Download umbilical vesicle (yolk sac)

HUMAN EMBRYOLOGY
(First eight weeks of early human development)
Recommended reading:
Keith Moore, Vid Persaud
The developing human: Clinically oriented
embryology. 8th ed. Elsevier, 2008: 1-91, 110-143
Progenesis – includes formation
of spermatozoa and oocytes male and female germ cells
• Spermatogenesis – spermatozoa
development - is maintained in the
seminiferous tubules of testes
• Oogenesis – oocyte production - is
maintained within the ovarian follicles
MORPHOLOGY OF
SPERMATOZOON AND OOCYTE
FERTILIZATION
•
Fusion of a spermatozoon with oocyte
resulting in formation of one-cell embryo –
zygote
•
Fertilization in most of the cases occurs
within the ampullary region of the uterine
tube on days 14-15 of ovarian-menstrual
cycle (after the ovulation – oocyte release
from the ovary) and insemination with
sperm
•
Fertilization is accounted as the first day of
embryogenesis
MAIN EVENTS DURING FERTILIZATION
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Capacitation
Acrosomal reaction
Penetration of corona radiata and zona pellucida
Fusion of the oocyte and sperm cell membranes
Cortical and zona pellucida reactions
Resumption of 2nd meiotic division
Metabolic activation of the egg
Restoration of diploid number of chromosomes
Fertilization
The main results of fertilization:
• Restoration of the diploid number of
chromosomes
• Determination of the sex of embryo
• Initiation of a cleavage
CLEAVAGE of the zygote
(first week of development)
Zygote → blastomeres → 8-cell stage
(compaction) →16-cell stage (morula,
or mullberry) → blastocyst
↓
inner cell mass → embryoblast
outer cell mass → trophoblast
Cleavage means enlargement of total cell number
with simultaneous reduction of their size
resulting from the abbreviated mitotic cycle
Cleavage of the zygote and formation
of the blastocyst
Summary of the ovarian cycle, fertilization and
human development during the first week
IMPLANTATION
• Adhesion – attachment of blastocyst to
the surface of endometrium – day 5
• Invasion – introduction of blastocyst
inside the uterine wall resulting from its
lytic digestion – days 6-12
• Uteroplacental circulation – starts day
10, when maternal blood from eroded
uterine vessels provide nutritional
support to the developing embryo
Early stages of implantation: A – 6 days – blasocyst
attachment; B – 7 days – invading blastocyst
BILAMINAR EMBRYONIC DISC FORMATION:
SECOND WEEK OF DEVELOPMENT
Embryoblast differentiates into bilaminar embryonic disc:
• Epiblast
• Hypoblast (local thickening of which forms prechordal
plate, indicating future cranial region of the embryo)
Trophoblast differentiates into:
• Cytotrophoblast
• Syncytiotrophoblast, which erodes blood vessels of
endometrium and establish haematotrophic circulation
Cavities formation:
• Amniotic cavity
• Umbilical vesicle (yolk sac)
Implanted blastocysts: A – 10 days; B – 12 days
End of the second week (day 14): formation of
chorionic sac and chorionic villi
THIRD WEEK OF DEVELOPMENT:
GASTRULATION – formation three of germ layers
• Formation of the primitive streak with primitive
node and primitive pit
• Migration of cells from epiblast
• Epiblast gives birth to three primary tissues:
- ectoderm
- mesoderm
- endoderm
Resulting from cellular migration and cavities
formation embryo is becoming three laminar,
surrounded with two cavities – umbilical vesicle
(yolk sac) and amniotic cavity
Origin of embryonic tissues
Dorsal view of a 16-day embryo
Third week: further events
• Notochord formation
• Development of somites (each of it including
dermatome, sclerotome and myotome)
• Neural tube and neural crest formation
(neurulation)
• Development of primordial cardiovascular system
• Development of intraembryonic coelom
• Development of secondary and tertiary chorionic
villi
18 day: formation of notochord and neural groove
Embryos of days 19 to 21: formation of somites
Embryonic days 19-21: neurulation
Day 21: primordial cardiovascular system and
tertiary chorionic villi
Organogenesis: fourth to eight weeks
During the fourth week a flat trilaminar embryonic
disc is folding with the formation of a cylindrical
shape embryo
• Folding of the ends of the embryo in the median
plane results in formation of cranial and caudal
regions moving ventrally as the embryo elongates
• Folding of the embryo in the horizontal plane
produces right and left lateral folds
Folding of embryos during days 21-22-26-28
Derivatives of three germ layers:
endoderm, ectoderm and mesoderm
ENDODERMAL GERM LAYER DERIVATIVES
• Epithelial lining of
- gastrointestinal tract
- respiratory tract
- urinary bladder
• Parenchyma of
- thyroid, parathyroid glands
- liver and pancreas
• Epithelial lining of the tympanic cavity and
auditory tube
ECTODERMAL GERM LAYER DERIVATIVES:
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central nervous system
peripheral nervous system
sensory epithelium of ear, nose and eye
epidermis (including hair and nails)
pituitary, mammary and sweat glands
oral cavity mucosa, enamel of the teeth
MESODERMAL GERM LAYER DERIVATES
• splanchnic, or visceral mesoderm layer
• intermediate mesoderm
• somatic, or parietal mesoderm layer
• dorsal mesoderm, which forms somites
Each somit consists of three components:
- sclerotome
- dermatome
- myotome
Human embryo 28 days with somites
Number of somites correlated to approximate
age in days
Approximate age
(days)
20
21
22
23
24
25
26
27
28
30
No. of somites
1-4
4-7
7-10
10-13
13-17
17-20
20-23
23-26
26-29
34-35
EXTRAEMBRYONIC ORGANS
• Placenta
- embryonic part – chorionic membrane
(transformed trophoblast of the blastocyst)
- maternal part – decidual membrane (pregnancytransformed functional layer of endometrium)
• Umbilical cord (formerly connecting stalk)
• Umbilical vesicle (yolk sac)
• Allantois
• Amnion
Sagittal section through gravid uterus at 4 weeks
56 days old embryo with fetal membranes