HUMAN REPRODUCTIONBIOLOGY 269

Female Reproductive System

• Produce hormones (estrogen and progesterone) for secondary sexual characteristics.

• Produce one large oocyte at regular intervals and release it in such a way that it enters the oviduct (“Fallopian tube”,“uterine tube”).

• Receive semen from the male and transport it into the oviduct.• Produce hormones (estrogen and progesterone) which prepare

the uterus for pregnancy each cycle by thickening its inner lining; Getting rid of this thickened lining each cycle if pregnancy does not occur.

• Protect and nourish the embryo and fetus during pregnancy, then expel it through the vagina

• Produce milk to nourish the newborn

Ovary:

Located within pelvis, 2 to 5 cm (1 to 2 inches) lateral to the uterus

Approximately 3cm x 2cm x 1cm

Central region, or “medulla”, consists of connective tissue with many blood vessels.

Outer region, or “cortex”, has all of the oocytes. One or two of these oocytes may be developing before ovulation; undeveloped ones are stored here.

Also contains the remnant from the previous cycle’s ovulation, called a corpus luteum, which secretes progesterone to keep uterus ready for implantation of the embryo

Maturation of an oocyte is called oogenesis, which leads to ovulation

During oogenesis, the oocyte is surrounded by follicular cells granulosa cells in one or more layers, forming a follicle.

The developing oocyte and its follicle:Four stages before ovulation

1. Primordial Follicles:Primordial Follicles:

All formed before birth - hundreds in each ovary

Have not yet begun development – remain dormant until puberty

Consists of primary oocyte surrounded by a single layer

of flat follicular cells

The developing oocyte and its follicle:Four stages before ovulation

2. Primary FolliclesPrimary Follicles:

Develop only after puberty; 3 to 4 enter this stage each cycle

No change in primary oocyte

Follicle has begun development – cells are larger

The developing oocyte and its follicle:Four stages before ovulation

3. Secondary Follicles: Secondary Follicles:

1 or 2 primary follicles progress to this stage each cycle

Primary oocyte larger, finishes first division of meiosis to become a secondary oocyte, also forming a polar body.

Follicle grows larger as follicular cells divide to form many layers; spaces begin to develop in center of follicle

The developing oocyte and its follicle:Four stages before ovulation

4. Mature or Vesicular FollicleMature or Vesicular Follicle

Most follicles which made it to “secondary” will reach this stage

Follicle is very large; Many layers of cells surrounding fluid-filled center called antrum

Secondary oocyte covered with thick, clear membrane called zona pellucida; pushed to one side in follicle and surrounded by mass of follicular cells called cumulus oophorus

Big Question: How is this process of oogenesis regulated?

Oogenesis, as well as ovulation and many other things, is regulated by the pituitary gland, which lies just below the brain

Oogenesis and growth of the follicle in the ovary are initiated and stimulated by follicle stimulating hormone (FSH) from the pitutitary

During this oogenesis under the stimulation of FSH, follicular cells surrounding the oocyte secrete the hormone estrogen, which stimulates the inner lining of the uterus to thicken and begin secreting mucus, getting ready for pregnancy

This estrogen from the follicular cells also inhibits the pituitary gland from secreting any more FSH.

Thus, no more follicles will be stimulated to develop until it stops.

Rupture of the follicle is ovulation.

The secondary oocyte, surrounded by the zona pellucida and cumulus oophorus, is released from the surface of the ovary, where it can be captured by the open end of the oviduct.

The cumulus oophorus now called the corona radiata.

Luteinizing hormone also causes the follicular cells of the now empty follicle to develop into a corpus luteum. This structure secretes the hormome progesterone.

Ovulation is triggered when the pituitary gland secretes a different hormone, called luteinizing hormone.

Before ovulation: FSH stimulates cells of follicle to secrete estrogen, which stimulates the lining of the uterus to develop.

After ovulation: LH stimulates cells of the corpus luteum to secrete progesterone, which maintains the uterine lining in this “ready for pregnancy” condition

If fertilization and pregnancy occur, the placenta of the embryo secretes a hormone called chorionic gonadotropin, which stimulates the corpus luteum to keep secreting progesterone, which in turn keeps the lining of the uterus thick and full of blood vessels.

If fertilization and pregnancy do not occur, after a few days the corpus luteum deteriorates and stops producing progesterone. Without its stimulus, the uterus can not maintain its thick lining, so this falls off and is shed as menstruation.

When the oocyte is released from the surface of the ovary during ovulation, it is picked up by the oviduct, or Fallopian Tube, a hollow muscular tube which leads from near the uterus to the ovaries.

Oviduct:

Lateral End: Wide opening, or infundibulum, surrounds the surface of the ovary. The edge is surrounded by finger-like fimbria.

Middle Part: Wide ampulla

Medial End: Narrow isthmus leads into uterus. Cavity of oviduct is continuous with cavity of uterus

By mechanisms not completely understood:

• The oviduct moves the oocyte toward the uterus• The oviduct moves sperm away from the uterus

Fertilization occurs in ampulla of oviduct, forming the zygote, which goes through repeated mitotic cell divisions to eventually form the embryo.

Oviduct moves this growing mass of pre-embryo cells toward uterus: takes 3 or 4 days.

Secretions from oviduct are necessary to keep sperm, oocyte, and embryo alive.

Uterus:

Anterior to rectum; Posterior & superior to bladder

Narrowed inferiorly to form cervix, which projects into vagina

Uterus:

Outer layer = Myometrium: Thick layer of muscle

Inner layer = Endometrium: Lots of glands & blood vessels

Two layers of endometrium:

Basal layer: Closer to myometrium; Remains after menstruation; Regrows functional layer

Functional layer: Closer to cavity; Thickens every cycle; Embryo implants here during pregnancy; Dies and falls off during menstruation

Mass of Uterus:

Immediately before menstruation: 100 – 150 grams

Immediately after end of menstruation: 50 – 100 grams

End of pregnancy: 1,000 – 1,500 grams

At end of pregnancy:

Uterus extends from vagina inferiorly to bottom of sternum superiorly

Lies anterior to all other abdominal organs except bladder

Vagina:

Thin-walled, tubular

Inferior to uterus;

Anterior to rectum

Posterior to bladder & urethra

Receives penis and semen during sexual intercourse

Passage for delivery of fetus or menstrual flow

External Genitalia:

Major or outer labia

Minor or inner labia

Vestibule

Vaginal opening

Clitoris

Note relationships with pubic hair and the openings of the urethra and the anus

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