Pregnancy & Human Development

Chapter 29

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

Capacitation

• Must occur before spermatozoa can fertilize secondary oocyte:– contact with secretions of seminal vesicles– exposure to conditions in female reproductive

tract

Penetration

Secondaryoocyte

Head of sperm

1650X

Fertilization

Figure 29–1

Fertilization

Figure 29–1b (1 of 2)

Fertilization

Figure 29–1b (2 of 2)

Fusion Of the

pronuclei

Cleavage and Blastocyst FormationFigure 29–2

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.

Stages in Implantation

Figure 29–3

Implantation

Day 6

Implantation – Day 8

ImplantationDays 9 - 13and early

placentation

Ectopic Pregnancy

• Implantation occurs outside of uterus• Do not produce viable embryo• Can be life threatening

The Inner Cell Mass and Gastrulation

Figure 29–4

The Primary Germ LayersAll 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

The Fates of the Germ LayersTable 29–1

Extraembryonic Membranes and Placenta

Formation

Figure 29–5 (1 of 3)

Figure 29–5 (2 of 3)

Placenta FormationFigure 29–5 (3 of 3)

View of Placental Structure

Figure 29–6a

Placental StructureFigure 29–6b

Decidua:Decidua Capsularis • Thin portion of endometrium• No longer participates in nutrient exchange and

chorionic villi in region disappear

Decidua Basalis• Disc-shaped area in deepest portion of

endometrium• Where placental functions concentrated

Decidua Parietalis• Rest of the uterine endometrium• No contact with chorion

Hormones of Placenta

• Synthesized by syncytial trophoblast, released into maternal bloodstream:– human chorionic gonadotropin– human placental lactogen– placental prolactin– relaxin– progesterone– estrogens

Human Placental Lactogen (hPL)

• Helps prepare mammary glands for milk production

• Stimulatory effect on other tissues comparable to growth hormone (GH)

Placental Prolactin• Helps convert mammary glands to active status

Relaxin• Is a peptide hormone• Is secreted by placenta and corpus luteum

during pregnancy• Increases flexibility of pubic symphysis,

permitting pelvis to expand during deliveryCauses dilation of cervix

• Suppresses release of oxytocin by hypothalamus and delays labor contractions

An Overview of Prenatal DevelopmentTable 29–2 (1 of 4)

An Overview of Prenatal DevelopmentTable 29–2 (2 of 4)

An Overview of Prenatal DevelopmentTable 29–2 (3 of 4)

An Overview of Prenatal DevelopmentTable 29–2 (4 of 4)

Embryogenesis

• Body of embryo begins to separate from embryonic disc

• Body of embryo and internal organs start to form

• Folding, differential growth of embryonic disc produce bulge that projects into amniotic cavity:– projections are head fold and tail fold

The First Trimester

Figure 29–7a, b

The First Trimester

Figure 29–7c, d

Organogenesis

• Process of organ formation

The Second and Third Trimesters

Figure 29–8

Second Trimester

• Fetus grows faster than surrounding placenta

Third Trimester

• Most of the organ systems become ready• Growth rate starts to slow• Largest weight gain• Fetus and enlarged uterus displace many

of mother’s abdominal organs

Growth of the Uterus and Fetus

Figure 29–9a, b

Growth of the Uterus and Fetus

Progesterone

• Released by placenta • Has inhibitory effect on uterine smooth

muscle• Prevents extensive, powerful contractions

Opposition to Progesterone

• 3 major factors:– rising estrogen levels– rising oxytocin levels– prostaglandin production

Initiation of Labor and DeliveryFigure 29–10

False Labor

• Occasional spasms in uterine musculature• Contractions not regular or persistent

True Labor• Results from biochemical and mechanical factors• Continues due to positive feedback

Hormone levels

throughout pregnancy

Placental hormones

Contractions

• Begin near top of uterus, sweep in wave toward cervix

• Strong, occur at regular intervals, increase in force and frequency

• Change position of fetus, move it toward cervical canal

Stages of Labor

1. Dilation stage2. Expulsion stage3. Placental stage

Dilation Stage

• Begins with onset of true labor• Cervix dilates • Fetus begins to shift toward cervical canal• Highly variable in length:

– typically lasts over 8 hours

Dilation Stage

• Frequency of contractions steadily increase

• Amniochorionic membrane ruptures (water breaks)

The Stages of Labor

Figure 29–11 (1 of 2)

Expulsion Stage

• Begins as cervix completes dilation• Contractions reach maximum intensity• Continues until fetus has emerged from

vagina:– typically less than 2 hours

The Stages of Labor

Figure 29–11 (2 of 2)

Delivery

• Arrival of newborn infant into outside world

Episiotomy• Incision through perineal musculature• Needed if vaginal canal is too small to pass fetus• Repaired with sutures after delivery

Fetal circulation

The Beginning

Next - Inheritance

Cesarean Section

• Removal of infant by incision made through abdominal wall

• Opens uterus just enough to pass infant’s head

• Needed if complications arise during dilation or expulsion stages

Placental Stage

• Muscle tension builds in walls of partially empty uterus

• Tears connections between endometrium and placenta

• Ends within hour of delivery with ejection of placenta, or afterbirth

• Accompanied by a loss of blood

Actual placenta

Premature Labor

• Occurs when true labor begins before fetus has completed normal development

• Newborn’s chances of surviving are directly related to body weight at delivery

Immature Delivery

• Refers to fetuses born at 25–27 weeks of gestation

• Most die despite intensive neonatal care• Survivors have high risk of developmental

abnormalities

Premature Delivery

• Refers to birth at 28–36 weeks• Newborns have a good chance of

surviving and developing normally

Forceps Delivery

• Needed when fetus faces mother’s pubis instead of sacrum

• Risks to infant and mother are reduced using forceps:– forceps resemble large, curved salad tongs– used to grasp head of fetus

Breech Birth• Legs or buttocks of fetus enter vaginal

canal first instead of head• Umbilical cord can become constricted,

cutting off placental blood flow• Cervix may not dilate enough to pass head• Prolongs delivery• Subjects fetus to severe distress and

potential injury

5 Life Stages

1. Neonatal period - Extends from birth to 1 month

2. Infancy - 1 month to 2 years of age3. Childhood - 2 years until adolescence4. Adolescence - Period of sexual and

physical maturation5. Maturity

Colostrum

• Secretion from mammary glands• Ingested by infant during first 2–3 days• Contains more proteins and less fat than

breast milk:– many proteins are antibodies that help ward

off infections until immune system is functional

Colostrum

• Mucins present inhibit replication of rotaviruses

• As production drops, mammary glands convert to milk production

Breast Milk

• Consists of:– water– proteins– amino acids– lipids– sugars– salts– large quantities of lysozymes—enzymes with

antibiotic properties

Milk Let-Down Reflex

• Mammary gland secretion triggered when infant sucks on nipple

• Continues to function until weaning, typically 1–2 years

The Milk Let-Down

Reflex

Figure 29–12

Benefits of Breast-feeding• Acquired immune defenses

– Neutrophils, macrophages, T and B cells– Immunoglobulin A

• Reduced incidence of later diseases in child– Lymphoma, heart disease, gastrointestinal

disorders, diabetes mellitus & meningitis• In mother

– Reduced incidence of osteoporosis and breast cancer

– Stronger bonding, less post-partum depression,– More rapid weight loss, uterine recovery

Growth and Changes in Body Form and Proportion

Figure 29–13

From embryo to fetus