Reproductive System

Last Updated on Tuesday, 12 July 2011 04:41 Written by Sandesh Monday, 24 May 2010 06:01

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REPRODUCTIVE SYSTEM OF FROG

In vertebrates the reproductive cells or gametes are formed in special structures called gonads. These in females are ovaries and in males testes. The other organs which have developed in close association with gonads, and are meant for transport of gametes and for ensuring fusion as a result of external fertilization are all accessory.

Male Reproductive Organs:
Male reproductive system consists of:
1. Testes
2. Vasa efferentia
3. Urino-genital ducts

1. Testes: The testes are dark, yellow oval body lying ventral to kidneys to which they are suspended by a double fold of peritoneum, the mesochrium. Histologically, each testis is made up of mass of tubules, blood vessels, nerves and a small amount of supporting connective tissue. These tubules are called seminiferous tubules or crypts. The seminiferous tubules are lined by germinal epithelial cells which develop into sperms by the process of spermatogenesis.

Sperms are motile and have and have chemical specificity for the eggs. Each sperm is about 0.03 mm in length and consist of -
 small rounded anterior acrosome
 a long cylindrical head containing nucleus
 a short middle piece containing centrioles and mitochondria and
 much elongated posterior motile tail.

2. Vasa efferentia: All the seminiferous tubules in testes are connected to form 10 to 12 narrow tubes, the vasa efferentia. The sperm cells from seminiferous tubules pass into Bidder’s canal through vasa efferentia. The Bidder’s canal runs along the length of each kidney near its inner side. It is connected to the ureter through a series of collecting tubules of kidney. Thus vasa efferentia conduct mature sperm from testis to the ureter of kidney.

3. Urinogenital duct: In frog, the ureter serves as the passage for urine produced by kidneys and sperms produces by testis. Thus it is more aptly known as urinogenital duct. The ducts of the both sides open into the roof of cloaca separately. In some species of frog, the urinogenital ducts are enlarged near the kidneys or cloaca forming seminal vesicles for the temporary storage of sperms.

Thus on their way to exterior, the sperms in frog must pass through vasa efferentia, Bidder’s canal, transverse collecting tubules, ureter and cloaca.

Female reproductive organs:
Female reproductive organs include a pair of ovaries and oviducts.

1. Ovaries:
A pair of ovaries lie ventral to kidney attached to the dorsal abdominal wall and is suspended by a fold of peritoneum known as mesovarian. It is large, irregularly folded, multilobed, hollow sac-like grayish or blackish in colour. During breeding seasons, the ovaries appear to be greatly enlarged.
Histologically, each ovary contains numerous ova at different stages of development. The inner surface of each ovary is lined by germinal epithelial cells which undergo a complicated process known as oogenisis to give rise to ova. Groups of primordial germ cells form ovarian follicles and one of the many cells of follicles goes on to form future ovum while rest of the follicular cells thin follicular epithelium around it. Within the follicle, a vitelline membrane is secreted by each ovum around itself. Egg of frog is telolethical or polar since yolk is concentrated at one pole only. When ripe, the ova are shed into the body cavity by the rupture of ovarian wall.

2. Oviducts:
The two oviducts are relatively much longer and coiled tubes not connected directly to ovary. They extend forward to the base of the lungs, close to which they open into the body cavity. The anterior ends of oviduct are ciliated oviducal funnels which open through ostia. This ciliated funnel leads into first part of oviduct which is thin and narrow; the second part is the longest and coiled and its thick wall secretes an albuminous substance with which the eggs are coated as they pass downward. The final portion of oviduct is broad and thin-walled known as ovisac where eggs are temporarily stored before being laid. The two ovisacs of either side open on the dorsal wall of cloaca, opposite to the opening of the urinary bladder.

Eggs are released in the body cavity by the rupture of peritoneum covering the ovary. They are carried forward to the ostia of the oviduct. From here as they pass downwards they are coated with albuminous substance and then are stored in ovisac until they are being laid.

The finger-like fat body present in front of gonads serves as store-house of nourishment.