Normal Development - Milk

Introduction

Breast milk makes us mammals! This current page discusses some issues related to milk and neonatal nutrition, it is not a guide to breastfeeding, which is covered by many other resources. The composition of milk can vary over time, with colostrum the initial yellowish, sticky breast milk produced at the end of pregnancy.

The review article (abstract shown below) by Goldman in 2000^[1] may provide a way of thinking about gastrointestinal tract and human milk.

There are other resource pages that cover the topic of breast development (More? Integumentary Development - Mammary Glands). Milk also has important role in gastrointestinal tract postnatal development (More? Gastrointestinal Tract Development)

Some Recent Findings

Characterization of the correlation between ages at entry into breast and pubic hair development ^[2] "Human milk contains a multitude of bioactive proteins, with very diverse functions. Some of these proteins are involved in the synthesis and expression of milk, but the majority appears to have evolved to provide physiological activities in the breast-fed infant. These activities are exerted by a wide variety of mechanisms and have largely been unraveled by in vitro studies. To be active in the gastrointestinal tract, these proteins must be able to resist proteolytic degradation, at least for some time. We have evaluated the human milk proteins lactoferrin, haptocorrin, alpha(1)-antitrypsin, and transforming growth factor -beta in an in vitro digestion model, mimicking the conditions of the infant gastrointestinal milieu. These bioactive proteins are resistant against proteolysis and can remain intact or as larger fragments through passage of the gastrointestinal tract. In vitro digestibility assays can be helpful to assess which human milk proteins can resist proteolysis and to what extent."

Mammary Glands Pregnancy

During pregnancy raised estrogens and progesterone stimulate gland development, secretory alveolar structures form and differentiate, leading to milk production in late pregnancy and milk secretion during lactation. Breasts are hemispherical in shape due to fat deposition. After birth, neonatal lactation supports further growth/development.

Milk Composition

Hydrolysis of lactose

Most mammals produce milk containing similar components which may occur at different concentrations. Composition of the maternal diet can affect the concentration of some of these components. In addition, some materal environmental components can also appear in the milk.

Typical secreted milk contains:

Carbohydrate: lactose, glucose, galactose, and oligosaccharides
Electrolytes
Fats: triglycerides and fatty acids (omega-3 polyunsaturated fatty acids, such as docosahexanoic acid)
Minerals
Proteins: caseins, alpha-lactalbumin, immunoglobulins, albumin, lactoferrin, nonprotein nitrogen, enzymes, hormones, growth factors, and nucleotides
Trace elements: selenium and iodine
Vitamins: A, B1 (thiamin), B2 (riboflavin), B5 (pantothenic acid), B6 (pyridoxine), B12 (cobalamin), D, and E
Water

Human Milk

"Human milk contains agents that affect the growth, development and functions of the epithelium, immune system or nervous system of the gastrointestinal tract. Some human and animal studies indicate that human milk affects the growth of intestinal villi, the development of intestinal disaccharidases, the permeability of the gastrointestinal tract and resistance to certain inflammatory/immune-mediated diseases. Moreover, one cytokine in human milk, interleukin (IL)-10, protects infant mice genetically deficient in IL-10 against an enterocolitis that resembles necrotizing enterocolitis (NEC) in human premature infants.

There are seven overlapping evolutionary strategies regarding the relationships between the functions of the mammary gland and the infant’s gastrointestinal tract as follows:

certain immunologic agents in human milk compensate directly for developmental delays in those same agents in the recipient infant
other agents in human milk do not compensate directly for developmental delays in the production of those same agents, but nevertheless protect the recipient
agents in human milk enhance functions that are poorly expressed in the recipient
agents in human milk change the physiologic state of the intestines from one adapted to intrauterine life to one suited to extrauterine life
some agents in human milk prevent inflammation in the recipient’s gastrointestinal tract
survival of human milk agents in the gastrointestinal tract is enhanced because of delayed production of pancreatic proteases and gastric acid by newborn infants, antiproteases and inhibitors of gastric acid # # production in human milk, inherent resistance of some human milk agents to proteolysis, and protective binding of other factors in human milk
growth factors in human milk aid in establishing a commensal enteric microflora"

(Text from: Goldman AS. Modulation of the gastrointestinal tract of infants by human milk. Interfaces and interactions. An evolutionary perspective.^[1])

Milk Production

Development of the breasts and milk production is mainly regulated by the anterior pituitary hormone prolactin (PRL). The release of prolactin is regulated by the hypothalamus prolactin-releasing hormone (PRLH, prolactin-releasing peptide, PRRP)

Prolactin hormone other roles include:

regulating follicle stimulating hormone (FSH) effect on the ovary.
increased maternal myelination processes during pregnancy.

Prolactin-releasing hormone (PRLH, prolactin-releasing peptide, PRRP) is an 87 amino acid peptide hypothalamus hormone which regulates anterior pituitary release of prolactin.

Prolactin signaling Pathway

In the mammary gland:

Prolactin binds to its receptor (PRLR) and causes them to dimerize.
Receptor-associated tyrosine kinase Jak2 phosphorylates: the prolactin receptor and Stat5a and Stat5b (signal transducers and activators of transcription).
Activated Stat5a and -5b are transported into the nucleus
They specifically bind DNA of target genes (the GAS sequence, TTCNNNGAA).
Induce transcription that promote: proliferation, differentiation, and lactogenesis.

Abnormalities

Galactorrhoea is the inappropriate production of milk that is often associated with anterior pituitary tumours producing excess prolactin. This condition can occur in both females and males.

Necrotizing Enterocolitis (NE) is the death of intestinal tissue that occurs postnatally in mainly in premature and low birth weight infants (1 in 2,000 - 4,000 births). The underdeveloped gastointestinal tract appears to be susceptible to bacteria, normally found within the tract,to spread widely to other regions where they damage the tract wall and may enter the bloodstream.

References

↑ ^1.0 ^1.1 <pubmed>10721920</pubmed>
↑ <pubmed>20105661</pubmed>

Journals

Reviews

Articles

Search PubMed

Search Pubmed: Mammary Gland Development | lactation | human milk | milk | breastfeeding

External Links

External Links Notice - The dynamic nature of the internet may mean that some of these listed links may no longer function. If the link no longer works search the web with the link text or name. Links to any external commercial sites are provided for information purposes only and should never be considered an endorsement. UNSW Embryology is provided as an educational resource with no clinical information or commercial affiliation.

Australia ABC - Diet and nutrition | Healthy Living Pregnancy&Birth
New Zealand Medsafe - Drug Safety in Lactation | summary of drug distribution into breast milk
USA American Society for Nutrition
WHO Breastfeeding

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Cite this page: Hill, M.A. (2024, May 2) Embryology Normal Development - Milk. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Normal_Development_-_Milk

What Links Here?

[PMID10721920-1] 1.0 ^1.1 <pubmed>10721920</pubmed>

[2] <pubmed>20105661</pubmed>

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