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
the condition of carrying one or more embryos in the uterus preceded by fertilization/conception duration is correlated to size(mice/rats – 21 days;elephants – 600 days)
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- nervous system + epidermis Endoderm – digestive tract and lining Mesoderm – other organs and tissues
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
FIRST TRIMESTER High levels of Progesterone causes…
- mucus in cervix (as protective plug)- growth of maternal part of placenta- size of uterus- cessation / stopping of ovulation and menstruation due to negative feedback on hypothalamus and pituitary gland
Breasts engorge
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 barrier (trophoplast)Allows 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
*Local dampening of immune system
PARTURITION
Birth of fetus from uterus Labor = rhythmic uterine contractions 4 stages:
1. Dilation2. Fetal Exposure3. Umbilical Cord
Closure4. Placental Exposure/Delivery of Placenta
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
LACTATIONSecretion/formation of milk by the mammary glandsAspect of postnatal careDuring pregnancy…
– 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– Recall: 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