Physiology_of_spermatogenesis_and_semen_

Physiology of spermatogenesis : 

Physiology of spermatogenesis Dr. Asmita Mahla Fellow reproductive medicine IRM, MMM, Chennai

Embryology : 

Embryology Gonadal ridge: 5 wks : migration of germ cells from yolk sac SRY : differentiation of gonad into testes Primary sex cords: 7 wks Differentiation of sertoli cells: secrete MIF Sertoli cells surround germ cells Leydig cell differentiation from mesenchyme: 8 wks: secretes testosterone: wolffian structure differentiation

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Fetal leydig cells stimulated by placental betaHCG At birth these cells regress A second wave of Leydig cell differentiation at 2-3 months briefly elevating the male neonates’ testosterone levels Occurs in response to pituitary LH secretion Androgen produced during this period imprints the hypothalamus, liver, scrotum, phallus and prostate such that they respond to androgen stimulation later in life Absence of androgen surge in the newborn male may prevent normal development in affected males (Main et al, 2000)

Further development : 

Further development The HPO axis remains dormant in late infancy and early childhood At about 7-8 years- GnRH pulses increase at night time following an abatement of inhibition by melatonin Results in pulsatile LH release and FSH release. Other factors influencing puberty: - leptin, GH and IGF 1 Leydig cell ageing beyond 50 yrs. Due to accumulated oxygen free radicles– high LH, low testosterone slight decline in sperm quality and quantity (motility and number) with age 50% lower probability of pregnancy within a year in men older than 40 years than men at 25 yrs (Ford et al 2000)

Testes: gross structure : 

Testes: gross structure Paired gonads located in the scrotum Measure 4.5 to 5.1 cm 15- 25 cc in volume Enclosed in a capsule made of three layers Tunica vaginalis Tunica albuginea Tunica vasculosa

Histology : 

Histology Testes divided into lobules by fibrous septa which extend between the mediastinum testis and the tunica albuginea Number of lobules: 250-400 Lobules consist of 1-3, minute convoluted tubes, the seminiferous tubules. No. – about 840,average length of each is 70 to 80 cm. Diameter varies from 0.12 to 0.3 mm. The tubules are supported by loose connective tissue which contains scattered groups of “interstitial cells” containing yellow pigment granules: Leydig cells

System of tubules : 

System of tubules Tubuli recti :- straight tubules in the apices of the lobules Carry sperms and fluid from the seminiferous tubules About 25 larger ducts, of about 0.5 mm. in diameter Rete testis. a close net-work of anastomosing tubes at the mediastinum Drain Tubuli recti Ductuli efferentes At the upper end of the mediastinum, the vessels of the rete testis terminate in approx. 15 ducts Perforate the tunica albuginea, carry the seminal fluid from the testis to the epididymis.

Epididymis : 

Epididymis Mass of highly convoluted tubules formed by joining together of efferent ductules Measures 3-4 meters in length It increases in diameter and thickness as it approaches the ductus deferens. It is coiled on the posterior surface of each testis. Has three parts Caput (head) Corpus (body) Cauda (tail) Smooth muscle in the wall of the epididymis : propels the sperm into the ductus deferens

Seminiferous tubules : 

Seminiferous tubules The seminiferous tubules are the functional units of the testis, where spermatogenesis takes place. Each tubule consists of a basement layer formed of laminated connective tissue. Provide a unique environment for spermatogenesis Two types of cells found in ~ Germ cells Sertoli cells Sertoli cells are large and are arranged peripherally. Send out cytoplasmic projections into the adluminal compartment Between the sertoli cells are present germ cells These cells are arranged in three different groups

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Outer zone: cubical cells, with small nuclei, spermatogonia Middle zone: larger polyhedral cells, with clear nuclei, arranged in two or three layers; intermediate cells or spermatocytes. Inner zone: spermatids

Sertoli Cell : 

Sertoli Cell Large cell with irregular shape that rests on the basement membrane of seminiferous tubule Has cytoplasmic processes, irregular nucleus, prominent nucleoli, low mitotic index and forms unique tight junctional complexes with adjacent sertoli cells.

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Functions: Nurture the developing sperm cells through the stages of spermatogenesis. Provide both secretory and structural support. Secretory Produces androgen binding protein which is responsible for producing high local concentrations of testosterone in the seminiferous tubules – necessary for spermatogenesis Laminin, ceruloplasmin, growth factors, somatomedin, inhibin Structural Support germ cells through gap junctions Facilitate migration of differentiating germ cell Phagocytose extra cytoplasm fro the spermatid

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Form the blood-testis barrier, partitions the interstitial & blood compartment from the luminal compartment of the seminiferous tubules. control the entry and exit of nutrients, hormones and other chemicals into the tubules makes the luminal compartment an immune-privileged site: haploid gamete not recognized as self

Sertoli cell : 

Sertoli cell

Spermatogenesis : 

Spermatogenesis The process by which male spermatogonia develop into mature spermatozoa Occurs in the male testes and epididymis in a step wise fashion Begins at puberty, continues throughout life ; slight decline (motility and number) with age 50% lower probability of pregnancy within a year in men older than 40 years than men at 25 yrs Process directed by genes located on Y chromosome Optimal conditions needed for effective spermatogenesis

Spermatogenesis : 

Spermatogenesis 300,000 spermatogonia in each gonad at birth 600 million spermatocytes at puberty 100-200 million sperms produced each day 1 trillion during the entire lifetime

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spermatogonium Primary spermatocytes Secondary spermatocyte spermatid mitosis Meiosis 1 Meiosis 2 maturation sperm Slow mitosis Replenish stock

Stages of sperm development : 

Stages of sperm development Spermatocytogenesis: spermatogonium divide mitotically to produce primary spermatocytes in the seminiferous tubules Spermatidogenesis: production of spermatids by meiotic division of spermatocytes ( both 1 and 2) Spermiogenesis: production of sperms by maturation of spermatids

Stages of spermatogenesis : 

Stages of spermatogenesis

Functions of rete testes : 

Functions of rete testes Act as a valve with a built in mechanism for activating the flow of fluid and spermayozoa towards the epididymis Rete testes fluid is a dilute suspension of spermatozoa in a fluid that is iso-osmotic with plasma Reabsorption of fluid in the rete testes and efferent ductules is regulated by estrogen action In ERKO Mice, significant focal seminiferous tubular dysfunction found

Functions of Epididymis : 

Functions of Epididymis Epididymal function is androgen dependent Has three main functions: Sperm transport Sperm storage Sperm maturation

Sperm transport : 

Sperm transport Through the epididymis requires 2- 12 days Influenced by Daily testicular sperm production ( Amann 1981) Sexual activity: recent emissions reduce transit time through cauda by 68% Facilitated by spontaneous rhythmic contractions of smooth muscles surrounding the epididymis

Functions of epididymis : 

Functions of epididymis Sperm storage Retained in the cauda epididymis for varying length of time depending on the degree of sexual activity Studies in animals have shown that sperms can be retained in viable state for several weeks Data in unvasectomized men is lacking

Functions of epididymis : 

Functions of epididymis Sperm motility maturation Motility pattern in the cauda becomes mature; forward progressive: for at least 50% of sperms Sperm fertility maturation Capacity to fertilize zona free hamster eggs: occurs in the caput and is enhanced in the cauda Controversy exists concerning the outcome of fertilization by spermatozoa obtained from caput that have just acquired fertilizing capacity Embryonic mortality in animals high Relevance in humans is to be resolved by future studies

Biochemical changes in spermatozoa during epididymal maturation : 

Biochemical changes in spermatozoa during epididymal maturation Sperm surface membranes assume an increasingly negative charge Sulfhydryl groups undergo oxidation to disulfide bonds making sperm head and tail rigid structures that allow penetration of the egg. Acquisition of a greater capacity for glycolysis Changes in intracellular pH and calcium content Modification of adenylate cyclase activity Alteration incellular phospholipid content

Constituents of epididymal fluid : 

Constituents of epididymal fluid Glycerophosphorylcholine Carnitine Sialic acid Proteins Sperm survival factor Forward motility protein Progressive motility sustaining factor Sperm motility inhibiting factor Acidic epididymal glycoprotein EP2- EP3 proteins that induce sperm binding to zona pellucida

Structure of mature spermatozoa : 

Structure of mature spermatozoa 60 microns in length Enveloped by a plasma membrane that regulates transmembrane movement of ions Also contains receptors for ZP3 proteins that facilitates sperm egg interaction Divided into: Head – oval, 4.5x3 microns, consists of a nucleus, has highly compacted chromatin Acrosome: membrane bound organelle that contains enzymes required for penetration of zona Middle piece Highly organized consisting of helically arranged mitochondria: contain enzymes necessary for oxidative metabolism Centrally located axoneme around which are a set of dense fibres Tail : 55mic. In length

Structure of the sperm : 

Structure of the sperm

Effect of sex accessory gland secretions on spermatozoal function : 

Effect of sex accessory gland secretions on spermatozoal function Coagulum: from seminal vesicles:provides mechanical assistance to maintain spermatozoa in the vagina Liquefaction: by prostate specific antigen Fructose: seminal vesicles- energy source Antioxidants: glutathione peroxidase, superoxide dismutase, catalase, provide protection from oxidants Separation of spermatozoa from semen during centrifugation exposes sperm to an increased risk of oxidative damage

Composition of human semen : 

Composition of human semen The components of semen come from two sources: sperm, and seminal plasma Seminal plasma, in turn, is produced by contributions from the seminal vesicle, prostate, and bulbourethral glands. Seminal plasma of humans contains a complex range of organic and inorganic constituents.

Seminal Plasma : 

Seminal Plasma The seminal plasma provides a nutritive and protective medium for the spermatozoa during their journey through the female reproductive tract. The normal environment of the vagina is a hostile one for sperm cells, as it is very acidic, viscous, and patrolled by immune cells. The components in the seminal plasma attempt to compensate for this hostile environment. Basic amines such as putrescine, spermine, spermidine and cadaverine are responsible for the smell and flavor of semen. These alkaline bases counteract the acidic environment of the vaginal canal, and protect DNA inside the sperm from acidic denaturation

Hormonal control : 

Hormonal control GnRH : secreted from the hypothalamus in pulses: stimulates ant. pituitary to produce FSH, LH FSH acts on sertoli cells to aid spermatogenesis Secrete androgen binding protein

GnRH : 

GnRH Secreted from the hypothalamus Output displays 3 rhythms Seasonal – testosterone peaks: spring Circadian- testosterone secretion peaks – early morning Pulsatile: peaks every 90 - 120min. Seasonal and circadian rhythms modulated by melatonin signaling from pineal gland suprachiasmatic nuclei: intrinsic clock Other influences: from the olfactory and visual cortex The precursors of GnRH neurons migrate from olfactory placode during embryonic development

FSH / LH : 

FSH / LH Glycoprotein hormones secreted by the anterior pituitary Release stimulated by GnRH Pulses of LH secreted every 2hrs. Amplitude- 6 IU/L Tonic level of LH in blood- 10 IU/mL Maintains testosterone levels of 5 ng/mL

Regulation of spermatogenesis : 

Secrete ABP Regulation of spermatogenesis hypothalamus Anterior pituitary Leydig’s cells Sertoli cells GnRH LH FSH Spermato genesis Inhibin B - fb on pituitary testosterone Systemic testosterone action Activin + fb on pituitary DAZ Genes on Y chromosome

Feedback on hypothalamus and pituitary : 

Feedback on hypothalamus and pituitary Differential regulation of Gn secretion for different steroids in the male LH : Negative feedback by testosterone on secretion FSH : Negative feedback by estradiol (produced by aromatization of testosterone) Also by Inhibin B produced by sertoli cells The action of feedback are regulated by the respective receptors

Sperm capacitation : 

Sperm capacitation Discovered by Austin and Chang in 1951 Characterized by three accomplishments The ability to undergo acrosome reaction The ability to bind to zona pellucida The acquisition of hypermotility Uterus aids in capacitation Secretes sterol binding albumin, Lipoproteins Proteolytic glycosidasic enzymes such as heparin Non-mammalian spermatozoa do not require capacitation; ready to fertilize immediately after release from the male.

PROCESS OF CAPACITATION : 

PROCESS OF CAPACITATION Removal of seminal plasma factors from the surface Restriction of receptor mobility Modification of sperm cell membrane sterols, lipids and glycoproteins Decreased stability of plasma membrane and the outer acrosomal membrane increased permeability to Ca2+. Modification of surface charge increased intracellular cAMP levels Increase in sperm motility= +zona penetration Breakdown and merging of plasma membrane and acrosomal membrane Acrosomal reaction Release of enzymes from acrosomal cap Facilitates sperm fusion with egg membrane

Acrosomal reaction : 

Acrosomal reaction Breakdown and merging of plasma membrane of sperm head + outer acrosomal membrane Release of acrosomal enzymes Hyaluronidase Neuraminidase like factor Cumulus dispersing enzyme Acrosin (protease) Enzymes help in penetration of oocyte investments ~ Can also be induced by zona pellucida proteins and human follicular fluid in vitro