Jan 9, 2015

Reproduction of a Placental Mammal - Man

Placental mammals, including humans, have a complex reproductive system. After fertilization, the embryo develops inside the mother's uterus, attached to a placenta that provides nutrients and removes waste. The placenta also acts as a barrier, protecting the developing fetus from the mother's immune system. The fetus undergoes various stages of development until it is ready for birth, after which the placenta and other fetal tissues are expelled from the mother's body. The process of placental mammal reproduction is essential for the continuation of the species. 

The highest order of protection of the offspring is seen in man. It is evident from the internal development, birth and later care and feeding of the young.

 

Male Reproductive System

The reproductive function of the male is the production of sperm cells and their transfer to the female. Male reproductive system consists of:

(a) Testes

(b) Accessory Glands

(c) Penis

 

Testes

The paired testes (singular, testis) are formed inside the body cavity, but they descend shortly before birth into an external sac between the thigh, the scrotum. The testes contain about 15 to 20 coiled seminiferous tubules which produce sperm cells or spermatozoa at a daily rate of a billion or so in young man. The actual combined length of the seminiferous tubules in both testes is about 225 meters. 


Male Reproductive System

Accessory Ducts and Glands

Several structures are associated with the testes. Applied directly to each testis is an epididymis (Greek: upon the twins). It is a coiled tube that would be about six meters long if it were stretched. The haploid sperm cells are stored in the epididymis.

All the seminiferous tubules of the testis lead into the epididymis, which then fuses into the major seminal duct, the vas deferens (or ductus deferens). The vas deferens receives secretions from the seminal vesicles and the prostate gland as it passes alongside them to the urethra.

 

Penis

The urethra is the final tube that leads through the penis and transports sperm outside the male body. During ejaculation, fluids from the seminal vesicles and the prostate gland are secreted into the deferens, and these secretions plus the sperm cells make up the semen. Seminal fluid enhances sperm motility in various ways.

Testis And Sperm

With the onset of sexual excitement, the Cowper's glands secrete an alkaline fluid into the urethra to neutralize the acidity of any remaining urine. These fluids also act as a lubricant within the urethra to facilitate the ejaculation of semen. The function of the penis is twofold. It carries urine through the urethra to the outside, and it transports semen through the urethra during ejaculation.

 

Hormonal Regulation in Males

Among the seminiferous tubules in the testes are small masses of interstitial cells that secrete male sex hormones, especially testosterone. Testosterone affects (1) the production of sperm cells (2) the development of the sex organs (3) the appearance of secondary male sex characters.

FSH and LH produced by the pituitary gland are chiefly responsible for the stimulation of spermatogenesis and testosterone secretion in the testes.



Human Female Reproductive System 

Human Female Reproduction System

Sexuality in females is more complex than it is in males. Not only do females have to produce eggs, but after fertilization, they must nourish, carry and protect the developing embryo. They must also nourish it for a time after it is born. Besides all these, females have a monthly menstrual cycle. The female reproductive system consists of the ovaries, ducts, and external genitalia.

The egg-producing organs are the paired ovaries, elongated bodies, about 5cm long at maturity. Ovaries have two functions: (1) they release eggs (ova) fairly on a regular monthly schedule and (2) they produce female sex hormones. The ovaries contain follicles, the actual center of egg production. Each follicle contains a potential ovum, and follicles are always present in several stages of development.

Each ovary is partially covered by a funnel-shaped infundibulum, into which the egg passes when it is released from the ovary. From the infundibulum, an egg can pass down the fallopian tube (or oviduct) which leads to the uterus.

The uterus is a pear-shaped and sized organ located directly behind the urinary bladder. The uterus leads downward to the vagina, a muscle-lined tube about 8-10 cm long. A fold of skin called the hymen partially blocks the vaginal entrance. The vaginal canal ends at the cervix or the neck of the uterus.

Fertilization of the ovum usually takes place in the fallopian tube or oviduct, near the ovary. The zygote enters the uterus, and it is implanted in the wall of the uterus and undergoes further development.

The embryo establishes a close contact with the uterine tissue through which exchange of materials takes place. This tissue is called placenta. Together with the amnion, the chorion helps to form the placenta.

The gestation period, called pregnancy, is usually 280 days in human females. The edges of the amniotic folds come together around the stalk of the amnion and yolk sac and form a tube, which leads from the embryo to the placenta. This tube is called the umbilical cord. It contains the very important blood vessels that connect the embryo with the placenta.

During pregnancy, the uterus usually increases about 24 times. The process of childbirth or labor is started and controlled by a variety of hormonal events. Birth has been found to be brought about by a change in hormones produced by the placenta and also by a hormone produced by the pituitary glands. Oxytocin brings about the contractions of the uterus. The process of childbirth or parturition is commonly divided into three stages: (I) DILATION (II) EXPULSION (III) PLACENTAL STAGE OR AFTERBIRTH.

During the first stage of childbirth, the cervix dilates or becomes larger. This is the longest stage of childbirth. The opening of the cervix is initially quite small. It must enlarge from a diameter of one to two centimeters to 10 to 11 cm before the fetus can leave the uterus.

Dilation begins when the muscles of the uterus begin to contract. These muscle actions of the uterus gradually force the fetus and amniotic sac downwards. At first, the contractions are mild and occur at 20 minutes apart. Later, the contractions become stronger and may occur as close as a minute apart. Often towards the end of this stage, the amniotic sac breaks releasing the amniotic fluid.

Expulsion: The expulsion stage begins when the cervix is completely dilated and ends with the birth of the baby. This stage is characterized by strong and frequent contractions of the uterus. The expulsion of the fetus from the uterus to the outside of the mother's body usually takes from half an hour to two hours.

Placental stage: In the last stage of birth, also known as after-birth, contractions of the uterus dislodge the placenta from the wall of the uterus. Continued contractions expel blood, other tissue, fluids, and the placenta through the vagina. The placenta is squeezed as it passes out of the body. This action forces blood from the placental blood vessels into the body of the newborn baby. After the umbilical cord stops pulsating, the cord is tied, and then cut by a doctor. The placental stage is from ten to fifteen minutes.

Usually, at about the ninth month after the conception near the end of the human gestation period, the head is turned downwards towards the vagina. At birth, therefore, the head usually comes out first. However, very rarely the feet may appear first, causing complications. This results in breech birth. In other types of severe complications, for example, if the baby has been found to have a weak heart, cesarean section may be performed in which the abdominal and uterine wall is cut to remove the baby.

Placental remains: The contraction and relaxation movements of the uterus continue for some time after the birth. As a result, the placental remains are expelled out.


The three stages of birth 

Mammary Glands

There is a pair of mammary glands. They secrete milk and are accessory organs to the female reproductive system. They are converging to the nipple and opening on its surface in large numbers. The lymph nodes consist of many lobes. The lobes are divided into lobules.

During pregnancy, secretions of estrogen and progesterone cause the lobules to develop. The actual milk-producing hormone is prolactin.

Milk: It is produced by the mammary gland just at the time of childbirth. It contains a special lymph-like fluid known as colostrum. It is rich in antibodies. The true milk is not secreted until about the third day after childbirth. Usually, the baby is fed on the mother's milk up to two years. The baby may be fed on other sources of milk if required. As soon as the mother stops feeding the baby, her reproductive cycle begins again. Sometimes the reproductive cycles initiate even when the mother is breastfeeding.

 

Hormonal Control of the Female Reproductive Tract

Estrus: For the proper timing of internal fertilization, in many animals, the female informs the male that she is ready to mate. When the female is in estrus, or “heat” (becomes sexually receptive to the male), she undergoes physical and behavioral changes. These changes coincide with the maturation of eggs within the ovary.

Estrous Cycle: The physiological changes occurring from one period of estrus to the next are called the estrous cycle. It is controlled by hormones. The timing of the estrous cycle varies in different animals, e.g., rats: 5 days, bitch: every 6 months. The estrous cycle occurs in many animals. Example: rats, mice, cats, dogs.

Menstruation: The periodic shedding of blood and tissue from the uterine lining, the endometrium, is called menstruation.

Menstrual cycle: Unlike many other mammals, eggs of the human female mature quite often. Human females do not undergo a seasonal estrous cycle as dogs or cats do. About once every twenty-eight days, a mature egg is released from an ovary. At the same time, certain uterine changes occur to prepare for a possible pregnancy. This monthly egg maturation and uterine preparation is called the menstrual cycle.

The Role of the Ovary

The ovary consists of two types of cells: (a) germ cells (b) somatic cells. Germ cells give rise to the ova (eggs). Somatic cells are the follicle cells.

Follicle Cells: These are concerned with the support and nutrition of the developing ova. The follicle cells secrete estrogen. It is a female sex hormone. Estrogen acts on the uterus. It causes the endometrium of the uterus to divide rapidly. Some of the cells of the endometrium become glandular in nature. They start secreting proteinaceous secretions. When the embryo reaches the uterus, it starts taking up glandular type proteinaceous secretion. The wall of the uterus becomes thick, and more blood vessels develop in it due to estrogen."


Ovary and follicles

The ovum is surrounded by a follicle. The ovulation takes place, and as a result, the follicle ruptures. After rupture, the follicle is converted into a mass of specialized tissue called corpus luteum. If the discharged ovum is fertilized, the corpus luteum remains until the placenta is formed later. It gradually turns into scar tissue. If the ovum is not fertilized, the corpus luteum degenerates and turns into scar tissue. The corpus luteum is cream in color (corpus luteum means yellow body). Its cells become glandular, and they secrete a hormone called progesterone.

Functions of Progesterone: It prepares the myometrium, endometrium, and muscles of the uterine wall for the implantation of zygote.

Menstrual Cycle: There is periodic shedding of blood and tissue from the endometrium of the uterus. It is called menstruation (The Latin word for "month" is menses). The cycle is defined as a succession of events repeated regularly within a given period of time. So the occurrence of menstruation at regular intervals is called the menstrual cycle. Generally, it is repeated after every 28 days. However, the number of days is variable in different individuals or within the same individual at different times.

The menstrual cycle can be divided into four stages:

(a) Follicular stage

(b) Ovulation stage

(c) Luteal stage

(d) Menstrual stage

Follicular Stage: One or sometimes more than one egg and follicles start growing in size. The egg is surrounded by a follicle. The follicles arrange themselves in layers. So a large cavity appears between the follicle cells and the developing egg. Some of the follicle cells secrete estrogen. It affects the growth of the soft tissues of the uterus. Primarily estrogen affects the internal lining of the uterus. So the uterine tissue becomes spongy in nature.

Ovulation stage: At about the midpoint of twenty-eight to thirty days cycle, under the influence of Luteinizing hormone, ovulation takes place. The ovum then passes to the fallopian tube (or oviduct).

Luteal stage: After ovulation, continued LH stimulation causes the follicle cells to secrete glandular cells. These glandular cells constitute the corpus luteum. The maturing corpus luteum secretes progesterone. Progesterone acts on the uterus by stimulating vascularization (blood vessel development), glandular development, and glycogen accumulation in the endometrium. As a result, the uterine epithelium becomes folded, and the endometrium becomes soft and spongy.

Menstrual stage: In a typical menstrual cycle, when a pregnancy has not begun, the LH level falls. It causes the corpus luteum to begin to degenerate by the 21st or 22nd day. As the corpus luteum is degenerating, its hormone output decreases. The thickened spongy part of the uterus starts to break down, and menstruation begins. During this phase, the broken-down soft tissues of the uterus and a considerable amount of blood are released and pass out of the vagina. The day on which menstruation begins is commonly designated as "day one" of the menstrual cycle. Menstruation is a clear signal that pregnancy has not begun. The menstrual stage lasts for 5-6 days. The cycle is thus completed, and the uterus then enters into the next menstrual cycle.

Age for Menstruation: It begins usually at the age of 12-14 in females. It is variable.

Menopause: Between the ages of approximately 43 and early 50 a woman loses her ability to produce ova. The menstrual cycle ceases. Her breasts, genitalia and uterus all decrease in size. Sexual desire may also be reduced.


Menstrual Cycle 

Role of Pituitary Gland

Gonadotropic: The pituitary hormones that play an important role in the reproductive cycle in females are commonly called gonadotropins. The most important gonadotropins are:

(a) Follicle-stimulating hormone.

(b) Luteinizing hormone.

Both these hormones affect the ovary. FSH affects the developing oocyte. Oocyte is the cell that will give rise to the ovum. The follicle also increases due to FSFI (Female Sexual Function Index). The mature follicles are called Graafian follicles. LH at this stage causes ovulation. The follicle cells surrounding the developing oocyte secrete estrogen. So both FSH and LH cause the secretion of estrogen in an indirect way. Estrogen causes structural and functional changes in the uterus.

Luteotropic Hormone

It is secreted by the pituitary gland. LTH is also called prolactin. It stimulates the development and secretion of milk in the mammary gland. LTH also stimulates the corpus luteum to secrete progesterone.

Role of Placenta

Some cells of the placenta become specialized. These cells secrete hormones. The hormones are like the pituitary hormone in function.

Chorionic Gonadotropin

This hormone is secreted by the chorion of the placenta. Its function is similar to LH and LTH. Due to CG, the activity of the corpus luteum is maintained within the ovary. So there is secretion of progesterone for a considerable period. The placenta then starts secreting its own progesterone. At this stage, the secretion of ovarian progesterone ceases.

Functions: Major functions of human estrogen and progesterone during pregnancy are:

Estrogen

(1) Growth of mammary glands.

(2) Inhibits FSH release.

(3) Inhibits prolactin release.

(4) Prevents infection in the uterus.

(5) Increases the size of uterine muscle cells.

(6) Increases the sensitivity of myometrium to oxytocin.

Progesterone

(1) Growth of mammary glands.

(2) Inhibits FSH release.

(3) Inhibits prolactin release.

(4) Inhibits the contraction of myometrium.

The hypothalamus produces GnRH (gonadotropic-releasing hormone). GnRH stimulates the anterior pituitary to produce FSH (follicle-stimulating hormone) and LH (luteinizing hormone). FSH stimulates the follicle to produce estrogen, and LH stimulates the corpus luteum to produce progesterone.

Estrogen and progesterone affect the sex organs (e.g., uterus) and the secondary sex characteristics and exert feedback control over the hypothalamus and the anterior pituitary.


Role of hormones on testes and ovary

Test Tube Babies

Test tube babies refer to the in vitro fertilization (IVF) technique, which involves fertilizing one or more eggs outside the body in laboratory glassware and then transferring the fertilized egg, known as pre-embryos, back into the uterus. This technique was first developed in 1978 to help women with blocked oviducts, and it has since become a widely used method for couples struggling with infertility. The process of IVF typically involves stimulating the ovaries with fertility drugs to produce multiple eggs, collecting mature eggs, fertilizing the egg in the laboratory, culturing the pre-embryo, and finally transferring the embryo to the uterus. IVF is a highly effective procedure and has helped millions of couples around the world conceive and give birth to healthy babies.

 

#IVF #testtubebaby #in vitro fertilization #pre-embryo #fertility drugs #infertility

#TestTubeBaby #IVF #InVitroFertilization #preembryo #FertilityDrugs #Infertility

#IVFjourney #testtubebaby #in vitro fertilization #preembryos #fertilitytreatments #infertilitysupport

#AssistedReproductiveTechnology #IVF #testtubebaby #fertility #pre-embryo #infertilityawareness

#BabyAfterIVF #IVFsuccess #testtubebaby #pregnancy #infertilityjourney #fertilitytreatment



No comments:

Post a Comment