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

Intermediate (abdominal) The comparative anatomy of udder in mammalian Species Anterior (thoracic) Intermediate (abdominal) Posterior (inguinal) Total Goat, sheep, horse guinea pig 2 Cattle 4 Cat 8 Dog 2 or 4 8 or 10 Mouse 6 10 Rat 12 Pig 18 primates www.wikipedia.org

What is the difference between the animal udder? Cow: four glands and four teats Sheep and goats: two glands and two teats Sow: 12-14 teats and two glands per teat. Mare: four glands and only two teats.

The external anatomy the udder of cows

The udder of cows

The udder of cows

The weight of empty cows udder is about 12-30 kg. The udder weight is affected by: Age Stage of lactation Amount of milk in the udder Inherited differences among cows

Teat structure 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.

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.

The supportive system of udder

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

The supportive system of udder An illustrated view of the ligaments that permit udder suspension (Courtesy of Iowa State University)

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.

Mammary duct system

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 (150-220 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 150- 220 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)

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).

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)

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.

Major component of a secretory epithelial cell Apical membrane Secretory vesicles Golgi apparatus Tight junction Gap junction Rough Endoplasmic Reticulum Nucleus Smooth Endoplasmic Reticulum Lysosomes Basal and lateral membranes Cytoplasm Basement membrane

Mammary blood supply Milk synthesis rate depend to the rate of blood flow of udder. Blood flow in the mammary gland increase before parturition. The efficiency of extraction of the components from the blood while it passes through the udder is more important.

Mammary blood supply

Mammary blood supply

Mammary lymphatic network

Mammary lymphatic network

Udder edema Udder edema is swelling of the udder. Although it occurs to some degree in most cows at calving time, heifers calving for the first time are especially prone to having udder edema. Fluid accumulates between skin and glandular tissue, as well as in the gland. The skin is usually ~1/4" thick including subcutaneous layers, but during edema it can increase in thickness to 2". Severe edema can strain supportive structures of udder. Udder edema is often caused by an imbalance of hydrostatic and osmotic pressures, increasing fluid flow out of the capillaries into the interstitial spaces. This may occur because of damage to the capillary walls or obstruction of the lymphatic system. Don't know exactly why it happens, but from human medicine, increased salt intake, increased fluid intake, increased environmental temperature and damaged innervation can contribute to edema. Udder edema is a multi-factorial calving related disorder affecting mostly first parity dairy cows.

Udder edema Udder edema has a negative impact on the overall performance of the animal. The cause of udder edema is unknown; therefore any attempt to determine risk factors will improve the understanding of the pathophysiology and the prevention of the disease. Calf gender and calving season are important risk factors for udder edema in heifers raised under sub-tropical conditions.

Mammary nervous system

Mammary nervous system The efferent innervation of the mammary gland is entirely sympathetic in origin. Innervation of the udder is sparse compared with other tissues. Sensory nerves are involved in milk ejection and found in the teats and skins. There is no parasympathetic innervation to the gland. Sympathetic nerves are associated with the arteries but not with alveoli. There is no innervation of the secretory system. Few nerves go to the interior of the udder.