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-fertilization in the upper third of the oviduct/fallopian tube
-fertilization = union of egg and sperm to produce the zygote
-sperm must undergo capacitation after ejaculation – increase rate of tail beating
-happens in the female system – removal of glycoproteins and proteins from the acrosome
– allows for entry of calcium into the sperm – increase flagellar beating
-plasma membrane of the egg is surrounded by an extracellular matrix (glycoproteins) = zona pellucida
and a ring of follicular cells = corona radiata (nourishment in the follicle)
1. several sperm penetrate corona radiata
and enter zona pellucida
2. Sperm bind to the zona pellucida & triggers
release of acrosomal contents  zona digestion
3. ONE sperm contacts the plasma
membrane of the egg
4. immediate change in the zona pellucida
(depolarizes) – blocks polyspermy
5. entry of sperm nucleus into egg
6. fusion of the sperm’s pronucleus with the
pronucleus of the egg = zygote
embryonic stage:
1st week to week 8
-first cell division – within 24 hrs & takes 6 hrs
to complete
-second day – four cells
-end of third day – 16 cells
-fourth day – morula stage
-fourth to fifth day – blastocyst stage
-end of fifth day – hatching of blastocyst from
zona pellucida
-6th day – implantation of blastocyst into
-union of sperm and egg nuclei (zygote) -> first cell division within 24 hours to form
the embryo
- cell division continues -> formation of the morula at day 4
- morula = a mass of tiny, uniformly sized cells with equal amounts of cytoplasm
- -cells of the embryo = blastomeres
embryonic stage:
1st week to week 8
-first cell division – within 24 hrs & takes 6 hrs
to complete
-second day – four cells
-end of third day – 16 cells
-fourth day – morula stage
-fourth to fifth day – blastocyst stage
-end of fifth day – hatching of blastocyst from
zona pellucida
-6th day – implantation of blastocyst into
-day 5 – 6: morula forms a blastocyst = assymetrical ball of cells with a cavity
-end of day 5 – blastocyst breaks out of the zona pellucida
-day 6: -> implantation of blastocyst into the endometrium
-formation of the
blastocyst marks
the beginning of
morphogenesis shaping of the
migration of
dividing cells to
specific locations
-blastula = blastocyst - hollow ball of cells/blastomeres
-outer layer = trophoblast - forms extraembryonic tissues (e.g. placenta,
yolk sac)
-inner mass of cells at one end - totipotent embryonic stem cells
-second week of development (day 7 – 14) :
-amniotic cavity forms between the inner cell mass and the trophoblast
-the inner cell mass flattens = embryonic disc
-disc – upper layer = epiblast & lower layer =
-third week ( days 14 – 21): development of the gastrula via gastrulation (day 15)
-gastrulation = conversion of the two-layered embryonic disk -> three embryonic germ layers by
-differentiation - assumption of a specific cell structure and function
-three germ layers: -> ectoderm (integumentary system & nervous system)
-> mesoderm (mesenchyme  bones, fat, cartilage, blood, muscle)
-> endoderm (internal organs and membranes)
-primitive streak: groove on the epiblast that forms before gastrulation
-formation of the gastrula is identified through the formation of the primitive streak
-cells pass from the epiblast through the primitive streak and form the
mesoderm and endoderm during gastrulation through a process called invagination
-portions of the mesoderm that
do not form the notochord
segment into sections called
somites -> specific body
regions and structures
-four weeks into development (day 21 – 28): embryo forms a
tubular structure & undergoes neurulation
-in front of the primitive streak is the primitive node
- mesodermal cells from the primitive node become the notochord
– forms at 22 days
- neurulation: development of neural folds from ectoderm -> form
into a neural tube
- regions of the neural tube will form brain and spinal nerves,
yolk sac
-yolk sac: forms blood cells, gives rise to sex cells and the stem
cells of the immune system
-also forms part of the embryonic digestive tube
-portion of it becomes part of the umbilical cord
-embryo’s first connection is via a connecting stalk
-third week of development : formation of the allantois - tube
yolk sac
-chorion – forms after four weeks after implantation as slender
projections that grow out from the trophoblast = chorionic villi
-region of the chorion that is in contact with the
endometrium becomes the placenta
-produces human chorionic gondadotropin hormone –
hormone of pregnancy.
-amnion – surrounds the amniotic cavity (amnionic fluid +
developing embryo)
-amnion enfolds the connecting stalk and remnants of the
-fifth week (days 28 – 35): formation of lens, beginnings of maxilla and mandible &
paddle-shaped forelimb
-embryo is 10-12 mm long
-35 + 2 days: formation of eye, ear, forebrain, nasal pit, tail
-35 + 5 days: formation of midbrain, heart, external ear, primitive fingers
-sixth week: formation of primitive toes (22-24 mm)
-seventh week: formation of eyelids, webbed fingers
-eighth week: separation of toes and fingers (34-40 mm)
-fetal stage : end of the eighth week -> birth
-third month: body lengthens and head growth slows
-ossification of bones
-fourth month: reproductive organs appear
-rapid body growth
-lower limbs lengthen
-development of hair, eyebrows, lashes, nipples
-fifth month: growth slows
-skeletal muscles are active
-fetus curls into fetal position
-sixth to ninth month: weight gain
-skin smoothens as fat is deposited beneath skin
-eyelids open
-organs elaborate and grow (digestive and respiratory are last)
-neuronal networks form
of every 100 oocytes:
69 implant
42 survive more than 1 week
37 survive more than 6 weeks
31 born alive
-umbilical vessels carry blood between the
placenta and fetus
-fetal blood - greater concentration of oxygen
-enters fetus through umbilical vein and
bypasses the fetal liver via the ductus venosus
-enters the right atrium and can go to the right
ventricle and out through the pulmonary artery
(into the lungs) - bypasses the lungs through
a vessel that connects to the aorta - ductus
-blood in the right atrium can directly
flow into the left atrium (bypassing the lungs)
through a hole between the atria called
the foramen ovale
-blood enters the left ventricle and exits via
the aorta to travel to the rest of the body
-exits the fetus via the umbilical artery
(branch off the iliac artery)
Passive: breakdown of structures or failure of structures
-connective tissue breaks down
-DNA errors accumulate
-free radical damage accumulates (free radical has an unpaired
electron in its outer shell)
Active: e.g. autoimmunity
-begins before birth
-certain cells undergo a programmed death = apoptosis
-apoptosis starts in the embryo
e.g. death of neurons as pathways are created
Senescence: process of growing old
-continuation of degeneration that begins during adulthood
-result of normal wear and tear
Age-related changes:
Integumentary system: Loss of collagen and elastic fibers, decrease of sweat and
sebaceous glands, decrease in skin thickness, loss of hair pigments
Skeletal system: loss of bone matrix, thinner, brittle bones, loss of height
Muscular system: loss of muscle mass, degeneration of neuromuscular junctions
Nervous system: loss of dendrites and synaptic connections, decrease in sensory
sensitivity, decrease brain functions (memory, communication, smell & taste,
loss of lens elasticity)
Endocrine system: reduced hormone secretions, decreased metabolic rate
Digestive system:decreased gastric motility, reduced gastric juice secretion
CV system: degeneration of cardiac muscle, decrease in artery diameter