[034] Spermatogenesis (GB#110D01)

[034] Spermatogenesis (GB#110D01) | 基礎医学教育研究会(KIKKEN)Lab

● Father absent? The birth of sperm

In men, at the adolescent age, sperm will be mass-produced in the testes. Every day, it continues to produce tens of millions to over 100 million new spermatozoa through cell division for several decades. Because sperm is produced only by men, if you think that the original cell of the sperm is a “father”, in fact it is treated as a mother in the Japanese as a spermatocyte. Whether it is sperm or not, the parent who directly produces it can only be a mother.

Spermatocytes are likely to become “cells of sperm” when translated directly into Japanese, but they are not written anywhere as mothers. Just saying that neither father. Originally in biology, even outside of the country, there is a custom of calling cells just breaking up as daughter cells. As a daughter grows up and becomes a parent, it will be a mother, so it may not be strange that a cell before division is a mother cell. But, is it somewhat lonely to call sperm-making cells mothers? Even though it is mother or daughter, the sex chromosome that the cell has is XY. . .

● Sperm is made in the tunnel

In the testicle, there are a lot of thin long tubes filled with numerous convoluted seminiferous tubules. The length of one is written as 40 cm, 60 cm or 1 m, but in a looped tube, the thickness is roughly 0.2 mm, the description generally coincides. At the center of the tube is a continuous lumen penetrating throughout the testes. Approximately 500 tubes in one testicle are packed tightly in a complex winding manner. Sperm are made in this pipe. Basically, the cells in the wall of the duct (epithelium, seminiferous epithelium) are transformed while gradually moving towards the lumen of the vessel, becoming spermatozoa. The whole pipe is a sperm factory. However, the wall of the seminiferous tubule consists of not only cells that become spermatozoa but also roughly divided into two types: cells that become spermatozoa themselves and cells that support the process.

● The way to sperm is increasing the number first

The way sperm is made begins with spermatogonia sticking just inside the basal lamina surrounding the tube. Among them, there should be a mixture of cells (stem cells) that continue to proliferate and repeat cell division by the basement membrane and cells that turn away from the basement membrane into sperm. However, it is not much differentiated in textbooks. At this location, the spermatogonia seem to go on to the next stage after increasing the number by repeating 6 to 7 somatic cell divisions.
Somatic cell division of germ cells is somewhat strange. Even with cell division, the split cells are not completely cut off, and the cell membrane is partially connected, becoming a halfway state “syncytium”. So repeat the next cell division as it is. Therefore, if you do it well, the same line of dividing cells can be caught all at once and you know the approximate number. When dividing 6 times, that is, 64 pieces, when dividing 7 times, 128 spermatogonial cells are formed.

● The second stage is another world

Sertoli cell The zones of spermatogonia clumps are surrounded by Sertoli cells, which strongly support sperm production. Sertoli cells are tall and large cells that extend from the basement membrane to the lumen. Looking at the cross section of the seminiferous tubule, the spermatogonia cluster and Sertoli cells are alternately arranged side by side. Sertoli cells sandwiching spermatogonia are connected by tight junction on spermatogonia cells and form a kind of barrier separating spermatogonia side and luminal side. It is explained that this barrier named blood-testis barrier works to protect the cells of the subsequent process from the immune mechanism in the body. But from now on it is another world that is close to the outside of the body. As the number of cells increases from the spermatogonia, the properties of the cells change completely when passing through the gap of Sertoli cells and crossing this barrier. Cells that undergo meiosis, become spermatocytes.

● Meiosis occurs twice

Meiosis is a cell division in which two sets (2n) of chromosomes that somatic cells possess are divided into one set (n) of different cells. In the case of humans, since one set is 23, it becomes a division in which 46 are 23. When spermatozoa turn into spermatozoa, it happens twice. I hesitated to hear that. One time will be enough to be half. If you divide “meiosis” by half, it will be a quarter. That’s strange. However, “split in half” is indeed happening twice. Actually, the spermatocytes crossing the barrier replicate each chromosome before division, and the amount of chromosomes in each set is doubled (4 n), respectively. In the first round it becomes half (2 n), in the second it becomes half (n), and in the completed sperm cells the chromosome is one set (n).

Before the first meiosis, it was a secret ingredient that the chromosome doubled like somatic division. Well if it is the same as somatic cell division. However, how to separate chromosomes is different between somatic division and meiosis. In somatic division, chromosomes separate so as to make two of the same set of cells. If it is a male cell, the sex chromosome will always be distributed in sets of X and Y. On the other hand, in meiosis, X and Y are separated apart at the first time. Sex chromosomes are separated into twice the X set and the twice Y set. In the second meiosis, each cell is distributed in half as it is, then no more cell division.

● Sperm’s mother is also a daughter

Spermatocytes exit the barrier and undergo meiosis twice. The first meiotic division is called primary meiosis. The cells before the primary meiosis are called primary spermatocytes. Cells formed by primary meiosis will become secondary spermatocytes. Second spermatocytes with n × 2 times chromosomes do not replicate chromosomes, so they immediately enter the next secondary meiotic division and produce two spermatids. In Japan, Primary spermatocytes and Secondary spermatocytes are sometimes called spermatocytes (maternal cells) and prespermatid (daughter cells), respectively. Secondary spermatocytes will divide without growing, so if you take the view that you will not grow up into adults and give birth to their children as a daughter, this one might be better. As with spermatogonia, the division of spermatocytes does not completely separate, but it seems that the four siblings and sisters remain connected together even if they become sperm cells.

● Hentai sperm

sperm formation
Two sperm cells with X chromosome and two sperm cells with Y chromosome can be produced, respectively, through meiosis twice from one spermatocyte.Sertoli cells by the side have helped transform the spermatocytes. However, it is the last big work of Sertoli cells that wraps up sperm cells, gives necessary nutrition, removes unnecessary things, condenses the contents of the cell nucleus, grows the tail, and gradually completes to the shape of distinctive spermatozoa it is. This process is called transformation (sperm transformation). “Mature” as spermatozoa is still ahead but as a form it is completed while surrounded by the tip of Sertoli cells. And when the time comes, sperm are released into the lumen at the same time from the tip, and the sperm that has fallen apart goes to the epididymis outside the testes. Until it is finished, sperm are not given the ability to swim by themselves until they are ejaculated.

● Sperm is mass-produced everyday, like a wave

When reading the textbook, it says that it takes more than 70 days for the spermatozoa to be formed from the state of the spermatogenic cells within the convoluted seminiferous tubule. However, this is probably due to the calculation when it is considered to repeat a certain number of multiplications towards spermatocytes after switching from proliferation of stem cells to spermatogonia. In a recent study, even though it is split and proliferated as a spermatocyte, it sometimes tears back and returns to a short group, only one tears, returns to stem cells, and goes back and forth pretty well. Then you do not know where to count the number of days from where? Still the process of sperm formation seems to be pursued pursuantly on a proper schedule. When you look at only a narrow spot in the convoluted seminiferous tubule, sperm popping out is repeated with a constant rhythm every approximately 16 days (?) 【Broken link, details of the number of days unknown】. It seems that this cycle is shifted by one day along the long axis direction of the tube, so that sperm formation waves move inside the long tuning seminar tube every day every 16 days.

● Hormones necessary for the birth of sperm

This process of spermatogenesis is promoted by the male hormone called testosterone, which is made just outside the convoluted seminiferous tubule. Stromal cells named Leydig cells make this testosterone. Since testosterone is a steroid hormone, it diffuses to the seminal epithelium beyond the cell membrane and basement membrane, works strongly on Sertoli cells and boosts spermatogenesis.Sertoli cell1
No matter where you look at the process of sperm formation, there are no cells named “father” directly to cells that lead to sperm cells. But Sertoli cells that take a male hormone and promote the growth of sperm firmly with a strong force may be said to be father of sperm.

○ Referenced sites

Sperm development
Sperm development (Microscopic Image Gallery)
・Period of spermatogenesis and story of wave and vitamin A[Broken link / missing]
We discovered that the number of spermatozoa from one stem cell varies periodically,Kyoto University, Japan Science and Technology Agency (JST), August 9,
減数分裂:Tokyo Medical University Department of Paediatircs Genetics Study Group   
(March 19, 2010) Many, long, secret to keep making sperm ~ New differentiation model in mammalian spermatogenesis ~ 

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○ Referenced books

プロッパー細胞生物学: 細胞の基本原理を学ぶ,化学同人
カラー図解 人体の正常構造と機能 全10巻縮刷版,坂井 建雄,日本医事新報社
人体機能生理学,杉 晴夫,南江堂
トートラ人体解剖生理学 原書8版,丸善

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