Monday, April 30, 2012

1. Human reproduction

Reproduction is the production of young ones by an organism. Humans are sexually reproducing and viviparous.
HUMAN REPRODUCTIVE SYSTEM

1. Male reproductive system
It consists of Paired testes, Accessory ducts, Accessory glands and external genitalia.

a. Paired testes
-    Primary sex organs that produce sperms & testosterone.
-    Testes are formed within the abdomen. Soon after the birth or at the 8th month of pregnancy they descent into the scrotal sac (scrotum) through inguinal canal.
-    The low temperature (2-2.50 C less than the body temperature) of scrotum helps for proper functioning of testes and for spermatogenesis.
-    Each testis is oval in shape. Length 4-5 cm, width: 2-3 cm.
-    Each testis has about 250 testicular lobules. Each lobule contains 1-3 coiled seminiferous tubules.
-    Seminiferous tubule is lined internally with spermatogenic cells (spermatogonia or male germ cells) & Sertoli cells (supporting cells).
-    Sertoli cells give shape and nourishment to developing spermatogenic cells.
-    The regions outside the seminiferous tubules (interstitial spaces) contain
·   Small blood vessels.
·   Interstitial cells or Leydig cells. They secrete testicular hormones (androgens).
·   Immunologically competent cells.
b. Accessory ducts (Duct system)
-    Include rete testis, vasa efferentia, epididymis & vas deferens.
-    Seminiferous tubules unite  to form straight tubules → open into rete testis (irregular cavities) → Vasa efferentia (series of fine tubules that conduct sperms) → epididymis (stores sperms temporarily) vas deferens → join with a duct from seminal vesicle to form common ejaculatory duct → opens into urethra urethral meatus (external opening).
-    The urethra receives the ducts of prostate and Cowper’s glands and passes through the penis.
c. Accessory glands
-    Include a prostate gland, a pair of seminal vesicles and a pair of Cowper’s glands (bulbo-urethral glands).
-    Their collective secretion (seminal plasma) is rich in fructose, Ca and enzymes. It
·   Helps for transporting sperms
·   Supplies nutrients to sperms
·   Provides an alkaline medium to counteract the acidity of the uterus.
·   Secretions of Cowper’s glands lubricate the penis.

d. Penis (external genitalia)
-    It is a copulatory organ made up of special erectile spongy tissues which when filled with blood causes the erection. It facilitates insemination.
-    The cone-shaped tip of the penis is called glans penis which is covered by prepuce (foreskin).

2. Female reproductive system

It includes Ovaries, Accessory ducts & External genitalia.
a. Paired ovaries
- Primary sex organs which produce ova (female gamete) & steroid ovarian hormones (estrogen & progesterone).
-    Ovaries are located on both side of the lower abdomen.
-    Each ovary is about 2-4 cm in length and is connected to the pelvic wall and uterus by ligaments.
-    Each ovary is covered by a thin epithelium which encloses the ovarian stroma.
-    The stroma has outer cortex and inner medulla.
-    Ovary contains groups of cells (Ovarian or Graafian follicles). Each follicle carries a centrally placed ovum.
b. Accessory ducts (Duct system)
-    Include 2 oviducts (Fallopian tubes), a uterus & vagina.
-    Each oviduct (10-12 cm long) has 3 parts such as
·   Infundibulum: Funnel-shaped opening provided with many finger-like fimbriae. It helps to collect the ovum after the ovulation (release of ovum from ovary).
·   Ampulla: Wider part.
·   Isthmus: Narrow part. It joins the uterus.
-    The ciliated epithelium lined the lumen of the oviduct drives the ovum towards the uterus.
-    Uterus (womb) is inverted pear shaped.
-    It is supported by ligaments attached to the pelvic wall.
-    Uterus has 3 parts- Upper fundus, middle body and terminal cervix. Cervix opens to vagina.
-    Vagina opens to the exterior between urethra and anus.
-    The uterine wall has 3 layers such as
·   External perimetrium (thin membrane)
·   Middle myometrium (thick layer of smooth muscle)
·   Inner glandular and vascular endometrium.
-    The lumen of vagina is lined by a glycogen-rich mucous membrane consisting of sensitive papillae and Bartholin’s glands. The secretions of Bartholin’s glands lubricate the penis during sexual act.
c. External genitalia (vulva or pudendum)



-    Consist of Mons pubis, vestibule, hymen & clitoris.
-    Mons pubis: A cushion of fatty tissue covered by pubic hair.
-    Vestibule: A median channel. It includes
·   Labia majora: Large, fleshy, fatty and hairy outer folds. Surrounds vaginal opening.
·   Labia minora: Small, thin and hairless inner folds.
-    Hymen (Maiden head): A membrane which partially cover the vaginal opening. It is often torn during the first coitus. It can also be broken by a sudden fall or jolt, insertion of a vaginal tampon; active participation in some sports items etc. In some women, hymen persists even after coitus. So the hymen is not a reliable indicator of virginity.
-    Clitoris: A highly sensitive organ lying just in front of the urethral opening.

Mammary glands (breasts)

-    A pair of mammary glands contains glandular tissue & fat.
-    Glandular tissue of each breast has 15-20 mammary lobes containing clusters of cells (alveoli).
-    The cells of alveoli secrete milk which is stored in the cavities (lumen) of alveoli.
-    The alveoli open into mammary tubules.
-    The tubules of each lobe join to form a mammary duct.
-    Several mammary ducts join to form a wider mammary ampulla which is connected to lactiferous duct through which milk is sucked out.

GAMETOGENESIS

-    It is the formation of gametes in the gonads.
-    Includes Spermatogenesis and Oogenesis.
1. Spermatogenesis

-    It is the process of formation of sperms (spermatozoa) in seminiferous tubules of testis. It has 2 stages: Formation of spermatids and Spermiogenesis.
a. Formation of spermatids: In this, Spermatogonia (Sperm mother cells or immature male germ cells) produce spermatids.
       b.Spermiogenesis: Spermatids transform into sperm.

Schematic representation of spermatogenesis

Spermatogonia -2n (Diploid, 46 chromosomes)
Mitosis differentiation
Prim. spermatocytes (2n)
1st meiotic division
Sec. spermatocytes -n (Haploid, 23 chromosomes)
2nd meiotic division
Spermatids (n)
Differentiation  
Spermatozoa (n)
-    Each Primary spermatocyte produces 2 secondary spermatocytes. Each secondary spermatocyte produces 2 haploid spermatids. Thus 4 spermatids are formed from each primary spermatocyte.
- After spermiogenesis, sperm heads become embedded in the Sertoli cells and are finally released from seminiferous tubules. It is called spermiation.

Role of Hormones in Spermatogenesis

-    Hypothalamus releases large amount of Gonadotropin releasing hormone (GnRH).
-    GnRH stimulates the anterior pituitary gland to secrete 2 gonadotropins such as Luteinizing hormone (LH) & follicle stimulating hormone (FSH).
-    LH acts on the Leydig cells and stimulates secretion of androgens. Androgens stimulate the spermatogenesis.
-  FSH acts on the Sertoli cells and stimulates secretion of some factors which help in the process of spermiogenesis.
Structure of spermatozoa (Sperm)

-    A mature sperm measures about 60 µ (0.06 mm) long.
-    A plasma membrane envelops the whole body of sperm. Sperm consists of
a.    Head: Oval shaped. Formed of nucleus and acrosome. Acrosome is formed from Golgi complex. It contains lytic enzymes. Behind the head is a neck containing proximal and distal centrioles.
b.   Middle piece: Composed of axial filament surrounded by mitochondria and cytoplasm. Mitochondria produce energy for the sperm motility.
c.    Tail: Consists of a central axial filament. The sperm moves in fluid medium and female genital tract by the undulating movement of the tail.
-    Sperms are transported by the accessory ducts. Secretions of epididymis, vas deferens, seminal vesicle and prostate are essential for maturation and motility of sperms.
-    Seminal plasma & sperms together constitute the semen.
-    The human male ejaculates about 200-300 million sperms during a coitus.
-    For normal fertility at least 60% sperms must have normal shape and size. 40% of them must show vigorous motility.
2. Oogenesis

-    It is the process of formation and maturation of ovum.
-    It takes place in Graafian follicles.
-    Oogenesis is initiated in embryonic stage when millions of egg mother cells (oogonia) are formed within each ovary.
-    No more oogonia are formed and added after birth.
-    Oogonia multiply to form primary oocytes. They enter into prophase-I of the meiosis and get temporarily arrested at that stage.
-    Each primary oocyte gets surrounded by a layer of granulosa cells to form primary follicle.
-    A large number of primary follicles degenerate during the phase from birth to puberty. Therefore at puberty only 60,000-80,000 primary follicles are left in each ovary.
-    Primary follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles.
-    The secondary follicles transform into a tertiary follicle. It has a fluid filled cavity (antrum). The theca layer forms an inner theca interna and an outer theca externa. 
-    The primary oocyte within the tertiary follicle grows in size and undergoes first unequal meiotic division to form a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte retains the nutrient rich cytoplasm of the primary oocyte.
-    It is unknown that whether the first polar body divides further or degenerates.
-    The tertiary follicle further changes into the mature follicle (Graafian follicle).
-    Secondary oocyte forms a new membrane (zona pellucida).
-   Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary. This is called ovulation.
Schematic representation of oogenesis


Oogonia (egg mother cells)-2n (46 chromosomes)
  ↓ Mitosis differentiation
Primary oocyte- 2n (grow in size)
                   ↓ 1st meiotic division (prior to ovulation)
Secondary oocyte (n) & first polar body (n)
  ↓ 2nd meiotic division (during fertilization)
Ovum (n) & second Polar body (n)

Comparison between Spermatogenesis & Oogenesis

Spermatogenesis
Oogenesis
·   Occurs in testis
Occurs in ovary
·   Limited growth phase
Elaborated growth phase
·   Each primary spermatocyte gives 4 sperms
Each primary oocyte gives only one ovum. Polar bodies are formed.
·   It begins at puberty and extends up to senility
It begins at embryonic stage and suspended at the time of birth. The remaining part takes place only after puberty.

Structure of ovum (egg)
-    Spherical and non-motile. About 0.2 mm in diameter.
-    Ovum has 4 membranes:
a.    Plasma membrane (Oolemma): Innermost layer.
b.   Vitelline membrane:  Attached to plasma membrane.
c.    Zona pellucida: Transparent non-cellular layer found outer to the Vitelline membrane.
d.   Corona radiata: Outer layer formed of follicle cells. These cells are found together by hyaluronic acid.

MENSTRUAL CYCLE (REPRODUCTIVE CYCLE)

-    It is the cyclic events starting from one menstruation till the next that take place during the reproductive period (from puberty to menopause) of a woman’s life. Its duration is 28 or 29 days.
-    Menstrual cycle is also seen in other primates.
Menstrual cycle includes
a.     Ovarian cycle: Changes in ovary.
b.    Uterine cycle: Changes in uterus, oviduct and vagina.
Menstrual cycle has the following phases
I.  Menstrual phase: 1-5th day
-    The cycle starts with menstrual flow (bleeding).
-    It lasts for 3-5 days.
-    It is due to breakdown of endometrial lining and blood vessels of the uterus that comes out through vagina.
-    Menstruation occurs if the released ovum is not fertilized.
-    Lack of menstruation may be indicative of pregnancy.
It may also be caused due to stress, poor health etc.
II. Follicular (Proliferative) phase: 5-13th day
-    It starts from 5th day after menstruation and completed within 8-12 days.
-    In this phase, the action of gonadotropins (FSH &LH) from pituitary occurs. FSH stimulates
o Development of primary follicles into Graafian follicles.
o Secretion of oestrogens by Graafian follicles.
-    Oestrogens stimulate
o Proliferation of ruptured uterine endometrium and mucous lining of oviduct & vagina.
o Development of secondary sexual characters.
o Suppression of FSH secretion.
o Secretion of LH (Luteinizing hormone).
III. Ovulatory phase: 14th day
-    LH & FSH attain a peak level in the middle of cycle.
-    Rapid secretion of LH (LH surge) induces rupture of Graafian follicle and thereby ovulation (on 14th day).
IV. Secretory (Luteal) phase: 15-28th day
-    After ovulation, Graafian follicle is transformed into a yellow endocrine mass called Corpus luteum. It secretes progesterone. By the action of progesterone,
o Endometrium attains maximum vascularity, thickness and softness. Thus the uterus gets ready for implantation.
o FSH secretion is inhibited to prevent development of a second ovarian follicle.
-    During pregnancy all events of menstrual cycle stop and there is no menstruation.
-    If fertilization does not occur, corpus luteum degenerates and becomes a whitish mass (corpus albicans). As it has no secretory ability, progesterone level in blood decreases. It leads to menstruation indicating a new cycle.


§ Menarche: The first menstruation during puberty.
§ Menopause: Stopping of menstrual cycle (at about 50 yrs of age).

FERTILIZATION AND IMPLANTATION

-    During copulation, semen is released by the penis into the vagina (insemination).
  
Sperms → vagina → cervical canal → uterus → isthmus
                                                                            ↓
                       Fertilization ← Ampullary-isthmic Junction 
                                                                            ↑                      
Ovum (from ovary)fimbriae → infundibulum → ampulla

-    Fertilization (sperm + ovumzygote) occurs only if ovum & sperms are transported simultaneously. So all copulations do not lead to fertilization & pregnancy.
-    A sperm contacts with zona pellucida. It induces changes in the membrane that block entry of additional sperms.
-    With the help of secretions of the acrosome, sperm enter into the cytoplasm of the ovum. This causes second meiotic division of the secondary oocyte to form a second polar body and a haploid ovum (ootid).
-    The haploid nuclei of the sperm and ovum fuse together to form a diploid zygote.
-    Sex determination (See chapter 3).
-    The mitotic division (cleavage) starts as the zygote moves through the isthmus towards the uterus and forms 2, 4, 8, 16 daughter cells called blastomeres.
-    The embryo with 8-16 blastomeres is called a morula.
-    Morula continues to divide and transforms into blastocyst.
-    The blastomeres in the blastocyst are arranged into an outer layer (trophoblast) and an inner group of cells (inner cell mass) attached to trophoblast.
-    The trophoblast layer then gets attached to endometrium and the inner cell mass gets differentiated to 3 germ layers (outer ectoderm, middle mesoderm & inner endoderm). This 3-layered structure (gastrula) forms the embryo.
-    After attachment, uterine cells divide rapidly and cover the blastocyst. As a result, the blastocyst becomes embedded in the endometrium. This is called implantation.

PREGNANCY AND EMBRYONIC DEVELOPMENT

-    After implantation, finger-like projections (chorionic villi) appear on the trophoblast which is surrounded by the uterine tissue and maternal blood.
-    The chorionic villi & uterine tissue become interdigitated with each other and form placenta. It is a structural and functional unit b/w embryo (foetus) and maternal body.
-    Placenta is connected to the embryo by an umbilical cord. It transports substances to and from the embryo.
-    Functions of placenta:
·   Acts as barrier between the foetus and mother.
·   Soluble inorganic and organic materials, nutrients, hormones, antibodies, etc can pass through the placenta from the mother to foetus.
·   Helps in the gas exchange between mother and foetus.
·   Elimination of nitrogenous wastes of foetus.
·   Acts as an endocrine gland by secreting Human chorionic gonadotropin (hCG), human placental lactogen (hPL), oestrogens, progesterone and relaxin.
-    During pregnancy, levels of estrogens, progestogens, cortisol, prolactin, thyroxine etc are also increased in maternal blood. They support the fetal growth, metabolic changes in the mother and maintain pregnancy.
-    The germ layers give rise to all tissues (organs). The stem cells in inner cell mass have the potency to give rise to all the tissues and organs.
-    Human pregnancy (gestation period) lasts 9 months (for cats: 2 months, dogs: 2 months, elephants: 21 months).

Changes in embryo during pregnancy:

-    After one month of pregnancy: Heart is formed.
End of second month: Limbs and digits are developed.
End of 12 weeks (first trimester): The major organs (limbs, external genital organs etc) are well developed.
During 5th month: First movement of foetus and appearance of hair on the head.
End of 24 weeks (second trimester): Body is covered with fine hair, eyelids separate and eye lashes are formed.
End of 9 months: Ready for delivery.   

PARTURITION AND LACTATION

-    Parturition (labour): Process of giving birth to young ones.
-    Parturition is induced by neuroendocrine mechanism.
-    The signals originating from the foetus and placenta induce mild uterine contractions (fetal ejection reflex). This causes the release of oxytocin from maternal pituitary.
-    Oxytocin causes stronger uterine muscle contractions which in turn stimulate further secretion of oxytocin. This process is continued leading to expulsion of the baby out of the uterus through the birth canal.
-    After parturition, the umbilical cord is cut off.
-    The placenta & remnants of umbilical cord are expelled from the maternal body after parturition. It is called “after birth”.
-    The mammary glands produce milk towards the end of pregnancy by the process called lactation.
-    The yellowish milk produced during the initial few days of lactation is called colostrum. It contains several antibodies essential to develop resistance for the new born babies.

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