Human Reproduction

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HUMAN REPRODUCTION EVENTS OF HUMAN REPRODUCTION The process of human reproduction includes the following events.

1. Gametogenesis (formation Of Gametes) 2. Insemination(Transfer of gametes into female genital tract ) 3. Fertilization (Fusion of gametes)

4. Formation of zygote 5. Development of blastocyst 6. Implantation (Attachment of blastocyst to the uterine wall) 7. Gestation (Development of the embryo) 8. Parturition (Delivery of the baby) MALE REPRODUCTIVE SYSTEM Male reproductive system includes a pair of testes, Genital ducts Accessory glands and Penis. TESTES

• Testes is the organ in which sperms are produced. It is oval in shape with a length of about 4 to 5 cm and a width of about 2 to 3 cm .

Scrotum

• The testes are situated outside the abdominal cavity within a pouch called scrotum. Scrotum maintains the temperature of the testes 2- 2.5 degree Celsius lower than body temperature which is necessary for spermatogenesis.

Testicular lobules

• Each testes has about 250 compartments called testicular lobules . Seminiferous tubules

• Each testicular lobule contains one to three highly coiled tubules called seminiferous tubules . Sperms are produced here .

Spermatogonial cells

• Each seminiferous tubule is lined internally by cells called spermatogonial cells which undergo meiotic division and transforms to sperms .

Sertoli cells . • Each seminiferous tubule is lined internally by another type of cells

called Sertoli cells which provide shape and nourishment to the developing spermatogenic cells.

Interstitial cells or Leydig cells

• These are cells seen in between the seminiferous tubules.They synthesise and secrete the hormones called androgens which control spermatogenesis.

ACCESSORY DUCTS IN THE MALE REPRODUCTIVE SYSTEM

• The male sex accessory ducts include rete testis , vasa efferentia ,

epididymis and vas deferens.

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Rete testis

• These are the ducts in which seminiferous tubules of the testes open. Vasa Efferntia.

• Seminiferous tubules unite to form larger tubules called vasa efferentia . Seminiferous tubules open into vasa efferentia through rete testis .

Epididymis • Vasaefferntia opens in to folded and coiled tube called Epididymis

which stores sperm temporarily . Vas Deferens

• From the epididymus Vas Deferens arise and which join with the duct of seminal vesicle and a common ejaculatory duct is formed.

Urethral meatus

• The urethra serves as a common Urinogenital Duct and opens outside through the penis to its external opening called urethral meatus .

ACCESSORY GLANDS IN THE MALE REPRODUCTIVE SYSTEM • The male accessory glands include paired seminal vesicles , a prostrate

gland and paired bulbourethral glands .

• Secretion of these glands constitute the seminal plasma which is rich in fructose , calcium and certain enzymes . The secretion of bulbourethral glands also help in the lubrication of the penis

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Sectional view of seminiferous tubules

THE FEMALE REPRODUCTIVE SYSTEM

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The female reproductive system consists of a pair of ovaries , a pair of oviducts (fallopian tubes) ,Uterus , Cervix , Vagina ,Accessory glands and external genitalia. OVARY

• Ovary are the primary female sex organ that produce the female gamete ovum . It is about 2 to 4cm in length , 1.5cm wide and 1cm thick.

• Ovary has the outer cortex and inner medulla .

• Ovarian follicles are the sac like structures in the cortex of the ovary.

• Each ovarian follicle has an immature ovum called primary oocyte surrounded by follicle cells called primary follicles .

• A large number of primary follicles degenerate during birth to puberty and at puberty only 60,000 to 80,000 primary follicles are left in each ovary and out of these only 450 ova are produced in the entire life span of a female .

• Zona Pellucida is the thick coat of oocyte formed of glycoproteins.

• Mature ovarian follicle is called Graffian Foollicle.

• Antrum is the cavity in the graffian follicle

• Ovulation the process of release of ovum from the ovary.

• Corpus Luteum is the yellowish conical cellular structure developed from the ruptured ovarian follicular cells which secrete progestrone.

Sectional view of ovary

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OVIDUCTS (FALLOPIAN TUBES)

• Each oviduct is 10 to 12 .cm in length and opens behind the ovaries by funnel shaped openings called infundibulum .

• The finger like projections in the edges of infundibulam is called fimbriae which help in collection of the ovum after ovulation .

• Infundibulum leads to a wider part of the oviduct called ampulla.

• The last part of the oviduct is isthumus which connect the oviduct with uterus

UTERUS

• The uterus is single and it is also called womb.

• Cervix is the lower cylindrical part of uterus .

• Cervix opens into vagina. Vagina is the muscular tube projecting from the cervix.

• The cavity of the cervix is called cervical canal which along with vagina forms the birth canal.

Walls of uterus

• The wall of the Uterus has three layers of tissue .

• Perimetrium is the external thin membrane .

• Myometrium is the middle thick membrane made up of smooth muscles , which exhibit strong contraction during delivery of the baby .

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• Endometrium is the inner glandular layer that lines the uterine cavity Endometrium undergoes cyclical changes during menstrual cycle .

FEMALE EXTERNAL GENITALIA The female external genitalia includes 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 paired folds of tissue under the labia majora .

• Hymen is the thin ring of tissue partially covering the vaginal opening.

• Clitoris is the tiny finger like structure which lies at the upper junction of the two labia minora above the urethral opening.

• Female reproductive system

Sectional view of female reproductive system

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MAMMARY GLAND

• These are modified sweat gland which secrete milk. • Consists of 15 to 25 mammary lobules seperated by dense connective

tissue and adipose tissue. • Mammary lobes contains clusters of cells called alveoli which secrete milk • The alveoli opens into mammary tubules and which join to form

mammary duct. • Several mammary ducts join to form a wider mammary ampulla which is

connected to lactiferous duct . • Lactiferous duct carries milk to the nipple. Sectional view of mammary gland

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GAMETOGENESIS

• Gametogenesis is the formation of male and female gametes .

• Formation of male gamete is spermatogenesis and female gamete is oogenesis.

SPERMATOGENESIS

• Spermatogenesis begins at puberty .

• The spermatogonial cells present on the inside wall of the seminiferous tubules multiply by mitotic division and increase in numbers .

• Each spermatogonium is diploid and contains 46 chromosomes . Primary spermatocytes

• Spermatogonial cells synthesize cellular materials and stores food and grows. After growth it is known as primary spermatocytes.

Secondary spermatocyte

• Primary spermatocyte undergoes the first meiotic division and two haploid secondary spermatocyte are formed.(have 23 chromosomes)

Spermatids

• Secondary spermatocytes undergoes second meiotic division and four haploid spermatids are formed.

Spermiogenesis

• It is the transformation of spermatids in to functional spermatozoa or sperms.

Spermiation

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• After spermiogenesis sperm heads become embedded in the sertoli cells

,and are finally released from the seminiferous tubules by the process called spermiation .

Sectional view of seminiferous tubules

Flow chart of spermatogenesis

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HORMONAL CONTROL OF SPERMATOGENESIS

• Spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropin releasing hormone (GnRH) from the hypothalamus .

• GnRH acts at the anterior pituitary and stimulate the secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH).

• LH acts at the Leydig cells and stimulate the synthesis and secretion of androgens .

• Androgens stimulate the process of spermatogenesis .

• FSH acts on sertoli cells and stimulate the secretion of some factors which help in the process of spermatogenesis.

STRUCTURE OF SPERM It is about 50mm long

� Divisible in to head ,neck ,middle piece and tail Acrosome

� It is the structure found in the head of sperm. � Acrosome is formed from golgi apparatus and it contains the enzyme

hyaluronidase which help in sperm penetration Neck of sperm

� Neck of sperm has a proximal and distal centriole.

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� Proximal centriole initiates division of zygote � Distal centriole gives rise to axial complex of tail.

Mitochondria of sperm (Nebenkern ) � In the middle piece of sperm mitochondria are spirally arranged around

the axial filament. This mitochondrial spiral is called nebenkern. � Mitochondria produce energy for the movement of tail that facilitate

sperm motility essential for fertilization Sperm count

• The human male ejaculates about 200 to 300 million sperms during a coitus of which , for normal fertility , at least 60 per cent sperms must have normal shape and size and for at least 40 per cent of them must show vigorous motility .

Semen

• Sperms released from the seminiferous tubules mixes with the secretions of epididymis , vas deferens , seminal vesicles and prostate gland known as seminal plasma . The seminal plasma along with the sperms constitute the semen.

Structure of a sperm

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OOGENESIS

• It is the process of formation of mature female gamete in ovarian follicles

• At first Germinal epithelial cells undergoes repeated mitotic division and large no of oogonial cells are formed.

• Oogonial cells grows due to the accumulation of nutritive substances and transforms into primary oocyte

• Primary oocyte undergoes meiotic division and first meiotic division converts primary oocyte to large secondary oocyte and a small first polar body.

• second meiotic division converts secondary oocyte to large ootid or ovum

and second polar body.

• After oogenesis from a single oogonial cells one ootid and 3 polar bodies are formed.

Flow chart of oogenesis

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MENSTRUAL CYCLE

• Reproductive cycle in the female primates(eg. Monkeys, apes and human beings) is called menstrual cycle.

• The first menstruation begins at puberty and is called Menarche

• Menstrual cycle of human female is 28/29 days cycle

• The cycle of events starting from one menstruation till the next one is called menstrual cycle .

• It is accompanied by discharge of blood and degeneration products of uterine mucosa called menstruation.

Events of menstrual cycle 1.Menstrual phase

• If the released ovum is not fertilized Endometrial lining of uterine wall degenerates and blood vessels rupture.

• The blood , degenerated endometrial cells and secretory products are discharged through vagina and is called menstruation.

• The menstrual flow lasts for 3-5 days.

• Lack of menstruation may be the indicative of pregnancy . 2.Follicular phase (Proliferative phase)

• Starts from 5 th day to 14 th day of cycle.

• Ovarian follicles begin to develop and developing follicles are called graffian follicles.

• Graffian follicles secrete estrogen.

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• Endometrium become thicker , glandular and vascularised.

• LH and FSH secretion from pituitary increases and attains a peak level about 14 th day

3.Ovulatory phase

• Mature graffian follicle rupture and releases ovum known as ovulation.

• Ovulation takes place about 14 th day of menstrual cycle. 4. Luteal Phase

• The remaining part of the Graffian follicles transforms to corpus luteum.

• The corpus luteum secretes large amounts of progesterone which is essential for the maintenance of the endometrium .

Menopause

• In human beings , menstrual cycles stops around 50 years of age . This is known as menopause .

EVENTS OF MENSTRUAL CYCLE

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FERTILIZATION

• The process of fusion of a sperm with an ovum is called fertilization .

• The fertilization in human beings takes place in the fallopian tube .

• During fertilization , a sperm comes in contact with the zona pellucida layer of the ovum and induces the formation of the fertilization membrane which block the entry of additional sperms .

• The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum .

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• The entry of sperm induces the completion of second meiotic division and second polar body is released .

• The haploid nucleus of the sperm and ovum fuses to form the diploid zygote .

Ovum surrounded by few sperms

Sex determination in human beings Sex of the baby is determined by the father not by the mother . justify ?

• The chromosome pattern in the human female is XX and that in the male is XY.

• Females produce gametes with only X chromosomes and male produce gametes with either X or Y chromosomes .

• 50 percent of male gametes carry X chromosomes and 50 percent carry Y chromosomes .

• The XY zygote will be formed if the Y chromosome carrying male gamete fuses with the X gamete of female, and the child thus formed is male .

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• Female child will born from XX zygote if the X gamete of female fuses with the X gamete of male .

• Thus the sex of the baby is determined by the X and Y chromosomes of the father .

Cleavage � Cleavage is the repeated mitotic division of the zygote. � Blastomeres are the cells formed during cleavage . � Morula is the solid mass of 8 to16 celled embryo formed during cleavage. � Morula continues to divide and transforms into blastocyst .

� The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass .

� The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo .

Implantation � The process of attachment of embryo on the wall of uterus is called

implantation and it leads to pregnancy.

Transport of ovum , fertilization and passage of growing embryo Through fallopian tube

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Embryonic layers

� Immediately after implantation , the inner cell mass of embryo differentiates into embryonic layers .

� Ectoderm ,Mesoderm and Endoderm are the three embryonic layers formed.

� Ectoderm is the outer layer ,mesoderm is the middle layer and endoderm is the inner layer.

� These layers give rise to all tissues and organs in adults . � The inner cell mass are known as stem cells which have the capacity to

give rise to all tissues and organs . PLACENTA

• It is the physiological connection between maternal and foetal body.

• Placetna is formed from chorion and allantois .So human placenta is called chorioallantoic placenta.

• Umbilical cord is the tubular part of the placenta connecting foetus with placenta.

Functions of placenta

• It supplies nutrients ,hormones and antibodies to foetus .

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• It acts as a barrier between mother and foetus.

• It prevent mixing of maternal and foetal blood.

• It helps to exchange gases between foetus and mother.

• It helps to eliminate nitrogenous wastes.

• Placenta acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG) , human placental lactogen (hPL) estrogens , progestogens etc.

The human foetus within the uterus

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Pregnancy � The human pregnancy lasts 9 months. � After one month heart is formed . The first sign of growing foetus is the

heart beat . � By the end of second month foetus develops limbs and digits . � By the end of 12 weeks most of the major organ systems are formed . � During the fifth month , the first movement of the foetus and appearance

of the hair on the head observed . � By the end of 24 weeks the body is covered with fine hair , eye lids

separate , and eye lashes are formed . � By the end of nine months of pregnancy, the foetus is fully developed and

is ready for delivery. Parturition

• The process of expelling the young one from the mother’s uterus at the end of gestation is called parturition or child birth.

• Parturition is induced by complex neuroendocrine mechanism. The signals for parturition originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex .

• This reflex induces the release of oxytocin from the pituitary and oxytocin acts on the uterine muscles and stronger uterine contractions occurs which leads to parturition .

• After parturition the placenta and remains of umbulical cord are expelled and it is called after birth.

Lactation � The mammary glands of the female undergoes differentiation during

pregnancy and starts producing milk towards the end of pregnancy by the process called lactation .

� The first milk produced from the mammary glands after few days of lactation is called colostrum , which contains several antibodies essential for to develop immunity for the new born babies

• Secretion of milk is under the influence of prolactin Notes Prepared By Biju T L HSST Zoology GHSS Mylachal, Tvpm