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