Find below the important notes for the chapter, Human Reproduction, as per NEET Biology syllabus. This is helpful for aspirants of NEET and other exams during last-minute revision. Important notes for NEET Biology- Human Reproduction covers all the important topics and concepts useful for the exam. Check BYJU’S for the full set of important notes and study material for NEET Biology and solve the NEET Biology MCQs to check your understanding of the subject.
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Human Reproduction – Important Points, Summary, Revision, Highlights
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)
Male and female reproductive organs differ in the structure, function and development
Male Reproductive System
a) Testes: A pair of testes are present in the scrotum outside the body, it helps in maintaining an optimal temperature for sperm formation. There are ~250 testicular lobules present. Sperms are produced in seminiferous tubules. Each lobule contains 1-3 seminiferous tubules.
There are two types of cells present in the internal lining of seminiferous tubules:
- Spermatogonia are germ cells of males, that undergo meiotic division to form sperms
- Sertoli cells provide nutrition to the germ cells
Leydig cells (interstitial cells) are present in the interstitial spaces outside the seminiferous tubules. They produce and secrete androgens (male reproductive hormones)
b) Accessory ducts: There are accessory ducts present. Vasa efferentia, rete testis, epididymis and vas deferens. These ducts store and transport sperms from the testis to urethra. Vas deferens along with a duct from seminal vesicles form ejaculatory duct and opens into the urethra.
Semen has seminal plasma and sperms
c) Glands: There are three glands, prostate gland, a pair of seminal vesicles and bulbourethral glands. The glands secrete seminal plasma. It is rich in certain enzymes, calcium, fructose and provides lubrication to the penis.
Spermatogenesis 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).
LH stimulates Leydig cells to secrete androgens. The main androgens are testosterone and androstenedione. These hormones stimulate the process of spermatogenesis
FSH stimulates Sertoli cells to secrete factors, which help 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, which are 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 formation of sperms from spermatids
- Spermiation is the process by which sperms are released from seminiferous tubules
Structure of Sperm
Around 200-300 million sperms are ejaculated at once
- Head contains acrosome apically, which contains enzymes that facilitate entry of sperm into the ovum. It is followed by an elongated nucleus (haploid)
- Middle piece has multiple mitochondria, that provide energy for the movement of sperms
- Tail is a flagellum that protrudes out of the cell body and is responsible for the vigorous motility of sperms. The tail helps sperms in swimming so that they can reach towards the ovum
Female Reproductive System
a) Ovaries: There are two ovaries present. Ovaries produce ovum and female reproductive hormones. Peripheral cortex is present at the periphery of ovarian stroma and inner medullary region
b) Fallopian Tubes: A pair of fallopian tubes or oviducts connect ovaries to the uterus
Infundibulum is a funnel-shaped part of the fallopian tube, which is near the ovary. The infundibulum terminates into finger-like projections known as fimbriae, which collect the ovum
Infundibulum leads to the uterus through ampulla and isthmus
c) Uterus or womb: It is inverted pear-shaped. Embryo development takes place inside the uterus. The uterine wall is differentiated into three layers:
- Perimetrium- It is the outermost layer and is membranous
- Myometrium- It is the middle layer of smooth muscles. Strong contraction during delivery is due to the myometrium
- Endometrium- It is glandular and undergoes cyclic changes during the menstrual cycle. Blastocyst gets embedded in the endometrium for development
A narrow cervix connects the uterus to the vagina. The vagina and the cervical canal form the birth canal
d) External Genitalia: Mons pubis (fatty tissue), labia majora and labia minora (tissue folds) and clitoris form external female genitalia. The hymen membrane covers the vaginal opening.
e) Mammary glands: A pair of breasts are present in females. They contain fat and mammary glands. Alveolar cells secrete milk.
Oogenesis is the formation of a mature female gamete called the ovum.
Oogenesis differs from spermatogenesis as the process of oogenesis starts during embryonic development as compared to spermatogenesis, which only starts at puberty.
- Around two million oogonia are formed in the ovary of a foetus and no more oogonia are formed after that
- Oogonia divide further mitotically and enters the 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
- Most of the primary follicles degenerate and only 60,000 to 80,000 are left at puberty
- Primary follicles develop into secondary follicles and then to tertiary follicles, which contains 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 the formation of a haploid secondary oocyte, which is large and a tiny polar body
- The tertiary follicle changes into 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 of 28/29 days on an average and an ovum is released at the middle of each cycle (~14th day)
- Lack of menstrual cycle is an indicator of pregnancy or may be due to poor health or stress
- Menopause is the stage at which the menstrual cycle ceases (~45-50 yrs). Reproductive phase of a female is between menarche and menopause, whereas males continue to produce sperm throughout their life
The menstrual cycle is divided into three phases:
- Menstrual phase: When the ovum fails to get fertilised, menstrual flow results due to disintegration of the endometrial lining
Follicular phase:It is also known as the proliferative phase. Primary follicle develops into Graafian follicle and then to ovulation. The endometrium also regenerates.
Gonadotropins (LH and FSH) are released from the pituitary and their level increases gradually, stimulating follicular development. The level reaches a peak in the middle of the cycle and LH induces rupture of the Graafian follicle.
The growing follicle releases estrogen and the level of estrogen also increases at the midpoint
- Luteal phase: It is also known as the secretory phase.
Corpus luteum is formed from the remaining part of the Graafian follicle. It secretes the hormone progesterone, which is required for the maintenance of the endometrium and for the implantation of the fertilised ovum.
The corpus luteum degenerates in the absence of fertilisation. This is followed by menstrual phase and the new cycle begins
- The fertilization takes place at the ampullary region of the fallopian tube
- When one of the sperms comes in contact with the zona pellucida of the ovum, no more sperms can enter further due to changes induced in the membrane
- Second meiotic division of the secondary oocyte is induced on the entry of sperm. It forms a large ootid (haploid) and the secondary polar body
- The sperm and ovum fuse together to form the diploid zygote.
- The sex of the baby is determined at this stage. In spermatogenesis, there are two kinds of male gametes formed, one with the X chromosome and one with the Y chromosome. If the sperm containing X chromosome fuses with the ovum, the zygote develops into female and when sperm with the Y chromosome fuses with the ovum, the zygote develops into a male child
- The zygote divides mitotically forming blastomeres and moves towards uterus through the oviduct
- Morula is the stage with 8-16 blastomeres in the embryo
- The blastocyst is formed due to continuous division after the morula stage. At this stage, blastomeres get arranged in the outer layer called trophoblast and inner cells forming inner cell mass
- The trophoblast gets attached to the endometrium lining of the uterus and the inner cell mass gets differentiated into the embryo
- The blastocyst gets embedded in the endometrium as the dividing uterine cells completely cover it. The process is known as implantation. This is the start of pregnancy
- After implantation, cells of the embryo start differentiating into outer ectoderm, inner endoderm and then the middle mesoderm. These are called primary germinal layers and all the adult tissues are developed from these three layers
- Stem cells are present in the inner cell mass. They have the ability to develop into all types of tissues and are called pluripotent. Stem cells are now being used and researched for the treatment of various life-threatening diseases like cancer
- Placenta is the link between the growing foetus and the mother. It is formed by the interdigitation of uterine cells and chorionic villi (finger-like projections), protruding from trophoblasts
- Placenta provides the growing foetus with nutrients and oxygen and also helps in removing excretory products
- Umbilical cord connects the placenta to the foetus and transports nutrients
- Placenta produces many hormones, that are required to maintain pregnancy, for the fetal growth and at the time of delivery
- Human chorionic gonadotropin (hCG) and human placental lactogen (hPL) are only produced at the time of pregnancy. hCG level of the urine during pregnancy is the marker for detection of pregnancy by test strips
- Relaxin level increases at the later stages of pregnancy. Relaxin level also increases at the time of ovulation but in the absence of pregnancy, the level decreases and menstruation starts
- Level of progesterone, estrogen, prolactin, cortisol and thyroxine, etc. increases to support the fetal growth
Human- 9 months
Cats and dogs- 60-65 days
Elephants- 22 months
- 1st month- The heart is formed
- 2nd month- Limbs and digits are formed
- 3rd month (first trimester)- all the major organs are developed and the foetus is recognizable as a human being. External genitalia start differentiating and the sex of the foetus can be determined
- 5th month- foetus starts moving and hair on the head appear
- Second trimester- eyelashes and eyelids develop, the body gets covered with fine hair
- The third trimester- by the end of the third trimester, the child is fully developed and the body prepares for the delivery (parturition) of the infant
The process of childbirth is known as parturition. Parturition is initiated by neuroendocrine signals. Mild uterine contractions start when the foetus is fully developed. This is followed by secretion of oxytocin from the pituitary gland. Contractions become stronger and stronger as more oxytocin is secreted resulting in childbirth. After the infant, the placenta is also delivered. The umbilical cord is cut and often preserved these days as it contains stem cells, which can be used in future to cure diseases, not only of the person but also of the immediate family members.
To induce delivery, doctors inject oxytocin injection.
Lactation: Increased secretion of prolactin from the pituitary gland induces milk production in the mammary glands.
Colostrum is the first formed (yellowish) milk from the mammary glands. It is rich in antibodies and proteins and essential for the immunity of infants
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