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Pregnancy & Human
Development
Chapter 29 (28)
Fertilization: It’s all in the timing!
• Oocyte is only viable for ~ 24 hours.
• Sperm is viable for 12 – 24 hours (some
“super sperm” may be viable for up to 72
so be careful!)
• Therefore, usually, coitus must occur
within a 24 hour window on either side of
ovulation.
Barriers to fertilization
• Low vaginal pH
• Getting lost (50/50 chance of getting the
right uterine tube)
• Numerous defective sperm
• Uterine contractions
• Phagocytes
• By the time they get to the oocyte, there
are only a few dozen to a couple hundred
Penetration
Secondary
oocyte
Head of
sperm
1650X
No
Polyspermy!
Fertilization
&
First
cleavage
What’s this thing called, Love?
• Zygote – a the single cell after fusion of
the pronuclei of the oocyte & the sperm.
• Conceptus – covers the period of develop
following first cleavage and differentiation
of cells into an embryo.
– Morula – the conceptus as a solid ball of 16
cells (about day 3).
– Blastocyst – a hollow ball of cells, from day 4.
“Hatching” occurs at this stage, when the
blastocyst emerges from the zona pellucida.
Development from zygote to
implantation.
Then what ?
• The blastocyst differentiates into:
– the trophoblast, the outer ball of cells that
eventually becomes the placenta and
“extraembryonic” membranes.
– the inner cell mass (ICM) becomes the
embryo.
• The above occurs over the course of the
second week following conception.
• Implantation – occurs on about day 6 or
so, as the blastocyst burrows into the
endometrium.
Implantation – Day 6
Implantation – Day 7
Hormone
levels in
early
pregnancy
Placental hormones
Implantation
Placentation
Fetus & placenta: 13 weeks
Embryonic Development
Differentiation of the three primary germ layers
Germ layer formation, Part II
Neurulation: Part I
Neurulation: Part II
Gastrulation
Formation of the
primitive gut
Gasturlation
part II
Organogenisis
in a 5
week embryo
Differentiation
of Mesoderm
Differentiation of Mesoderm continued
The Primary Germ Layers
ECTODERM
MESODERM
ENDODERM
All nervous tissue
Muscle
Epidermis &
Derivatives
Cornea & lens
Connective tissue Digestive glands
Oral, nasal & anal
epithelium
Endothelium of
blood vessels
Tooth enamel
Serosae
Urethra & bladder
epithelium
Pineal, pituitary &
adrenal medulla
Eye’s fibrous &
vascular tunics
Respiratory tract
epithelium
Melanocytes
Synovia
Lymphoid tissue
Flat bones of cranium Urogenital organs
G.I. epithelium
Reproductive ducts &
gland epithelium
Thyroid, thymus &
parathyroid
Fig. 29.14a
Fig. 29.14b
Fig. 29.14c
Fig. 29.14d
Fig. 29.14e
Fig. 29.14f
Fig. 29.14g
Fig. 29.14h
From embryo to fetus
Fetal circulation
Pregnancy from the mother’s
perspective
Hey Ma!
It’s getting
crowded
in here!
Labor:
the
beginning
of the end
A Vertex
Presentation…
…and
welcome
to the
world!
The “let-down”
reflex
or, more fun
with oxytocin