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INTRODUCTION TO
HUMAN DEVELOPMENT
Dr. Nabil Khouri
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Objectives
To provide a general description of human embryological
processes as follows:
Germ cell formation & fertilization
Prenatal development
Induction, competence, and differentiation
Formation of three-layerd embryo
Formation of neural crest
Formation of human tissues
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What is Embryology
Embryology is literally “the study of the embryo”
More generally it refers to “the study of prenatal development”
Embryo = 2-8 weeks (our focus)
Foetus = 9 weeks to birth
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Embryology: From Egg to Embryo Terms
1. Pregnancy – events occurring from the time of fertilization (conception) until the infant is born2. Gestation period – extends from the last menstrualperiod until birth (280 days)3. Conceptus – refers to the developing offspring at any time during the pregnancy
A) Pre-embryo – first two weeks following fertilizationB) Embryo – from the third through the eighth weeks after fertilizationC) Fetus – ninth week through birthD) Infant – at birth
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Development involves:
1) Cell division and differentiation
Differentiation is the creation of different types of cells
2) changes in anatomical structures
Anatomical changes include gradual modification of physical and physiological characteristics
Development begins at fertilization
Differentiation and development
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Development can be divided into: Prenatal and postnatal
development
Prenatal development begins at fertilization and ends with birth
Prenatal development includes:
Embryological development
Changes occurring during the first two months after
fertilization
Fetal development
Begins at the start of the ninth week and continues until
birth
Postnatal development
Commences at birth and continues to maturity
Stages of development
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Fertilization is fusion of two haploid gametes (egg and
sperm) each with 23 chromosome to produce a zygote
that contains 46 chromosomes
Fertilization occurs in the uterine tubes
Within a day of ovulation
Spermatozoa cannot fertilize an ovum until after
capacitation
Fertilization (conception) “Definition”
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Embryology: Fertilization “fusion of sperm and egg”
1. Copulation must occur accordingly
A) Egg is viable for 12-24 hrs after ovulation
B) Sperm is viable for 24-72 hrs after ejaculation
C) Copulation must occur no more than three days before ovulation and no later than 24 hours after
2. Sperm must reach the egg
A) Only a few hundred thousand sperm in a males ejaculate actually make it to the uterine tubes
1) Millions leak immediately from the vagina
2) Millions are destroyed by acidity of the vagina
3) Only about 2,000-3,000 sperm actually make it to the egg
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Acrosomal reaction
3. The process of Sperm penetration requires 1. Capacitation, 2.
Acrosomal reaction, 3. Fertilization membrane formation
1) Capacitation – as the sperm swims towards the oocyte, the cholesterol that keeps the acrosome “tough” degrades causing the membrane to weaken and the enzymes within to be released Takes about 6-8 hours
2) Acrosomal reaction – release of the acrosomal enzymes triggered by the sperm coming into contact with oocyte membranes
a) First-arriving sperm create holes in the corona radiata it often takes hundreds of sperm to create the holes.
b) A later-arriving sperm reaches the zona pellucida and releases enzymes creating a hole in the zona pellucida
c) Once the sperm reaches the oocyte membrane, its nucleus is pulled into the oocyte cytoplasm
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Spermatozoa release hyaluronidase and acrosin
Enzymes required to penetrate corona radiate
Single spermatozoan contacts oocyte, fertilization begins
Oocyte activation
Polyspermy prevented by membrane depolarization and cortical reaction
3). Upon entry of the nucleus, the oocyte creates a Fertilization membrane just beneath the zona pellucida
forces out other entering sperm & prevents entry of
future sperm
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Ovulation occurs before the oocyte is completely mature
i.e. oocyte is ovulated before the process of meiosis is
completed
Ovulated oocyte is in metaphase of meiosis II
If the egg is fertilized the process of meiosis will
complete
The released oocyte is surrounded by a thick
specialised extracellular matrix, the ZONA PELLUCIDA,
that in turn is covered in layers of cells,
the GRANULOSA LAYER.
If fertilization does not occur, the oocyte disintegrate
without completing the process of meiosis
The Oocyte at Ovulation
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Fertilization
https://youtu.be/7G2rL5Cutd4
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Fertilization Steps
• Step 1 Ovulation
• At ovulation the oocyte is in metaphase of meiosis II
• Both the occyte and the polar body is surrounded by corona radiate
• Step 2 Fertilization
Oocyte is surrounded by the sperms
Acrosomal enzyme from several
sperms creates gaps in corona radiata
One sperm makes contact with the
oocyte membrane
• Sperm and oocyte fused
• The process of meiosis will complete
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The first trimester is the period of embryological and
early fetal development
Four processes occur during the first trimeter
1) Cleavage
2) Implantation
3) Placentation
4) Embryogenesis
The First Trimester
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Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.
Discussed
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Pre-embryonic Development
1. CLEAVAGE occurs as the zygote travels through the
uterine tube and into the uterus
A) Period of rapid mitotic divisions
1) 2-cell stage – 36 hours after fertilization
2) 4-cell stage – about 48 hours
3) 8-cell stage – about 72 hours
4) Morula – solid ball of cells that is 16 or more
cells big
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Cleavage and
Formation of
Morula and
Blastocyst
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2. The morula hollows out and fills with fluid (now known as a blastocyst)
A) Blastocoel – hollowed-out region of the blastocyst
3. The zona pellucida disintegrates and releases the blastocyst
A) The blastocyst is composed of 2 cell layers
1) Trophoblasts –the large flattened cells of the outer layer
a) will take part in placenta formation
b) b) secrete hCG to prompt the corpus luteum to continue secreting progesterone in order to maintain the endometrium
2) Inner Cell Mass (embryoblast) – a cluster of small
rounded cells of the inner layer becomes the actual embryo
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Division of
the zygote Zygote. Formation of
the morula
Morula
Blastula Or
Blastocyt
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Structure of the blastocyst (blastula):
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Implantation
A) When the blastocyst reaches the uterus, it initially floats
freely, receiving nourishment from the endometrial secretions
(uterine milk)
B) Six or seven days after ovulation, the trophoblast cells embed
into the endometrium and begin secreting digestive enzymes
that degrade the endometrial surface
C) As the endometrium is eroded, the blastocyst burrows into the
lining
D) The endometrial lining reacts by growing over the blastocyst
E) The chorion develops from the trophoblast cells starting to give
rise to the placenta
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Copyright © 2006/07 The
University of Sharjahslide 24
Implantation of the blastocyte:
Uterine wall
Endometrium
Digested
endometrium
Blastocyte
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Relation of
Blastocyst to
Endometrium –
Time of
Implantation
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Implantation
Decidua
As the blastocyst implants, usually on the posterior wall of the fundus or body of the uterus, it is oriented so that the inner cell mass faces the endometrium
Different regions of the decidua are named based on their positions relative to the site of the implanted blastocyst
1. Decidua basalis
2. Decidua capsularis
3. Decidua parietalis
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Relation of Blastocyst to Endometrium – Time
of Implantation
Implantationhttps://youtu.be/pO84bn6IhGU?list=PLG4nig
1r_0xyqML6OvgCX0QXAq8cRkQsp
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Embryonic Development
A. The blastocyst is converted into the gastrula in which the
embryonic membranes develop and three primary germ
layers form
1. Gastrulation – process by which the embryonic
tissues are formed
2. The embryonic membranes form as the inner cell
mass splits to form upper and lower cell layers
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During the Second Week of Development
Development of the trophoblast: the trophoblast
develops two layers:
1. outer syncytiotrophoblast
2. inner cytotrophoblast
Trophoblast produces human chorionic gonadotropin
(hCG)
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Events of Second Week of Development
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Second Week of Development
Development of the bilaminar embryonic disc
hypoblast (primitive endoderm)
epiblast (primitive ectoderm)
bilaminar embryonic disc
Amniotic cavity
Amniotic fluid
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Events of Second Week of Development
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Second Week of Development
Development of the yolk sac
Exocoelomic membrane
in humans, the yolk sac is relatively empty, small, and
progressively decreases in size
it has several important functions
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Embryology
A) Amnion (amniotic sac) – forms from the upper cell layer
This sac fills with amniotic fluid that provides a buoyant
environment that protects the developing embryo
B) Yolk sac – forms from the lower cell layer It serves to form part
of the digestive tube, produces the earliest blood cells and blood
vessels, and is the source of primordial germ cells of the embryo’s
gonads
C) Allantois – forms as a small out-pocketing of the yolk sac
1) Acts as the structural base of the umbilical cord and becomes
part of the urinary bladder
D) Chorion – develops from proliferating trophoblast cells giving
rise to the placenta
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Second Week of Development
Development of sinusoids: small spaces called
lacunae
Lacunar networks
Sinusoids
maternal blood and endometrial secretions enter the
lacunar networks to provide embryonic nutrition and to
serve as a disposal site for embryonic wastes
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Second Week of Development
Development of the chorion
it surrounds the embryo and, later, the fetus
it eventually becomes the major embryonic component of the
placenta
Gonadotropin (hCG)
Chorionic cavity
Connecting (body) stalk, the future umbilical cord
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Third Week of Development
Gastrulation
the bilaminar embryonic disc is transformed into a trilaminar embryonic disc consisting of three primary germ layers:
ectoderm, mesoderm, and endoderm
Gastrulation
primitive streak
primitive node
Invagination results in formation of the three primary germ layers
Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.
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Formation of the tri-laminar disc:
Primitive streak
Bilaminar disc
Epiblast layer
Hypoblast layer
Third week
Formation of primitive streak at the midline causing the disc to have right
and left half, each half will be as mirror to the other
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Ectoderm layer
Endoderm layer
Mesoderm layer
Formation of the mesodermal layer:
Movement of some cells of epiblast toward hypoblast at the
primitive streak forms a third embryonic layer called mesoderm.
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During the third week
The primary germ layers form along the embryo
A) Ectoderm – gives rise to skin and nervous system
B) Endoderm – gives rise to the functional linings of the
digestive, respiratory, and urogenital systems
C) Mesoderm – gives rise to muscle, bone, blood vessels,
kidneys and all the other components of organs (except
linings)
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Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.
Gastrulation
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Third Week of Development
Notochordal process develops
notochord
The (1) oropharyngeal membrane and the (2)
cloacal membranes develop
Gastrulation:https://www.youtube.com/watch?v=3AOoikTEfeo&list=PLG
4nig1r_0xyqML6OvgCX0QXAq8cRkQsp&index=3
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Development of Notochordal Process
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The embryonic layers are distinct from each other and
give rise to specific tissues such as:
Ectoderm
Nervous system.
Sensory epi. of eye , ear & nose.
Epidermis, hair and nails.
Mammary & cutaneous glands.
Epi.of sinuses,oral &
nasal cavities.
Intra-oral glands.
Tooth enamel.
Formation of the tissues:
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Formation of the tissues:
Mesoderm
Muscle tissue
C.T
Bone .
Cartilage.
Pulp.
Dentin .
Cementum.
PL.
Ecto-mesoderm
Alveolar Bone .
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By the end of the Third Week of
Development
Allantois develops
Neurulation
The head end of the neural tube develops into the
brain
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Formation of Neural
Crest
Neural plate: will fold to
form neural groove
Neural groove: fuses to
form neural tube
Neural tube: Gives rise to
brain and spinal cord
Neurulation:
https://www.youtube.com/w
atch?v=lGLexQR9xGs&list=
PLG4nig1r_0xyqML6OvgCX
0QXAq8cRkQsp&index=4
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Neurulation and Development of Somites
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Fourth Week of Development
Organogenesis
Embryonic folding
head fold, tail fold, and two lateral folds
Formation of the primitive gut
Pharyngeal (throat)
Future ears and eyes occurs
Upper limb buds, lower limb buds, and a heart
prominence
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Embryonic
Folding
https://www.yout
ube.com/watch?v
=yXUv4MPuNTA&
list=PLG4nig1r_0
xyqML6OvgCX0Q
XAq8cRkQsp&ind
ex=5
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Development of
Pharyngeal
Arches, Clefts,
and Pouches
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Fifth Through Eighth Weeks of Development
Rapid development of the brain and head
Development of distinct limb regions
Further development of the eyes and ears
Tail disappears
External genitals begin to differentiate
End of the eighth week, the embryo has clearly
human characteristics
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Placenta Formation
A. Functions to exchange waste products and blood gases
B. The chorion develops chorionic villi which extend
into the endometrium where they come into
contact with maternal blood supply
C. Placenta takes over the role of secreting hCG
and also secretes relaxin: causes the pubic
symphysis to soften and become more flexible
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Placentation
The process of forming the placenta
Shaped like a pancake, it consists of a fetal portion
formed by chorionic villi and a maternal portion
formed by the decidua basalis
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Development of the Chorionic Villi and
Placenta
Development of lacunae
Chorionic villi grow and blood capillaries develop
within them
Maternal and fetal blood do not normally mix
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Development of Chorionic Villi
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Development of Chorionic Villi
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Development of Chorionic Villi
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Placenta
Umbilical cord consists of:
two umbilical arteries that carry deoxygenated fetal
blood and wastes from the fetus to the placenta
one umbilical vein that carries oxygenated blood and
nutrients from the placenta into the fetus
Wharton’s jelly
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Placenta and Umbilical Cord
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Placenta and Umbilical Cord
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Embryology
4. Circulation in fetus versus newborn
A) Fetal circulation has several adaptations so that the
lungs and liver are largely bypassed because they are
non-functional
1) The umbilical vein carries oxygen- and nutrient-
rich blood from the placenta to the fetus
2) The umbilical arteries carry deoxygenated,
waste-laden blood from the fetus to the placenta