EMBRYOLOGY: AN OVERVIEW

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EMBRYOLOGY AN OVERVIEW
Dr. SARATHCHANDRAN Department of Zoology Sri
Venkateswera College University of
Delhi Delhi-110021
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EVOLUTIONARY TRENDS
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STAGES IN SEXUAL REPRODUCTION
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GAMETOGENESIS
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SPERMATOGENESIS
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STRUCTURE OF SPERM
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OOGENESIS
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Need to communication between ovary and uterus
(a)
Control by hypothalamus
Estrogen Progesterone
GnRH FSH, LH
Hypothalamus
Brain
High estrogen
GnRH
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Low estrogen
Anterior pituitary
FSH
LH
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(b)
Pituitary gonadotropins in blood
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Ovulation control
LH
FSH
FSH and LH stimulate follicle to grow
LH surge triggers ovulation
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(c)
Ovarian cycle
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Estrogen, Progesterone
Ovary
Corpus luteum
Degenerating corpus luteum
Growing follicle
Mature follicle
Luteal phase
Ovulation
Follicular phase
Progesterone and estrogen secreted by corpus
luteum
Estrogen secreted by growing follicle
in increasing amounts
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(d)
Ovarian hormones in blood
Peak causes LH surge
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Progesterone
Estrogen
Progesterone and estro- gen promote thickening of
endometrium
Estrogen level very low
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(e)
Uterine (menstrual) cycle
Endometrium preparation
Endometrium
Uterus
Menstrual flow phase
Secretory phase
Proliferative phase
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Days
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Ovary and Uterus Coordination
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Prenatal Development

• Embryonic development
• fertilization - 8 weeks

• Fetal development
• 9 weeks - birth

time period from fertilization to birth
gestation
Postnatal Development
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FERTILIZATION
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• Fertilisation thus has some consequences
• Restoration of diploidy
• Chromosomal sex determination
• Completion of meiosis II in the oöcyte
• Initiation of the first cell division (cleavage)

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Fertilization and the Events of the First 6 Days
of Development
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Cleavage

• Holoblastic (complete)
• Xenopus, mouse, human

• Meroblastic (incomplete)
• Fish, Chick

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COMPACTION
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BLASTOCYST

• Inner cell mass
• Trophoblast
• Zona pellucida disappears
• Implantation

• Implantation is the process of embedding the
  embryo in the uterine wall.
• Disappearance of the zona pellucida and the
  invasive nature of trophoblastic tissue are the
  major factors in implantation.
• The second picture shows significant changes in
  the embryo these are explained in the next two
  slides.

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Formation of blastocyst

• Two cell types
• inner cell mass (ICM)
• internal cells at morula, form embryo and
  associated yolk sac, allantois and amnion
• trophoblast cells
• external cells at morula, give rise to
  chorion
• Cavitation and formation of the blastocoel
• trophoblast cells secret fluid into the morula
  to create a blastocoel

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Implantation of the Blastocyst
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The Primitive Streak
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Derivation of Mammalian Tissues
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Fig. 8.24
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Germ layers endoderm, mesoderm and ectoderm
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Formation of the Mesoderm and Notochord
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Changes in the Embryo
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Changes in the Embryo
Figure 3.7c, d
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Table 16.2 Structures Produced by the three germ
layers Ectoderm All nervous tissue epidermis
of skin, parts of eyes and ears, hair, pituitary
gland, adrenal medulla Endoderm most linings of
digestive system, of respiratory system Mesoderm
muscle, cartilage, bone, blood, Kidneys and gonads
Neurulation.
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Developmental Events of the Fetal Period
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Developmental Events of the Fetal Period
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Developmental Events of the Fetal Period
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? At the beginning of the 2nd week, the embryo
has developed extensive membranes that lie in
close contact with the mother's tissues.
? Well into the 3rd week, the chorionic membrane
has continued to penetrate the mother's
endometrium. The balloonlike structure is the
yolk sac.
The embryo at the 4th week?. The dark lies
protected in its amniotic sac. The dark eye is
prominent and the enormous brain lies trucked
against the embryonic heart.
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?The human embryo is shown at 42 days with the
surrounding membrane removed .
It is about 16 mm long.
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At 6 weeks, with the amnion removed, the fingers
are apparent and the bulbous brain still
dominates the embryo. Notice the tail trucked
under the abdomen. The tiny pit above the arm
will become the ear.
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At about 7 weeks, the embryo afloat in its
amniotic fluid, is clearly anchored to its
placenta by the twisted umbilical cord through
which great blood vessels pass. The abdomen is
swollen due to the rapid growth of the liver, the
main blood-forming organ at this time.
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The 8-week embryo is shown here is front
view. The organs are now more or less complete.
The skeletal system is among the last to form,
but bones are now evident in the arms and legs.
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At 9 week lids have begun to grow down over the
eyes, and the outer ear begins to from. The fetus
may begin to move, wave its arms and legs, and
may even suck its thumb. It is now beginning to
fill its amniotic space and will assume the
typical upside-down fetal posture.
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At 10 weeks the skeleton is well along in its
development.
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At 14 weeks the fetus is e fist-sized. Ribs and
blood vessels are visible through the translucent
skin. The vigorous movements of the fetus can now
be felt by the mother. Refinements such as
fingerprints and fingernails have not yet
developed.
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By the end of 5 months the fetus is covered with
fine-downy hair and its head may have already
started to grow its own crop. The heart is
beating now at a rate of 120 to 160 times per
minute.
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Fig. 16-9 The development of the human embryo
showing relative size at different ages. Note
that at 4 weeks, every few features are clearly
distinguishable. The major organs have begun to
form. The embryo at this time is strangely
vulnerable to a variety of dangers from drugs to
radiation.