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What Are Gametes?
• Gametes
– mature form of cells set aside which will
undergo meiosis and contribute a haploid
chromosomal complement at fertilization to
reconstitute the diploid
• Egg or oocyte
– large, storage
• Sperm
– small, motility
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Gametogenesis
• Formation of gametes
– primordial germ cell
– proliferation
– become oo- or spermato- gonia
– undergo meiosis
– become gametes
• Mutations here really matter
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Stages of Gametogenesis
• Nomenclature
– gonium -> 1o -cytes -> 2o -cytes -> -tids
• Progression
– 1 premeiotic S-phase (DNA synthesis)
– 2 meiotic cell divisions
– all 4 spermatids = in size
– 1 large oocyte + 2 or 3 small polar bodies
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When do Gametes Differentiate?
• Time of differentiation
– for males from haploid spermatid mature
spermatozoa
– for females during primary oocyte prophase I
• vitellogenesis
• recombination
• growth
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How to Make a Sperm
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Structure of a Mammalian Testis
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What Cell Lineages Contribute to
the Testis?
• Somatic cells
– Leydig
– peritubular
– Sertoli
• Germ cells
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Somatic Testicular Cells
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SEM of Tubule
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Cross Section of Tubule
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What Are Stem Cells?
• Stem cells
– divide to produce 1 cell the same and one
different from mother cell
– thus produces unlimited supply of cells while
retaining a copy in reserve of stem cell
– there are also embryonic and adult stem cells of
various potentials
– in testis, the stem cell population is found
among the spermatogonia
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What Controls Stem Cells?
• Spermatogonial stem cells
– mutations W (white-spotting) and Sl (steel)
produce defects in pigment, blood and sterility
– affect stem cell signaling
– W = c-kit gene encoding growth factor receptor
• transmembrane protein
– Sl = KL encoding kit ligand
• both on cell surface and secreted
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How Do We Know W or Sl Are
Important?: Mutants in Kit
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How Do We Know? Experiment
• KL signaling
– c-kit expressed in 1o germ cells and
spermatogonia
– KL expressed in Sertoli and other cells
interacting with germ cells
– experiment: inject antibody to c-kit into
peritoneum
• blocks binding of KL to c-kit and inhibits
proliferation
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Are All Testis Cells Subject to
Same Signals?
• Blood testis barrier
– Sertoli cells make tight separation between the basal and adlumenal (near the lumen) compartments
– spermatogonia are in the basal compartment but the others are in the microenvironment of the adluminal compartment controlled by the Sertoli cells
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Cross Section of Tubule
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Are Cells Only Controlled by
External Signals?
• Male germ cells often form nests connected by cytoplasmic bridges by incomplete cell division
• May contribute to synchrony of development in the differentiation phase (spermiogenesis)
• It takes 65 days from spermatogonia to spermatozoa in humans
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Spermatogonial Syncitium
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Syncitia in Spermiogenesis
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Do Testicular Somatic Cells
Communicate?
Sertoli
Peritubular
Leydig
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Do Pituitary and Testis
Communicate?• Communication axes
– brain testis
• brain: luteinizing and follicle stimulating hormones
(LH, FSH)
• testis: inhibin, activin, follistatin
– somatic somatic
• peritubular PModS Sertoli
• Leydig testosterone Sertoli and peritubular
• Sertoli stimulatory and inhibitory factors
Leydig
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Do Somatic and Germ Line
Communicate in Testis?
• Communication axes (cont.)
– somatic germ line
• germ line: basic fibroblast growth factor, nerve
growth factor
• somatic: testosterone, various growth factors
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How is Cell Differentiation
Accomplished to Make Sperm?
• Major alterations to make mature sperm cell
– Golgi acrosomal vesicle
– nucleus compacted and new DNA binding
proteins
– centrosomes associate with head
– mitochondria fused or collected in midpiece
– flagellum
– shedding of excess cytoplasm
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Morphogenesis of Spermatids
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Is Spermiogenesis Progressive?
All spermatids
All haploid
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Are There Functional Differences in
Mature Sperm Surface?
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Internal Specialization of the
Head: Nucleus and Acrosome
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Is There Great Diversity in
Mature Sperm Morphology?
E.B. Wilson, 1898
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To Be a Good Sperm, It’s Not Enough to
Be Beautiful and Run Fast
Sperm are differentiated for many tasks: e.g. swimming, egg
recognition, penetration of egg coats, fusion, delivery of genome
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Is There Biochemical Diversity
Among Animals?
• DNA packaging proteins
– highly conserved histones in somatic cells
• basic DNA binding nucleosome forming proteins
– in spermatozoa
• wide array of different types
• Hypothesis• no replication, transcription, metaphase chromosomes in
spermiogenesis
• removes all evolutionary constraints associated with these
processes
• proteins free to diverge