Spermatogenesis and the Male Tract I Flashcards
Testis
Main Exocrine and Endocrine product
Exocrine: spermatozoa
Endocrine gland: mainly testosterone
Germ cell origin and early development of the Testis.
Germ cells can dervie to be M/F so early on cannot be distinguished.
Found outside the embryo on the posterior side of the yolk sac.
- Therefore these germ cells migrate early on, up around the hindgut, through the dorsal mesentry, opposite to where the aorta is developing, split and then come to lie in two long genital ridges (in what will become the pelvic cavity).*
- They take up an association with migrating epithelial cells covering the genital ridge.*
If a female, the epi. cells become Granulosa cells of the follicle
if a male, the epi cells become Sertoli Cells feed+nourish development
Describe the structure and subsequent differentiated of an ‘indifferent Gonad’
Genital ridge swells and shortens, and hangs from a mesentry of the body wall.
Germ cells (with their epi. associations) are distributed all over the gonad. Through gene signals, you develop into a specific gender.
If Ovary: germ cells are spread around the edge of the structure
If Testis: germ cells cluster and form cords in the centre (forerunner of the seminiferous tubules)
What are the origins of the these early development Testis Cell Types.
- Spermatogonia
- Sertoli Cells
- Leydig Cells
- Myeloid Cells
- Spermatogonia from Germ cells
- Sertoli Cells from epithelial cells
- Leydig Cells from Interstitual tissue between cords
- Myeloid Cells from interstitual tissue between cords wrap around seminiferous tubules; are contractile
When and how does the descent of the Testis occur?
In the 7th to 8th month of Fetal Life.
- Testis in the abdominal position starts to move
- Move inferiorly, come up against the lower abdominal wall.
- A weakening will form, the inguinal canal develops
- They move outside of the body, into the scrotal sacs (as the temperature is lower which is required for spermatogenesis)
What controls the Descent of the Testis?
This is under Hormonal control:
From abdominal cavity → internal inguinal ring: Anti-mullerian Hormone (AMH)
From inguinal canal to scrotal sac: Testosterone
Mechanism unclear, but we know they influence/act on the gubernaculum.
gubernaculum: Attached to the lower pole of the testis, effectively a ligament with some striated muscle cells, may be some muscular action that helps pull/draw down the testis OR a change/shortening of the gubernaculum itself
What happens when there’s a failure of the Testis to descend?
A condition called ‘Cryptorchidism’
- Can be unilateral or bilateral
- Can be Incomplete (90%) or Maldescent
Maldescent: gets to anterior abdominal wall, perineum or thigh- ectopic. Goes in wrong direction! (whereas ‘incomplete’ is right direction but gets blocked)
- Affects 3% at full term, and 30% of premature babies
- Majority of these self-correct within 3 months
- Can correct surgically
- True incidence is much lower, 0.1-0.2%
What are the consequences of Cryptochidism?
Failure to descend leads to infertility due to the body temperature too high for spermatogenesis!
Therefore it’s important to check for testes in scrotal sacs at birth!!
How do the Testosterone levels change in the early stages of development
Start low, and then
- Peak in the fetal period for “brain programming” to ‘maleness’
- Drops right done so very low at birth
- Peak at 2-3yr neonatal stage for “gonadal programming” for preprogramming of gonadal tissue
- Drops once again until puberty
- During Puberty hugely increase, and this remains relatively constant so there’s no male menopause!
Changes of testes at puberty
- Marked increase in proliferation of spermatogonia
- Cords develop a lumen; become seminiferous tubules
- Beginning of Sperm production
The Seminiferour tubules are divided into….
Describe the passage of sperm from within the testis
The Seminiferous Tubules (100m) are divided into fibrous compartments with fibrous divisions.
- The tubes are double ended and join into the Rete Testis network.
- Rete testis feeds out into another coiled structure that runs around the side of the testis, the Epididymus (6m).
- The Head (larger, more packed w tubule), body and tail of the epididymus leads into a section which is less folded, with a thicker muscular coat and this leads into the Vas Deferens (4m)
Histology of the Seminiferous Tubules
- Spermatogonia: found at the periphery of tubule, in a single layer of cells. Have a full set (2n) of chromosomes that divide and move towards the Lumen.
- Primary Spermatocytes: quite Large
- Spermatids: small rounded cells
- Thick walled for lots of activity
- Lots of Interstitual tissue/space around the tubules
- Lots of BV to get Testosterone to general circulation + germ cells
- Sertoli Cells: Often look like ‘►’ sitting on the BM, where in actual fact you are only seeing the nucleus and the cell body extends to the lumen. Nurse ‘nourishing’ cells.
- Leydig Cells: Pale cytoplasm, rounded cells in the interstitual space. Produce Androgens ‘testosterone’
What’s interesting about the shape of the Sertoli Cells?
- Sits on the Basement Membrane and extends out to the lumen
- Groups of Sertoli cells with germ cells between them. Germ cells move up through this convuluted pathway during development, and are enveloped in the Sertoli cell membrane.
Have Junctional Complexes: tight junctions between cells, that seperate the spermatocyte (undergoing meiosis) from Spermatogonia (mitosis). This avoids messing up and crossing of mis-matched genetic information. Spermatogonia are allowed to pass through only!
What are Myofibroblasts?
Little cells that sit on the outside of the Seminiferous tubule, can squeeze the tubule to move the fluid down the tubule lumen!
What are the 3 main stages of Spermatogenesis?
- Spermatocytogenesis: mitosis
- Meiosis: reduced # chromosome, 2 divisions
First division: halves number of chromosomes
Second Division: double number of cells again to 4 - Spermiogenesis
See pg 28 of module book!
Look and describe pg 29 spermatogenesis diagram
The spermatatogonia divide,
- One half stays at the basement membrane, as a back up layer
- The other half just keeps dividing and moves through the Junctional Complex to become a 1º spermatocyte.
- This is now a slow process of Meiosis; takes around 3 weeks (which is why you can usually see lots of them)
- First division produces 2º spermatocyte, which are only around for a short time (chromosome number has reduced ‘haploid’)
- Once undergone 2nd meiotic division, it is now a spermatid. Th.ese are little Round cells → elongated with dark nuclei
The Golgi Phase of Spermatid Development
Golgi Phase: Rounded cells, opposite to a forming flagellum there is a Golgi; (where proteins get added to carbs produced in the ER) and vesicles of protein+carbs fuse and become large, membrane bound Proacrosomal Granules (which will form the acrosome)
The Cap Phase of Spermatid Development
Cap Phase: granules keep fusing, and eventual form a cap of acrosomal cap, and the whole structure is called an acrosome that sits atop the nucelus like a beanie.
Describe Spermatid Release into the lumen!
- Tails alway hang into the lumen
- Heads of the sperm are embedded in sertoli cells
- Cytoplasm gets pulled off the sperm, helping the streamlining process,
Leftover cytoplasm forms a residual bulb that gets phagocytosed/internalised and recycled by Sertoli cells, forming nutrients for future spermatids/spermatogonia coming up from the basement membrane
What ende product are you left with after Spermatogenesis?
A mature Spermatozoan.
One of the most specialised cells in the body.
Highly streamlined, lots of mitochondria in the midpiece.
fibrous sheath within the tail.
Periodic Release of Spermatozoa into the lumen comes every ____ days in ___________.
Why does this occur?
Periodic Release of Spermatozoa into the lumen comes every 16 days in successive waves.
The interval at any point in the lumen is 16 days.
This is because sperm are all linked from the initial spermatogonia to the spermatozoa by cytoplasmic bridges, that are all marching together in unision to the lumen.
Sperm is produced 300-600/second
