Human Reproduction – part II
In part I the processes underlying formation and
maturation of gametes in both males and females was
covered.
In this part we’ll follow the processes of fertilization,
development, and birth.
Fertilization
Successful fertilization
depends on the structure,
adaptations, and
performance of sperm.
Structure first:
The sperm absorbs fructose, ascorbic acid (vitamin
C) and amino acids from semen, and the long
mitochondrion makes metabolic energy (ATP)
available for the flagellum to power movement
through the upper vagina, the uterus, and the oviduct.
When sperm reach the egg (there must be many), the
acrosome at the head releases enzymes that digest a
path through a jelly coat surrounding the egg.
Then they reach the vitelline layer, on which there
are species-specific binding sites. After binding,
sperm can penetrate this layer.
One sperm’s plasma membrane fuses with the egg
plasma membrane, and its nucleus enters the egg cell.
This is what the egg cell and supporting cells look like
in humans as they move into the oviduct. You can see
why potent enzymatic digestion is necessary for the
sperm to reach the egg cell membrane.
(Jelly coat)
Plasma membrane fusion causes the egg cell plasma
membrane to become impermeable; only one sperm
gets to fertilize the egg and create a diploid embryo.
The fusion also brings the egg out of metabolic
dormancy, and begins the development process.
The vitelline layer separates from the plasma
membrane and becomes a fertilization envelope
within a few minutes. The intervening space fills with
fluid.
Here’s the process diagrammatically: (note that
human fertilization occurs in an inaccessible place, so
this diagram reflects what happens in an externally
fertilized species)
The fertilized egg is called a zygote. Activation of
the metabolic machinery of the zygote also begins a
process of repeated cell division called cleavage.
About 3 days after fertilization the embryo is a ball
of ~8 cells. This stage is called a morula.
Cleavage continues, forming a ball of cells (> 1000)
that is initially solid, but then becomes hollow with a
fluid filled center. This stage is called a blastula.
It is at this stage that the embryo implants in the wall
of the uterus. Essentially as soon as implantation, the
embryo begins secreting Human Chorionic
Gonadotropin.
HCG causes the corpus luteum to enlarge and
produce more progesterone, which prevents
menstruation. It is also soon detectable in urine and
blood, indicating pregnancy.
Later, the placenta produces Human Chorionic
Somatomammotropin. This is the baby’s way of
influencing its food source.
Here’ a diagram of the implantation process and the
establishment of a circulatory link between mother
and embryo:
More detail than you need is
incorporated into this
diagram. What is important?
The trophoblast grows into
the endometrium, and
induces the expansion of the
mother’s circulatory system
in the vicinity of
implantation. The chorion
develops from the
trophoblast. It produces
HCG. The amnion also
comes from embryonic cells.
The next phase of embryonic development is called
gastrulation. During gastrulation cells migrate.
Initially an opening forms
from the surface into the
fluid filled blastocoel. A
smaller, separate fluid-filled
chamber forms, that is called
the archenteron. It is the
digestive cavity.
The cells migrating inward
continue dividing, and the
three basic tissue layers form:
ectoderm, mesoderm, and
endoderm.
The gastrulation diagrammed on the previous slide
was for a frog embryo. Ectoderm forms the outer
layer of the embryo. Endoderm forms the inner layer
and a yolk plug that fills the blastopore. Mesoderm
differentiates between those other two layers.
Gastrulation is deemed to have finished when the 3
layers are all formed.
What was the blastopore will eventually be open
again, as the organism’s anus.
The next phase of development is neurulation.
In this phase a notochord forms from a portion of
the mesoderm, and the ectoderm above it thickens to
become the neural plate.
Then continued growth of the neural plate leads to it
‘folding over on itself’. The folded over part
becomes a tube that separates from the ectoderm. It
is the neural tube, which is the beginning of the
nervous system.
The neural tube will become
the brain and spinal cord of
the organism.
The neural tube is located
just above the notochord,
which will eventually be
replaced by the vertebral
column; it will enclose the
spinal cord.
Somites form as segmented
portions of mesoderm. They
become vertebrae and
muscles
In vertebrates the mesoderm ‘splits’ leaving a cavity,
which is the body cavity still evident in you. It’s
called the coelom, and this kind of development is
called schizocoelous.
Very early on at least primitive versions of most
mature tissues and organs are formed.
From ectoderm: epidermis, epithelial lining of mouth
and rectum, sense receptors, nervous system,
adrenal medulla
From mesoderm: skeleton, muscles, circulatory
system, excretory system, reproductive system
(except germ cells, adrenal cortex
From endoderm: lining of the respiratory
system, liver, pancreas, thyroid & parathyroid
glands, linings of the reproductive system
After organs have been formed growth dominates. In
humans, that’s all that occurs in the third trimester.
There are some important basic processes that go on
along the way:
Apoptosis – programmed cell death. For example,
your hands and feet begin as pads. It is cell death that
separates your fingers and toes. It’s also important in
development of the nervous system.
Induction – An enormous amount of signaling goes
on between cells, where one cell causes another to
differentiate in a particular way. Development of the
eye results from signals between optic vesicle and
overlying ectoderm.
Formation of the optic vesicle and the shape change
that occurs as it develops towards becoming the retina
was induced during gastrulation.
The optic vesicle in turn induces overlying ectoderm to
differentiate into lens, then it induces surface ectoderm
to differentiate into the cornea.
Signals and the differentiation process show very
strong positional effects. Development itself also has
an ‘axis’. The head end of the organism tends to
develop earlier than the ‘tail’ or lower end.
Timing is also very critically integrated. Altered
development can result from exposure to various kinds
of teratogens. Measles, thalidomide, etc. alter the
expression of genes, producing various physical
abnormalities, but don’t alter the genes themselves.
Depending on when a pregnant woman took
thalidomide, her child could have ‘seal-like’ arms or
legs. The effect on legs would have come from a
slightly later exposure to the drug.
It is typical to divide pregnancy in humans into 3
month blocks or trimesters.
Within the first trimester the fetus looks like a very
miniature human. It has all body parts – a beating
heart, arms, legs, eyes, … and, with a little luck an
ultrasound image can detect its sex.
During the second trimester features and organs are
refined. Fingers and toes become fully distinct;
eyebrows, eyelids, and eyelashes develop, the
placenta takes over HCG production to maintain the
level of progesterone.
During the third trimester everything grows and
strengthens.
It is now time for birth (formally: parturition.
Hormones at birth and beyond
Approaching parturition (fancy word for birth) the
Mother’s blood increases its estrogen concentration.
That increases the number of oxytocin receptors in the
uterus.
The fetus and pituitary release oxytocin. This hormone
stimulates uterine contraction and causes fetal
membranes to release prostaglandins that increase the
strength of contractions.
When the fetus reaches “birth position” and presses
against the cervix, this mechanical stimulation
increases oxytocin release. More pressure…more
oxytocin - a positive feedback cycle.
Labor occurs, the cervix dilates (full dilation = ~10cm.)
Uterine contractions force the baby through the birth
canal, then parturition.
Hormones are also important after birth…
After birth, hormones are critical to nursing.
The baby’s suckling stimulates the mother’s pituitary
to release prolactin. Prolactin stimulates the milk
glands to produce milk.
The suckling also stimulates the pituitary to produce
oxytocin. Oxytocin again acts as a muscle stimulant.
This time it is muscles around the milk glands, causing
them to eject milk out the nipple into the baby’s
mouth.