Bovine biology series
Part - 35 Mammary gland (1/5)
Just over 20 years ago, I drove into the parking lot of a slaughterhouse in Visalia, California. I walked into the office with four large garbage bags.
I ask for four udders, tissues that would be placed in the bags and taken back to campus.
The day before, I had ask a dairyman in Tulare if I could look at the cows he was shipping to this slaughterhouse. I identified four cows, took pictures of them, and now I was about to obtain their udders.
These things were heavy, somewhere between 50 to 80 pounds. Well, I placed them in the trunk of the car and drove back to Cal Poly.
Once there, I placed each udder on the lab counter, and using a rolling and squeezing action, physically removed as much milk and blood as possible from the openings. Next, I mixed a solution of 10 percent formalin and 90 percent distilled water and perfused the udder with a positive pressure, filling the cavities of space and vessels with this preservative.
Then I suspended the udder in a fish tank for two weeks. The tissue turned a pale cream color, and the tank liquid of two- percent formalin turned a murky white.
At the end of the preservative period, I drained the tank, and emptied the udder the best I could by once more physically rolling and squeezing the solution out.
I borrowed a very sharp meat cutter and cut each quarter in half, drawing the knife through the central portion of each teat, halving it in such a way that we could see a cross section of the teat end, teat orifice, and gland cavity.
I photographed the whole thing and it became my Senior Project that enabled me to graduate and become a biologist.
What a wonderful learning experience for me. For I remember that when my turn came in Dr. Herman Rickard's Senior Project lab, I got to put on a long white coat, the kind medical doctors wear, and talk all about tissue preservation, the various structures and tissues and vessels of the udder tissue. Finally, I discovered with my own eyes the very tissue that is secretory, from which blood is coming in and going out, leaving the accumulation of a liquid that is milk, as the udder tissue expands and expands further until there is no more secretion as intramammary pressure sufficiently ceases the making of milk.
Such a lesson is one of the most formative in my life, for really, the act of holding an udder in my hands, visually observing vessels and cavities and secretory tissue full of milk is clearly an unforgettable one.
With this story I introduce our lesson for the next several months.....the mammary glands.
The word mammary is derived from the Latin word "mammarius" and that word is related to the root Latin word "mamma" that has two meanings: mother and breast.
The simplest form of the mammary gland is found in the Monotremes. This word is used in describing the lowest order of mammals. They nourish their young with a mammary gland devoid of a nipple, as milk is withdrawn from the gland through a series of tubules that are connected to hairs on the gland's exterior surface. The young suck on these hairs, and milk flows from the gland, on the surface of the hair, and into the baby’s mouth.
There are two examples of these Monotremes, the spiny anteater and the duck-billed platypus. These mammals actually lay eggs, and upon hatching, the young snuggle up to mom and nurse milk through these hairs on the gland.
The next level of sophistication is found in a mammal called the Marsupial. The young in these mammals are born alive instead of in an egg. They find their way into a pouch where they become attached to the tubular teats of the mother. Examples of marsupials are opossums, kangaroos, wallabies, koala bears, and a wombat (a small bear-like creature found in Australia).
The more advanced forms of mammary glands are found in the higher animals, those growing their young in a uterus with a placenta. The number of glands and the location of the teats vary with specie, but generally the determinant factor is the number of young born at one time.
In a mammary fetus, setting within the confines of a uterus and enveloped in the warm, placental membranes, we may barely find the mammary line, a bit of tissue upon the skin's surface that is defined by a slightly different type of epidermal skin. This mammary line is the forerunner of the developed mammary gland.
At this point in the fetus, the cells near the mammary line are very similar to other secretory glands, namely the sweat glands. That is, in this bit of epidermal skin tissue there becomes some form of vascularization, where increased blood flow will supply cells that perform the role of sifting through the blood and taking nutrients, accumulating them and discharging them into an opening or sinus, until they can be removed from the body physically.
Thus, the mammary glands are derived from epidermal - skin tissue. The mammary gland bud, that very small protrusion of skin-like tissue in the fetus can be divided into two distinct types of dermal tissue.
The more inward and thus deeper tissue type is called the mesenchyme. This tissue contains the blood vessels, the nerve cells, and the supporting-connective tissue that will feed, interpret the environment, and keep the gland securely in place.
The more outward portion of the mammary bud, the ectoderm, contains the developing cisterns, openings and ducts.
During the later stages of fetal growth, the mammary bud elongates, while in the mesenchyme blood vessels increase in number and size. Interestingly enough, in the fetus, the mammary bud actually develops a primary sprout, characterized by a physical split, the actual dividing of the mammary bud so that a cistern or opening can form. This is the first actual space that is made so that milk can be stored. Further, secondary sprouts begin developing. These sprouts become the ducts that will drain milk of the mature gland into the gland cistern formed by the primary sprout. Then tertiary sprouts occur, further adding drainage pathways for milk so that all the secretory tissue can be drained into the gland cistern. We might think of a highway system, whereas eventually all the smaller, narrower farm roads lead to bigger highways, with the freeway being the ultimate source of getting us from one point to another.
Hair follicles begin to produce hair on the surface of fetal mammary glands at four months. Throughout the remainder of gestation, fat cells are grown in the mesenchyme region. At birth, the mammary gland is a sort of fat pad, and in the calf, it can be felt by careful palpation.
From birth to puberty is a relative quiet time for mammary growth. This is no surprise, really, for the body is growing in size, the brain is developing and the muscle-skeleton frame, as well as the internal organs are rapidly increasing in weight.
Puberty, however, does bring about a tremendous change, largely driven by the internal clock that signals the hormone system that now the body is ready to reproduce.
There is some influence of estrogen on the immature mammary gland before puberty. Estrogen is responsible for increased duct growth and development of the connective tissue that later will support the weight of the mammary gland.
At puberty, estrogen secreted from the now functional ovary further develops the duct system in rapid manner. With each subsequent estrus, marked by the secretion of estrogen so a follicular egg may be formed, the duct system is developed further. Additionally, prolactin and somatotrophin from the anterior pituitary gland (brain) supplant the work of estrogen by elongating and branching further the network of ducts.
Progesterone, the pregnancy hormone, plays a role as well. This hormone, secreted by the corpus luteum, stimulates the growth of the secretory tissue, the alveolar cells where milk is made.
If a heifer continues to cycle and is not bred, the immature mammary gland develops further with each estrus cycle. The result can be some discharge of a watery type of milk like fluid from the gland. The udder will in fact be comprised of four quarters, each individual units of production. The watery fluid will have some of the regular components of milk, lactose, casein and fat.
But the virgin heifer's mammary gland never fully develops into a mature gland until pregnancy occurs. The change in hormone type at pregnancy and especially very near the time of calving represents dramatic physiological and physical change in the mammary gland, as we know that defines a springing heifer.
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