Evolutionary Trends in the Mammalian Reproductive System

The primary reproductive organs are the ovaries; their functions include production of sex hormones and oocytes and secretions by glands in the reproductive system.  The accessory ducts include the uterine tubes, where fertilization occurs; the uterus, where the embryo develops; and the vagina, which acts as a birth canal and receives the penis during sexual reproduction. 

In amniotes, fertilization is usually internal, with sperm transfer occasionally aided by a male intromittent organ.  In placental mammals, the reproductive structures have become specialized to facilitate giving live birth.

  The three living groups of mammals vary in their methods of reproduction. 

~Placental Mammals, like the cat, are viviparous.  Viviparity, or birth of live young, has independently evolved more than one hundred times in vertebrates.  However, many vertebrates retain the shelled egg laying method of giving birth due to the nutrients provided by the shell and passed to the embyo.

~Monotremes are the most primitive living mammals.  They have retained the reptilian oviparous method of reproduction and lay shelled eggs.   

~Marsupials undergo a distorted version of viviparity.  They give birth to underdeveloped live young.  When the young are born, they make their own way to the permanent brood pouch, or marsupium.  Development of the young is completed in the pouch.

~Ovary~   Paired, almond-shaped organs that are supported by the mesovarium, a portion of the broad ligament.  The ovary is the site of oogenesis and hormone production. 

 ~Oviducts~   These structures, also referred to as uterine tubes, receive the oocyte and provide a site for fertilization.

~Infundabulum~  This is the lateral region of the oviduct that resembles an open funnel.  The structure is surrounded by ciliated, finger-like projections called fimbrae that drape over the ovary.  Movements of the fimbrae sweep the oocyte into the tube.

~Ostium Tubae~  The infundabulum opens medially by way of the ostium tubae.

~Uterus~  The oviducts merge into the larger uterine horn.  The two uterine horns fuse to form the uterine body.  The entire structure is Y-shaped.  The mammalian uterus has evolved a highly vascularized lining whose function is to receive, retain, and nourish a fertilized egg throughout pregnancy.  During the pregnancy, the uterine horns fill the abdominal cavity.  The other organs are pushed in all directions to make room for the developing fetus.

~Vagina~ The vagina is a thin-walled tube that lies posterior to the uterus.  It provides a passageway for the delivery of an offspring.

Mammary glands are unique to living mammals.  They are derived from specialized skin glands.  Mammary glands function only in lactating females when they produce milk to nourish an offspring.

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