Breeding cycle

In the life of placental mammals generally, there are periods where the ovary becomes regularly active in the adult female. These cycles are periodic, and coincide with mating, and production of young. Each is known as breeding cycle. The period of such cycles differs in various mammals:

• In case of lion and tiger, the cycle is annual.

• In case of cats and dogs, there are two breeding cycles per year.

• In case of rabbits and cats, the breeding season occurs frequently per month.

• In Human being, breeding cycle is known as the menstrual cycle which lasts for 28 days. The two ovaries alternate with each other to produce the matured ova.

The menstrual cycle:

This cycle is classified into three phases:

1. Phase of proliferation:

The pituitary gland releases the Follicle stimulating hormone (FSH). This stimulates few follicles to develop in the ovary, to form a matured one which is known as Graafian follicle, inside which the ovum grows and matures within 10 days. Graafian follicle secretes Estrogen hormone which stimulates the growth of the Endometrium.

2. Phase of ovulation:

The anterior lobe of the pituitary gland secretes the Luteinizing hormone (LH) in the 14^th.day from the beginning of the previous menstruation. This stimulates the ovum to liberate from the Graafian follicle. So, the Graafian follicle is transformed into the Corpus luteum, which produces Progesterone and Estrogen. These hormones act to increase the thickness of the Endometrium, and its blood supply. This phase lasts for 14 days.

3. Phase of menstruation:

• If the ovum is not fertilized:

The Corpus luteum degenerates gradually, and its secretion of Progesterone stops, so the Endometrium degenerates and the blood vessels tear due to the successive contractions of the uterus. Thus menstrual bleeding takes place, which lasts for 3 – 5 days. And new cycle for the other ovary begins.

• If the ovum is fertilized:

The Corpus luteum remains to secrete the Progesterone and Estrogen, these inhibit ovulation, and thus the menstrual cycle stops till after birth. The Corpus luteum reaches to its maximum growth at the end of the 3^rd. month of pregnancy. Then it starts to degenerate during the 4^th. month, when the Placenta that grows in the uterus takes over its function in secreting Progesterone, that preserves the endometrium, and stimulates the maternal mammary glands to develop gradually. So, if the Corpus luteum is removed before the 4^th. Month, abortion occurs.

Fertilization

It’s the fusion of the male gamete (sperm) with the female gamete (ovum) to form the zygote, which divides forming the embryo.

Events of fertilization:

1. After the ovum is released in the 14^th. day from the beginning of menstruation. It will be ready for fertilization through 1-2 days. Fertilization takes place in the 1^st. third part of the Fallopian tube.

2. The number of sperms ranges between 300 – 500 million per ejaculation, many of them are lost during their journey to the ovum, therefore the Human male is considered infertile if the number of sperms is less than 20 million.

3. The sperms share in secreting the Hyaluronic enzyme which dissolves part of the ovum coat through which one sperm only enters (the sperm and the neck only).

4. The sperms can stay alive about 2 – 3 days inside the female genital system.

5. After fertilization, the ovum surrounds itself with a coat, that prevents the entrance of any other sperm.

Pregnancy and embryonic development:

1. By about 24 hours after fertilization, the zygote converts (by mitotic division) to two cells (two blastomeres) in the upper part of fallopian tube, then to four cells in the next day.

2. The cellular division speeds up to form a mass of small cells, known as morula. Morula is pushed along the uterine tube by ciliary action and muscle contraction till it reaches the uterus.

3. In the uterus, morula is implanted among the folds of the thick-walled uterus (the endometrium), at the end of the first week. The endometrium is characterized by rich blood supply necessary for the development of embryo along the nine months of pregnancy.

The embryonic membranes:

The rate of embryo growth is increased. Organogenesis, and tissue differentiation gradually takes place. Two embryonic membranes are formed which are the Chorion, and the inner one is the Amnion:

1. The Amnion:

It surrounds the embryo with a fluid that serves to protect the embryo against shocks, and dryness.

2. The Chorion:

It surrounds the amnion, its function is to protect the embryo. Finger-like projections grow from the chorion membrane to be inserted in the endometrium in which the capillaries of both embryo and mother touch, these intermingle forming the Placenta.

The Umbilical cord:

The embryo is connected with the Placenta by the Umbilical cord; its length is about 70 cm and increases to give more freedom for the motility of the embryo. The Umbilical cord is a tissue rich in blood vessels which transfer digested food, Vitamins, water, salts, and Oxygen from the Placenta to the embryo’s intestine, and it transfer the excretory wastes and Carbon dioxide from the embryo’s intestine to the Placenta.

The importance of the Placenta:

1. It transfers digested food, water, Oxygen, and Vitamins form mother’s blood to the embryo’s blood by diffusion, and get rid of the embryo’s excretory wastes. Foetus blood does not normally mix with mother blood.

2. It secretes the Progesterone hormone at the beginning of the 4^th. month of pregnancy when the Corpus luteum degenerates and Placenta becomes the source of the Progesterone hormone.

3. It transfers the drugs, and harmful substances such as Alcohol, Nicotine, Viruses from mother’s blood to the embryo, these cause great harms, serious deformities, and diseases to the embryo.

Stages of the embryonic development:

The period of the embryonic development is classified into three stages, which are:

1. The first stage:

It includes the 1^st. three months of pregnancy, in which:

- The nervous system and the heart start its development. (in the 1^st. month)

- The hands and eyes become differentiated.

- The two sexes become differentiated. (the testes are developed in the 6^th. week, and

the ovaries are developed in the 12^th. week)

- Response to stimuli becomes established.

2. The second stage:

It includes the middle three months, in which:

- The development of the heart is completed, and its beats can be heard.

- Ossification and support of the skeletal system takes place.

- The sense organs are completed.

- The growth of the embryo in size increases.

3. The last stage:

It includes the last three months, in which:

- The development of the brain is completed.

- The growth slows down.

- The development of the other internal organs is completed.

Events of the ninth month:

a. The Placenta dissociates gradually, thus Progesterone decreases in its secretion, and the attachment between the foetus, and the uterus becomes loosened preparing for birth.

b. Labor begins with series of contractions in the uterine wall till the foetus is expelled outside. The baby starts his life with a distinct cry, which stimulates the respiratory system.

c. The Placenta separates from the uterine wall, and moves outside the body, then the umbilical cord is cut from the baby side, and the food supply of the newborn changes to the mother’s milk. A pituitary hormone stimulates the production of milk in the mammary glands of the mother, milk flows out to feed the baby with the most valuable nutritive and emotional supply, protecting him from many somatic and psychic diseases in his future.

N.B.:

1. The suitable female age for pregnancy ranges between 18 and 35 years. If the age decreases or exceeds that range, both the mother and the embryo will be subjected to serious problems, and the possibility to produce deformed babies will increase. On the other hand, if the father age is too old, similar results will occur to his babies.

2. The time interval of pregnancy differs according to the living organism:

- It is 21 days in case of rats.

- It is 150 days in case of sheep.

- It is 270 days in case of Man.

- It is 330 days in case of cattle.

- It is 900 days in case of elephant.

Means of contraceptive:

Several contraceptive methods are used in birth control:

1. The pills:

They contain a combination of synthetic Estrogen and synthetic Progesterone hormones. A woman starts using them after the menstrual cycle and continues for 3 weeks. These pills prevent pregnancy by inhibiting ovulation.

2. The intrauterine device (the coil):

It is inserted into the lumen of the uterus. It prevents the fertilized ovum to be implanted in the uterus.

3. Condum:

It prevents the sperms from entering the vagina.

4. Surgical sterilization:

It involves the ligation of the two fallopian tubes in the woman and cutting them. So, fertilization does not occur for the ova produced by the ovary. In a similar way, each vas deferens is tied, separated from other structures and is cut in case of males.

Multiple births:

Usually, one baby is born in each birth. Sometimes, multiple births take place reaching six babies in the same time. The most common is the twins. The international percentage of twins is once in about 86 births. Multiple births are rare. There are two types of twins:

1. Fraternal (Dizygotic) twins:

Two matured ova are liberated (from one or both ovaries) at the same time. The two ova are fertilized (with two sperms), and each one will develop to an independent embryo. The two embryos differ genetically from each other. Each embryo has its own embryonic sac, and separate placenta in the uterus. They never exceed brothers and sisters of the same age.

2. Identical (Monozygotic) twins:

This type of twins results from the fertilization of one ovum by a single sperm. The developing cells separate into two masses of cells, each of which develops independently. They form two embryos, identical genetically and share one embryonic sac and mostly one placenta. They have the same genetic characters. These twins may be born partially attached to each other in some place in the body, a case known as Siamese twins which can be surgically separated in most cases.

Test tube babies:

A matured ovum is obtained from a wife’s ovary, and being fertilized externally with the husband’s semen inside test tube in a certain nutritive medium till reaches to the morula stage. Then it is implanted in the wife’s uterus to complete its embryonic development till birth.

The embryo culture experiments:

Experiment No. 1:

Experiments have been carried out on rabbits, and mice. Cells from early embryonic stages were separated, and each embryonic cell was cultured in a female uterus to produce a complete individual. This can be considered as Asexual reproduction.

Experiment No. 2:

Two early embryos (8 cells stage) were removed from two different mice. The two embryos were cultured together in a nutritive medium, then they were pushed to fuse into single embryo. Then it was implanted in the uterus of another female mouse. It was developed and born normally. This newborn is produced from two mothers, two fathers, besides the foster mother.

Experiment No. 3 (Renucleation):

Very fine surgical instruments were used to remove nuclei from an Amphibian (toad) embryonic cells at different stages of development, and to transplant them into unfertilized eggs of the same species, whose nuclei had been removed or destroyed by radiation. Normal development was proceeded giving rise to individuals identical in characters to the individual from which the cultured nuclei were taken. This proves that the early embryonic cell nucleus is capable to direct the embryonic development in a manner similar to that of the zygote nucleus itself.

Gamete banks:

These banks are used to store selected animal gametes especially those of cattle and horses to keep them available for reproduction till the time of need. The gametes are stored in a frozen conditions (- 120° C) for up to 20 years. After that period, they can be used in artificial fertilization, even after the death of their producer individuals, or if some rare animal species are liable to extinction. Some people desire to store their gametes in such banks, to ensure the continuity of their generations, even after their death with several years.

On farm animals, researches are carried out to control the sex of their newborn. It is possible to separate the sperms with (X) chromosome from sperms with (Y) chromosome by laboratory means such as centrifugation, or exposure to a limited electric field. This technique is applied on cattle to produce only males for meat, or females for reproduction, and milk production as required by breeders.

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