Sperm, sperm
Structure of typical sperm
head
neck
tail
middle piece
part and parcel of sth
Terminal lobule
Atypical sperm structure
spermatogenesis
Hormonal regulation of spermatogenesis
Gene regulation of spermatogenesis
The germ cells of male animals. Its shape is quite different from that of ordinary cells. Sperm of various animals can be divided into typical and atypical types. It is typically tadpole-shaped, with a nearly cylindrical head (different animals) and a slender tail, such as flagella. Atypical sperm have various shapes, but none of them have flagella.
An unexplained phenomenon is the production of so-called mutant sperm. In mammals (including humans), birds, amphibians, fish, insects and annelids, in the same body, in addition to producing typical sperm, there will be extra small, extra large or even more than one flagella variant, not due to degeneration or pathological reasons. The shape of mutant sperm in marine and freshwater gill (mollusk) is completely different from that of normal sperm.
Structure of typical sperm
Since Levin Hook observed the sperm of human beings and some higher animals in 1677, more than 1,000 kinds of animal sperm have been studied in 1000, most of which are tadpole-shaped. Since 1950s, the understanding of the biological characteristics of sperm has made rapid progress. Taking mammals as an example, the structure of sperm can be divided into three parts: head, neck and tail.
The head is mainly composed of nucleus and acrosome, which is spherical, long cylindrical, spiral, pear-shaped and axe-shaped. These shapes are determined by the shapes of the nucleus and acrosome.
The nucleus of mature sperm contains highly dense chromatin, so it is difficult to distinguish its structure under optical microscope and electron microscope. There is an acrosome at the front end of the nucleus, which is a hat-shaped structure consisting of two membranes covering the first two thirds of the nucleus. The layer near the plasma membrane is called the acrosomal membrane, and the layer near the nucleus is called the acrosomal membrane. There are hydrolase particles in acrosome, which is related to sperm passing through various egg membranes outside the egg. The cavity between acrosome and nucleus is called inferior acrosomal cavity, which contains actin. Some invertebrates have acrosome reaction during sperm fertilization: actin polymerizes to form acrosome processes or acrosome filaments; Sperm can attach to the plasma membrane of the egg, leading to the fusion of sperm and egg-fertilization.
Although the nuclear membrane is a double-layer membrane structure, the distance between the two layers is very small, and there are nuclear membrane holes only in the folds connected with the neck at the back end of the nuclear.
This part of the neck is the shortest. Located behind the head, it is cylindrical or funnel-shaped, also known as the connecting section. It is connected to the back end of the front nucleus and then to the tail. There is a matrix at the front end, which is composed of dense substances and just falls into a depression called implantation fossa at the back end of the nucleus. There is a slightly thicker head plate behind the base plate, and there is a transparent area between them, in which the fine fibers are connected with the nuclear membrane at the back end of the core through the base plate. Behind the head plate is the proximal centriole, which is slightly inclined, but almost perpendicular to the axis formed by the distal centriole. Around these structures, there are 9 nodal columns composed of longitudinal fibers, showing the depth interval, and mitochondria are distributed around the nodal columns. These nine segmented columns are closely connected with the heads of the nine coarse fibers behind them.
The tail is divided into three parts: middle section, main section and tail section. The main structure is an axial wire passing through the center.
The middle part from the distal centriole to the ring is called the middle section, and its length varies greatly among mammals, but its structure is generally similar. The main structures are axoneme and peripheral mitochondrial sheath.
Axle filament: the moving organ of sperm, which is formed by the distal centriole and extends to the terminal segment of sperm. The structure of sperm axoneme is similar to animal flagella (or cilia), and the basic composition is 9+2 type, that is, the two microtubules in the center are single and the nine microtubules in pairs are surrounded by them.
The fiber sheath outside the axial filament is composed of 9 coarse fibers. They are connected with nine segmented pillars on the neck. This is unique to mammalian sperm, so people classify mammalian sperm as 9+9+2 type (Figure 3), although its size and shape are different in various animals. There are similar structures in the sperm of birds and some invertebrates.
② Mitochondrial sheath or mitochondrial helix: Mitochondria are connected with each other, and the helix wraps around the coarse fiber, so it is called mitochondrial sheath. It is a continuous structure in which mitochondria gather together and fuse with each other during sperm formation. The number of turns of each mammal's spiral varies greatly, from a dozen to hundreds.
③ Ring: located at the rear end of the middle section. After the last revolution of the mitochondrial sheath, the plasma membrane turns inward. It is unique to mammalian sperm and may be related to preventing mitochondria from moving backwards during sperm movement.
The longest part of the tail of the main joint, which consists of axial filament and cylindrical fiber sheath outside. Two fibers extend into the longitudinal ridge in the fiber sheath. Because the longitudinal ridge is located on both sides of the dorsal abdomen, the transverse section of the sperm tail is oval.
The terminal segment enters the terminal segment with the main segment, and the fiber sheath gradually becomes thinner and disappears.
Atypical sperm structure
* * * is characterized by no flagella, but its shape is quite different. This sperm is widely distributed in invertebrates. Some sperm, such as those of lower crustaceans, are spherical or banded, and are more like cells than typical sperm. Some grow many slender protrusions, which may help adhesion and prevent them from being washed away by the water in the incubation room; The sperm of higher crustaceans are complex in shape, with slender protrusions and chitin capsules. The atypical sperm of nematode is as simple as amoeba, while the sperm of Ascaris equisetifolia has a unique lens. Because most atypical sperm are simple in shape, it is easy to think that they are in the early stage of sperm formation. However, after comparative studies, such as centriole and flagella produced by it, it can still be seen in some sperm (although it has not grown out of the body). It is generally believed that this simple shape is a degenerate secondary figure.
spermatogenesis
The process from spermatogonial stem cells to sperm in higher animals is similar, all of which are carried out in seminiferous tubules of testis.
Mammalian spermatogonia can proliferate as stem cells to produce new stem cells and differentiated cells. This not only preserves the generation of stem cells themselves, but also constantly produces differentiated cells, and then produces primary spermatocytes. As for how many times they undergo mitosis to produce primary spermatocytes, various animals are different. Except for the earliest spermatogonia, after each mitosis in spermatogenesis, the cytoplasm is not completely separated, and the cells are connected by bridges, which looks like syncytium. This may help to maintain strict synchronization between cells and produce a large number of sperm at the same time.
After spermatocytes are produced, they enter the growth period and increase in size, which is called primary spermatocytes at this time. Their nuclei synthesize DNA, and their chromatin undergoes a series of complex changes to prepare for the first mature division (see meiosis). After cleavage, each primary spermatocyte produces two haploid secondary spermatocytes. The latter does not copy DNA, but enters the second mature division after a short stay, forming two sperm cells. Therefore, a primary spermatocyte undergoes two mature divisions to form four haploid spermatocytes. It's just that this stage of spermatogenesis in various animals is roughly similar.
The process from sperm cells to sperm is called sperm formation, also known as sperm metamorphosis. This process is extremely complicated, mainly because the nucleus and organelles have undergone tremendous changes. The composition of nuclear protein in the nucleus has changed significantly, resulting in chromatin densification and nuclear volume reduction. In some animals, protamine replaces histone in the nucleus. Golgi apparatus, centriole and mitochondria have also undergone great changes. Golgi apparatus consists of a series of vesicles, some of which produce acrosome particles. The vesicles continue to expand and merge into larger vacuoles, covering the front end of the nucleus and further evolving into cap-shaped acrosome. Acrosomal precursor particles also aggregate into larger acrosomal particles, showing mucopolysaccharide reaction. When Golgi apparatus changes, centriole splits into two and moves away from each other. The proximal centriole is located in the depression of the posterior end of the nucleus, and the distal centriole forms flagella axoneme, which disappears later. Mitochondria redistribute and form a spiral around the axoneme. This movement is related to actin fibers around mitochondria. At the same time of these changes, most of the cytoplasm gathered in the neck and connected with sperm only through a thin stem. At this time, the tail of sperm has grown from the back end. When the petiole breaks, the sperm is separated from the cytoplasm (called residue) and enters the lumen of the convoluted tubule.
The whole process of spermatogenesis is closely related to sertoli cells. The epithelium of seminiferous tubules consists of long columnar supporting cells and germ cells, with a wide bottom and a narrow top. Spermatocytes are located between sertoli cells and the basement membrane of convoluted tubules, and there is desmosome-like connection between them (see intercellular connection). Mature spermatocytes gradually move into the tubule cavity, mainly relying on the movement of supporting cells themselves (possibly related to abundant microfilaments). Spermatocytes at all levels are located in the pits of sertoli cells, or in the pits formed by two adjacent sertoli cells, and form gap connections with the cell membranes of sertoli cells, thus connecting them. Sperm cells at all levels are arranged according to maturity, and the abnormal sperm cells are closer to the top. Here, sertoli cells are connected with sperm cells through two structures: one is the connecting structure formed by actin fibers in the ectoplasm; The other is zonule complex, which is formed by the top surface of sertoli cells and the head surface of sperm cells. Because spermatocytes are constantly replaced from spermatogonia, such an accurate arrangement is produced (Figure 6).
Hormones regulating spermatogenesis are regulated by luteinizing hormone (LH), follicle stimulating hormone (FSH) secreted by pituitary and testosterone secreted by leydig cells. Interstitial cells, also known as interstitial cells, are located in interstitial tissue between convoluted tubules. They synthesize and secrete testosterone into convoluted tubules to promote spermatogenesis. The production of testosterone is controlled by LH released by pituitary gland. FSH secreted by pituitary stimulates cells to synthesize and secrete androgen binding protein, which has strong affinity with testosterone to maintain the concentration of testosterone in convoluted tubules and its effect on spermatogenesis. In addition, FSH can directly start spermatogonial division and stimulate the development of early germ cells.
Genes in spermatogenesis control chromatin concentration during spermatogenesis, making DNA unable to be transcribed, which is completed before sperm is completely formed. Transcription stops at different times during spermatogenesis in various animals. For example, in Drosophila, RNA synthesis stops during primary spermatocytes, while in mice, it is still going on in spermatocytes shortly after mature division, and it will stop completely when the nucleus begins to elongate.
The formation of sperm depends on the synthesis of protein. Since RNA synthesis has stopped, protein synthesis required by sperm metamorphosis must rely on stable RNA produced and stored in the early stage, and it will not be translated until sperm metamorphosis. This is the regulation of post-transcriptional level and the mechanism of delayed gene expression. Sperm, for example, is synthesized in sperm cytoplasm, enters the nucleus to replace histone, and has been transcribed in primary spermatocytes. The RNA synthesized in the nucleus was transferred to the cytoplasm and combined with protein to form 16 ~ 18s nucleoprotein particles, which were stored in the cytoplasm until the sperm cell stage. In this case, there is a long time gap between transcription and translation, and there is still a lack of understanding of the factors that control post-transcriptional gene expression. Terminal differentiation of other types of cells may also encounter similar phenomena.
Sperm is a mature male germ cell, which is formed in testis.
Mature human sperm is shaped like a tadpole, about 60 microns long, and consists of a head with parental genetic material and a tail with motor function, which is divided into four parts: head, neck, middle and tail. I also collected a sperm movement map (69. 1k), which is very beautiful and worth seeing.
Sperm abnormalities include head, body and tail morphological variation, or head-body mixed malformation. Head deformities include giant head, inversion of head nucleus and cytoplasm, mushroom head and double head; Body deformities include big, thick, wedge, triangle, etc. Tail deformity includes thick tail, short fork tail and double tail; Mixed malformation of head and body includes enlargement of head and body, elongation of nuclear malformation and mixed elongation of head and body.
Sperm movement
There are several types of sperm movement, but there are two most common ones. One is to go straight ahead, but sperm actually swim forward. The second movement is rocking. Sperm only wags its tail and does not move forward. The movement types of epididymal sperm and ejaculated sperm are different. Sperm with different components in semen have different types of exercise. Because of the relatively high speed of sperm movement in the early stage of ejaculation, sperm-rich ejaculation is often used for artificial insemination. When sperm encounters changes in the ionic microenvironment and biophysical state of epididymal fluid, seminal plasma, cervical mucus, endometrial fluid, oviduct fluid and peritoneal fluid, the types of sperm movement also change. Sperm moves faster in the presence of oviduct mucus and follicular fluid. The optimal ratio between prostate secretion and seminal vesicle secretion also affects sperm motility and movement. Seminal seminal vesicle secretion contains several components that are harmful to sperm movement and vitality. Prostate secretion stimulates sperm movement. Mixing kallikrein or kallikrein with semen samples can improve sperm motility. Regular continuous administration of kallikrein to patients with oligozoospermia for several months can increase the number of sperm and improve sperm motility. Of the hundreds of millions of sperm injected into the female reproductive tract at any one time, less than 100 sperm can reach the fertilization site.
Sperm flagella repeatedly propagates sine waves in a coordinated order. In this way, the movement of sperm is regulated by the energy produced by flagella. Flagella has anatomical longitudinal contraction protein, crude fiber and related microfilaments and microtubules. Therefore, in order to overcome the resistance of viscous cavity fluid such as cervical mucus, such driving force is needed for a long time. In order to move forward effectively, sperm cells must coordinate their motion waves and maintain them as a result of development.
There are many contradictions about the three-dimensional model of sperm tail movement, which may be because the photos taken are basically two-dimensional structures. However, it is considered that there is a main wave motion from the base to the tip on a plane, which is equivalent to the wide surface of the sperm head. A rotating component attached to this motion wave causes a spiral motion. But it is not clear whether this rotation component is clockwise or counterclockwise.
From the base to the tip, the head-to-head agglutination of human sperm often rotates counterclockwise, that is, in the opposite direction to the forearm of the peripheral fiber of axoneme. If the axoneme conducts the contraction pulse in some way, the pulse will pass through the axoneme in the same direction.
There are adenosine triphosphate (ATP) and adenosine triphosphate enzyme (ATPase) in sperm, which play an important role in muscle contraction and establish a connection between energy response and sperm movement. Just as ATP provides a lot of energy when muscle fibers contract, ATP decomposition provides a lot of energy for sperm fibers to contract. ATP consumed by sperm can be supplemented by glycolysis and respiration.
The energy produced by metabolic process (ATP is its final form) is transferred to the structure that can convert chemical potential energy into mechanical kinetic energy. This energy supply is partly supplemented by substrate metabolism of culture medium and partly produced by in-situ metabolic pathway, including adenosine triphosphatase. It seems that the initiation of flagella movement is partly controlled by endocrine through cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). However, in a series of reactions, flagella movement is mainly triggered by protein complex activated by calcium ions.
The bioelectrical properties of sperm plasma membrane, neurochemical regulation and activation of special ion transfer enzyme systems, namely Na+ and K+, depend on magnesium ATPase, which may have a causal relationship with the coordination of motion waves and the movement speed of sperm cells. The speed of sperm movement may also be affected by the change of flagella movement frequency.
The occurrence, maturation and transportation of sperm are in human embryos, and primordial germ cells can be identified on the 24th day of development. At this time, they appear in the yolk. The cells undergo mitosis and go to the primordial gonad in the reproductive valley in the 4th-5th week. Primordial germ cells increased rapidly during migration and after cell division. By the 42nd day, as many as 1300 primordial germ cells could be found, which later became either oogonia or spermatogonia and existed in undifferentiated gonads. In the early stage of halo differentiation, primordial germ cells were evenly distributed in seminiferous tubules. They remain static throughout childhood, but in early adolescence, spermatogonia begin to proliferate and become primary spermatocytes through mitosis, and meiosis does not begin until adolescence.
Male adolescence is similar to female adolescence in many aspects, but there are also some obvious differences. The growth peak is measured by height, and men appear two years later. There is no exact time point like menarche to indicate that a certain stage of sexual maturity has been reached. Just like female development, the development stage of a certain age can vary greatly among individuals. Testosterone causes the growth of the body and the maturation of male accessory organs, as well as the characteristic distribution of secondary sexual characteristics such as deep voice and hair.
The fully developed cross section of the slaughtered testis shows that the germ cells in the seminiferous tubules are at different stages of development. These cells come from seminiferous epithelium and are surrounded by the basement membrane of seminiferous tubules. Interstitial cells that produce and secrete testosterone are located between seminiferous tubules. Sertoli cells are another kind of special cells, which are adjacent to the germ cells in seminiferous tubules. These cells are not part of the germ cell line. They are located on the basement membrane of seminiferous tubules and extend into human cavity, and their branches extend into the middle of germ cells at different stages. For many years, it was thought that sertoli cells only provided structural support in testis. Now people realize that they have many important functions. In adults, they play an important role in the release of sperm into seminiferous tubules. They also provide an important barrier to separate the lumen from the spermatogenesis of the remaining seminiferous tubules and produce androgen-binding proteins. Spermatogonial cells and adjacent supporting cells are juxtaposed on the basement membrane. This serves as a functional barrier between the gap and the fluid infiltrating the reproductive epithelium. The fluid in seminiferous tubules is essentially different from the blood, plasma or lymph components of testis. It contains less protein and three times as much potassium as glucose. There are significant differences in amino acid concentrations. The contents of glutamic acid and aspartic acid in testicular fluid are higher than those in blood. The most important thing is that the basement membrane prevents protein from entering seminiferous tubules. This can protect the contents from blood antibodies, which will react with sperm. Testosterone can diffuse freely through the basement membrane. It has been found that the concentration of testicular reticulum fluid is similar to that of testicular venous blood. Therefore, hormones important for spermatogenesis can be effectively transported, while other substances in the blood circulation that can interfere with this process are excluded.
Spermatogenesis is a series of processes from undifferentiated spermatogonia with 46 chromosomes (diploid, 2N) to sperm with only 23 chromosomes (haploid, n). Throughout adult life, the testis constantly provides sperm, which is transported and stored in the accessory reproductive organs. Evidence of normal spermatogenesis is found in men in their eighties and nineties. This effective and continuous process provides incredible reproductive capacity.
In seminiferous tubules, spermatogonia undergo mitosis and eventually form spermatocytes. Primary spermatocytes in adolescence undergo meiosis to form smaller secondary spermatocytes, which contain haploid chromosomes and further divide into spermatocytes. This division is not accompanied by a further reduction in the number of chromosomes. The transformation of sperm cells into sperm is a mature process, with neither meiosis nor mitosis. The last process is called sperm formation. Although the division of nucleus has been completed in the primary spermatocyte and secondary spermatocyte stages, the division of cytoplasm is not over. The components of sperm cells are still connected by cytoplasmic valleys; Therefore, sperm development is interrelated.
The number of sperm in the initial area of epididymis is relatively small, and its concentration increases with the absorption of fluid by epididymal epithelium. Healthy young men have curly tubules filled with sperm at the tail of appendages. Little is known about the environment of the halo attachment distal region, which allows sperm storage and maintains the integrity of physiological functions. In rabbits and other animals, sperm in epididymis can maintain fertility for up to 30 days.
A series of changes have taken place in the process of sperm maturation under the attachment of Gu. Sperm with halo on the head have neither motility nor abnormal swimming, and tend to move in circles. When they pass through the aura, their swimming ability becomes one-way. The surface characteristics of sperm plasma membrane have undergone important changes, but there is no structural acrosome change after human sperm matures.
The penis consists of three long cylindrical erectile tissues. It is wrapped in an elastic sheath. Each cylinder contains blood vessels and a meeting space that can be filled with blood flow during sexual arousal. The above two columns are called corpus cavernosum, which can harden the penis and increase the length and width of the penis when erect. The cavernous body of penis is divided into penile feet at the root of penis body. The penis foot is attached to the human body, and when dispersed into healthy fibers, it is anatomically attached to the pelvis. The third column of penis, urethral cavernous body, is located in the lower part of penis. Stop at the glans penis. The urethra passes through the urethral cavernous body. When erect, urethral sponge is softer than penile sponge. At the climax, the glans penis itself will increase twice as much as usual. Give it a lot of sensory nerve endings and have the strongest sexual desire. At the climax, the penis becomes hard, the urethral orifice is enlarged, the testis is partially raised, and the halo skin and tunica vaginalis increase. When sexual excitement reaches the stage, semen gathers in the prostate urethra, and at this time there is a feeling that ejaculation is inevitable. Cooper's gland (urethral bulbar gland) located at the root of urethra releases its contents. Testicular volume increases and varus, scrotum thickens. At orgasm, the internal sphincter of bladder contracts, the seminal vesicle contracts, the vas deferens contracts, the erectile sphincter contracts, and the prostate contracts. When the penis itself contracts, these contractive forces force semen to pass through the urethra.
Ejaculation involves the coordinated contraction of epididymis and smooth muscle of vas deferens, prostate and seminal vesicle wall. At the distal end of epididymis and vas deferens, the muscle thickening of these catheter walls is particularly obvious. The final result of stimulating contractile activity at orgasm is under the control of adrenergic nerve fibers. On the contrary, erection depends on the stimulation of parasympathetic nerve. Only the distal region with halo tail and vas deferens contract obviously under the stimulation of adrenergic fibers of lower abdominal nerve. This type of nerve distribution prevents a large number of immature sperm from entering the seminal plasma through the vas deferens during ejaculation. The function of accessory organs is that semen receives fluid from seminal vesicle, prostate and Cooper gland (urethral ball gland) during ejaculation. Seminal vesicles are paired structures located on the dorsal side of bladder triangle. They secrete fructose-rich products and enter the urethra through the ejaculatory duct. The prostate surrounds the base of the urethra, and its contents are sent into the urethra through many small catheters during ejaculation. Prostate secretes a clear liquid, which is slightly acidic and rich in acid phosphatase. Acid, zinc and some proteolytic enzymes. Secretions from bulbar urethral glands or Cooper glands enter bulbar urethra. They provide lubricating fluid to keep the urethra moist. During orgasm and complete erection, their contents can be released before ejaculation, sometimes with a small amount of sperm. Testicular androgen is known to be responsible for maintaining the function of accessory organs. The main function of seminal vesicle and prostatic fluid is to transport sperm during ejaculation, but they may also play some roles in sperm metabolism.