The functional anatomy of the lactating nipple-areolar complex

The female Homo sapiens nipple is anatomically variable

Normal female nipple and areolar size, shape, and colour in lactation are remarkably diverse.

I use the terms nipple, nipple walls (visible exterior sides of the nipple), nipple face (visible exterior which comprises the top part of the nipple), nipple stroma (internal tissue), base of nipple (where the nipple meets the areola), and areola to discuss the anatomy of the nipple-areolar complex (NAC).

NDC argues that it usually doesn't help breastfeeding women (and may even worsen outcomes) to have normal anatomic variability pathologised or blamed for breastfeeding problems. Many different anatomic factors affect how the complex adaptive system of the breastfeeding mother and baby interact and fit together, physically.

Our role as clinicians is to offer strategies which help her optimise how she and her baby fit together across the very wide range of human anatomic diversities relevant to breastfeeding.

• In a group of 300 Japanese women, 60% had nipples that were cylindrical in shape. In 25%, the nipple was narrower at the base than at the tip. The other 15% were oval. Nipples measured between 6 mm and 23 mm wide at the nipple base, and could be level with the areola, measured at 0 millimetres or up to 20 millimetres high.1

• In a study of 119 Californian women, who were 69% white and 23% Hispanic/Latino, nipple base width ranged between 15 to 34mm, and height between 5 to 20 mm high (with right nipples significantly longer than left nipples).2

Nipple-areolar complex skin

Human skin is a layered and composite tissue which acts as a physical and chemical barrier to the external environment.

The skin of the lactating nipple-areolar complex is characterised by both unique protective factors and unique exposure to risk. You can a table showing these various factors here.

NAC epidermis or epithelium

The top surface or face of a woman’s nipple is covered with convoluted epidermal ridges. NDC proposes that from an evolutionary perspective, these corrugations have evolved to enhance epidermal elasticity and resilience during breastfeeding. The epidermis of the nipple lacks sensory nerves or blood vessels.

At the base of the epidermis, in the stratum basale, keratinocytes differentiate from epidermal stem cells. These columnar epithelial keratinocytes rest on a basement membrane and are structurally welded together by specialised junctions known as desmosomes. Desmosomes behave like a spring and transmit physical forces throughout the cells. Keratinocytes then differentiate and migrate up through two other layers (the stratum spinosum and the stratum granulosum) towards the surface of the epidermis, producing pro- and anti-inflammatory cytokines.

By the time keratinocytes reach the outermost layer, the stratum corneum, they have lost their nucleus and organelles. The stratum corneum is comprised of about 15 layers of these flattened dead keratinocytes, referred to as corneocytes. Each corneocyte is surrounded by a lipid rich matrix and contains a tough, complex network of keratin filaments.

The stratum corneum constantly sheds corneocytes, and is impermeable, preventing water loss.

NAC dermis

The dermis lies underneath the epidermis, and is a thick fibrous elastic structure, dense in collagen, which gives skin its mechanical strength. The dermis is highly vascular and densely innervated with sensory nerve endings, including nociceptors and mechanoreceptors.3

The nipple-areolar complex lacks subcutaneous tissue

Most human skin has a subcutaneous base under the dermis, a soft layer of loose connective tissue and fat which contains larger blood vessels and nerves. This subcutaneous layer helps regulate body temperature and acts as a shock-absorber.4 However, the nipple-areolar complex lacks subcutaneous tissue. Below the areolar muscle there is just a thin layer of fat with blood vessels, which disappears as it approaches the nipple papilla.

NDC proposes that the nipple-areolar complex lack of subcutaneous tissue has three evolutionary advantages.

1. The vacuum of milk removal acts directly upon superficial lactiferous ducts without the interference of an added cushioning layer.

2. Ductal dilation is optimized without the added intra-oral volume of subcutaneous tissue.

3. The nipple is able to achieve a firm shape and definition, in the absence of softening subcutaneous tissue.

Nipple stroma

Here are key facts to know about nipple stroma and areolar smooth muscle, which will help as we think about nipple inflammation in breastfeeding.

• The nipple core is composed of irregular, very dense fibrous or collagen tissue, which supports the lactiferous ducts, and upon which the dermis rests.

• Circular and radial smooth muscle fibres run under the dermis of the nipple, and contract with tactile, mechanical or temperature stimulation.5 Contractile radial and circular smooth muscle fibres are also found beneath the dermis of the areola, continuous with nipple smooth muscle fibres. Contraction of the nipple and areolar smooth muscle is moderated through the sympathetic adrenergic nerves, and occurs in tandem.6

• Elevation of the nipple papilla has both a visual and tactile effect, orienting the infant for suckling.

• The nipple warms during breastfeeding due to both vasodilation and the warmth of the infant’s mouth. Nipple duct diameters decrease with cold applications.6

• The face of the lactating nipple has on average nine lactiferous duct orifices (range 4-18).7 The circular and radial smooth muscle fibres form a mesh-like structure in the connective tissue around the ducts at the tip of the nipple, with larger fibres found along the ducts as they extend into the nipple stroma.

• Lactiferous ducts are lined with cuboidal epithelium, but squamous epithelium lines the last one of two millimetres of the duct leading up to the orifice.

• Nipple stroma is threaded through with densely interlaced ducts and ductules, small blood vessels, nerves which run to the sensory nerve endings of the dermis, and bundles of smooth muscle tissue. Large nerve bundles have been visualised in the nipple stroma, at the base of the nipple, and in the glandular tissue adjacent to the ducts draining into the nipple.

References

1. Sanuki J-i, Fukuma E, Uchida Y. Morphologic study of nipple-areola complex in 600 breasts. Aesthetic Plastic Surgery. 2008;33(3):295-297.
2. Ventura AK, Lore B, Mireles O. Associations between variations in breast anatomy and early breastfeeding challenges. Journal of Human Lactation. 2021;37(2):403-413.
3. Pawlaczyk M, Lelonkiewicz M, Wieczorowski M. Age-dependent biomechanical properties of the skin. Postep Der Alergol. 2013;5:302-306.
4. Tepole AB, Gosain AK, Kuhl E. Stretching skin: the physiological limit and beyond. International Journal of Non Linear Mechanics. 2012;47(8):938-949.
5. Geddes DT. Ultrasound imaging of the lactating breast: methodology and application. International Breastfeeding Journal. 2009;4:doi:10.1186/1746-4358-1184-1184.
6. Gardner H, Lai CT, Ward LC, Geddes DT. Thermal physiology of the lactating nipple infuences the removal of human milk. Scientific Reports. 2019;9:11854.
7. Ramsay DT, Kent JC, Hartmann RA, Hartmann PE. Anatomy of the lactating human breast redefined with ultrasound imaging. Journal of Anatomy. 2005;206:525-534.