FERTILIZATION & DEVELOPMENT

FERTILIZATIONInitiation of biological reproduction (through insemination or pollination) Main Functions:

1. Combine haploid sets of chromosomes from male and female zygote

2. Activate egg cell (Sperm + Egg contact = metabolic reactions)

ACROSOMAL REACTION Release of hydrolytic

enzymes to break down coating of the egg Lock and Key Recognition (importance of specificity) Leads to fusion of sperm + egg membranes and entry of a single sperm Present in sea urchins

SEA URCHIN FERTILIZATION

MAMMALIAN FERTILIZATION

FUSION OF MEMBRANESEffect #1: Fast Block to Polyspermy (1/10th sec) 1. Triggering of electrical response in egg membrane2. Ion channel gates open3. Na+ changes membrane potential (depolarization)

Effect #2: Cortical Reaction (changes in egg cytoplasm)1.Production of IP32.Increase of Ca2+

3.Fusion of cortical granules with membrane4.Creation of osmotic gradient (water drawn in)5.Swelling (pushes outer layer away from membrane)

FUSION OF MEMBRANES

… and then the outer layer will serve as the FERTILIZATION MEMBRANE.

(prevents multiple fertilization)

… and then the outer layer will serve as the FERTILIZATION MEMBRANE.

(prevents multiple fertilization)

CORTICAL REACTIONCauses granules from cortex to be released via exocytosis Enzymes released during this reaction hardens the fertilization membrane (zona pellucida for humans) Slow Block to Polyspermy

SUMMARY OF FERTILIZATION1. Contact of sperm and egg

2. Acrosomal reaction3. Fusion of egg and sperm

membranes (Entry of only 1 sperm)4. Fast Block to Polyspermy5. Cortical Reaction 6. Slow Block to Polyspermy

PREGNANCY/GESTATION

divided into trimesters (“3 months”) 1st Trimester: conception - Week 14 2nd Trimester: Week 14 – Week 28 3rd Trimester: Week 28 – Delivery/Labor/Week 40

PREGNANCY/GESTATION

FIRST TRIMESTER Most profound development/rapid period of growth Fetus is most susceptible to damage (disease, alcohol, etc.)1. Fertilization2. Cleavage - succession of rapid cell divisions3. Zygote Blastomere Morula Blastocyst 4. Implantation – attachment of blastocyst to endometrium5. Start of Differentiation

6. Embedding of blastocyst in endometrium7. Embryo directly gets nutrients from endometrium 8. Forming of placenta from tissues that grow out of

embryo9. Organogenesis

Ectoderm- forms outer layer of gastrula Endoderm – lines embryonic digestive tract Mesoderm – fills space between ectoderm + endoderm

10. Secretion of hormones to signal presence and control mother’s reproductive system (ex. HCG)

FIRST TRIMESTER

Human Chroionic Gonadotropin (HCG) – peptide hormone produced by embryo which keeps progesterone levels up to make uterus lining thick

Human Chroionic Gonadotropin (HCG) – peptide hormone produced by embryo which keeps progesterone levels up to make uterus lining thick

SECOND TRIMESTER1. Growth up to 30cm; Activity

may be visible through abdominal wall

2. Hormone levels stabilize as HCG declines.

3. Corpus Luteum / uterine wall deteriorates.

4. Placenta secretes own progesterone to maintain pregnancy.

5. Uterus grows large enough to make pregnancy obvious.

THIRD TRIMESTER1. Growth up to 3.5kg in

weight and 50cm in length

2. Fetal activity decreases3. Uterus expansion4. Displacement of

mother’s abdominal organs

* Hormones to induce and regulate labor

HORMONES TO INDUCE AND REGULATE LABOR

1. Highest level of estrogen formation of oxytocin2. Oxytocin

- stimulates powerful contractions through smooth muscles of the uterus- stimulates placenta to secrete prostaglandins to enhance contraction

3. Positive Feedback: Physical and emotional stress associated with contractions stimulate release of more oxytocin and prostaglandins

MOMMY & BABY WHY DOESN’T THE MOM’S BODY REJECT THE BABY?Recall: Immune System-Self and non-self recognition-The baby is considered non-self because it contains half a different set of chromosomes.

THE PLACENTAPhysical barrierAllows red blood cells & nutrients to pass, but not lymphocytesT cells are restricted

MOMMY & BABY WHY DOESN’T THE MOM’S BODY REJECT THE BABY?

FETUSlymphocytic suppressor cells that inhibit response to interleukin-2 masks fetus from interleukin-2’s detection of foreign objects

PARTURITION

Birth of fetus from uterus 3 stages:

1. Dilation2. Fetal Exposure3. Umbilical Cord Closure and Placental Exposure

DILATION• Thinned cervix opens up to around 3cm•Cervix is integrated in lower part of the uterus and on top of baby’s head• Full Dilation: 10 cm• Duration varies(4-8 hours)

FETAL EXPOSURE• pressure on cervix, uterine contraction

• head must pass through narrowest part of pelvis, pubic arch and itroitus = PUSHING• burning sensation in labia• after birth of fetal head:

1. delivery by extension2. head points to side3. rotation to expose

shoulders

UMBILICAL CORD CLOSURE• After fetal exposure• Umbilical cord is cut and would close normally if not clamped

PLACENTAL EXPOSURE• physiological separation done 15 – 30 mins after delivery • done with or without assistance (intramuscular injection of oxytocin followed by contraction)

HORMONES INVOLVEDIN PARTURITION

a) To make cervix ready: - relaxin, prostaglandin,

estrogenb) For uterine contraction:

- prostaglandin, progesterone, oxytocin, specific positive feedback in oxytocin

LACTATION• Secretion/formation of milk by the mammary glands• During pregnancy…– remarkable growth in breast size due to the action of

estrogen, adrenal steroids, and growth hormones which make ducts and alveoli in breast grow

– prolactin, progesterone also involved

START OF SECRETORY ACTIVITY• in Milk producing unit / alveoli• caused by prolactin when estrogen and

progesterone are withdrawn from the blood

• Colostrum: – also known as the ‘first milk’

of the mother– good for baby’s health– expected at pregnancy or

childbirth– milk secretion increases

after, usually 3rd or 4th day after birth

– breasts are engorged, tense, tender, feel warm

START OF SECRETORY ACTIVITY

• Depends on baby’s suction• Contractile mechanism– moves milk fr. alveoli to ducts

• During suckling..– pressure in mother’s breasts– nerve impulse fr. sucking of baby– path of impulse : sensory neurons in nipples

hypothalamus in the brain pituitary gland– Pituitary gland secretes oxytocin (for contraction of

myoepithelial cells of alveoli and ducts containing milk)

EJECTION OF MILK

EJECTION OF MILK

• Maintenance of Breastmilk Production– What is needed?

• Prolactin• Sucking – for removal of

milk, release of prolactin• Milk pressure, hence

periodic breast feeding• Continuous unless there is

congestion, emotional disturbance