Muscular Abnormalities

Congenital absence of the sternocostal head of the pectoralis major muscle and its associated abnormalities has been described earlier in this chapter. In 5% of cadavers, a sternalis muscle can be found lying longitudinally between the sternal insertion of the sternocleidomastoid muscle and the rectus abdominis muscle.

The pectoralis minor muscle is inserted into the head of the humerus and the coracoid process of the scapula in 15% of cases. Part of the tendon then passes between the two parts of the coracoacromial ligament to insert into the coracohumeral ligament.

Rarely, the axillopectoral muscle arises as a separate part of the latissimus dorsi muscle and crosses the base of the axilla superficially, then passes deep to the pectoralis major muscle to join its insertion or to continue to the coracoid process (the Langer axillary arch). This anatomic arrangement can cause compression of the axillary vessels and difficulty in orientation during axillary dissection.

Microanatomy of Breast Development
The developing breast at puberty has been described in detail by Russo and Russo as growing and dividing ducts that form club-shaped terminal end buds. Growing terminal end buds form new branches, twigs, and small ductules termed alveolar buds. Alveolar buds subsequently differentiate into the terminal structure of the resting breast, named the acines by German pathologists or the ductule by Dawson. The term alveolus is best applied to the resting secretory unit and acines to the fully developed secretory unit of pregnancy and lactation.

TABLE 3. Characteristics of Human Breast Lobules. Lobules develop during the first few years after menarche. The alveolar buds cluster around a terminal duct and form type I (virginal) lobules, comprising approximately 11 alveolar buds lined by two layers of epithelium. Full differentiation of the mammary gland proceeds through puberty, takes many years, and may not be fully completed if interrupted by pregnancies.

Detailed microanatomic studies of the breast have shown the presence of three distinct types of lobules. Type I lobules, previously described, are the first generation of lobules that develop just after the menarche. The transition to type II and type III gradually results from continued sprouting of new alveolar buds. The characteristics of the four lobular types are described in Table 1-3 and Table 1-4.

TABLE 4. Proliferative Activity of Human Breast Terminal Duct–Lobular Unit Components as Measured by DNA-Labeling Index

Microscopic Anatomy of the Adult Breast