I. Male Reproductive System

1. Testes: There is a pair of oval, pinkish testes, that lie outside the abdominal cavity, in special pouch called scrotum [because in scrotum the temperature is less than (2°C - 2.5°C) that of abdomen, which is necessary for spermatogenesis].

The cavity of scrotum is connected to the abdominal cavity through inguinal canal. Testes are held in position in scrotum by spermatic cords (extends between testes and abdomen) and gubernaculum. Each spermatic cord is formed by an artery, a vein, a nerve and other tissues.

A testis is enclosed by a tunica vaginalis. Below the tunica vaginalis is a fibrous sheath called tunica albuginea. It projects inwards as septa and divides the testis into many lobules. Each lobule contains 1 - 3 highly coiled seminiferous tubules. They are lined by germinal epithelium. Germinal epithelial cells divide and form spermatogonia (sperm mother cells). They divide further and produce primary spermatocytes. These primary spermatocytes undergo meiotic divisions and form the sperms/ spermatozoa [The process of formation of sperms is known as spermatogenesis].

Among the germinal epithelial cells sertoli cells are present. They provide nutrition for developing sperms and secrete a hormone called inhibin, which inhibits the secretion of FSH. Among the seminiferous tubules, there are Leydig cells, or interstial cells. They secrete testosterone. It is necessary for the development of secondary sexual characters in male.

Seminiferous tubules open into the vasa efferentia through a network of minute tubules called rete testis.

2. Epididymis: It is a long, narrow and highly coiled tube present along the posterior surface of each kidney. In epididymis, sperms are stored, where they mature. Epididymis is divisible into three parts, namely, caput epididymis (receives sperms through vasa efferentia), corpus epididymis (connects caput epididymis and cauda epididymis) and cauda epididymis (it is attached to the scrotum through gubernaculum).

3. Vasa deferentia: The long narrow, muscular tube arising from the cauda epididymis is the vasa deferns. The inner lining of vas diferens is formed by pseudostratified epithelium and lamina propria (areolar tissue). Vas deferens passes through the inguinal canal, loops over the urinary bladder, receives a duct from seminal vesicle and become an ejaculatory duct. The ejaculatory ducts of both sides converge and open into the urethra.

4. Urethra: It is the common passage for urine and sperms. It originates in the urinary bladder, receives the ejaculatory ducts, passes through the penis and opens out through urethral meatus.

5. Penis: Penis is the copulatory organ that transfers the sperms to the vagina of female. It is formed by two dorsal (corpora cavernosa) and a ventral (corpus spongiosum) columns of special tissue that helps in erection. The enlarged tip of the penis is called glans penis, which is covered by loose skin called prepuce. Urethra lies in corpus spongiosum.

6. Accessory glands:

a. Seminal vesicles: Near the posterior part of urinary bladder there is a pair of simple tubular glands known as seminal vesicles. Each seminal vesicle opens into the corresponding vas deferens. Seminal vesicles secrete about 60% of the seminal fluid. It is an alkaline, viscous fluid that contains prostaglandins, fructose, proteins, citric acid, inorganic phosphorus and potassium. Prostaglandins cause the mucus lining of cervix to be more receptive to the sperms and activate sperms. Fructose is the energy source for sperms.

b. Prostate gland: It lies beneath the urinary bladder. It surrounds the urethra and sends its secretions into it. Prostate secretion forms 15 - 30% of seminal fluid. It activates the sperms and provides nutrition.

c. Bulbo urethral glands/ Cowper's glands: Behind the prostate gland, at the beginning of penis, is a pair of Cowper's glands. The secretions of these glands are alkaline, lubricates the urethra and makes it alkaline by flushing out the acidic urine residues.

II. SPERMATOGENESIS

The process of formation of sperms from the seminiferous tubules of testis is known as spermatogenesis. The germinal epithelial cells of seminiferous tubules divide and produce spermatogonia and each spermatogonium is a diploid cell (having 23 pairs of chromosomes). Spermatogonia divide mitotically and produce primary spermatocytes (diploid). Each spermatocyte undergo Meiosis I and form two haploid secondary spermatocytes. They undergo Meiosis II and form four haploid spermatids. Spermatids metamorphase into functional sperms. [Process of formation of sperms from spermatids is know as spermiogenesis]. After spermiogenesis (sperm heads become embedded in the sertoli cells), sperms are released from seminiferous tubules by a process called spermiation.

Spermatogenesis begins at the age of puberty due to increase in release of gonadotropin releasing hormone by the hypothalamus. It stimulates the secretion of follicle stimulating hormone and luteinising hormone by the adenohypophysis of pituitary. LH stimulates the Leydig cells to secrete testosterone, which stimulates the spermatogenesis. FSH stimulates the secretion of some factors necessary for spermatogenesis.

        A sperm is a minute, actively motile cell having three parts, viz. head, middle piece and tail. Entire sperm is covered by a thin plasma membrane.
i. Head contains a nucleus (haploid). Anteriorly, head is covered by a cap, known as
acrosome. It contains enzymes like hyaluronidase, acid phosphatase, proteases that help in fertilisation. Head is followed by a neck.
ii. Neck is followed by the middle piece, which mainly contains mitochondria.
iii. Behind the middle piece is the tail, which helps in swimming in semen/fluids of female reproductive organs.
* The seminal plasma along with sperms is know as semen.
 

III. Female Reproductive System

1. Ovaries: A pair of ovaries lie in the abdominal cavity behind the kidneys. They are attached to the inner surface of dorsal abdominal wall by a peritoneum, known as mesovarium. Ovaries are covered by a layer of simple cuboidal epithelium (germinal epithelium). Below this is a dense connective tissue layer is present called tunica albuginea. The stroma of ovary is formed by an outer cortex and inner ovary. Cortex contains ovarian follicles at various stages of development. Stroma contains blood vessels, lymph vessels and nerves.

2. Fallopian tubes (oviducts): There are two fallopian tubes, one each side, extending between the ovary and uterus. Fallopian tubes are attached to the abdominal wall by mesosalpinx. The free end of fallopian tube is funnel like and is known as infundibulum. Its opening is known as ostium. The edges of infundibulum are fimbriated.

Fimbriae help in collecting the ovum after ovulation. Infundibulum leads to a wider part called ampulla. It opens into the uterus through an isthmus having a narrow lumen. Ovum is fertilised in the fallopian tubes.

3. Uterus or Womb: It is a triangular, highly vascular and highly muscular inverted pear shaped sac lies in the pelvis between the urinary bladder and rectum. Uterus is attached to abdominal wall by mesometrium. Uterus opens into vagina through cervix. Cavity in cervix is called cervical canal.
        Uterine wall contains three layers. Outer layer is thin and is called perimetrium. Middle thick, muscular layer (smooth muscles) is the myometrium. The inner glandular and vascular layer is the endometrium. Endometrium undergoes cyclical changes during menstrual cycle.

4. Vagina: It is a long, median fibro muscular tube extending between cervical canal and vestibule. It is lined by non keratinised stratified epithelium. It is highly vascular and opens into the vestibule by vaginal orifice.

5. Ulva/ Pudendum: External genitalia of female forms the ulva. Vestibule contains two openings- The upper urethral orifice and the lower vaginal orifice. Vaginal orifice is partially covered by a mucus membrane called hymen. Vestibule is bound by a pair of labia minora (inner) and a pair of labia majora (outer). At the upper junction, where the two labia minora joins is a clitoris. It is an erectile, sensitive structure, which is homologous to penis of male.
6. Accessory glands:
a. Bartholin glands/Greater vestibular glands: A pair of Bartholin glands lie one on each side of opening of vagina. Their secretions lubricate the vaginal passage (Bartholin glands are homologous to bulbo urethral glands of male).
b. Skene glands/ Lesser vestibular glands: These glands are located in the anterior wall of vagina (These are homologous to prostate gland of male). When stimulated, they secrete a lubricating fluid.
c. Mammary glands (Milk glands): The mammary glands are paired structures containing glandular tissue and fat. Glandular tissue of each breast is divided into 15-20 mammary lobes containing clusters of cells called alveoli. Cells of alveoli secrete milk, and is stored in cavities of alveoli. They open into the tubules, which join with each other and form a mammary duct. Several such ducts join to form a wide mammary ampulla. It is connected to a lactiferous duct that opens on the nipple, through which the milk is sucked by the baby.

IV. Oogenesis: The process of formation of ovum from ovary is known as oogenesis. It is initiated during embryonic development, about two millions oogonia are formed in the foetal ovary. They stop their division at prophase I of Meiosis I. At this stage, they are called primary oocytes.
1. Formation of Ovarian follicles:
i. Primary oocyte is encircled by a squamous epithelial layer. Now it is called primordial follicle.
ii. Gradually squamous epithelium becomes cubiodal epithelium and then becomes stratified epithelium. It is called membrana granulosa. The cells are called granulosa cells. Now the follicle is called primary follicle.
iii. Then primary oocyte is covered by a homogeneous membrane called zona pellucida.
iv. Inner layer of granulosa cells are firmly attached to zona pellucida and form corona radiata.
v. A cavity appears within the membrana granulosa and increases in size. It is called antrum. Still the primary oocyte is attached to wall of follicle at one side.
vi. As the follicle grows, the stroma cells surrounding the membrana granulosa form a covering around the follicle called theca. Now the follicle is called secondary follicle.

vii. Inner cells of theca secrete oestrogen.
viii. At this stage, primary oocyte completes Meiosis I and form a large, haploid secondary oocyte and a small first polar body.
ix. Secondary oocyte begins Meiosis II but stops at Metaphase II . Now the follicle projects out from the ovary and is called mature or Graafian follicle.

2. Ovulation:
i. Release of ovum (at secondary oocyte stage) from the Graafian follicle of ovary is called ovulation.
ii. After ovulation, the empty follicle is filled with yellowish glandular substance called corpus luteum.
iii. It secretes a hormone progesterone.
iv. If the ovum is fertilised, corpus luteum remains in the follicle upto four months. Progesterone is essential for maintenance pregnancy in first few months. Later placenta secretes progesterone. Hence degeneration of corpus luteum takes place.
v. If ovum is not fertilised, corpus luteum persists for about 14 days.
vi. After functional existence, corpus luteum degenerates leaving a mass of fibrous tissue called corpus albicans.
 

3. Ovum: Ovum is surrounded by zona pellucida. Between the cell membrane/ vitelline membrane and zona pellucida, perivitelline space is present.

4. Reproductive Period: In females, the formation ova begins at the age of puberty (10 - 14 years) and continues upto 45 - 50 years. With the onset of menopause, oogenesis is impaired. In males also formation of sperms begins at 10 - 14 years and continues much longer period than that of females.
 

V. Menstrual Cycle:
The reproductive cycle in the primates (females) (e.g.: rhesus monkey, man) is known as menstrual cycle. Menstruation is the term applied to the periodic discharge of blood, mucus, and cellular debris from the uterus in non pregnant primates (females). In human beings the period of menstrual cycle is 28 days. Menstruation begins at puberty and is called menarche. One ovum is released in the middle of each menstrual cycle.

i. Menstruation: It lasts for about 3 - 5 days. When the ovum is not fertilised, the endometrium along with its vascular tissues are discharged (menstruation) through vagina. Absence of menstruation is generally an indication of pregnancy.
ii. Follicular phase: In this phase, the primary follicles in ovary becomes Graafian follicles. Simultaneously the uterine endometrium becomes thick and highly vascular.
iii. Ovulation: In the middle of the menstrual cycle (about 14th day). Ovulation takes place.
iv. Luteal phase: In this phase, the empty follicle develops corpus luteum. It secretes progesterone, necessary for maintenance of endometrium for implantation, if ovum is fertilised. In the absence of pregnancy, corpus luteum degenerates (= corpus albicans), which is followed by menstruation.

VI. Copulation:
i. During sexual intercourse (i.e., copulation), the penis of male ejaculates the semen in the vagina of female.

ii. In the female genital tract, sperms undergo certain changes and acquire ability to fertilise the ovum. It is known as capacitation.
iii. Changes that occur in zona pellucida constitute the zona reaction.
 

VII. Fertilisation
i. Sperms reach the ampullary isthmatic region of the Fallopian tube. The ovum also reaches the Fallopian tube, where fertilisation (in humans it is called conception) takes place.
ii. The sperm makes its way through craona radiata and zona pellucida of the ovum.
→ Hyaluronidase of acrosome of sperm dissolves the hyaluronic acid present around the ovum.
→ Acrosin enzyme released from acrosome dissolves the zona pellucida (acrosome reaction)
iii. Many sperms enter the perivitelline space, but only one of them fertilises the ovum.

                                   
iv. Acrosome reaction induces the Meiosis II in secondary oocyte. As a result, a large cell called ootid (becomes ovum) and second polar body (degenerates) are formed.
v. The nucleus of ovum (female pronucleus) and that of sperm (male pronucleus) are fused to form a zygote nucleus or synkaryon [Union of pronuclei (male and female) is known as amphimixis]. Now the ovum with synkaryon is called zygote or egg.
vi. Now carona radiata disappears.

VIII. Development
1. Cleavages:
i. The egg of human beings is described as microlecithal type.
ii. Cleavages are holoblastic, radial, indeterminate and unequal.
iii. The resultant cells of cleavages are known as blastomeres.
 

2. Morula:
i. After 3 - 4 cleavages, 8 - 16 blastomeres are formed. This solid mass of cells is called morula.
ii. Morula is surrounded by zona pellucida.
iii. Morula reaches the uterus, passes through a process called compaction.
iv. After many cleavages, number of cells increase. The superficial layer of flat cells called trophoblast, which encloses the inner cell mass (becomes embryo) [embryoblast]
 

3. Blastocyst:
i. Some fluid diffuses into the morula from the uterine cavity and separates the inner cell mass from trophoblast. (still inner cell mass is attached to the trophoblast at one end)
ii. Now the trophoblast becomes flat, in the region where inner cell mass is attached. This side is called embryonic pole.

iii. The morula is now called blastocyst.
iv. The cavity of blastocyst is called primary body cavity or segmentation cavity.
v. The cells of trophoblast present over the inner cell mass are called cells of Rauber.

4. Implantation:
i. Gradually the zona pellucida disappears and the cells of trophoblast (of blastocyst) comes in direct contact with the uterine endometrium.
ii. The trophoblast cells invade the uterine epithelium and firmly attached to it. It is called interstitial implantation (it occurs 6th day after fertilisation.)
iii. After implantation, in that place, the uterine endometrium differentiates into decidua.
→ The part of decidua where placenta is formed is known as decidua basalis.
→ The part of decidua that separates the embryo from the lumen of uterus is called decidua capsularis.
→ The remaining part of uterus is called decidua parietalis.

iv. Implantation will be completed by the end of 2nd week.
 

5. Bilaminar embryonic disc:
i. The inner cell mass is formed into a disc like structure.
ii. Then the cells of Rauber disappear.
iii. From the embryonic disc, some cells are separated by a process called delamination and form a new layer just beneath the embryonic disc. It is called hypoblast.

iv. The remaining part of embryonic disc is known as epiblast.
v. The cells of hypoblast divide and form a layer under the trophoblast and ultimately lines the segmentation cavity. Now the segmentation cavity is called umbellical vesicle or yolk sac (which contains no yolk.)
vi. Gradually, the embryonic disc acquires oval shape. Its posterior end is more thick.

6. Gastrulation:
i. In the posterior part of the embryonic disc, a primitive streak is formed.
→ Longitudinal groove in primitive streak is called primitive groove.
→ On either side of the primitive groove, are the primitive folds.
→ Anteriorly, the primitive streak has a primitive pit.
→ In front of the primitive pit, is Hensen's node or primitive knot.
ii. The endodermal cells replaces the hypoblast and forms the endoderm.
iii. The mesodermal cells of epiblast converge towards the primitive folds, involute through the primitive groove, and reaches the middle part of epiblast and endoderm. Thus mesoderm is formed.
iv. Now the epiblast is the ectoderm.
v. Thus the three primary germ layers (ectoderm, mesoderm and endoderm) are established (and all are derived from the epiblast.)
 

7. Foetal membranes/ extra embryonic membranes:
i. During embryonic development, four extra embryonic membranes (amnion, chorion, yolk sac and allantois) are formed.

ii. From the embryonic disc/ blastodisc an amniotic head fold a tail fold and a pair of lateral folds are formed. All the four folds are joined (above the embryo) at a point called amniotic raphe, where they are fused. 
→ Thus embryo is enclosed by a membrane called amnion. Its cavity is called amniotic cavity, which is gradually filled with amniotic fluid. Amnion and its fluid protects the embryo from shocks, dessication and malformations.
→ Simultaneously the chorion is also formed along with amnion and it encloses the embryo along with other foetal membranes. Cavity enclosed by chorion is extra embryonic coelom.

iii. Allantois and yolk sac are formed from ventral part of gut.
→ Yolk sac is formed from the mid gut but it has no nutritive value.
→ Allantois is formed from the hind gut, grow in size. It stores the excretory materials.
iv. Chorion and allantois are fused to form a chorio allantoic membrane. It forms the foetal part of placenta.

8. Organogenesis:
i. The chorda mesodermal cells converge towards the Hensen's node, involute and extend anteriorly as a ribbon between epiblast and endoderm. It forms the notochord.
ii. The notochord induces the overlying ectodermal cells to form the neural plate (Induction). Neural plate elongates over the notochord. A longitudinal neural groove appears, which deepens. The neural folds of either side fuse to form a neural tube (Neurulation).
iii. The mesodermal cells spreads in all directions between ectoderm and endoderm.
→ The mesoderm present an either side of notochord and neural tube is called epimere.
→ The mesoderm around the gut is known as hypomere.
→ The mesoderm between epimere and hypomere is called mesomere.
iv. Epimere becomes mesodermal sometes. Sometes differentiates into myotomes (form voluntary muscles), sclerotomes (form vertebral column) and dermatomes (form dermis of skin and other connective tissues)
v. Mesomere forms the urinogenital organs and their ducts.
vi. Hypomere splits into outer somatic mesoderm and inner splanchnic mesoderm. Cavity between these layers is called intra embryonic coelom (near the embryo) and forms pericardial, pleural and peritoneal cavities.

9. Placenta:
Trophoblast of chorio allantoic membrane forms finger like projections called chorionic villi. They are accommodated in the sockets called crypts in the Uterine wall. They are integrated to form the placenta. But the maternal and foetus blood never mix with one another because of membrane barriers.
     The foetus part of placenta includes the trophoblast (ectoderm of chorio allantoic membrane), syndesmos of foetus (connective tissue) and foetal blood capillaries (lined by endothelium only). Maternal part of placenta includes the endometrium, syndesmos of uterus and uterine blood capillaries.
        Placenta of man is described as chorio allantoic placenta and is discoidal (chorionic villi are restricted to dorsal side of blastodisc) and haemochorial type (chorionic villi of foetus are directly dipped in blood pools of uterine wall)
        During child birth (that is parturition), placenta is cast off along with the encapsulating maternal tissues (decidua) causing damage to uterine wall. Hence, placenta in man in also described as deciduate placenta.
        Placenta facilitates the exchange of respiratory gases, nutrients and excretory materials between maternal blood and foetal blood. (placenta is connected to the foetus through umbellical cord) placenta secretes the following hormones.

i. Progesterone - Essential for maintenance of pregnancy.
ii. Oestrogen (especially estradiol) - Promote uterine growth and development of mammary glands.
iii. Human chorionic gonadotropin (hcG) acts similar to that of LH

10. Gestation/ Pregnancy:
The period of intra uterine development of foetus is known as gestation. It is about 38 weeks (266 days) after fertilisation of ovum. (or 40 weeks from the start of last menstrual cycle)

i. In the first trimester (three months) - organogenesis takes place.
ii. In the second trimester - fine hair will develop, eye lids are separated and eye lashes are formed.
iii. In the third trimester - by the end of this trimester the foetus is fully developed and ready for delivery.
 

11. Parturition:
The process of child birth is known as parturition. It begins with labour. Signals of parturition originate from the fully developed foetus and the placenta which induces mild contractions of uterus (foetal ejection reflex). It induces the release of oxytocin by the pituitary gland. Oxytocin causes stronger contractions of uterine wall. It leads to expulsion of the baby out. Soon after the infant comes out, the decidua is also expelled out.
 

12. Lactation:
i. Mammary glands stars producing milk by the end of gestation (lactation).
ii. The milk produced during initial few days after parturition is know as colostrum. It is absolutely essential to protect the new born from the infections, because it contains several types of antibodies.
iii. Breast feeding is always good for the health of mother and baby.