Endocrine and Reproductive Systems



Overview and Homeostasis

Nervous System and Drugs

Skeletal, Muscular and Integumentary

Circulatory and Respiratory

Digestive and Excretory

Endocrine and Reproductive

Immune System and Disease

Vocabulary: hormone, target cell/organ, progesterone, asexual reproduction, mitosis, sexual reproduction, meiosis, zygote, differentiation, testes, scrotum, vas deferens, prostate, urethra, penis, ovary, oviduct/fallopian tube, uterus, vagina, testosterone, estrogen, corpus luteum, follicle, fetus, menstrual cycle, ovulation, placenta, umbilical cord, amniotic fluid, embryo, aging, reproductive technology

Cell Membrane Receptors

Cell Membrane Receptors

Many cell membranes have receptor molecules on their surface. These receptor sites play an important role in allowing cells and organs to communicate with one another.

Hormonal Regulation
Hormones provide a primary way for cells to communicate with each other. A hormone is a chemical messenger with a specific shape that travels through the bloodstream influencing another target cell or target organ. Upon reaching the cell the hormone is targeted for, the hormone often activates a gene within a cell to make another necessary compound. One example of this is provided by the pituitary gland. This gland at the base of the brain makes a hormone called LH (luteinizing hormone). This hormone travels through the bloodstream and stimulates the ovary to produce yellow tissue that produces the hormone progesterone, which maintains the thickness of the uterus lining. The graphic below illustrates how this kind of hormonal regulation can work in a plant cell. Animal cell hormonal regulation involves a similar mechanism.

A Hormonal Feedback Mechanism 

The animation at the right illustrates how a hormone can bind to receptors on a cell membrane and trigger that cell to produce a needed compound.

Any change in nerve or hormone signals will change the communication between cells and organs in an organism and thus may cause problems for organismís stability and ability to maintain homeostasis.

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.

Sexual Reproduction 
Sexual reproduction
produces offspring that have a combination of genes inherited from two parents sex cells or gametes. These gametes are produced by the process of meiosis. The single cell formed by the union of egg and sperm is called a zygote. The zygote contains all the information necessary for growth, development, and eventual reproduction of the organism.  

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.

Human Development
The development of humans and other sexually reproducing organisms is a highly regulated process involving mitosis and differentiation. Reproduction and development are subject to environmental impact. The general process of birth, human development, and aging involves a predictable series of events. 

Reproductive technology has medical, agricultural, and ecological applications. This technology has also stirred ethical concerns as well, especially where this technology applies to humans.

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

 -- internal 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

Endocrine Interactions
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.

             Human Menstrual Cycle

Note the influence of the hormone progesterone in beginning the thickening of the uterus lining and the role of the hormone estrogen in maintaining the thickness of that lining. Ovulation or release of the egg occurs at the midpoint of this cycle, while the uterine lining thins and is shed (menstruation) when the level of estrogen begins to decline to a large extent.

Menstrual Cycle Stages

1. follicle stage (10-14 days average duration)

  •  production of ova/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
  • "yellow body" 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

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.

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.

  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.   

While the patterns of development discussed previously hold true for humans, these developmental patterns vary between different plants and different animals.  

Aging is a complex series of developmental changes which occur with the passage of time. This
process is influenced by both heredity and the environment. This process eventually leads to the
death of the organism.

Reproduction and development are subject to environmental impact. Human development, birth, and aging should be viewed as a predictable pattern of events.

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.

The in vitro Fertilization Process

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