Human breast milk is the primary source of nutrition for the newborn, which can solely fulfill the nutrient requirements up to six months of age. When a woman conceives, several physiological changes occur in her body under the influence of hormones. One of them is the development of breasts and the production of milk. Several hormones play a role in this process. Hypothalamus secretes stimulatory or inhibitory factor, pituitary gland secretes prolactin and oxytocin, while placenta keeps a regulated level of estrogen and progesterone in the blood by producing this hormones. Other hormones as mentioned below in the text are also essential for the production of milk as they are required for the provision of the basic nutrient components of the milk.

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breastfeeding - cross cradle position

Image: “Breastfeeding – Cross Cradle Position.” by BruceBlaus – Own work. License: CC BY-SA 4.0


Structure of the Female Breast

An adult woman breast is an exocrine gland that is composed of 15-20 separate compartments called lobes. Each lobe has its own ductal system called the lactiferous duct. The main parts of the female breast are:

breast structure

Image: “Breast structure.” by Susan Spangler (Illustrator) – This image was released by the National Cancer Institute, an agency part of the National Institutes of Health, with the ID 1810. License: Public Domain

https://creativecommons.org/licenses/by-sa/4.0/

  • Lobules, which are the milk producing glands and are composed of several alveoli. The alveoli are lined by cuboidal epithelium and surrounded by the myoepithelial cells, which have contractile ability.
  • Lactiferous ducts form the channel passage for milk from alveoli to the nipple.
  • Stroma is composed mainly of fatty tissue and ligaments which surround the lobules, ducts, lymphatic and blood vessels.
  • The lactiferous ducts empty into nipple, which is the central part of areola. Areola is the dark colored skin consisting of sweat glands. The figure shows the structure of the female breast.

Development of the Breasts

Breasts begin to develop during puberty under the influence of female sex hormones. This is a part of secondary sexual characteristics in females. Although the breast development is also influenced by the diet of an individual, hormonal stimulation plays a major role. This explains a remarkable increase in breast size during pregnancy.

Under normal physiological conditions, the process of lactation and milk production starts after parturition, which again is under hormonal control. These hormones are discussed in detail below:

Role of estrogen in growth of ductal system during pregnancy

A high amount of estrogen is released by the placenta during pregnancy.

  • Estrogen causes the development and branching of the ductal system.

  • It causes an increase in stromal tissue.

  • Deposition of fat in the breasts resulting in an overall increase in weight and size. The average increase in the weight of breasts during pregnancy is 2 pounds.

Role of progesterone in development of lobular system during pregnancy

After the initial development of the ductal system, progesterone works synergistically with estrogen. Its main role in breast development is highlighted below:

  • There is an overall growth of the lobules and budding of the alveoli.

  • The alveoli gain secretoryfunction.

  • Its function is further enhanced by the action of growth hormone, prolactin, insulin and glucocorticoids.

Initiation of lactation and role of prolactin

As mentioned previously, the breasts are able to produce sufficient amount of milk after delivery. However, the initiation of lactation occurs in the fifth week of pregnancy under the influence of prolactin hormone.

  • Prolactin is secreted by the anterior pituitary gland. Its blood levels begin to rise in the fifth week of pregnancy until delivery, at which its level is 10 times greater than in a non-pregnant female.
  • The lactogenic effect of prolactin is supported by a hormone produced by the placenta called human chorionic somatomammotropin.
  • The secretory effect of prolactin is inhibited by estrogen and progesterone during pregnancy, even though a few milliliters of fluid are secreted every day. This is a physiological response as during pregnancy the fetus gets all the nutrients from the maternal blood and does not require to be breast fed.
  • The fluid which is secreted in the last few days of pregnancy and just before parturition is called colostrum. It contains exactly the same amount of lactose and protein as breast milk, yet has no fats. The rate of production of colostrum is 100 times less than the rate of production of breast milk.
  • Soon after the birth of the baby, the inhibitory role of estrogen and progesterone diminishes and the process of milk production starts. In 1-7 hours postpartum, copious amount of milk is produced under the lactogenic effect of prolactin.
  • As mentioned above, other hormones also play their role in the production of milk as they provide the required amino acids, fatty acids, glucose and calcium.
  • The basal level of prolactin soon becomes equal to that of a non-pregnant female. If the mother feeds her new born on regular intervals, every time she nurses him, neuronal signals go to the hypothalamus, increasing the level of prolactin to 20 folds. This is called prolactin surge and is required for milk production.
  • Prolactin is therefore required to keep the mammary glands functional. If the mother stops feeding her child, prolactin surge does not occur and the milk production stops. However, if she continues feeding, the milk production continues for a few years postpartum, but the quantity may decline over 8-9 months.

Role of hypothalamus in milk production

Hypothalamus is known to stimulate the pituitary hormones. However, in case of prolactin, it has an inhibitory role. If the baby suckles the nipples, neuronal signals are sent to the hypothalamus and the inhibitory effect on secretion of prolactin is lost. Thus, prolactin is secreted by the pituitary gland leading to milk production. I

f there is an injury of the hypothalamic-hypophysial system, the inhibitory response is permanently lost causing an increase in prolactin secretion. There are two factors secreted by the hypothalamus:

  • Prolactin inhibitory hormone (PIH) which remains dominant most of the time and inhibits prolactin secretion. Dopamine, a PIH tends to decrease prolactin by 10-folds.
  • Prolactin releasing factor (PRF), which has an intermittent effect in releasing prolactin.

Prolactin, on the other hand is thought to decrease the production of gonadotropin releasing hormone (GnRH) by the hypothalamus. This eventually suppresses the production of follicle stimulating hormone (FSH) and luteinizing hormone (LH) by the pituitary gland.

Nursing mothers therefore do not ovulate as follicles do not undergo maturation and development. This occurs in approximately half of the nursing mothers. With continuous feeding, the pituitary gland overcomes the inhibitory effect and starts producing FSH and LH. Breast feeding therefore acts as a natural contraceptive.

Role of oxytocin in secretion of milk

let down reflex

Image: “Let down reflex.” by OpenStax College – Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013. License: CC BY 3.0

Although the milk that is produced is continuously being secreted in the alveoli, it has to travel through the ductal system to reach to the baby’s mouth. When the baby suckles the nipples, initially he is not sucking any milk. Sensory signals travelling through the somatic nerves reach the spinal cord and finally to the hypothalamus, which secretes another hormone called oxytocin.

Oxytocin travels in the blood to reach the myoepithelial cells surrounding the outer wall of the alveoli, causing them to contract. This increases the pressure inside the alveoli to 30mmHg and forces milk into the ductal system.

Oxytocin and prolactin are secreted simultaneously and after 30 minutes of suckling milk starts to flow in the ductal system. This is known as ‘let down’ process, well demonstrated in the figure.

Several other factors also influence the release of oxytocin such as the sound of baby crying, fondling the baby by the mother and other psychogenic factors. Therefore, undisturbed puerperium is essential for successful nursing.

Sucking Reflex

The sucking reflex is a primitive instinct present in all newborn of mammals. It is linked to the rooting reflex and allows the baby to suckle anything that comes into contact with the roof of the mouth. It comprises of two components:

  • Expression: when nipple is placed between the child’s lip and palate, he will move the tongue to press the nipple.
  • Milking: the child will move his tongue away from the nipple towards areola, to be able to swallow the milk expressed.

Composition of Breast Milk

Breast milk is the primary source of nutrition for the newborn. It not only contains the required nutrients but also protects the baby against infections as his immune system has not been developed yet. As per World Health Organization (WHO) guidelines, a mother should breast feed her child up to two years.

Human breast milk consists of the following components:

  • Total fats up to 4.2g/ml with poly unsaturated fatty acids of 0.6g/ml
  • Total protein up to 1.1g/ml. It includes casein, lactalbumin, immunoglobulins IgG and IgA, albumin, lysozyme and beta lactoglobulin.
  • Lactose is the major carbohydrate present up to 7g/ml. Other oligosaccharides are 0.5g/ml.
  • Minerals such as calcium, iron, phosphorus, sodium, potassium and chlorine are also present.
  • Folate and vitamin C content is remarkably greater in human breast milk, when compared to goat’s or cow’s milk.

Galactorrhea

Galactorrhea is the spontaneous flow of milk from the mammary glands, unrelated to pregnancy or breast feeding. It can occur in both males and females irrespective of their age.

Hormonal irregularities such as hyperprolactinemia, elevated levels of thyrotropin releasing hormone (TRH) and thyroid stimulating hormone (TSH) are frequently associated.

History of drug intake such as methyldopa, opiods, antipsychotics, selective serotonin reuptake inhibitors (SSRIs), oral contraceptive pills (OCPs), and behavioral causes need to be ruled out in a patient with galactorrhea.

Certain pituitary adenomas also produce a high level of prolactin in the blood. As mentioned above, prolactin inhibits ovulation and is one of the causes of infertility.

Agalactorrhea or Lactation Failure

It is a medical condition in which there is an insufficient production of milk or an absence of ‘let down’ reflex in response to suckling. The mother is therefore unable to breast feed her new born baby.

It can be investigated by doing blood prolactin levels, which are usually low in such cases. It can be treated with galactagogues such as domeperidone, certain antipsychotics, TRH or TSH. Lactation failure with normal to high level of prolactin may be because of Sheehan’s syndrome or lymphocytic hypophysitis.

Review Questions on Lactation and Breastfeeding

The correct answers can be found below the references.

  1. A 46 years old male presented to his family physician with a compliant of discharge from the nipples along with off and on low grade fever. He has compliant of headache and vomiting early in the morning. On examination, he has HR 105/min, RR 28/min, BP 140/90 mmHg and discharge from both the nipples more at night. CBC shows hb 10 mg/dl, normal TLC with lymphocytosis. Local examination showed no swelling or tenderness. Which of the following could be the most probable cause to these compliants?
    1. Hirsutism
    2. Mastitis
    3. Pituitary adenoma
    4. Hyperthyroidism
  2. A 30-years-old fresh mother came to her family gynecologist with a compliant of insufficient breast milk production. On examination, there was no deformity of the breasts and they are normally developed. Child is also active and his sucking reflex is satisfactory. Child was born by a C-section at the start of 9th month of gestation. Which of the following medication would you advise?
    1. Phenobarbital
    2. Domperidome
    3. Acetazolamide
    4. Misoprostol
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