Unit 4A – Reproduction and Development
Asexual Reproduction
Species are maintained in existence through the life spans process of reproduction. Asexual reproduction
produces genetically identical offspring from a single parent cell. The process of mitosis is associated with
asexual reproduction and the growth and repair of cells in sexually reproducing organisms.
Asexual reproduction is sometimes called cloning. Cloning is the production of identical genetic copies. All
forms of asexual reproduction are variations of the cell division process of mitosis.
Some Methods of Asexual Reproduction (not required for 2007 NYS Living Environment Regents)
1. binary fission -- involves an equal division of both the
organism cytoplasm and nucleus to form two identical
organisms
-- the diagram of the protist at the right is example of this
2. budding -- involves one parent dividing its nucleus
(genetic material) equally, but cytoplasm unequally
-- the diagram of a yeast at the right is an example of this
3. sporulation (spore formation) -- is reproduction
involving specialized single cells coming from one parent
-- the diagram of mold spores being formed at the right is
an example of this
Sexual Reproduction
The process of sexual reproduction involves two parents. Both parents normally contribute one gamete or sex
cell to the process. This process assures that the genetic information given to the offspring will be obtained
equally from each parent. The female gamete is called the egg or the ovum and the male gamete is called a
sperm. These gametes are formed in specialized reproductive structures called gonads. The sperm is much
smaller than the egg, but is capable of moving on its own power using a whip-like tail called a flagellum.
Male System
The structure and function of the human male reproductive system, is very similar to that of many other
mammals. The male system is designed to make sperm or male gametes and is adapted to provide for the
delivery of these gametes to the female to allow for fertilization.
Male Reproductive System
Male Reproductive System Structures
1. testes -- produces sperm and the hormone testosterone
2. scrotum -- pouch enclosing the testes keeping the sperm at an optimum temperature for
development
3. vas deferens -- tube carrying sperm away from the testes
4. prostate gland -- the largest of several glands which add lubricating and other fluids to the sperm
-- this combination of sperm and fluids is called semen
5. urethra -- tube through the penis carrying sperm to the outside of the body
6. penis -- adaptation for internal fertilization of the female
Female System
The structure and function of the human female reproductive system, is very similar to that of many other
mammals. It is designed to produce female gametes or eggs, allow for internal fertilization, support the
internal development of the embryo and fetus, and provide nutrition through milk for the newborn.
Female Reproductive System
Female Reproductive System Structures
1. ovary -- (females have two of these) -- produce female gametes or eggs and the hormone estrogen
2. oviduct (fallopian tube) -- carries the egg away from the uterus
-- fertilization normally occurs here
3. uterus -- implantation and development of the embryo and fetus before birth occurs here
4. vagina or birth canal -- entry point for sperm from the male and exit tube for the baby when it is born
Human Reproduction
Human sexual reproduction occurs in a very similar manner to other sexually reproducing animals. Both males
and females contain specialized reproductive structures designed to produce gametes and facilitate
development. Both the male and female have specialized chemicals or hormones which aid this process as
well.
Sperm and Egg (fertilization)
The sperm and egg unite in a process called fertilization. This process forms a single celled structure called a
zygote which contains the complete genetic information to develop into a complete new organism having
characteristics of both parents.
Process of Fertilization
The zygote, which is a fertilized egg consisting of one cell, will begin to divide rapidly by mitosis forming the early
developing human embryo. Fertilization and the initial stages of this mitotic cell division occur in the oviduct. The
early embryo is migrates down the fallopian tube and completes most of its development in the wall of the uterus.
Initial Development and Differentiation
The processes of gamete production, fertilization, and development follow an orderly sequence of events.
Zygotes contain all the information necessary for growth, development, and eventual reproduction of the
organism.
Fertilization and Initial Development of the Embryo
The placenta is a combination of maternal and fetal tissue which allows for the exchange of materials with the
fetus and mother. Needed materials such as food and oxygen diffuse through the placenta to the fetus, while
wastes from the fetus diffuse to the mother. The umbilical cord is a fetal structure containing blood vessels
which allows materials to be carried between the fetus and placenta in both directions. The amniotic fluid
surrounds the fetus and helps to provide a shock absorber to protect the fetus against mechanical injury in the
event the mother is shaken or injured in some manner.
Fetal Development in the Uterus
The embryo will eventually develop into a three cell layered structure. This structure is called a gastrula and will
eventually differentiate to form the specialized cells. Differentiation means that the cells will develop specific
jobs and develop into specific tissues in the maturing organism. An example of this is that the outer cell layer of
the developing gastrula will develop into the skin and nervous system of a mature human organisms. Most
multicellular animals undergo a similar pattern of development and differentiation.
Fetal Development
Development is a highly regulated process After this small cluster of cells called the gastrula forms in humans,
tissues begin to form. In humans, the embryonic development of essential organs occurs in early stages of
pregnancy. During the first three months of human development, organs begin to form. The human embryo is
usually referred to as a fetus when human like features become visible in its structure. All organs and body
features are developed by the end of the sixth month. During the last three months of pregnancy, organs and
features develop well enough to function after birth.
Human Development at 6 weeks
Human Development at 4 months
**The embryo (or fetus) may encounter risks from faults in its genes and from its mother's exposure to
environmental factors such as inadequate diet, use of alcohol, tobacco, drugs, other toxins, or infections.
Hormone Control
Human reproduction and development are influenced by factors such as gene expression, hormones, and the
environment. The reproductive cycle in both males and females is regulated by several different hormones.
Some of these hormones include:
testosterone --
produced by the testes in the male and stimulates the development of male secondary
sex characteristics (like facial hair and deeper voice).
estrogen --
produced by ovaries in the female and stimulates the development of female secondary
sex characteristics (wider hips and mammary glands) as well as starting the thickening of
the uterus lining in preparation for a possible pregnancy after the egg is released by the
female each month.
progesterone --
produced by yellow tissue called corpus luteum in the empty ovarian follicle (place in
ovary producing and releasing the egg) -- this hormone maintains the thickness of the
uterus lining in case fertilization occurs and development of a fetus occurs.
In human females of reproductive age, these hormones interact in a cyclic pattern called the menstrual cycle.
This pattern of events repeats itself on average every 28 days unless a pregnancy or other disruption occurs. A
graphic representation and written description of the stages of the human menstrual cycle is provided below.
Menstrual Cycle Stages
1. follicle stage (10-14 days average duration)
 FSH (follicle stimulated hormone) stimulates production of eggs occurs in tiny cavities in the ovary called
follicles
 enlarging follicle produces estrogen which causes the uterus to get ready for embryo implantation (uterus
thickens its lining)
2. ovulation (1 day)
 follicle enlarges and ruptures ovary wall
 egg is released to the oviduct (usually only 1 is released at a time)
3. corpus luteum stage (10 -14 days average duration)
 yellow tissue fills the follicle after ovulation called the corpus luteum, stimulated by LH (a hormone)
 Corpus luteum secretes progesterone which maintains the thickness of the uterus in case a pregnancy
occurs
4. menstruation (3-5 days average duration)
 periodic shedding of the thickened lining of the uterus which occurs if fertilization does not occur
Reproductive Technologies
Reproductive technology has medical, agricultural, and ecological applications. In many instances, these
technologies have progressed at a faster rate than the ethical considerations resulting from these technologies.
Some of these techniques include birth control methods used to block the process of fertilization. Many
technologies now exist to enhance the process of fertilization and development in humans and other organisms.
Hormone therapy can cause increased egg production. Surgery can open blocked fallopian tubes in females and
the vas deferens in males. In vitro fertilization (test-tube babies) is a widely used technique to aid infertile couples,
allowing them to have children where this otherwise would not be possible.
In-vitro fertilization