Sperm in the Female Tract - Anatomy & Physiology

Overview

• There is immediate retrograde loss at the entrance to the cervix/uterus with phagocytosis by leukocytes.
• In the cervix, sperm must travel through privileged pathways. This serves to eliminate non-motile sperm and remove some abnormalities.
• In the uterus, capacitation is initiated with some phagocytosis occuring.
• In the oviduct, capacitation is completed and sperm show hyperactive motility.
• Sperm attach to the cillia of the epithelium in the isthmus of the oviduct to form a functional reservoir. Only thousands from the initial millions in the ejaculate form this store, as 5-98% may be eliminated by retrograde flow or phagocytosis in the early female tract.
• At the uterotubal junction, the acrosome reaction occurs and the spermatozoon penetrates the oocyte in fertilization with the formation of male and female pronuclei.

Capacitation

• If semen is deposited in the cranial vagina, capacitation may begin as sperm ascend the cervix.
• If semen is deposited in the caudal cervix (mare) or mid-cervix (sow) and immediately enters the uterus, capacitation is initiated in the uterus and completed in the isthmus of the oviduct.
• During capacitation, the plasma membrane of the sperm undergoes marked biochemical changes.
• During mixing of sperm with seminal plasma, epididymal sperm become coated in seminal plasma proteins. These are stripped by the female tract environment. These proteins were the decapacitating agents that were rendering the sperm incapable of fertilization.
• Once the plasma proteins have been stripped, the portions of molecules that can bind to the zona pellucida of the oocyte are exposed.
• Membrane lipases and protein kinases destabilize the plasma membrane, enabling it to undergo fusion if it is brought close to another membrane (it is said to become 'fusogenic').
• Normally, intracellular calcium levels are low. Entry of calcium ions causes a sharp rise in intracellular calcium that signals for the acrosome reaction to occur.
• Sperm remain in the female tract awaiting fertilization by an ovulated oocyte, this can be:
  • ~24 hours in a cow
  • ~40 hours in a sheep
  • ~7/8 days in a bitch
• Sperm are a heterogenous population, so they are not all capacitated at the same rate.

Privileged Pathways

• Sperm must travel through the highly convoluted cervix.
• During oestrus, the cervix produces mucus.
  • Sialomucin: Low viscosity mucus produced by basal areas of cervical crypts.
  • Sulfomucin: More viscous mucus produced in apical portions of the cervical epithelium covering the tips of cervical folds.
• The difference in viscosity of the mucus creates two distinct environments within the cervix.
• Sperm encountering sulfomucin are washed out of the tract.
• Sperm encountering sialomucin swim into it, creating 'privileged pathways' in deeper cervical crypts. Sperm then traverse the cervix through these pathways.
• Thus, the cervix acts as a filter to immotile sperm because in order to survive; the sperm must actively swim into the 'privileged pathways'.

Hyperactivation

• In the oviduct, the motility pattern of sperm becomes hyperactive.
• Motility changes from linear (swim in a straight line, like a swimmer) to non linear (localised movement in one small area, like dancers in a nightclub).
• Occurs in the ampulla of the oviduct, induced by molecules within the epithelium here.
• Hyperactivity facillitates sperm-oocyte contact.

Acrosome Reaction

• Enables spermatozoa to penetrate the zona pellucida.
• Exposes the equitorial segment, so it can later fuse with the oocyte plasma membrane.
• The plasma membrane of the spermatozoon forms multiple fusion sites with the outer acrosomal membrane.
• When the two membranes fuse, many small vesicles are formed, this is known as vesiculation.
• After this has occured, acrosomal contents are dispersed and the sperm nucleus is left with the inner acrosomal membrane surrounding it.
• Damaged acrosomes (sudden heating/cooling etc) do not vesiculate, but rupture all at once.

Penetrating the Zona Pellucida

• Spermatozoa contain zona binding proteins on their plasma membrane surfaces overlying the acrosome, which bind specifically to zona pellucida proteins (these must be exposed during capacitation before zona binding can occur).
• The zona pellucida consistes of three glycoproteins, zona proteins 1,2 and 3.
  • Zona proteins 1&2: structural proteins, provide structural integrity of the zona.
  • Zona protein 3: binds to proteins on the spermatozoal membrane.
• The sperm plasma membrane contains two zona binding sites:
  • Primary zona binding region: responsible for adherence to the zona pellucida.
  • Acrosome reaction promoting ligand: when this binds to the zona protein 3, signal transduction occurs to initiate the acrosome reaction.
• After the acrosome reaction, zona penetration is a rapid process (minutes).
• The acrosome reaction releases enzymes:
  • Acrosin: hydrolyses zona proteins and enhances the ability of sperm to bind to the zona.
  • Proacrosin: innactive form of acrosin with a strong affinity for the zona pellucida. It aids in binding spermatozoon to the zona as the acrosome reaction proceeds.
• As proacrosin is converted to acrosin, sperm begins to penetrate the zona pellucida.
• Sperm digest a small hole in the zona pellucida through which it can pass. This small,regional dissolution leaves the zona predominantly intact. Maintainence of the zona is important to prevent blastomeres in the early embryo from separating.
• Mechanical force generated by the flagellar action of the tail is sufficient to then push sperm through the zona.