Notes on "HUMAN REPRODUCTION"

 Notes on "HUMAN REPRODUCTION"

By Nagraj Sir

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

1. 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 contains one to three seminiferous tubules.

·        Seminiferous tubules lined by male germ cells and Sertoli cells (provide nutrition to the germ cell and the sperm.)

·        In between the seminiferous tubule there is interstitial cell or Leydig cell. which produce hormones androgen (testosterone)

2.Accessory ducts:

 

·        Includes rete testis, vasa efferentia, epididymis, vas deferens. Ejaculatory duct

and  urethral duct

·        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 duct.

3.Accessory glands:

 

·        Includes

o    Paired seminal vesicle

o    Unpaired prostate gland

o    Paired bulbourethral gland.

·        Secretion of these glands constitutes the seminal plasma.

·        Seminal provide alkaline medium and  rich in fructose, calcium, and certain enzyme.

·        Secretion of bulbo-urethral glands which gives mucus and helps in lubrication of penis.

 

4.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.

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,Cervix,Vagina

o    External genitalia.

  o    A pair of mammary gland.

1.Ovaries:

 

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

·        The ovaries located in the lower abdomen and 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

2.Oviduct:

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

       ·        Each fallopian tube is about 10-12 cm long

·        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.

3.Uterus:

 

        ·        It is single and pear shaped is called womb.

        ·        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

o    Perimetrium: external thin membranous.

o    Myometrium: middle thick layer of smooth muscles

o    Endometrium: inner glandular layer.

        ·        Endometrium undergoes cyclical changes during menstrual cycle.

        ·        Myometrium exhibits strong contraction during delivery of the baby.

 

4.External genitalia:

 

·        It includes following:-

o    Mons Pubis: cushion of fatty covered by skin and pubic hair.

o    Labia majora: fleshy folds of tissue which extends down from the mons pubis and surrounds the  vaginal opening.

o    Labia minora: are paired folds of tissue under the labia majora.

o    Hymen: the opening of vagina is often covered partially by a membrane called hymen.

o    Clitoris: a tiny finger-like structure lies at the upper junction of two labia minora above the urethral opening.

5.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 which secrete milk, stored in the lumen of alveoli.

·        The alveoli open into mammary tubules which further join to form a mammary duct.

·        Several mammary ducts join to form a wider mammary ampulla which connected to lactiferous duct, through which milk is sucked out.

GAMETOGENESIS: which means formation of gametes

 

1.Spermatogenesis:

 

·        Formation of sperm from the germ cell in the testes is spermatogenesis.

·        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.

o    Luteinizing hormone (LH) and

o    Follicle stimulating Hormone (FSH)

·        LH acts on Leydig cells and stimulates synthesis of androgens which stimulates spermatogenesis.

·        FSH acts on Sertoli cells and stimulates spermatogenesis

Structure of sperm:

·        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 (Hyaluronidase) 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.

2.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.

·        Oogonia enters into meiosis-I and proceeds upto 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.

·        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.

o    Large haploid cell is called secondary oocyte.

o    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

·        At this stage Graafian follicle releases secondary oocyte from the ovary by the process called ovulation.

Menstrual cycle:

·        Reproductive cycle of female 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:

 

1.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.

2.Follicular phase(proliferative 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.

 

3.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.

4.Luteal phase(Secretory 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:

·        The motile sperm swim rapidly, pass through cervix, uterus and finally reach to the ampulla

·        The ovum released from the ovary also transported to ampulla where fertilization takes place.

·        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.

·        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.

Cleavage:

·        Repeated mitotic division of the zygote without growth resulting a multicellular ball like embryo is called cleavage.

·        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.

·        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:

o    Human chorionic gonadotrophins (hCG)

o    Human placental lactogen (hPL)

o    Estrogen.

o    Progesterone

o    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.

 

PARTURATION :

 

·        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.