Lecture 8: Spermatogenesis Flashcards
Spermatogenesis - Definition
production of spermatozoa (male gamete) by the testis
consists of all cell divisions and morphological changes of the developing germ cells
takes place entirely in the seminiferous tubules
3 Phases of Spermatogenesis
- PROLIFERATION
- mitotic divisions of spermatogonia (A-spermatogonia –> B-spermatogonia)
- stem cell renewal- supply of spermatogonia continues indefinitely - MEIOTIC
- B-spermatogonia differentiate into primary spermatocyte and undergo meiosis
- Meiosis I- DNA replication and crossing over
- Meiosis II- secondary spermatocyte divides and forms two haploid spermatids - DIFFERENTIATION
- no cell divisions
- spherical spermatid differentiates into a highly specialized spermatozoon
Crossing Over in Meiosis
segments of one chromosome crossing to a homologous chromosome, resulting in random assortment of DNA segments between chromatids and insuring genetic diversity in daughter cells
at the end of meiosis, each newly formed spermatid is genetically unique
as consequence, spermatids carry potential antigens for the immune system, the blood testis barrier is therefore, critical to spermatogenesis
Role of a spermatozoon
travel (or swim) through the female reproductive tract
deliver the male genetic material
activate the oocyte for zygote formation
The differentiation phase is subdivided into 4 phases
- GOLGI PHASE
small vesicles of the Golgi fuse and form proacrosomic granules, which stay located on one side of the nucleus. centrioles migrate to the other side of the nucleus
Golgi vesicle fusion continues and forms large acrosomic granules. proximal centriole forms the attachment point of the flagellum. distal centriole gives rise to the flagellum itself
- CAP PHASE
acrosomic vesicle forms a cap over one side of the nucleus (anterior). Golgi and other cytoplasmic organelles migrate towards the “distal portion”
Flagellum starts to be formed from distal centriole - ACROSOMAL PHASE
nucleus beings to elongate. microtubules are reorganized and form the Manchette
neck and annulus are formed. flagellum continues development - MATURATION PHASE
manchette forms the postnuclear cap
mitochondria migrates to posterior side of the nucleus and form a spiral assembly that defines the middle piece
annulus forms the connection between middle and principle pieces of the flagellum
Head of Spermatozoon
nucleus- oval, flattened, condensed DNA (disulfide cross-linking; protamines)
acrosome- membrane vesicle with multiple hydrolytic ensymes
- cover the anterior two thirds of the nucleus
- ensymes used for acrosomal reaction in fertilization
post-nuceluar cap- membrane component posterior to the acrosome
plasma membrane- integrity is crucial for survival and function
DNA condensed and highly compacted nucleus
DNA remains fundamentally inert until fertilization
Flagellum (Tail)
capitulum- is the neck of the sperm. attaches the flagellum to the head
Middle piece- mitochondria sheath, powerhouse of the spermatozoon
principle piece - major piece of the flagellum. important for movement
terminal piece- end of microtubule fibers
plasma membrane- covers all segments
mitochondrial helix= spiral assembly of the mitochondria around the middle piece of the flagellum = high production of ATP energy
Spermiation
release of fully differentiated spermatozoa into the lumen of seminiferous tubules
spermatozoa are formed in cycles (or waves), which vary in length according to species
nonetheless, there are always segments of the seminiferous tubules undergoing spermiation, which makes it a continuous and uniform process
constant and uniform endocrine control
Apoptosis of Germ Cells
it is a common event in spermatogenesis in all species, but the rate of apoptosis might be affected by:
- heat stress
- welfare
- trauma
- diseases
- hormone levels
- nutrition
- season
in addition to apoptosis, these factors might also affect the quality of developing germ cells
