Question

Describe the path an egg takes starting in the ovary, continuing in the oviduct, and ending in the uterus. Include the following terms in your description: meiosis, ovulation, sperm, fertilization, embryo, endometrium.

Answer 1

FEMALE REPRODUCTIVE SYSTEM:

The female reproductive system consists of pair of ovaries and oviduct, the uterus, vagina, external genitalia and mammary glands. All these structures have evolved to perform the function of ovulation, fertilization of an ovum by a sperm, support of developing embryo and foetus and the birds and care of the infant. The ovarian follicle is the basic structural unit of the ovary. Each ovary contains number of follicle depending on the age of a female.

OVULATION:

During menstrual cycle 20 large tertiary follicle form in both the ovaries. But generally only one of these follicles will ovulate. When germ cell begin to appear in the ovaries they undergo the process of meiosis to form haploid cells with 23 chromosomes. When haploid egg fuses with the haploid sperm the diploid condition again appears. The process of ovulation/oogenesis result in the formation of female gamete called egg or ovum.

MEIOSIS:

At birth, the ovaries of a female infant contain primary oocytes in primordial follicles and  these primary oocytes are arrested with the first meiotic division and this first meiotic arrest is maintained in the oocytes of follicles of all stages except the most mature.

The membrane of the Graffian follicle oocyte nucleus disintegrates, in a process called germinal vesicle breakdown. Here the first meiotic division is completed and produces tiny ( polar body) and large( secondary oocyte) two haploid daughter cells. The secondary meiotic division in the secondary oocyte is completed after the entry of sperm into the ovum.

PATH OF THE EGG:

The egg released from the ovary passes into the oviducts where the infundibulum helps in collection of ovum after ovulation.

OVIDUCTS:

The oviducts are paired tubes extending from near each ovary to the top of the uterus.Nearest the ovary is a funnel-shaped portion, the oviductal infundibulum. The opening into the infundibulum, into which the ovulated egg enters, is the ostium. Estrogens cause secretion of mucus in the oviducts; they also cause the oviductal cilia to beat faster and the smooth muscle to contract more frequently.

UTERUS:

Uterus is a single, inverted pear-shaped organ situated in the pelvic cavity above the urinary bladder and in front of the rectum. The uterus is supported by bands or cords of tissue, the uterine ligaments. The dome-shaped region above the points of entrance of the oviducts is the uterine fundus. The wall of the uterine fundus and corpus has three layers of tissue. The outer perimetrium and middle myometrium and inner to it is Endometrium. This endometrium layer can be divided further into an internal surface layer, the stratum functionalis and a deeper layer, the stratum basalis. The stratum functionalis is shed during menstruation. The underlying stratum basalis is not shed during menstruation but contains blood vessels that produce part of the menstrual flow.

FERTILIZATION:

The egg is released into oviduct wait for the entry of sperm for fertilization.The motile sperms swim and pass through cervix enter uterus and reach ampullary region of the fallopian tube. The ovum released by the ovary is also at the ampullary region where fertilization takes place. The process of fusion of sperm and ovum is called fertilization. During fertilization sperm comes in contact with the zona pellucida layer of the ovum and induces the changes in the membrane that block the entry of additional sperms. It ensure that only one sperm can fertilize and ovum.

The haploid nucleus of sperm and ovum fuse together to form a diploid zygote. The mitotic division starts as zygote moves through the isthmus of oviduct  called cleavage towards uterus and forms daughter cells called blastomeres( morula) . The morula divide and forms blastocyst. The blastomere in the blastocyst are arranged into an outer layer called trophoblast and inner cell mass. Trophoblast layer gets attached to the endometrium and the inner cell mass gets differentiated as the embryo. After attachment the uterine cell divide rapidly and covers the blastocyst. As a result the blastocyst becomes embedded in the endometrium of the uterus and this is called implantation and leads to pregnancy.