HUMAN REPRODUCTION

HUMAN REPRODUCTION

 

THE MALE REPRODUCTIVE SYSTEM.

·         Located in the pelvis region.

·         Male reproductive system includes

o    A pair of testes.

o    Accessory ducts.

o    Accessory glands.

o    External genitalia

Testes:

·         Located outside the abdominal cavity within a pouch called scrotum.

·         Scrotum provides low temperature required for spermatogenesis.

·         Each testis is about 4 to 5 cm length and 2 to 3 cm width.

·         Each testis has about 250 compartments called testicular lobules.

·         Each lobule contains one to three seminiferous tubules.

·         Seminiferous tubules lined by male germ cells and Sertoli cells.

·         Male germ cell undergoes meiosis and produce sperm.

·         Sertoli cells provide nutrition to the germ cell and the sperm.

·         In between the seminiferous tubule there is interstitial cell or leydig cell.

·         Leydig cells produce testicular hormones called androgen (testosterone).

Accessory ducts:

·         Includes rete testis, vasa efferentia, epididymis and vas deferens.

·         Seminiferous tubules open into vasa efferentia through rete testis.

·         The vasa efferentia leaves the testis and open into epididymis.

·         The epididymis leads to vas deferens that ascends to the abdomen through inguinal canal and loops over the urinary bladder.

·         Vas deferens receives a duct from seminal vesicle and opens into the urethra as the ejaculatory duct.

·         Urethra originates from the urinary bladder and extends through the penis to its external opening called urethral meatus.

Accessory glands:

·         Includes

o    Paired seminal vesicle

o    A prostate gland

o    Paired bulbourethral gland.

·         Secretion of these glands constitutes the seminal plasma.

·         Seminal plasma rich in fructose, calcium, and certain enzyme.

·         Secretion of bulbo-urethral glands helps in lubrication of penis.

External genitalia:

·         Penis is the external genitalia.

·         It is made of special tissue that helps in erection of the penis to facilitate insemination.

·         The enlarged end of penis is called glans penis.

·         Glans penis is covered by a loose fold of skin called foreskin.

 

THE FEMALE REPRODUCTIVE SYSTEM

·         Located in the pelvic region of the female.

·         The female reproductive system includes:

o    A pair of ovaries

o    A pair of oviduct.

o    Uterus

o    Cervix

o    Vagina

o    External genitalia.

o    A pair of mammary gland.

Ovaries:

·         It is the primary female sex organs that produce the female gamete (ovum).

·         It also produces several steroid hormones.

·         The ovaries located in the lower abdomen.

·         Each ovary is about 2-4 cm in length.

·         Connected to the pelvic wall and uterus by ligaments.

·         Each ovary is covered by thin epithelium which encloses the ovarian stroma

·         The ovarian stroma has two zones

§  A peripheral cortex.

§  An inner medulla.

Oviduct:

·         Oviducts, uterus and vagina constitute the female accessory ducts.

·         Each fallopian tube is about 10-12 cm long and extends from the periphery of each ovary to the uterus.

·         Close to the ovary the oviduct has a funnel shaped structure called infundibulum?

·         The edges of the infundibulum possess finger-like projections called fimbriae, which helps in collection of the ovum after ovulation.

·         The infundibulum leads to a wider part of the oviduct called ampulla.

·         The last part of the oviduct is called isthmus which joined to uterus.

 

Uterus:

• It is single and is called womb.
• It is inverted pear shaped.
• Attached the pelvic wall by ligaments.
• The uterus opens into vagina through a narrow cervix.
• The lumen of cervix is called cervical canal.
• Cervical canal along with vagina form the birth canal.
• The wall of the uterus has three layers of tissues
• Perimetrium: external thin membranous.
• Myometrium: middle thick layer of smooth muscles
• Endometrium: inner glandular layer.
• Endometrium undergoes cyclical changes during menstrual cycle.
• Myometrium exhibits strong contraction during delivery of the baby.

 

External genitalia:

• It includes following structure:
• Mons Pubis: cushion of fatty covered by skin and pubic hair.
• Labia majora: fleshy folds of tissue which extends down from the mons pubis and surrounds the vaginal opening.
• Labia minora: are paired folds of tissue under the labia majora.
• Hymen: the opening of vagina is often covered partially by a membrane called hymen.
• Clitoris: a tiny finger-like structure lies at the upper junction of two labia minora above the urethral opening.

Mammary glands:

• Mammary gland consists of glandular tissue and fat.
• Glandular tissue of each breast divided into 15-20 mammary lobes.
• Mammary lobes contain cluster of cells called alveoli.
• The cells of alveoli secrete milk, stored in the lumen of alveoli.
• The alveoli open into mammary tubules.
• The tubules of each lobe join to form a mammary duct.
• Several mammary ducts join to form a wider mammary ampulla.
• Mammary ampulla connected to lactiferous duct, through which milk is sucked out.

GAMETOGENESIS: (formation of gametes)

Spermatogenesis:

• Formation of sperm from the germ cell in the testes is spermatogenesis.
• The process begins at puberty.
• Spermatogonia present in the lining of seminiferous tubules undergo mitotic division to increase their number.
• Each spermatogonium is diploid (2n) which contain 46 chromosomes.
• Innermost layer of spermatogonial becomes larger called primary spermatocyte.
• Primary spermatocyte undergoes meiosis-I to form two equal haploid (n) secondary spermatocytes (n).
• Each secondary spermatocyte undergoes meiosis-II to form two equal, haploid spermatids.
• Each primary spermatocyte produces four spermatids.
• Spermatids transformed into spermatozoa (sperms) by the process called spermiogenesis.
• The sperm head embedded in the Sertoli cell.
• Release of sperm from the seminiferous tubule is called spermiation.

Hormonal control of spermatogenesis:

• This process is initiated at puberty due to secretion of gonadotrophins releasing hormone (GnRH)
• GnRH secreted form hypothalamus and stimulate anterior pituitary to secrete two gonadotrophins.
• Luteinizing hormone (LH) and
• Follicle stimulating Hormone (FSH)
• LH acts on Leydig cells and stimulates synthesis of androgens.
• Androgen stimulates spermatogenesis.
• FSH acts on Sertoli cells and stimulates spermatogenesis in other ways.

Structure of sperm:

• Ultrastructure of sperm consists of a head, neck, a middle piece and a tail.
• Whole body of sperm surrounded by plasma membrane.
• The sperm head contain an elongated haploid nucleus.
• Above the nucleus a cap like structure present called acrosome.
• The acrosome contains enzymes which help in fertilization of ovum.
• The middle piece contains mitochondria, which provide energy for movement of tail that facilitate sperm motility.
• Human male ejaculates 200-300 million sperms during coitus.
• 60 percent must have normal shape and size and 40 percent of them must show vigorous motility.
• Sperm released from seminiferous tubules enters into accessory ducts.
• On their way fluids from seminal vesicle and prostate gland added which collectively called as Semen.
• The function of male accessory ducts and glands are maintained by testicular hormone androgen.

Oogenesis:

• Formation of a mature female gamete or ovum is called oogenesis.
• Oogenesis starts during embryonic stage, 25th week of the fetal age.
• Germinal epithelium of ovary divided mitotically to produce millions of gamete mother cell or oogonia.
• No oogonia formed or added after birth.
• Oogonia enters into meiosis-I and proceeds upto diakinesis of Prophase-I and get suspended, at this stage called primary Oocytes.
• Each primary oocyte surrounded by layers of granulose cells and then called primary follicle.
• At puberty only 60,000 to 80,000 primary oocytes are left in each ovary.
• After puberty primary follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles.
• The secondary follicle transformed into tertiary follicle, characterized by a fluid filled cavity called antrum.
• The theca layers organized into an inner theca interna and outer theca externa.
• During the growth of primary follicle into tertiary follicle during puberty, the primary oocyte restarts its first meiotic division and completes it within tertiary follicle resulting two unequal haploid cells.
• Large haploid cell is called secondary oocyte.
• A tiny cell called first polar body.
• The secondary oocyte retains bulk of the nutrient rich cytoplasm of primary oocyte.
• The tertiary follicle having secondary oocyte further changes into Graafian follicle.
• The secondary oocyte surrounded by a new membrane, zona pellucida.
• The secondary oocyte undergoes second meiotic division continued upto metaphase-II and get suspended until entry of sperm.
• At this stage Graafian follicle releases secondary oocyte from the ovary by the process called ovulation.
• On entry of a sperm into the secondary oocytes stimulates it to complete meiosis-II and there is formation of a haploid ovum and a second polar body (n).

Menstrual cycle:

• Reproductive cycle of female primates is called menstrual cycle.
• The first menstruation begins at puberty is called Menarche.
• Menstrual cycle repeated at an average interval of 28/29 days.
• One ovum is released in the middle of each menstrual cycle.

Menstrual cycle has following phases:

Menstrual phase:

·         1st phase of menstrual cycle.

·         Menstrual flow occurs.

·         Lasts for 3-5 days.

·         Breakdown of endometrial lining and blood vessel.

·         Mucus and blood comes out through vagina.

·         It occurs only when ovum released but no fertilization.

·         Lack of menstruation is the indication of pregnancy.

Follicular phase:

·         Menstrual phase followed by follicular phase.

·         Primary follicle becomes Graafian follicle.

·         Regeneration and proliferation of uterine endometrium.

·         LH and FSH level increases gradually in follicular phase.

·         Level of estrogen increases as it is secreted from growing follicle.

·         It lasts for 5-13 days.

Ovulatory phase:

·         FSH and LH attain peak level in this period (14th day).

·         This is called LH surge, which induces rupture of Graafian follicle and release of ovum from the ovary called ovulation.

Luteal phase:

·         Remaining part of Graafian follicle transformed into corpus luteum.

·         Coupus luteum produces large amount of progesterone.

·         Progesterone maintains the uterine endometrium, and prepares it for implantation.

·         Thickness of uterine endometrium increase in many folds, due to proliferation.

·         If there is fertilization, corpus luteum grows further and pregnancy continued, menstrual cycle stopped.

·         In the absence of fertilization corpus luteum degenerates.

·         Disintegration of endometrium leading to menstruation.

·         Menstrual cycle ceases around 50 years of age, called menopause.

FERTILIZATION AND IMPLANTATION:

• During copulation (coitus) semen is released by the penis into the vagina is called insemination.
• The motile sperm swim rapidly, pass through cervix, uterus and finally reach the junction of isthmus and ampulla (ammpullary-isthmic junction).
• The ovum released from the ovary also transported to ampullary isthmic junction where fertilization takes place.
• Fertilization only takes place if both sperm and ovum reach ampullary – isthmic junction simultaneously.
• The process of fusion of a sperm and ovum is called fertilization.
• Acrosome of sperm secretes enzymes helps in penetration into the ovum.
• Once a sperm comes contact with the zona pellucida of ovum and induces the changes in the membrane that blocks the entry of additional sperms.
• That ensures monospermy and prevents polyspermy.
• Only one sperm fertilize with one ovum.
• Entry of sperm into the ovum induces the ovum to complete its second meiotic division of secondary oocyte.
• Meiosis-II is also unequal cytokinesis resulting production of one large ovum (ootid) and one small second polar body.
• Haploid nucleus of sperm fused with the haploid nucleus of ovum to form a diploid zygote.

Sex determination:

• Sex of a baby has been decided during fertilization and in the zygote.
• Sex is determined by the sex-chromosomes present in gametes.
• Human female contain two XX chromosomes.
• Human male contain XY chromosomes.
• All the female gametes produced with only ‘X’ chromosome.
• Sperms produced by male, 50% with ‘X’ and 50 % with ‘Y’ chromosome.
• After fertilization zygote either carries XX or XY chromosomes.
• Zygote with XX chromosomes develop into female and with XY chromosome develops into male.

Cleavage:

• Repeated mitotic division of the zygote without growth resulting a multicellular ball like embryo is called cleavage.
• Cleavage starts soon after fertilization.
• Daughter cells produced during cleavage are called blastomeres.
• The product of cleavage is called Morula, which is 8 to 16 celled.
• The morula continues to divide and grow and transformed into blastocyst.
• The blastomeres in blastocyst arranged into an outer layer called trophoblast and an inner mass of cells attached to trophoblast called inner cell mass.
• Trophoblast cells attached to the endometrium helps development of placenta.
• Inner cell mass gets differentiated into the embryo.
• After attachment the uterine cells divide rapidly and cover the blastocyst.
• Blastocyst completely embedded in the uterine endometrium. This is called implantation.

Pregnancy and embryonic development:

• After implantation, finger like projections appears on the trophoblast called chorionic villi.
• Chorionic villi surrounded by uterine tissue and maternal blood.
• Temporary association between the fetal tissue (chorionic villi) and maternal tissue (uterine endometrium) is called placenta.

Function of placenta:

• The embryo connected to the placenta by umbilical cord, which transports substances to and from the embryo.
• Facilitate transport of oxygen and nutrient from mother to embryo.
• Removes CO₂ and waste material from the embryo.
• Acts as endocrine gland and produces several hormones like:
• Human chorionic gonadotrophins (hCG)
• Human placental lactogen (hPL)
• Estrogen.
• Progesterone
• Relaxin produced from the ovary in the later stage of pregnancy.

Embryonic development:

• After implantation the inner cell mass of blastocyst differentiated into an outer layer called ectoderm and an inner layer called endoderm.
• Mesoderm differentiated in-between ectoderm and endoderm.
• The inner cell mass thus called stem cells, having potency to produce all types of cell, tissues and organs by differentiation.

Organogenesis:

• Formation of different organs in the embryo is called organogenesis.
• Human pregnancy lasts for 9 months.
• After one month of pregnancy heart is formed in the embryo.
• By the end of 2nd month the foetus develops limbs and digits.
• By the end of 12 weeks (first trimester) most of organ system is formed (limbs and external genitalia are well developed).
• First movement of foetus and appearance of hairs observed in 5th month.
• By the end of 24th week (2nd trimesters) the body is covered with fine hairs, eye-lids separate, and eyelashes are formed.
• By the end of 9 months the foetus is fully developed and is ready for delivery.

PARTURATION AND LACTATION:

• The period of pregnancy is called gestation period. (9 months).
• Ejection or expulsion or delivery of foetus is called parturition.
• Parturition is due to vigorous contraction of uterine Myometrium.
• The signal of parturition is originated from the fully developed foetus and the placenta which induces mild contraction of uterus called fetal ejection reflex.
• Fetal ejection reflex triggers the release of Oxytocin from pituitary.
• Oxytocin induces stronger contraction of uterine endometrium.
• Stimulatory reflex continues stronger contraction leads to expulsion.
• After delivery the placenta is also expelled out of the uterus.

Lactation:

• The mammary gland of the female more differentiated during pregnancy,
• Mammary gland starts producing milk towards the end of the pregnancy.
• Process of milk production in mammary gland is called lactation.
• Milk produced during initial days of lactation is called colostrum.
• Colostrum contains several antibodies which provide immunity to the new born baby.