Plus Two Zoology Notes Chapter 1 Human Reproduction is part of Plus Two Zoology Notes. Here we have given Plus Two Zoology Notes Chapter 1 Human Reproduction.
|Text Book||NCERT Based|
|Chapter Name||Human Reproduction|
|Category||Plus Two Kerala|
Kerala Plus Two Zoology Notes Chapter 1 Human Reproduction
Simplified Detailed Notes
Reproductive Events in Humans
It is the formation of gametes, ie., sperms in male and ovum in female.
Transfer of sperms into the female genital tract.
Fusion of male and female gamate leading to the formation of zygote.
Formation and development of blastocyst.
Attachment of blastocyst to the uterine wall for nourishment.
Delivery of the baby : These reproductive events occur after puberty.
The Male Reproductive System
The male reproductive system is located in the pelvis region
a. A pair of testes
b. Accessory ducts
c. Accessory glands
d. External genitalia
a. A pair of testes
- It is located in abdominal cavity with in a pouch called scrotum.
- Scrotum maintaining the low temperature (between 2 – 2.5°C) of the testes.
- Each testis is oval in shape.
- It’s length is 4 to 5 cm and a width of about 2 to 3 cm.
- The testis is covered by a dense covering.
- Each testis is divided into 250 compartments called testicular lobules.
- Each lobule contains 1-3 highly coiled tubles called seminiferous tubules.
- Each seminiferous tubule is lined internally by two types of cells:
- Male germ cells (Spermatogonia) Undergo meiotic cell division to produce sperms.
- Sertoli cells They provide nutrition to the germ cells.
- The regions out side the seminiferous tubules called interstitial space.
- It contains small blood vessels and interstitial cells or leydig cells.
- Leydig cells synthesise and secrete the male hormones, androgens (testosterone) which maintain the male sex characteristics
b. Accessory ducts
The male sex accessory ducts include:
- Rete testis
Set of tubules connected to both ends of seminiferous tubule
- Vasa efferentia
Rete testis opens to small tubules called vasa efferentia.
Tubules of vasa efferentia combines to form epididymis, where sperms are stored.
- Vas deferens
It is a long muscular duct, transfer sperm from epididymis to urethra during ejaculation.
- Ejaculatory duct
It is a short duct that joins two vas deferens and the ducts open into i urethra.
- Urethra arises from urinary bladder and joins the ejaculatory duct to form urinogenital canal.
- It passes through the penis and finally opens outside by the opening called urethral meatus.
c. Accessory glands
- A pair of seminal vesicle
- A prostate gland
- Paired bulbourethral glands
- It’s secretion also helps in the lubricaton of the penis.
d. External genitalia
- Erectile copulatory organ in males.
- Both urine and semen are passed through the penis.
- The enlarged end of the penis is called glans penis, covered by a loose fold of skin called foreskin.
The female reproductive system
The female reproductive system consists of:
- A pair of ovaries
- External genitalia
- Mammary gland
A pair of ovaries
- Primary sex organ of female.
- Located on each side of the lower abdomen.
- It is about 2 – 4 cm in length and is connected to the pelvic wall and uterus by ligaments.
- Each ovary is covered by a thin epithelium which encloses the ovarian stroma.
- The stroma is divided into two zones: a peripheral cortex and an inner medulla.
The oviducts (fallopian tubes) uterus and vagina constitute the female accessory duct
Oviducts (fallopian tubes)
- The muscular tubes which carry the ovum from the ovary to the uterus are called oviducts or fallopian tubes.
- Each fallopian tube is about 10-12 cm long.
- Each oviduct has three parts they are,
The opening of the fallopian tube is expanded into funnel-shaped infundibulum.
The edges of the infundibulum has finger like projections called fimbriae.
It helps in the collection ofthe ovum after ovulation.
The infundibulum leads to a wider part of the oviduct called ampulla.
It is the part of the oviduct having a narrow lumen which joins the uterus.
- It is a single, hollow, muscular organ, which is inverted pear shaped.
- It is supported by ligaments and attached to the pelvic wall.
- It is formed of three parts fundus, body and cervix.
- Cervix is the narrow part of the uterus which opens into the vagina.
- The cavity of the cervix is called cervical canal.
- The cervical canal along with vagina forms the birth canal.
- The wall of the uterus has three layers of tissue
- Perimetrium: outer thin covering of the uterus wall.
- Myometrium : middle thick layer of smooth muscle fibres which contract strongly during delivery of the baby.
- Endometrium: inner layer that contains glands and many blood vessels. It undergoes cyclical changes during the menstrual cycle.
- It is the lowermost part of female reproductive part.
- It opens to the exterior between urethra and anus.
- It act both as copulation canal and birth canal during parturition.
Female external genitalia
It includes mons pubis, labia majora, labia minora, hymen and clitoris.
- Mons pubis
It is a cushion of fatty tissue covered by skin and pubis hair.
- Labia majora
It is a large, fleshy, fatty and hairy outer folds.
It extends from mons pubis and surrounds the vaginal opening.
- Labia minora
They are paired folds of tissue under the labia majora.
A tiny finger-like structure, lies at the upper junction of the two labia minora above the urethral opening.
A thin mucous membrane that covers the vaginal opening either partially or entirely.
The hymen often breaks during the first intercourse or during sports like horse riding, cycling or sudden fall or jolt, etc.
Mammary glands (breasts)
- It is paired, contains glandular tissue
and fatty tissues.
- Each glandular tissue is divided into 15-20 mammary lobes with alveoli.
- The cells of alveoli secrete milk which is stored in the cavities (lumens) of alveoli.
- The alveoli open into mammary tubules.
- The mammary tubules of each lobe open into small mammary duct.
- Mammary ducts join to form a wider mammary ampulla that is connected to lactiferous duct through which milk is sucked out.
- It is the process of formation and differenciation of haploid gametes (sperms and ova) from the diploid primary germ cells.
- it includes :
- It consists of two stages : Formation of spermatids and spermiogenesis.
- The process of formation of sperms (spermatozoa) from diploid spermatogonia is called spermatogenesis.
- It includes the following phases:
- Spermatogonia present on the inner wall of seminiferous tubules multiply by mitotic division and increase in numbers.
- Spermatogonia grow and increase in size and forms primary spermatocytes.
- Each spermatogonia is diploid and contains 46 chromosomes.
Maturation phase (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 the 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.
- 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 tubles by the process called spermiation.
Hormonal control of spermatogenesis
- Spermatogenesis starts at the age of puberty by the gonadotropin releasing hormone (GnRH).
- GnRH is a hypothalamic hormone.
- The increased levels of GnRH stimu-late the anterior pituitary which then secretes:
a. LH (luteinising hormone) LH acts at the Leydig cells and stimulates the secretion of androgens which stimulates the process of spermatogenesis.
b. FSH (follicle stimulating hormone) FSH acts on sertoli cells and helps in spermiogenesis.
Horizontal control of spermatogenesis
Structure of a sperm
- Human sperm or spermatozoan is a microscopic structure.
- It is composed of a head, neck, middle piece and tail.
- It is oval shaped, contains an elongated haploid nucleus.
- Anterior end is coverd by cap like acrosome, filled with enzymes that helps in fertilisation of ovum.
It is very short and seen behind the head.
It consists of two centrioles :
- Proximal centriole
It initiates cell division in fertilised egg.
- Distal centriole
It forms the axial filament of the sperm tail.
It contains numerous mitochondria.
That provide energy for the movement of the tail that facilitate sperm motility essential for fertilisation.
It consists of axial filaments surrounded by the plasma membrane.
It helps the sperm to swim in a fluid medium.
- A human male ejaculates about 200-300 million sperms during a coitus.
- Seminal plasma along with the sperms constitute the semen.
- It is process of formation of a mature female gamete, ovum in the ovaries.
- It is divided into three phases: Multiplication phase, growth phase and maturation phase.
- It is initiated in embryonic stage when millions of egg mother cells (Oogonia) are formed within each foetal ovary.
- No more oogonia are formed after birth.
- The oogonia start division and enter into prophase I of meiosis and get temporarily arrested in this stage and are called primary oocytes.
- 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.
- Each secondary follicle soon transforms into tertiary follicle.
- The tertiary follicle is characterised by a fluid-filled cavity called antrum.
- The theca layer has 2 parts :
- Theca externa (outer)
- Theca interna (inner)
- At this stage, the primary oocyte with the tertiary follicle grows in size and becomes mature for meiotic division
- During the phase of maturation, the diploid primary oocyte undergoes meiosis -1 (reduction division) to form 2 unequal haploid cells.
- The larger cell is called the secondary oocyte and the smaller cell is called the first polar body.
- The tertiary follicle changes into a Graafian follicle.
- The secondary oocyte forms a new membrane called zona pellucida.
- The release of secondary oocyte (ovum) from the ovary by the rupture of Graafian follicle is called ovulation.
- Ovaries and female reproductive tract of female primates undergo a series of cyclic changes meant to prepare fertilisation and pregnancy. This is called menstrual cycle.
- This occurs in primates, eg. monkeys, apes and human beings.
- It is repeated at an average interval of about 28/29 days.
- The ovum is released (ovulation) during the middle of each menstrual cycle.
The first appearance of menstruation at puberty.
In human beings, menstrual cycle stops around 50 years of age. This is called menopause.
It includes four phases as follows:
- It lasts for 3-5 days.
- If fertilisation does not occur, then the corpus luteum starts degenerating. This results in decreased progesterone. Endometrium fails to be maintained.
- The unfertilised ovum along with ruptured endometrium and blood vessels go out through the vaginal opening.
- This process of bleeding is known as menstruation or menses.
Follicular phase (Proliferative phase)
- It lasts for about 14 days.
- The anterior pituitary produce gonadotropins (LH and FSH).
- FSH (Follicle Stimulating Hormone) stimulates the growth of ovarian follicle.
- The follicular cells of Graafian follicle secretes the female hormones, estrogen.
- The hormone estrogen promotes the growth (proliferation) of endometrium.
- This happens on the 14th day after menstruation.
- Rapid secretion of LH leading to its maximum level during the midcycle called LH Surge.
- The LH stimulates the rupture of the Graafian follicle and release of the mature ovum this process is called ovulation.
- Ovum remains alive and functional for 24 hours.
- LH also starts the change of empty Graafian follicle into corpus luteum and secretion of progesterone from it.
Luteal Phase (Secretory phase)
- It lasts for 12-14 days after ovulation.
- After the release of ovum the remaing parts of Graafian follicle cells form a yellow body called corpus luteum.
- Corpus luteum secretes progesterone hormone which is essential for the maintenance of endometrium for the implantation of the embryo.
- During pregnancy all events of the menstrual cycle stop and there is no menstruation.
- In the absence of fertilisation, the corpus luteum degenerates. This causes disintegration of the endometrium leading to menstruation, making a new cycle.
Fertilisation & Implantation
- The process of fusion of a sperm with an ovum is called fertilisation.
- It takes place in ampullary-isthmic junction of the fallopian tube.
- During copulation (coitus) semen is released by the penis into the vagina (insemination).
- The motile sperms pass through the cervix, enter into the uterus.
- The sperms finally reach the junction of isthmus and ampulla (ampullary-isthmic junction).
- A sperm comes in contact with the zona pellucida layer of the ovum.
- The enzymatic secretion of acrosome help the sperm to enter the cytoplasm of the ovum.
- This induces the completion of the meiotic division of the secondary oocyte.
- This results in a second polar body and a haploid ovum (ootid).
- The haploid sperm nuclei and that of the ovum fuse together to form a diploid zygote.
- The sex of the child is determined by the father and not by the mother.
- The presence of ‘X’ or ‘Y’ chromosome in the sperm determines the sex of the embryo.
- Sex chromosome pattern in human female is XX and in male is XY.
- When the egg get fertilised, the sex of the child will depend on the kind of sperm that fertilises the ovum.
- The zygote carrying XX sex chromosomes would be female baby.
- XY would be a male baby.
- After fertilisation, the zygote undergoes a series of mitotic division which is called cleavage.
- As a result 2,4,8,16 daughter cells are produced which are termed as blastomeres.
- Embryo with 8-16 blastomeres is called a morula.
- Morula continuous to divide and transforms into blastocyst.
- Blastomers in the blastocyst are arranged into an outer layer called tro-phoblast and an inner group of cells attached to trophoblast called inner cell mass.
- The trophoblast layer gets attached to the endometrium and the inner cell mass gives rise to the embryo.
- After attatchment uterine cells divide rapidly and cover the blastocyst.
- The embedding of the blastocyst in the endometrium of the uterus is called implantation.
- Implantation leads to pregnancy.
Pregnancy and Embryonic Development
- After implantation, finger like projections appears on the trophoblast called chorionic villi.
- Chorionic villi and uterine tissue gets integrated with each other to form placenta.
- Placenta is a structural and functional unit between embryo and maternal body.
- Placenta is connected to the embryo by an umbilical cord. It transports substances to and from the embryo
- Placenta also secretes relaxin. This helps in parturition by softening the connective tissue of the pubic symphysis.
- During pregnancy, the level of some hormones like estrogen, progesteron, cortisol, prolactin, thyroxine, etc. are increased several times in the mothers blood.
- Increased production of these hormones is necessary for supporting the foetal growth, metabolic changes in the mother, and maintenance of pregnancy.
- hCG, hPL and relaxin are produced in women only during pregnancy.
Funtions of placenta
- Act as a barrier between foetus and embryo.
- It acts as the nutritive, respiratory and excretory organ of the foetus.
- Provides nutrients and oxygen to the developing embryo.
- Removes CO, and waste materials from the embryo.
- Acts as an endocrine tissue and produces several hormones like,
- Human chorionic gonadotro pin (hCG)
- Human placental lactogen (hPL)
All are essential to maintain pregnancy.
Formation of germ layers
- The inner cell mass of blastocyst develops into three layers:
- outer layer called ectoderm
- inner layer called endoderm
- middle layer called mesoderm
- Inner cell mass contain certain cells called stem cells that have the potency to give rise all the tissue and organs.
- The period of pregnancy is called gestation.
- Human pregnancy last for 9 months.
- For cats: 2 month, dogs: 2 months, elephants: 21 months.
Embryonic development at various stages of pregnancy
After 1 month of pregnancy
The embryos heart is formed.
By the end of 2nd month
The foetus develops limbs and digits.
By the end of 12 weeks (Ist trimester)
Most of the major organ systems are formed. eg: the limbs and external genital organs are well developed.
During the 5th month
The first movements of the foetus and appearance of hair on the head are observed.
By the end of 24 weeks (2nd trimester)
The body is covered with fine hair, eye-lids separate, and eyelashes are formed.
By the end of 9 months
The foetus is fully developed and it is ready for delivery.
Parturition and Lactation
- The average duration of human pregnancy is about 9 months which is called the gestation period.
- The vigorous contraction of the uterus at the end of pregnancy causes expulsion of the foetus. This process of delivery of the foetus is called parturition.
- It is induced by a complex neuroendocrine mechanism.
- Parturition signals originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex.
- 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.
- Mammary glands of female undergo differentiation and start producing milk at the end of pregnancy. This is called lactation.
- This helps the mother in feeding the newborn.
- The milk that comes out of the mammary glands during initial days of lactation is called colostrum.
- Colostrum contains several antibodies essential to develop resistance for the newborn babies.
Human Reproduction at a sight
Gametogenesis – Formation of sperm in males and ovum in females
Insemination – Transfer of sperms into female genital tract
Fertilisation- Fusion of male and female gamete
Formation of Zygote (by fusion of gametes)
Formation and development of blastocyst
Implantation – Attatchment of blastocyst to the uterine wall
Gestation or embryonic development
Parturition – delivery of the baby
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