2. Lactation Physiology (part 2)
Isfahan University of Technology, Isfahan, Iran
Lactation Physiology (part 2)
By: A. Riasi
(PhD in Animal Nutrition & Physiology)

3. The udder is a complex system
A supportive system.
A secretory system composed of epithelial cells.
A duct system for storage and conveyance of milk.
Blood, lymph, and nerve systems.

4. The udder of cows
In the cow, the udder is separated into two halves by a longitudinal groove, the intermammary groove. It consists of four separate mammary glands (quarters), each gland has one teat and each teat has one opening. The right and left halves are entirely separate. The rear quarters are normally larger than the fore quarters and secrete about 60% of the daily milk yield. The milk from each gland is emptied through the teat. Rear teats are usually shorter than the fore teats.
About 25-50% of all cows have extra teats. Extra teats should be removed during calfhood.

5. The udder of cows
60% of milk production
40% of milk production

6. The supportive system of udder
A strong udder suspensory system is required to maintain proper attachments of the gland to the body. Remember that the mammary gland is a skin gland, and is therefore external to the body cavity. An average Holstein cow easily may have 50 kg (>100 lbs) of weight hanging from her body when she walks into the milking parlor to be milked. The system of ligaments and other tissues which attach the udder to the cow are critical for successful lactation.

7. There are seven tissues that provide support for the udder:
Skin (covering the gland is only of very minor support)
Superficial fascia or Areolar subcutaneous tissue
Coarse areolar or cordlike tissue
Subpelvic tendon
Superficial layers of lateral suspensory ligament
Deep lateral suspensory ligament
Median suspensory ligament
There are seven tissues that provide support for the udder:
1- Skin (covering the gland is only of very minor support)
2- Superficial fascia or Areolar subcutaneous tissue - This attaches the skin to underlying the tissue. It, too is only of minor support for the cow's udder (areolar tissue is a kind of loose connective tissue. Actually loose connective tissue includes areolar tissue, reticular tissue, and adipose tissue).
3- Coarse areolar or cordlike tissue - This tissue forms a loose bond between the dorsal surface of the front quarters and abdominal wall. These are important for keeping the fore quarters closely attached to the body wall, but are not the major support of the udder.
4- Subpelvic tendon - This tendon not actually part of the suspensory apparatus, but gives rise to the superficial and the deep lateral suspensory ligaments. It is not a continuous tissue sheet but is attached to the pelvis at several points.
5- Superficial layers of lateral suspensory ligament - These are mostly composed of fibrous tissue (with some elastic tissue), arising from the subpelvic tendon. They extend downward and forward from the pubic area. When it reaches the udder it spreads out, continuing downward over the external udder surface beneath the skin and attaching to the areolar tissue.
6- Deep lateral suspensory ligament - The inner part of the lateral suspensory ligament also arises from the subpelvic tendon, but is thicker than the superficial layer, mostly fibrous tissue. It extends down over the udder and almost enveloping it.
7- Median Suspensory Ligament (MSL) - This is the most important part of the suspensory system in cattle. It is composed of two adjacent heavy yellow elastic sheets of tissue that arise from the abdominal wall and that attach to the medial flat surfaces of the two udder halves. The median suspensory ligament has great tensile strength. It is able to stretch somewhat as the gland fills with milk to allow for the increased weight of the gland. It is located at the center of gravity of the udder to give balanced suspension, so that even if rest of the layers are cut away except for the median suspensory ligament, the gland stays balanced under the animal.

8. An illustrated view of the ligaments that permit udder suspension
When the cow is viewed from behind, a distinct midline groove marks the position of median suspensory ligament (MSL). The tissues of the MSL provide the necessary elasticity needed by the udder as it fills with milk to expand away from the body.
Lateral Suspensory Ligaments (LSL): These structures contain more collagen than elastin and thus provide support but not elasticity to the udder. The LSL arise from the tendons above and posterior to the udder and extend along both sides of the udder providing support to the
interior of the udder. The LSL extend to the midline on the floor of the udder joining the MSL at the base of the udder. However, they don’t join under the bottom of the udder.
An illustrated view of the ligaments that permit udder suspension
(Courtesy of Iowa State University)

9. Teat structure Annular (cricoid) rings Furstenburg’s rosette
Cows have four teats (one teat drains on gland). Teat is otherwise known as Papilla mammae. The skin of the teat is characterized by the absence of hair and glands (sweat or sebaceous glands). Teat size and shape is independent of the size, shape or milk production of the udder. Average size for the fore teats is ~6.6 cm long and ~ 2.9 cm in diameter. The corresponding values for the rear teats are 5.2 and 2.6 cm, respectively.
Teat (streak) canal: About 7 to 16 mm long. It is closed by circular sphincter muscles. The streak canal retains the milk in the udder against the pressure developed by the accumulation of milk. It is also the main barrier against infection and is lines with skin like epidermis cells that produce a bacteriostatic secretion.
Furstenburg’s rosette: Mucosal folds of the streak canal lining at the internal end of the canal. It folds over the canal opening due to pressure when the udder is full, which aids in retention of milk. It has a protective leukocyte population which are thought to leave the teat wall and enter the cistern via Furstenberg's Rosette. It contains bactericidal cationic proteins (e.g. ubiquitin).
Annular (cricoid) rings: Located at the proximal end of the teat cistern, marking the boundary between the teat cistern and the gland cistern.
Teat cistern: The cavity within the teat and is located just below the gland cistern. It is continuous with the gland cistern. It is lined with numerous longitudinal and circular mucosal folds, which from pocket on the inner lining of the teat.

10. Teat structure

11. Interior anatomy of the Mammary Gland
The interior structure of mammary gland:
Connective tissue (Stroma)
Ductular system
Secretory tissue
The mammary gland is a complex organ that proceeds through the early embryonic stages, into pregnancy, lactation and finally regression.
Structure of the mammary gland:
1- Connective tissue
2- Ductular system
3- Secretory tissue
Connective tissue consists of fibrous tissues made of collagen. This vascularized connective-tissue stroma contains lipid-depleted adipocytes and fibroblasts. The function of the connective tissue is to support the ductular and the secretory tissues. A desirable udder should contain a minimal amount of connective tissue and maximal amounts of serectory tissue.

12. Mammary duct system
The ductular system of the mammary gland starts at the alveoli and ends at the streak canal. No modification of the milk composition occurs once it has moved into the duct. Milk is first collected in the lumen of the alveoli and secretory (terminal) ductules. It then drains into larger ducts until it enters the primary (mammary) ducts attached to the gland cistern. The gland cistern is the largest collecting point of milk. The gland cistern is linked to the teat cistern. The two cisterns are partially separated by an annular fold. Usually ducts lead into each gland cistern, but as many as 20 ducts can be found. Milk in the teat cistern is prevented from escaping to the outside by a sphincter muscle which contracts around the teat (streak) canal through which milk is drawn to the outside. The size and the shape of the gland cistern of each quarter vary considerably. The capacity of the gland cistern varies from ml of milk. No relationship exists between the size of the cistern and the amount of milk secreted by the quarters.

13. Secretory tissue (Adapted from Akers & Denbow, 2103) Secretory tissues
The secretory epithelial cells comprise about 1/2 of the total cells in the secretory tissue during lactation. Secretory tissue is arranged into lobes with each lobes consists of many lobules. Each lobule containing clusters or groups of alveoli ( alveoli) which are surrounded by a network of blood vessels. Each alveolus has all the components required to produce and deliver milk to the duct system. Each alveolus consists of a single layer of epithelial cells (secretory cells) surrounding the central lumen into which the epithelial cells eject the milk they synthesize.
Alveoli (acini): Alveoli are sack-like structure where milk is synthesized and secreted. The lumen of the alveolus is lined by a single layer of secretory epithelial cells. The epithelial lining is surrounded by contractile epithelial cells (myoepithelial cells), which contact in response to the hormone oxytocin, resulting in milk being squeezed out of the alveolar lumen and into the small ducts. Outside the myoepithelial cells, the alveolus is surrounded by a connective tissue basement membrane. The capillary bed on the outside the alveolus is part of the stromal tissue between alveoli.
Clusters of alveoli are encapsulated by a connective tissue sheath and are organized as a lobule (about 0.7 – 0.8mm diameter in cow). Group of lobules are surrounded by a connective tissue sheaths and comprise a lobe. Each mammary gland is made of numerous lobes.
(Adapted from Akers & Denbow, 2103)

14. Mammary alveolus. This diagram illustrates the three dimensional structure of the mammary alveolus. The hollow center of the alveolus provide a space for the accumalation of milk components that have been synthesized and secreted by the secretory cells that compose the internal wall of the structure. The outside of the alveolus has a network of myoepithelial cells that contract in response of release of oxytocin at the time of milking. This forces stored milk into the terminal duct, which exits the lumen the alveolus. The milk progresses through larger ducts to be emptied at the nipple or teat end. (Adapted from Akers & Denbow)

15. A photomicrograph of a developing mammary duct
A photomicrograph of a developing mammary duct. Taken from a Holstein calf, this tissue stained with specific cytokeratin 18 (red, a marker specific for epithelial cells), CD10 (green , a marker of myoepithelial cells), and Ki67 (yellow, a protein produced in nuclei of cells that are about to divide). The tissue section is from a study to evaluate the effects of the ovary on ontogeny of myoepithelial cells in the bovine mammary gland. (Adapted from Akers & Denbow, 2103)

16. Secretory tissue
A lactating secretory cell is the basic unit of milk synthesis
Milk precursors are taken from the blood into the cell
The secretory cell have two kind of junctions with neighbor cells:
Tight junction around the apical portion
Gap junction in lateral portion
A lactating secretory cell is the basic unit of milk synthesis. Milk precursors are taken from the blood into the cell through the basal and the lateral membrane and milk is discharged into the lumen through the apical membrane. Individual cells are joined to their neighbor cells on all sides by tight junctional complex structure located around the apical portion of the apical portion that forms a tight barrier, which prevents the passage of materials between cells under normal conditions. Secretory cells are probably also bound to adjacent cells through gap junction, which allow low molecular weight materials to pass from one cell to another. The intracellular exchange may help a given alveolus to synchronized milk discharge from various secretory cells into the lumen.

17. Major component of a secretory epithelial cell

18. Major component of a secretory epithelial cell
Apical membrane
As expected for highly active secretory cells, the cytoplasm of lactating alveolar cells is filled with numerous mitochondria and an extensive rough endoplasmic reticulum network. In addition there is a well-developed Golgi apparatus, and secretory vesicles containing casein micelles are present in the apical region of the cell.
1- Nucleus The nucleus is somewhat located at the basal half of the cells. It contains the genetic materials that control the functions of cell as well as the genetic information for the synthesis of milk protein and several enzymes.
2- Endopalsmic Reticulum Membrane-bound channels that begin with the nucleus and extent through the cytoplasm. They are the site of lipid synthesis and part of the flow of membranes that form inner vesicles. The ER has two parts; A- Rough ER: Covered with ribosomes, and are the sites of milk protein (export protein) biosynthesis. B- Smooth ER: Devoid of ribosomes and serve as intracellular channels for transferring materials within the cell.
3- Golgi Apparatus Site of protein packaging (addition of Ca & P to casein, formation casein micelles). It is also the site of lactose biosynthesis. Other non-fat milk constituents are also packed in the Golgi apparatus before being secreted. The golgi apparatus usually just apical to the nucleus and most of the secretory structures (secretory vesicles, fat droplets) generally on the apical half of the cell.
4- Secretory vesicles Originate in the Golgi apparatus to carry non-fat constituents and vesicular membranes to the apical membrane
5- Lysosomes Contain enzymes which degrade unwanted materials in the cell. The enzymes are released during involution (e.g. dry period or during mastitis).
6- Cytoplasm The fluid matrix of the secertory cell. It includes enzymes, macromolecules, nutrients. It is also the site of anaerobic breakdown of glucose.
7- Junctions: Structures that join adjacent cells. a- Tight junctions: Located around the apical portion of the cell. Join adjacent cells and form tight barriers, which stop the passage of materials between cells. b- Gap junctions: Join adjacent cells and allow materials of low molecular weights to move between cells.
8- Membranes: a- Basal and lateral membranes: Milk precursors are taken from blood into secretory cells through these membranes. b- Apical membrane: Milk is discharged through this membrane into the lumen. c- Basement membrane: Milk precursors coming from the blood through this membrane and are located in the interstitial space below the secretory cell. The basement membrane is a thin layer of connective tissue proteins that provide structural support for the epithelial cells.
The basal side of alveolar epithelial cells contacts myoepithelial cells and the basement membrane, which separates the epithelial compartment from the stroma and the vascular system. Thus, there are a number of potential barriers to the transfer of exogenous substances from blood or stromal cells: (1) vascular or stromal membranes; (2) the basement membrane; (3) basal epithelial membranes; (4) paracellular junctional complexes; (5) Golgi membranes and (6) apical epithelial membranes.
Secretory vesicles
Golgi apparatus
Tight junction
Gap junction
Rough Endoplasmic Reticulum
Nucleus
Smooth Endoplasmic Reticulum
Lysosomes
Basal and lateral membranes
Cytoplasm
Basement membrane

19. Different membrane of a epithelial cells
Basal membrane
Lateral membranes
Milk precursors are taken from blood into secretory cells through the basal and lateral membranes.
Apical membrane
Milk is discharged through this membrane into the lumen.
Basement membrane
Milk precursors coming from the blood through this membrane and are located in the interstitial space below the secretory cell.

20. Secretory tissue
The basal side of alveolar epithelial cells contacts myoepithelial cells and the basement membrane.
The potential barriers for transfer of exogenous substances from blood or stromal cells:
vascular or stromal membranes
The basement membrane
Basal epithelial membranes
Paracellular junctional complexes
Golgi membranes
Apical epithelial membranes