Learning Objectives:
- Understand the role of meiosis in gametogenesis
- Understand the similarities and differences between oogenesis and spermatogenesis
- Identify the microscopic anatomy of the testes
- Know the stages of spermatogenesis, what is happening to the cells at each stage and the DNA content at each stage.
- Identify the microscopic anatomy of the ovary
- Know the stages of oogenesis, what is happening to the cells at each stage and the DNA content at each stage.
• Gametogenesis is the general term for the production of gametes.
• Gametes are often referred to as germ cells.
• The mature male gamete is called sperm.
• The mature female gamete is called an egg, ovum or oocyte.
• Gametes are the cells that will fuse during fertilization to produce a single celled zygote. The zygote will increase in size and complexity by dividing to produce more cells.
• In organisms the number of chromosomes found in the adult is specific for that species. For example, humans have 23 pairs of chromosomes (46 total chromosomes). We have 2 of every chromosome. We refer to this number as diploid (or 2n). In order for each member of the species to keep the same number of chromosomes the gametes need to undergo a special type of cell division called meiosis.
• During meiosis the number of chromosomes is halved. The gametes are haploid (n).
• Meiosis only occurs in organs called gonads. In males the gonad is the testes, in females it is the ovaries.
Meiosis
• The process that produces haploid gamete cells. Haploid cells have half as many chromosomes as the parent (diploid) cell.
• Gametes are genetically distinct from BOTH the parent diploid cells that produced them and different from each other.
• Gametes are produced in organs called gonads (testes and ovaries) though meiotic cell division.
• Review Figure 2.1 to see the differences between Mitosis and Meiosis.
• NOTE that there are 4 stages of Mitosis (prophase, metaphase, anaphase and telophase). At the end of Mitosis there are 2 diploid daughter cells that are genetically identical to each other and the original parent cell.
• In Meiosis there are two rounds of division (Meiosis I in which homologous chromosomes separate, and Meiosis II, in which the sister chromatids separate). Each of these rounds has the same 4 stages as mitosis. The result of Meiosis is 4 haploid cells that are genetically different from each other and from the original diploid parent cell.
Karyotype
• A karyotype is a display of chromosome trapped in their most condensed form.
• This is during metaphase. The chromosomes are easiest to see and identify at this stage.
• A picture of the chromosomes is obtained and then the chromosomes are sorted and aligned. They are sorted such that the homologous pairs of chromosomes are paired with each other.
• They are aligned so that the largest chromosome pair, chromosome 1, is in the upper left-hand corner and the smallest chromosome displayed last.
• This can be used to see if the proper number of chromosomes is present. It can also be used to see if there are any gross chromosomal defects present.
Gametogenesis
• The term for gametogenesis in males is spermatogenesis. In females it is called oogenesis.
• Review Figure 2.2 for the similarities and differences between spermatogenesis and oogenesis.
• Primordial germ cells are an early population of cells that give rise to the germ cells. Primordial germ cells are typically identifiable early in development.
• Germ cell is a generic term for any cells that are, or will become, gametes (sperm or eggs).
• In both spermatogenesis and oogenesis we start with a single diploid cell in the Gonial stage (a spermatogonia or an oogonia). These cells divide my mitosis to increase the number of gonial cells.
• The gonial cells grow and then enter meiosis 1 as primary (1') gonocytes (1' spermatocyte or 1' oocyte). The 1' gonocytes then completes meiosis 1.
• Once these cells have completed meiosis 1 they become haploid secondary (2') gonocytes (2' spermatocytes or 2' oocytes).
• Notice that the primary spermatocyte is the same size as the original spermatogonia. However the primary oocyte is larger. During the primary oocyte stage the ovum is growing. It needs to obtain nutients to help nurture it through the first few divisions should it become fertilized.
• The secondary gonocytes undergo meiosis II to produce gonotid cells (spermatids or ootids),
• Although we start out with a single diploid cell in both spermatogenesis and oogenesis. we do not end up with the same number of viable gametes.
• At the end of spermatogenesis we have 4 haploid spermatids. These 4 spermatids will undergo a process called spermiogenesis (not to be confused with spermatogenesis) to mature into 4 spermatozoa (mature sperm).
• In oogenesis we end up with one very large oocyte and 3 small polar bodies. These seems like a lot of genetic waste! In order for the oocyte to have enough cytoplasm, rich in nutrients and organelles, needed for the first few cell divisions, all of the cytoplasm is given to one cell.
• Let's look at spermatogenesis and oogenesis in more detail.
The Testes
• The testes are surrounded by tough sheath of connective tissue called the Tunica albuginea (not seen in Figure 2.3)
• Figure 2.3 shows a cross section of the testes that is magnified. Within the testes notice the many cross sections through tube like structures. These are the Seminiferous Tubules, an epithelial tissue that produces the sperm.
• The seminiferous tubules have a free apical surface facing the lumen and a basal surface sitting on a basement membrane made of type IV collagen, laminin, fibronectin and heparin sulfate proteoglycan
• Let us zoom in and look at epithelium more carefully.
• In Figure 2.4 we are zoomed on one of the seminiferous tubules. Notice there are 2 types of cells:
1. Sertoli cells – non-dividing columnar cells that extend from basement membrane to lumen. They have irregular shaped nuclei with a light stain. that make an unbroken continuous ring around tubule.
2. Interstitial cells (Leydig) cells lie between Seminiferous Tubules. The Leydig cells secrete testosterone in response to another hormone called Luteinizing hormone produced by the anterior pituitary gland.
• Testosterone is needed for both spermatogenesis and the development of of secondary sexual characteristics
another hormone produced by the anterior pituitary gland, Follicle Stimulating Hormone (FSH) targets Sertoli cells causing them to increase cAMP levels and secrete androgen proteins.
• Most males make many sperm – up to 300 million/day. This requires an enormous surface area. Quantity is important sperm counts less than 20 million/ejaculation is considered sterile.
• Many things can cause sterility. Anything that increases body temperature like tight jeans, fever, hot baths ca cause sterility.
• Sperm are very temperature sensitive. They require a temperature that is 1.5-2 degrees colder than normal body temperature to mature properly. This is the reason why they are held in scrotal sac outside the body cavity In humans sperm production begins to decline at about age 45, but not enough to cause sterility.
• We will be looking at spermatogenesis in mammals.
Spermatogenesis
• In Figure 2.4 you should notice the male germ cells are at various stages of differentiation. Notice that they mature as the move closer to lumen.
• Germ cells at all stages of development are nestled on indentations in Sertoli cells.
• The Function of Sertoli cells is to support and nourish the germ cells and translocation of sperm toward lumen for eventual release.
• The Sertoli cells create a blood-testis barrier which isolate sperm from the immune system.
Notice each of the following stages of sperm development in the diagram of spermatogenesis in Figure 2.2 and the image of a magnified seminiferous tubule in Figure 2.4:
1. Spermatogonia (2n) – dark, granular, round or slightly oval nuclei near the basement membrane. They multiply by mitosis.
2. Primary (1') spermatocytes (2n) in the layer above spermatogonia. They are in period of growth. The nuclei are large and stain relatively dark.
3. Secondary (2’) spermatocytes (1n). You may not always see any because they almost immediately go onto 2nd meiotic division. You may find them by looking for cells that are ½ the size of the 1’ spermatocyte.
4 . Spermatids – ¼ size of 1’ spermatocytes and closest to lumen
5. Spermatozoa - easily identified by their long tails extending into the lumen. If you can't see these long skinny tails you may need to increase contrast bey decreasing the light.
Spermiogenesis
● Is the process by which spermatids mature into spermatozoa.
● Spermatids in some tubules form a layer 3-4 cells thick are undergoing the complex process of differentiating into spermatozoa
● During spermiogenesis the spermatids undergo the following changes:
1. Elongation of nucleus in what is becoming the head of the sperm and formation of the acrosomal cap.
2. Formation of flagellum extending into lumen.
3. Excessive shedding of cytoplasm which is pinched off and appears as dark residual bodies near lumen. This will be phagocytized by Sertoli cells.
4 . When fully formed the tip of the head remains attached to Sertoli cells for a short period.
5. Eventually they are released into lumen.
● The entire process takes about 48 days in rats, and about 64 days in humans (from spermatogonia to spermatozoa).
● Figure 2.5 shows the parts of a mature spermatozoan.
The Ovary
● We will be focusing on oogenesis in mammals. Oogenesis occurs in the Ovary.
● This process produces far fewer germ cells than in males.
● Females are born with about 800,000 eggs, most in oogonial stage (undergo mitosis). This occurs before birth. At the time of birth most germ cells exist as primary (1’) oocytes, but some are still oogonia.
● For 30-40 years that a female has her menstrual cycle she will ovulate about 400 eggs. The remaining 99% will degenerate. There are many more unused sperm.
● The teardrop-shaped ovary is surrounded by a layer of connective tissue. Just like in the testes this is called the Tunica albuginea.
● The Hilum is at narrow end of the ovary. Often on prepared slides of the ovary you see remnants of the mesovium, a narrow rope like connective tissue that attached the ovary to broad ligament within the body cavity
● Look at Figure 2.6. You should notice the ovary has an thick outer cortex and an inner medulla
● The Inner medulla contains large blood vessels, lymphatic tissue, nerves and connective tissue.
● The Outer cortex contains many circular follicles.
● Follicles contain a single egg surrounded by one or more layers of nurse or follicle cells.
● The function of the nurse or follicle cells is to support and nourish the developing germ cells.
Oogenesis and the follicle
● Oocytes develop inside a layer of cells called the follicle cells.
● The primordial follicle contains a primary oocyte surrounded by a single layer of squamous epithelial cells arrested in first meiotic division. ● Most follicles in the ovarian cortex will be at this stage.
● Starting at puberty until menopause, each menstrual cycle begins with the growth of 20-50 primordial follicles are stimulated by Follicle Stimulating Hormone (FSH) to begin to grow.
● As soon as it begins to grow it is now called a –Primary Follicle. The Follicle cells become cuboidal and/ or columnar and increase in number. The many layers of follicle cells surrounding the primary oocyte are called the Stratum granulosum. These follicle cells provide nutrients and secrete fluid, called liquor folliculi that accumulates in spaces between the follicular cells.
● Once this space, called the Antrum, appears the follicle is now a Secondary Follicle. The Antrum eventually becomes enormous and fills most of the follicle.
● The primary oocyte is still surrounded by follicle cells. These cells are called the Cumulus oophorous.
Once the antrum becomes huge the follicle is now considered a mature Tertiary Follicle or Graafian follicle.
● The Graafian Follicle (Figure 2.8) contains a large amount of fluid. Some substances in the fluid keep oocyte in meiotic arrest.
● Not all growing follicles become Graafian follicles. In humans usually only 1 will reach this stage each month.
● The other follicles degenerate (or atresia– atretic follicles)
● These will be numerous on the slide. The cells in these follicles look misshaped and the nuclei stain darkly.
● Look carefully at oocytes in each follicle type in Figure2.7 Notice it is much larger during follicular growth. Oocytes in primordial follicle are about 25-30um in diameter.
● Observe the Graafian follicle in Figure 2.8. As follicles mature cells outer follicle cells coalesce around the follicle and form the Theca.
● The Theca eventually forms 2 layers; The Theca interna is closest to the follicle. It secretes a nutritive fluid for follicle cells and some estrogen and androgen intermediates that follicle cells convert to estrogen
● The Theca externa is the more fibrous outer layer.
● Immediately surrounding the oocyte is a translucent area called the zona pellucida. The zona pellucida contains glycoproteins called ZP1, ZP2, and -ZP3. These glycoproteins play a major role in fertilization, inducing activating the acrosome reaction in the sperm and eventually being altered by cortical granule substances released by the fertilized egg. In their altered form they prevent polyspermy.
● The Corona radiata (meaning “radiating crown”) is a layer of follicle cells immediately surrounding the oocyte.
● When the oocyte is ovulated these follicle cells remain attached. At fertilization, the sperm will have to penetrate the corona radiata and the zona pellucida in order to get to the oocyte surface.
● Ovulation of the oocyte is triggered by a surge in luteinizing hormone (LH) secreted by the anterior pituitary gland.
● Just prior to ovulation, the oocyte completes the first meiotic division and is ovulated as a secondary oocyte. The second meiotic division will begin in the upper end of the oviduct, only to enter meiotic arrest once again. It will not complete meiosis II until after fertilization
● The Graafian Follicle now becomes an endocrine gland, secreting progesterone and estrogen and is now called the Corpus luteum. These hormones maintain the uterine lining in preparation of an implantation.
● The Corpus luteum looks collapsed with folding edges and are very large.
● If there is no pregnancy the corpus luteum decline (~14 days) and decline brings on menses.
● The Corpus luteum is replaced with scar tissue and is now called the corpus albicans.
● If pregnancy occurs, the Corpus luteum grows to about 5cm and continues to secrete hormones until about the 9th or 10th week of pregnancy. Then placenta takes over the job of secreting these hormones.