SUDHIR MEMORIAL INSTITUTE
A.I.S.S.C.E 2022-2023
BiologyInvestigatoryproject
TOPIC:Reproduction
Inhumans
NAME - SK REJU
CLASS - XII (SCIENCE)
BOARD ROLL NO -
Index
TOPICS
Certificate
Acknowledgement
Human Reproduction : Introduction
Male Reproductive System
Female Reproductive System
Gametogenesis
Menstrual Cycle
Fertilisation And Implantation
Pregnancy
And
Embryonic
Development
Parturition And Lactation
Conclusion
Bibliography
CERTIFICATE
I certify that the investigatory project on the topic
"Reproduction In Humans" has been done by Sk
Reju of class XII under my supervision for the
partial completion of A.I.S.S.C.E. Practical
Examination 2022-2023. I do hereby wish them
success for all the hard labour put in by them for
the completion of the project
Internal's Signature
External's Signature
Date:
Date:
ACKNOWLEDGEMENT
I, Sk Reju student of class 12, Sudhir Memorial
Institute hereby acknowledge that the project on
the topic "Reproduction In Humans" has been
done under the guidance of the teachers of the
department of biology. I would also like to extend
my gratitude to our principal sir, all my teachers
and the school authority for their support. I would
also like to thank my friends who helped me
throughout in this project.
Human Reproduction : Introduction
Reproduction is the process of giving birth to their
young ones which are identical to their parents. As
we all are aware of, the process of reproduction in
humans is sexual reproduction, which involves
internal fertilization by sexual intercourse.
The reproductive events in humans include
formation of gametes (gametogenesis), i.e., sperms in
males and ovum in females, transfer of sperms into
the female genital tract (insemination) and fusion of
male and female gametes (fertilisation) leading to
formation of zygote.
Male Reproductive System
The male reproductive
system is positioned
in the pelvis region and
comprises a pair of
testes in addition to
the accessory glands,
ducts, and the external
genitalia.
A pouch-like structure known as scrotum encloses the
testes located outside the abdominal cavity
Each testis has
close to 250 testicular
lobules(compartments).
These lobules comprise
1-3 seminiferous tubules
wherein the sperms are
produced. the lining of
these tubules consists of two types of cells – male
germ cells and sertoli cells
The exterior of these tubules consist of spaces
containing blood vessels and Leydig cells
Male sex accessory ducts comprises rete testis, vasa
efferntia, epididymis and vas deferens
The urethra opens externally to the urethral meatus
The male external genitalia, the penis is covered by
foreskin which is a loose fold of skin
The female reproductive
system is made up of the
internal and external sex
organs, which consists
of a pair of ovaries and
oviducts, cervix, uterus,
vagina and the external
genitalia situated in the
pelvic region. Along with the mammary glands,
these female reproductive organs are combined
both structurally and functionally in order to
support the complete processes of reproduction
including ovulation, fertilization, pregnancy, and
the birth of a child.
The female accessory ducts are constituted by
the oviducts, vagina and uterus
The section closer to the ovary is funnel-shaped
infundibulum that possesses the fimbriae –
finger-like projections facilitating the assimilation
of ovum post ovulation
The infundibulum directs to a wider section of
oviduct known as ampulla.
The last section of the oviduct, isthmus, has a
narrow lumen joining the uterus
Uterus is also known as the womb
The cervical cavity is known as the cervical canal
which goes onto form the birth canal along with
the vagina
Female external genitalia comprises – mons
pubis, labia minora, labia majora, clitoris and
hymen
Mammary Glands are paired structures (breasts)
that contain glandular tissue and variable
amount of fat.
Glandular tissue has 15-20 mammary lobes
containing clusters of cells called alveoli.
The cells of alveoli secrete milk, which is stored in
the cavities (lumens) of alveoli. The alveoli opens
into mammary tubules
The tubules of each lobe join to form a mammary
duct.
Several mammary ducts join to form a wider
mammary ampulla which is connected to
lactiferous duct through which milk is sucked
Gametogenesis
Gametogenesis is the process of
division of diploid cells to
produce new haploid cells. In
humans, two different types of
gametes are present. Male
gametes are called sperm and
female gametes are called the
ovum.
Spermatogenesis
In the male, immature germ cells are
produced in the testes. At puberty, in
males, these immature germ cells or
spermatogonia are converted into
sperms
by
the
process
of
spermatogenesis. Spermatogonia are
diploid cells that undergo mitotic
division and their number increases.
Primary
spermatocytes
undergo
meiosis and produce haploid cellssecondary
spermatocytes.
These
secondary spermatocytes undergo the
second meiotic division to produce
immature sperms or spermatids. These
spermatids undergo spermiogenesis to
transform
into
sperms.
Various
hormones like GnRH, LH, FSH and
androgens are involved in stimulating
spermatogenesis.
Oogenesis
In females, the oogonia are converted to the mature ovum. This process is called
oogenesis. In the female ovary, millions of oogonia or mother cells are formed
during fetal development. These mother cells undergo the meiotic cell division, the
meiotic division rests at the prophase-I and lead to the production of primary
oocytes. Primary oocytes are embedded within the primary follicles on the outer
layer. Primary follicles get surrounded by more granulosa cell layer and forms
secondary follicles. Secondary follicles then turn into the tertiary follicle. At the
stage of female puberty, the primary oocytes present in the tertiary follicles
complete meiosis and form secondary oocytes (haploid) and the polar body by
unequal division. The tertiary follicle undergoes some structural and functional
changes and produces mature Graafian follicle. Secondary oocyte undergoes
second meiotic division to form an ovum. Ovum is released from the Graafian
follicle during the menstrual cycle. The release of an ovum from the Graafian follicle
is called ovulation. Ovulation is controlled by the female reproductive hormone
which is stimulated by the pituitary gland.
Menstrual Cycle
In a life cycle, a woman’s body is vulnerable to a variety of changes.
The cycle of these changes occur in women every month, positively
for pregnancy is called the menstrual cycle. When an ovum is
unfertilized, the uterus lining sheds and leads to a haemorrhage,
called menstruation. In a girl, menstruation starts from the age of 10
to 15 when she attains puberty and this beginning is known as
menarche. The ending of menstruation is known as menopause
which takes place at the age range of 50. The first day of bleeding is
marked as the first day of a menstrual cycle and the period from
one menstrual cycle to another can vary from 28 to 30 days. Before
discussing the different phases of the menstrual cycle, it is
important to have a glimpse of the female reproductive system and
organs involved in this cycle. They mainly include:
A pair of ovaries that store, nourish and release ova.
Uterus (womb), where implantation of a fertilised egg takes
place and the foetus develops.
Pair of the fallopian tubes connecting the ovaries and uterus.
The count of the ovum in each ovary is decided and fixed before the
birth of a girl. As she reaches puberty, hormones stimulate the
development and release of one ovum per month. This continues till
menopause.
Phases Of Menstrual Cycle
The menstrual cycle is divided into four phases, namely:
1. Menstrual phase: Day 1, uterus lining which is prepared for implantation starts
to shed which lasts 3 to 5 days.
2. Follicular phase: In this phase, the primary follicle starts developing into a
mature Graffian follicle. The endometrium also starts proliferating. The uterus
starts preparation for another pregnancy.
3. Ovulatory phase: Mid-cycle phase, this is the phase in which ovulation takes
place i.e., day 13-17. The end of the follicular phase along with the ovulation
period defines the fertilisation period.
4. Luteal phase: It is the post-ovulation phase, where the fate of the corpus
luteum is decided. If fertilisation occurs, pregnancy starts. If fertilisation
doesn’t occur, it marks the onset of another cycle.
Fertilisation And Implantation
Fertilisation or fusion of gametes is the
most vital event in the process of sexual
reproduction as it results in new life. In
humans, while the male gamete or sperm is
motile; the female gamete or ovum is nonmotile. Therefore, for fertilisation to occur
the two gametes must be brought together.
This is achieved through insemination
where the penis releases semen, filled with
thousands of sperms, into the vagina.
Interestingly, while half of the total sperms
released carry the X-chromosome, the
other half contain Y-chromosome.
These sperms then rapidly move through the cervix, pass through the uterus and finally
reach the ampullary-isthmic junction of the fallopian tube. In the meantime, the ovum,
after completing the first meiotic division, gets released into the fallopian tube with the
rupture of the Graafian follicle. Upon its release, the ovum begins to undergo the
second meiotic division. However, the division does not go beyond the first phase.
Interestingly, the ovum contains only the X-chromosome. Studies have shown that the
ovum is filled with cytoplasm called the yolk or vitellus surrounded by a membrane
called the vitelline membrane.
This vitelline membrane is ensconced in another membrane called zona pellucida. Once
it reaches the ampullary-isthmic junction, the ovum gets bombarded by several
thousand sperms. However, only one sperm penetrates the zona pellucida and initiates
the fertilisation process. After gaining entry through the zona pellucida, the acrosome
present over the sperm’s nucleus, starts secreting enzymes that aids the sperm head to
get through the ovum’s cytoplasm, while the sperm sheds its tail. The fertilised egg now
resumes the remaining phases of the second meiotic division.
About 3 days after fertilisation, the zygote contains nearly 8 to 16 cells and is called a
morula. Over a period of two days, the morula descends down into the uterus and
transforms itself into a blastocyst- a hollow ball composed of about 100 blastomeres
which are arranged in two layers. The outer layer, called the trophoblast, eventually
gives rise to the placenta while the inner layer consisting of a group of cells called the
inner cell mass eventually differentiates to form the embryo. Interestingly, at the
blastocyst stage, the zygote, now called the embryo gets attached to the uterus as the
tropoblast grows outwards and penetrates the endometrial lining of the uterus. In
response, the endometrial cells divide and start surrounding the blastocyst. This causes
the blastocyst to sink and get implanted into the uterus. Once implantation has
occurred, pregnancy gets initiated. Moreover, after implantation, the inner mass cells,
get differentiated and begin to form the embryo. Embryo formation is thus a complex
process that begins with fertilisation and ends with implantation of the embryo.
Pregnancy And Embryonic
Development
Pregnancy is the term used to describe the period in which a fetus develops inside a
woman's womb or uterus. Pregnancy usually lasts about 40 weeks, or just over 9
months, as measured from the last menstrual period to delivery. Health care providers
refer to three segments of pregnancy, called trimesters.
After fertilization, the zygote is formed. The zygote divides mitotically to form 2, 4, 8,
16 celled stages. These cells are known as blastomeres.
Morula- Embryo having 8 to 16 blastomeres.
The morula continues dividing mitotically and gets transformed to blastocyst. The
outer layer of the blastocyst is called trophoblast and it gets attached to the uterine
wall known as the endometrium. The implantation starts in the first week but gets
completed by 2nd week.
The inner cell mass of blastocyst forms embryo. Blastocyst differentiates further to
embryonic and extraembryonic tissues. The implantation completes at the 2nd
week.
The interdigitated chronic villi of trophoblast and uterine cells form the placenta,
which is the connection between the mother and the growing foetus.
The placenta provides nourishment and oxygen to the embryo and helps in
removing carbon dioxide and waste produced by the embryo. It also acts as an
endocrine gland and secretes various hormones like hCG (Human Chorionic
Gonadotropin), estrogen, progestogens, etc. for maintenance of pregnancy.
Gastrulation starts in the 3rd week, the inner cell or embryo starts differentiating
into three germinal layers, i.e. ectoderm, endoderm and mesoderm. These cells
transform and get differentiated to all the tissues and organs, like nerve, blood,
muscle, bone, digestive tract, etc.
Ectoderm- nervous system, brain, spinal cord, epidermis, hair, nails, etc.
Mesoderm- connective tissue, muscles, circulatory system, notochord, bone,
kidney, gonads
Endoderm- internal organs, stomach, liver, pancreas, bladder, lung, gut lining
To sum up, the heart is the first organ to start working. After the 1st month of
pregnancy, the heart develops.
Limbs and digits develop in the 2nd month.
By the end of 1st-trimester or 3rd month all the major organ systems develop. Genital
organs are visible.
During 5th month the embryo starts moving and hairs start appearing on the head.
By the end of 2nd trimester (24 weeks or 6 months), eyelashes are formed, eyelids
separate and the body gets covered with fine hair.
By the end of the 9th month, the foetus fully develops and is ready for birth.
Parturition And Lactation
The average duration of human pregnancy is about 9 months which is
called the gestation period. Vigorous contraction of the uterus at the end
of pregnancy causes expulsion/ delivery of the foetus. This process of
delivery of the foetus (childbirth) is called parturition. Oxytocin is the
hormone responsible for this. The mammary glands of the female
undergo differentiation during pregnancy and starts producing milk
towards the end of pregnancy by the process called lactation.
Conclusion
Both the male and female reproductive systems play an important role
in the process of reproduction. Other than these reproductive organs,
there are sex hormones which are produced by the respective glands
and are mainly involved in the development of secondary sexual
characteristics and proper functioning of the reproductive tracts.
Bibliography
Biology class 12 : NCERT
BYJU'S
Vedantu
Wikipedia