Reproductive systems
- The reproductive system in both males and females consists of structures that produce reproductive cells, or gametes, and secrete sex hormones.
- A gamete is a haploid cell that combines with another haploid gamete during fertilization.
- Sex hormones are chemical messengers that control sexual development and reproduction. The male reproductive system consists of structures that produce male gametes called sperm and secrete the male sex hormone testosterone
- The reproductive system is a collection of internal and external organs in both males and females that work together for the purpose of procreating. Due to its vital role in the survival of the species, many scientists argue that the reproductive system is among the most important systems in the entire body.
- The male reproductive system consists of two major parts: the testes, where sperm are produced, and the penis.
- The penis and urethra belong to both the urinary and reproductive systems in males. The testes are carried in an external pouch known as the scrotum, where they normally remain slightly cooler than body temperature to facilitate sperm production.
- The external structures of the female reproductive system include the clitoris, labia minora, labia majora and Bartholin’s glands..
- The major internal organs of the female reproductive system include the vagina and uterus which act as the receptacle for semen and the ovaries, which produce the female’s ova.
- The vagina is attached to the uterus through the cervix, while the fallopian tubes connect the uterus to the ovaries.
- In response to hormonal changes, one ovum, or egg or more in the case of multiple births is released and sent down the fallopian tube during ovulation. If not fertilized, this egg is eliminated as a result of menstruation.
- Fertilization occurs if a sperm enters the fallopian tube and burrows into the egg.
- While the fertilization usually occurs in the oviducts, it can also happen in the uterus itself.
- The egg then becomes implanted in the lining of the uterus, where it begins the processes of embryogenesis (in which the embryo forms) and morphogenesis (in which the fetus begins to take shape).
- When the fetus is mature enough to survive outside of the womb, the cervix dilates and contractions of the uterus propel it through the birth canal
Male Reproductive System
- The penis is the organ of copulation and is an accessory organ.
- The reproductive organs in the male are the testes.
- The dartos is involuntary muscle that puts the wrinkles in the scrotum; remember the cremaster is the muscle that lifts the testes towards the body or lowers them from the body.
- The epididymis is 4-6 meters in length and is where spermiogenesis (sperm maturation) occurs.
- It takes sperm about 12 days to traverse the epididymis.
- Spermatogenesis (production of sperm) takes place, specifically, in the seminiferous tubules.
- The testes produce the sperm and secrete testosterone.
- The seminal vesical secretes fructose, vitamin C, prostaglandins, amino acids and the bulk of the semen.
- It also contains clotting precursors (fibrinogen) and is a yellowish, alkaline fluid.
- The prostate gland is about the size of a chestnut, contains two lobes and is a firm organ.
- It secretes “plasmin”.
- The fluid released from the prostate is thin, milky, alkaline and makes up about a third of the semen.
- Cowper’s glands are utilized to flush the urethra of residual urine or other substances that will damage the sperm when they are ejaculated through the urethra.
- These secretions are alkaline and mucous-like; they provide only about 2-3 drops of lubricant, so these glands aren’t of great significance in terms of lubrication for intercourse.
- In general, the volume of semen runs around 3-6 mL and contains in the neighborhood of 300 to 400 million sperm.
- Succeeding ejaculates in a short period of time contain a smaller volume of semen.
Female Reproductive System
- A female’s reproductive system produces eggs. This system is also the place where a fertilized egg can grow and develop into a baby. Recall that a male begins producing sperm when he reaches puberty. A female begins producing eggs before she is born.
- Female Reproductive Organs Unlike a male, all the reproductive organs of a female are located inside her abdomen.
- Two folds of skin, called labia, protect the opening to a female’s reproductive system.
- Beyond the opening, inside the female’s body is a thin-walled chamber called the vagina. This is where semen is deposited.
- Uterus Above the vagina, further inside the body, is the uterus It is a thick, muscular organ inside which a fertilized egg can develop.
- A uterus is normally about the size and shape of a pear, but it enlarges during pregnancy. A tissue called the endometrium lines the uterus.
- The neck, or opening, of the uterus into the vagina is called the cervix.
- During childbirth, the cervix gets wider, or dilates. This enables the baby to move into the vagina and out of the mother’s body. Ovaries and Fallopian Tubes A pair of organs called ovaries (singular, ovary) produces eggs.
- An egg released from an ovary moves into a fallopian tube or oviduct that connects the ovary to the uterus.
- Fertilization usually occurs while the egg is in a fallopian tube. An egg cell has no flagellum, so it cannot move on its own like a sperm cell can.
From Fertilization to Old Age
- A day or two after an ovary releases an egg, the egg may unite with a sperm. Sperm are deposited in the vagina during sexual intercourse. They propel themselves through the uterus and enter a fallopian tube. This is where fertilization usually takes place.
Cleavage and Implantation
- When a sperm penetrates the egg, it triggers the egg to complete meiosis. The sperm also undergoes changes. Its tail falls off, and its nucleus fuses with the nucleus of the egg. The resulting cell, called a zygote, contains all the chromosomes needed for a new human organism. Half the chromosomes come from the egg and half from the sperm.
Morula and Blastocyst Stages
- The zygote spends the next few days traveling down the fallopian tube toward the uterus, where it will take up residence. As it travels, it divides by mitosis several times to form a ball of cells called a morula.
- The cell divisions are called cleavage. They increase the number of cells but not the overall size of the new organism. As more cell divisions occur, a fluid-filled cavity forms inside the ball of cells. At this stage, the ball of cells is called a blastocyst.
- The cells of the blastocyst form an inner cell mass and an outer cell layer. The inner cell mass is called the embryoblast. These cells will soon develop into an embryo. The outer cell layer is called the trophoblast. These cells will develop into other structures needed to support and nourish the embryo.
Implantation
- The blastocyst continues down the fallopian tube and reaches the uterus about 4 or 5 days after fertilization.
- When the outer cells of the blastocyst contact cells of the endometrium lining the uterus, the blastocyst embeds in the endometrium.
- The process of embedding is called implantation. It generally occurs about a week after fertilization.
Growth and Development of the Embryo
- After implantation occurs, the blastocyst is called an embryo. The embryonic stage lasts through the eighth week following fertilization. During this time, the embryo grows in size and becomes more complex. It develops specialized cells and tissues and starts to form most organs
Formation of Cell Layers
- During the second week after fertilization, cells in the embryo migrate to form three distinct cell layers, called the ectoderm, mesoderm, and endoderm.
- Each layer will soon develop into different types of cells and tissues, as shown in Figure below.
Differentiation of Cells
- A zygote is a single cell. How does a single cell develop into many different types of cells? During the third week after fertilization, the embryo begins to undergo cellular differentiation.
- Differentiation is the process by which unspecialized cells become specialized. As illustrated in Figure below, differentiation occurs as certain genes are expressed while other genes are switched off.
- Because of this process, cells develop unique structures and abilities that suit them for their specialized functions
Organ Formation
- After cells differentiate, all the major organs begin to form during the remaining weeks of embryonic development.
- A few of the developments that occur in the embryo during weeks 4 through 8 are listed in Figure below. As the embryo develops, it also grows in size.
- By the eighth week of development, the embryo is about 30 millimeters (just over 1 inch) in length. It may also have begun to move
Growth and Development of the Fetus
- From the end of the eighth week until birth, the developing organism is referred to as a fetus. Birth typically occurs at about 38 weeks after fertilization, so the fetal period generally lasts about 30 weeks.
- During this time, as outlined in Figure below, the organs complete their development. The fetus also grows rapidly in length and weight.
By the 38th week, the fetus is fully developed and ready to be born. A 38-week fetus normally ranges from 36 to 51 centimeters (14–20 inches) in length and weighs between 2.7 and 4.6 kilograms (about 6–10 pounds).
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