Development Mammary gland

1 development

1.1 biochemistry
1.2 timeline

1.2.1 before birth
1.2.2 growth
1.2.3 pregnancy
1.2.4 postmenopausal

development

mammary glands develop during different growth cycles. exist in both sexes during embryonic stage, forming rudimentary duct tree @ birth. in stage, mammary gland development depends on systemic (and maternal) hormones, under (local) regulation of paracrine communication between neighboring epithelial , mesenchymal cells parathyroid hormone-related protein (pthrp). locally secreted factor gives rise series of outside-in , inside-out positive feedback between these 2 types of cells, mammary bud epithelial cells can proliferate , sprout down mesenchymal layer until reach fat pad begin first round of branching. @ same time, embryonic mesenchymal cells around epithelial bud receive secreting factors activated pthrp, such bmp4. these mesenchymal cells can transform dense, mammary-specific mesenchyme, later develop connective tissue fibrous threads, forming blood vessels , lymph system. basement membrane, containing laminin , collagen, formed afterward differentiated myoepithelial cells, keeps polarity of primary duct tree. these components of extracellular matrix strong determinants of duct morphogenesis.

biochemistry

estrogen , growth hormone (gh) essential ductal component of mammary gland development, , act synergistically mediate it. neither estrogen nor gh capable of inducing ductal development without other. role of gh in ductal development has been found mediated induction of secretion of insulin-like growth factor 1 (igf-1), occurs both systemically (mainly originating liver) , locally in mammary fat pad through activation of growth hormone receptor (ghr). however, gh acts independently of igf-1 stimulate ductal development upregulating estrogen receptor (er) expression in mammary gland tissue, downstream effect of mammary gland ghr activation. in case, unlike igf-1, gh not essential mammary gland development, , igf-1 in conjunction estrogen can induce normal mammary gland development without presence of gh. in addition igf-1, other paracrine growth factors such epidermal growth factor (egf), transforming growth factor beta (tgf-β), amphiregulin, fibroblast growth factor (fgf), , hepatocyte growth factor (hgf) involved in breast development mediators downstream sex hormones , gh/igf-1.

during embryonic development, igf-1 levels low, , gradually increase birth puberty. @ puberty, levels of gh , igf-1 reach highest levels in life , estrogen begins secreted in high amounts in females, when ductal development takes place. under influence of estrogen, stromal , fat tissue surrounding ductal system in mammary glands grows. after puberty, gh , igf-1 levels progressively decrease, limits further development until pregnancy, if occurs. during pregnancy, progesterone , prolactin essential mediating lobuloalveolar development in estrogen-primed mammary gland tissue, occurs in preparation of lactation , nursing.

androgens such testosterone inhibit estrogen-mediated mammary gland development (e.g., reducing local er expression) through activation of androgen receptors expressed in mammary gland tissue, , in conjunction relatively low estrogen levels, cause of lack of developed mammary glands in males.

timeline
before birth

mammary gland development characterized unique process epithelium invades stroma. development of mammary gland occurs after birth. during puberty, tubule formation coupled branching morphogenesis establishes basic arboreal network of ducts emanating nipple.

developmentally, mammary gland epithelium produced , maintained rare epithelial cells, dubbed mammary progenitors thought derived tissue-resident stem cells.

embryonic mammary gland development can divided series of specific stages. initially, formation of milk lines run between fore , hind limbs bilaterally on each side of midline occurs around embryonic day 10.5 (e10.5). second stage occurs @ e11.5 when placode formation begins along mammary milk line. give rise nipple. lastly, third stage occurs @ e12.5 , involves invagination of cells within placode mesenchyme, leading mammary anlage (biology).

the primitive (stem) cells detected in embryo , numbers increase steadily during development

growth

postnatally, mammary ducts elongate mammary fat pad. then, starting around 4 weeks of age, mammary ductal growth increases ducts invading towards lymph node. terminal end buds, highly proliferative structures found @ tips of invading ducts, expand , increase during stage. developmental period characterized emergence of terminal end buds , lasts until age of 7–8 weeks.

by pubertal stage, mammary ducts have invaded end of mammary fat pad. @ point, terminal end buds become less proliferative , decrease in size. side branches form primary ducts , begin fill mammary fat pad. ductal development decreases arrival of sexual maturity , undergoes estrous cycles (proestrus, estrus, metestrus, , diestrus). result of estrous cycling, mammary gland undergoes dynamic changes cells proliferate , regress in ordered fashion.

pregnancy

during pregnancy, ductal systems undergo rapid proliferation , form alveolar structures within branches used milk production. after delivery, lactation occurs within mammary gland; lactation involves secretion of milk luminal cells in alveoli. contraction of myoepithelial cells surrounding alveoli cause milk ejected through ducts , nipple nursing infant. upon weaning of infant, lactation stops , mammary gland turns in on itself, process called involution. process involves controlled collapse of mammary epithelial cells cells begin apoptosis in controlled manner, reverting mammary gland pubertal state.

postmenopausal

during postmenopause, due lower levels of estrogen, , due lower levels of gh , igf-1, decrease age, mammary gland tissue atrophies , mammary glands become smaller.