Spermatogenesis: Formation of Sperm

General Embryology

• Gametogenesis refers to the formation and maturation of sperm and ovum
• Site of Gametogenesis is in the gonads (testis and ovaries)
• Spermatogenesis is the formation of mature sperm from primitive germ cells
• Spermatogenesis onset happens at puberty (13-16 years) and continues into old age
• Spermatogenesis occurs in the seminiferous tubules of the testis
• The duration of Spermatogenesis is about 64 days

Spermatogenesis Stages (At Puberty)

• Primordial germ cells divide by mitosis to form different types of spermatogonia (type A & B)
• Spermatogonia enters several mitotic divisions to increase in number and form daughter spermatogonia
• Each contains (46, 2n) chromosomes
• Daughter spermatogonia (type B) enlarge in size and differentiate into primary spermatocyte (46, 2n)
• Primary spermatocyte replicates its 2 copies of DNA to 4 copies of DNA before it enters the first meiotic division
• Each primary spermatocyte divides by the 1st meiotic division into two secondary spermatocytes
• Each secondary spermatocyte contains a haploid number of d-chromosomes (23)
• Each secondary spermatocyte contains a normal amount of DNA (2 copies of DNA)
• Each secondary spermatocyte divides by the second meiotic division into 2 spermatids
• Each spermatid contains a haploid number of single chromosomes
• Each spermatid has half the normal amount of DNA (1 copy)
• Each spermatid, transforms into a mature sperm via spermiogenesis, without any division

Hormonal Control of Spermatogenesis

• Testosterone activates genes that promote the differentiation of spermatogonia
• FSH or Follicle-stimulating hormone is secreted by the pituitary gland
• FSH is responsible for nourishing the sperm and promoting their development and maturation
• LH or Luteinizing hormone is secreted by the pituitary gland
• Luteinizing hormone's main function is to activate the release of testosterone in the Leydig cells.
• Inhibin inhibits the release of FSH in the pituitary gland and therefore stops spermatogenesis through negative feedback

Mature Sperm Structure

• Length of mature sperm is 55 microns
• Mature sperm consists of the head, neck and tail
• The Tail is made up of the Body (mid piece), principal piece and end piece
• The head is four microns long and forms most of the bulk of the sperm
• The head contains a condensed haploid nucleus, which carries hereditary material of the father (22 autosomes & X- or Y- sex chromosomes)
• The head is covered by an acrosomal cap and has minimal cytoplasm and a cell membrane.
• The neck or connecting piece is the narrow junction between the head and tail
• The neck contains two centrioles (basal body)
• The tail is 45 microns long and consists of three segments
• The tail provides the motility of the sperm that assists its transport

The three segments of the tail

• The Body or mid piece is swollen and is 6 microns long
• Body / Mid piece consists of cell membrane, cytoplasm, mitochondrial sheath and an axial or axinome filament
• Annulus (Jensengs ring) is distal to the mid piece
• Principal piece and end piece are the additional parts

Spermiogenesis

• Spermiogenesis is the post-meiotic series of morphological transformation of spermatids into mature sperms.

The Spermiogenesis Process

• Formation of the head
• The head contains the nucleus, which forms the greater part of it
• Formation of the acrosome or head cap
• It covers the anterior two-thirds of the nucleus and contains enzymes that facilitate penetration of the ovum during fertilization
• Formation of the neck, which is the narrowest part
• Formation of the body or middle piece
• Contains mitochondria around the axial filament
• Formation of the tail that provides motility
• The axial filament forms the core of the tail

Chromosome Number in Spermatogenesis

• The numbers of chromosomes is diploid (44+XY) in Spermatogonia and primary spermatocyte
• The numbers of chromosomes is haploid (22+X or 22+Y) in secondary spermatocyte, spermatid and sperm

Sperm types

• Androsperm represent 50% of the total number and contains Chromosomes -22 Y, is smaller in sizes
• The Androsperm has a shorter life span, swims faster
• Androsperm is less resistant to vaginal pH; alkaline medium increases activity and life, they are Resposnsible for male child
• Gynosperm accounts for 50% of the total and contains chromosomes -22X and is bigger in size
• The Gynosperm has a longer life span, swims slower, is more resistant to vaginal pH
• Gynosperm benefits from Alkaline medium –activity and life increases and is Responsible for female child

Sperm Transport

• In males, sperms from the epididymis (in the tail) are rapidly transported to the urethra via peristaltic contractions of the vas deferens
• As the sperms pass by the seminal vesicles, prostate and bulbourethral glands, secretions are added to them

Sperm Transport - Female Reporoductive Tract

• During sexual intercourse, about 200-600 million sperms are deposited via ejaculation within the posterior fornix of the vagina around the external os of the cervix
• The sperms pass slowly through the cervical canal by tail movements
• Passage of sperms through the uterus and uterine tubes results mainly from muscular contractions of the walls
• Only about 200 sperms reach the lateral end of the uterine tube
• Sperms are typically found in the ampulla of the uterine tube after 5 minutes
• Only one sperm enters ovum

Seminal Fluid

• Seminal fluid consists of 10% sperms and 90% secretions
• It is a thick white gelatinous fluid rich in fructose and glucose
• Characterized by a seminiferous odor
• Seminal fluid is alkaline in reaction
• Volume is 3-5 ml per ejaculation.
• Volume typically has More than 80 % motile sperm; more than 40% should be motile after 2 hours

Seminal Fluid Functions

• Contains fructose for sperm nutrition.
• Seminal fluid is alkaline to neutralize the acidity of the vaginal secretions.
• Seminal fluid contains prostaglandins that help in the sperm motility

Newly Formed Sperms

Newly formed sperm is immature

• They are non Motile so can not fertilize any ovum

To be able to fertilize, the Sperm must acquire these traits

• Maturity: can be achieved in the epididymis and vas deferens
• Motility: happens from the ampulla of vas deferens to vagina
• Capacitation achieved in the cervix of the uterus
• Undergo Acrosome reaction once inside the uterine tube

Abnormalities of the Seminal Fluid

• Under normal conditons, less than 10% of the sperms are of abnormal form

Numerical Defects in Sperm Volume

• Aspermia: no seminal fluid is produced
• Azoospermia: sperms are absent
• Oligospermia: the sperm count is less than 20 million per cm2
• Polyspermia: the sperm count is more than 250 million per cm2
• Asthenospermia: poor motility
• Necrospermia: sperms are dead

Abnormalities to Sperm Shape

• Dwarf sperms: small sperms
• Giant sperms: big sperms
• Sperms with two heads
• Sperms with two tails