CHAPTER 29 DEVELOPMENT & INHERITANCE
Fertilization occurs when genetic
material from ovum and sperm merge into a single nucleus. Only about 1% of the
300-500 million sperm ejaculated reach the secondary oocyte. Fertilization
usually occurs in the Fallopian tube 12-24 hours after ovulation. Sperm can
travel up the Fallopian tube and reach the oocyte within minutes, but they are
not capable of fertilization at that point.
The oocyte is surrounded by several layers of follicular cells, the corona
radiata, and a glycoprotein layer, the zona pellucida. After several hours in
the female reproductive tract sperm, have undergone capacitation and are
capable of fertilization. Release of enzymes of the acrosomes of a number of
sperm enable one sperm to penetrate the protective layers of the
oocyte--syngamy. Syngamy causes the secondary oocyte to complete Meiosis II and
form the ovum. The fertilized ovum contains a nucleus that is a combination of
chromosomal material from both ovum and sperm and is diploid.
Dizygotic (fraternal) twins are
produced when 2 ova are released and both are fertilized by separate sperm.
They may or may not be the same sex and are no more closely related than other
siblings.
Monozygotic (identical) twins develop
from a single fertilized ovum that splits at an early stage of development.
They are always the same sex and contain identical genetic material.
After fertilization, rapid mitotic cell
division called cleavage takes place. A solid mass of cells called the morula
is produced (still in the uterine tube). Although there are greater numbers of
cells, the morula is still about the same size as the zygote. The progressively
smaller cells it contains are called blastomeres.
By the fifth day the morula has changed
into a hollow ball of cells, the blastocyst, and reached the uterine cavity.
The blastocyst has these parts:
1. Trophoblast---outer covering that comes in contact with the uterine
wall and enables the blastocyst to burrow into the endometrium
2. Inner cell mass
3. Blastocele---fluid-filled cavity
The blastocyst remains free in the
uterus for 1 - 2 days. About 6 - 7 days after fertilization the blastocyst
attaches to the endometrium--implantation. The blastocyst then proceeds to
burrow into the endometrium.
At this stage, the trophoblast has 3
functions:
1. Secretes enzymes that allow the blastocyst to burrow into the uterine
wall
2. Develops into the chorion (one of the fetal membranes)
3. Secretes hCG, which causes the corpus luteum, to continue to secrete
estrogens and progesterones to maintain the pregnancy.
Secondary oocyte
Mature ovum (already fertilized)
Zygote
Morula
Blastocyst---this stage implants (burrows in to the wall of the uterus)
Following implantation, the endometrium
is known as the decidua and consists of:
1. Decidua basalis—portion between embryo and stratum basalis of the
uterus—will form the maternal part of the placenta
2. Decidua capsularis—portion between embryo and uterine cavity
3. Decidua parietalis—remaining endometrium
The first 2 months of development are
the period of the embryo--study is embryology. After implantation the inner
cell mass of the blastocyst differentiates into the 3 primary germ layers:
ectoderm, endoderm and mesoderm. Cell migrations that establish the 3 germ
layers are called gastrulation. All tissues and organs of the body will develop
from these.
Endoderm forms the epithelial lining of
most internal body structures, such as the GI tract, the respiratory tract,
etc. and the epithelium of many glands
Mesoderm forms muscle, bone and other
connective tissue, the peritoneum and the endothelium of blood vessels
Ectoderm forms skin and nervous system
Embryonic membranes, which protect and
nourish the embryo and later the fetus also form during this time:
1. Yolk sac--early site of blood cell formation, contains cells that
migrate to ovaries or testes and differentiate into oogonia or spermatogonia
2. Amnion--entirely surrounds embryo and creates a cavity filled with
amniotic fluid in which the embryo floats. It acts as a shock absorber and
helps regulate fetal body temperature. Embryonic cells are sloughed off into
this fluid and can be removed and examined by amniocentesis.
3. Chorion--forms the embryonic part of the placenta, also surrounds the
fetus outside the amnion
4. Allantois--its blood vessels help form the connection between mother
and baby
The placenta is developed by the 3rd
month of pregnancy. It is formed by the chorion of the embryo and the
endometrium of the mother. It allows oxygen, carbon dioxide, nutrients and
wastes to diffuse back and forth between the blood of mother and baby. Equally
important, the placenta produces several hormones that maintain pregnancy. At
first, these hormones come from the corpus luteum, but it could not sustain an
entire pregnancy.
One thing that the placenta does NOT do
is screen out nearly all harmful substances from reaching the baby. Not too
long ago, medicine gave the placenta way too much credit for this. Pregnant
women were not warned much about smoking, alcohol, medications, etc. Now we
have learned that while the placenta does give some protection, most things in
the mother’s blood do cross over to the baby. Fortunately, most microorganisms
do not cross the placenta, although some viruses pass through.
Fingerlike projections of the chorion,
chorionic villi, grow into the endometrium and contain blood vessels of the
allantois which will come in close contact with the blood of the mother in
intervillous spaces. Following exchange, blood travels from the placenta to the
baby by way of the umbilical vein. Blood flows through 2 umbilical arteries
from the baby's circulation to the placenta. The umbilical arteries and vein
are located in the umbilical cord.
At delivery the placenta detaches from
the uterus and is called the afterbirth.
1. Fetal ultrasonography--the big
advantage is no known risk to mother or baby. Sound waves are reflected back
from the fetus and converted to an image (sonogram) on a screen. It is used to
determine fetal age, viability and growth, to determine fetal position,
multiple pregnancies and some abnormalities.
2. Amniocentesis--some of the amniotic
fluid is withdrawn and fetal cells and dissolved substances are studied. It can
detect certain genetic disorders as well as determining fetal maturity and
well-being. It can be used at 14-16 weeks of gestation and can detect almost
350 genetic defects. Guided by ultrasound, a needle is inserted into the
amniotic fluid and about 10 ml of fluid is withdrawn. There is a small risk of
spontaneous abortion (.5%).
3. Chorionic villus sampling--this test
can give the same results as amniocentesis and can be performed at about 8
weeks. Guided by ultrasound, either a catheter is inserted vaginally or a
needle through the abdominal wall is used to remove some chorionic villus
tissue. The risk of spontaneous abortion is slightly higher (1-2%).
The period of the fetus is months 3-9
of development. By the end of the period of the embryo all organs are present.
They will continue to grow and develop for the next 7 months.
The corpus luteum secretes estrogens
and progesterone for the first 3-4 months. These hormones maintain the lining
of the uterus and prepare the mammary glands for lactation. In the 3rd month
the placenta begins to secrete higher levels of these hormones and the corpus
luteum degenerates. The chorion secretes hCG to maintain the corpus luteum so
long as it is needed.
The placenta also secretes:
Human chorionic somatomammotropin, which encourages development of
breast tissue and regulates metabolism.
Relaxin (also produced by ovaries), which relaxes symphysis pubis and
pelvic ligaments and helps dilate the cervix
Corticotropin-releasing hormone (CRH)---may help establish timing for
birth. Cortisol is produced in response and encourages maturation of the fetal lungs.
Gestation is the period during which
zygote, embryo or fetus is carried in the female reproductive tract. In humans
this is 266 days. Obstetrics is the branch of medicine.
Labor--process by which the fetus is
expelled
At the end of gestation secretion of
progesterone, which inhibits uterine contractions, drops as the level of
estrogens rises. This plays a role in causing labor to begin. The placenta
produces corticotrophin releasing hormone, which causes the baby's pituitary to
secrete ACTH. In response, the fetal adrenal glands secrete cortisol and DHEA.
The cortisol encourages maturation of the baby's lungs, and some of the DHEA is
converted to estrogens. The estrogens cause smooth muscle fibers of the uterus
to develop oxytocin receptors. Prostaglandins also play a role.
In a positive feedback cycle, oxytocin
from the posterior pituitary stimulates uterine contractions. These begin at
the top of the uterus and move downward, pushing the baby against the cervix
and, when cervical dilation is complete, out through the vagina. 3 stages of
labor:
1. Dilation--regular contractions lead to complete dilation of the
cervix (10cm). The amniotic sac usually ruptures during this stage.
2. Expulsion--time from complete cervical dilation to delivery
3. Placental--time from delivery to expulsion of the placenta by
continued uterine contractions. These contractions also constrict blood vessels
in the uterine wall, controlling bleeding.
In response to the stress of birth, the
adrenal medulla of the baby secretes high levels of epinephrine and
norepinephrine, which helps clear the lungs, mobilize nutrients and promote a
rich blood supply to the brain and heart.
During the 6 weeks following delivery
the uterus is reduced in size by the process of involution. The cervix returns
to pre-pregnancy condition.
Respiratory system--this system is
fairly well developed by the 7th month of gestation, but the lungs remain
collapsed or filled with amniotic fluid. Immediately after delivery carbon
dioxide begins to build up and the baby begins to breathe. The fluid is either
suctioned out or drains out with breathing.
Cardiovascular system--with the first
breath, the foramen ovale (between atria) closes as two flaps of heart tissue
fold together and fuse. Muscle in the walls of the ductus arteriosus contract
and begin to close the vessel off, but closure is usually not complete until
about 3 months after birth.
Both pulse and respiration are rapid in
newborns (resp. 45/minute, pulse 120-160/minute). These gradually decline.
The infant's liver may not immediately
function adequately, causing a temporary jaundice to develop in up to 50% of
normal newborns.
This is the passage of hereditary traits
from one generation to another. Genetics is the branch of biology. Genetic
counseling offers advice on genetic problems.
Genome--complete genetic makeup of an
organism (about 100,000 genes in a human)
Genotype--all genes present, both those expressed
and those not expressed
Nuclei of all human cells except
gametes contain 23 pairs of chromosomes (diploid number). Homologous
chromosomes contain genes that control the same traits. Genes at the same
location on homologous chromosomes that control the same traits are called
alleles.
Dominance---a dominant gene is one that
dominates other genes present for a trait--if one fully dominant gene for a
trait is present the trait will be expressed regardless of the presence of
other genes.
A recessive gene is one that may be
present but will not be expressed unless no dominant allele is present.
A person with the same gene for a trait
present on both homologous chromosomes (identical alleles) is said to be
homozygous. A person with 2 different alleles is heterozygous.
For example, the gene that causes a
person to have freckles is dominant (no freckles is recessive). Dominant genes
are represented by capital letters, recessive by lower case:
F = freckled
f = no freckles
Everyone has 2 of these genes, so the
possibilities are:
FF (homozygous)
These possibilities are described as the
ff (homozygous)
possible genotypes.
Ff (heterozygous)
Phenotype is the way the genetic makeup is
expressed. Although the genotype differs, FF and Ff would have the same
phenotype (both freckled). The phenotype
of ff would be no freckles.
Charts called Punnett squares are used
to determine the possible ways gametes can combine to produce diploid
offspring.
Normal traits are most often dominant,
abnormal traits are most often recessive, particularly if the defect is
serious. This is because few individuals with the problem may live to
reproduce--in some cases they will even die in utero, so dominant abnormalities
tend to die out.
A cell having missing or added
chromosomes is said to be aneuploid. This is usually the result of
nondisjunction---homologous chromosomes do not separate during meiosis I.
Monosomic cell--missing a chromosome
Trisomic cell--extra chromosome (trisomy 21 is Down syndrome)
Although it is easiest to understand,
simple dominant-recessive inheritance is not the only possibility. Many traits
are influenced by more than 1 pair of genes and other factors such as environment
can also play a role in gene expression.
Incomplete
dominance--neither allele
is completely dominant over the other and heterozygous individuals have a
phenotype intermediate between the 2. In flowers (snapdragons):
PP=red
pp=white
Pp=pink
In humans, sickle cell anemia is an
example:
HbA = normal hemoglobin HbA HbA =normal
HbS = abnormal hemoglobin HbA HbS =minor symptoms
HbS
HbS=severe disease
Hair texture is another:
H = curly hair HH =
curly hair
H1 = straight hair H1 H1
= straight hair
H H1 =
wavy hair
Multiple
allele inheritance--although
a single individual can have only 2 alleles for a gene, some genes may have
more than 2 alternate forms.
Human ABO blood groups: 3 alleles, A, B, and O. Also these alleles are
codominant. This means that if more than one is present, both can be expressed.
|
GENOTYPE |
PHENOTYPE |
|
A A |
A |
|
A O |
A |
|
B B |
B |
|
B O |
B |
|
A B |
A B |
|
O O |
O |
Polygenic
inheritance—there is not
always one single gene that controls a trait. In fact, quite frequently the
combined effects of many genes influence a particular trait. Examples in humans
are skin color, eye color, hair color, height and body build. Polygenic
inheritance is also influenced by environment.
Chromosome pair #23 differs between
males and females. Females have 2 full-size X chromosomes. The #23 chromosome
pair is called the sex chromosomes--the other 22 pairs are called autosomes.
Males have one X chromosome and a smaller Y chromosome in this pair. Females
have 2 X chromosomes.
Male gametes (sperm) can carry either
the X or the Y chromosome; female gametes only an X. Sex is determined at
fertilization by the sperm. All embryos develop alike until the 7th week. At
that point, if the Y chromosome is present, it will initiate male development.
The X chromosome carries genes for
traits other than sex determination. Males, having only one X chromosome, will
have only 1 gene present for those traits located on the "missing"
part of the Y chromosome. Females will have 2 copies of these genes, since they
have two X chromosomes. These are called sex-linked traits and they show up
much more often in males. In females they usually appear only when both X
chromosomes carry the abnormal trait.
Red-green Colorblindness--due to
missing photopigments, red and green will look the same.
C = normal color vision
c = red-green colorblindness
XC XC =normal female
XC Xc =female carrying recessive gene (normal
color vision)
Xc Xc =colorblind female
XC Y =normal male
Xc Y =colorblind male
Other examples:
Classic hemophilia
Fragile X Syndrome
Non-functional sweat glands
Juvenile muscular dystrophy
Any agent or influence that causes
developmental defects in the embryo a teratogen. Many of these do the most
damage in the very earliest weeks of gestation, sometimes even before the
mother is aware of the pregnancy.
1. Chemicals & drugs--almost all of these cross the placenta and
many can harm the embryo/fetus
Pesticides
Defoliants
Some hormones
Some antibiotics
LSD
Marijuana
Cocaine
2. Alcohol--number one teratogen. Excessive intake during pregnancy
causes fetal alcohol syndrome, which includes characteristic physical
deformities and learning and behavior problems.
3. Smoking--low birth weight and higher fetal mortality. Associated with
cardiac abnormalities, cleft lip & palate and sudden infant death syndrome
4. Irradiation--X-rays or other radioactivity may cause small head size,
mental retardation, and skeletal abnormalities