|“I realized that nature had invented reproduction as a mechanism for life to move forward, as a life force that passes right through us and makes us a link in the evolution of life. Rarely seen by the naked eye, this intersection between the animal world and the plant world is truly a magic moment.”
– Louie Schwartzberg
The creation of life is a highly debated topic that has earned many religious, social, and moral connotations to its name. From a scientific point of view, it is most assuredly one of the most complex and marvelous developments of nature in the biological world. Reproduction is a biological process by which living organisms procreate and give birth to their offspring to ensure life continuity on earth. Depending on the mode of reproduction, it is categorized into two types. Sexual reproduction involves the creation of life by the fusion of male and female gametes. Asexual reproduction occurs when only a single organism is involved in the birth of the offspring.
This is an important chapter while preparing for Class 12th biology board exam.
Whether the zygote develops inside or outside gives us two kinds of organism termed as oviparous if fertilization occurs outside the body and viviparous if it occurs inside. Humans are viviparous by nature as we directly give birth to our babies instead of laying eggs. Since humans are one of the most advanced species, reproduction is highly complicated. It involves multiple processes sequentially, each more fascinating than the next.
Testes: Pair of testes is present in the scrotum that is kept outside the body to safeguard against the excessive heat of the human body and maintain optimum temperature. A total of 250 testicular lobules are there, where each lobule contains one-three seminiferous tubules where the sperms are created.
Two types of cells are present in the internal lining of seminiferous tubules:
- Spermatogonia: Germ cells of males that undergo meiotic division to form sperms
- Sertoli cells: To provide nutrition to the germ cells
Leydig cells (interstitial cells) are present in the interstitial spaces outside the seminiferous tubules that produce and secrete androgens (male reproductive hormones)
Accessory ducts: Vasa efferentia, rete testis, epididymis and vas deferens are the four accessory ducts present. Their job is to store and transport the sperms from the testis to the urethra. Vas deferens with a duct from seminal vesicles function as an ejaculatory duct that opens into the urethra.
Glands: Prostate glands, a pair of seminal vesicles, and the bulbourethral glands are the three glands responsible for the secretion of seminal plasma that is rich in certain enzymes, calcium, and fructose and also provides lubrication to the penis.
Starts at puberty. There is increased secretion of GnRH (gonadotropin-releasing hormone) from the hypothalamus. GnRH stimulates the pituitary to secrete LH (luteinizing hormone) and FSH (follicle-stimulating hormone).
Spermatogenesis begins at the advent of puberty with increased secretion of GnRH (gonadotropin-releasing hormone) from the hypothalamus that helps to stimulate the pituitary to secrete LH (luteinizing hormone) and FSH (follicle-stimulating hormone).
LH stimulates Leydig cells to secrete androgens. Testosterone and androstenedione are the main androgens that stimulate the process of spermatogenesis.
FSH stimulates Sertoli cells to secrete factors for helping in the process of spermiogenesis
- Spermatogonia are diploid (2n=46) and divide by mitotic division to form multiple cells called primary spermatocytes.
- Each primary spermatocyte cell divides meiotically to first form two haploid (n=23) secondary spermatocytes, equal in size.
- Secondary spermatocytes further divide in the 2nd meiotic division to form haploid spermatids of equal size.
- So each diploid primary spermatocyte cell forms four haploid spermatids.
- Spermiogenesis is the process of the formation of sperms from spermatids.
- The formation of sperms from spermatids is termed spermiogenesis.
- The process of releasing the sperms from seminiferous tubules is termed spermiation
Semen consists of seminal plasma and sperms. At a time, 200-300 million sperms are ejaculated at once. The Head contains acrosome apically, which contains enzymes that facilitate the entry of sperm into the ovum. It is followed by an elongated nucleus (haploid).
- The middle piece has multiple mitochondria that deliver energy for the sperm’s movement.
- The tail is a flagellum that protrudes out of the cell body and is responsible for the vigorous motility of sperms. The tail helps sperm in swimming to reach towards the ovum.
Female Reproductive System
Ovaries: Two ovaries are present in females responsible for producing ovum and reproductive hormones. The peripheral cortex is present at the periphery of the ovarian stroma and inner medullary region.
Fallopian Tubes: A pair of fallopian tubes or oviducts helps connect the ovaries to the uterus.
The infundibulum is a funnel-shaped part of the fallopian tube near the ovary and terminates into finger-like projections known as fimbriae, which collect the ovum. It leads to the uterus through the ampulla and isthmus.
Uterus or womb: An inverted pear-shaped region where the embryo develops. Its walls are made up of three layers:
- Perimetrium: Outermost membranous layer
- Myometrium: Middle layer consisting of smooth muscles that give strong contractions because of the myometrium during the delivery
- Endometrium: Glandular layer where blastocyst is embedded for development. It undergoes cyclic changes during the menstrual cycle.
A narrow cervix connects the vagina and the uterus. Together the vagina and cervical canal constitute the birth canal.
External Genitalia: Mons pubis (fatty tissue), labia majora and labia minora (tissue folds) and clitoris form external female genitalia along with the hymen membrane that protects the vaginal opening.
Mammary glands: Pair of breasts that contain fat and mammary glands. Alveolar cells secrete milk.
It is the formation of a mature female gamete called the ovum. It differs from spermatogenesis as It begins during embryonic development, unlike spermatogenesis, which starts only at puberty.
- Ovary of the fetus only forms around two million oogonia.
- Oogonia divide further mitotically and enter prophase-1 of meiotic division. At this stage, the division stops temporarily, and the cells are called primary oocytes.
- Primary oocytes get surrounded by granulosa cells to form primary follicles.
- Only 60,000 to 80,000 primary follicles are left at puberty, as most have degenerated.
- Primary follicles develop into secondary and tertiary follicles, containing a fluid-filled cavity known as the antrum.
- The first meiotic division of primary oocytes occurs inside the tertiary follicle. It is an unequal division leading to a large haploid secondary oocyte, which is large and has a tiny polar body.
- The tertiary follicle changes into the Graafian follicle (mature follicle). The membrane zona pellucida develops around the secondary oocyte.
- Ovulation is the process by which the ovum (secondary oocyte) is released on the rupture of the Graafian follicle.
- Menarche: First menstruation at puberty
- The menstrual cycle is 28/29 days on average, and an ovum is released at the middle of each cycle (~14th day).
- Lack of menstrual cycle indicates pregnancy or may be due to poor health or stress.
- Menopause is the stage at which the menstrual cycle ceases (~45-50 yrs). A female’s reproductive phase is between menarche and menopause, whereas males continue to produce sperm throughout their lives.
There are three phases of the menstrual cycle:
- Menstrual phase: On failure of fertilisation of the ovum, the endometrial lining disintegrates and leads to menstrual flow.
- Follicular phase: Also known as the proliferative phase, it includes the development of primary follicle and Graafian follicle, and then to ovulation, along with the regeneration of endometrium.
The pituitary releases Gonadotropins (LH and FSH) with a gradual increase in their level to stimulate follicular development. In the middle of the cycle, the level reaches its peak, and LH induces the rupture of the Graafian follicle.
The growing follicle releases estrogen, and estrogen levels increase at the midpoint.
- Luteal phase: Also known as the secretory phase, it involves the formation of the Corpus luteum from the remaining portion of the Graafian follicle. It secretes progesterone needed for endometrium maintenance and implantation of the fertilized ovum. In the absence of fertilization, the corpus luteum degenerates, followed by the menstrual phase, after which the new cycle begins.
- Occurs at the ampullary region of the fallopian tube
- Once a sperm comes into contact with the zona pellucida of the ovum, it induces certain changes in the membrane to prevent the entry of any further sperms.
- Entry of sperm induces the second meiotic division of the secondary oocyte that forms a large ootid (haploid) and the secondary polar body.
- The sperm and ovum fuse together to form the diploid zygote.
- This stage is responsible for the sex of the baby. Spermatogenesis involves the formation of male gametes with either the X chromosome or the Y chromosome. The zygote develops into a female if the X chromosome containing sperm fuses with the ovum, else it develops into a female if the Y chromosome containing sperm fuses with the ovum.
- The zygote divides mitotically, forming blastomeres, and moves towards the uterus through the oviduct.
- Morula is the stage with 8-16 blastomeres in the embryo.
- The continuous division after the morula stage forms the blastocyst. The blastomeres that arrange themselves in the outer layer are termed trophoblast, and those on the inner cells lead to the formation of the inner cell mass.
- The trophoblast gets attached to the uterus’s endometrium lining, and the inner cell mass gets differentiated into the embryo.
- Implantation is the start of pregnancy, which includes embedding the blastocyst in the endometrium when the dividing uterine cells completely cover it.
Stages of Embryo Development
|Weeks after Fertilisation||Development of Embryo|
|3rd Week||The start of the third week of gestation sees the formation of clusters of cells and three germinal layers.|
|4th Week or 1st month||The heart is the first organ to function. Foetal arm buds and optic pits also become visible.|
|5th Week||At the fifth week, the embryo grows approx. 4mm and begins to curve into a C-shape. Foetal liver, pancreas, gallbladder, spleen, the inner part of the ear and the pharyngeal arches also start developing.|
|6th Week||In the sixth week, the embryo grows from 4 mm to 9 mm in length. Baby’s external features like the eyes and nose, leg buds and hand, stomach and kidney also start developing.|
|7th Week||In the seventh week, the baby continues to grow fast from 9mm to 13mm, along with the lengthening of arms and legs. Internal organs like lungs, primary sex organs and lymphatic system also develop in this stage.|
|8th Week or 2nd Month||At the 7th week, most of the internal and external organs like the baby’s hair follicles, external ear and nipples start developing.|
|1st-trimester||By the end of the third month, all the major organ systems are well developed and the genital organs are visible.|
|5th Month||In the fifth month, hairs start appearing on the head and the baby starts to show movement.|
|24 weeks or 6 months||By the end of the 2nd trimester, the baby’s eyelashes are formed, eyelids separate and the body gets covered with fine hair.|
|9th month||By the end of the 9th month, the foetus fully develops and is ready for birth.|
It is the process of childbirth that is initiated by neuroendocrine signals. Once the foetus is completely developed, mild uterine contractions start to occur, followed by oxytocin secretion from the pituitary gland. As more and more oxytocin is secreted, the contractions become stronger in strength and lead to childbirth. The placenta is also delivered after the infant. Nowadays, the umbilical cord is cut and preserved for stem cells that can help cure future diseases for the person and the immediate family members.
Doctors often inject oxytocin to help induce delivery.
Lactation: Mammary glands start milk production due to increased prolactin secretion from the pituitary gland.
Colostrum: First formed (yellowish) milk from the mammary glands is rich in antibodies and proteins and essential for the immunity of infants.
Frequently Asked Questions on Reproduction of Humans
- What are the three stages in the reproduction process in humans?
There are three stages in human reproduction – Pre-fertilisation, Fertilisation, and Post-fertilization.
- What is the process of human fertilization?
Fertilization is termed the fusion of male and female gametes in the ampulla of the fallopian tube.
- What is meant by insemination?
The transfer of male gamete to the female genital tract is termed insemination.
- What is the period of fertilization of the egg?
The egg stays alive for 12-24 hours after release from the ovary. If not fertilized within this time, it disintegrates and gets shed off by the uterine lining.
- What is the period of gestation in humans?
Humans have a gestation period of nine months.