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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
Secondaryoocyte
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
GastrulationFormation of the
primitive gut
Gasturlationpart II
Organogenisisin a 5
week embryo
Differentiation of Mesoderm
Differentiation of Mesoderm continued
The Primary Germ Layers
All nervous tissue Muscle G.I. epithelium
Epidermis & Derivatives
Connective tissue Digestive glands
Cornea & lens Lymphoid tissue Reproductive ducts & gland epithelium
Oral, nasal & anal epithelium
Endothelium of blood vessels
Thyroid, thymus & parathyroid
Tooth enamel Serosae Urethra & bladder epithelium
Pineal, pituitary & adrenal medulla
Eye’s fibrous & vascular tunics
Respiratory tract epithelium
Melanocytes Synovia
Flat bones of cranium Urogenital organs
ECTODERM MESODERM ENDODERM
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