Unit 16 Human Reproductive System and Gametogenesis
Key Unit Competence
To be able to relate the structures of the human reproductive system to their functions anddescribe gamete formation.
LEARNING OBJECTIVES
At the end of this unit, the learner will be able to:
• Describe the structure of human male and female reproductive systems.
• State where female and male gametes are produced.
• Describe the histology of mammalian ovary and testis.
• Relate the histology of the testis and ovary to their functions.
• Outline gametogenesis in a male and a female human as a process involving mitosis, growth,
meiosis and maturation.
• Explain how spermatozoa are produced.
• Explain how oocytes are produced.
• Explain the significance of gametogenesis.
• Analyse and interpret chart diagrams of spermatogenesis and oogenesis.
• Prepare slides well to study the structure of gametes.
• Research about gametes and their formation and deduce their findings.
• Appreciate the significance of the process of gametogenesis at puberty as a key characteristic
of sexual maturity.
• Acknowledge the relevance of meiosis during gametogenesis as an essential tool inmaintaining the diploid condition after fertilisation.
ACTIVITY 1
Have you ever wondered how is the generation transferred from one to another? Look into
internet for matter related to the process of reproduction and reproductive organs. Make areport on it and present to the class.
INTRODUCTORY ACTIVITY
Observe the photo below and answer the questions that follow
a. The woman on the left side is pregnant. What do you think about the origin of the fetus
in the womb of the pregnant woman?
b. Use the photo on the right side to make notes on the dembryonic development
c. Use the photo and try to identify the parts of the female reproductive system and their
functions
d. The child on the photo is a boy. Can you identify the parts of the male reproductive
system and their functions
e. What do you think are the causes to the changes observed on the woman on the rightside of the photo
16.1 REPRODUCTION IN HUMANS
ACTIVITY 2
Aim: To dissect and identify structures of male and female reproductive system of rat.
Theory: The sex of the rat can be determined by looking for external testes and teats. Large
testes are visible at the ventro-posterior side of the male rat and 6 pairs of teats ventrally in
females. The major male reproductive organs of the male rat are the testes (singular: testis)
which are located in the scrotal sac, along with internally placed duct systems and associated
glands. Female reproductive system consists of a pair of ovary, duct system and glands.
Procedure:
• Obtain and sacrifice a rat. Place it in your dissecting tray with ventral side facing up so as to
observe the external features and determine the sex of the rate you have taken.
• Examine the anal opening just posterior to the tail. Insert the scissor blades through the anus
along the midline lifting the skin carefully so as not to damage the underlying organs.
In male rat, cut through the scrotum carefully to reveal the testis and also observe other
internal structures—epididymis, vas deferens, seminal vesicles, prostate gland and
seminiferous tubules.
In female rat, locate the urethral and vaginal openings on the ventro-posterior side of the
body, under the tail, just posterior to the last pair of teats. Also, observe other reproductiveorgans—uterus, ovaries, oviducts and vagina.
Precautions:
1. Be careful not to cut too deeply. Keep the tip of your scissors pointed upwards.
2. Always wear gloves and safety goggles when handling the rat to protect your hands and eye
from chemical splattering or debris.
3. Once an incision is made, allow the rat to be drained off the fluid.
4. Handling and dissection of animals should be done in the presence of experts, followingethical guidelines.
Human beings reproduce by sexual means where the male and female involve in sexual
intercourse, resulting in fertilization. During sexual intercourse, the interaction between the
male and female reproductive systems results in fertilization of the woman’s ovum by the
man’s sperm. The ovum and sperm are specialized reproductive cells called gametes, generated
by a process called gametogenesis (i.e., spermatogenesis in males and oogenesis in females).
The gametes are haploid in nature and it is when these two cells merge into one zygote cell
that genetic recombination occurs and diploid condition is achieved back. After a gestation
period, i.e., nine months in humans, childbirth takes place. In the present unit, we will discuss
the histological and anatomical details of organs involved in male and female reproductive
systems, the process of formation of gametes i.e., spermatogenesis and oogenesis, structure ofgametes i.e., spermatozoan and ovum in details.
16.2 MALE REPRODUCTIVE SYSTEM
The male reproductive system consists of the male sex organs, a system of various ducts and
accessory sex glands.
Figure 16.1: The overview of components of male reproductive system
16.2.1 Male Sex Organs
Scrotum: Scrotum is a bag like structure which is an out pouching of abdominal wall. Externally,
it appears as a single pouch separated with a median ridge called raphe. Internally, it is divided
into two sacs each of which contains a testis.
Figure 16.2: Scrotum from anterior view
The temperature of testes is maintained 2°C–3°C below the core body temperature which
provides favourable environment for spermatogenesis. This lowering of temperature is regulated
by scrotal muscle reflexes and venous pampiniform plexus present around the testicular artery.
Testes: Present inside the scrotal sacs are the oval shaped male sex organs, called testes. A
human testis is about 4.5 cm long and 2.5 cm broad with 3 cm antero-posterior diameter,
weighing approximately 10-15 grams. The testes are surrounded by serous sheath called tunica
vaginalis, anteriorly and laterally. Behind tunica vaginalis is present a thick, white, fibrous
capsule called tunica albugenia. Interior to it lies the tunica vasculosa, which is rich in vascular
supply. Posteriorly, the tunica albuginea thickens greatly and is projected into the interior of
the testis as the Corpus Highmori or mediastinum testis. The testes are held in position by
mesenteries called mesorchium.
The ducts, blood & lymphatic vessels and nerves enter or leave the testis through the mediastinum.
The connective tissue septa, called septula testis (singular- septum of testis) radiate from
mediastinum into the testis. These septulae subdivide the interior of the testis into a number of
pyramidal lobules called the testicular lobules. Each testicular lobule further contains several
sperm producing convoluted tubules known as the seminiferous tubules (semin = seed; fer =to carry).
Figure 16.3: (A) Entire testis section at low magnification. (B) Histological sections of testis
observed at different magnifications of objective lens. (C) Diagrammatic view of testis histology
The seminiferous tubules contain specialized epithelial cells, the spermatogenic cells and the
supporting cells, also known as the sertoli cells. Sertoli cells provide support and nourishment,
help in cell-to-cell communication, secrete inhibin and androgen binding protein etc. These are
surrounded and supported by intertubular connective tissue which is rich in blood vessels and
groups of epithelial cells, known as the Leydig cells (also called the interstitial cells or interstitialendocrinocytes). The leydig cells produce androgen and the male sex hormone, testosterone.
16.2.2 Male Reproductive Ducts
The ducts in male reproductive system help in storage and transportation of spermatozoa.
The spermatozoa along with testicular fluid after their release into the lumen of seminiferoustubules (spermiation process) are propelled towards the rete testis. Rete testis is composed
of various channels through which spermatozoa travel to efferent ducts, i.e., vasa efferentia
(also called ductuli efferentes). Vasa efferentia further joins with epididymis. Epididymis
is divided into three parts: head (caput), body (corpus) and tail (cauda) epididymis. The
cauda epididymis is attached to scrotal sacs by a connective tissue called gubernaculum.
Vasa efferentia joins epididymis from head region and vas deferens leaves the epididymis
from tail region. The epididymis performs functions such as storage of spermatozoa,
disposal of aged and abnormal spermatozoa and carries out maturational changes by
secretion and absorption. Spermatozoa then pass through a single tube known as the vas
deferens, also called ductus deferens or ductus epididymis. It joins with duct of seminal
vesicle to form the ejaculatory duct. Both the ejaculatory ducts from either side further
penetrate into prostate gland and empty into urethra. Urethra finally opens to the exterior
through penis. Penis is both the excretory and reproductive functions as it passes both the
urine and spermatozoa. Penis is divided into root, body i.e., shaft and glans penis. The
shaft is composed of three columns of erectile tissues i.e. pair of corpora cavernosa on
dorsal side and single corpus spongiosum on ventral side. Each column is surrounded by
dense connective tissue called tunica albuginea. The root is the attached proximal portion
and consists of bulb of penis. Glans penis is the distal, sensitive, cone shaped, hairless
structure that contains sensory receptors for sexual stimulation. Prepuce or foreskin is the
loose skin fold that covers the glans penis as sheath. Surgical removal of prepuce is calledcircumcision.
Figure 16.4: Diagrammatic view of Male Reproductive Ducts
Figure 16.5: Anatomy of Penis, the Male Reproductive Organ
16.2.3 Accessory Male Reproductive Glands
These include a pair of seminal vesicles, a prostate and a pair of bulbourethral glands.
(i) The seminal vesicles are one pair of sac like structures near the base of the bladder. Their
ducts join the vasa deferfintia to form the ejaculatory ducts. They produce an alkaline
secretion which forms 70% of the volume of semen. The secretion of the seminal
vesicle contains spermatozoa activating substances such as fructose, citrate, inositol,
prostaglandins and several proteins. Fructose is a source of energy for the sperm.
Prostaglandins stimulate uterine contractions and thus, may help the sperm to move
towards the female’s oviducts, where fertilization takes place. Some proteins of the
secretion help coagulation of semen after ejaculation. Alkalinity of the seminal fluid
helps to neutralize the acidic environment of the male urethra as well as that of female
reproductive tract which would otherwise inactivate and kill sperms.
(ii) The prostate gland is of a size of a golf ball and surrounds the proximal portion of urethra.
It produces a milky, slightly acidic secretion (pH = 6.5) which form 25% of the volume
of semen. The secretion of the prostate gland nourishes the spermatozoa and aids intheir motality.
(iii) The bulbourethral glands or cowper’s glands are present on either side of the proximal
portion of urethra. They secrete mucus that helps in the lubrication of penis and linning
of urethra.
The secretions of the above glands constitute seminal plasma which is rich in fructose, calcium
and certain enzymes. Fructose which is produced by the seminal vesicles, is not present anywhere
else in the body, provides a forensic test, for rape. The presence of fructose in the female’s genital
tract confirms sexual intercourse.
Semen is the mixture of spermatozoa and seminal plasma (the secretions of the seminal vesicles,
prostate gland and cowper’s glands). It is ejected from the penis during ejaculation. A single
ejaculate may contain 200 to 300 million spermatozoa (sperms). Semen is slightly alkaline
(pH 7.35 to 7.50). Its alkalinity helps to neutralize the acidity of urethra left from the passageof urine and protects the sperms from the acidity of vagina.
Figure 16.6: Sagittal view of human male reproductive system
Testosterone is the principal hormone of the testes which is synthesized in the Leydig cells
from cholesterol and is also formed in the adrenal cortex. The major functions performed by
testosterone in male reproductive system are:
• Gonadotropin regulation
• Spermatogenesis
• Sexual differentiation
• Wolffian stimulation
• Sexual maturation at puberty
Inhibin is another hormone of testicular origin that inhibits FSH secretion. Estradiol and estroneare also synthesized in testis in minor quantities.
APPLICATION 16.1
1. Complete with appropriate terms:
(i) Male sex organs include ...................... and ...................... .
(ii) The seminiferous tubules contain specialized cells, the spermatogenic and supporting
cells called ...................... .
(iii) ...................... joins epididymis from head region and ...................... leaves the
epididymis from tail region.
(iv) ............... provide alkaline environment that protects the passing spermatids
2. What is the function of the prostate gland
3. In which ovarian structures do oocytes develop
4. List the sequence of structures through which a sperm passes from its site of production to
the site of fertilization of an ovum5.How do egg cells move from the ovary to the uterus
16.3 FEMALE REPRODUCTIVE SYSTEM
The human female reproductive system consists of the primary sex organs or the gonads
(ovaries), the genital ducts (oviducts or the uterine/fallopian tubes, uterus, cervix and vagina)
and the external genitalia, along with a pair of mammary glands.
Ovaries (singular: Ovary, Latin: Ovarium, literally meaning ‘egg’ or ‘nut’) — the primary sexorgans in females are egg-shaped, paired structures, located in the upper pelvic cavity, one on
Figure 16.7: An overview of human female reproductive system
either side of the uterus in front of the ureter, embedded in the connective tissue matrix called
ovarian fossa. A single ovary is about 2 to 4 cm long, 2 cm wide and 1.5 cm thick and weighs
about 15 grams. Each ovary is held in place by three ligaments: mesovarium or the broadligament, suspensory ligament and the uteroovarian ligament.
Figure 16.8: The relation between various structures of human female reproductive system.
Ovarian ligaments can be seen clearly
Histology of the ovary: A typical human ovary is composed of connective tissue called stroma,
wrapped by germinal epithelium which is further layered by tunica albuginea. The stroma
is divided into two zones: an outer dense cortex and a less dense inner medulla. The medulla
consists of loose connective tissue, blood vessels, lymphatics, smooth muscle fibres and nerves.
The cortex consists of rounded structures called ovarian or the Graafian follicles, at various
stages of development. Each follicle consists of a large ovum surrounded by several layers of
follicular or granulosa cells.
A fully grown ovarian or the Graafian follicle typically consists of:
• an oocyte (15-30 μm wide) with a nucleus called the germinal vesicle, bounded by vitelline
membrane which is further surrounded by zona pellucida.
• surrounding the zona pellucida is present membrane granulosa, consisting of granulosa
cells or the follicular cells. The granulosa cells lying in close vicinity of the oocyte may
become elongated to form the corona radiata.
• membrana granulosa is further covered on the outside by theca interna and theca externa.
• a cavity called follicular antrum/cavity filled with a fluid, the liquor folliculi.
• the oocyte anchors to the wall of the follicle by a thin layer of follicle cells called cumulusoophorus, which nourishes the oocyte.
Figure 16.9: Diagrammatic view of the cross section of a human ovary.
Ovarian follicles can be seen at various stages of development
Figure 16.10: Structure of a mature Graafian follicle
Ovaries perform two important functions:
(a) Oogenesis: production of female gamete (ovum, pl. ova).(b) Production of hormones: some of the follicular cells produce the hormone estrogen, while
the follicle is developing and progesterone and relaxin while the follicle is degenerating
as corpus luteum. Estrogen stimulates the growth and functions of female sex organs
and development of secondary sexual characteristics. Progesterone prepares the uterine
lining to receive embryo and maintains it during pregnancy. It also stimulates the growth
of mammary glands. Relaxin helps in the relaxation of pelvic ligaments and softeningand widening of the cervix during delivery of the baby (parturition).
ACTIVITY 3
Aim: To study the histology of human testis and ovary using prepared slides and micrographs.
Theory: A typical testis is composed of various cell types i.e., sertoli cells, leydig cells,
fibroblasts,
primary and secondary spermatocytes, spermatids and basal lamina etc. A typical
human ovary is composed of connective tissue consisting of rounded follicles, at various stages
of development. Each follicle consists of a large ovum surrounded by several layers of follicular
or granulosa cells. Germinal epithelium is the outermost boundary of the ovary.
Procedure:
Observe a permanent slide of transverse section (T.S.) of human ovary and testis under lowpower and then under high power microscope and locate various cell types.
On the basis of your observations;
1. Label the cell types in following micrograph of testis and ovary.2. Complete the table given below based on histological analysis ofovary:
16.3.1 Female Reproductive Ducts
Oviducts: Also known as the uterine ducts or the fallopian ducts, lined with ciliated epithelia,
they function to transfer the ovum from the ovary to the uterus and serve as the site of fertilization
of the male and female gametes. Each oviduct, 10-12 cm long, extends from the margins of
the ovary to the uterus and can be divided into four continuous regions as:
• Infundibulum — the part closest to ovary. It has finger-like projections called fimbriae that
drape over the ovary and serve to receive the ovum released by the ovary during ovulation.
• Ampulla — the widest and major part of the tube, the site of fertlization.
• Isthmus — the narrower part, that links to the uterine wall.
• Interstitial or the intramural or the uterine part—that lies within the uterine wall.
Uterus: This is an ‘inverted pear’-shaped, muscular, hollow (uterine cavity lies within),
hormone-responsive organ that serves to house, nourish and protect the growing foetus till
birth. Anatomically, the uterus consists of three parts:
• Fundus—the dome shaped part above the openings of the uterine parts of the fallopian tubes.
• Corpus uteri or the ‘body’—the main centrally expanded portion.
• Cervix—the inferior narrow portion that opens into the vagina. A narrow, constricted
region, about 1 cm long called isthmus joins the uterus with the cervix. The cervical canal
or the cavity of the cervix communicates with the uterus internally by an aperture called
internal os and with the vagina by external os.
Cervix and vagina together form the birth canal. During childbirth, it dilates widely to allow the
baby to pass through. The wall of the uterus is composed of tissue layer called endometrium.
The endometrium is shed during menstruation and is regenerated by the basal layer after eachmenstruation.
Vagina: It is a distensible, muscular tube, about 10 cm long, which extends from vulva (external
genitalia) to the uterus. The vaginal opening on the vulva is termed the vaginal orifice. The
orifice is partially covered by a membrane called hymen that ruptures during the first act of
intercourse. However, it may get ripped off during some strenuous activities like sports or due
to some disease. Vagina serves as a receptacle for the male copulatory organ during sexual
intercourse, provides a passageway for the menstrual flow and forms part of the birth canalduring childbirth.
16.3.2 External Genitalia (Vulva)
The female external genitalia include mons pubis, labia majora, labia minora, hymen and clitoris.
Mons pubis is a cushion of fatty tissue covered by skin and pubic hair. The labia majora are fleshy
folds of tissue which extend down from the mons pubis and surround the vaginal opening. The
labia minora are smaller folds which lie under the labia majora. Anteriorly, labia minora merge
together to form a small eractile organ called clitoris. It is Homologous to the male’s glans penis.
Posteriorly, the labia minora are fused together to form the fourchette. They also containnumerous sebaceous glands.
Figure 16.11: The external genitalia in human female
16.3.3 Female Reproductive Glands
The glands associated with vestibular region are broadly of two types:
• Glands of Skene or Paraurethral glands (lesser vestibular glands): Those are numerous
minute glands present around the urethral orifice. They secrete mucus and are homologousto the male prostate glands.
• Bartholin’s glands (greater vestibular glands): These are located on each side of vagina
beside the vaginal opening and produce a mucus secretion to lubricate the vestibule that aids
in sexual intercourse. They are considered homologous to the bulbourethral glands in males.
• Mammary glands or Breasts: These are modified sweat glands that produce milk. Though
characteristic of all female mammals, they are also present in a rudimentary form in males.
In females also, they remain underdeveloped until puberty. Each breast is covered with
skin and bears a central protruding nipple surrounded by a pigmented area called areola.
Internally, each breast consists of the glandular tissue, the fibrous connective tissue and the
fatty or the adipose tissue:
o The glandular tissue of each breast is divided into 15-20 mammary lobes. Each
lobe is further divided into a number of lobules containing clusters of cells called
alveoli. The cells of alveoli secrete milk, which is stored in the cavities (lumens) of
alveoli. The alveoli open into mammary tubules. The tubules of each lobe join to
form a lactiferous or mammary duct, which opens on the nipple. Just before their
opening, the lactiferous duct swells to form the mammary ampulla or lactiferous
sinus, which acts as a reservoir for milk during lactation. In non-pregnant and
non-nursing women, the glandular structure of the breast is largely underdevelopedand the duct system is rudimentary.
Figure 16.12: (a) Front view human breast with right half showing internal structure
(b) Sagittal section of human breast
Human milk consists of water, organic and inorganic components. The main constituents
include fat (droplets), casein (milk protein), lactose (milk sugar), mineral salts (sodium, calcium,
potassium, phosphorus, etc.) and vitamins. Human milk is poor in iron and vitamin C content.
With childbirth, the anterior lobe of pituitary secretes the hormone prolactin, which stimulates
the production of milk. Another hormone oxytosin secreted by the posterior lobe of pituitary
stimulates the release of milk, though the psychic state and nutrition of the mother also governs
milk production.The first milk produced by each breast after childbirth is termed the colostrum.
It is thick, yellowish fluid rich in proteins and antibodies, that provides passive immunity to
the breastfed baby. Colostrum also helps the newborn’s digestive system to grow and function
properly.
16.3.4 Menstrual Cycle
Menstrual cycle is the reproductive cycle involving cyclic in the ovary and uterus that occurs in
the female primates, e.g., humans, monkeys, apes, etc. This cycle is responsible for maturation of
the ovum and its release from the ovary (ovulation) and preparation of the uterus for pregnancy.
The menstrual cycle is regulated by hormones. A typical menstrual cycle in human female is
about 28 days long with 3–5 days plus or minus variation. The onset of this cycle (known as
menarche) generally starts at the age 12–15 (puberty) in girls and ends (known as menopause)
by the age of 45–50 in humans.
One cycle is completed in three phases: the uterine cycle is divided into menstrual phase,
proliferative phase, and secretory phase and correspondingly the ovarian cycle consists of thefollicular phase, ovulation, and luteal phase
1. Menstrual phase or bleeding period or the menses or period (days 1–4): Low levels
of hormones progesterone and estrogen make the blood vessels of the endometrium
constrict. This causes the endometrial lining to shed along with blood and unfertilized
ovum.
At the same time, under the influence of Follicle stimulating hormone (FSH) secreted
by the anterior lobe of pituitary, ovaries are beginning follicular phase i.e., growth and
proliferation of one of the follicles to become Graafian follicle or tertiary follicle.
2. Proliferative phase (days 5-14): Gonadotrophic releasing factor (GnRF) secreted by
the hypothalamus stimulates the release of FSH and Luteinizing hormone (LH). High
levels of FSH stimulate the ovarian follicle to secrete estrogens. The combination of FSH
and LH, and estrogen, has a positive feedback effect on anterior pituitary, causing therelease of more and more FSH and LH and thus, more estrogen.
Estrogens cause the endometrial lining of the uterus to proliferate, rapid cell multiplication
occurs with development of uterine glands and blood vessels. Ovary is still undergoing
follicular phase.
Towards the end of proliferative phase, high levels of estrogen trigger a sudden
increase in the levels of LH. This LH surge, as it is called, lasts for 24-48 hours, causes
the rupture of ovarian follicle and release of ovum from the ovary into the oviduct(= ovulation phase of the ovarian cycle).
Figure 16.13: Diagrammatic presentation of various events during a menstrual cycle
3. Secretory phase (days 14-28): The final phase of the uterine cycle corresponds to the
luteal phase of the ovarian cycle that lasts for about 10 days. The Luteinizing hormone(LH) secreted by the anterior lobe of pituitary stimulates the development of corpus
luteum from degenerating cells of the ovarian follicle after ovulation. Corpus luteum
secretes large amounts of progesterone and some estrogen.
The combination of estrogen and progesterone inhibits the release of FSH and LH from
pituitary (negative feedback). Progesterone prepares the uterus to receive, implant and
nourish a fertilized egg during the second half of the menstrual cycle.
If implantation does not occur, the endometrium breaks down and menstruation occurs.
Corpus luteum degresses and production of progesterone is lowered. This releases theinhibition of FSH and LH, thus initiating the next menstrual cycle.
Corpus Luteum
Corpus luteum (Latin means “yellow body”; plural: corpora lutea) is a temporary endocrine
structure that develops from degenerating cells of the ovarian follicle after the release of ovum,
under the influence of LH. It produces large amounts of progesterone.
If the oocyte is fertilized, the corpus luteum continues to proliferate and increases hormone
production. By the end of the third month of pregnancy, luteal cells occupy a large part of the
ovary and keep releasing progesterone. However, by the end of the fourth month, they regress
slowly.
If the oocyte is not fertilized, the corpus luteum degenerates in 10–12 days after ovulation.It is visible only in the form of a white scar, the corpus albicans, on the outside of the ovary.
APPLICATION 16.2
1.Complete with appropriate terms:
(i)...................... stimulates the release of milk.
(ii) Ovary is held in place by three ligaments called .............., .............., and ................. .
(iii) Follicular cells produce ...................... , ...................... and ...................... .
(iv) ...................... acts for reservs for milk during lactation.
2. The diagram shows a section through part of a human testis
a) Giving a reason for your answer in each case, what percentage of each of the following
types of cell would you expect to contain a Y chromosome
i. type A cells
ii. type C cells?
b) Many of the type B cells will undergo mitosis. Explain the importance of mitosis in a
mature testis
c) Give two ways in which cell division results in the type C cells being genetically differentfrom one another
16.4 GAMETOGENESIS
Gametogenesis is the process of formation of haploid gametes from undifferentiated, diploid
germ cells in the gonads for sexual reproduction. Male and female sex cells or gametes i.e.,
sperms and ova are formed respectively in the male and female gonads (testes and ovaries).
Types and phases of gametogenesis: Formation of male gametes (sperms) is termed
spermatogenesis and that of female gametes (ova, singular: ovum) is referred to as oogenesis.
Both the processes undergo three basic phases, common to both:
1. Proliferative or the multiplication phase
2. Growth phase3. Maturation or differentiation phase
ACTIVITY 4
Aim: To understand the process of gametogenesis and haploid nature of gametes.
Theory: Gametes are haploid cells that are formed from diploid germ cells through the process
of gametogenesis. The significance of developing haploid gametes lies in the fact that after
fertilization, the developing zygote attains the diploid status back. In this way, the developing
embryo gets the single copy of all the chromosomes from each parent.
Procedure: Based on the chart diagrams of spermatogenesis and oogenesis shown below,compute the number of chromosomes at each stage, assuming 2n = 46.
16.4.1 Spermatogenesis
The process of formation of haploid male gametes or spermatozoa from diploid reproductive
cells in males is called spermatogenesis. The complete process is broadly divided into two parts:(i) Formation of spermatids and (ii) Spermiogenesis or spermatoleosis.
Formation of Spermatids
The process of formation of spermatids is further divided into three stages as:
(a) Multiplication phase: The primordial germ cells or sperm mother cells differentiate from
germinal epithelium of testis and increase in size with prominent nuclei. These cells
divide repeatedly by mitosis (i.e., equational division) and produce a number of diploid
daughter cells, known as spermatogonia. Thus, in this stage, multiplication of germ cells
takes place mitotically.
(b) Growth phase: In this phase, spermatogonia increase in size by accumulating food
reserves and are now called primary spermatocytes.
(c) Maturation phase: The primary spermatocytes (which are diploid) undergo first
maturation division which is meiotic division (or reductional division) to produce two
haploid secondary spermatocytes. These haploid secondary spermatocytes divide further
by mitosis to give rise to four haploid spermatids. This mitotic division is called secondmaturation division.
The spermatids so produced are non-motile rounded structures that metamorphose into functional
and motile spermatozoa through a process known as spermiogenesis or spermatoleosis.
The spermatozoa from testis are incapable of fertilizing an ovum. They undergo several
morphological, physiological and biochemical changes as they move through the epididymis
to attain this structural and physiological maturity. The epididymis i) provides a favourable
environment to spermatozoa in acquiring fertilizing ability and ii) stores them until they are
ejaculated or move down to the vas deferens.
The morphological changes include structural remodelling of acrosome and formation of
disulfide linkages. The physiological and biochemical changes include increase in net negative
charge on spermatozoa, change in pattern of motility, change in content of sialic acid, increase
in specific activity and reflection power, resistance to pH and temperature and changes inmetabolic patterns.
Spermiogenesis
A series of changes in spermiogenesis that transform a non-motile spermatid into motile,
functional spermatozoa are listed below:
• The nucleus shrinks and flattens by losing water. Only DNA is left in the nucleus, making
cells very light that aids to its motility.
• The two centrioles of a centrosome form proximal and distal centrioles. The proximal
centriole lies at the posterior end of nucleus and the distal centriole gives rise to axial
filament of the flagellum and acts as a basal granule.
• The mitochondria gather around axial filament and gradually unite to form spiral sheath
or nebenkern. It acts as power house of the sperm and provides energy.
• The golgi bodies form the covering over nucleus called acrosome. During acrosome
formation, one or more vacuoles start enlarging with a small, dense body called
pro-acrosomal granule which further enlarges to form acrosomal granule. The vacuole loses
its liquid content and forms the cap of spermatozoan. The remaining part of golgi apparatus
is reduced and discarded from sperm.
During all these steps, head of the developing sperm remains embedded in sertoli
cells for nourishment. At the end, fully formed spermatozoan shows distinct head,middle piece and tail region.
Figure 16.14: (a) Process of spermatogenesis showing chromosome numbers at various stages and
the cross-section of a seminiferous tubule showing histological arrangement of various cell types
(b) Stages in the formation of spermatozoan from spermatid and acrosome formation fromgolgi apparatus during spermiogenesis.
Structure of Spermatozoa
The sperms are microscopic and motile cell. Each sperm is composed of four parts—a head, a
neck, a middle piece and a tail. A plasma membrane covers the whole body of sperm.
(i) Head is the enlarged end of the sperm, containing an elongated haploid nucleus. The
anterior of the nucleus is covered by a cap-like structure called acrosome. The acrosome
contains enzymes sperm lyftins or hyaluronidases, which are used to contact and
penetrate the ovum at the time of fertilization.
(ii) Neck is very short and is present between the head and middle piece. It contains the
proximal centriole towards the nucleus which plays a role in the first cleavage of thezygote and the distal centriole which gives rise to the axial filament of the sperm.
Figure 16.15: Structure of a sperm
(iii) Middle piece possesses numerous mitochondria which produce energy for the movement
of the sperm. At the end of the middle piece, there is a ring centriole (annulus) with
unknown function.
(iv) Tail is several times longer than the head. It consists of an axial filament surrounded by
a thin layer of cytoplasm. The tail provides motality to the sperm, which is essential forfertilization.
The male ejaculates about 200 to 300 million sperms during a coitus. For a normal fertility,
at least 60 per cent sperms of the ejaculate must have normal shape and size, and at least 40
per cent of the normal sperms must show vigorous motility. Sperms remain alive and retain
their ability to fertilize an ovum from 24 to 48 hours after having been released in the femalegenital tract.
16.4.2 Oogenesis
The process of oogenesis occurs in the ovaries. The three phases of proliferation, growth and
maturation occur in discontinuous steps.
(a) Proliferative or multiplication phase: During early foetal development, certain cells
within the germinal epithelium of the ovary become enlarged. These cells proliferate by
mitosis, producing undifferentiated germ cells called egg mother cells or oogonia (2n).The oogonia divide mitotically to produce groups of oogonia, termed follicles.
Figure 16.17: Schematic representation of oogenesis
(b) Growth and differentiation phase: During this long phase, which may last upto years,
one cell in a follicle prepares for the formation of ovum. It starts meiotic division but
gets arrested at prophase-I stage and is called primary oocyte. The remaining cells of
the follicle lose the potential to become primary oocyte and are known as the follicular
cells or granulosa cells. These follicular cells serve to protect and nourish the primary
oocyte. The complete follicle with a primary oocyte surrounded by a layer of follicular
cells is called the primary or the ovarian follicle.
(c) Maturation phase: At puberty, only one of the primary oocytes resumes division per
menstrual cycle, alternately in each ovary. The tertiary follicle matures into a Graafian
follicle, within which the primary oocyte divides to form two very unequal cells - a large
secondary oocyteand a very small 1st polar body or polocyte
may further be divided into two polar bodies. However, the secondary oocyte again gets
arrested at metaphase stage of meiosis-II and is released from the ovary during ovulation.
It waits in the oviduct for the sperm to arrive. If fertilization occurs, sperm entry into
the secondary oocyte marks the resumption of meiosis. The 2nd maturation division
(meiosis-II) again divides the secondary oocyte into two unequal daughter cells—a large
ootid and a very small 2nd polar body. The ootid undergoes maturation into a functional
haploid ovum. A thin vitelline membrane develops outside the plasma membrane of theovum that protects and nourishes the latter.
Figure 16.18: Maturation of a follicle shown in a clockwise direction proceeding from the primordial
follicles. FSH stimulates the growth of a tertiary follicle, and LH stimulates the production of estrogen
by granulosa and theca cells. Once the follicle is mature, it ruptures and releases the oocyte. Cellsremaining in the follicle then develop into the corpus luteum.
Thus, from one oogonium (egg mother cell), one ovum and three polar bodies are formed.
The ovum is the functional female gamete while the polar bodies take no part in reproduction
and soon degenerate. The formation of polar bodies only helps the egg to get rid of one set of
chromosomes and still enables the ovum to retain most of the cytoplasm and food for the future
embryo. In case fertilization does not occur, the secondary oocyte undergoes degeneration andis driven out of the body.
Structure of Ovum
An ovum is a spherical, non-motile cell, in the secondary oocyte stage of oogenesis, where the
second maturation division is yet to occur. Human ovum is extremely small in size i.e., 0.15
mm in diameter, polar and microlecithal. The large nucleus is called germinal vesicle or later
the female pronucleus. The nucleolus is called the germinal spot and cytoplasm is known as
ooplasm. The peripheral layer of ooplasm, known as cortex, is more viscous and contains
cytoskeletal structures like microtubules and microfilaments, pigment granules and cortical
granules of mucopolysaccharides. The inner part of cytoplasm, called the endoplasm is withcell-organelles, informosomes, tRNAs, histones, enzymes etc. The ovum is covered over by a
Figure 16.19: Structure of a mature human ovum with corona radiata surrounding it
thin, transparent vitelline membrane which is further covered over by zona pellucida. There isa narrow space between these two membranes known as perivitelline space. During discharge
of ovum from the Graafian follicle, several layers of follicular cells adhere to the outer surfaceof zonapellucida and are arranged radially forming corona radiata.
Differences between spermatogenesis and oogenesis
ACTIVITY 5
Study the chart diagrams of spermatogenesis and oogenesis. Identify various stages and writedown the points of similarities and dissimilarities between the two processes.
Differences between sperm and ovum
ACTIVITY 6
On the basis of your observations, draw the structure of a human spermatozoan and an ovum,labelling the following parts along with the functions of each:
APPLICATION 16.3
1.Complete with appropriate terms:
(i) The three basic steps of gametogenesis includes ..............., ............... and ............... .
(ii) One spermatogonium produces ............... and one oogonium produces ............... .
(iii) ............... cells provide nutrition in spermatogenesis.
(iv) The tertiary follicle matures into ............... .2. The diagram shows some stages in the formation of a mammalian egg cell
a.How many chromosomes will there be in
i. cell B
ii. cell C
b.Suggest one advantage in the way in which the cytoplasm divides during meiosis
c. Describe and explain two ways in which the events of meiosis cause the egg cells to begenetically different from one another
16.5 ONSET OF PUBERTY IN HUMANS
Males attain sexual maturity, or puberty as it is called, at the age of 11–13 years. At puberty,
the primary sex organs (=gonads) mature and secondary (=external) sex organs mature and
secondary sexual characters appear viz. development of pubic, chest and underarm hairs, growth
of larynx resulting in deepening of voice, masculine pattern of fat distribution, thick secretion
from skin oil glands, bone and muscle development, etc., are some of the prominent male
secondary sexual characters. Testosterone controls the onset of puberty in males. Hypothalamus
starts producing GnRH, which stimulates the anterior lobe of pituitary to secrete luteinizing
hormone (LH) and follicle stimulating hormone (FSH).
Females attain puberty at the age of 12–15 years. At this age, pituitary gland starts producing
follicle-stimulating hormone (FSH). The FSH induces the development of ovaries, which
produce the hormone estrogens, chiefly estradiol. Estrogen is responsible for the development of
secondary sex organs and secondary sexual characters in females, which include development
of breasts and external genitalia, pigmentation of the areola, growth of pubic hair, axillary
hair (in the armpits), widening of the pelvis and deposition of fat in thighs, buttocks and face.Puberty marks the onset of menstruation or menarche in females.
Differences between primary and secondary sex organs
16.6 SUMMARY
• Human male reproductive system carries out the functions of spermatogenesis i.e.,
formation of functionally active, motile sperms along with seminal plasma.
• The system comprises male sex organs i.e., testes (paired organ) and scrotum, a series
of ducts that help in transportation and maturation of spermatozoa and the accessory
glands which secrete the essential components of semen plasma.
• The temperature of testes is maintained 2°C –3°C below the core body temperature which
provides favourable environment of spermatogenesis.
• The male reproductive ducts include vasa efferentia, vasa defrentia, epididymis, ejaculatory
ducts and urethra primarily.
• The accessory glands of reproduction in human reproductive system are seminal vesicles,
prostate gland, urethral (Littre’s) glands and bulbourethral (Cowper’s) glands.
• Human female reproductive system is designated to carry out the functions of ovulation,
carry male and female gametes, fertilization, gestation and childbirth.
• Human female reproductive system consists of female sex organs- ovaries, duct system
- oviducts or the uterine/fallopian tubes, uterus, cervix, vagina, the external genitalia –
vulva and a pair of mammary glands.
• Ovaries produce female gametes- ova. They contain ovarian follicles at various stages of
development. An ovarian follicle consists of ovum surrounded by granulosa cells and
other layers for protection and nutrition.
• Oviducts transfer ovum from ovary to the uterus and serve as the site of fertilization.
Each oviduct is divided into an infundibulum, ampulla, isthmus and uterine parts.
• Uterus is a hollow organ that serves as the site of implantation and nourishment of the
embryo till birth. It consists of three parts – fundus, body and cervix. The inner lining
is called endometrium.
• Cervix and vagina form the birth canal.
• External genitalia or vulva consists of mons pubis, clitoris, labia majora, labia minora
and perineum.
• Glands include lesser vestibular and greater vestibular glands and a pair of mammary
glands. Mammary glands function in the production of milk for the young one.
• Menstrual cycle consists of three phases: menstrual, proliferative and secretory in the
uterus, corresponding to follicular, ovulation and luteal phases of ovarian cycle.
• The onset of menstrual cycle at puberty is termed menarche and end is called menopause.
• The menstrual cycle is governed by hormones.
• The process of formation of haploid male gametes or spermatozoa from diploid
reproductive cells in males is called spermatogenesis.
• The complete process of spermatogenesis is broadly divided into two parts:
(i) Formation of spermatids and (ii) Spermiogenesis or spermatoleosis.
• Spermatid formation is further divided into three phases as multiplication, growth and
maturation phase.
• Spermiogenesis is the process of series of changes to transform a non-motile spermatid
into motile, functional spermatozoa.
• Oogenesis is the formation of haploid ovum from diploid undifferentiated germ cells in
the ovary.
• Oogenesis is completed in three phases of discontinuous steps.
• Proliferative or multiplication phase involves proliferation of oogonial cells by mitosis.
• Growth phase is all about growth and differentiation of primary oocyte and development
of mature ovarian follicle.
• Maturation phase provides time for the oocyte to undergo two meiotic divisions toproduce functional haploid ovum.
16.7 GLOSSARY
• Castration: The surgical removal of testis is called castration.
• Coitus: The sexual act of transferring sperms of male to vagina of female.
• Cryptorchidism: Abnormality wherein the testes do not descend down from the abdomen
to scrotum is called cryptorchidism (crypt = hidden; orchid = testis).
• Ectopic pregnancy: A pregnancy occurring in the fallopian tube or outside of the uterine
lining.
• Ejaculation: The process of propulsion of semen out of the urethra at the time of orgasm.
• Emission: Emission is the phenomenon of movement of sperms from the testes into the
urethra along with secretions from the various accessory glands of reproduction, where they
mix to form semen.
• Erection: The accumulation of blood in penile erectile tissue leading to temporary
swelling and elongation of penis is called erection. This accumulation of blood is because
of dilation of arteries and compression of veins of penis.
• Gynecomastia: The condition when mammary glands become functional in males.
• Hypermastia: Presence of more than the normal number of breasts.
• Hysterectomy: Surgical removal of the uterus is known as hysterectomy.
• Impotence or erectile dysfunction: Impotence is the failure to achieve and/or maintain
the erection for coitus.
• Orgasm: Orgasm is a pleasurable feeling of physiological and psychological release
associated with the culmination of sexual stimulation. Emission and ejaculation of semen
accompany orgasm in males.
• Tubectomy: Surgical procedure for sterilization in which a woman’s fallopian tubes are
clamped and blocked, or severed and sealed.
• Vasectomy: Bilateral ligation of vas deferens as a contraceptive measure in males iscalled vasectomy.
END UNIT ASSESSMENT 16
I. Choose whether the following statements are True (T) or False (F)
1. Germinal vesicle is the mitochondria of the ovum.
2. Urethral gland in human males is also called gland of Littre.
3. The proliferative phase involves two meiotic divisions of the oocyte.
4. Clitoris is considered homologous to penis of males.5. Testosterone is synthesized in sertoli cells.
II. Fill in the blanks
1. Cowper’s gland in human male is also called ............................. .
2. Testes are present in the sac called ..............................
3. ....................... hormone helps in the relaxation of pelvic ligaments during childbirth.
4. The site of fertilization in human female is ............................. .
5. Corpus luteum secretes large amounts of hormone ............................... .
6. The onset of menstrual cycle at puberty is termed ............................... .
7. The caudaepididymis is attached to scrotal sacs by a connective tissue called
............................. .
8. The hormone responsible for most of the secondary sex characters in human maleis ............................. .
III. Long Answer Type Questions
1. Give an account of various processes involved in oogenesis in human females.
2. Draw a well labelled diagram of human spermatozoan.
3. Give a detailed account of male reproductive accessory glands and discuss their role
in male reproductive system.
4. Describe the histological structure of human testis.
5. With the help of suitable diagrams, explain the structure of human female reproductive
system in detail.
6. Describe the internal anatomy of a typical human ovary.
7. Explain the steps leading to the formation of corpus luteus? What is its significance?
8. State where male and female gametes are produced.
9. Explain the significance of gametogenesis.
10. (i) Identify the structure shown in figure.
(ii) Name the parts marked 1–8 in the figure.
(iii) Write the functions of the parts marked 7 and 8.
(iv) Name the chemical substance present in the part marked 2.(v) What is the importance of the chemical substance present in the part marked 2?
11. Genetic disparity has since ages disturbed the masses on Earth. Reproduction is a natural
process but still the work for male and females is defined in the society. Assess your
understanding to support gender equality. Also, list the names of organizations supportingthe cause in Rwanda.