Development of the Embryonic Mammary Gland

Summary

Although the major part of mammary gland development takes place in adults during pregnacy, the organ is formed already in the embryo. During this period the gland displays responsiveness to steroid hormones and is influenced by signals from the surrounding mesenchyme.

Growth

In mouse embryos the anlage of the mammary gland is visible as a small placode around day 11.5. A small epithelial bud forms on day 12.5 which increases in size very slowly until day 15.5 (resting period). Around day 16 a primary sprout starts to grow into the fat pad. Several ducts are present at birth.

Tissue interaction

As in other derivatives of the epidermis, reciprocal interactions between epithelium and mesenchyme take place during embryonic mammary gland development. During embryonic development the mammary epithelium is associated with two types of mesenchyme:

1. The mammary mesenchyme which surrounds the epidermal bud. It consists of several layers of concentrically organized fibroblasts which are more densely packed than the dermal cells. These cells contain receptors for testosterone and estrogen (Wasner et al.,1983). (Slide 1: androgen and estrogen receptors in mammary mesenchyme). These cells also express the receptor for PTH/PTHrP and PTHrP signaling is crucial for outgrowth of the primary sprout (see here). The extracellular matrix of the mammary mesenchyme is rich in tenascin (Chiquet-Ehrismann et al.,1986). Mesenchymal steroid receptors (Heuberger et al.,1982) and tenascin (Inaguma et al.,1988) are induced by the mammary epithelium. Recent experiments have confirmed earlier observations (Propper, 1968) that a combination of mammary mesenchyme from 12 to 13 day rat or mouse embryos induces mammary specific differentiation in mid-ventral or dorsal epidermis (Cunha et al.,1995).

2. The future fat pad. This tissue is positioned below the epithelial bud in the deeper mesenchyme and consists of preadipocytes. It is required for typical mammary epithelial morphogenesis.

Morphogenesis of the mammary epithelium is strongly influenced by the mesenchyme. Combinations of mammary epithelia with different mesenchymes were grown as transplants under the kidney capsule of adult female mice. After several weeks the hosts were mated and allowed to lactate to provide the correct lactogenic stimuli to evaluate the influence of the mesenchyme on cellular differentiation. Mammary epithelia from 17 day embryos combined with fat pad mesenchyme developed normal ducts while ductal hyperplasias were induced in epithelia combined with the mammary mesenchyme (Sakakura et al.,1982). Mammary epithelia grown in the mesenchyme of the salivary gland developed a salivary-like morphology (Sakakura et al.,1976). Despite their different morphological appearance all transplants were able to lactate as assessed by the presence of secretion in the lumina and galactosyl-transferase activity attributed to a-lactalbumin.

Sexual dimorphism and hormone responsiveness

In most strains of mice and rats a pronounced sexual dimorphism exists (Slide 2: embryonic day 14 male and female mammary anlage). Mammary rudiments are formed in both sexes. In male embryos on day 14 the mammary mesenchyme condenses around the epithelium and especially the stalk and causes a detachment of the gland from the epidermis. This process is associated with massive necroses in the epithelium. Destruction of the mammary epithelium is caused by androgens and is also seen in organ culture (Kratochwil and Schwartz,1976; Kratochwil,1977). Responsiveness to androgens is lost during day 15 of development although androgen receptor contents remain high until birth (Kratochwil,1977; Wasner et al.,1983). Recently it was shown that development of the mammary mesenchyme and sexual dimorphism depend on PTHrP signaling (Dunbar et al, 1999).
Embryonic mammary glands are able to respond to lactogenic hormones in culture. Ceriani (1970) could show that mammary glands from 16 day rat embryos grown with combinations of different hormones produced a "casein-like material", i.e., phosphoprotein that comigrates with authentic casein. Insulin alone was sufficient to induce basal levels. Insulin and aldosterone gave slightly higher levels. Insulin plus prolactin plus aldosterone increased the synthesis even further. Insulin plus prolactin plus aldosterone plus progesterone was the most effective hormone combination.

Literature

Reviews:

Kratochwil, K. (1975) Experimental analysis of the prenatal development of the mammary gland. In: Milk and Lactation. Mod. Probl. Paediat.,15, 1-15. (Karger, Basel)

Kratochwil, K. (1986) Tissue combination and organ culture studies in the development of the embryonic mammary gland. In: Developmental Biology, Vol. 4, 315-333. Ed. by R. B. L. Gwatkin (Plenum Publ. Corp.)

Sakakura, T. (1987) Mammary Embryogenesis. In: The Mammary Gland. Development, Regulation, and Function. 37-66. Ed. by M. C. Neville and C. W. Daniel (Plenum Press)

Sakakura, T. (1991) New aspects of stroma-parenchyma relations in mammary gland differentiation. Int. Review Cytol. 125, 165-202.

Robinson, G.W., Karpf, A.B.C., Kratochwil, K. Regulation of mammary gland development by tissue interaction. J. Mam. Gland Biol. Neopl. 4, 9-19, 1999

Original papers

Ceriani (1970) Fetal mammary gland differentiation in vitro in response to hormones. II. Biochemical findings. Develop. Biol., 21, 530-546.

Chiquet-Ehrisman, R., Mackie, E.J., Pearson, C.A., Sakakura, T. (1986) Tenascin: an extracellular matrix protein involved in tissue interaction during fetal development and oncogenesis. Cell 47, 131-139.

Cunha, G., Young, P., Christov, K., Nandi, S., Talamantes, F., Thordarson, G. (1995) Mammary phenotypic expression induced in epidermal cells by embryonic mammary mesenchyme. Acta Anat., 152, 195-204.

Dunbar, M.E., Dann, P.R., Robinson, G.W., Hennighausen, L., Zhang, J.-P., Wysolmerski, J.J. Parathyroid hormone-related protein signaling is necessary for sexual dimorphism during embryonic mammary development. Development 126, 3485-3493, 1999.

Heuberger, B., Fitzka, I., Wasner, G., Kratochwil, K. (1982) Induction of androgen receptor formation by epithelium-mesenchyme interaction in embryonic mouse mammary gland. Proc. Natl. Acad. Sci., 79, 2957-2961.

Inaguma, Y., Kusakabe, M., Mackie, E.J., Pearson, C.A., Chiquet-Ehrismann, R., Sakakura, T. (1988) Epithelial induction of stromal tenascin in the mouse mammary gland: From embryogenesis to carcinogenesis. Develop. Biol., 128, 245-55.

Kratochwil, K., Schwartz, P. (1976) Tissue interaction in androgen response of embryonic mammary rudiment of mouse: Identification of target tissue for testosterone. Proc. Natl. Acad. Sci., 73, 4041-4044.

Kratochwil, K. (1977) Development and loss of androgen responsiveness in the embryonic rudiment of the mouse mammary gland. Develop. Biol., 61, 358-365.

Propper, A.Y. (1968) Relations epidermo-mesodermiques dans la differenciation de l'ebauche mammaire d'embryon de lapin. Ann. Embryol. Morphog. 2, 151-160.

Sakakura, T., Nishizuka, Y., Dawe, C.J. (1976) Mesenchyme-dependent morphogenesis and epithelium-specific cytodifferentiation in mouse mammary gland. Science, 194, 1439-1441.

Sakakura, T., Sakagami, Y., Nishizuka, Y. (1982) Dual origin of mesenchymal tissues participating in mouse mammary gland embryogenesis. Develop. Biol., 91, 202-207.

Wasner, G., Hennermann, I., Kratochwil, K. (1983) Ontogeny of mesenchymal androgen receptors in the embryonic mouse mammary gland. Endocrinology, 113, 1771-1780.

key words

mammary anlage, embryonic development, sexual dimorphism, testosterone

Contributed by:

Gertraud W. Robinson
National Institutes of Health
Bldg. 8, Rm. 105
Bethesda, MD 20892
Phone: 301-496-5004
Fax: 301-480-7312
e-mail: gertraur@bdg10.niddk.nih.gov