Sexual Reproduction In HumansGametogenesis - Production of Egg and Sperm
STOP: Did you take the Meiosis Quia Quiz? Do not proceed until you have taken it.

Brief Review --

Sexual Reproduction

-- is the production of genetically unique offspring. Facilitates genetic diversity within a population, allowing the species to evolve and adapt over time. In sexual reproduction, the offspring are the result of the union of two unique gametes (egg and sperm cells). 99% of all organisms on Earth, including humans use sexual reproduction.

Genetically Unique Gametes
  • Eukaryotic body (somatic) cells are diploid (contain two sets of chromosomes). Gametes must be haploid (one set of chromosomes) to ensure that when the two gametes unite, a diploid cell will be formed. This is achieved through the process of meiosis. In addition to decreasing the number of chromosomes (chromosomal reduction), meiosis produces unique gametes. Unique gametes are produced through the processes of crossing-over and independent assortment.

FYI
Meiosis animation

Crossing-over tutorial and animation

independent assortment animation

crossing-over and independent assortment - a brief explanation


hum_gametes.jpg

Sex Organs of the Human Male

The reproductive system of the male has two major functions:
  • production of sperm
  • delivery of these to the reproductive tract of the female.
external image male_reproductive.gifSperm production — spermatogenesis — takes place in the testes.
Each testis is packed with seminiferous tubules (laid end to end, they would extend more than 20 meters) where spermatogenesis occurs.








Spermatogenesis
  1. Go to the following webpage: http://wps.aw.com/bc_martini_eap_4/40/10469/2680298.cw/content/index.html
  2. Click on the first link: spermatogenesis and watch the entire movie.
  3. Where does spermatogenesis occur?
  4. During puberty which hormone stimulates cells to begin meiosis?
  5. How many sperm cells are created from each meiotic division?
    1. Open another window and type in the following URL:
http://highered.mcgraw-hill.com/olc/dl/120112/anim0043.swf
  1. Watch the animation. Draw the process of spermatogenesis. Use the image and word bank below to help you!


Spermatgonia
Mitosis
Meiosis I
Meiosis II
Primary Spermatocyte
Secondary Spermatocytes
Spermatids
Differentiation
Sperm Cell








  1. Complete the following questions / matching section. Continue to watch the animation as needed so that you can do this.
    1. Are spermatogonia diploid or haploid cells?
    2. After the first division, mitosis, one cell remains a spermatogonia while the other continues on to form sperm. Why do you think it might be important to have one cell remain as a spermatogonia? (HINT: Men start making sperm at puberty and continue for their entire lives!)
    3. How many chromosomes are in a sperm?
    4. What is differentiation?

  1. Review the picture of a sperm to the right. Notice it is divided into 3 segments, the head, midpeice and tail. We will talk more about the structure of a sperm later on.




Part III: Oogenesis
  1. Return to the first webpage: http://wps.aw.com/bc_martini_eap_4/40/10469/2680298.cw/content/index.html
  2. Click on the second link: reproductive system of the human female and watch the FIRST of the SEVEN tutorials
    1. What are the four main parts of the female reproductive system?
    2. Of these 4, which one is responsible for making the eggs?
    3. Of these 4, which one do you think is most commonly referred to as the fallopian tubes?
    4. Of these 4, which one is the sight of fertilization (where the egg and the sperm unite)?
    5. Of these 4, which one is responsible for housing the baby while he/she matures?

  1. Return to the first webpage: http://wps.aw.com/bc_martini_eap_4/40/10469/2680298.cw/content/index.html
  2. Click on the third link: oogenesis and watch the animation (sound ON)
    1. Where does oogenesis occur in females?
    2. Mitosis and meiosis I begin BEFORE birth in females and then stops until what time in a female’s life?
    3. The video states that each woman has a finite number of primary oocytes at birth. What does that mean?
    4. When females perform meiosis I the cytoplasm of the cells is not divided equally. As a result the primary oocyte creates a secondary oocyte and a .
    5. The secondary oocyte then enters the oviduct (fallopian tube) and begins meiosis II. The process of meiosis II will not finish, though, unless what occurs?
    6. How many actual ova (eggs) are created in one meiotic division?
Part IV: Comparing Spermatogenesis and Oogenesis
  1. Return to the first webpage: http://wps.aw.com/bc_martini_eap_4/40/10469/2680298.cw/content/index.html
  2. Click on the fourth link: comparison of spermatogeneis and oogenesis and watch the animation.
  3. Copy the timelines (see below) and fill in: (1) when mitosis begins, (2) when mitosis ends, (3) when meiosis begins, (4) when meiosis ends

Spermatogenesis Timeline


Embryo Birth Childhood Puberty Death

Oogenesis Timeline


Embryo Birth Childhood Puberty Menopause Death



















Spermatogenesis

The walls of the seminiferous tubules consist of diploid spermatogonia, stem cells that are the precursors of sperm.
Spermatogonia
  • divide by mitosis to produce more spermatogonia or
  • differentiate (change) into spermatocytes.
Meiosis of each spermatocyte produces 4 haploid spermatids. This process takes over three weeks to complete.
Then the spermatids differentiate into sperm, losing most of their cytoplasm in the process.

spermatogenesis.jpg


The picture on the left shows
  • on the left you can see the parts of the testis, sperm are produced in the seminiferous tubules. The epididymis stores the mature sperm until they are released.
  • in the center is a close-up of the process of spermatogenesis occurring inside the seminiferous tubules of the testis
  • on the right you can see how the process of meiosis produces four fully functioning, genetically unique sperm cells.


27_09bSpermCellStructure_L.jpg

Sperm

Sperm cells are little more than flagellated nuclei. Each consists of
  • a head, which has
    • an acrosome at its tip. The acrosome contains digestive enzymes that will help the sperm penetrate the egg's defenses.
    • contains a haploid set of chromosomes in a compact, inactive, state.
  • a midpiece containing mitochondria
  • a tail which is a flagellum.


external image bat_sperm.jpg
This electron micrograph (courtesy of Dr. Don W. Fawcett and Susumu Ito) shows the sperm cell of a bat. Note the orderly arrangement of the mitochondria. They supply the ATP to power the whiplike motion of the tail.
An adult male manufactures over 100 million sperm cells each day. These gradually move into the epididymis where they undergo further maturation. The acidic environment in the epididymis keeps the mature sperm inactive.
In addition to making sperm, the testis is an endocrine gland. Its principal hormone, testosterone, is responsible for the development of the secondary sex characteristics of men such as the beard, deep voice, and masculine body shape.Testosterone is also essential for making sperm.
Link to more on testosterone.
Testosterone is made in the interstitial cells (also called Leydig cells) that lie between the seminiferous tubules.

LH

Interstitial cells are, in turn, the targets for a hormone often called interstitial cell stimulating hormone (ICSH). It is a product of the anterior lobe of the pituitary gland. However, ICSH is identical to the luteinizing hormone (LH) found in females, and I prefer to call it LH.

FSH

Follicle-stimulating hormone (also named for its role in females) acts directly on spermatogonia to stimulate sperm production (aided by the LH needed for testosterone synthesis). [Discussion]

Sex Organs of the Human Female

The responsibility of the female mammal for successful reproduction is considerably greater than that of the male.
external image female_reproductive.gif
She must
  • manufacture eggs
  • be equipped to receive sperm from the male
  • provide an environment conducive to fertilization and implantation
  • nourish the developing baby not only before birth but after.

Oogenesis

external image oogenesis.gif
Egg formation takes place in the ovaries.

In contrast to males, the initial steps in egg production occur prior to birth. While she is still in her mother's womb, diploid stem cells called oogonia divide by mitosis to produce more oogonia and primary oocytes. By the time the fetus is 20 weeks old, the process reaches its peak and all the oocytes that she will ever possess (~4 million of them) have been formed and then begin to die. By the time she is born, only about 1 million of these remain. Each ovum has begun the first steps of the first meiotic division stopping at the end of prophase I.

No further development occurs until years later when the girl becomes sexually mature during puberty. Then the primary oocytes recommence their development, usually one at a time and once a month. The ovum is released in a process called ovulation.

Just before ovulation occurs, the primary oocyte grows much larger and completes meiosis I, forming a large secondary oocyte and a small polar body that receives little more than one set of chromosomes. Which chromosomes end up in the egg and which in the polar body is entirely a matter of chance.

FYI: What is a polar body? Click here to find out more about polar bodies

In humans (and most vertebrates), the first polar body does not go on to meiosis II, but the secondary oocyte does proceed as far as metaphase of meiosis II and then stops.

The ovum is released from the ovaries into the fallopian tubes during ovulation. The egg will wait inside the fallopian tubes for the sperm. The ova needs to be fertilized within 24 hours. Only if fertilization occurs will meiosis II ever be completed. Entry of the sperm restarts the cell cycle. The ovum will now complete meiosis II becoming a fertilized egg or zygote (and also a second polar body).

STOP: TAKE THE GAMETOGENESIS QUIA QUIZ
Click here to go to the Pregnancy Webpage
FYI:
Link to a discussion of the **menstrual cycle** and the hormones that regulate it.
Link to WebMD site discussing the menstrual cycle, ovulation and fertilization.
Several sexually-transmitted diseases (STDs), especially gonorrhea and infections by chlamydia can cause scarring and blocking of the tubes and are a major cause of infertility.
In tubal ligation, the fallopian tubes are surgically cut and their ends tied to prevent pregnancy.

Sources:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Sexual_Reproduction.html
http://www.3dscience.com/Resources/Reproductive_Cells.php
http://www.pearsonsuccessnet.com/snpapp/iText/BrowseITEXTServlet?eventType=openIEXT&ISBN=0-13-181118-5&ISBNUrl=%2FiText%2Fproducts%2F0-13-181118-5%2Findex.html&ITEXTOID=0-13-181118-5&EnrollmentOID=AB13AD1491204E62E040A00A7F3463FE&DisplayTitle=Prentice+Hall+Biology+%C2%A92006&TitleInFrame=Y&classPeriodOid=ba2b63679e424011a129c2b6d69953b9&isbnUrlIsJavascript=false