Nipple

The nipple is a protuberance that sits upon the surface of the breast from which the milk produced by the breast leaves the breast through the lactiferous ducts.^[1] The milk can flow through the nipple passively or it can be ejected by muscular contractions that occur along the ductal system. The nipple is surrounded by the areola which varies in pigmentation.^[2] It is often called a teat when referring to non-humans. It can also be used to describe the flexible mouthpiece of a baby bottle. In humans, nipples of both males and females be stimulated as part of sexual arousal. In many cultures, human female nipples are sexualized,^[3] or "...regarded as sex objects and evaluated in terms of their physical characteristics and sexiness."^[4]

Etymology

The word "nipple" most likely originates as a diminutive of neb, an Old English word meaning "beak", "nose", or "face", and which is of Germanic origin.^[5] The words "teat" and "tit" share a Germanic ancestor. The second of the two, tit, was inherited directly from Proto-Germanic, while the first entered English via Old French.^[6][7]

Anatomy

The breast: cross-section scheme of the mammary gland:

1. Chest wall
2. Pectoralis muscles
3. Lobules
4. Nipple
5. Areola
6. Milk duct
7. Fatty tissue
8. Skin

In the anatomy of mammals, a nipple, mammary papilla or teat is a small projection of skin containing the outlets for 15–20 lactiferous ducts arranged cylindrically around the tip. The skin of the nipple is rich in a supply of special nerves that are sensitive to certain stimuli: these are slowly-adapting and rapidly-adapting cutaneous mechanoreceptors. Mechanoreceptors are identified respectively by Type I slowly-adapting with multiple Merkel corpuscle end-organs and Type II slowly-adapting with single Ruffini corpuscle end-organs, as well as Type I rapidly-adapting with multiple Meissner corpuscle end-organs and Type II rapidly-adapting with single Pacinian corpuscle end-organs.

The dominant innervation to the nipple is derived from the lateral cutaneous branches of fourth intercostal nerve.^[8] The physiological purpose of nipples is to deliver milk to the infant, produced in the female mammary glands during lactation. Marsupials and eutherian mammals typically have an even number of nipples arranged bilaterally, from as few as two to as many as 19.^[9]

In humans

Changes in size

The average projection and size of human female nipples is slightly more than 3/8 of an inch (10mm).^[10] This is equal to about five stacked American quarters.^[11] Pregnancy and nursing tend to increase nipple size, sometimes permanently. Pregnancy also deepens the pigmentation.

Nipple stimulation

During breastfeeding, nipple stimulation by an infant will simulate the release of oxytocin from the hypothalmus. Oxytocin is a hormone that increases during pregnancy and acts on the breast to help produce the milk-ejection reflex. Oxytocin also causes uterine contractions postpartum when the infant begins to nurse and stimulates the nipple.^[12] The strong uterine contractions that are caused by the stimulation of the mother's nipples help the uterus contract. These contractions are necessary to prevent post-partum hemorrhage.^[13] When the baby suckles or stimulates the nipple, oxytocin levels rise and small muscles in the breast contract and move the milk through the milk ducts. The result of nipple stimulation by the newborn helps to move out through the ducts and to the nipple. This contraction of milk is called the “let-down reflex.”^[14] Latching on refers to the baby fastening onto the nipple to breastfeeding. A good attacment is when the bottom of the areola (the area around the nipple) is in the baby's mouth and the nipple is drawn back inside his or her mouth. A poor latch results in insuffienct nipple stimulation. The nipple is poorly stimulated when the baby latches on too close to the tip. This can cause cracked nipples and a reluctance to continue to breastfeed.^[15][16] After the birth of the infant, the milk supply increases due to increased stimulation of the nipple. If the baby increases nursing time at the nipple, the mammary glands respond to this stimulation by increasing milk production,

Nipples can be sensitive to touch and can lead to sexual arousal.^[17] Few women report experiencing orgasm from nipple stimulation.^[18] Before Komisaruk et al.'s functional magnetic resonance (fMRI) research on nipple stimulation in 2011, reports of women achieving orgasm from nipple stimulation relied solely on anecdotal evidence.^[19] Komisaruk's study was the first to map the female genitals onto the sensory portion of the brain; it indicates that sensation from the nipples travels to the same part of the brain as sensations from the vagina, clitoris and cervix, and that these reported orgasms are genital orgasms caused by nipple stimulation, and may be directly linked to the genital sensory cortex ("the genital area of the brain").^[19][20][21]

Breast/nipple cancer

Symptoms of breast cancer can often be seen in changes in the nipple and areola. Not all women have the same symptoms. Some people do not have any signs or symptoms at all. A person may find out they have breast cancer after a routine mammogram. Warning signs can be:^[22][23]

• New lump in the nipple, or breast or armpit.
• Thickening or swelling of part of the breast, areola or nipple.
• Irritation or dimpling of breast skin.
• Redness or flaky skin in the nipple area or the breast.
• Pulling in of the nipple or pain in the nipple area.
• Nipple discharge other than breast milk, including blood.
• Any change in the size or the shape of the breast or nipple.
• Pain in any area of the breast.^[22][23]

Changes in the nipple are not necessarily symptoms or signs of breast cancer. Other conditions of the nipple can mimic the signs and symptoms of breast cancer.^[22]

Nipple pain

Nipple pain can be a disincentive for breastfeeding.^[24] Sore nipples that progress to cracked nipples is of concern since many woman cease breasting due to the pain. In some instances an ulcer will form on the nipple.^[25] One reason for the development of cracked and sore nipples is the incorrect latching-on of the infant to the nipple. If a nipple appears to be wedge-shaped, white and flattened, this may indicates that the attachment of the infant is not good and there is a potential of developing cracked nipples.^[26] Herpes infection of the nipple is painful.^[27]

Nipple pain can also be caused by excessive friction of clothing against the nipple that causes a fissure.

Vertical transmission

Some infections are transmitted through the nipple, especially if irritation or injury to the nipple has occurred.In these circumstances, the nipple itself can become infected with Candida that is present in the mouth of the breastfeeding infant. The infant will transmit the infection to the mother. Most of the time, this infection is localized to the area of the nipple. In some cases the infection can can progress to become a full-blown case of mastitis or breast infection.^[28] In some cases, if the mother has an infection with no nipple cracks or ulcerations, it is still safe to breastfeed the infant.

Herpes infection of the nipple can go unnoticed because the lesions are small but usually are quite painful. Herpes in the newborn is a serious and sometimes fatal infection.^[27] Transmission of Hepatitis C and B to the infant can occur if the nipples are cracked.^[29]

Other infections can be transmitted through a break of the skin of the nipple and can infect the infant.

Nipple discharge

Nipple discharge refers to any fluid that seeps out of the nipple of the breast. Discharge from the nipple does not occur in lactating women. And discharge in non-pregnant women or women who are not breasfeeding may not cause concern. Men that have discharge from their nipples are not typical. Discharge from the nipples of men or boys may indicate a problem. Discharge from the nipples can appear without squeezing or may only be noticeable if the nipples are squeezed. One nipple can have discharge while the other does not. The discharge can be clear, green, bloody, brown or straw-colored. The consistenct can be thick, thin, sticky or watery.^[30][31]

Some cases of nipple discharge will clear on their own without treatment. Nipple discharge is most often not cancer (benign), but rarely, it can be a sign of breast cancer. It is important to find out what is causing it and to get treatment. Here are some reasons for nipple discharge:^[30]

• Pregnancy
• Recent breastfeeding
• Rubbing on the area from a bra or t-shirt
• Injury to the breast
• Infection
• Inflammation and clogging of the breast ducts
• Noncancerous pituitary tumors
• Small growth in the breast that is usually not cancer
• Severe underactive thyroid gland (hypothyroidism)
• Fibrocystic breast (normal lumpiness in the breast)
• Use of certain medicines
• Use of certain herbs, such as anise and fennel
• Widening of the milk ducts^[30]

Sometimes, babies can have nipple discharge. This is caused by hormones from the mother before birth. It usually goes away in 2 weeks. Cancers such as Paget disease (a rare type of cancer involving the skin of the nipple) can also cause nipple discharge.^[30]

Nipple discharge that is NOT normal is bloody, comes from only one nipple, or comes out on its own without squeezing or touching the nipple. Nipple discharge is more likely to be normal if it comes out of both nipples or happens when the nipple is squeezed your nipples. Squeezing the nipple to check for discharge can make it worse. Leaving the nipple alone may make the discharge stop.^[30] Some genetically-males develop a condition known as gynecomastia, in which the breast tissue under the nipple develops and grows. Discharge from the nipple can occur. The nipple may swell in some genetically-males possibly due to increased levels of estrogen.

Nipple disorders

In male mammals

A human male nipple

Mammalian fetuses within the same species look the same, regardless of sex. In humans, this lasts for around 6 weeks, after which genetically-male fetuses begin producing male hormones such as testosterone.^[34] Usually, males' nipples do not change after this.

The uncoupling of male and female traits occurs if there is selection for it: if the trait is important to the reproductive success of both males and females but the best or "optimal" trait is different for a male and a female. We would not expect such an uncoupling if the attribute is important in both sexes and the "optimal" value is similar in both sexes, nor would we expect uncoupling to evolve if the attribute is important to one sex but unimportant in the other. The latter is the case for nipples. Their advantage in females, in terms of reproductive success, is clear. But because the genetic "default" is for males and females to share characters, the presence of nipples in males is probably best explained as a genetic correlation that persists through lack of selection against them, rather than selection for them. Interestingly, though, it could be argued that the occurrence of problems associated with the male nipple, such as carcinoma, constitutes contemporary selection against them.

In a paper, Stephen Jay Gould and Richard C. Lewontin emphasize that we should not immediately assume that every trait has an adaptive explanation. Just as the spandrels of St. Mark's domed cathedral in Venice are simply an architectural consequence of the meeting of a vaulted ceiling with its supporting pillars, the presence of nipples in male mammals is a genetic architectural by-product of nipples in females. This idea implies men have nipples because females do.^[35]

See also

References

Wikimedia Commons has media related to Nipples.

Look up nipple, teat, or papilla in Wiktionary, the free dictionary.

Bibliography

• Walker, Marsha (2011). Breastfeeding management for the clinician : using the evidence. Sudbury, Mass: Jones and Bartlett Publishers. ISBN 9780763766511.