The Breasts: Some Morphological Aspects
Dr. Nelson Soucasaux , Brazilian gynecologist
As is well known, the basic structures that, in women, give rise to
the breasts, already exist in both sexes since embryonic life: they consist
of the nipple, the areola and a rudimentary system of very small branching
tubules that originate in the nipple and spread into the subareolar connective
tissue. This branching system of tubules is formed by 15 to 20 main ducts
(canals). By the time of the puberal growth of the breasts, the proliferation
of each one of these ducts results in the formation of an isolated glandular
structure that corresponds to a mammary lobe. Ham observes that each breast
is constituted by the reunion of many individualized glands, each one of
them possessing an independent excretory canal through which its secretion
is conducted towards the nipple (Ham, A.W. "Histologia," Guanabara
Koogan, Rio de Janeiro, 1967). Each mammary lobe, in turn, consists of a
great number of lobules, and each one of these lobules by 10 to 100 acini
grouped together around a small collecting duct (Haagensen, C.D. "Diseases
of the Breast," Saunders, Philadelphia, London, 1956).
Just like all the other woman's sexual organs, both the development
and the trophicity of the breasts depend on the estrogens. Thus, the puberal
growth of the breasts is mostly due to the estrogen stimulation. There is
also a small complementary action on the part of progesterone as the biphasic
(ovulatory) cycles are established.
The puberal development of the breasts is characterized by: 1) great
proliferation of the ductal system (the branching canalicular structures
of the breasts) and of the connective and adipose tissue; 2) beginning of
the development of the acinar or alveolar system (the mammary secretory
structures); 3) increase in size and pigmentation of the nipples and areolas.
All these changes make the girl's originally "flat breasts" acquire
the features of those of the adult woman.
The estrogenic action on the breasts mostly stimulates the growth and
proliferation of the ductal (canalicular) system, the loose and fibrous
connective tissue that surrounds the ducts and lobules, as well as the deposition
of fat. Progesterone, besides reducing some excessive proliferative effects
of the estrogens, acting conjointly with the latter, seems to be the main
hormonal factor responsible for the development of the mammary secretory
acini (the alveolar structures of the breasts). There are reports that the
entire development of the lobular-alveolar system is only completed during
The gestational hypertrophy of the breasts is basically a result of
the heightened levels of estrogens and progesterone that characterize this
event of women's lives. The heightened prolactin levels that are equally
typical of pregnancy possibly also contribute to this gestational enlargement
of the breasts. Though the main actions of prolactin on the breasts consist
of preparing these organs for lactation and stimulating milk secretion,
there seems to exist a synergistic action between this pituitary hormone,
the estrogens and progesterone on the puberal and gestational development
of the mammary glandular structures.
Here we must observe that, since associated with the estrogens, progesterone
and prolactin, other hormones characterized by general metabolic actions
also seem to exert a non-specific and complementary action on the mammary
development and physiology. Among them we can mention the growth hormone,
thyroxine and triiodothyronine and cortisol.
During the puberal development of the breasts, the ducts proliferate
according to a typical, increasingly branching pattern (like the growth
of the branches in a tree) and, at the final extremities of their smaller
branches, epithelial sprouts that will originate the mammary acini are formed.
As already stated, this great proliferation of the mammary ducts or tubules
is fundamentally due to the estrogenic action. The development of the acinar
system (the mammary secretory structures) only seems to begin with the establishment
of the ovulatory cycles and the consequent appearance of progesterone along
the second phase of the cycles. Even so, the entire development of the acinar
system takes place only during pregnancy.
Also as a result of the estrogenic action and simultaneously with the
remarkable branching growth of the ductal system, there is a great proliferation
of periductal, intralobular, interlobular and interlobar connective tissue,
as well as the deposition of fat. Thus, according to what is being exposed,
we can verify that the inner structure of the breasts is basically formed
by an epithelial parenchyma and a connective and adipose stroma.
As already mentioned, the mammary parenchyma is formed by an intricate
and complex branching system of ducts and glandular acini. In it, the diverse
groupings of acini with their respective ducts and surrounding connective
tissue constitute the mammary lobules and lobes. According to Netter's description,
the mammary ducts ". . . extend radially from the nipple toward the
chest wall, and from them sprout variable numbers of secondary tubules.
These end in epithelial masses forming the lobules or acinar structures
of the breast," (Netter, F.H. "The Ciba Collection of Medical
Illustrations, Vol. 2, Reproductive System," U.S.A., 1954.)
The mammary secretory acini are surrounded by the myoepithelial cells,
capable of contracting under the action of oxytocin. Thus, the release of
oxytocin that takes place during the act of breast feeding causes the contraction
of these myoepithelial cells and the consequent ejection of milk.
Both the quantity and the dimensions of the ductal, acinar and lobular
structures vary greatly from woman to woman. Great variations are also found
in the same woman not only according to the phases of life and their respective
hormonal influences, but also even from one breast to the other. Almost
always the major concentration of glandular structures occupies the upper-external
quadrants of the breasts. For that reason, at clinical examination the density
and consistency of the mammary tissues is usually greater at the upper-external
quadrants than at the other ones.
As we have seen, the mammary stroma, in the middle of which the ducts
and acini are located, consist of a mixture of fibrous and loose connective
and fatty tissues. The loose and fibrous connective tissue predominates
at the parts of the breasts in which the amount of ductal and acinar (glandular)
structures is greater, while the fatty tissue predominates at the parts
possessing less glandular structures. As to the importance of the fibrous
and fatty tissue that constitute the mammary stroma on the degree of firmness
or flaccidity of the breasts, Netter observes that ". . . in the absence
of pregnancy and lactation, the relative amounts of fatty and fibrous tissue
determine the size and consistency of the breast."
The suspensory ligaments of the breasts constitute another group of
structures of the mammary stroma that also play an important role in the
inner architecture of these organs. They consist of thin layers of dense
fibrous tissue that "encase" and "envelop" the mammary
lobes and lobules and indirectly attach them both to the breast subcutaneous
tissue and to the deep pectoral fascia. (The deep pectoral fascia is located
behind the breasts, covering the great pectoral muscles). Thus, the suspensory
ligaments of the breasts are responsible for the somewhat difficult anatomic
task of "holding up" the entire breast structure in its position
by keeping it "anchored" both on the skin and on the great pectoral
As to the fatty tissue that is part of the mammary stroma, most of it
is located at the subcutaneous region, between the skin and most of the
glandular structure. A thinner layer of this tissue is found at the retromammary
region, above the aforementioned pectoral fascia. Small amounts of fatty
tissue are also present in the middle of the connective tissue that surrounds
the ductal and acinar structures.
The nipples are formed mostly by dense connective tissue and smooth
muscle fibers and each one of them is internally crossed by the 15 to 20
main mammary ducts that open up on its surface. The smooth muscle fibers
surround the mammary ducts, and their contraction cause the erection of
the nipples. The nipples also contain nerve endings especially sensitive
to pressure, whose stimulation triggers a neuroendocrine reflex responsible
both for the hypothalamic release of oxytocin and for the pituitary momentary
outputs of increased amounts of prolactin. (These neuroendocrine events,
triggered by sucking the nipples, are fundamental during breast feeding
because the release of oxytocin cause the ejection of milk, and the pituitary
"peaks" of prolactin maintain the lacteal production.) The nerve
endings of the nipples are also responsible for their enormous sensitivity
to sexual stimulation.
The areolas also consist of dense connective tissue containing many
smooth muscle fibers. They also contain several modified sebaceous glands
that can clearly be seen on their surface. At the subareolar region and
just below the nipple the mammary ducts exhibit short small dilations called
Asymmetries and differences between one breast and the other in the
same woman are very frequent. They include differences not only regarding
size and shape, but also the distribution, density and relative amounts
of the several elements that constitute the mammary parenchyma and stroma.
These differences existing from one breast to the other in the same woman
are due to variations on the individual responsivity of the mammary tissues
to the hormonal stimulations.
The text above is an adapted excerpt from my book "Os Órgãos
Sexuais Femininos: Forma, Função, Símbolo e Arquétipo"
("The Female Sexual Organs: Shape, Function, Symbol and Archetype"),
published by Imago Editora, Rio de Janeiro, 1993. For more information on
the book, see page http://www.nelsonginecologia.med.br/orgaos.htm
from my website www.nelsonginecologia.med.br .
Copyright Nelson Soucasaux 1993, 2003 (text and illustrations)
Nelson Soucasaux is a gynecologist dedicated to clinical, preventive
and psychosomatic gynecology. Graduated in 1974 by Faculdade de Medicina
da Universidade Federal do Rio de Janeiro, Brazil, he is the author of several
articles published in medical journals, and of the books "Novas Perspectivas
em Ginecologia" ("New Perspectives in Gynecology") and "Os
Órgãos Sexuais Femininos: Forma, Função, Símbolo
e Arquétipo" ("The Female Sexual Organs: Shape, Function,
Symbol and Archetype"), published by Imago Editora, Rio de Janeiro,
Website (Portuguese-English): www.nelsonginecologia.med.br