Bovine biology series
Part - 36 Mammary gland (2/5)
We continue this month with a study of the mammary gland, actually a study of the mammary glands, plural, for there are four glands in the dairy cow.
Last month we examined the birth of the udder tissues in the newborn calf and followed the maturation of these tissues into the secretory tissues which define a cutaneous gland, or skin-like gland. Remember that the mammary glands closely resemble another type of cutaneous gland, the sweat glands that help cool the body by delivering water to the surface of the skin so that it can evaporate, carrying along with it heat of work or digestion.
Let us begin this month with a description of a fully developed udder, one that is secreting milk following calving and thus is lactating. The udder is defined as follows: consisting of four glands, four teats, which can be described as two halves, the left and the right. The quarters are distinct; that is, they are separated by connective tissue, and each quarter consists fully of secretory tissue, ducts and sinuses-cisterns that fill with milk, blood systems for nutrient delivery and removal of by-products, and a teat opening from which milk is withdrawn by nursing or mechanical withdrawal.
In addition, there are usually supernumerary teats. These are associated with a small gland that usually is almost indistinguishable. In some cases these extra teats are removed, although rarely do they produce any secretion. They are capable of obtaining an infection, however, for just like the normal teat opening leading to a gland or quarter, once pathogenic bacteria breach that opening and get inside, they can cause the onset of infection if the defense mechanism cannot eradicate the invader. In most cases, however, these extra glands, if secretory at all, are for just a short while, usually immediately after calving.
The empty weight of the lactating udder is quite variable, but measures have ranged from 14 to 32 kilograms or 31 to 70 pounds. Capacity of the udder in not necessarily closely correlated with empty udder weight since the ratio of secretory tissue to connective tissue also varies widely. The weight and capacity of the udder usually increase until the cow reaches maturity, usually the fourth lactation.
The two halves of the udder left and right, are separated by the median suspensory ligament, which is formed by tough, slightly elastic connective tissue originating from the abdominal tissue. This is a remarkable ligament, for it provides the balance that suspends the udder from front to back, suspending it perfectly between the legs and yet there is some flexibility so that as the cow ages and especially following years of tremendous milk volumes weighing heavily on this ligament, it remains very strong because it can flex slightly, back and forth. If the median suspensory ligament did not have this degree of flexibility, then very soon after freshening, the full udder weight of milk would stretch the ligament further and further until the udder became so pendulous, we could not milk it.
Additionally, there are the ligaments that run near the outside of the udder, beneath the skin. The lateral suspensory ligaments, of which there are two, a left one for the left half and the right one for the right side of the udder, consist of fibrous, non-elastic connective tissue. The lateral suspensory ligaments are attached to the pelvic area, that region sitting higher than and further behind the median suspensory ligament. The lateral suspensory ligaments are the primary supporting structures of the udder. You may think of them as a bread sack, encompassing the entire udder half like plastic wrapping around a loaf of bread. Where the plastic is tied is analogous to the pelvic area.
The skin provides an insignificant amount of support; rather its function is protection and thus serving as the first line of defense for the more delicate tissues found beneath.
The lateral suspensory ligaments are tied to the median suspensory tissues near the bottom portion of the udder. Thus, in order that this remarkable gland, when empty at 50 pounds or so, can contain another 50 pounds of milk as the cow walks into the parlor, is quite remarkable. A 100 pounds of weight is suspended like a weight between the legs, but the truly remarkableness of this cow is the fact that this happens twice daily, or even more, day after day, year after year. The flexibility of the median suspensory ligament, its ability to remain strong so that the udder stays in place as indicated by the presence of the intramammary groove and properly placed teats, and the lateral suspensory ligament, which does not flex, so that the udder retains its genetic conformity without falling apart quickly after a few udder-fulls of milk might make it pendulous, work in concert so that we have a productive, functional cow year after year.
The teat has a small cistern terminating with the steak canal, the teat opening. Radiating upwards from the streak canal into the teat is the structure known as Furstenberg's rosette. This small tissue is composed of seven or eight loose folds of skin like dermal tissue with a layer of connective tissue; the purpose is helping retain milk from leaking out of the teat. As the weight of milk volume increase, the Furstenberg's rosette layers serve as a barrier, holding the milk back from the teat opening.
The primary milk retaining tissue is the sphincter muscle found at the just-inside portion of the teat opening. This muscle is involuntary as defined by nervous control; the cow cannot think about relaxing this muscle, it must be constantly constricted. However it does relax in times of milk flow, as the calf nurses or the machine pulls milk via negative air pressure, vacuum, through the teat opening. As we all know fully, immediately after milk is drawn through the teat opening, the sphincter muscle is relaxed in its maximum point, over time the muscle constricts back to normal.
Whereas the properly functioning sphincter muscle is another first line defense mechanism for keeping bacteria away from the teat cistern, immediately after milking it is less effective, so we keep cows standing after leaving the milking parlor so that this sphincter muscle can close to its normal size. But we also know that age and the many, many milkings do tend to relax this muscle to the point that in some cows, they milk faster as the opening is greater, but they can more easily admit bacteria as the physical diameter of the teat opening is larger.
In fact this is one reason for milking cows three times a day, for it seems to me in cows producing large amounts of milk, 8-10 gallons daily or more in which the udder reaches its maximum intramammary pressure by eight hours and due to this pressure and the relaxation of the sphincter muscle, the cow becomes a leaker, removal of milk is superior to having a cow sit in a free stall, leaking milk, and exposing the teat opening to potential pathogen bacteria. Although we know there are other things to consider than just this fact when deciding milking strategies.
The teat cistern is the open space in the central portion of the teat. We know that in hand milking a cow, it is this volume that we force out through the teat opening by closing off the top of the teat cistern and under pressure, the milk is forced throughout the teat bottom. The milking machine draws this milk out by creating negative air pressure, or vacuum.
The teat cistern is directly connected to the gland cistern, which is larger is space, can hold more milk, and most importantly, is the ultimate culmination of all the ducts in the udder secretory tissue. For just like the blood vessels in the body either lead to or from the heart, the duct system from the point of the gland cistern eventually reaches all the secretory tissue so that the milk can be drained from the udder.
One interesting thing I did not know about the juncture of glands is one just learned.....that at the point of intersection, most ducts have a constriction, or reduction is physical size. Why? Well, this constriction helps milk stay in place, so to speak, so that milk does not flow into the gland cistern by gravity. In effect, this helps serve the role of preventing leaking in high producing cows.
The actual milk-producing unit is the alveolus, or the alveoli (plural). The word alveolus is derived from the Latin word " alveus" meaning "hollow".
The alveolus is actually a unit of production. It comes with oxygen and nutrient rich blood supply as supplied by an arteriole coming from the heart and lungs, the venule portion of the blood system that carries away the by-products of milk synthesis, various capillaries that feed and obtain these products to and from the blood system, the myoepithelial cell, those smooth muscles that act in response to external stimuli, we know it as milk letdown driven by oxytocin from the brain, the epithelial cell that actually makes the milk itself from blood, and the lumen or opening of the alveolus where once milk is made, the milk volume is kept in storage within this producing unit until forced out by the contracting myoepithelial cells.
The alveoli are grouped together in units known as lobules, each of which is surrounded by distinctive connective tissue. We may liken this to a cluster of grapes that must somehow be bundled together so that some form of continuity is retained; otherwise the grape units might physically move around too much and the resultant tissue damage would reduce potential milk production. In turn, the lobules are grouped in larger bundles, surrounded by more contractile myoepithelial cells that aid in milk let down.
As we have learned about milk synthesis, we know that the range of blood to milk volume is 500 to 650 to 1. Quite a remarkable ratio, in that a tremendous blood volume is required to make milk. The primary blood flow rises from the heart of course, specifically the abdominal artery, leaving the heart, making its way near the backbone of the cow, then bifurcating, splitting in two in the pelvic region becoming the left and right external pudic artery. These large arteries feed blood into the halves of the udder.
These arteries enter the mammary gland from the abdominal cavity through the inguinal canal, a literal hole in the abdominal cavity so that nerves and blood vessels may reach the udder. As they enter the mammary glands, there is a sigmoid flexure of the vessels, for they are lengthened here so that when the udder fills or eventually grows larger, there is some bit of extra blood vessel length to account for this elongation. Quite remarkable, I think, for without this flexure, the blood vessels might stretch beyond the point of elasticity and break.
Once the external pudic arteries enter the mammary gland, they divide once more into two main arteries, the one feeding the front quarter is the anterior, or cranial artery; the other one is the posterior, or caudal artery feeding the back quarter. Branches of the cranial and caudal arteries further divide until all reaches of the secretory, connective, muscular and dermal tissues are fed with fresh blood.
The blood leaves the udder by two main routes: the external pudic vein and the mammary vein. The external vein closely follows the route of the arteriole system described above, through the inguinal canal along the backbone entering the posterior vena cava and then the heart.
The other exit route is the milk vein, easily observable on the belly of a cow. This large milk vein, also called the subcutaneous abdominal vein, drains used blood from the cranial branch of the mammary veins, out the front portion of the udder. It enters the body cavity at the milk well, eventually entering the heart muscle through the anterior vena cava.
This arrangement of veins provides two direct routes for blood leaving the udder: out the top or bottom probably depending on the position (standing or laying down) of the cow.
The mammary gland lessons continue next month.
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