Fertilization, Development, and Birth

Animation

Fertilization

• During fertilization, the sperm nucleus fuses with the egg nucleus, which results in a zygote

• As a sperm gets close to the egg, it has to undergo capacitation to activate the acrosome

Fertilization

• The plasma membrane of the egg is surrounded by a region called the zona pellucida

• The zona pellucida is surrounded by a few layers of follicular cells, called the corona radiata

Animation

Polyspermy

• For proper development, only one sperm should enter an egg

• To prevent polyspermy, as soon as the sperm touches the egg:

1. The zona pellucida hardens to prevent any other sperm from gettng to the egg

2. The zona pellucida lifts away from the surface of the egg

Development

1. Cleavage: zygote begins to divide; lots of cell division; no increase in size

2. Growth: cell division is now accompanied by an increase in size

3. Morphogenesis: the embryo is beginning to take shape

4. Differentiation: cells start to take on specific structure and function

Extraembryonic Membranes

1. Chorion

(fetal half of the placenta)

2. Yolk sac

(makes red blood cells)

3. Allantois

(umbilical cord)

4. Amnion

(shock absorber)

Stages of Development

• Events that occur from fertilization to birth; also called the gestation period

• 280 days in humans • Pre-embryonic development: events

of the first week include ovulation, fertilization, cleavage, and implantation

Fig 8.3 Pre-embryonic development

Embryonic Development

• Embryonic development begins with the second week and lasts until the end of the second month of development

• Trophoblast secretes enzyme to burrow into the wall of the endometrium

• Also secretes HCG

Gastrulation

• Cells begin to move (morphogenesis) and form the primary germ layers:

• Nervous system develops first (week 3)

• Heart develops next (weeks 3-4)

Placenta

• Formed by the end of the second month

Fetal Development and Birth

• Includes the third through ninth months of development

• Third month:

• Head growth slows so body can catch up

• Formation of fingernails, eyelashes, eyebrows, nipples

• Bone formation

• Distinguish males from females

• Fourth month:

• Fetal heartbeat can be heard with a stethoscope

• Possible fetal movement

• End of 4th month, fetus is about 6 inches and weight is about 6 oz

Fifth Through Seventh Months

• Fetal movement increases

• Skin is covered with fine hair called lanugo, and a white greasy coating called vernix caseosa

• End of 7th month, fetus is 12 inches long and weighs about 3 lbs

Eighth Through Ninth Months

• Fetus turns so its head is pointed down toward the cervix

• If the baby is breech (feet first), a cesarean section may be performed

• At the end of the 9th month, length is about 20 ½ inches and weight is about 7 ½ lbs

Birth

• Oxytocin stimulates the uterine muscles to contract

– Pushes fetus downward

– Stretches the cervix

• Uterine contractions occur regularly every 15-20 minutes

• The process of giving birth is called parturition

Stage 1

• Uterine contractions occur to dilate the cervix to 10 cm

• If amniotic fluid hasn’t already ruptured, it does during this stage, releasing amniotic fluid

Epidural

Epidural

Stage 2

• Contractions every 1-2 minutes, lasting ~1 minute each, accompanied by a desire to push

• Head descends into the vagina, episiotomy may be done

• Head is delivered, followed by shoulders and rest of the body

• Umbilical cord is cut and tied

Stage 3 • The placenta, or

afterbirth is delivered during this stage

• About 15 minutes after birth, uterine contractions shrink the uterus and dislodges the placenta

• Placenta is delivered through the vagina

Effects of Pregnancy on the Mother

• Nausea/vomiting, loss of appetite, fatigue

• Weight gain • Hormonal changes • Blood volume increases • Heartburn • Incontinence • Edema and varicose veins in legs • Gestational diabetes • Stretch marks

Female Breast and Lactation

• After birth, prolactin is secreted to begin milk production

• Meanwhile, the breasts produce colostrum

• Suckling signal the brain to release oxytocin, which causes milk letdown