Reproduction
Reproduction is a biological process by which an organism gives rise to young ones of their own kind.
✓Reproduction enables the continuity of the species, generation after generation.
✓Sexual reproduction is responsible for variation in a population and its inheritance to future generations.
Modes of reproduction;
a. Asexual reproduction
b. Sexual reproduction
Asexual reproduction
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A type of reproduction by which offspring is produced from s single parent with or without the involvement of gamete formation. (uniparental).
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Offsprings are morphologically and genetically identical to their parents (clones).
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Asexual reproduction is common among unicellular organisms, and in plants and animals with relatively simple organizations.
Sexual Reproduction
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Sexual reproduction involves the fusion of male and female gametes to form a zygote.
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All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. This period of growth is called Juvenile phase. Juvenile phase is also known as vegetative phase in plants.
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In non-primates (cows, sheep and dogs), the female reproductive cycle is called oestrus cycle.
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In primates (monkey, apes and humans), the female reproductive cycle is called menstrual cycle.
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The phase of life cycle where an organism possesses all the capacity and potential to reproduce sexually is called reproductive phase.
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The post-reproductive phase of life cycle where an organism slowly loses the rate of metabolism, reproductive potential and show deterioration of the physiological activity of the body is called senescent phase.
The Human Male Reproductive System
The male reproductive system consists of the following:
I. Primary sex organ: testes.
II. Secondary or accessory sex organs:
a. Accessory ducts: rete testis, vasa efferentia, epididymis and vas deferens.
b. Accessory glands: seminal vesicle, prostate and bulbourethral glands (Cowper's gland).
III. External genitalia: penis.
I. Testes
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A pair of testes is situated outside the abdominal cavity in a sac of skin called the scrotum. Scrotum keeps the testes at a temperature 2-2.5 oC lower than the internal body temperature, which is necessary for the synthesis of sperms.
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Each testis is oval, measures about 4-5 cm in length and is 2-3 cm in width.
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The outermost covering of the testis is formed by a dense fibrous membrane called tunica albuginea.
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Each testis is divided into 250 compartments called testicular lobules.
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Each lobule contains 1-3 highly coiled tubules known as seminiferous tubules in which sperms are produced.
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Each seminiferous tubule is lined on its inside, by two highly specialised cells called male germ cells (spermatogonia) and Sertoli cells.
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Spermatogonia undergo meiotic cell division to produce sperms.
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Sertoli cells or nurse cells provide nutrition to the germ cells.
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The regions outside the seminiferous tubules contain masses of cells called interstitial cells or Leydig cells. Leydig cells synthesise and secrete the male hormones called androgens (testosterone) which maintain male sex characteristics.
II. Accessory Duct System
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The seminiferous tubules open into the vasa efferentia through rete testis.
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Several vasa efferentia open into the epididymis and carry sperms outside the testes.
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Epididymis is a long, coiled tube present along the posterior surface of each testis. It continues as the vas deferens and ascends into the abdomen to loop over the urinary bladder. Epididymis temporarily stores non-motile and immature sperms.
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The duct from seminal vesicle and vas deferens together form the ejaculatory duct. They pass through the prostate gland and join the urethra. They carry secretions of seminal vesicle and sperms from the testes to the outside through urethra.
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Urethra originates from the urinary bladder and extends through the penis to its external opening called urethral meatus. It carries urine from the bladder and sperms from the vas deferens through the penis.
III. Accessory Glands
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The accessory or secondary glands include a prostate gland, two seminal vesicles and two bulbourethral glands.
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Prostate gland: It surrounds the urethra and produces a milky secretion which forms a considerable part of the semen. This secretion contains citric acid, lipids and enzymes. Secretion of the prostate gland nourishes and activates the spermatozoa to swim.
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Seminal vesicles: These secrete mucus and a watery alkaline fluid that contains fructose which provides energy to the sperms.
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Bulbourethral glands or Cowper's glands are attached to the urethra below the prostate gland. They secrete mucus fluid for the lubrication of the penis.
IV. External Genitalia
Penis: It is the male copulatory organ having erectile tissues and vascular spaces. When the male is sexually excited these spaces fill with blood, causing the penis to erect. The distal end of the penis is called glans penis. The glans penis is covered with a smooth skin called foreskin or prepuce.
The female reproductive system consists of the following:
i. Primary sex organ: ovary
ii. Secondary sex organs:
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Accessory ducts: pair of oviducts (fallopian tubes), uterus, cervix and vagina.
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Accessory gland: mammary gland.
iii. External genitalia: vulva.
I. Ovaries
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Ovaries are the primary female sex organs that produce the female gametes (ovum) and several steroid hormones (ovarian hormones).
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Ovaries are located one on each side of the lower abdomen and remain attached to the pelvic wall and uterus by ovarian ligaments.
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Each ovary is almond-like flattened body, measuring about 2-4 cm in length.
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The outer region of the ovary is composed of developing follicles and the middle region forms the stroma which contains connective tissue, blood vessels and mature follicles.
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The stroma is divided into two regions: a peripheral cortex and an inner medulla.
Spermatogenesis
The process of formation of spermatozoa (sperms) from diploid spermatogonia is called spermatogenesis.
It includes the following phases:
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Multiplication phase: The male germ cells (spermatogonia) present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers.
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Growth phase: Spermatogonia grow and increase in size and form primary spermatocytes. Each spermatogonium is diploid and contains 46 chromosomes.
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Maturation phase or formation of spermatids: Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal haploid cells called secondary spermatocytes, which have only 23 chromosomes each. The secondary spermatocytes undergo the second meiotic division to produce four equal haploid spermatids.
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Differentiation phase: The spermatids are transformed into spermatozoa (sperms) by the process of spermiogenesis. The sperm's head gets attached to Sertoli cells to draw nourishment and are finally released from the seminiferous tubules by the process called spermiation.
Hormonal control of spermatogenesis:
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Spermatogenesis is initiated at the age of puberty by the gonadotropin releasing hormone (GnRH) secreted by the hypothalamus.
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The increased levels of GnRH stimulate the anterior pituitary which then secretes the FSH (follicle stimulating hormone) and LH (luteinising hormone).
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FSH stimulates Sertoli cells to secrete some factors which help in spermiogenesis.
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LH acts on the Leydig cells and stimulates synthesis and secretion of androgen.
Structure of a sperm (Spermatozoa)
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A sperm consists of four parts—head, neck, middle piece and tail.
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A plasma membrane envelops the whole body of the sperm.
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Head is the enlarged end of a sperm, contains an elongated haploid nucleus, anterior portion of nucleus capped by acrosome. The acrosome contains hydrolytic enzymes that help in dissolving membranes of the ovum during fertilisation.
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Neck contains proximal centriole which is necessary for the first cleavage division of zygote and the distal centriole that is connected to the tail filament.
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Middle piece contains a number of mitochondria that provide energy for the movement of the tail that facilitate sperm motility, essential for fertilisation.
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Tail consists of axial filaments surrounded by the plasma membrane. It helps the sperms to swim in a fluid medium.
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A human male ejaculate about 200-300 million sperms during a coitus. Seminal plasma along with the sperms constitute the semen.
Oogenesis
The process of formation of a mature haploid female gamete from diploid oogonium is called oogenesis. It occurs in the ovaries.
It consists of the following three phases:
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Multiplication phase: Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each foetal ovary. No more oogonia are formed and added after birth. These cells start division and enter into prophase-I of the meiotic division. They get temporarily arrested at this stage and are called primary oocytes.
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Growth phase: Each primary oocyte then gets surrounded by a layer of granulosa cells. This structure is called the primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. At puberty, only 60,000 to 80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles.
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Maturation phase: In the first maturation phase, the secondary follicle soon transforms into a tertiary follicle. The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division to form a large, haploid, secondary oocyte and a tiny first polar body. The tertiary follicle changes into a mature follicle - the Graafian follicle - which ruptures to release the secondary oocyte (ovum) from the ovary by a process called ovulation. The second maturation phase occurs after fertilisation when the meiotic division of the secondary oocyte is complete. This second meiotic division results in the formation of a second polar body and a haploid ovum (ootid).
Menstrual Cycle
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The rhythmic series of changes that occur in the reproductive organs of female primates (monkeys, apes and human beings) is called menstrual cycle.
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It is repeated at an average interval of about 28/29 days.
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The first appearance of menstruation at puberty is called menarche.
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During pregnancy all events of the menstrual cycle stop and there is no menstruation.
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A natural decline or cease of menstrual cycle at the age of 45-50 years is called menopause.
The menstrual cycle has four phases. These are:
A. Menstrual Phase:
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The soft tissue of endometrial lining of the uterus disintegrates causing bleeding.
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The unfertilised egg and soft tissues are discharged.
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It lasts for 3-5 days.
B. Follicular Phase/Proliferative Phase:
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The primary follicles in the ovary grow and become a fully mature Graafian follicle.
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The endometrium of the uterus is regenerated due to the secretion of LH and FSH from anterior pituitary and ovarian hormone, estrogen.
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It lasts for about 10 to 14 days.
C. Ovulatory Phase:
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Rapid secretion of LH (LH surge) induces rupture of Graafian follicle, thereby leading to ovulation (release of ovum).
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It lasts for only about 48 hours.
D. Luteal Phase/Secretory Phase:
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In this phase the ruptured follicle changes into corpus luteum in the ovary and it begins to secrete the hormone progesterone.
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The endometrium thickens further, and their glands secrete a fluid into the uterus.
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If ovum is not fertilised, the corpus luteum undergoes degeneration and this causes disintegration of the endometrium leading to menstruation.
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Estrogen and progesterone levels rise during this phase.
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It lasts for only 1 day.
Fertilisation
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The process of fusion of a sperm (male gamete) with an ovum (female gamete) is called fertilisation.
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During coitus, semen is released by the penis into the vagina (insemination). The motile sperms swim rapidly through the cervix, enter into the uterus and reach the ampullary-isthmic junction of the oviduct.
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A sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane to block the entry of additional sperms.
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The hydrolytic enzymes of the acrosome of sperm help to dissolve zona pellucida and plasma membrane of the ovum and sperm head is allowed to enter into the cytoplasm of the ovum, i.e., secondary oocyte.
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Finally, diploid zygote is formed by the fusion of sperm and an ovum.
Implantation
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Zygote divides rapidly by mitotic division as it moves through isthmus of oviduct towards uterus. This is called cleavage.
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As a result, 2, 4, 8, 16 daughter cells are produced which are termed as blastomeres.
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Embryo with 8-16 blastomeres is called a morula.
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The morula continues to divide and transforms into a large mass of cells called blastocyst, which passes further into the uterus.
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Blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called inner cell mass.
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The trophoblast layer gets attached to the cells of the endometrium and the inner cell mass gives rise to the embryo.
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After attachment, the cells of endometrium divide rapidly and cover the blastocyst. So, the blastocyst gets embedded in the endometrium of the uterus. This is called implantation, which leads to pregnancy.
Pregnancy and Embryonic Development
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The cells of the trophoblast differentiate into an inner layer and an outer layer. The outer layer is called the chorion and forms the chorionic villi, which are finger-like projections that grow into the endometrium.
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An intimate connection is established between chorionic Villi and uterine tissue and forms a structural and functional unit between the developing embryo and the maternal body called placenta.
Functions of Placenta
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Provides nutrients and oxygen to the developing embryo.
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Removes CO2 and waste materials from the embryo.
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Acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progesterones that are essential to maintain pregnancy.
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An umbilical cord connects placenta with the embryo that helps in the transport of substances to and from the embryo.
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The inner cell mass of blastocyst develops into three layers:
a. outer layer called ectoderm
b. inner layer called endoderm
c. middle layer called mesoderm
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Inner cell mass contains certain cells called stem cells that have the potency to give rise to all the tissues and organs.
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After one month of pregnancy, the embryo's heart is formed.
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By the end of second month of pregnancy, the foetus develops limbs and digits.
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By the end of third month, most of the organ systems are formed.
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Appearance of hair on the head and foetus movement is observed during fifth month.
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After six months, the body is covered with fine hair, eye-lids separate and eyelashes are also formed.
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By the end of nine months of pregnancy, the foetus is completely developed and is ready for its delivery.
Parturition
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The average duration of human pregnancy is about 9 months which is called the gestation period.
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The act of expelling the full term foetus from the mother's uterus at the end of gestation period is called parturition.
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It is induced by a complex neuroendocrine mechanism.
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Parturition signals originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex.
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This triggers the release of oxytocin from the maternal pituitary. Oxytocin induces stronger uterine muscle contractions which lead to expulsion of the baby from the uterus through the birth canal.
Lactation
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Mammary glands of female undergo differentiation and start producing milk at the end of pregnancy. This is called lactation.
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This helps the mother in feeding the newborn.
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The milk that comes out of the mammary glands during initial days of lactation is called colostrum. It contains several antibodies (IgA) and nutrients (like calcium, fats, lactose) for the baby. It provides passive immunity to the baby.
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Thus, breast-feeding is recommended by doctors for bringing up a healthy baby.