HUMAN REPRODUCTION

HUMAN REPRODUCTION

Ø MALE REPRODUCTIVE SYSTEM:-[ Paired Testis  + Accessory Ducts + External Genitalia]

(Human Male Reproductive system -Front View)

   SCROTUM: - extra abdominal pouch to contain & protect Testicles. They maintain a low temperature (2-2.5⁰C lower than body temp.) necessary for spermatogenesis.

   Adult Testis is Oval shaped (length 5cm; width 3cm) covered by Tunica albugenia and fibrous tissue. Each testis has ~ 250 compartments (Testicular Lobules), each contain 1-3 highly coiled SEMINIFEROUS TUBULES (each Tubule is lined inside by GERMINAL EPITHELIUM/ SPERM MOTHER CELL/ SPERMATOGONIUM and SERTOLI CELLS/ Nourishing Cells).

         Between Seminiferous Tubules, are present INTERSTITIAL CELLS OF LEYDIG/ INTERSTITIAL CELLS/ LEYDIG CELLS (Synthesize ANDROGENS  eg. Testosterone).

   ACCESSORY DUCTS:-

(A) Seminiferous Tubule opens in to Vas Efferentia through a network of Rete Testis.

(B) Vas Efferentia leaves testis & opens in to EPIDIDYMUS located on posterior side of Testis.

(C) Epididymus :- (Head-Body-Tail) final maturation of sperms & storage before release.

(D) Vas Deference:- carry sperms from Epididymus , loops over Urinary bladder & opens into Urethra as Ejaculatory Duct  after getting secretions from other glands/ducts.

(E) Urethra originates from Urinary bladder & extends through the penis to its external opening (URETHRAL MEATUS).

   ACCESSORY GLANDS:-

(A) Paired SEMINAL VESICLES (long, chilly shaped, located behind Urinary bladder, secrets alkaline fluid to save sperms from Vaginal Acidity)

 (B) PROSTATE GLAND (dispersed, cloudy located between seminal vesicle & Cowper’s Gland & fluid rich in Fructose, Calcium, other enzymes to nourish & increase sperm mobility in vaginal tract)

(C) Paired BULBOURETHRAL GLAND or COWPER’S GLAND (bulb shaped, size of pea seeds, located at the origin of Urethra, secretes thick viscous fluid for lubrication & reduction of friction during coitus)

   PENIS:- (Male External Genitalia) made of special Erectile Tissue to facilitates Insemination. Its enlarged end (Glans Penis) is covered by a loose skin fold (Foreskin).

Ø FEMALE REPRODUCTIVE SYSTEM:- [Paired Ovaries + Oviduct + Uterus +Cervix + Vagina] {Mammary Glands}.

   OVARIES:- located on each side of lower abdomen (2-4 cm long) & connected to Pelvic wall & Uterus by Ligaments. Each ovary covered by thin Germinal Epithelium enclosing Ovarian Stroma (cortex + Medulla). Ovaries produce OVA & HORMONES (ESTROGEN; PROGESTERONE)..

( HUMAN OVARY - T.S )

   OVIDUCT/ FALLOPIAN TUBES:- (10-12 cm long) extends from periphery of each ovary to uterus. Close to ovary it is funnel shapes (INFUNDIBULUM) with edges finger like (FIMBRIAE) which collects ovum after ovulation.. Infundibulum leads to a wider part (AMPULLA) where fertilization takes place. Last part is ISTHMUS with narrow lumen & joins the Uterus.

   UTERUS (WOMB):-  Single & shape like an inverted Pear. Supported by Pelvic ligaments. Opens in to Vagina by a narrow Cervix (Cervical Canal) which along with Vagina forms the BIRTH CANAL. Uterine wall is 3 – layered :-

PERIMETRIUM (External thin membranous)

 MYOMETRIUM (middle thick smooth muscles)

 ENDOMETRIUM (inner glandular)

 Endometrium undergoes cyclic changes during Menstrual Cycle while Myometrium exhibits strong contraction during Parturition (BIRTH) of the child.

   EXTERNAL GENITALIA:- Cluster of fatty tissue (MONS PUBIS) covered by skin & pubic hairs. Fleshy Folds of Tissues (LABIA MAJORA) extend down surrounding of Vaginal Opening; LABIA MINORA are paired folds under Labia Majora. Vaginal Opening is partially covered by a membranous HYMEN. A tiny finger like structure (CLITORIS) lies at the upper juncture of Labia Minora above urethral opening (Embryologically Homologous to Penis).

   MAMMARY GLANDS:-Paired structures (Breasts) containing Glandular Tissue (divided into 15-20 lobes containing clusters of cells- ALVEOLI, secreting milk, which is stored in the lumen/cavity of alveoli. Alveoli opens in to Mammary Ducts which joins to form wider Mammary Ampulla connected to Lactiferous Ducts or Nipples, through which milk is sucked out)  & variable amount of Fat.

Ø GAMETOGENESIS :-( Production of Gametes through Meiosis, in Primary Reproductive Organs- Testis/Ovary).

   SPERMATOGENESIS:- (Formation of sperms in testis from Spermatogonia, after puberty)

(SPERMATOGENESIS)

   SPERMATOGONIUM (2n/diploid; present on inner wall of germinal epithelium of Seminiferous Tubules) divides mitotically to increase in number (A to G stages).

                 Later becomes Primary Spermatocytes (2n) which undergo Meiosis/ R.D-I to become Two haploid SECONDARY SPERMATOCYTES (n) , which further divides (RD-II) to produce TWO SPERMATIDS  (n) each. A total  of 4- spermatids from one Primary spermatocytes, are formed. These Spermatids are transformed/ metamorphose in to SPERMATOZOA/SPERMS by a process called SPERMIOGENESIS or SPERMIOTELEOSIS. After this, Sperm heads are embedded in Sertoli Cells & are finally released from the Seminiferous Tubules by a process called SPERMIATION.

   Spermatogenesis starts at Puberty due to the action of Gonadotropin Releasing Hormone (GnRH) from Hypothalamus over Anterior Pituitary Gland to secret two hormones –

1.     Luteinizing Hormone (LH) acts at Leydig’s Cells to produce Androgens for Spermatogenesis;

2.     Follicle Stimulating Hormone (FSH) acts on Sertoli Cells to stimulate factors helping Spermatogenesis.

   STRUCTURE OF SPERM:- [HEAD + NECK + MID PIECE + TAIL] 

(HUMAN SPERMATOZOA- SKETCH)

( HUMAN SPERMATOZOA -SKETCH)

Entire body is covered with Plasma Membrane (Animal Cell).

1. Head contains Haploid NUCLEUS (n); ACROSOME (modified Golgi apparatus containing Lytic enzymes/ Sperm Lysins eg. Hyaluronidase, to penetrate layers of egg).

2. Mid Piece contains Mitochondrial Spire (Nebenckern) to provide energy for motility).

3. Tail produces concentric lashing movement to move sperm.

   Human Males release/ejaculates 200-300 million sperms. For normal fertility, 60% sperms must have normal shape & size, 40% must show vigorous motility.

      Secretions of Epididymus, Vas deferens, Seminal Vesicles & Prostate Glands are essential for maturation & motility of sperms (CAPACITATION). Complete Fluid is called SEMEN (alkaline).

   OOGENESIS: - (process of formation of Female gamete).

      Initiated during Embryogenesis, when a few million Gamete Mother Cells (OOGONIA) are formed in each Fetal Ovary. NO more Oogonium is formed after the Birth of the girl child. They become Primary Oocytes (2n) and get temporarily arrested in Prophase-I of Meiosis.

      They are surrounded by Granulosa cells (PRIMARY FOLLICLE).  Most follicles degenerate up to puberty (only 60,000 – 80,000 left in each ovary). These are surrounded by more Granulosa cells (SECONDARY FOLLICLE) later more cells added & a fluid filled cavity (ANTRUM) is formed (TERTIARY FOLLICLES).

      The Theca is differentiated in to Theca Interna & Theca Externa. {at this time primary Oocytes grows in size & complete its final meiotic division to form a Haploid SECONDARY OOCYTE + a polar body).The Tertiary Follicle further changes in to a mature follicle/ GRAAFIAN FOLLICLE.

      The secondary Oocytes form a new membrane (ZONA PELLUCIDA). The Graafian Follicle ruptures to release secondary Oocytes/ovum from ovary in to Fallopian Tube (OVULATION).

( HUMAN OVUM)

( BOTH PROCESSES COMPARED )

   MENSTRUAL CYCLE:- (The reproductive cycle in the female primates)

   In Human females, the first Menstruation begins at puberty (MENARCH) & the cycle ceases at 50 years of age (MENOPAUSE).

   In Human Females, Menstruation is repeated every 28/29 days & the cycle of events starting from one menstruation till next is called MENSTRUAL CYCLE. It is divided in to Three Phases:-

1.    MENSTRUAL PHASE (3-5 days):- Breakdown of Endometrial linings of Uterus & its blood vessels forming a fluid that comes out of vagina along with ripe/unfertilized ova (secondary oocyte).  Lack of Menses is indicative of Pregnancy (sometimes due to poor health, stress etc..)

2.    FOLLICULAR/PROLIFERATORY PHASE (12-14 days):-Primary Follicles grow to become Mature/Graafian Follicle. Endometrium of Uterus also regenerates through PROLIFERATION. These changes are induced by changes in PITUITARY & OVARIAN Hormonal Levels. The secretions of Gonadotropin (LH & FSH) increase gradually & stimulate Follicular development as (well as secretion of Estrogen by growing Graafian Follicle). Both FSH & LH attains a peak level in mid cycle (14^th Day). Surge in LH –level induce rupture of Graafian Follicle (OVULATION).

3.    LUTEAL PHASE (10 days):- Ovulation results in the formation of a yellow cloudy mass of remnants of Follicular cells (CORPUS LUTEUM) which secrets another hormone PROGESTERONE (essential for maintaining endometrial linings; implantation & other events of pregnancy) In case of Pregnancy, the Ovarian events are arrested in this condition & no menstruation occurs. In case of no fertilization, Corpus Luteum degenerates in to a white body (CORPUS ALBICANS) & level of LH – drops down sharply resulting in to breakdown of uterine Endometrium (MENSES).

   FERTILIZATION & IMPLANTATION:- 

   PRE FERTILIZATION EVENTS: Semen delivered in Vagina by Penis (INSEMINATION). Motile sperms pass through cervix, enter uterus & finally Ampullary-Isthmic Junction of Fallopian Tube. Acidity of female tract is neutralized by semen. Some sperms are killed through phagocytosis by uterine wall, some move to wrong Tube/ Oviduct, some exhausted. Some reach their destination (50-250). Ovary also release ovum/secondary Oocytes in Ampulla (OVULATION).

   FERTILIZATION EVENTS:-

1.    Fertilization occurs only when both sperm & ova reach isthmic-ampullary junction simultaneously. (That’s why NOT all copulations result in pregnancies).

2.    The process of fusion of sperm & ovum is called FERTILIZATION. Many sperms try to enter, only one succeeds, Acrosome burst & release its sperm Lysins to digest corona radiate+ zona pellucida + Vitelline membrane,

3.     Formation of fertilization membrane, Cortical Reaction (to prevent polyspermy),

4.     Conversion of secondary Oocytes to ova releasing second polar body, fusion of nucleus of sperm & ovum (AMPHIMIXIS) resulting as Zygote.

   POST FERTILIZATION EVENTS: -

1.    Mitotic divisions (CLEAVAGE) start as zygote moves from isthmus to uterus.

2.     Cleavage is HOLOBLASTIC, COMPLETE & UNEQUAL in general. It results in 2, 4,8,16 (MORULA), 32(BLASTULA), 64(GASTRULA) celled stages called Blastomers.

3.     BLASTULA/BLASTOCYST has outer flat cells (TROPOBLASTS / FEEDER LAYER) which absorbs nourishment & stores it in a cavity inside (BLASTOCOEL) surrounded by cells of INNER CELL MASS...

( HUMAN BLASTULA - SEM Picture)

4.    Trophoblast gets attached to Endometrium (IMPLANTATION) within 5-7 days of fertilization.

(IMPLANTATION OF EMBRYO)

5.    Inner Cell Mass develops in to Embryo.

   PREGNANCY & EMBRYONIC DEVLOPMENT:-

   After Implantation, finger like CHORIONIC VILLI appears on Trophoblast, surrounded by uterine tissue & maternal blood {PLACENTA FORMATION}.

   ROLE OF PLACENTA:- supply of oxygen & nutrition, removal of carbon dioxide & Excretory products by an Umbilical Chord. Placenta as Endocrine tissue (Hormones like hCG, hPL, estrogen, progesterone)

   Relaxin is produced under ovarian influence. Very High metabolic rate & Hormonal levels are maintained during GESTATION PERIOD (280 days) which is essential to support fetal growth.

   THREE GERM LAYERS:- After implantation, inner cell mass differentiates in to an outer layer (ECTODERM), an inner layer (ENDODERM) & a middle layer in between (MESODERM) which give rise to tissue systems/Organs in adults. Inner cell Mass contains STEM CELLS (TOTIPOTENT).

( Early Embryo with Extra Embryonic Membrane)

(Developed Fetes with Extra Embryonic Membranes)

   ORGANOGENESIS:- Heart (1 month),  Limbs & digits (2 month), most of major organ systems formed in 12 weeks (3 months), First fetal movement &hairs on head (5 month), body hairs-eye lid separates-eye lashes formed (second trimester/24 weeks/6 month), fully developed foetus & delivery/ PARTURITION/BIRTH (9 month)

   PARTURITION/CHILD BIRTH (process of delivery of the foetus):- Gestation period (280 days/ a little over 9 months) . It is induced by a complex Neuro-endocrine Mechanism. Signals released from fully developed foetus & placenta which induces mild uterine contraction (FOETAL EJECTION REFLEX).

This triggers release of OXYTOSIN (Posterior Pituitary) acting on uterine muscles causing strong Uterine Contractions, which in turns stimulate further secretion of Oxytocin. This cycle continues resulting in stronger contractions (LABOUR PAIN) &leads to expulsion of baby out through birth canal (PARTURITION). Soon after, placenta is also expelled out.

C-Section:

   LACTATION:-Mammary Glands undergo differentiation under hormonal influence, during pregnancy & starts producing milk during the end og pregnancy period (LACTATION) to feed the new born. The milk produced during the first few days of lactation (COLOSTRUM) is yellowish & dense. It contains several Antibodies very necessary for developing immunity in the new born baby.

(Breast feeding during initial period is highly recommended by doctors for a healthy baby).