Male reproductive organs include: Primary sex organs & Accessory sex organs
Primary sex organs (gonads): Testes
Pathway for passage or sperms:
Testes contain the seminiferous tubules (which produce sperms) à Rete testes à continue as the vas efferens à head of epididymis à duct of epididymis à vas deferens à ampulla (terminal portion of vas deferens) [after joining duct of seminal vesicle forms) à ejaculatory duct à (passes thru prostate to form) internal urethra.
Accessory sex organs:
· Seminal Vesicles: situated on either side of prostate gland. Its secretions are emptied into the ampulla of vas deferens.
· Prostate gland: consists of many secretory glands, secretions from which are emptied into internal urethra thru utriculus prostaticus.
· Urethra: consists of two parts:
o Internal urethra – which is the continuation of ejaculatory duct.
o External urethra – continuation of internal urethra thru the penis.
It contains mucus glands throughout its length – called as Glands of Littre. The bilateral bulbourethral glands also open into the urethra.
· Penis: It is the male genital organ. It is formed by 3 erectile tissue masses: paired corpora cavernosa & an unpaired corpus spongiosum (which surrounds the urethra & terminates distally to form the glans penis).
External genitalia: scrotum, penis & urethra.
Internal genitalia: the remaining sex organs.
Functional anatomy of testes:
· Testes are ovoid having the organization of compound tubular gland.
· They located in the scrotum.
· Each testis is enclosed by 3 coverings:
o Tunica vasculosa (innermost): made up of connective tissue & is rich in blood vessels.
o Tunica albuginea: dense fibrous capsule
o Tunica Vaginalis (outermost): closed cleft like covering constituted by mesothelial cells. This layer is formed of:
§ Visceral layer: adheres to tunica albuginea.
§ Parietal layer: lines the inner surface of scrotum.
The 2 layers glide on one another to allow free movt. Of testes.
· All the 3 layers cover the ant. & lateral surfaces of testis but the post. surface is covered by tunica albuginea only, which is thickened post. to form the mediastinum testis.
· From the mediastinum testis, C.T septa called septula testis radiate into testis & bind with tunica albuginea at various points.
· Due to septula testis, the interior of testis is divided into a no. of pyramidal lobules (bases direceted towards the periphery & apices towards the mediastinum) which are not completely separated & anastomose with one another at many places.
· Each testis has 200-300 lobules.
· Each lobule contains 1-4 coiled seminiferous tubules. They are surrounded & supported by interlobular C.T.
· Interstitial cells of Leydig are present in btwn the seminiferous tubules. They are the hormone secreting cells of testis.
· The s.tubules do not end bluntly but, form single, double or triple arches, whose limbs are not in the same lobule.
· Towards the apex, the convoluted tubules unite with a narrow straight tubule.
· The straight tubules from all the lobules pass into mediastium, where they join & form a network of thin walled channels called the rete testis.
Seminiferous tubules:
The stratified epithelium consists of 2 types of cells: germ cells & sertoli cells.
Germ cells (spermatogenic cells):
· They are the precursor cells of spermatozoa. They occupy the spaces btwn basement membrane & lumen of seminiferous tubules.
· In children, only primitive germ cells called spermatogonia are present.
· With the onset of puberty, different stages of spermatogenic cells are seen: (from periphery to lumen):
Spermatogonium à primary spermatocyte à secondary spermatocyte à spermatid.
Sertoli cells (sustentacular cells / nurse cells):
· They are large irregular columnar cells extending from the basement membrane to the lumen.
· Supporting cells that provide protection & nourishment for spermatogenic cells till the spermatozoa are released from them.
Functions of sertoli cells:
· Convert androgens into estrogens via the enzyme aromatase present in sertoli cells.
· Secrete androgen binding protein (ABP) & EstrogenBP (EBP).
· Secrete inhibin & activin– which inhibit & stimulate the release of FSH from ant. Pituitary respectively.
· Secrete Mullerian regression factor (MRF) or Mullerian inhibiting substance (MIS) in fetal testes. MRF is responsible for regression of Mullerian duct during sex differentiation in fetus.
Blood-Testis Barrier
· It is a mechanical barrier that separates seminiferous tubules of testes from blood & other testicular tissues.
· It is formed by tight junctions btwn the adjacent sertoli cells near the basal membrane of s.tubule.
· The testosterone must be bound to androgen- binding protein (ABP) to cross the barrier.
· Functions:
Prevents the entry of large molecules (such as proteins & cytotoxic substances, galactose)
Allows the passage of water, urea, nutritive substances & hormones for spermatogenesis.
· Damage of the barrier: by trauma or viral infection like mumps. The sperms enter the blood à activation of immune system à autoantibodies against sperms à destruction of germ cells à sterility.
Stages of spermatogenesis
Spermatogonia are primitive sex cells. At puberty, a spermatogonium divides mitotically to form two daughter cells:
· One of these is kept as a lifetime stem cell reservoir.
· The other undergoes several divisions to form a primary spermatocyte.
The 1° spermatocyte undergoes MD1 à two 2° spermatocytes, each of which undergoes MD2 à 4 spermatids à ultimately differentiate into spermatozoa.
After MD1, the spermatocytes have a single (haploid) set of chromosomes.
Stage of transformation:
Spermeogenesis: it is the process by which the spermatids become matured spermatozoa. There is condensation of spermatozoa, formation of acrosome, removal of extraneous cytoplasm etc.
Spermiogenesis: Process by which the matured sperms are released form sertoli cells into the lumen of seminiferous tubules.
Androgens (male sex hormones):
· They are C19 steroid hormones.
· They include potent hormones like testosterone (T) & 5α-dihydrotestosterone (DHT) and
Less potent 17-ketosteroids (17-KS) such as DHEA.
· In males: synthesis of testosterone:
95% à by the testes
5% à by the adrenal cortex.
· In females: testosterone is synthesized by Ovaries & adrenal cortex.
· Plasma testosterone conc. in males is 15 times higher than in females.
· 98% of testosterone circulating in blood = bound to plasma proteins (albumin and sex hormone-binding globulin, SHBG).
· The testes secrete also small quantities of DHT and estradiol (E2).
· Larger quantities of DHT (via 5-α-reductase) & estradiol are synthesized from testosterone (via aromatase) by their respective target cells.
· DHT and testosterone bind to the same intracellular receptor.
· Estradiol influences many functions in the male, e.g., epiphyseal cartilage & ejaculate formation.
Regulation of Testosterone secretion:
· It is regulated by LH (also called ICSH – interstitial cell stimulating hormone), the pulsatile secretion of which is controlled by Gn-RH.
· LH stimulates the release of testosterone from Leydig’s cells in the testes.
· Testosterone & estradiol inhibit LH & Gn-RH secretion (-ve feedback).
· Gn-RH also induces the release of FSH which:
o Stimulates the secretion of inhibin
o Induces the expression of androgen-binding protein (ABP) in Sertoli cells of the testes.
o Induces the formation of LH receptors in the interstitial cells of Leydig.
· Testosterone, DHT, estradiol & inhibin inhibit the secretion of FSH (-ve feedback).
· Activin inhibits FSH secretion.
Testosterone cannot induce spermatogenesis without the help of ABP.
Effects of testosterone:
Target organ of testosterone – Testes
· It affects male sexual differentiation, spermatogenesis & sperm growth.
· It influences the functions of genitalia, prostate & seminal vesicle.
· It induces the secondary sex characteristics that occur in males around the time of puberty.
· It is necessary for normal sex drive (libido), procreative capacity (fertility) and coital capacity (potentia coeundi) in the male.
· It stimulates hematopoiesis & has anabolic properties à muscle mass in males.
· It also has CNS effects & can influence behavior— e.g., cause aggressiveness.
C:N - High conc. of testosterone, either natural or synthetic (anabolic steroids) à masculinization (virilization) of the female.
Sexual development and differentiation.
· The genetic sex determines the devp. of the sex-specific gonads (gamete-producing glands).
· The germ cells (spermatogonia) then migrate into the gonads.
· The somatic sex is female when the somatic sex development & sex differentiation occurs in the absence of testosterone.
· Male development requires the presence of testosterone in both steps with or without the aid of additional factors (e.g., CGRP) in certain stages of development (e.g., descent of testes into scrotum).
Semen
· White or grey fluid that contains spermatozoa.
· Fluid expelled during ejaculation (2–6 mL) contains 35–200 million sperm in a nutrient fluid - seminal plasma.
· Semen: 10% - sperms from the testes.
· 90% - Seminal plasma, which is a collection of fluids form:
o Prostate à gives milky appearance to semen (due to alkaline secretion), PG’s stimulate uterine contraction.
o Seminal vesicles & Bulbourethral gland à provide mucoid consistency to semen.
· During intercourse, the alkaline seminal plasma à vaginal pH à sperm motility.
· Sperm count = 100 – 150 million / ml.
· Sterility occurs when the sperm count falls below 20 millions/ml.
Prostate:
· The acidic vaginal secretions in the females are neutralized by the alkaline secretions of the prostate.
· Its clotting enzymes convert fibrinogen (from seminal vesicles) into coagulum, essential for holding the sperms in uterine cervix.
· The coagulum is dissolved by fibrinolysin of the prostatic fluid à so sperms become motile.
Structure of Sperm
· Has 4 parts: head, neck, body & tail.
· The ant. 2/3rd of the head is like a thick cap – acrosome (Galea capitis), which develops from golgi apparatus.
· It is made up of mucopolysacharide & acid phosphatase. It also contains hyaluronidase & proteolytic enzymes essential for fertilization of ovum.
Azoospermia: lack of sperm in semen (congenital or due to excess use of corticosteroids)
Oligozoospermia: Low sperm count < 20 million/ml of semen à infertility
Teratozoospermia: Presence of sperms with abnormal morphology.
Aspermia: Lack of semen due to retrograde ejaculation of semen into the urinary bladder.
Oligospermia: Low volume of semen (congenital)
Hematospermia: Appearance of blood in semen due to infection of urethra.
Fertilization
· Fusion of sperm & egg. Occurs in the ampulla of the fallopian tube.
· Sperm ascension: Small percentage of the sperm expelled during ejaculation reaches the fallopian tubes.
· The mucous plug sealing the cervix has to be penetrated for sperm ascension.
· The mucous plug also acts as a sperm reservoir for a few days.
· Capacitation: The sperm must undergo certain changes to be able to fertilize an ovum. It is done during the time required for the sperms to reach the ampulla of fallopian tube (about 5 hours).
· After ovulation the ovum is released into the abdominal cavity & enters the fallopian tubes via fimbriated end.
· Ovum of mature follicle in the ovary is in the primary oocyte stage with diploid no. of chromosomes. Just before ovulation, 2nd meiotic division takes place forming 2ndary oocyte which is haploid.
· When a sperm makes contact with the egg (via chemotaxis), species-specific sperm-binding receptors on the ovum are exposed & the proteolytic enzyme acrosin is thereby activated (acrosomal reaction).
· Acrosin allows the sperm to penetrate the cells surrounding the egg (corona radiata).
· The sperm bind to receptors on the envelope surrounding the ovum (zona pellucida) & enters the egg. The membranes of both cells then fuse.
· The ovum now undergoes a 2nd meiotic division, which concludes the act of fertilization.
· Rapid proteolytic changes in the receptors on the ovum (zona pellucida reaction) prevent other sperm from entering the egg.
· Fertilization usually takes place on the 1st day after intercourse & is only possible within 24 hours after ovulation.
Sexual response in the male
· Impulses from tactile receptors on the skin in the genital region (especially the glans penis) & other erogenous areas are transmitted to the erection center in S2–S4, which conducts them to parasympathetic neurons of the pelvic splanchnic nerves à sexual arousal.
· Sexual arousal is decisively influenced by stimulatory or inhibitory impulses from the brain triggered by sensual perceptions, imagination etc.
· Via nitric oxide, efferent impulses lead to dilatation of deep penile artery branches (helicine arteries) in the erectile body (corpus cavernosum), while the veins are compressed to restrict the drainage of blood.
· Moreover, voluntary & involuntary contractions of the ischiocavernosus muscle the pressure in the erectile body above the Ps. This causes the penis to stiffen & rise (erection).
· The ejaculatory center in the spinal cord (L2 –L3) is activated when arousal reaches a certain threshold. Immediately prior to ejaculation, efferent sympathetic impulses trigger the partial evacuation of the prostate gland and the emission of semen from the vas deferens to the post. part of the urethra.
· This triggers the ejaculation reflex and is accompanied by orgasm, the apex of sexual excitement.
· The effects of orgasm are reflected by perspiration & an increase in respiratory rate, HR, BP & skeletal muscle tone.
· During ejaculation, the internal sphincter muscle closes off the urinary bladder while the vas deferens, seminal vesicles and bulbospongiosus & ischiocavernosus muscles contract rhythmically to propel the semen out of the urethra.
Sexual response in the female
· Due to impulses similar to those in the male, the erectile tissues of the clitoris & vestibule of the vagina engorge with blood during the erection phase.
· Sexual arousal triggers the release of secretions from glands in the labia minora & transudates from the vaginal wall, both of which lubricate the vagina & the nipples become erect.
· On continued stimulation, afferent impulses are transmitted to the lumbar spinal cord, where sympathetic impulses trigger orgasm (climax).
· The vaginal walls contract rhythmically (orgasmic cuff), the vagina lengthens & widens, and the uterus becomes erect, thereby creating a space for the semen.
· The cervical os also widens & remains open for about a half an hour after orgasm.
· Uterine contractions begin shortly after orgasm (& are induced locally by oxytocin).
· Erection and orgasm are not essential for conception.
No comments:
Post a Comment