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Fertilization
Fertilization is the union (fusion) of monoploid sperm nucleus
(n) with a monoploid egg nucleus (n). During meiosis the
chromosome number (2n) was reduced. In fertilization, the
resulting diploid zygote has its species number of
homologous chromosomes (2n) restored. Fertilization must
occur in a moist environment.
male gamete + female gamete → zygote
(n)
(n)
(2n)
External vs. Internal Fertilization
• External Fertilization – fertilization occurs
outside the body of the female in the
water. Large numbers required due to
predators. Ex: fish, amphibians and other
aquatic vertebrate animals.
• Internal Fertilization – the gametes fuse in
the moist reproductive tract of a female.
Ex: terrestrial vertebrate animals, birds,
reptiles, and mammals.
Embryonic Development
• After internal or external fertilization, the zygote (fertilized
egg) undergoes a series of rapid mitotic cell divisions
called cleavage. Although there is very little growth in
size, cleavage causes in increase in the number of cells
with a decrease in the size of each cell. Once cleavage
begins, the zygote becomes a developing embryo.
Stages of Development
• Cleavage – a series of rapid mitotic cell divisions that
leads to the formation of a mass of cells, the morula.
Additional mitotic cell divisions leads to the formation of
the blastula, a single layer of cells which is a hollow balltype structure. During cleavage, there is little or no
increase in individual cell size.
• Gastrulation – one side of the blastula becomes
indented forming the gastrula, which has an inner layer,
the endoderm, and an outer layer, the ectoderm. The
mesoderm forms between ectoderm and endoderm.
• Differentiation: Three embryonic layers differentiate
and give rise to the various tissues, organs, and systems
of the multicellular animal.
 The nervous system and the skin originate from the
ectoderm layer
 The muscles, circulatory system, skeleton, excretory
system, and gonads originate from the mesoderm layer.
 The lining of the digestive and repiratory tracts, and
portions of the liver and pancreas, originate from the
endoderm.
L. variegatus zygotes, viewed from the side. A.
1-cell zygote. The fertilization envelope is
visible as a large "halo" around the embryo.
The arrow points to the site of sperm
penetration. B. 2-cell. C. 8-cell. D. 16-cell. E.
32-cell. F
Cleavage
Gastrulation & Differentiation
• Laminins correspond to a large number of
heretotrimeric glycoproteins, playing and a
major role in several cell functions,
including differentiation, proliferation,
adhesion, and migration
Growth
• The cell number and cell
size increases. Early
development consists of
differentiation, growth of
cells, tissues and organs.
• Site of developmentinternal development
occurs inside the female’s
body while external
development occurs
outside the female’s body
in both terrestrial and
aquatic environments.
• In water: The eggs of
many fish and
amphibians are fertilized
externally and develop
externally in an aquatic
environment. The survival
rate is generally low,
which accounts for the
large number of fertilized
eggs produced. The
developing embryo’s food
is the yolk stored in the
egg.
On land : Amniotic Egg
• Eggs of birds, many
reptiles, and a few
mammals develop
externally on a land
environment after internal
fertilization. Since there is
a better survival rate, a
somewhat fewer number
of fertilized eggs are
produced. The
developing embryo’s
source of food is the yolk.
p. 785 textbook, Chapter 35
A – Yolk
B – Shell
C – Chorion
D – Embryo
E – Amnion
F - Allantois
• Some adaptations for animals which develop externally on land are
a shell which provides protection and membranes which help
provide a favorable environment for embryonic development. These
embryonic membranes include:
 The amnion contains the amniotic fluid. This fluid provides a water
environment, protects the embryo from shock, and prevents
adhesion of the embryonic tissues to the shell.
 The yolk sac surrounds the yolk. Blood vessels which penetrate the
yolk sac transport food to the developing embryo.
 The allantois functions as a respiratory membrane and a storage
site for the nitrogenous waste, uric acid.
 The chorion is an outer membrane surrounding the other embryonic
membranes and separates them from the environment.
• Internal development involves the growth of the embryo within the
body of the parent which provides nutrition and protection.
Generally, a relatively high survival rate allows the production of
fewer fertilized eggs.
Reproduction & Development in
Human Males
•
The male
reproductive system
performs two major
functions:
1. The production of
sperm cells.
2. The deposition of
these cells within
female reproductive
tract.
p. 1007 textbook,
Chapter 44
• Sperm production occurs in the testes. The testes are
located in the scrotum where the temperature is 1-2
degrees Celsius cooler than normal body temperature.
This provides an optimum temperature for sperm
production and storage. From the testes sperm move
through the sperm duct and urethra. The urethra is a
tube contained within the penis. The penis is a structural
adaptation for internal fertilization.
• Glands secrete a liquid into these tubes. The liquid
serves as a transport medium for the sperm which is and
adaptation for life on land. This liquid and the sperm cells
constitute semen. *Secretions from prostate and
bulbourethral glands also secrete alkaline fluid that
neutralize acids in female reproductive tract.
• In addition to producing sperm, the testes also produce
the male sex hormone, testosterone. This regulates the
maturation of sperm and the development of secondary
characteristics, such as beard development and the
lower voice pitch.
Reproduction & Development in
Human Females
• Ovaries are paired structures
located within the lower portion
of the body cavity. Ovaries
produce eggs (ova) in tiny
cavities called follicles.
Following ovulation (release of
the egg from the ovary), the
egg cell is transported through
an oviduct (Fallopian tube) to
the uterus. At the lower end of
the uterus, known as the
cervix, is a muscular tube, the
vagina. The vagina receives
the sperm from the male and is
also the birth canal.
• At the birth of a female, all of the potential eggs
that she will ovulate are present in immature
form. Usually only 1 egg is released at the time
of each ovulation.
• In addition to eggs, the ovaries also produce the
female sex hormones estrogen and
progesterone. These regulate the development
of secondary sex characteristics such as the
development of the mammary glands and the
broadening of the pelvis. The hormones also
have coordinating role in the menstrual cycle.
Menstrual Cycle
• The menstrual cycle
begins at puberty and
ceases at menopause.
Menopause is the
permanent cessation of
the menstrual cycle. The
duration of this cycle is
approximately 28 days
but may vary
considerably and may be
interrupted by pregnancy,
illness, or other factors.
• There are four stages.
Four stages of Menstrual Cycle:
• Follicle Stage – involves the maturation of an egg within
the follicle and the secretion of the hormone estrogen,
which initiates thickening blood tissue in uterine lining.
• Ovulation – the release of an egg from the follicle.
• Corpus Luteum Stage – forms from the follicle following
ovulation. It secretes progesterone which enhances the
thickening of the uterine lining.
• Menstruation – periodic shedding of the thickened
uterine lining. It lasts for a few days and occurs when
fertilization does not take place.
Roles of Hormones in the
Menstrual Cycle
• The reproductive cycle of the human female involves the interaction
of the hormones from the hypothalamus, pituitary gland, and
ovaries. During the menstrual cycle, under the influence of the
hormones from the hypothalamus, the pituitary gland releases
hormones (FSH and LH) which influence the functioning of the
ovaries. FSH stimulates follicle growth and the ovary to produce
estrogen. LH stimulates the growth of the corpus luteum (in the
ovary) to secrete progesterone which inhibits estrogen production.
The ovaries, in turn, secrete estrogen and progesterone which
produce changes in the uterus. Estrogen causes the release of FSH
to be stopped. And increase in progesterone causes the reduction in
the production of FSH and LH. In addition, the hormones from the
ovaries regulate the secretion of hormones by the pituitary gland
and the hypothalamus. This is an example of a negative feedback
mechanism.
Fertilization
• Fertilization usually occurs in the upper one third of the oviduct. If
the egg is not fertilized within approximately 24 hours after ovulation,
it deteriorates. If fertilization occurs, cleavage of the fertilized egg
(zygote) begins in the oviduct, and six to ten days later, the resulting
embryo may become implanted in the uterine lining. After this stage
of development, the yolk of the egg has been depleted. But with
implantation, the embryo can obtain nutrients from the uterus.
• Identical twins develop from one zygote which separates during
cleavage. If more than one egg is released and fertilized, multiple
births may result. Fraternal twins develop from two eggs, each
fertilized by separate sperm cells.
• In vitro fertilization is commonly called test tube babies.
Prenatal Development
• Prenatal development includes
the following processes:
 Cleavage – occurs in the
oviduct.
 Gastrulation – occurs after the
embryo is implanted in the
uterus.
 Differentiation and Growth –
result in the formation of
specialized tissues and
organs.
• The placenta is the connection
between the mother and the
developing embryo which
allows nutrients, respiratory
gases, and wastes to be
exchanged. Since there is no
direct connection between the
embryo’s blood and the
mother’s, substances move
between them by active and
passive transport.
• The amnion is the membrane
that surrounds the embryo and
contains amniotic fluid which
protects the developing
embryo from shock and
temperature changes.
Placental Development
• The umbilical cord holds the
blood vessels that carry
materials between the embryo
and the placenta.
• Birth usually occurs after a
gestation period of
approximately nine months.
During birth, strong
contractions of the uterine
muscles forces the baby
through the cervix and vagina
to the outside of the mother’s
body. The expulsion of the
placenta occurs after the
delivery of the baby.
•
•
And now for the rest of development...
Here come the organs!
Week 4: How you looked at 1 month past
conception....
Week 4
Week 4-8 is when all the major
organ systems of the body are
formed.
Week 7 / Day 48: First brain waves can be
detected
Week 8
•
•
•
Week 9 - Week 40: (Fetus):
After 12 weeks or so, the baby's development is
largely "finished" - except brain and lung
development
The fetus just spends much of the 2nd and 3rd
trimesters just growing (and doing various flip-turns
and kicks inside the amniotic fluid)
And then, before you know it...
Week 20 - Love that
thumb!
Week 16
Week 36 - 40 - Partruition....