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Structure and Function of the Mammary System in Dogs

By: Virginia Wells

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What Is the Mammary System?

The mammary system is comprised of the mammary glands or breasts and is present in all mammals. In males, mammary glands exist in a rudimentary state.

Where Is the Mammary System Located?

Mammary glands are typically arranged in two parallel rows extending from the underside of the chest to the groin area, along the outside of the body wall. In dogs, there are usually five mammary glands on each side, joined together in a chain.

What Is the General Structure of the Mammary System?

Mammary glands are composed of glandular tissue and connective tissue. The secretory tissue of the mammary glands become active during pregnancy, pseudopregnancy, the period of lactation when puppies are nursing, and often remain active for four to 50 days after weaning.

Each breast consists of a glandular complex and its associated nipple or teat (papillae mammae), which is covered by skin. The teats indicate the position of the glands in both the male and the female. The number of teats varies from eight to 12, with four to six gland complexes on each side of the midline. Ten is the most common number in larger breeds, while four pairs are more common in smaller breeds. In bitches with 10 normal teats, the pattern is two pairs of chest teats, two pairs of abdominal teats, and one pair of inguinal or groin teats. The pattern is often staggered, which makes all teats equally accessible to the pups when suckling with the bitch lying on her side.

The number of ducts opening into a teat varies from 7 to 16, and these are located on the blunt end in an irregular pattern. The streak canal, or teat canal, is 1/4 to 1/3 the length of the teat. The teat sinus extends upward from the teat canal into the gland. The teat sinuses are small uniformly wide passages.

The mammary glands have a large blood supply, and veins are more extensive than arteries. The thoracic (chest) mammary glands receive their arterial blood supply from the artery that passes through the ribs. The abdominal and inguinal (groin) glands are supplied by the arteries that come from the abdomen. Mammary veins usually lie next to the arteries.

Nerve fibers accompany the blood vessels to the mammary glands. These nerves are distributed to the tissue of the gland, to the blood vessels, to the smooth muscle of the teat, and to the skin. Secretion of the mammary glands is influenced by hormones from the brain and other organs, and by the nervous system.

What Are the Functions of the Mammary System?

The chief function of the mammary glands is to provide milk and nourishment to the newborn.

What Are the Common Diseases of the Mammary System?

  • Mastitis is inflammation or infection of the mammary glands. It occurs most commonly in the lactating or pseudopregnant animal. Mastitis is usually caused by a bacterial infection that travels up the teat opening. Clinical signs vary but may include pain, heat and swelling of the affected gland and abnormality of the milk.

  • Galactostasis is the abnormal accumulation of unexcreted milk in the mammary gland. The condition may be associated with infected or noninfected mastitis, and may also be noted postweaning or during pseudopregnancy. The engorgement of the gland with residual milk causes mild to moderate inflammation, swelling, and discomfort.

  • Agalactia is the failure to produce and excrete milk. It may arise with developmental abnormalities in the mammary glands, when there is failure of milk letdown, or from other diseases of the mammary glands or mother. Such diseases include poor nutrition or systemic illness of the bitch, and anxiety on the part of the bitch, especially young, first time mothers.

  • Galactorrhea is the excessive or inappropriate production and release of milk. This milk production is not associated with pregnancy or impending delivery of puppies. It occurs most commonly during pseudopregnancy in the dog, a hormonal state with outward clinical signs that resemble pregnancy. The condition generally resolves spontaneously as the pseudopregnancy subsides. Persistent or recurrent episodes may be treated with spaying (ovariohysterectomy).

  • Mammary gland tumors are the most common tumor to develop in female dogs, particularly in those that have not been spayed. Mammary gland tumors are frequently malignant, although some do not behave very aggressively. Spaying the dog before its second heat cycle can prevent these tumors.

    What Types of Diagnostic Tests Are Used to Evaluate the Mammary System?

  • A complete blood count, organ profile, and urinalysis are often used to look for evidence of infection or other related organ abnormalities.

  • Chest X-rays may be indicated if a tumor is present, in order to search for metastasis.

  • Cytology can be performed on milk samples, and is helpful to search for signs of inflammation and infection.

  • Bacterial culture and sensitivity testing may be performed. Material is retrieved using a special culture swab or collection tube, and attempts are made to grow the bacteria and identify them. Several antibiotics are then tested on the bacterial culture to determine which antibiotic works best to kill the bacteria.

  • A fine needle aspirate and examination of the cells is often useful when assessing masses or tumors.

  • Biopsy of masses and abnormal tissue may also be performed.

  • Removal of the affected gland(s) and submitting them for biopsy may be needed in some cases to identify the underlying problem and its cause.

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