- Fertilization
Occurs where in the female reproductive system? The
fallopian tubes (oviducts).
- Deposition of sperm in female reproductive
tract
From the vagina, the sperm must move up through the
cervix and into the fallopian tubes, where there is the possibility of
a sperm and an egg fusing (fertilization).
- Average number of sperm per ejaculation
Average: 300 million - 400 million sperm/ejaculation.
Minimally, a male must produce between 125 million-150 million to be considered
fertile. Of the millions of sperm ejaculated into the vagina, several thousand
will actually arrive at the oviduct (fallopian tubes). At least several
hundred are required to to ensure fertilization, though only one will actually
fuse with the egg. The others are necessary to loosen the outer layers
of the egg, eventually allowing one to enter.
- Capacitation of the sperm
After being deposited into the female reproductive
tract, the sperm must go through certain changes before they are able to
swim/ fuse with the egg. (Once deposited, capacitation of the sperm takes
45 - 60 minutes.)
- Viability of the sperm
Because the female reproductive tract is hostile
to the sperm, most are killed. Viable sperm remain in the remain in the
female reproductive tract for up to 48 hours.
There are three basic parts to a sperm cell: 1-the head (contains the chromosomes,
and the acrosome, which contains hydrolytic enzymes that help break up
the cell mass surrounding the egg.); 2-the middle piece (the engine); and
3-the tail and end piece. Males produce multiple millions of cells. Every
meiotic event in human males is preceeded by mitosis. The process of sperm
maturation is continual and takes about 60 days. Each meiotic sequence
in males results in 4 sperm cells.
- Fusion of egg and sperm
Fertilization. Once a sperm has penetrated the egg,
there is an instantaneous change in the outer membrane of the egg that
prevents other sperm from entering.
- Number of sperm required for the
process of of fertilization
(Noted above.) At least hundreds must arrive at the
egg. They cooperate in breaking down the egg's outer mass.
- Viability of the egg
After ovulation, (day 14) the egg is viable for 15
- 24 hours. (Females have been known to conceive in every part of the menstrual
cycle, but are far more likely to conceive when ovulating, which is difficult
to predict.) Of the thousands of eggs in the female ovaries, only around
400 will be matured and ovulated during the course of a woman's lifetime.
With each meiotic sequence, only one egg is produced. The other 3 meiotically
produced cells are termed polar bodies.
- Fetal development
(Interesting photographs.)
- Early stages
The mitotic divisions take place as the zygote (after
the first mitotic division: the embryo) moves down the fallopian tube toward
the uterus. A general time scale: female ovulation at day 14; fertilization
on day 15; implantation 5-7 days after fertilization: the young embryo
has completed several mitotic divisions and begun to implant in the uterus.
- 1st mitotic division
The 1st mitotic division (from a one cell 'zygote'
to a two cell 'embryo') occurs about 36 hours after fertilization.
- 2nd mitotic division
The 2nd mitotic division (from a two cell 'embryo'
to a four cell 'embryo') occurs about 60 hours after fertilization.
- 3rd mitotic division
The 3rd mitotic division (from four to eight cells)
occurs about 72 hours after fertilization.
- 1st trimester: (0-3 months)
1st month. Day 17 after conception: blood cells begin
to form. Day 18: heart (cell-cluster) begins to form. Day 24: cluster of
heart cells begin to contract (rhythmic heartbeat).
Size increases by 40 times in the first month. For about the first 35 days,
it's difficult to distinguish between the embryonic development of any
mammal from any other mammal. At about 40 days, discernible developmental
features become apparent.
- 2nd trimester: (4-6 months)
The lips and finger-pads (leave fingerprints) have
developed. Potential eggs begin to develop in females. The heart is pumping
25 quarts of blood/day. Ossification (formation of bone from cartilage)
begins.
- 3rd trimester: (7-9 months)
Obviously a human being: covered with downy-like
hair and lubricant material. The brain is well-developed. Most of the basic
organ systems are refined and, basically, completed during the third trimester.
- Birth (parturition)
- Timing
- Overall length of pregnancy
266 days from conception (280 days from first day
of last menstrual flow). To predict birth-date from the first day of the
last menstrual cycle: formula (for the test): take the first day of the
mother's last menstrual cycle (i.e. Sept. 13), add 7 days (Sept. 20), count
back 3 months (June 20), and adjust for the year (June 20, 1998).
- Time of day and birth frequency
The most frequent time of day for human births is
3-4 am.
- Initiation of labor: complex hormonal
interplay
The estrogen and progesterone increase steadily over
the course of the pregnancy. For the first 2 months the corpus luteum (empty
transformed follicle after ovulation--in the ovary) maintains the pregnancy
by maintaining levels of these hormones (estrogen and progesterone). From
the second month on, the placenta produces the estrogen and progesterone,
maintaining the pregnancy. Toward the end of the pregnancy, there is a
precipitous drop in the estrogen and progesterone--the signal that the
baby is ready to be born. Mother produces two hormones in response to to
this drop: oxytocin and prostaglandins. Oxytocin and prostaglandins initiate
contractions of the uterus. (Mother has no control over uterine contractions--they're
involuntary.)
- Stages of labor
(Labor is an appropriate title.)
- Dilation
The increase in diameter of the cervix. The cervix
must dilate (expand) to full dilation: 10 cm. Events associated with dilation:
duration: 2-16 hours (though it is not unusual to go 24 hours or so); the
initial uterine contractions recur every 15-20 minutes; later contractions
(at the end of dilation) recur every 1-2 minutes. The breaking of the water,
or rupturing of the amniotic sac, is associated with the dilation phase.
- Expulsion
Duration: 2-60 minutes. Begins with passage of baby
into the birth canal (vagina). The vagina responds to hormones (as did
the cervix in the dilation phase) and becomes more physically conducive
to the birthing process. Crowning: the appearance of the baby's head at
the opening of the birth canal. Contractions occur every 1-2 minutes and
last between 50-90 seconds. The intensity of the contractions has increased.
Mother is able to use abdominal muscles to complement the uterine contractions.
The expulsion phase ends with birth.
- Placental
Expelling the placenta, attached umbilical chord,
and some blood (ordinarily around 8 oz.). Duration: 15-30 minutes. The
hardest uterine contractions occur during the placental phase, the purpose
being to constrict the uterus.
Questions:
What is an epesiotomy? During the expulsion phase a surgical cut can be
made from the bottom of the vagina down to the rectum. This allows more
space, and prevents tearing, which takes longer to heal than a surgical
cut.
How does the baby breathe? Throughout
the pregnancy, the baby breathes the amniotic fluid. At birth, the pressure
of the birthing process flushes the fluid from the baby's lungs, preparing
him/her to breathe air.
'Breech' is when a baby is coming
rear-end first.
How soon after birth does the menstrual
cycle return? Variable: from 30 days to 6-8 months after delivery. If the
mother nurses, oxytocin is produced, delaying the onset of menstruation.
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