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An Amazing Journey

by Donald "DOC" Skinner, VMD

Fertilization occurs in the oviduct of the bitch, this is the tube that forms a tunnel so to speak that enables the egg to pass from the Ovary to the uterus. Unlike many other species, the canine ovulates the egg as a primary oocyte. This means that when the egg is ovulated it contains 78 chromosomes just like any other cell in the body. The egg must reduce its chromosome number by one half so that when it is fertilized by the male’s sperm, which already contains 39 chromosomes, it too will contribute 39 chromosomes and the fertilized egg will contain 78 chromosomes. This will provide the genetic code to produce a pup that has received half of its 78 chromosomes from the mother and half from the father.

Body cells all contain 78 chromosomes in the dog. When cells grow or replace other cells, they do so by a process known as Mitosis. There are five distinct phases in this process of cell division known as mitosis. Suffice to say the result is the formation of two new cells from the original and this is the process by which the body is constantly growing and also replacing old cells (as in the skin).

Reproductive or germinal cells contain 78 chromosomes as do body cells therefore they must reduce their number by half before they unite in the process of fertilization. This is accomplished through a division known as meiosis. In the first two phases of meiosis the germinal cells in the male and female undergo mitosis as earlier described. In the third phase the reduction of chromosomes occur by producing two cells each of which contains 39 chromosomes. From this step on the process differs in the male and female, although the next two steps involve mitotic duplication. In the male, the result is the formation of four sperm each containing 39 chromosomes from the original non-functional primordial germ cell in the testicle. This process continues incessantly in the healthy fertile male producing billions of viable sperm. In the female the meiosis reduces the chromosome number from 78 to 39 as in the male resulting in two primary oocytes. It is the primary oocyte that is released during ovulation. However the primary acolytes undergo another mitosis producing four cells as in the male but in the female only one cell becomes a functional egg the other three are non-functional, much smaller and discarded. The surviving egg contains more yolk to nourish itself after fertilization.

Bitches have quite an extended period of fertility during their heat so conception after a single natural breeding is quite high. One reason being the sperm may actually initiate the process of fertilization on the primary oocyte before it undergoes mitosis. This being the case one sperm lucks out and the other three are discarded with the non-functional polar bodies (the three discarded eggs).

The early process of fertilization is the dissolving of the eggs outer shell making it penetrable by a single sperm. This is accomplished by release of a proteolysis enzyme by the thousands of sperm swimming around the egg. The enzyme called hyaluronidase helps digest the protein shell of the egg thus making it more penetrable. Although only one sperm fertilizes the egg, sufficient numbers are necessary for it to accomplish its goal. If the egg has passed its last mitotic division and the three non-functional polar bodies have been discarded it will survive for 12 to 36 hours during which time it may be fertilized. Thus 48 hours for the primary oocyte to undergo two mitotic divisions and a life of 36 hours for the functional egg gives a fertile period of about three and one half days. In a very fertile male sperm have survived for seven days. So theoretically fertilization may not take place for a week after breeding. It has been proven that the bitch will whelp her litter 63+or-1 days after her day of ovulation, the plus or minus may be due to whether or not the sperm has initiated but not completed fertilization in the primary ocyte. Since we do not know when ovulation occurs, the breeding date is not a reliable tool in determining the time of whelping. Bitches are neither early or late in delivery based on the time of breeding only the breeding is early or late. If a progesterone assay has been run by blood test then the actual time of ovulation can be reasonably predicted, this was the method used to prove the whelping time is sixty-three days after ovulation. This test proves useful in bitches that have an ovulation time that falls out of the usual.

Since we have discussed the formation of the reproductive cells, lets get it on. Once the sperm has penetrated the egg it must make its way through the cytoplasm to the nucleus of the egg. The sperm head containing its 39 chromosomes enters the nucleus of the egg which contains39 chromosomes forming the fertilized cell or zygote containing 78 chromosomes. Once a sperm has entered the egg, the point of penetration is sealed off chemically to prevent other sperm from entering. The zygote spends 9 or 10 days for its journey out of the oviduct to the uterus. During this time the cell has undergone many mitotic divisions taking on the shape of a hollow sphere embryonically known as a blastocyst. When does a zygote become an embryo, an embryo a fetus and a fetus an individual never more controversial than now due to stem cell research. Stem cells being cells that have not genetically become specific organs bout due have the genetic code to do so. It is the genetic code carried by the D.N.A. that reduced the chromosome number in the egg and sperm and is now orchestrating the development of a pup and depending on the content of those chromosomes actually what breed, size, color etc. that develops.

The zygotes pass freely from one horn to the other so they may or may not implant in the horn that they first entered. The uterine wall undergoes changes prior to fertilization in preparation for the implanting zygote. The first of which is recognized by the bloody discharge that is excreted, this is Mother Nature doing a D&C. The uterine lining has sloughed off and is replaced by a thicker lining with a richer blood supply.

Implantation of the zygote occurs about two weeks after ovulation. After implantation the fetus will receive its nutrition from the placenta. The area of the zygote that penetrates the uterine lining, actually destroying some of the linings cells, is coded to become part of the umbilical cord, which will connect the fetal placenta to the maternal placenta. The fetal and maternal placentas have an intimate contact that is composed of four layers, uterine endothelia blood vessels, the chorion, the fetal mesenchyme and the fetal endothelium. The maternal placenta forms a girdle the full circumference of the inner uterus around the chorion (fetal placenta). Depending on the breed it may be from one to six inches wide. The edges of this girdle-like placenta hemorrhage and the blood clot so formed is the early nutrition for the developing embryo providing among other things, iron. The fetus is suspended in the fluid-filled amniotic sac. This sac is a shock absorber, bactericidal bath and a perfect environment for the growing fetus and also acts as a lubricant during the whelping. The developing fetus is protected by the mother’s body wall, the wall of the uterus, the amniotic sac and the chorion. The umbilical cord serves as a pipeline for nutrition, excretion and respiration (an actual exchange of oxygen and carbon dioxide). It is imperative that the puppy be born shortly after the separation of the placenta from the uterus or it will die from a lack of oxygen. It is also important if you are present to shake out any amniotic fluid that may be blocking its airway. The umbilical vein contains two arteries and one vein, no nerves so it does not receive any nervous stimuli from the mother. It is developing its own nervous system. The fetus has nothing to do but grow as determined by its genetic code until birth.

The fetus undergoes during its growth an Ontogeny that recapitulates phylogeny. Wow what does that mean? This is known as the recapitulation

Theory and what it means is that this growing embryo during one stage of development will resemble an aquatic form I.e. fish, an amphibian form i.e. frog, a reptile i.e. snake, a primitive mammal i.e. shrew and finally a puppy. In other words this developing fetus in nine weeks went through evolutionary changes that took his species over a billion years to experience. There are many very thick embryology texts that describe in detail the many changes that I have only cameoed in its development. So if someone comes up to you at the next field trial and says to you ontogeny recapitulates phylogeny, you will know exactly what they are talking about.

It is noteworthy to mention that the mother will only provide 5% to 10% of the pup’s immunoglobulin, which is the basis of its passive immunity. Most of the pup’s passive immunity is derived from the colostrum. Since the large molecular immunoglobulins can only be absorbed by the pup’s intestine during the first 48 hours, it is imperative that the pup nurse as much as possible during this time frame. It is also imperative that the mother has an adequate quantity of milk for the nursing puppy. Covering both agendas will help to insure a healthy litter.

The first major organ to develop is? Exactly, the heart, first as a 2 chambered pump as seen in the worm then near the end of its embryonic development the four chambered heart as seen in the adult. The lungs, kidneys, liver and intestines develop later than the heart because their functions are temporarily carried out by the mother’s placenta.

During embryonic growth some cells are differentiated to become the germ plasm or reproductive organs. In females ovaries and in males testicles are formed these organs contain reproductive cells containing 78 chromosomes but which we have mentioned earlier will through meiosis reduce to the contributing number of 39 by each sex. Thus via genetic coding nature has insured continuity of the species.

Hormones orchestrate this grand symphony of growth and preparation. The ovary produces progesterone to maintain the pregnancy until birth. The maternal placenta produces relaxin a hormone that relaxes the pelvis providing a wider birth canal. Prolactin is released to promote milk production. The endocrine system of the pup is complete to direct its physiology for a normal and healthy start.

Two cells one from the male and one from the female have united to produce in nine weeks a beagle pup. Isn’t it hard to visualize your field champion as once being a hollow sphere of cells rolling around in his mother’s uterus? It truly was an amazing nine-week journey.

Should you have a concern regarding the health of your Beagle(s), you should contact your veterinarian. All information on this site is presented solely for educational and informational purposes and should not, at any time, be considered a substitute for seeking or receiving veterinary care for your Beagle(s).