Stacey Breast part 1 embryology

  It should be noted, however, that what constitutes “normal”  histology in the breast varies according to sex, age, menopausal status, phase of the menstrual cycle, pregnancy, and  lactation, among other factors. Therefore, determination of  whether a given breast specimen is normal or shows pathologic alterations must take these variables into consideration. 

EMBRYOLOGY 

Development of the human mammary gland begins during  the 

5th week of gestation, 

at which time thickenings of  the ectoderm appear on the ventral surface of the fetus. 

⬇️ 

These mammary ridges, also known as milk lines, 

extend  from the axilla to the groin. 

⬇️

Except for a small area in the pectoral region, the bulk of these ridges normally regress  

Failure of regression of the milk lines can result in the appearance  in postnatal life of ectopic mammary tissue or accessory  nipples anywhere along the milk lines; 

commonly encountered in the 

1. axilla, 

2. inframammary  fold, and

3.  vulva (5–7). 

The earliest stages of breast development are largely  independent of sex steroid hormones (8). 

After the 15th  week

20th and 32nd weeks mesenchyme differentiate into fat within  the collagenous stroma between the  of gestation.  last 8 weeks  the epithelial cords canalize and branch, forming lobuloalveolar  structures as a result of mesenchymal paracrine effects.  A  depression in the epidermis, the mammary pit, forms at the convergence of the lactiferous ducts.  last few weeks of gestation responsive to maternal and placental steroid  hormones,  epithelial cells in the acinar  units exhibit secretory activity.  birth nipple forms  by evagination of the mammary pit near the time of.

transient  sensitivity to testosterone, which acts primarily on the  mesenchyme.  mesenchyme condenses around an epithelial stalk on the  chest wall to form the breast bud, the site of mammary  gland development.  Solid epithelial columns then develop within the mesenchyme, and these ultimately give rise to  the lobes or segments of the mammary gland.  Portions of  the fetal papillary dermis encase the developing epithelial  cords and eventually give rise to the vascularized fibrous  stroma that surrounds and invests the mammary  ducts and lobules.  More collagen-rich reticular dermis extends into the breast to form the suspensory ligaments of  Cooper, which attach the breast parenchyma to the skin.

 At birth, 

• withdrawal of the maternal and placental sex steroids stimulates  prolactin secretion, stimulates colostrum  secretion.

• both male and female neonates -palpable enlargement of the breast bud. 

first month of life,

• sex steroid hormones and prolactin decline during the secretory activity ends and the gland regresses and becomes  inactive until puberty,

•  the breast consists  primarily of lactiferous ducts that exhibit some branching 

• ❌🚫without evidence of progressive alveolar differentiation,  although some rudimentary lobular structures may persist. 

until 4 months of age

• extramedullary hematopoiesis, and this may persist in the  periductal stroma (9) (Figure 3.1)