Gametogenesis (that's just a fancy word for the process in which sex cells are produced)
The sex cells are produced through a process involving several cell divisions. These include mitosis (see my post on Cell Structure and Mitosis for more details) and meiosis, which I'm going to talk about here. Meiosis differs from mitosis in that, while mitosis results in two "daughter" cells, each with a full set of chromosomes (46 in humans), meiosis results in four "daughter" cells, each with only half a set of chromosomes (23 in humans). Halving the number of chromosomes is necessary since a zygote is formed from two sex cells, and if each sex cell had the standard number of chromosomes, the resulting zygote would have double the number of chromosomes.
Meiosis starts off the same way as mitosis: the information in the cell duplicates and chromosomes become visible. There are 23 pairs of chromosomes, the first 22 numbered 1-22, and the last pair being the sex chromosomes (XX in females and XY in males). During prophase, the chromosomes pair off in a process called synapsis, each pair appearing as four strands twisted together. During metaphase, the pairs line up along the equator of the cell. During anaphase, the pairs separate, so the two chromosomes from each pair move to opposite ends of the cell. At the end of this division, two daughter cells are produced, each containing half the number of parental chromosomes. Since they contain half the amount of information, they're known as haploid cells. (Cells containing a full set of chromosomes are known as diploid cells.)
But wait.
...
...
...
...
There's more.
After this first division, the two daughter cells divide yet again. The stages in this division are simply referred to as prophase II, metaphase II, anaphase II and telophase II (there's no second interphase as the genetic material does not need to duplicate again). This division is pretty similar to mitosis in that the chromatids of the chromosomes are separated and sent to opposite ends of the cell. This results in four new cells which contain half the number of chromosomes of the original cell.
Let's have a closer look now at the two types of gametogenesis (gamete development)- spermatogenesis and oogenesis.
Spermatogenesis
Spermatogenesis is the production of sperm. It occurs in the seminiferous tubules of each testis (for more info on the bits and pieces of the reproductive system, take a look at yesterday's post). These tubules are lined with spermatogonia, or sperm mother cells, which are immature cells containing full sets of 46 chromosomes. When puberty begins, these cells begin dividing by mitosis, resulting in more spermatogonia cells. This means that there is a continuous source of spermatogonia for the production of sperm.
Of course, if spermatogonia just kept dividing by mitosis forever, there wouldn't be any sperm! Therefore, some spermatogonia are instead pushed towards the centre of the tubule, where they begin to grow and are called primary spermatocytes. These are also diploid cells (i.e. 46 chromosomes), which then undergo meiosis to produce secondary spermatocytes (after the first division), which are haploid, and then spermatids (after the second division), which are also haploid. Now the spermatids begin to mature: they lose much of their cytoplasm and form tails to become spermatozoa (a.k.a. sperm). Special cells that extend from the outer portion of the seminiferous tubule into the centre keep the maturing sperm nourished. This process, from spermatogonium to spermatozoa, takes roughly 72 days, and occurs continuously after puberty.
Sperm are only around 0.06mm long, and are made up of a head, neck, middle piece and tail. The tip of the head contains a fluid filled vesicle which, in turn, contains enzymes. These enzymes help to break down the layers surrounding the egg. The rest of the head consists almost entirely of the nuclear material. The middle piece contains mitochondria and a tiny bit of cytoplasm. The tail has a simple but necessary function: to propel the cell forward, towards the egg.
Since most of the cytoplasm is lost after the spermatids mature into sperm, sperm have a very short survival period. They are nourished mainly from the semen.
Oogenesis
(Completely random and unnecessary warning: whoever discovered this process must have really loved the letter "o" and/or the sound "oo," as you shall soon see.)
The production of ova is known as oogenesis. It's somewhat similar to spermatogenesis, but while four sperm are produced from one spermatogonium, only one ovum is produced from one oogonium (the other 3 products are small pathetic things that die off).
Now for a bit more detail!
As I stated in my previous posts, millions of egg mother cells are already present at birth. These are called oogonia (singular oogonium), which are diploid cells. Additionally, by the time of birth, these oogonia have grown into primary oocytes and have reached the first prophase of the first meiotic division. Each primary oocyte is surrounded by a single layer of cells, forming a primary follicle.
Follicles begin to grow and mature when puberty begins. When a follicle matures, the primary oocyte undergoes the first meiotic division, resulting in two haploid cells of unequal size. You see, even though each cell gets the same amount of chromosomes, one gets much more cytoplasm than the other. The one that gets hardly any cytoplasm is known as the first polar body, which may or may not divide again to produce two additional polar bodies, but in the end they all disintegrate anyway. The daughter cell that gets much more cytoplasm, on the other hand, becomes a secondary oocyte.
The secondary oocyte initially doesn't get past metaphase. Ovulation then occurs, pushing the secondary oocyte and the aforementioned first polar body into the Fallopian tube. If the ovum meets sperm, meiosis is completed, producing two haploid cells of unequal size. The smaller one is the second polar body, which eventually disintegrates. The larger one is the one that eventually matures into the ovum.
(There's also another type of cell called an "ootid" that appears in one of the diagrams in the book between "secondary oocyte" and "mature ovum," but the text helpfully doesn't cover that part. I'm assuming it's some kind of intermediate stage between the two.)
Fertilisation
In order for the sperm to meet the egg and fertilise, sexual intercourse, or coitus, has to take place. I'm sure you know what it is so I'm just going to rattle off a few random "fun facts" about semen here:
- An ejaculation occurs from rhythmic contractions of the epididymis, the vas deferens, the seminal vesicles and the prostate gland, which push the semen (containing sperm) out of the body.
- An ejaculation expels around 3mL of semen, containing 250-300 million sperm as well as the secretions of the seminal vesicles, bulbo-urethral glands and prostate gland.
- The semen is slightly alkaline, which helps to neutralise the acid nature of the male urethra and female vagina.
If you read that second point, you might think that 250-300 million sperm is an awfully large amount of sperm to be going after one egg. Well, there's a reason why so many sperm are needed, and that is that they die easily (i.e. they have a high sperm mortality). Only a few thousand reach the uterine tubes, and several thousand of these have to surround the egg to break down the stuff around it, since the amount of enzymes in each sperm is quite small. However, only one lucky sperm will enter the egg- once the sperm enters, a fertilisation membrane forms, preventing other sperm from entering. Oh, and once the sperm enters, the egg cell finally gets to complete meiosis. Yay!
I've spoken a couple of times about the "stuff surrounding the egg" that the sperm has to break down without really talking about what it is! Well, this "stuff surrounding the egg" is basically a layer of cells known as the corona radiata, which are held together by an acid. It's this acid that is broken down by the enzymes of the cell.
Anyway, back to the sperm entering the egg. Once it's entered the egg, the tail is absorbed and the head becomes the male pronucleus, which moves through the cytoplasm towards the female pronucleus- the nucleus of the egg. The two cells fuse to form a zygote, which is a diploid cell. It's this cell that's eventually going to become the baby. Yay! (Or ARGH!!! NOOOOOOOOO!!!!!!!!!!! if you hate kids.)
Now fertilisation can't just happen at any time. Sperm can hang around for about 72 hours, and the egg can only be fertilised for around 12-24 hours. Hence fertilisation can only occur if sperm is deposited up to 72 hours before ovulation, or up to 24 hours after ovulation. Also, successful ovulation doesn't necessarily result in a successful pregnancy. There are lots of things that can go wrong, as I'm sure you all know.
Hormones
Various features of the reproductive system depend on hormones, secreted by the endocrine glands ("endocrine"= excrete stuff into the body- see my post on tissues), for regulation and control. Hormones are secreted into the extracellular fluid around the cells in the gland. After that, the hormones go into the capillaries to be transported to the target organ(s).
One important endocrine gland is the pituitary gland, located below the brain and above the mouth. There are two hormones secreted by the pituitary gland that affect the gonads (sex organs).These are known as gonadotropic hormones, or gonadotropins.
One of these hormones is called the follicle-stimulating hormone (FSH). In females, this stimulates the development of the ovarian follicle. During the development of the ovarian follicle, oestrogen is stimulated, and as oestrogen is produced, less FSH is secreted. In males, FSH stimulates the production of sperm.
The other gonadotropic hormone secreted by the pituitary gland is luteinising hormone (LH). In females, this promotes final maturation of the ovarian follicle, ovulation and the formation of the corpus luteum. The corpus luteum produces progesterone as well as oestrogens. As progesterone levels increase, less LH is secreted. In males, LH stimulates cells in the testes to secrete the hormone testosterone, which is important for sperm production.
Another hormone in females involved in the reproductive system is human chorionic gonadotropin (HCG). This hormone is produced in the developing placenta in a pregnant woman, and it maintains the corpus luteum.
Aside from gonadotropic hormones, the pituitary gland also secretes lactogenic hormones, or prolactin. This affects the breasts of women, and is important in milk production.
Next up- pregnancy and birth! (When will all this end?!)
No comments:
Post a Comment