Transcript ecology and evolution

Mr. Lajos Papp
The British International School, Budapest
2014/2015
11.1 Antibody production and vaccination
Antibodies are made by lymphocytes called B-cells. A
few B-cells that can make each type of antibody are
produced and if these cells encounter an antigen to
which their antibody binds, they multiply to form a
clone of many cells. This is called clonal selection.
Sometimes, several different types of antibody can bind
to the same antigen, so more than one clone cell is
formed. This is called polyclonal selection.
The B-cells will then differentiate into plasma cell and
memory cells. The plasma cells will make large
amounts of antibodies.
A clone of B-cells can produce large amounts of
antibody quickly and so give immunity to the disease
with which the antigen is associated.
Immunity to a disease is only developed if the immune
system is challenged by the disease. This is called the
principle of challenge and response.
Outline the principle of challenge and response, clonal
selection and memory cells as the basis of immunity.
The idea of a polyclonal response can be introduced
here.
Active immunity: immunity due to the production of
antibodies by the organism itself after the body’s
defence mechanisms have been stimulated by antigens.
Passive immunity: immunity due to acquisition of
antibodies from another organism in which active
immunity has been stimulated, including via the
placenta, colostrum, or by injection of antibodies.
Define active and passive immunity.
Antibody production by B-cells usually depends on
other types of white blood cells, including macrophages
(phagocytes) and helper T-cells (lymphocytes).
Macrophages take in antigens by endocytosis, process
them and then attach them to proteins called MHC
proteins. The MHC proteins carrying the antigens are then
moved to the plasma membrane by exocytosis and the
antigens are displayed on the surface of the macrophage.
This is called antigen presentation.
Helper T-cells have receptors in their plasma membrane
that can bind to antigens presented by macrophages.
Each helper T-cell has receptors with the same antigenbinding domain as an antibody. These receptors allow a
helper T-cell to recognize an antigen presented by a
macrophage and bind to the macrophage.
The macrophage passes a signal to the helper T-cell
changing it from an inactive to an active state. This is
called activation of helper T-cells.
B-cells have antibodies in their plasma membrane.
These antibodies recognize an antigen and the antigen
binds to the antibody.
An activated helper T-cell with receptors for the same
antigen binds to the B-cell. The activated helper T-cell
sends a signal to the B-cell, causing it to change from an
inactive to an active state. This is called activation of
B-cells. Activated B-cells start to divide by mitosis to
form a clone of cells.
Plasma cells are active B-cells with an extensive
network of rough endoplasmic reticulum. This is used
for synthesis of large amounts of antibody (protein).
Memory cells are B-cells that are formed at the same
time as plasma B-cells. The memory cells persist and
allow a rapid response if the disease is encountered
again.
Memory cells give long-term immunity to a disease.
Explain antibody production. Limit the explanation to
antigen presentation by macrophages and activation of
helper T-cells leading to activation of B-cells which
divide to form clones of antibody-secreting plasma
cells and memory cells.
Large quantities of a single type of antibody can be
made using a clever technique.
Antigens that correspond to a desired antibody are
injected into an animal.
B-cells producing the desired antibody are extracted
from the animal.
Tumour cells are obtained. These cells grow and divide
endlessly.
The B-cells are fused with the tumour cells producing
hybridoma cells that divide endlessly.
The hybridoma cells are cultured and the antibodies that
they produce are extracted and purified.
There are many ways in which monoclonal antibodies
can be used.
Treatment of rabies
Rabies usually causes death in humans before antibodies
produced by the immune system controls it. If a person
becomes infected, an effective strategy is to vaccinate
against rabies and at the same time inject monoclonal
antibodies. These control rabies virus until antibodies are
produced as a result of vaccination.
Diagnosis of pregnancy
Obtain monoclonal antibodies against the HCG (human
chorionic gonadotropin). Fix them in place on a testing
strip. Add urine to the testing strip.
If the HCG is present in the urine (if the woman is
pregnant), it will attach to the antibodies. The test has
been so designed that this will give a colour showing a
positive test.
Describe the production of monoclonal antibodies and
their use in diagnosis and in treatment. Production
should be limited to the fusion of tumour and B-cells
and their subsequent proliferation and production of
antibodies.
Limit the uses to one example of diagnosis and one of
treatment. Detection of antibodies to HIV is one
example in diagnosis. Other is the detection of HCG in
pregnancy test kits.
Examples of the use of these antibodies for treatment
include targeting of cancer cells with drugs attached to
monoclonal antibodies, emergency treatment of rabies,
blood and tissue typing for transplant compatibility
and purification of industrially made interferon.
A vaccine is a modified form of a disease-causing
micro-organism that stimulates the body to develop
immunity to the disease, without fully developing the
disease. The principle of vaccination is that antigens in
the vaccine cause the production of the antibodies
needed
to
antibodies.
stimulate
the
production
of
enough
The first vaccination causes a little antibody production
and the production of some memory cells. The second
vaccination causes a response from the memory cells
and therefore faster and greater production
of
antibodies.
Explain the principle of vaccination. Emphasise the
role of memory cells. The primary and secondary
responses can be clearly illustrated by a graph.
Benefits
Some
diseases
may
be
completely
eliminated.
Smallpox, for example, has already been eradicated by
vaccination.
Deaths due to disease can be prevented. For example,
measles is a major cause of death of small children in
some parts of the world.
Long-term disabilities due to disease can be prevented.
For example, if pregnant women are infected with
rubella their babies can be born with deafness, blindness
and heart and brain damage. Mumps can cause
infertility in men.
Dangers
Excessive amounts of vaccination may reduce the ability of
the immune system to respond to new diseases.
The immunity developed after vaccination may not be as
effective as immunity due to actually catching a disease.
Vaccination of children might make them vulnerable to
more severe infection as adults, for example with measles.
There is a danger of side effects from some vaccines,
which can cause long-term disability. Whooping cough
vaccination sometimes causes brain damage and MMR
vaccine may increase the chance of autism.
Discuss the benefits and dangers of vaccination. The
benefits should include total elimination of diseases,
prevention of pandemics and epidemics, decreased
health-care costs and prevention of harmful sideeffects of diseases. The dangers should include the
possible toxic effects of mercury in vaccines, possible
overload of the immune system and possible links with
autism.
11.2 Movement
The roles of the musculoskeletal system are movement,
support and protection.
Bones: move, support, and protect various organs of the
body.
Bones and exoskeletons provide anchorage for muscles
and act as levers.
The point where bones meet is called a joint. Most
joints allow the bones to move in relation to each other
– this is called articulation. Joints have a similar
structure, including cartilage, synovial fluid and joint
capsule.
Ligaments: hold bones together, limit their movement,
resist the dislocation of joints.
Muscles: produce force and cause motion. Voluntary
contraction of the skeletal muscles is used to move the
body and can be controlled.
Tendons: connects muscle to bone and is capable of
withstanding tension. Tendons and muscles work
together and can only exert a pulling force.
Different joints allow different ranges of movement.
Synovial joints allow certain movements but not others.
Skill
Annotation of a diagram of the human elbow.
Guidance
Elbow diagram should include cartilage, synovial fluid,
joint capsule, named bones and named antagonistic
muscles.
Source: http://www.ib.bioninja.com.au/higher-level/topic-11-human-health-and/112-muscles-and-movement.html
Movement of the body requires muscles to work in
antagonistic pairs. Antagonistic muscles produce
opposite movements at a joint.
Muscle cells have the ability to contract when
stimulated and are therefore able to exert a force in one
direction. An external force must be applied to restore
the fibres to their original length. Before a muscle can
be contracted second time, it must relax and be extended
by the action of another muscle.
This means that muscles operate in pairs with each
member of the pair acting in the opposite direction to
the other. These muscle pairs are termed antagonistic.
Muscles may be classified according to the type of
movement they bring about. A flexor muscle bends a
limb, whereas an extensor straightens it. Flexors and
extensors therefore form an antagonistic pair.
Each muscle is attached to a bone at both ends. One
attachment (origin) is fixed to a rigid part of the
skeleton, while the other (insertion) is attached to a
moveable part.
There may be a number of points of insertion and / or
origin. For example, two antagonistic muscles which
bend the arm about the elbow are the biceps (flexor) and
triceps (extensor).
The biceps has two origins both on the scapula
(shoulder blade) and a single insertion on the radius.
The triceps has three origins, two on the humerus and
one on the scapula and a single insertion on the ulna.
Cartilage: covers surface of bone, reduces friction.
Synovial fluid: lubricates the joint, nourishes the
cartilage.
Joint capsule: is an envelope surrounding a synovial
joint.
Radius: It provides structure and support for the lower
arm with the ulna. It connects to the elbow and provides
a point of attachment for muscles and ligaments for the
arm and hand.
Ulna: It is a bone in the forearm. It functions as part of
the elbow joint.
Humerus: important projections on the humerus serve
as locations for muscle and ligament attachments.
Biceps: is to flex / bend the elbow and to turn / rotate
the forearm. (flexor)
Triceps: is an extensor muscle of the elbow joint.
Application
Antagonistic pairs of muscles in an insect leg.
Muscles are group of cells working together. Muscle
cells (also called muscle fibres) can be more than 1 cm
long. They are arranged together and have connective
tissue around them. Skeletal muscle fibres are
multinucleate and contain specialized endoplasmic
reticulum.
Inside a muscle cell, you find many thin myofibrils.
These thin fibres cause the typical striated pattern of
skeletal muscles. Myofibrils contain two types of
myofilaments: myosin and actin.
Each myofibril is made up of contractile sarcomeres. A
sarcomere contains a light section, a dark section, an
intermediate section, a dark section and a light section.
Skill
Drawing labelled diagrams of the structure of a
sarcomere.
Guidance
Drawing labelled diagrams of the structure of a
sarcomere should include Z lines, actin filaments,
myosin filaments with heads, and the resultant light and
dark bands.
The thin filaments are attached to the Z line. This light
section is called the I band. They partly overlap with the
thick myosin filaments which appears as a dark section,
the A band. In the middle between the two Z lines, you
find only myosin, the H band.
Across the fibres, you find the T system. These are
tubules touching the sarcolemma and associated with
vesicles which are part of the sarcoplasmic reticulum.
The vesicles are of great importance because they
regulate the movement of calcium ions to/from the
sarcoplasm. Since the Ca2+ concentration determines
the activity of ATPase, this essentially determines the
activity of the muscle.
Sarcolemma: membrane surrounding muscle cells /
fibres.
Sarcomere: section of the myofibril between two Z
lines.
Sarcoplasm: cytoplasm in a muscle fibre.
Sarcoplasmic reticulum: internal membranes within
sarcoplasm.
Guidance
Measurement of the length of sarcomeres will require
calibration of the eyepiece scale of the microscope.
The contraction of the skeletal muscle is achieved by
the sliding of actin and myosin filaments. It was
discovered that A band (myosin) is the same length in
contracted and relaxed muscles.
The sliding filament theory says that the actin and
myosin filament slide over each other to make the
muscle shorter. Little hooks on the myosin filaments
attach to the actin and pull them closer.
ATP hydrolysis and cross bridge formation are
necessary for the filaments to slide.
Calcium ions and the proteins tropomyosin and
troponin control muscle contractions.
Actin filaments contain actin as well as two proteins:
tropomyosin and troponin. Tropomyosin forms two
strands which wind around the actin filament, covering
the binding site for the myosin hooks.
The muscle cannot contract now. When a nerve impulse
arrives at the muscle, it causes the vesicles of the
sarcoplasmic reticulum to release Ca2+ into the
sarcoplasm.
The calcium ions attach to the troponin which is
attached to the tropomyosin. This uncovers the binding
sites on the actin for the myosin hooks. The muscle will
now contract.
When no more impulses arrive, calcium ions are moved
back into vesicles of the sarcoplasmic reticulum by
active transport. The binding sites on the actin will then
be covered again and the muscle will relax.
Skill
Analysis of electron micrographs to find the state of
contraction of muscle fibers.
11.3 The kidney and osmoregulation
Excretion: the removal from the body of the waste
products of metabolic pathways.
Osmoregulation: the control of the water balance of the
blood, tissue or cytoplasm of a living organism.
Animals
are
either
osmoregulators
or
osmoconformers.
Osmoregulators maintain a constant internal solute
concentration,
environments.
even
when
living
in
marine
Osmoconformers are animals whose internal solute
concentration tends to be the same as the concentration
of solutes in the environment.
The Malpighian tubule system in insects and the
kidney carry out osmoregulation and removal of
nitrogenous wastes.
Hemolymph: circulating fluid.
Skill
Drawing and labelling a diagram of the human kidney.
The composition of blood in the renal artery is different
from that in the renal vein.
Renal artery – large molecules, more toxins,
oxygenated blood, more salts / ions, more H2O, less
CO2, more nutrients, supplies kidneys with O2.
Renal vein - less toxins, deoxygenated blood, less salt,
less H2O, more CO2, less nutrients, returns blood from
kidneys to heart, blood contains no wastes, with less O2,
urea, salt/ions, and more CO2.
Blood plasma
Glomerular
filtrate
Urine
Urea
+
+
++++
Proteins
+
-
-
Glucose
+
+
-
The ultrastructure of the glomerulus and Bowman’s
capsule facilitate ultrafiltration.
Afferent arterioles: are a group of blood vessels that
supply the nephrons. The afferent arterioles branch from
the renal artery. The afferent arterioles later diverge into
the capillaries of the glomerulus.
Efferent arterioles: they form from a convergence of
the capillaries of the glomerulus. They play an
important role in maintaining the glomerular filtration
rate despite fluctuations in blood pressure.
Podocytes: are cells of the epithelium in the kidneys
and form a component of the glomerular filtration
barrier, contributing size selectivity and maintaining a
massive filtration surface.
Adjacent podocytes interdigitate (to interlock like the
fingers of clasped hands) to cover the basement
membrane which is associated with the glomerular
capillaries, but the podocytes leave gaps or thin
filtration slits.
The slits are covered by slit diaphragms (fenestrations)
which are composed of a number of cell-surface
proteins, which ensure that large macromolecules such
as serum albumin and gamma globulin remain in the
bloodstream.
Small molecules such as water, glucose, and ionic salts
are able to pass through the slit diaphragms and form an
ultrafiltrate which is further processed by the nephron to
produce urine.
Skill
Annotation of diagrams of the nephron.
Guidance
The diagram of the nephron should include glomerulus,
Bowman’s capsule, proximal convoluted tubule, loop of
Henle, distal convoluted tubule; the relationship
between the nephron and the collecting duct should be
included.
The activity of the nephron is based on the principles of
ultrafiltration,
reabsorption
and
secretion.
In
ultrafiltration, the part of the fluid in the blood is pushed
out of a blood vessel into the nephron.
Further in the nephron the substances which the body
does not want to lose are reabsorbed into the blood (e.g.
glucose). Finally some substances (e.g. ammonia) are
secreted into the filtrate to be removed with the urine by
the cells of the nephron.
The renal artery supplies the kidney with blood. It splits
into many smaller blood vessels and each nephron has
an afferent vessel which carries the blood to the
glomerulus.
From the glomerulus, the efferent vessel carries the
blood around the other parts of the nephron. After this,
the blood passes into larger blood vessels which
eventually become the renal vein.
In the glomerulus, the blood is under a high pressure
and ultrafiltration takes place. This means that some of
the liquid and dissolved particles are pushed out of the
blood vessel; the cells and the larger molecules (e.g.
proteins) are too big to pass through and will not be
found in Bowman’s capsule.
All in the blood in the body passes through the kidney
every five minutes. Approximately 15-20% of the fluid
in the blood will pass into Bowman’s capsule.
The filtrate needs to pass a barrier made of three
different layers: the wall of the glomerulus, the
basement membrane of the glomerulus and the inner
wall of Bowman’s capsule.
Proximal convoluted tubule: is the portion of the duct
system of the nephron leading from Bowman's capsule
to the loop of Henle. It selectively reabsorbs useful
substances by active transport.
The fluid in the proximal convoluted tubule is similar to
plasma and contains glucose, amino acids, vitamins,
hormones, urea, ions and water. Most of the
reabsorption in the nephron occurs here, all the
glucose, amino acids, vitamins, hormones and most of
the sodium, chloride and water are reabsorbed into the
blood vessels.
Osmosis drives the reabsorption of water as it follows
the active transport of glucose and Na+. Cl- passively
follows the actively transported Na+.
All these substances need to move across the wall of the
proximal convulated tubule.
To facilitate this, the cells lining the lumen of the
proximal convulated tubule have a brush border: a row
of microvilli (fingerlike extensions of the cell) which
greatly increase the available surface area. Mitochondria
are also prominent in these cells, providing the energy
for active transport.
The loop of Henle maintains hypertonic conditions in
the medulla. In the descending limb of the loop of Henle
water leaves the nephron by osmosis due to the
increasing concentration of salt.
This water immediately passes into the blood capillaries
and is removed from the area. The ascending limb is
impermeable to water and salt is lost from the filtrate by
active transport.
The salt remains near the loop of Henle (it is not
immediately removed by the blood) and helps to
maintain a concentration gradient in the medulla.
The fluid which leaves the loop of Henle is less
concentrated than the tissue fluid around it. The
concentration gradient in the medulla is maintained by
the vasa recta countercurrent exchange.
The vasa recta are the blood vessels running along the
loop of Henle. There is no direct exchange between the
filtrate and the blood but substances pass through the
interstitial region of the medulla.
The length of the loop of Henle is positively correlated
with the need for water conservation in animals. The
longer the loop of Henle, the more water can be
reclaimed. Animals adapted to dry habitats will have
long loops of Henle.
Guidance
ADH will be used in preference to vasopressin.
ADH controls reabsorption of water in the collecting
duct. ADH increases the permeability of the walls of the
distal convoluted tubule and the collecting duct. ADH is
released from the posterior lobe of the pituitary gland
when there is a lack of water.
The dilute filtrate coming from the loop of Henle can
then lose water (by osmosis) in the distal convoluted
tubule and in the collecting duct.
The water is reabsorbed by the blood. When ADH is
absent and the walls are impermeable, water is not
removed from the filtrate in the distal convoluted tubule
and the collecting duct and ends up in the bladder as
dilute urine.
The type of nitrogenous waste in animals is correlated
with evolutionary history and habitat.
Application
Consequences of dehydration and overhydration.
page 496
Definition, medical outcomes.
Application
Treatment of kidney failure by hemodialysis or kidney
transplant.
Application
Blood cells, glucose, proteins and drugs are detected in
urinary tests.
Urinalysis. A urine test strip: pH, protein, glucose.
Drugs: monoclonal antibodies.
Microscopy: presence of WBC, RBC.
11.4 Sexual reproduction
Annotate a light micrograph of testis tissue to show the
location and function of interstitial cells (Leydig cells),
germinal epithelium cells, developing spermatozoa and
Sertoli cells.
An outer layer of germ cells called spermatogoni (2n) divide
by mitosis to produce spermatogonia. Spermatogonia grow
into larger cells called primary spermatocytes (2n). Each
primary spermatocyte carries out the first division of
meiosis to produce two secondary spermatocytes (n). Each
secondary spermatocyte carries out the second division of
meiosis to produce two spermatids (n).
Spermatids become associated with nurse cells, called Sertoli cells,
which help the spermatids to develop into spermatozoa (n). This is
an example of cell differentiation. Sperm detach from Sertoli cells
and eventually are carried out of the testis by the fluid in the centre
of the seminiferous tubule.
Outline the processes involved in spermatogenesis within the
testis, including mitosis, cell growth, the two divisions of meiosis
and cell differentiation. The names of the intermediate stages in
spermatogenesis are not required.
State the role of FSH, testosterone and LH in
spermatogenesis.
hormone
source
role
stimulates primary spermatocytes
to undergo the first division of
FSH
pituitary gland
meiosis,
to
form
secondary
spermatocytes
stimulates
testosterone
interstitial
cells
in the testis
secondary
pituitary gland
development
spermatocytes
of
into
mature sperm
stimulates
LH
the
the
secretion
testosterone by the testis
of
Germinal epithelium: the surface of the ovary is
covered by a layer of simple cells. The germinal
epithelium gives the ovary a dull gray colour. The
germinal epithelium is the main origin of tumours in the
ovaries.
Primary follicles: an immature ovarian follicle in which the
developing oocyte is surrounded by a layer of follicular
cells.
Mature follicle: a follicle ready for ovulation; a first
maturation (meiotic) division of the ovum usually occurs
just prior to the rupture of the follicle.
Secondary oocyte: arises from the primary oocyte after
it completes the first meiotic division after the body has
become sexually mature. The secondary oocyte divides
into the mature egg and a polar body, thus ending the
second meiotic division.
Annotate a diagram of the ovary to show the location
and function of germinal epithelium, primary follicles,
mature follicle and secondary oocyte.
Meiotic division I gives two haploid secondary oocytes with
unequal cytoplasm and then ovulation occurs. Meiotic
division II occurs after sperm penetration.
In the ovaries of a female foetus, germ cells called oogonia
(2n) divide by mitosis to form more oogonia. Oogonia grow
into larger cells called primary oocytes (2n).
Primary oocytes start the first division of meiosis but
stop during prophase I. The primary oocyte and a single
layer of follicle cells around form a primary follicle.
When a baby girl is born the ovaries contain about
400 000 primary follicles.
Every menstrual cycle a few primary follicles start to
develop. The primary oocyte completes the first division of
meiosis, forming two haploid nuclei.
The cytoplasm of the primary oocyte is divided unequally
forming a large secondary oocyte (n) and a small polar cell
(n). The secondary oocyte starts the second division of
meiosis but stops in prophase II.
The follicle cells meanwhile are proliferating and follicular
fluid is forming. When the mature follicle bursts, at the time
of ovulation, the egg that is released is actually still a
secondary oocyte.
After fertilization the secondary oocyte completes the
second division of meiosis to form an ovum, (with a sperm
nucleus already inside it) and a second polar body.
The first and second polar bodies do not develop and
eventually degenerate.
Outline the processes involved in oogenesis within the
ovary, including mitosis, cell growth, the two divisions
of meiosis, the unequal division of cytoplasm and the
degeneration of polar body. The terms oogonia and
primary oocyte are not required.
Draw and label a diagram of a mature sperm and egg.
cell type
chromosomes chromatids process time of completion
primary
in prophase I until
diploid / 46
4N
meiosis I
oocyte
ovulation
secondary
meiosis halted in metaphase
oocyte
haploid / 23
2N
II
II until fertilization
The sperm cells travel to the head of the epididymis (via
vasa efferentia). Here they mature and become motile.
During one ejaculation, approximately 3 cm3 of semen
is produced. Only 10% of this is sperm cells. Most of
the fluid in the semen is produced by the seminal
vesicles.
The fluid they produce contains fructose for energy and
prostaglandins which cause contractions in the female
reproductive system (helping the sperm move towards
the egg cell).
The fluid from the prostate is alkaline and helps to neutralise
the normally acidic environment of the female reproductive
tract. Normal pH is around 4 but the presence of prostate
fluid will make it around pH 6 which is the optimum pH for
sperm motility.
Outline the role of the epididymis, seminal vesicle and
prostate gland in the production of semen.
Similarities
both start with proliferation of cells by mitosis;
both involve the cell growth before meiosis;
both involve the two divisions of meiosis;
both produce gametes by meiosis in gonads.
Differences
number of gametes produced in total:
spermatogenesis produces large numbers of gametes
(millions daily),
oogenesis produces few (one every 28 days);
number of gametes per cell:
spermatogenesis produces four sperm cells from one
primary spermatocyte,
oogenesis produces one ovum from one primary oocyte;
time of formation:
males produce gametes continuously, from puberty until
old age, females produce primary oocytes before birth
and then one ovum per month, from puberty until
menopause;
release of gametes:
males can release gametes at any time during ejaculation,
females are on a monthly cycle (on about day 14 of
menstrual cycle by ovulation).
Compare the processes of spermatogenesis and oogenesis
including the number of gametes and the timing of the
formation and release of gametes.
Arrival of sperm: sperm cells are attracted by a
chemical signal and swim up the oviduct to reach the
egg. Fertilization is only successful if many sperm cells
reach the egg.
Binding: the first sperm cell to break through the layer
of follicle cells binds to the zona pellucida. This
triggers the acrosome reaction.
Acrosome reaction: the contents of the acrosome are
released, by the separation of the acrosomal cap from
the sperm. Proteases from the acrosome digest a route
for the sperm cell through the zona pellucida, allowing
the sperm cell to reach the plasma membrane of the egg.
Fusion: the plasma membranes of the sperm cell and
egg fuse and the sperm nucleus enters the egg and joins
the egg nucleus. Fusion causes the cortical reaction.
Cortical reaction: small vesicles called cortical granules
move to plasma membrane of the egg and fuse with it,
releasing their contents by exocytosis. Enzymes from the
cortical granules cause cross-linking of glycoproteins in the
zona pellucida, making it hard and preventing the entry of
any more sperm cell.
Mitosis: the nuclei from the sperm cell and egg do not fuse
together. Instead, both nuclei carry out mitosis, using the
same centrioles and spindle of microtubules. A two-cell
embryo is produced.
Describe the process of fertilization including the
acrosome reaction, penetration of the egg membrane by a
sperm, and the cortical reaction.
1. The fertilised egg has developed into a blastocyst that
will implant into the endometrium.
2. Implantation of the blastocyst which begins to secrete
human chorionic gonadotrophin (HCG).
3. HCG passes into the maternal blood. The concentration
doubles every 2-3 days and reaches a peak at 8-10 weeks.
4. The HCG targets the ovary and the corpus luteum.
5. The corpus luteum secretes progesterone and oestrogen
at high levels. The oestrogen and progesterone continue to
inhibit FSH and LH secretion from the pituitary.
6. The progesterone prevents the breakdown of the
endometrium and so the embryo can continue its
development into a foetus.
Oestrogen and progesterone are needed throughout pregnancy
to stimulate the development of the uterus lining. During the
first few days after ovulation the corpus luteum secretes these
hormones whether or not there has been fertilization. After
implanting in the uterus wall, the embryo starts to secrete a
hormone called HCG (human chorionic gonadotrophin).
HCG prevents degeneration of the corpus luteum, which
would happen at the end of a menstrual cycle. HCG
stimulates the corpus luteum to grow and to continue
secretion of oestrogen and progesterone. This is
essential to allow the pregnancy to continue.
By the middle of the pregnancy, the corpus luteum starts to
degenerate, but by then cells in the placenta are secreting
oestrogen and progesterone and these cells secrete
increasing amounts until the end of the pregnancy.
Outline the role of human chorionic gonadotrophin
(HCG) in early pregnancy.
After fertilization, the zygote passes down the oviduct and
undergoes
cleavage
(cell
division
in
embryonic
development). The zygote divides to form a two-celled
embryo (24 hours). Then, each cell undergoes mitosis and
divides. Repeated divisions increase the number of cells,
called blastomeres that make up the embryo. At about the
32-cell stage the embryo is a solid ball of cells called a
morula.
Morula reaches uterus (3 days). Cleavage at this stage does not
result in an increase in size of the morula because the cells
continue to be retained within the cell layer. The blastomeres
form a wall of cells enclosing a central cavity in the morula. The
outer layer of blastomeres differentiates at one point to form a
thickened mass of cells, the inner cell mass. This stage is called
the blastocyst and is reached about 4-5 days following ovulation.
When the blastocyst arrives in the uterus it spends about two
days. Between the sixth and ninth days after ovulation the
blastocyst
becomes
embedded
within
cells
of
the
endometrium. This process is called implantation.
Outline early embryo development up to the implantation of
the blastocyst. Limited this to several mitotic divisions
resulting in a hollow ball of cells called the blastocyst.
A foetus is surrounded by amniotic fluid which in turn is
surrounded by the amniotic sac. The amniotic sac keeps the
fluid from leaking out and protects the foetus against
infections. The amniotic fluid buffers shocks and protects
the baby from mechanical harm. Babies drink amniotic fluid
and urinate in it. It is constantly made and filtered by the
mother.
The foetus is supported and protected by the amniotic sac
and amniotic fluid.
The placenta is a foetal tissue which invades maternal uterine
tissue. The baby’s blood runs through blood vessels which go
through blood spaces filled with maternal blood. An exchange of
substances takes place (diffusion). The foetal blood returns to the
foetus enriched with nutrients and oxygen, the maternal blood has
taken up the carbon dioxide and other waste products from the
foetal metabolism, which it will excrete.
Materials are exchanged between the maternal and foetal blood
in the placenta.
1.
Umbilical cord connects the foetus to the placenta.
2.
Umbilical arteries carry the deoxygenated blood to the
placenta. The umbilical vein returns the blood to the rest of the
foetal circulation.
3.
The myometrium is composed on smooth muscle that
produces the contraction in labour.
4.
The endometrium which is maintained through out
pregnancy by progesterone. Initially from the corpus luteum
and later from the placenta itself.
6.
The female blood supply which supplies the foetus with
oxygen and nutrient. It will also remove waste from the foetal
blood and excrete this through the maternal systems.
7.
Open ended blood arterioles and capillaries that produce the
inter-villous 'blood pool'.
8.
Inter-villus spaces filled with maternal blood. These surround
the placental villi and allow for very efficient exchange.
9.
Placental-villi with large surface area for the exchange of
nutrient and waste.
Placenta – a disc-shaped structure.
Placental villi – small projections that give a large surface area for
gas exchange and exchange of other materials. Foetal blood flows
through capillaries in the villi.
Inter-villous space (pool) – maternal blood flows through these
spaces, brought by uterine arteries and carried away by uterine
veins.
Explain how the structure and functions of the placenta, including
its hormonal role in secretion of oestrogen and progesterone,
maintain pregnancy.
1. At the end of gestation there is a drop in the high
levels of progesterone and the progesterone receptor is
also blocked. Therefore the progesterone is not
effective.
2. With the fall in progesterone the pituitary secretes
oxytocin.
3. Stretching of the lower uterus walls by the foetus
and its production of prostaglandin add to the
stimulus for the pituitary to secrete oxytocin.
4. The oxytocin causes the smooth muscle in the walls
of the uterus (myometrium) to contract and labour has
begun.
Positive feedback
1. In this system the stimuli to the brain increases the
oxytocin production.
2. In turn the oxytocin stimulates myometrial contraction.
3. Myometrial contraction further stimulates the pituitary
of the mother to release more oxytocin.
4. The
strength and frequency of the myometrial
contractions is further increased.
5. In turn this further stimulates more oxytocin production.
6. The
process
builds
with
stronger
and
stronger
contractions.
7. Final the child passes though the cervix and vagina to be
born.
8. Contractions continue for a further period until the
placenta is delivered (after birth).
It was thought that hormonal activities within the
mother controlled the timing of birth but recent
evidence obtained from research on several mammals
has suggested there is a high degree of foetal
involvement in the timing of birth.
The initial stages of birth are believed to result from stimuli
influencing the foetal hypothalamus to release a hormone
from the foetal pituitary. As foetal hormone is released it
stimulates the foetal adrenal gland to release corticosteroids
which cross the placental barrier and enter the maternal
circulation causing a decrease in progesterone and an
increase in secretion of prostaglandins.
The reduction in progesterone level allows the maternal
pituitary gland to release oxytocin, and removes the
inhibitory effect on contraction of the myometrium. Whilst
oxytocin causes contraction of the smooth muscle of the
myometrium, prostaglandins increase the power of the
contractions.
The release of oxytocin occurs in waves during labour and
provides the force to expel the foetus from the uterus.
The onset of contractions of the myometrium, so called
labour pains, are accompanied by the dilation of the cervix,
the rupture of the amnion releasing amniotic fluid from the
cervix, and the stimulation of stretch receptors in the walls
of the uterus and cervix.
The latter activate the autonomic nervous system and autonomic
reflexes induce contraction of the uterus wall. Other impulses
pass up the spinal cord to stimulate the hypothalamus to release
oxytocin.
The pressure of the head of the foetus engaged in the pelvis
pressing against the cervix with the face towards the mother's
anus irritates the cervix and leads to stronger contractions of the
myometrium.
The birth process is divided into three stages. During
the first stage (labour), which typically lasts about 12
hours, the contractions of the uterus move the foetus
toward the cervix, causing the cervix to dilate (open).
The cervix also becomes effaced; that is, it loses its
normal shape and flattens so that foetal head can pass
through. During this stage of birth the amnion usually
ruptures, releasing about a litre of amniotic fluid, which
flows out through the vagina.
During the second stage (delivery), which normally lasts
between 20 minutes and an hour, the foetus passes through
the cervix and the vagina and is born.
With each uterine contraction the woman holds her breath
and bears down so that the foetus is expelled from the uterus
by the combined forces of uterine contractions and
contractions of the abdominal wall muscles.
After the baby is born, the contractions of the uterus
squeeze much of the foetal blood from the placenta back
into the infant. The cord is tied and cut, separating the
child from the mother.
During the third stage of birth (afterbirth), which lasts 10 or
15 minutes after the birth, the placenta and the foetal
membranes are loosened from the lining of the uterus by
another series of contractions, and expelled.
Outline the process of birth and its hormonal control,
including the changes in progesterone and oxytocin levels
and positive feedback.