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Course 114
General EmbryologyPART V
Dr. Karim M. Khalil
Lecturer of Veterinary Anatomy and EmbryologyCairo University
/ Karim Khalil
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Four mammalian embryos at various stages of development:
• A Pig
• B Calf
• C Rabbit
• D Human
“Please, note the similarities in the different developing mammalian species!”
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Refers to the study of the development of an
embryo immediately after conception to the
fetal stage.
Embryology
Embryo = unborn ology = science
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Development
Prenatal Postnatal
From fertilization onset up to time of birth
After birth up to time of death .
Fertilization stage: 1st week
Embryonic stage: 2nd – 8th week
Fetal stage: 8th – birth date 4
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Embryogenesis
Action Day
Fertilization - Zygote1 – 2
Zygotic stage - Morula 2 – 5
Blastocyst – Implantation6 – 7
Embryonic disc & Germinal layer8 – 12
Neural tube - Notochord 15 – 20
Fetal membranes & Placenta21 - 25
Organogenesis & Heart beating26 – 355
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Embryogenesis
• The process by which the embryo is formed and
develops.
• It starts with the fertilization of the ovum (or
egg) which, after fertilization, is referred to as
a zygote.
• The zygote undergoes rapid mitotic divisionswith no significant growth (a process knownas cleavage) and cellular differentiation,leading to development of an embryo. 7
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Division
Mitosis Meiosis
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Differences between Mitosis and Meiosis
MEIOSISMITOSISEvents
Only in the germ
cells.
In somatic & germ
cells.Occurrence
reductional
division.
It is an equational
division.Definition
FourOnly twoDaughter cells
Occurs Does not occur.Synapsis
Occurs Does not occur.Crossing over
Do not split Each centromere
splits into two.Centromeres
HaploidDiploidChromosomes No. 11
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Gametogenesis
Spermatogenesis Oogenesis
The term is applied for the development of male gametes
(Spermatogenesis) and female gametes (Oogenesis)
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The difference between spermatogenesis and oogenesis
OogenesisSpermatogenesisComparison
Ovary, cortexSeminefrous Tubules (Testes)Occurrence
Ootid + 3 polar bodies4 spermEnd result
LargerSmallerSize of Gametes
Puberty up to
menopausePuberty up to deathContinuity
Immotile ovaMotile spermMotility
Only XX or YChromosomes
AnabolicCatabolicMetabolic activity
AbsentPresentSpermiogenesis
Stimulate follicular
development
Act on sertoli cells (stimulate
spermatogenesis)FsH
Ho
rmo
nal
acti
vity
Stimulate ovulationAct on leydig cells to secrete
testosteroneLH 15
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A repeated cyclic changes occurs in the
uterus and ovary accompanied by shedding
of endometrium.
Oestrous cycle Menstrual cycle
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Animals are classified according to repetition of oestrous cycle into:
Type of estrous
Polyestrous
Cow Sow
Woman
Monoestrous
Wild Animals
Seasonal Polyestrous
MareCatEwe
Seasonal Monoestrous
BitchBear
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Classification of ova according to Amount and Distribution of yolk
Acc
ord
ing
to: Amount of
yolk
Microlecithal
(oligolecithal)Mesolecithal
Macrolecithal
(megalecithal)
Distribution
of yolkisolecithal
Heterolecithal
Extreme
telolecithalTelolecithal Centrolecithal
CleavageTotal (complete) Holoblastic
Partial (incomplete)
Meroblastic
Equal Unequal Discoidal Superficial
Animal speciesMammals
and
Amphioxus
Amphibian Birds Insects
Figure
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• Corpus luteum of menstruation (cyclic) persists
for 10-14 days in non fertilizing ovum.
• Corpus luteum of pregnancy persists up to 6
months then degenerated as its function
being carried out by the placenta.
Fate of Corpus Luteum
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Fertilization
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Cleavage: rapid succession of cell divisions... without cellgrowth - no increase in size, only an increase incell numbers forms hollow ball of cells calledBLASTULA or blastocyst, with internal fluidcavity is called the blastocoel.
• Animal pole: portion of embryo housing primary tissues.• Vegetal pole: portion of embryo containing "yolk“.
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Classification of ova according to Amount and distribution of yolk
Acc
ord
ing
to: Amount of
yolk
Microlecithal
(oligolecithal)Mesolecithal
Macrolecithal
(megalecithal)
Distribution
of yolkisolecithal
Heterolecithal
Extreme
telolecithalTelolecithal Centrolecithal
CleavageTotal (complete) Holoblastic
Partial (incomplete)
Meroblastic
Equal Unequal Discoidal Superficial
Animal speciesMammals
and
Amphioxus
Amphibian Birds Insects
Figure
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Embryogenesis
Action Day
Fertilization - Zygote1 – 2
Zygotic stage - Morula 2 – 5
Blastocyst – Implantation6 – 7
Embryonic disc & Germinal layer8 – 12
Neural tube - Notochord 15 – 20
Fetal membranes & Placenta21 - 25
Organogenesis & Heart beating26 – 3533
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1. Cleavage
2. Morula
3. Blastula
4. Implantation
Blastogenesis includes:
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Implantation
The process of embedding the blastula in the endometrium of the uterus.
• It occurs on 7th day after
fertilization.
• The blastocyst is
implanted inside the
maternal endometrium
by its embryonic pole.36
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How the blastocyst is implanted?
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The eroded endometrium
is now called
Decidua
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Complete implantation Incomplete implantation
Blactocyst is completely
hidden inside the
endometrium
Chorionic villi lie in
opposition to endometrium
without penetration
All mammals except >>Ruminant ,mare ,sow and
dolphin (Marine mammals)
True placenta False placenta
Types of Implantation
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Gastrulation
• Means the process of development of the 3
germinal layers of the embryo.
• The developmental events from the Blastula
up to Gastrula (Chorionic vesicle) including
germ layers, neural tube and notochord.
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Day 8 - 1042
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Day 1243
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• Formation of Notochord.
• Development of the neural tube and neural
crest.
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Derivatives of the neural tube and neural crest
C. N. S. (Brain and spinal cord)
1. Sensory ganglia.2. Autonomic ganglia.3. Pigment cells
(melanocytes).4. Schwann cells
(neurolemma).5. Sympathatic chain.6. Adrenal medulla.7. Arachnoid and pia
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Differentiation of intraembryonic
mesoderm
1. Dorsal (Paraxial) mesoderm. (Somites)
a) Sclerotome ->> bones, cartilage, connective tissue.b) Myotome ->> skeletal Muscles.c) Dermotome ->> Dermisof the skin.
2. Intermediate mesoderm.
3. Lateral mesoderm.54
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Ectoderm Mesoderm Endoderm
• C. N. S. • (Brain and spinal
cord)• Epidermis of the
skin.• Ear and Eye.• Pituitary gland• Adrenal medulla
• Connective tissue• Cartilage and
Bones.• Muscles• Dermis of the skin• Urogenital system.• Cardiovascular• Lymphatic• Body Cavities• Respiratory system• Alimentary tract• Adrenal cortex
• Epithelial lining of respiratory system.
• Epithelial lining of Digestive system.
• Epithelial lining of urinary bladder and urethra.
• Epithelial lining of ear
• Parenchyma of glands
Main Derivatives of the 3 germ layers
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Embryogenesis
Action Day
Fertilization - Zygote1 – 2
Zygotic stage - Morula 2 – 5
Blastocyst – Implantation6 – 7
Embryonic disc & Germinal layer8 – 12
Neural tube - Notochord 15 – 20
Fetal membranes & Placenta21 - 25
Organogenesis & Heart beating26 – 3557
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Day 8 - 1059
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Day 1260
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Development of the chorionic villi
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Placenta
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Placenta
A circular disc (15 cm. diameter, 4cm thickness and 500 gm/weight) .
It has 2 surfaces:
• Maternal surface: rough irregular has from 15-20
cotyledons.
• Fetal surface : smooth and shiny covered by the
amnion. It gives attachment to the umbilical cord
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I. The pattern of implantation.
II. Anatomically based on placental shape and
contact points or distribution of the chorionic
villi
III. Histologically based on the structure of
attached surface (penetration of villi into the
decidua)
Classification of placenta according to
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Complete implantation Incomplete implantation
Blactocyst is completely
hidden inside the
endometrium
Chorionic villi lie in
opposition to endometrium
without penetration
All mammals except >>Ruminant ,mare ,sow and
dolphin (Marine mammals)
True placenta False placenta
I. Patterns of Implantation
Classification of placenta according to
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II. Anatomically, based on placental shape
Classification of placenta according to
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III. Histologically based on the structure
Classification of placenta according to
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The
pat
tern
of
imp
lan
tati
on
Placenta Non deciduated
False placenta
Deciduated
True placentaImplantation Incomplete Complete Bleeding in
parturation - +
Shedding of
endometrium - +
Anatomically Diffuse Cotyledonary Zonary Discoidal
HistologicallyEpithelio
chorial
Syndesmo
chorial
endothelio
chorial
Hemo
chorial
Animal species Mare and
SowCow and Ewe
Bitch and
Queen
Man
,Monkey
and
Rodent
Types of Placenta
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Foetal membrane of mammals & umbilical cord
Foetal membrane Embryonic origin Function
AmnionExtraembryonic mesoderm
lined by ectoderm
1. Protection against external risk.
2. Prevent adhesion.
3. Give aseptic medium.
4. Facilitate free movement.
5. Good cervical dilator during
delivery.
6. Provide equal pressure on fetus.
Primary yolk
sac
Roof by endoderm while the
rest by exocoelomic membrane Give rise: primary sex cells ,midgut
and allantois.
Its main function is nutrition
through the vitellinointestinal
duct.
Secondary yolk
sacCompletely lined by endoderm
Definitive yolk
sac
Lined by endoderm and
covered by mesoderm derived
from trophoblast
AllantoisEndodermal protrusion from
the caudal wall of yolk sac Excretion
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Foetal membrane Embryonic origin Function
Chorion Syncitio and cytotrophoblast Part of placenta
PlacentaChorioallantoic membrane +
endometrium
1. Nutritive material.
2. Oxygen.
3. Waste products.
4. Act as barrier against
bacteria and drugs.
5. Endocrine function
(progesterone).
Umbilical cordMesoderm from connecting
stalk covered by amnion.
It is forms the Wharton,s jelly.
Connects the fetus with mother
through 2 umbilical artery and 1
left umbilical vein
Foetal membrane of mammals & umbilical cord