Humans reproduce sexually. They are viviparous, i.e. giving birth to the young ones. Human reproduction comprises a number of sequential steps.
Gametogenesis- production of male (sperm) and female (ovum) gametes
Insemination- transfer of male gamete to female genital tract
Fertilization- the fusion of gametes to form a zygote
Blastocyst development- the continuous mitotic division in zygote
Implantation- attachment of blastocyst to the endometrium of the uterine wall
Gestation- embryonic development (9 months in human)
Parturition- childbirth
Male and female reproductive organs differ in the structure, function and development
Human male reproductive system consists of:
○ The primary sex organs, i.e., a pair of testes.
○ The secondary sex organs i.e., the duct system and associated glands and
○ External genitalia.
Located outside the abdominal cavity (extraabdominal) with in a pouch called scrotum; scrotum provides a temperature 2.0 – 2.5°C lower than the body temperature necessary for spermatogenesis.
Each testis is oval in shape and measure about 4 to 5 cm in length, 2–3 cm in width and about 3 cm in thickness.
The testis is covered by a dense covering called tunica albuginea.
Internally the testis is divided into about 250 compartments, called testicular lobules.
Each lobule contains 1–3 highly coiled seminiferous tubules (the structural and functional units of testis) in which sperms are produced.
The seminiferous tubule has a tunic of connective tissue and is internally lined by seminiferous or germinal epithelium.
The seminiferous epithelium consists of two types of cells:
(1) Sertoli cells and (2) spermatogonia, that form the male germ cells or spermatozoa.
Sertoli cell provide nutrition and shape to the developing germ cells and spermatozoa.
The spermatogonia undergo cell divisions to form spermatozoa.
The spaces called interstitial spaces, between the seminiferous tubules have connective tissue, which includes blood vessels and Leydig cells (also called Interstitial cells);
Leydig cells synthesise and secrete the male sex hormones called androgens of which testosterone is the principal one.
The intratesticular duct system starts with tubuli refti, which are the short and straight end segments of the seminiferous tubules; these tubles open into vasa efferentia, through rete testis.
Vasa efferentia conduct the sperms out from the testis and open into the epididymis.
Epididymis is a single convoluted tubule that is located along the posterior surface of the testis.
The epididymis continues as the vas deferens that ascends into the abdominal or pelvic cavity through inguinal canal.
It loops over the urinary bladder and receives a duct from the seminal vesicle (of its side) to form ejaculatory duct that runs through the prostate and opened into the prostatic urethra, just after its other gene from the urinary bladder.
The urethra receives the ducts of prostate gland and the bulbourethral (Cowper’s) glands a little ahead and runs through the penis to its external opening; called urethral meatus.
The accessory glands of male reproductive system include: ○ a pair of seminal vesicles, ○ a prostate gland and ○ a pair of bulbourethral glands (also called Cowper’s glands)
Their secretions are collectively called as seminal plasma.
Seminal plasma is rich fructose, calcium, and certain enzymes; it provides nutrition and a medium for the spermatozoa to swim in the female reproductive tract.
The seminal plasma along with the spermatozoa, is called semen.
Penis is the external genitalia in human males.
It is made up of special erectile tissue, that helps in erection of the penis to facilitate insemination.
The enlarged tip of the penis is called glans penis; it covered by a loose fold of skin called foreskin of prepuce.
The vulva is the external opening in a female’s body. The vagina is part of the vulva. Further, the vulva comprises of-
The labia often termed as lips are folds of skin around the vaginal opening. Similarly, the outer lips are labia major Labia majora are fleshy and covered with pubic hairs whereas the inner lips are labia minora. These folds protect the opening of the vagina and urethra.
The clitoris is present at the point of meeting of inner labia and forming a little hood. This hood is the clitoral hood. The clitoris contains many nerve endings, therefore, it is the center of female pleasure.
The mons pubis is a tissue found anterior to the pubic bone and pubic hair covers it. It provides cushioning during intercourse.
These glands secrete mucus which helps during copulation. It is present on each side of the vaginal opening. These glands were discovered by Danish anatomist Caspar Bartholin.
The Internal Female Genitalia
The internal female organs are-
It is the muscular and flexible part of the female reproductive system. Further, it is the canal that during copulation receives sperms. Moreover, it is responsible for childbirth and menstrual flow. It is present between the anus and urethra. The average length of the vagina is 3-4 inches.
In addition, it is extremely elastic which is why it is for childbirth. The thicker and extremely elastic property of muscle walls helps in childbirth. Further, It also helps in the penetration of the penis during intercourse.
The walls of the vagina have a mucous gland that secretes mucus. The walls of the vagina are acidic due to the presence of lactic acid. Lactic acid plays a great role in preventing the penetration of disease-causing microorganisms. Finally, the pH of the vagina is 4.5. Thus, lactobacilli are responsible for the balancing pH of the vagina.
We often refer uterus as the womb which is an upturn pear-shaped reproductive organ. This organ is muscular and hollow. The uterus is the womb as it is the very place where a fetus grows. Also, it is somewhat located between the bladder and the rectum. The adult uterus is about 3-4 by 2-3 inches. Moreover, the uterus of the female reproductive system consists of three distinct layers-
The innermost layer of the uterus is the endometrium. It is the layer that sheds every month during menstruation. Epithelial tissues make endometrium. During the menstrual cycle, the walls of the endometrium grow thick as a result they are ready for implantation.
When there is no fusion of gametes this layer is shed off with blood. In addition, the proliferation is under the influence of female sex hormones (androgens) i.e., estrogen and progesterone.
A fallopian tube extends from the periphery of each ovary to the uterus; it is a muscular tube of about 10–12 cm long situated above and behind the urinary bladder.
The part of fallopian tube closer to the ovary is funnel-shaped and is called infundibulum.
The edges of the infundibulum possess fingerlike projections, called fimbriae, that help in collecting the ovum during ovulation.
The infundibulum leads into a wider part of the fallopian tube or oviduct, called ampulla.
The last part of the fallopian tube that joins the uterus is narrow and it is called isthmus.
A human female has a pair of functional mammary glands that consist of glandular tissues and variable quantity of fat.
The glandular tissue is divided into 15–20 mammary lobes; each lobe contains clusters of cells, called alveoli, which open into mammary tubules.
The mammary tubules of each lobe join to form a mammary duct.
Several mammary ducts join to form a wider mammary ampulla that is connected to lactiferous duct, through which milk comes out.
The milk is actually secreted by the cell of alveoli and is stored in the lumen of alveoli.
It is the process of formation of spermatozoa in the testis; the process starts at puberty.
The spermatogonial cells present on the inner wall of seminiferous tubule multiply by mitotic division.
Some of the spermatogonia serve as stem cells for spermatogenic lineage.
Other spermatogonia function as progenitor cells or the precursors of spermatozoa.
They undergo changes by accumulating food materials and grow almost double in size and become the primary spermatocytes.
Each primary spermatocyte undergoes the first meiotic division and forms two haploid secondary spermatocytes.
The secondary spermatocytes undergo the second meiotic division leading to the formation four equal sized, haploid spermatids.
The spermatids are transformed into spermatozoa by process of spermiogenesis.
The sperm heads become embedded in the Sertoli cells and are ultimately released from the seminiferous tubules by the process of spermiation.
Spermatogenesis is initiated due to an increase in the secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus at the age of puberty.
The increased level of GnRH act on the anterior pituitary and stimulate the secretion of two gonadotropins, i.e., luteinizing hormone (LH) and follicle stimulating hormone (FSH).
LH acts on the Leydig cells and stimulates them to secrete testosterone.
FSH acts on the Sertoli cells and stimulates secretion of some factors, which help in spermiogenesis.
The whole body of a spermatozoan is enveloped by a plasma membrane. A human sperm is composed of four parts; heat, neck, middle piece and tail. Head contain an elongated, compact, haploid nucleus, i.e., concentrated DNA. The anterior end has a cap-like structure, called acrosome; the acrosome contains enzymes that helps in dissolving membranes of the ovum for fertilization. Neck contains two centrioles, a maximal centriole, which is necessary for the first cleavage division of zygote and a distal centriole, that is connected to the tail filament. Middle piece contains a number of mitochondria, that provide energy for the motility of sperms.
Tail - It consists of an axial filament surrounded by the plasma membrane.
It helps the sperm to move in the female reproductive tract towards the female gamete (ovum) for fertilization.
Oogenesis is the process of formation of female gametes or ova in the ovary.
Oogenesis is initiated during embryonic development in a female foetus.
All the oogonia (about a million) are formed in the ovary of the foetus of 25 weeks; no new oogonia are formed after birth.
The oogonial cells which enter meiosis I, are the primary oocytes but the division remains suspended in prophase I; other cells form the granulosa/follicle cells and forego the potential of forming ova.
The primary oocyte becomes surrounded by a layer of granulosa cells and is called the primary follicle.
A large number of these follicle degenerate before puberty (follicular atresia) and only about 60000 – 80000 are left in the ovary.
The primary follicles become surrounded by more layers of granulosa cells and a thecal layer; at this stage they are called secondary follicles.
As development continues, a cavity develops in the secondary follicle around the primary oocyte; it is called antrum and is filled with a fluid called, liquor folliculin.
Simultaneously, the thecal layer becomes organized into an outer theca externa and at this stage the structure, is called tertiary follicle.
As the tertiary follicle continues its development, the primary oocyte completes its meiosis I and forms a large-cells, the secondary oocyte and a tiny cell, first polar body.
The secondary oocyte grows in size with a bulk of nutrient-rich cytoplasm.
The tertiary follicle becomes changed into a mature follicle, called Graafian follicle.
The secondary oocyte forms a new membrane, called zona pellucida, around it.
The secondary oocyte starts its second meiotic division, but it is suspended in metaphase II, until a sperm enters it.
At this stage, the Graafian follicle ruptures to release the secondary oocyte form the ovary.
In primates (human, ape, monkey) the cycle of 28 days is marked by the vaginal bleeding called menstruation or “weeping of uterus” as major event.
Depending upon the changes in ovary, uterus i.e., preparation for conception and pregnancy, and associated hormonal changes,
High level of LH from pituitary induces ovulation, FSH level comes down :
Luteal phase (or Secretory phase)
After ovulation till the onset (or 1st day) of next menstruation it is the longest phase of about 13 days.
Ruptured follicle under the influence of LH, develops as corpus luteum to secrete progesterone. After a fall for 2-3 days the estrogen level rises again. High level of estrogen and progesterone during this period inhibits FSH, hence, maturation of follicle stops, but the growth of uterine wall increases multifold. Menstrual cycle remain suspended during pregnancy.
Amenorrhoea – Menstruation does not occur due to many physiological/hormonal or mental defects this is the sign of sterility.
Fertilisation
During copulation, the semen is released by the penis into the vagina; this process is called insemination.
The motile sperms move through the cervix, enter the uterus and reach the ampullary-isthmic junction of the fallopian tube.
The ovum released from the ovary also reaches the ampullary-isthmic junction, where fertilization takes place.
As sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane to block entry of other sperms; thus, only one sperm can enter an ovum(secondary oocyte).
The enzymes of the acrosome of sperm help to dissolve zona pellucida and plasma membrane of the ovum for the entry of the sperm into the cytoplasm of the ovum (secondary oocyte).
The entry of sperm induces the completion of second meiotic division of the secondary oocyte, that results in the formation of a haploid ootid and a small polar body.
Even before the nucleus of the ootid is organised, it fuses with the sperm nucleus (fertilization) to form a diploid zygote.
Development of zygote
Cleavage
The zygote undergoes successive (mitotic) division, called cleavage, as it moves through the isthmus of fallopian tube towards the uterus.
The daughter cells are called blastomeres.
Morula - The embryo with 8 to 16 blastomeres, is called morula; at this stage it is a solid sphere. Blastocyst - Cell divisions continue in the morula and the blastomeres become arranged along the periphery leaving a central cavity, called blastocoel; the embryo at this stage, is called a Blastocyst - The cells now become arranged as an outer layer, called trophoblast and an inner group of cells, called inner cells mass, that is attached to trophoblast and projecting into the blastocoels.
Implantation and placenta formation
The trophoblast layer gets attached to the endometrium.
The cells of endometrium divide rapidly and cover the blastocyst. So, the blastocyst gets embedded in the endometrium; this process is called implantation; it leads to pregnancy.
After implantation, finger-like projection, called chorionic villi appear from the trophoblast; they are surrounded by the uterine tissue and material blood vessels.
The chorionic villi and the uterine tissue become interdigitated to form placenta, the structural and functional unit, between the foetus/embryo and the uterine wall of the mother.
Umbilical cord, the structure that connects the placenta with the foetus, helps in transport of substances to and from the foetus.
Placenta performs the following functions:
○ It facilitates supply of oxygen and nutrients to the foetus.
○ It also helps in the removal of carbon dioxide and other excretory products from the foetus.
○ It acts as an endocrine tissue and secretes and following hormones:
human chorionic gonadotropin (hCG)
human placental lactogen (hPL)
progestogens and estrogens; these are necessary for the foetal growth and maintenance of pregnancy.
The levels of other hormones like estrogens, progestogens, control, prolactin, thyroxine, etc also increase several-folds in the material blood.
Parturition
Parturition means childbirth. It is also known as labour. It is the mechanism of signalling the onset of labour (or) a procedure of delivering a child after the completion of pregnancy period. The developed child is born with the release of cortisol.
During parturition process, the cervix dilates and relaxes. Along with cortisol, oxytocin and estrogen hormones are released to begin the milk production and labour.
During parturition, the uterus contracts to push the fetus towards the cervix and continues until the fetus comes down the birth canal. In this process, the head should be first to come out.
After parturition, the uterus releases the placenta and it passes out immediately after the fetus is born. Lactation begins and the first milk is called colostrum. This milk contains antibodies, which is required for a newborn baby to protect it against infectious diseases and other allergies.
Lactation
The mammary glands also undergo certain development during pregnancy under the influence of hormones like prolactin (hPL) and progesterone.
They start producing milk towards the end of pregnancy and the milk is used for feeding the new-born (lactation).
The milk that comes out of the mammary glands of the mother during the initial few days of lactation, is called colostrum; it contains several antibodies needed to develop resistance in the new-born baby.