The acrosome is an organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....

 that develops over the anterior half of the head in the spermatozoa
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...

 (sperm cells) of many animals. It is a cap-like structure derived from the Golgi apparatus
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....

. Acrosome formation is completed during testicular maturation. In Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...

n mammals the acrosome contains digestive enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s (including hyaluronidase
The hyaluronidases are a family of enzymes that degrade hyaluronic acid.In humans, there are six associated genes, including HYAL1, HYAL2, HYAL3, and PH-20/SPAM1.-Use as a drug:...

 and acrosin
Acrosin is a digestive enzyme that acts as a protease. In humans, acrosin is encoded by the ACR gene. Acrosin is released from the acrosome of spermatozoa as a consequence of the acrosome reaction. It aids in the penetration of the Zona Pellucida....

). These enzymes break down the outer membrane of the ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

, called the zona pellucida
Zona pellucida
The zona pellucida is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the latter, external but of essential importance to it...

, allowing the haploid nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 in the sperm cell to join with the haploid nucleus in the ovum.

This shedding of the acrosome or acrosome reaction
Acrosome reaction
During fertilization, a sperm must first fuse with the plasma membrane and then penetrate the female egg in order to fertilize it. Fusing to the egg usually causes little problem, whereas penetrating through the egg's hard shell can present more of a problem to the sperm...

 can be stimulated in vitro by substances a sperm cell may encounter naturally such as progesterone
Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...

 or follicular fluid
Follicular fluid
Follicular fluid is a liquid which fills the follicular antrum and surrounds the ovum in an ovarian follicle. This fluid is rich in hyaluronic acid.-External links:* *...

, as well as the more commonly used calcium ionophore
An ionophore is a lipid-soluble molecule usually synthesized by microorganisms to transport ions across the lipid bilayer of the cell membrane...

 A23187. This can be done to serve as a positive control when assessing the acrosome reaction of a sperm sample by flow cytometry
Flow cytometry
Flow cytometry is a technique for counting and examining microscopic particles, such as cells and chromosomes, by suspending them in a stream of fluid and passing them by an electronic detection apparatus. It allows simultaneous multiparametric analysis of the physical and/or chemical...

  or fluorescence microscopy
Fluorescence microscope
A fluorescence microscope is an optical microscope used to study properties of organic or inorganic substances using the phenomena of fluorescence and phosphorescence instead of, or in addition to, reflection and absorption...

. This is usually done after staining with a fluoresceinated lectin
Lectins are sugar-binding proteins that are highly specific for their sugar moieties. They play a role in biological recognition phenomena involving cells and proteins. For example, some viruses use lectins to attach themselves to the cells of the host organism during infection...

 such as FITC-PNA, FITC-PSA, FITC-ConA, or fluoresceinated antibody such as FITC-CD46.
In the case of globozoospermia (sperm with round heads), the Golgi apparatus
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....

 is not transformed into the acrosome, causing male infertility
Male infertility
Male infertility refers to the inability of a male to achieve a pregnancy in a fertile female. In humans it accounts for 40-50% of infertility. Male infertility is commonly due to deficiencies in the semen, and semen quality is used as a surrogate measure of male fecundity.-Pre-testicular...

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