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LIFE CYCLE
STAGE ONE – GESTATION
1. MEIOSIS
a. We already know that through meiosis sperm and eggs
(gametes) are made with ½ the genetic information
b. The male gamete will mature and divide to make
spermatids which mature into spermatozoon. (haploid
chromosome in a capsule with a long flagellum)
c. Each haploid sperm has different mixes of chromosomes
– lots of variation.
d. Ovaries produce eggs and are in the lower part of the
abdomen
e. Ovarian cycle prepares egg for fertilization once a month
(this is called ovulation)
f. If egg is not fertilized, then another will be produced for
next month
g. Maternal gamete (egg/ovum) divides and makes one
large (becomes the ovum) and the other is a polar body
(can’t support embryo)
h. Egg is released from ovary and moves into the fallopian
tube
i. Fallopian tube carries the egg into the uterus
j. During fertilization the egg and sperm join together and
the genetic information
2. Fertilization
a. This is fusion of gametes
b. During ejaculation 150 million to 350 million sperm are
deposited in vagina near fallopian tubes where fertilization
can occur
c. To reach egg sperm must
i. Pass through cervix
ii. Cross uterus
iii. Swim up fallopian tube
iv. Many will not make it past vagina (trapped in
mucus)
v. ½ will swim up the wrong tube
d. many sperm arrive at the egg at the same time
e. Enzymes in the head loosen the protective layer of cells
around the egg
f. Head of sperm enters egg and fuses with genetic
information inside the egg
g. Protective shield goes around the egg so no more sperm
can get through
h. Sperm (haploid) fuse with egg (haploid) to form a diploid
zygote (23 chromosomes each – zygote with 46
chromosomes)
3. EMBRYONIC DEVELOPMENT
a. Newly fertilized egg is called a zygote
b. Begins mitotic division (cleavage) after 30 hours (embryo)
i. These cells don’t increase in size
ii. Cleavage produces a ball of cells called a morula
(not much larger than zygote)
iii. Cells of the morula divide and form a blastocyst
(ball of cells with a fluid filled cavity)
c. Travels through fallopian tubes down to uterus as divides
d. 4-5 days, the embryo reaches the uterus and is made up
of 100 or more cells;
e. By the time it reaches the uterus, the morula becomes a
blastocyst
f. When it reaches the uterus, the morula embeds itself in
the thick uterine lining; implantation
g. Pregnancy begins at implantation (6 days after
fertilization)
h. from 9th week to birth it is called a fetus
Pregnancy
1. First Trimester
a. For first 2-3 weeks, the embryo looks like embryo of other
animals
b. The embryo develops from the mass of cells on the inner
surface of the blastocyst
c. At first, all of the cells look alike but then they organize into
three major layers: ectoderm, mesoderm and endoderm.
d. Different parts of the body develop from these different
layers
i. Ectoderm; nervous tissue and skin
e.
f.
g.
h.
i.
j.
k.
ii. Mesoderm: muscle, connective tissue, organs
iii. Endoderm: linings of digestive, respiratory and urinary
tract; also liver, glands, pancreas, thyroid and thymus
Placenta
i. 6-8” in diameter and 1” thick
ii. nutrients, gasses, pathogens, drugs pass from mother
to fetus through placenta
iii. Developing fetus is attached to placenta by umbilical
cord which has arteries and veins that carry blood
between fetus and placenta
iv. Blood between mother and fetus never mixes;
materials are exchanged through diffusion
Brain, nervous system and spinal cord form in third week
Heart beats at 21 days and is smooth beating at 28 days
By fifth week: arms, legs, eyes, ears, and digestive system
develop
By 6th week: fingers and toes and some brain activity;
embryo begins to move
From 8 weeks to birth: the embryo is called a fetus
End of 1st trimester – 2” long
2. Second Trimester
a. Mother’s abdomen begins to swell as uterus enlarges
b. Fetuses heartbeat can be heard and skeleton begins to form
c. Layer of soft hair grows over skin
d. Fetus begins to wake and sleep
e. Can feel fetus moving around: swallows, hiccups, sucks its
thumb and makes a fist; kicks feet and curls toes
f. By end of second trimester, it is about 1 foot long and eyes
are open
3. Third trimester
a. Fetus grows quickly and undergoes changes so it can
survive outside the mother
b. Can see light and darkness through abdominal wall, and can
react to music and loud sounds
c. During last half of the trimester, the fetus develops fat under
its skin to insulate the body for a steady temperature.