LACTATING MAMMARY GLAND EPITHELIAL CELL

Course: Advanced Cell Biology Instructor: Klaudia Brix Author: Zaina Batool

Secretory Granules Secretory granules are membrane enclosed vesicles transporting milk proteins.4

Lipids* Lipid droplets moving freely in the cytoplasm coalesce to form large droplets near the apical part of the cell. Through merocrine secretion, they project into the lumen of the acinus in the form of colloidal micelles and are invested with a plasma membrane envelope in addition to a layer of cytoplasm that is so thin that the cytoplasmic loss is minimal. 2

Peroxisomes Peroxisomes are enclosed by a single membrane and contain various enzymes involved with the production and degradation of hydrogen peroxide in addition to the oxidation of organic molecules which are toxic or unwanted by the cell.4

Golgi Apparatus The Golgi apparatus is a membrane-bound stack that modifies folded proteins received from the endoplasmic reticulum and sorts them in cellular or extra-cellular compartments by vesicular transport. It is usually found near the nucleus and enlarges during lactation. It has the greatest volume during the peak of cellular milk synthesis, just before its release into the alveolar lumen. Lactose synthesis occurs here when glucose and UDP-galactose react in the presence of lactose synthetase to form lactose and UDP. 2

Lysosomes Lysosomes are bound by a single membrane and contain digestive enzymes. The acidic lumen of lysosomes results in optimal activity by these enzymes.4

Rough Endoplasmic Reticulum (rER) Rough endoplasmic reticulum (rER) plays a major role in the synthesis of membrane-bound and secreted proteins. The de novo synthesis of milk proteins such as casein, α-lactalbumin, and β-lactoglobulin from plasma amino acids takes place on the ribosomes found on the cystolic surface of rER. 2 The proteins are then modified and packaged into membrane-enclosed transport vesicles called secretory granules by the Golgi apparatus. These proteins are released from the cell when the vesicle’s membrane fuses with the plasma membrane of the epithelial cell. 1

Mitochondria Mitochondria have a double membrane which encloses the intermembrane space. Their other compartments include cristae and a matrix. 3 A moderate amount of large mitochondria are found in this type of cell. A mitochondrion has an independent genome and its main role is generating adenosine triphosphate (ATP) through oxidative phosphorylation.

Smooth endoplasmic reticulum Smooth endoplasmic reticulum is involved mainly in lipid synthesis. The de novo synthesis of fat from plasma fatty acids through the malonyl coA pathway takes place here in lactating cells. 2

Process of myoepthelial cell* Myoepithelial cells originate from the ectodermis and are located between the surface of mammary gland epithelial cells and the basal lamina. 1 They are arranged in a basketlike network and are present in the secretory portion of mammary glands as well as larger ducts. 2

References: 1. Ross, Michael H., Lynn J. Romrell, and Gordon I. Kaye. Histology: A Text and Atlas. Baltimore: Williams & Wilkins, 1995. Print. 2. Vorherr, Helmuth. The Breast: Morphology, Physiology, and Lactation. New York: Academic, 1974. Print. 3. Kierszenbaum, Abraham L. Histology and Cell Biology: An Introduction to Pathology. Philadelphia, PA: Mosby Elsevier, 2007. Print. 4. Davey, John, and Mike Lord. Essential Cell Biology. a Practical Approach. Oxford: Oxford UP, 2003. Print.

Neurohormonal regulation of milk synthesis by lactating mammary gland epithelial cells Pituitary prolactin is the main stimulus for synthesis and secretion of milk, while other metabolic hormones such as cortisol, insulin, thyroid hormone, parthyroid hormone, and growth hormone play less central roles. 2 Prolactin and human chorionic somatolactotropin (hCS) levels increase as pregnancy progresses but are suppressed by estrogen and progesterone. Suckling simulates receptors in the nipple which send impulses to the hypothalamus, resulting in the inhibition of the release of prolactin-inhibiting factor. They also signal the release of oxytocin, which stimulates the myoepithelial cells. This leads to a contraction of alveolar secretory cells and the cells in larger ducts and milk is ejected from the alveoli and the ducts.1 Suckling also increases the pituitary release of adrenocorticotropic hormone (ACTH), which supports alveolar milk synthesis.

*Labeled despite not being organelles for greater clarity and understanding

Nucleus The nucleus is the storehouse for genetic information encoded on DNA, which is organized in a complex of DNA, histones, and nonhistone proteins called the chromatin. The double-membrane of the nucleus compartmentalizes the processes taking place inside the nucleus and is perforated with nuclear pore complexes (NPCs) which control traffic in and out of this organelle. The lumen enclosed by this double-membrane is continuous with that of the endoplasmic reticulum. Each nucleus contains a nucleolus where ribosome synthesis and assembly takes place.4