Sperm has a strong flagellum that helps it to propel it through aqueous medium
Devoid of ribosomes, Endoplasmic reticulum (ER) or Golgi apparatus (GA)
There is a mitochondria at its neck which provides energy for motility to the tail
Has 2 distinct regions surrounded by plasma membrane – tail and head
DNA is tightly packed, so that its volume is minimized for transport and transcription is hut off. Chromosome are packed with simple, positively charged proteins called protamines instead of histones
At the tip of the head, there is a secretory vesicle – acrosome which secretes hyaluronidase. This enzyme helps to dissolve the egg coat and aids penetration. When the sperm contcts the egg, the contents of acrosome are released by exocytosis. This is called acrosome reaction. It also secretes some proteins that help attach the sperm tightly to the egg
The tail is basically a flagellum whose axoneme starts from a basal body just posterior to the head. Axoneme has 2 central singlet microtubules surrounded by 9 evenly spaced doublet microtubules. This is urrounded by 9 outer dense fibres – stiff and non-cntractile with unknown function
Flagellar movement is driven by dynein motor proteins which utilizes energy from ATP to slide along microtubules. ATP is generated by mitochondria in the midpiece (anterior part of tail) hydrolysis
Human sperm
Spermatogenesis
Begins at puberty. Occurs in the epithelial lining of seminiferous tubules. Spermatogonia are found on the outer edge of the tube adjacent to basal lamina. Here they proliferate continuously by mitosis
Some daughter cells stop proliferating and differentiate into primary spermatocyte then undergo meiosis I developing into secondary spermatocyte. This then undergoes meiosis II forming 4 spermatids. Spermatids differentiate into mature sperms which are then eventually released into lumen of seminiferous tubules. Mature sperms then pass into epididymis and are stored there until release
Secondary spermatocytes contain 22 duplicate autosomes and a duplicated X or Y chromosome. Out of the 4 spermatids, 2 contain X and the other 2 contain Y chromosome
Meiosis proceeds without cytokinesis. Thus, uptil the stage of spermatids, they are held together by cytoplasmic bridges forming syncytium. Thus, it can be supplied with all the products of a diploid genome, eg. Sperm with Y chromosome could obtain essential proteins encoded by genes on X chromosome. This allows sperm differentiation
DAZ gene produces a protein which is an RNA-binding protein which is very essential for spermatogenesis. If deleted, it results in infertile men
Spermatogenesis
Fertilization
Fertilization is the process of gamete fusion. When the sperms are released into the female genital, they are attracted to the ovum by chemoattractant released by follicle cells around the ovum
The sperm is first modified which requires about 5-6 hours in humans in order to fuse with the ovum. This is called capacitation
This is triggered by bicarbonate ions in vagina which enters the sperm, activates adenylyl cyclase, increasing cAMP concentration in the cell. This alter lipids and glycoproteins present on the plasma membrane of the sperm leading to increase in metabolism and motility and a decrease in membrane potential (hyperpolarization)
The sperm penetrates the layer of follicular cells and binds to zona pellucida which is a species specific barrier
Zona pellucida has 3 glycoproteins – ZP1, ZP2, ZP3. These are produced exclusively in the growing oocyte. ZP2 and 3 form filaments which are cross-linked by ZP1, thereby creating a 3D network around the ovum. ZP3 is crucial as eggs lacking this glycoprotein have been found to be infertile
Sperm surface proteins bind to specific O-linked oligosaccharides on ZP3 which are thought to be the sperm receptors. This stimulates acrosomal reaction
In mouse, trigger for acrosomal reaction is initiated by ZP3 on zona which induces Ca2+ influx in sperm cytosol causing exocytosis of hyaluronidase
Hyaluronidase, so released dissolves zona pellucida and allows penetration. Various sperm proteins are also exposed which then bind protein component of ZP2 glycoproteins. It also exposes some other proteins on the plasma membrane of sperms allowing fusion with the plasma membrane of ovum
After a sperm nucleus enters the egg cytoplasm, the egg acts to prevent polyspermy. This is ensured by 2 mechanisms :
Depolymerization, after the entry of first sperm, of the ovum plasma membrane. However, the membrane potential soon returns to normal
Cortical reaction
A sperm protein acts to increase the cytosolic Ca2+ level that spreads throughout the cell as wave. This is known as Ca2+This increase in Ca2+ level leads to something called as cortical reaction wherein cortical granules in the cortex release contents by exocytosis. Zona pellucida is hardened and thus, block entry of other sperms. Changes include cleavage of ZP2 and hydrolysis of sugar groups of ZP3
The sperm then reaches the plasma membrane of the egg and interacts with it. Neighbouring microvilli rapidly elongate and cluster around the sperm and bring it inside head-first
In mouse, the interaction is mediated by a TMP (transmembrane protein) called fertilin. It gets exposed on the plasma membrane of the sperm due to acrosomal reaction
Fertilin has 2 glycosylated subunits – α and β, held by non-covalent bonds. Extracellular N-terminal domain of the subunits are thought to bind integrins on ovum’s surface which in turn are associated with a member of tetraspan family of membrane protein
Extracellular domain of α subunit has hydrophobic region which resembles fusogenic region of viral fusion proteins, thereby mediating fusion of sperm and ovum
Besides fertilin some other sperm proteins are also required for fusion
This fused fertilized egg is called zygote. The 2 pronuclei (haploid) fuse to give a diploid nucleus and fertilization is complete. In mammals, this does not happen until the membrane of each pronucleus has been broken down in preparation for zygote’s first mitotic division
Sperms also contributes centriole, absent in ovum. It replicates and assembles mitotic spindles in zygote for mitosis
Thus, this zygote can now develop into an individual or organism
