Glándula mamaria

Introduction to Mammary Gland Development

Overview of the Lecture

• Fernando Pérez introduces the topic of mammary gland development, linking it to histology and embryology at the National University of Rosario.

Initial Stages of Mammary Gland Formation

• The mammary glands begin as ectodermal epithelial buds that grow into the mesenchyme, driven by inductive factors from the mesenchyme. This process starts in the fifth week of embryonic development.

• An imaginary line called the mammary line extends from the mid-clavicle to the iliac crest, where this growth occurs, with more pronounced development at the cephalic end.

Growth Patterns and Structural Changes

Cephalic vs. Caudal Growth

• There is greater proliferation at the cephalic end while involution occurs at the caudal portion, leading to a future thoracic structure. This pattern continues until about week 17.

Internal Structures Development

• By week seven, septa form within the mesenchyme creating compartments; internal epithelial cords develop and branch out into ducts during this period.

Mammary Gland Architecture

Duct Formation and Capillary Network

• As development progresses towards week 25, these internal cords canalize into ducts surrounded by differentiated connective tissue that supports vascularization around them.

Prominence and Lobular Structure

• After week 25, there is an increase in prominence towards the surface due to collagen fibers forming septa between lobules; supernumerary glands may also appear along mammary lines.

Inductive Factors in Mammary Gland Differentiation

Role of Extracellular Matrix

• Inductive signals for mammary gland differentiation are mediated primarily through connective tissue factors produced in its extracellular matrix, influencing further developmental processes.

Anatomical Structure and Functionality

Anatomy of Mature Mammary Glands

• The mature structure consists of branched tubular glands with multiple lobules converging into larger ducts for efficient milk transport during lactation; this design minimizes energy costs compared to a single large duct system.

Postnatal Development Changes

Development of Mammary Glands and Hormonal Influence

Structure and Growth of Mammary Glands

• The mammary gland begins as a branched structure that develops until puberty, where menstrual cycles initiate estrogen stimulation leading to mitosis and growth of ductal ends.

• Alveoli formation occurs only during pregnancy; the cyclical evolution generates characteristic adipose distribution in the hypodermis, contributing to secondary sexual characteristics in females.

Changes During Pregnancy

• In late pregnancy, alveoli appear along with epithelial cells that assist in milk secretion through contraction during lactation.

• A sagittal cut shows an underdeveloped mammary gland with terminal ducts; post-puberty, increased adipose tissue enhances surface prominence.

Structural Comparisons

• Observations reveal independent terminal ducts draining into the nipple, with skin pigmentation changes due to melanocyte activity.

• Images compare inactive mammary glands to those during gestation, highlighting ductal growth and branching leading to alveolar structures by the third trimester.

Lactation and Post-Pregnancy Changes

• Comparison between inactive glands and those in lactation shows significant structural expansion with numerous alveoli filling lobules.

• Inactive glands exhibit connective tissue septa but lack developed lobules or alveoli; they primarily consist of dense connective tissue.

Hormonal Regulation of Mammary Development

• Hormones regulate mammary development throughout menstrual cycles; estrogens promote tissue growth while progestogens facilitate cell differentiation.

• Without pregnancy, hormonal influence ceases until the next cycle. However, if pregnant, continuous estrogen production supports ongoing ductal growth.

Role of Placental Hormones

• Early pregnancy maintains estrogen levels via luteal support; this leads to progressive ductal growth characterized by extensive branching.

Functioning of the Corpus Luteum and Lactation

Hormonal Changes During Pregnancy

• The corpus luteum functions well initially, but it undergoes involution as the placenta takes over hormone production, specifically estrogen and progesterone, which act on maternal tissues.

• In the last trimester, alveoli differentiate under prolactin stimulation to produce milk secretion; lactation is primarily stimulated by prolactin post-birth.

Role of Prolactin and Oxytocin

• Estrogens and progestogens stimulate mammary tissue throughout pregnancy, while prolactin specifically targets alveoli for milk production in the final trimester.

• Oxytocin facilitates milk ejection by causing contractions in myoepithelial cells surrounding the alveoli.

Development of Mammary Glands

• During pregnancy, lobules increase in diameter and number due to growth and branching; connective tissue partitions thin out as mammary structures expand.

• The proliferation of ducts continues with increased branching leading to a more complex structure filled with developing alveoli towards the end of pregnancy.

Alveolar Formation

• Alveoli begin to appear at the distal ends of ducts during late pregnancy; most alveolar development occurs in the last month before birth.

• Connective septa become thinner as more structures develop within lobules, indicating an increase in functional elements like alveoli.

Postpartum Changes in Mammary Glands

• After childbirth, there is significant expansion of lobules filled with secretory alveoli resembling "moth-eaten" structures due to lipid droplets and protein secretion.

• Myoepithelial cells assist in transporting milk through ducts; lobules are highly expanded with minimal connective tissue visible between them.

Mechanisms of Milk Secretion

• Alveoli exhibit extensive dilation post-birth with various vesicles containing lipids and proteins ready for secretion into ducts.

• Two mechanisms for secretion are identified: apocrine (where part of the cytoplasm is lost during lipid release), and merocrine (for proteins without cytoplasmic loss).

Composition of Breast Milk