II. Oogenesis- the production of female gametes (egg/ovum/ova). A. Structure 1. Oogenisis occurs in the female gonad- the ovaries. 2. One egg and 3 polar bodies are produced.

B. Process I. Events Before Birth a. Within the female fetus OOGONIUM (s)(diploid=46) divide repeatedly by MITOSIS. b. These cells grow into larger diploid cells called PRIMARY OOCYTES. PRIMARY OOCYTES begin the process of Meiosis I but are arrested in the early stages. c. Follicle cells divide by mitosis and surround each PRIMARY OOCYTE to form a PRIMARY FOLLICLE. (remains unchanged until puberty)

2. Events with the Menstrual Cycle a. Each cycle a few PRIMARY FOLLICLES will complete Meiosis I with an UNEVEN division of the cell. Results in ONE very large (haploid=23) SECONDARY OOCYTE and ONE polar body (genetic waste that disintegrates). Secondary Oocyte and Polar Body After Meiosis I

3. The single ring of follicle cells begin dividing and start to produce fluid. Two rings of follicle cells form with a fluid filled cavity separating them.

4. The SECONDARY OOCYTE begins Meiosis II but is again stopped early in the process. An increase in fluid between the two follicle layers creates a bulge on the surface of the ovary and leads to OVULATION. The SECONDARY OOCYTE or egg is released from the ovary.

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5. MEIOSIS II is completed at fertilization 5. MEIOSIS II is completed at fertilization. Resulting in ONE large haploid egg and 3 polar bodies. The egg is the largest cell in the body. Due to the uneven division of the cytoplasm the egg receives all of the cytoplasm, nutrients, and organelles.

Yolk- nutrients and fat droplets in the ovum (egg) Haploid nucleus- contains 23 chromosomes. Cortical granules- secretory vesicles that release their content to prevent fertilization by more then one sperm. Zona Pellucida – glycoprotein membrane responsible for the binding of sperm. Corona Radiata- protective layers of follicle cells.

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An egg cell is 150 microns in size, and is colourless and semi-transparent. It is the shape of a sphere and its outer part is surrounded by a soft yet tough membrane. Within the structure of the egg there are extra nutrients like fat, sugar and proteins. This reserve of nutrients will ensure that the egg cell is nourished on its journey and, if fertilization occurs, will take care of its needs until it reaches the uterus.

Ovary Structure The light microscope image shows the mature secondary follicle. At the centre of the follicle is the oocyte. Note the Zona pellucida which surrounds the oocyte. This surrounds the oocyte and is composed glycoproteins that take part in the acrosome reaction

Summary:

Comparison of Spermatogenesis and Oogenisis SIMILARITIES: Both processes start with MITOSIS to make more germ cells. The -gonia cells must grow before they undergo Meiosis. Both processes go through Meiosis I and Meiosis II. Both produce cells with haploid nuclei.

Differences Millions of sperm produced each day. Four small sperm produced per germinal cell. Released during ejaculation Sperm production continues all through life starting at puberty. Sperm formation begins at puberty. One secondary oocyte ovulated each menstrual cycle. One large egg produced (and three polar bodies) Released during ovulation. Ovulation starts at puberty and stops at menopause. Early stages of egg production happens before birth.

Human Chorionic Gonadotrophin (HCG)

Fertilization Occurs in the Oviducts

Fertilization: Sperm Cell Penetrates Egg with Acrosomal Enzymes