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Basic Embryology: Topics to be covered
Introduction and Clinical Relevance
Gametogenesis
Ovulation to Implantation
Bilaminar Germ Disc
Trilaminar Germ Disc
The Embryonic period
Fetus and Placenta
Birth Defects and Prenatal Diagnosis
Embryology is the branch of biology and medicine
concerned with the study of embryos and their
development.
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Objectives of Learning Embryology
Understanding the fundamental processes that convert a
microscopic single cell into a multitrillion structure of the a
human adult
Understanding the mechanisms responsible for congenital
malformations
Understanding the basic principles of reproduction in the
practice of Infertility, Genetics and Fetal Medicine
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Knowledge of Embryology is essential for a
health care provider---
It includes an investigation of molecular, cellular and systemic factors
contributing to the formation of an adult
It helps in creating health care strategies for better reproductive
outcomes
It results in newer techniques
• For prenatal diagnosis and treatments
• Therapeutic procedures to treat infertility
• To prevent birth defects
• To reduce infant mortality and morbidity
Helps us understand the mechanism of postnatal diseases and their
management
Helps us understand how the prenatal experiences play an important
role in our postnatal health.
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Embryology: History in brief--
Aristotle (384-322 BC)- Founder of Embryology
• Study of chick embryo- theory that developing organisms go
through a series of stages before acquiring their final form, a theory
later called epigenesis.
• Embryo rose from menstrual blood after activation by sperm
W. Harvey- Could not observe early stage of embryo.
So, he concluded that “Embryos are secreted by uterus”
Leeuwenhoek (17th Century)- Used improved
microscope and observed the spermatozoa
Mistakenly thought sperm contains the miniature preformed
human being that enlarged when it deposited in the female
genital tract
Von Baer (1827)- Father of Embryology: described
oocyte in the ovarian follicle in dog
Haeckel- “Ontogeny recapitulates Phylogeny”
Drawing of sperm (17th Century)
The miniature human being
within it was thought to enlarge
when the sperm enters the
ovum
Others believed that the oocyte
contained a miniature human
being that enlarged when it was
stimulated by a sperm
8. Gametogenesis:
Primordial Germ
Cells
§ Primordial Germ Cells give
rise to gametes- sperms
and eggs
Meiosis- to reduce the
number of
chromosomes- 23 in
each gamete
Cytodifferentiation to
complete maturation
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9. Primordial Germ
Cells
§ Gametogenesis: Primordial
Germ Cells give rise to
gametes- sperms and eggs
Meiosis- to reduce the
number of
chromosomes- 23 in
each gamete
Cytodifferentiation to
complete maturation
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10. The Chromosomal
Theory of Inheritance
§ Chromosomes- Diploid, 23 pairs
in body cells
§ Haploid, 23 in each gamete
§ 22 pairs of autosomes, one
pair of sex chromosomes
§ Sex Chromosomes- XX in
females, XY in males
§ Genes- 21300
§ Fertilization restores 46
chromosomes
§ 23 from oocyte and 23 from
sperm
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11. Mitosis
§ One cell divides into two
daughter cells, each with
genetic material identical to
the parent cell- 46
chromosomes
§ Four phases-
§ Prophase
§ Metaphase
§ Anaphase
§ Telophase
§ Centromere of each
chromosome divides during
anaphase
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12. Meiosis
§ Meiosis I- Actual Meiosis
o Prophase 1
o Metaphase 1
o Anaphase 1
o Telophase 1
§ Meiosis II – (Mitosis)
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2
§ Meiosis results in four
daughter cells, each with half
the number of chromosomes,
that are recombined
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13. Meiosis- Crossing over
and Recombination
§ Occurs In prophase I of Meiosis
§ Leads to exchange of
chromosome segments between
sister chromatids
§ Creates new combinations of
genes
§ Pairing creates a tetrad/ bivalent
§ Point of attachment of two
chromosomes is Chiasmata
§ Leads to
§ different combination of
genes than parents
§ Haploid chromosomes in each
gamete
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14. Polar Bodies
§ One primary oocyte gives
rise to four daughter cells
§ Each daughter cell has 22
autosomes and one X
Chromosome
§ Only one matures into
oocyte and the other three
are polar bodies
§ Polar bodies do not have
cytoplasm
§ Inn case of
spermatogenesis, each
spematogonia forms four
mature sperms
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15. Chromosomal
Abnormalities
Numerical
§ Polyploidies
o Triploidy
o Tetra ploidy
§ Aneuploidies
o Trisomy
o Monosomy
o Tetrasomy
Structural
o Deletion
o Duplication
o Inversion
o Insertion
o Ring
o Marker
o Unbalanced
o Balanced
o Robertsonian Translocation
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TRISOMY 21 (Down Syndrome)
21. Oogenesis
§ Ovulation- between menarche and
menopause
§ Oocytes in ovaries-
§ 2 Million at birth
§ 40, 000 at puberty
§ 400 ovulated over lifetime
§ Primordial germ cells 2N
§ Oogonium- 2N
§ Primary Oocyte 2N
§ Between birth and Puberty- Prophase I of
meiosis
§ At Puberty FSH completes Meiosis I
§ Secondary Oocyte 1N; Polar body
§ Meiosis II- Stimulated by fertilization
§ Ovum 1N; second polar body
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23. Spermatogenesis
§ Mitosis
§ Meiosis
§ Spermiogenesis
§ Head
§ Midpiece
§ Tail
§ Capacitation
§ Lifespan of a spermatozoa
§ Puberty through life
§ 40- 1000 million p d
§ 72 days for sperm
production
§ 10-14 days for transport
(epididymis)
§ 20-100 million per ml of
ejaculate
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24. Spermiogenesis
§ Transformation of a
spermatid into a mature
sperm
§ Formation of the
acrosome, which covers
the nuclear surface
§ Condensation of nucleus
§ Formation of neck, middle
piece and tail
§ Shedding of the cytoplasm
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