Comparative Anatomy - Rat and Frog

The Common Grass Frog

Urogenital System

In both male and female frogs, the kidneys are a pair of large, dark, flattened oval structures lying dorsally on either side of the backbone in the most dorsal part of the body cavity. Note the adrenal gland, an irregular lighter colored band of tissue running the length of the ventral surface of each kidney.

In the male frog, testes appear as a pair of cream colored oval structures, attached to the anterior, ventral surface of each kidney. Sperm passes from the testes via fine ducts through the kidneys to the ureters and then to the cloaca. A slight enlargement in the posterior portion of each ureter is the seminal sesicle. Look for a small tube that runs parallel to the ureter in the males of some species of frogs. This is the vestigial oviduct.

Examine the male frog

In the female, locate the ovaries. They will vary greatly in size depending upon the season of the year that the frog was collected. In the spring, prior the egg laying they are greatly distended and fill a large portion of the body cavity. Black and white spherical ova will be clearly visible in these frogs. After egg laying till the fall the ovaries are much smaller, and appear as a pair of irregular shaped, lacy structures attached to the kidneys in a similar position to the testes in males.

Examine the female frog prior to egg laying

The oviducts (Mullerian ducts) are convoluted tubules lying dorsally in the body cavity and extending from the ovaries to the cloaca. The anterior end of the oviduct is enlarged to form the ostium. The posterior end of the duct widens and forms the ovisac.

Examine the female frog after egg laying

Endocrine System

Endocrine glands perform a wide variety of functions. They are grouped into a common system because of a similarity in their mode of secretion, which entails secretion into the blood stream rather than through a system of ducts. Some of the endocrine glands are highly specialized for endocrine function and do little else of note: others, such as the pancreas and gonads, perform an endocrine function in addition to other functions performed within the primary system of which they are a part. Because of this dual function of some organs, you will have previously seen much of the material to be considered in this section. One major endocrine gland, the pituitary, is closely associated with the ventral surface of the brain and hence is best studied, after the brain has been removed from the braincase which will be done in the next laboratory.

Thyroid Gland:
In the rat this gland is on the ventral surface of the trachea at its anterior end where it joins the larynx. The thyroid gland is in two main lobes lying on either side of the midline and joined by a narrow isthmus. In the frog, the thyroid consists of two separate structures. Their location is similar to that described for the rat.

Examine the Thyroid Gland as it appears in the:

    1. Rat
    2. Frog

Parathyroid Glands:

Those glands are small and usually difficult to locate. In the frog, there are two pairs, lying close to and just lateral to the external jugular vein. The rat has only one pair and they are embedded in the dorsal surface of the thyroid gland.

Adrenal Glands:

Note that in the frog, the adrenal glands are somewhat diffuse elongated structures embedded in the ventral aspect of the kidney. In the rat, the adrenal glands are separate structures, lying just anterior to the kidneys. They have their own blood supply in the rat.

Examine the Adrenal Glands as they appear in the:

    1. Rat
    2. Frog

Pancreas:

In both the Amphibians and Mammals, the pancreas consists of two functionally and anatomically separate parts, the main body of the gland that secretes digestive enzymes into the duodenum by way of one or more pancreatic ducts and the endocrine portion of the gland which secretes hormones into the blood stream. The latter portion consists of many small Islets of Langerhans, scattered throughout the gland.

Examine the Pancreas as it appears in the:

    1. Rat
    2. Frog

Gonads:

In both frog and rat, the gonads produce sex hormones. In the male the testes contain glandular interstitial tissue in addition to the sperm producing seminiferous tubules. The interstitial cells function in the synthesis of the male sex hormone, testosterone. In the female, the ovarian follicles produce estrogens. These follicles, which also contain the eggs, rupture when an egg becomes ripe (this is controlled by pituitary hormones). The old follicle then develops rapidly into a new structure, the corpus luteum, which also secretes hormones. The ovaries of the female frog are much larger and less compact than the rat, hence the eggs lying in the ovarian follicles will normally be more readily seen in the frog.

Examine the following gonad images:

    1. Rat Ovaries
    2. Rat Testes
    3. Frog Gonads

First published Oct 95: Modified Aug 05
Copyright © Michael Shaw 2005 (Images and Text)