FEMALE REPRODUCTIVE SYSTEM

Ovaries

The ovaries are a pair of small glands about the size and shape of almonds, located on the left and right sides of the pelvic body cavity lateral to the superior portion of the uterus. Ovaries produce female sex hormones such as estrogen and progesterone as well as ova (commonly called "eggs"), the female gametes. Ova are produced from oocyte cells that slowly develop throughout a woman’s early life and reach maturity after puberty. Each month during ovulation, a mature ovum is released. The ovum travels from the ovary to the fallopian tube, where it may be fertilized before reaching the uterus.

Fallopian Tubes

The fallopian tubes are a pair of muscular tubes that extend from the left and right superior corners of the uterus to the edge of the ovaries. The fallopian tubes end in a funnel-shaped structure called the infundibulum, which is covered with small finger-like projections called fimbriae. The fimbriae swipe over the outside of the ovaries to pick up released ova and carry them into the infundibulum for transport to the uterus. The inside of each fallopian tube is covered in cilia that work with the smooth muscle of the tube to carry the ovum to the uterus.

Uterus

The uterus is a hollow, muscular, pear-shaped organ located posterior and superior to the urinary bladder. Connected to the two fallopian tubes on its superior end and to the vagina (via the cervix) on its inferior end, the uterus is also known as the womb, as it surrounds and supports the developing fetus during pregnancy. The inner lining of the uterus, known as the endometrium, provides support to the embryo during early development. The visceral muscles of the uterus contract during childbirth to push the fetus through the birth canal.

Vagina

The vagina is an elastic, muscular tube that connects the cervix of the uterus to the exterior of the body. It is located inferior to the uterus and posterior to the urinary bladder. The vagina functions as the receptacle for the penis during sexual intercourse and carries sperm to the uterus and fallopian tubes. It also serves as the birth canal by stretching to allow delivery of the fetus during childbirth. During menstruation, the menstrual flow exits the body via the vagina.

Vulva
The vulva is the collective name for the external female genitalia located in the pubic region of the body. The vulva surrounds the external ends of the urethral opening and the vagina and includes the mons pubis, labia majora, labia minora, and clitoris. The mons pubis, or pubic mound, is a raised layer of adipose tissue between the skin and thepubic bone that provides cushioning to the vulva. The inferior portion of the mons pubis splits into left and right halves called the labia majora. The mons pubis and labia majora are covered with pubic hairs. Inside of the labia majora are smaller, hairless folds of skin called the labia minora that surround the vaginal and urethral openings. On the superior end of the labia minora is a small mass of erectile tissue known as the clitoristhat contains many nerve endings for sensing sexual pleasure.

Breasts and Mammary Glands

The breasts are specialized organs of the female body that contain mammary glands, milk ducts, and adipose tissue. The two breasts are located on the left and right sides of the thoracic region of the body. In the center of each breast is a highly pigmented nipple that releases milk when stimulated. The areola, a thickened, highly pigmented band of skin that surrounds the nipple, protects the underlying tissues during breastfeeding. The mammary glands are a special type of sudoriferous glands that have been modified to produce milk to feed infants. Within each breast, 15 to 20 clusters of mammary glands become active during pregnancy and remain active until milk is no longer needed. The milk passes through milk ducts on its way to the nipple, where it exits the body.

           

The Reproductive Cycle

The female reproductive cycle is the process of producing an ovum and readying the uterus to receive a fertilized ovum to begin pregnancy. If an ovum is produced but not fertilized and implanted in the uterine wall, the reproductive cycle resets itself through menstruation. The entire reproductive cycle takes about 28 days on average, but may be as short as 24 days or as long as 36 days for some women.

Oogenesis and Ovulation

Under the influence of follicle stimulating hormone (FSH), and luteinizing hormone (LH), the ovaries produce a mature ovum in a process known as ovulation. By about 14 days into the reproductive cycle, an oocyte reaches maturity and is released as an ovum. Although the ovaries begin to mature many oocytes each month, usually only one ovum per cycle is released.

Fertilization

Once the mature ovum is released from the ovary, the fimbriae catch the egg and direct it down the fallopian tube to the uterus. It takes about a week for the ovum to travel to the uterus. If sperm are able to reach and penetrate the ovum, the ovum becomes a fertilized zygote containing a full complement of DNA. After a two-week period of rapid cell division known as the germinal period of development, the zygote forms an embryo. The embryo will then implant itself into the uterine wall and develop there during pregnancy.

Menstruation

While the ovum matures and travels through the fallopian tube, the endometrium grows and develops in preparation for the embryo. If the ovum is not fertilized in time or if it fails to implant into the endometrium, the arteries of the uterus constrict to cut off blood flow to the endometrium. The lack of blood flow causes cell death in the endometrium and the eventual shedding of tissue in a process known as menstruation. In a normal menstrual cycle, this shedding begins around day 28 and continues into the first few days of the new reproductive cycle.

Pregnancy

If the ovum is fertilized by a sperm cell, the fertilized embryo will implant itself into the endometrium and begin to form an amniotic cavity, umbilical cord, and placenta. For the first 8 weeks, the embryo will develop almost all of the tissues and organs present in the adult before entering the fetal period of development during weeks 9 through 38. During the fetal period, the fetus grows larger and more complex until it is ready to be born.

Lactation

Lactation is the production and release of milk to feed an infant. The production of milk begins prior to birth under the control of the hormone prolactin. Prolactin is produced in response to the suckling of an infant on the nipple, so milk is produced as long as active breastfeeding occurs. As soon as an infant is weaned, prolactin and milk production end soon after. The release of milk by the nipples is known as the “milk-letdown reflex” and is controlled by the hormone oxytocin. Oxytocin is also produced in response to infant suckling so that milk is only released when an infant is actively feeding. 




HOW DID LIFE STARTED ON THE EARTH ?

Scientists do not know how life began on Earth. They do know that the early Earth’s atmosphere was very different from the atmosphere now.

In 1952, Stanley Miller was working with Harold C. Urey designed an experiment to see how complex organic molecules might have formed under the conditions of early Earth. They believed the early Earth atmosphere would have been composed of methane, ammonia, hydrogen and water vapor. They sealed these gases in an airtight container, and then exposed the gases to sparks of electricity to simulate lightning. They continued the lightning for a week, and by the end, a reddish-brown substance had coated the walls of the container. This substance contained 11 of the 20 amino acids used by life on earth. Since Miller and Urey performed this experiment, its results have been confirmed many times by other scientists. Many scientists now believe that the early Earth’s atmosphere was composed of carbon dioxide, nitrogen and water vapor.

Modern experiments with this mixture of gases produce similar results suggesting that early conditions on Earth produced complex organic molecules that probably became the basis for the development of more complex organisms. However, scientists have not been able to replicate the formation of even simple organisms, or anything that can really replicate itself. There are several theories as to how the amino acids might have made the leap into the complex, self-replicating life we see today.

Metabolism - First

Some scientists believe that metabolism, in other words - the ability to break down carbon dioxide in the presence of a catalyst into small organic molecules - was how the first life developed. These reactions might have evolved to become more complex, and then genetic molecules somehow formed and joined in later. There are many different theories as to exactly what types of molecules and catalysts would have been involved.

Genes - First

Other scientists believe that the first living organisms were genes. These genes were single molecules that had developed in such a way as to be able to catalyze their own replication. This theory seems more likely, since even simple systems such as crystals, have been demonstrated to evolve with modifications that breed true. Some scientists have suggested that certain compositions of clay create the right environment for these reactions to propagate.

RNA

RNA is a complex molecule found in all living things that seems to be able to catalyze its own reproduction. Many scientists believe that simple RNA molecules developed and eventually became more complex and developed into the organisms we see today.

LUCA

Astrobiologists and biochemists want to understand something they call LUCA (the Last Universal Common Ancestor). The idea is that all life on Earth has a common ancestor, kind of like a great-great-great-great grandmother. They search for traits that are common across all life forms and assume that any traits that are common to all life forms today must have been inherited from LUCA, who had them all as well. Biochemists know quite a bit about LUCA and her biochemistry. She stored her genetic information in DNA, she had several hundred proteins performing a variety of functions, and she used the same 20 amino acids we use in our proteins. She used RNA and had some kind of double-layer lipid membrane. She was probably the ancestor of the three kingdoms of life: Archaea, Eukaryotes and Bacteria. LUCA lived at least 2 billion years ago, before there was much oxygen in the atmosphere. She used enzymes containing iron in her metabolic pathways the way much life on early Earth did. Studying how life arose on Earth is useful to astrobiologists, but they keep in mind that the way life formed on Earth is not the only way life could have formed. It is simply one way that it did.

ANNELIDA-COELOM AND COELOMODUCTS

Annelids are the first EUCOELOMETE animals in the evolutionary order. The coelom is SCHIZOCOELOM and it is divided into compartments by intersegmental septa.

Some structures in annelids are arranged segmentally and are described as SEGMENTAL STRUCTURES. Among them there are COELOMODUCTS and NEPHRIDIA . These structures will be useful for the passage of reproductive and excretory products from coelom to the exterior. The segmental structures mainly divided into     

                             I.            COELOMODUCTS

                           II.            NEPHRIDIA

COELOMODUCTS: These are generally wide tubes of MESODERMIC origin and develop as ENVAGINATIONS of coelomic epithelium. Each coelomoduct opens into coelom by a ciliated funnel called COELOMOSTOME which opens out by a genital aperture. Coelomoducts primarily function as GONODUCTS. So these will be limited to the with reproductive structures. In some forms they function as excretory ducts.

NEPHRIDIA: These are generally coiled tubes which are formed by the INVAGINATIONS OF ECTODERM. Each nephridium coummunicates with the outside through a NEPHRIDIOPORE. If the nephridium opens out into the coelom through nephrostome,it is described as OPEN. If it is absent,it is CLOSED. Nephridia without nephridiostome is called PROTONEPHRIDIUM, and if present it is METANEPHRIDIUM. Nephrostome may communicate with prostome of the same segment or anterior to it. Nephridia are primarily excretory(osmoregulation)in function and secondarily serves as REPRODUCTIVE DUCTS(gonoducts).

Nephridia without a nephrostome are considered as PRIMITIVE. In some cases,the closed end of the tube may have SOLENOCYTES which occur singly or in group.

Nephridia with a nephrostome are considered as ADVANCED. In a majority of polychaetes,it opens into coelom through CILIATED NEPHROSTOME.

Based on size, nephridia are classified into MICRONEPHRIDIA and MEGANEPHRIDIA. Generally micronephridia will be more in segments and meganephridia will be anepair per segment.

If nephridium communicates outside through nephridiophore directly, it is termed as EXONEPHRIC,  whose primary function is EXCRETORY and if the terminal ducts ultimately open into alimentary canal it is called as ENDONEPHRIC  nephridia, which are basically OSMOREGULATORY in function.

MIXONEPHRIDIA OR NEPHROMIXIUM:

            Incase of oligocheates and hirudineans the nephridia and coelomoducts are separate structurally and functionally. Where as the nephridia of polycheates and aschiannelids, they function as excretory and reproductive ducts and so they are described as NEPHROMIXIA or MIXONEPHRIDIA