaments, dermis, etc.)
Cranial somatic sensory neurons
All postcranial sensory neurons
All postganglionic neurons
Enteric neurons
Adrenal medulla
Melanocytes
Blood vessels of head
Adrenal cortex
Visceral membranes and
mesenteries
Epicardium
Smooth muscle, connective
tissue, and blood vessels
of the gut tube
Somatic mesoderm
Splanchnic mesoderm
Trapezius
Brain and spinal
Lateral mesoderm
Sternocleidomastoid
cord (all motor and
interneurons),
Seminiferous tubules
Retina and optic
Epididymis
Basioccipital
bone
nerve
Vertebral column and Ductus deferens
Uterine tubes
ribs
Neural tube
Neural crest
Uterus
Body wall muscles
Upper vagina
Limb
muscles
Lens
Urinary tubes
Dermis of back
Inner ear
Hairs
Branchial (pharyngeal) Ureters
Olfactory nerves
Nails
muscles
Cranial visceral sensory
Sweat glands
neurons
Intermediate
Sebaceous glands
mesoderm
Extraocular
Mammary glands
Paraxial
Placodes
muscles
Glands of eyes
mesoderm
Ceruminous glands
Prechordal mesoderm
Nasal cavity lining
Cornea
Oral cavity lining
Conjunctiva
Parotid gland
Endocardium
Tooth enamel
Myocardium
Epidermis
Anterior pituitary
Ectoderm
Anal canal
Cardiogenic mesoderm
Mesoderm
Fascia, tendons, ligaments,
adipose tissue, general
connective tissue
Body ectoderm
Endoderm
Mesenchyme
Yolk sac
Epithelial lining of tympanic
cavity and pharyngotympanic
tube. Cells of thyroid gland,
parathyroid glands, tonsillar
recess, thymus, ultimobranchial
bodies. Lining of submandibular
and sublingual glands.
Epithelial lining of
respiratory system from
larynx to alveoli
E m b r y o l o g y
42
L e c t u r e
Gametes
(spermatocytes and
oocytes) of testes
and ovaries
Epithelial lining of digestive
tract from pharynx to rectum
Epithelial lining
of biliary tubes of
the liver and the
pancreatic ducts and
tubules
M a n u a l
b y
M a r k
N i e l s e n
Epithelial lining of
urinary bladder,
urethra, and lower
part of vagina