Part Two: A Hen’s Reproductive System
Biology of the Chicken, Part 2
By Thomas L. Fuller, New York
Have you ever been asked, “Which came first, the chicken or the egg?” When I was teaching reproduction in junior high science, I would fall back on my love and knowledge of poultry for examples. It was inevitable that this question would be directed to me. My answer: “The first chicken must have laid the first chicken egg.”
It was simple and usually sufficed. An egg is defined by biologyonline.org as an organic vessel where an embryo develops, and one in which the female of the species lay as a means of reproduction. The chicken reproductive system is designed to perpetuate the species while enduring heavy losses to nature. Birds do this by having the ability to produce more young than are needed for survival of the species. This reproduction ability in chickens has been cultured, selected, and controlled to produce, in abundance, one of the most nutritional foods known to man.
The reproduction system of the chicken differs significantly from that of our own reproductive system. Although most of the reproductive organs of the chicken bear similar names to mammalian organs, the chicken organs differ widely in form and function. Chickens, like most other birds, are considered prey animals in the animal kingdom. In this article we will explore a reproductive system designed to compensate for being a prey animal and still maintain the species.
Henrietta, our female chicken, has two basic parts to her reproductive system: the ovary and the oviduct. The ovary is located midway between the base of the neck and the tail. An ovary consists of ova (plural of ovum) or yolks. It is interesting to note that from the time she hatched, Henrietta had a fully formed ovary. This miniature of a mature organ already contains tens of thousands
of potential eggs (ova). Many more than she will ever produce. At this same early stage of life, our chick has two sets of ovaries and oviducts. Inherently the left side develops and the right side regresses and becomes nonfunctional in adult birds. It is not known why only one side dominates. In mammals both ovaries are functional. There have been cases in poultry when the left ovary has been damaged. In these cases the right side will develop and take over. This is another example of nature finding a way.
While Henrietta was growing up, so was her ovary and ova. Each ovum starts as a single cell surrounded by a vetilline membrane, a clear casing that encloses the egg yolk. As our pullet approaches puberty, the ova mature and additional yolk forms on each ovum. My poultry mentor, Professor Edward Schano from Cornell University, left me with a mental picture of this process I will never forget. It all begins with a layer of fat forming on a single egg cell. The next day the first egg cell gets a second layer of fat and another egg cell gets its first layer of fat. The day after that the first egg cell gets a third layer of fat, the second egg cell gets a second layer of fat and another egg cell gets its first layer of fat. This process goes on each day until there is a grape-like structure of ova of varying sizes.
At this point a pullet, or young hen, is ready to start laying eggs. The first step in this process is ovulation. Frequency of ovulation is a direct result of the amount of light exposure. With natural or artificial light exposure of about 14 hours a day, a hen may ovulate again from 30 minutes to just over an hour from the time the previous egg has been laid. Contrary to some beliefs, a hen can not lay an egg every day. If an egg is laid too late in the day the next ovulation will wait until the next day. This gives Henrietta a well deserved break. In poultry this is the start of a process that is similar to an assembly line. The mature ovum or layered egg cell is released into the oviduct. The sack that has enclosed the egg cell now ruptures naturally and the yolk begins its 26-hour trip through the oviduct. The oviduct has five divisions, sections, included in a serpentine structure some 27-inches long. These sections include the infundibulum, magnum, isthmus, shell gland and the vagina.
The beginning of the oviduct is the infundibulum. The infundibulum is 3 to 4 inches in length. Its Latin meaning, “funnel,” implies a hit or miss drop into a hoop as if our valued ovum were a basketball. Its true physiology is to muscularly engulf the stationary yolk. It is also here that fertilization of the egg would occur. It should be noted that mating has no influence on ovulation and egg production. During the 15 to 18 minutes the yolk is in this section the suspensory ligaments of the yolk known as chalaze are produced. They serve to keep the yolk properly oriented in the center of the egg.
The next 13 inches of the oviduct is the magnum. Its Latin meaning “large” appropriately identifies this section of the oviduct for its length. The developing egg remains in the magnum for approximately three hours. It is at this time the yolk gets its covering of albumin, or egg white. It is interesting to note that there is more albumin than is needed to cover a yolk at any given time. This abundance of albumin can actually cover two yolks that may have been released at the same time. It creates two formed egg yolks in one egg shell. These are the infamous “double yolkers.”
The third section of the oviduct is called the isthmus. An anatomical definition for the isthmus is a narrow band of tissue that connects two larger parts of a structure. Its function in chicken reproduction is to create the inner and outer shell membrane. Constriction occurs on the forming egg while progressing through the isthmus’ four inches of length. Our future egg remains here for about 75 minutes. The membrane has a look and texture similar to onion skin. You may have noticed the shell membrane attached to the shell when you have broken open an egg. This membrane protects the contents of the egg from bacterial invasion and prevents rapid moisture loss.
Nearing the end of our assembly line the egg enters the shell gland. It is four to five inches in length. The egg remains here for the longest time during its assembly. More than 20 hours of the 26 hours needed to create an egg will be spent in this region of the oviduct. This is where the egg’s shell is formed. Made largely of calcium carbonate, it is a tremendous drain on the body calcium of Henrietta. Almost half of the calcium needed to produce this protected shell is taken from the hen’s bones. The rest of the calcium demand comes from the feed. I am a strong believer in free choice oyster shell along with a good egg production feed. One other influence occurs at this time if the heritage of the hen dictates it. Pigment deposition or the coloring of the egg shells gives its appearance.
The last part of the oviduct is the vagina. It is about four to five inches in length. It has no part in the egg’s formation. It is however, critical to the process of laying the egg. The vagina is a muscular tube that pushes and turns the egg 180 degrees so as to be laid large end first. This rotation allows the egg to be in its strongest position for proper laying. It is nearly impossible to break an egg by simply squeezing it with one hand from end to end. Consider trying this with an egg that has no flaws and proper calcium content. Squeeze the egg from each end with both palms of your hands. However, hold it over the sink, just in case!
Just before the egg is laid, while still in the vagina, it is covered with the bloom, or cuticle. This coating seals the pores and prevents bacteria from getting inside the shell, and also reduces moisture loss. Considering chicken reproduction and not breakfast, Henrietta needs her clutch of eggs to stay uncontaminated and fresh enough for her to start incubation. This clutch may be a dozen eggs and take two weeks to produce. From the vagina the completed egg enters the cloaca and through the vent to a soft nest.
The reproductive system of the female chicken is a fascinating assembly line that produces one of the world’s most perfect foods. More importantly, if you are a bird, it affords a way to ensure survival of your species by producing a number of young with minimal care. In an upcoming article we will address the reproductive system of the male chicken, or rooster. We will also investigate some secondary sex traits as they apply to both sexes. I trust you now understand better some to the demands on our friend Henrietta in the production of an egg. It’s no a wonder she celebrates with a resounding cackle after accomplishing such a feat.