REPRODUCTIVE SYSTEM AND GROWTH IN MAMMALS

Objectives

This blog post provides readers with the following objectives. The reader will be able to:

·        Describe the internal structure of the testis and ovary.

·        Outline the process of gamete formation.

·        Describe fertilization, development of the embryo and birth.

·        Outline the secondary sexual characteristics in male and female human beings.

·        Explain the different birth control methods.

·        Explain prenatal(antenatal) care

 

Sexual Reproduction

Sexual reproduction involves the fusion of two gametes. The gonads (the ovaries and testes) form the gametes (sex cells).  The fertilized egg develops within the female reproductive system and culminates in the birth of a baby.  Sexual maturation and development of sex cells are regulated by hormones secreted by the pituitary gland and gonads.

Male Reproductive System

The organs consist of the gonads and the accessory organs

Testes (singular: testis): are the reproductive glands of the male.  Each testis consists of hundreds of tiny sperm tubules called seminiferous tubules bound together by interstitial cells. The tubules combine to form epididymis which lies in close contact with the external surface of the testis. The epididymis is connected to vas deferens (sperm duct) The testes are suspended outside the body within the scrotum by spermatic cords. 

Scrotum: is a sac of pigmented skin, connective tissues and smooth muscles that hangs outside the body. It is divided into two compartments by a midline septum, each containing one testis.  

Epididymis: is tightly coiled semiferous tubules which lies outside the testis. It stores sperms temporarily.

Sperm duct: carries sperms from the epididymis to the urethra during ejaculation.

Urethra: The urethra is the terminal portion of the male duct system.  It conveys both urine and semen (at different times). 

Penis: is composed mainly of erectile tissue. It is divided into a root, an elongated tubular (or body) and glans penis. The body is composed of three cylindrical columns of erectile tissue, bound together by fibrous tissue and covered with skin.  Erectile tissue is a spongy tissue containing blood spaces. The penis erects when the spaces are filled with blood. The glans penis is the cone-shaped terminal portion of the penis.  The skin of the body of the penis is loosely attached and continuous over the glans penis as a retractable sheath called prepuce or foreskin.  The prepuce is commonly removed in infants by a procedure called circumcision. 

Accessory Glands

1. Seminal Vesicle: are paired glands. They secrete seminal fluid which provides nourishment and oxygen to the sperms. The fluid also provides medium for the sperms to swim towards the oviduct in the female reproductive system.

2. Prostate Gland: it secretes a thin, milky alkaline fluid which gives semen its milky appearance.  It also contains a clotting enzyme, which thickens the semen in the vagina.

3. Bulbourethral Glands: are spherical glands below the prostate gland.  They secrete mucus like fluid responsible for sexual stimulation.  The fluid neutralizes the acidic urine, lubricates the urethra and the end of the penis.

Semen/Seminal Fluid

The seminal or semen, is the fluid passed from the urethra during ejaculation.  It consists of the fluids secreted by the bulbourethral gland, seminal vesicles and prostate gland along with sperm cells from the testes.  It is slightly alkaline (pH7.5) and contains nutrients for sperm cells and substances that activate their mobility. 

Internal Structure of the Testis

The testis is surrounded by fibrous tissue called tunica albuginea. The testis is divided into several shaped lobules or compartments by septa extending from the tunica albuginea. Each lobule contains tightly coiled seminiferous tubules which is site for the production of sperms. The seminiferous tubules are lined by germinal epithelium.  The seminiferous tubules unite to form epididymis on the outer surface of the testis. In between the tubules are groups of interstitial cells that secrete the hormone testosterone

Spermatogenesis

Spermatogenesis is the process of production of sperm cells (spermatozoa) by the division of spermatogenic cells in the germinal epithelium.  Sperm production are controlled by follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland and by testosterone from the testes.

The outermost cells of a seminiferous tubule are known as spermatozoid or primordial germinal cells.  They are formed from the germinal epithelium. These cells divide to form diploid spermatogonium (plural: spermatogonia). The spermatogonium divides by mitosis and undergoes changes to become a primary spermatocyte.

Each primary spermatocyte, divides in the first meiotic division to form two haploid secondary spermatocytes.

In the second meiotic division, each secondary spermatocyte divides to form two haploid spermatids.  Each spermatid attaches to a supporting cell (Sertoli cell) and develops a flagellum to form a sperm cell.

 Sperm cells are carried into the epididymis where they are temporally stored and mature.

Structure of the Sperm 

A mature sperm cell consists of a head, middle piece, and flagellum or tail.  The flattened head is composed of a compact nucleus containing 23 chromosomes.  The anterior portion of the head is covered by a cuplike structure, called the acrosome.  The acrosome   contains an enzyme that helps the sperm cell penetrate the female sex cell.  The body contains mitochondria, where ATP is formed, required for lashing the flagellum.

Functions of the Male Reproductive System

1.  The production of male hormones

2. The production of male gametes (spermatozoa)

3. Transmission of spermatozoa to the female.

Female Reproductive System

The female reproductive system consists of a pair of ovaries, which are the primary sex organs and the secondary sex organs (vagina, uterus, fallopian tubes) and the external genitalia.

Ovaries: are small oval organs situated in the abdominal cavity just below to the kidneys. It produces female sex cells or ova. It also produces female sex hormones, estrogen and progesterone. A cluster of cells called the follicle surrounds and nourishes each ovum.

Oviduct or Fallopian tube: is the site at which fertilization takes place. It serves as passageways from ovaries to uterus. It is not connected to the ovary.

Uterus: The uterus is a hollow, thick-walled muscular organ. It receives, nourishes and protects the fetus during pregnancy. They are able to expand and contract and push the baby out during birth.

Cervix: This is the narrow region of the uterus where it meets the vagina. It prevents infection from entering the uterus.

Vagina: is made up of three layers, an inner mucosal layer, a middle muscularis layer, and an outer fibrous layer. The inner layer is made of ridges or rugae that stretch and allow penetration to occur. They also stimulate the penis during intercourse. Vagina receives sperm during coitus and also acts as a birth canal at the end of pregnancy.

In virgins, a thin sheet of tissue with one or more holes in it, called the hymen, partially covers the opening of the vagina. The hymen is very vascular and tends to bleed when it is ruptured during the first coitus.

External Genitalia  

The external genitalia also known as Vulva include the mons pubis, labia, and clitoris. 

1. Mons Pubis: is a fatty, rounds area overlying the symphysis pubis.  This area is covered with pubic hair. 

2. Labia: are folds of skin near the opening of the vagina.

3. Labia majora are two thick outer folds of skin with hairs that surrounds the opening of vagina and the urethra.

4. Labia minora are two small folds of skin that lies inside the labia majora and join anteriorly to form a hood like covering of the clitoris.

5. Clitoris: is a small, sensitive, rod-like and erectile organ located in front of the vestibule.  The tip of the clitoris, contains numerous sensory nerve endings which leads to sexual response, when stimulated (i.e., female excitatory organ).

Internal Structure of the Ovary

The outermost layer covering the surface of the ovary is called the ovarian, or germinal, epithelium.  The denser outer part of the ovary is called the cortex and looser inner part is called the medulla.  Blood vessels, lymphatic vessels and small vesicles called ovarian follicles, which contains oocytes are distributed throughout the cortex.

Oogenesis

Oogenesis is the production of secondary oocytes within the ovaries.

The germinal epithelium divides mitotically to form oogonia (singular: oogonium). Each oogonium grows into a large size structure called the primary oocytes.

Females are born with all the primary oocytes that they will ever possess (about 20 million). From birth to puberty, most of them break down and around 300,000 – 400,000 remain in the ovary. Out of these, only about 400 complete their development and ovulated.

The primary oocyte is surrounded by a layer of cells to form a structure called primary follicle. The primary follicles continue development and become secondary follicles. The secondary follicle continues to enlarge and become a fluid-filled chamber follicle, called the mature or Graafian follicle.

The primary oocyte (inside Graafian follicle), undergoes first meiotic division to produce a secondary oocyte and a polar body. 

The Graafian follicle expands and bursts to release the secondary oocyte from the ovary into the fallopian tube. The release of the secondary oocyte is called ovulation. The secondary oocyte undergoes a second meiotic division to become the ovum.

Once the ovum has been shed, a blood clot forms in the empty follicle. This develops into a tissue called the corpus luteum (yellow body) that produces the hormone progesterone. If pregnant occur the corpus luteum persists, but if there is no pregnancy it degenerates and a new ovarian cycle begins.

Functions of the Female Reproductive System

1. produce gametes (ova)

2. secrete sex hormones

3. receive the sperm from the male during sexual intercourse or coitus

4. provide sites for fertilization, implantation and embryonic and fetal development 

5. facilitate parturition or delivery of the baby

Fertilization and Development of Embryo

Fertilization

After ovulation the ovum is released into the opening of the fallopian tube. Copulation (coitus, intercourse or mating) releases several sperms into the vagina. They swim through the cervix and uterus into the fallopian tubes where they meet the ovum. The acrosome of sperm comes in contact with the ovum and releases digestive enzymes that break down a gelatinous layer around the ovum called, the zona pellucida to facilitate sperm entry. Only one sperm succeeds in entering the egg. The sperm loses its tail and the two nuclei fuse to form a diploid zygote, a process known as fertilization.

Implantation and Gestation

After fertilization, the zygote passes along the oviduct to the uterus by beating of the cilia and wave-like contractions of the wall.  As the fertilized egg travels down the fallopian tube, it starts to divide mitotically. It divides into two cells and then to four, eight, sixteen, etc. until a solid ball of cells called a morula is formed. The division continues and a hollow ball of cells called blastocyst develops. The outer most layer of the blastocyst, called trophoblast, contribute to the formation of placenta.  The blastocyst embeds itself into the endometrium of the uterine wall, a process called implantation. The embedded embryo will then develop into a foetus. The period of development of the fetus is called pregnancy or gestation, which in humans, lasts approximately 40 weeks. During pregnancy, more estrogen and progesterone are produced. They maintain endometrium and suppressed the development of new ovarian follicles.

Placenta and Fetal Membranes

The placenta, umbilical cord and fetal membranes develop around the fetus which provide it with nutrients and remove waste products. Placenta attaches the fetus to the wall of the uterus. The fetus is attached to the placenta by the umbilical cord. The umbilical cord is highly vascularized, containing arteries and veins. Four embryonic membranes (chorion, amnion, yolk sac and allantois), formed around the developing fetus.

1.   The chorion forms the outer covering, protecting the inner membranes and the fetus.
2.   The amnion/amniotic cavity; the inner membrane that encloses amniotic fluid. It functions in supporting and cushioning the foetus. It also acts as ‘‘shock absorber’’.  During birth, the amniotic fluid lubricates and reduces friction in the vagina.

3.   The yolk and allantois sac are relatively small and are not important.

Functions of the placenta

1. Prevent passage of pathogen from the mother to fetus

2. Convert some glucose into fructose

3. Allow passage of drugs/vaccines from the mother to fetus

4. Produce hormones progesterone that prevent ovulation and menstruation during pregnancy

5. Carries oxygen, water, nutrients, antibodies from mother’s blood to fetus

6. Removes waste products from the fetus blood to mother’s blood for elimination

7. Prevent mixing of mother’s blood and fetal blood

8. Filters out urea/nitrogen waste of the baby’s blood and transfers it to the mother’s blood kidney

9. Produces progesterone which maintains uterine wall in healthy state during pregnancy

Birth or Parturition

Parturition occurs at the end of gestation period. The walls of the uterus start to contract (labor). They are stimulated by the release of hormone oxytocin. The contractions cause the cervix to widen and begin to open. As labor progresses, the amniotic sac rupture causing a fluid to flow through the birth canal. The fetus is then pushed through the cervix and along the vagina before being delivered. In the final stage of labor the placenta or “afterbirth” is expelled.

Twins

1.  Dizygotic or Non-identical or Fraternal Twins: occurs by separate fertilization of two ova by two sperms. These twins may differ genetically and physically not alike.

2. Monozygotic or Identical Twins:  occurs when one ovum is fertilized by a single sperm and the fertilized ovum divides into two separate halves. Each halve develops into separate embryo. Such children are genetically identical and difficult to differentiate between them. They have the same blood group type and also inheritance of hemophilia.

4. Siamese twins or Conjoined twins: identical twins which fail to separate up completely during the course of their division and development. Such twins are joined together at some part and may share some vital organs such kidney.

Parental (Antenatal) Care

Parental is very high in mammals with human exhibiting the highest parental care. All mammals feed their young on milk which provides the young with a balanced diet.

The hormones oestrogen and progesterone stimulate the mammary glands to develop and prolactin promotes the secretion of the milk. The first milk is called colostrum. It is a rich in nutrients and contains protective antibodies from the mother. Milk contains fat, protein and milk sugar as well as vitamins and most minerals although it contains little iron. The milk is the normal source of nutrient for newly-born mammals.

Secondary Sexual Characteristics

Males: When puberty begins, usually between the ages of 10 and 14, the pituitary gland secretes hormones that stimulate the testicles to produce testosterone. The production of testosterone brings about many physical changes.  

1. the scrotum and testes grow large 

2. the penis becomes longer

3. hair begins to grow in the pubic region, face and underarms

4. voice also deepens 

5. the chest and shoulder becomes broader   

6. production of sperm begins

Females: Girls experience very rapid physical growth, accompanied by the beginning of ovulation and menstruation, between the ages of 9 and 13. The ovaries produce estrogen and progesterone responsible for these changes; 

1. Ovulation and menstruation begins

2. The vagina and external genitalia begin to enlarge

3. the breasts and the hips develops and enlarge

4. Pubic hair grows around the genitals and under the armpit

5. voices changes

6. body contour become rounded

7. Development of sexual drive

These changes are referred to as Secondary Sexual Characteristics.  Puberty is the stage within which a male or female develops secondary sexual characteristics.

Birth Control

It is any method that allows people to prevent unwanted pregnancy. It controls birth rate, over population, reduce maternal death and also enhances standard of living for family. Some birth control methods prevent STI and STDs

Methods of Birth Control

Natural Method 

1. Abstinence: Continuous abstinence, or abstaining from sexual intercourse, is the only form of contraception that is 100 percent effective.  

2. Withdrawal Method or Coitus Interruptus: The man withdraws the penis out of the vagina before ejaculation. This is not a reliable method as sperms may leak out early during intercourse.

3. Rhythm Method: This involved abstaining oneself from sexual intercourse during the fertile period of the menstrual cycle (between day 11 to day 15 after the first day of menstruation). Intercourse is restricted to 'safe period' when ovum is not released. This is not reliable because it depends on the regularity of the menstrual period of the women.

Chemical Method

1. Spermicide: is a chemical which kills sperms. It’s applied into before coitus. Not very effective on its own.

2. Hormonal method (Oral contraceptive pills): is a synthetic hormone (progesterone and estrogen) taken orally to prevent ovulation or production of eggs. It is very effective and safe if instructions are followed closely. One pill must be taken a day for 21 consecutive days beginning on the 5th day of menstruation.

Mechanical Method

1. Intrauterine Device (IUD): It is a small plastic or metal device placed in the uterus by a doctor. It prevents implantation of the fertilized ovum.

2. Barrier (Femidom and Condom): Both are made of thin sheath of rubber. Condom is worn over penis during sexual intercourse. Femidom is inserted into the vagina during sexual intercourse. Both function in preventing sperms from entering the vagina during ejaculation. They are very effective in preventing STDs.

3. Cervical Cap (and Diaphragm): It is placed in the vagina to cover the cervix. It prevents sperms from entering the uterus.

Surgical Method

1.  Vasectomy (male sterilization): The sperm ducts are cut and tied. It prevents fertilization. It’s very effective long term birth control method but irreversible.

2.  Tubal Ligation (female sterilization): The oviducts are clipped or tied and cut. It prevents the ovum from meeting with the sperms and thus preventing fertilization. It is very effective long term birth control method but irreversible

Family Planning

Family planning refer to the control of one’s family size to ease pressure on parents and public amenities. The idea in family planning is to plan the number of children couples are to have.

This is necessary for the following reasons:

1.  It promotes maternal or child health
2.  It prevents population explosion

3. It reduces maternal or infant mortality rate

4. It prevents indiscriminate abortion

5. It increases standard of living or financial burden of parents is reduced

Family planning is done by practicing contraception.  Contraception is prevention of pregnancy. It is done by means of contraceptive methods (birth control methods) discussed above.

Click Here for WAEC Past Questions and Answers on Reproduction in Human