Main Page Dynamic Development
The Foundations of Developmental Biology
From Sperm and Egg to Embryo
Genetic Regulation of Development
Organizing the Multicellular Embryo
Generating Cell Diversity
Dynamic Development at a Glance
The Developmental Biology Journal Club
Developmental Biology Tutorial
How do transcriptionally inert spermatids differentiate into spermatozoa?
Spermatids are spherical cells with centrally located nuclei. How are they transformed
into functional spermatozoa? The morphological events during spermiogenesis are discussed
in your textbook, and you should read them carefully. We shall focus here on nuclear
modifications and their consequences.
The major events in nuclear modification are: chromatin condensation and morphological
Chromatin condensation helps to streamline the cell by reducing volume. It also may serve
a protective function, reducing the susceptibility of the DNA to mutation or physical
Condensation is facilitated by formation of specific DNA-protein complexes. Proteins that
may be involved include protamines (small, highly basic, arginine-rich proteins),
histone-like proteins or other sperm-specific proteins.
Replacement may be gradual, with somatic histones being replaced by sperm-specific
proteins, or it may involve discrete steps in which transitional proteins interact with
the DNA after the histones are removed and before the protamines are added.
In salmonid fishes, somatic histones become hyperacetylated or undergo similar
modifications that reduce the binding between the histones and DNA.
In mammals, somatic histones are removed by protease digestion. Remodeling of the
chromatin in mammals is a two-step process. In the first step, the histones are replaced
by small, highly basic transition proteins. This eliminates nucleosomes and stops
transcription. The replacement of the transition proteins with protamines stabilizes and
further compacts the chromatin via the formation of disulfide cross-bridges.
The replacement of histones by sperm-specific proteins results in a termination of
transcription. This has important consequences for gene expression during spermiogenesis.
It means that all events after this must rely upon post-transcriptional processes.
In Drosophila, this occurs in the primary spermatocyte stage. Thus, no
transcription occurs in secondary spermatocytes or spermatids. In mammals and birds, some
transcription has been detected after meiosis. It is a generalized phenomenon that
spermatids lack transcription.
How, then, are proteins synthesized that are needed to complete spermiogenesis? For
example, protamine synthesis occurs after transcription has been terminated by replacement
of somatic histones with transitional proteins. The protamine is synthesized on
transcripts that were themselves synthesized before loss of somatic histones. In fact,
these transcripts are stored in the cytoplasm of developing mouse spermatids for 7 days
before they are translated. How are they maintained in a repressed state and subsequently
de-repressed and translated?
The 3' untranslated region (UTRs) of mouse protamine transcripts appears to determine when
they are translated (reviewed by Hecht, 1995). The role of the 3' UTR was demonstrated by
fusing the 3' UTR of mouse protamine 1 mRNA to the human growth hormone gene. Mice made
transgenic for this transgene transcribed it before histone replacement but did not
translate it until late in spermiogenesis when the endogenous protamine messengers were
translated. Conversely, replacement of the protamine 3' UTR in the protamine 1 gene with
that of human growth hormone allowed translation to commence immediately after
transcription (Braun et al., 1989). (At this point, you should review the technique for
making transgenic mice (see this resource also as well as
Browder et al., p. A-19)
A specific 18 kDa protein binds to a sequence element located in the 3' UTR of the
protamine 2 transcripts. This sequence element is also present in the 3' UTR of transition
protein 1 and protamine 1. The binding of the RNA-binding protein to the conserved
sequences represses translation in vitro. Translational regulation mediated by 3'
UTR sequences is a common mechanism of translational control. We shall discuss other
examples during the course. It will be essential to learn how the repression is
subsequently relieved to facilitate the translation of these repressed transcripts. The
RNA-binding protein is a phosphoprotein. Dephosphorylation of the protein prevents its
binding to RNA. Hence, one possibility is that a change in its phosphorylation status
relieves the inhibitory effect.
Another mechanism that regulates translation during mammalian spermiogenesis may function
by controlling the level of polyadenylation of RNA. Several of these transcripts are
stored with poly(A) tails of about 160 nucleotides and become partially deadenylated to
about 30 nucleotides when they are translated. Polyadenylation is a commonly used strategy
in translational regulation during development.
- Summarize the major nuclear events involved in transformation of spermatids into
- What are the consequences of terminating transcription before differentiation has been
- How is synthesis of protamines facilitated? Review the experiments demonstrating the
putative roles of the protamine 3' UTRs.
- How can phosphorylation function as a binary switch? (Why did I ask this question?)
Braun, R.E. et al. 1989. Protamine 3'-untranslated sequences regulate temporal
translational control and subcellular localization of growth hormone in spermatids of
transgenic mice. Genes & Dev. 3: 793-802.
Browder, L.W., Erickson, C.A. and Jeffery, W.R. 1991. Developmental Biology. Third
edition. Saunders College Pub. Philadelphia.
Hecht, N.B. 1995. The making of a spermatozoon: a molecular perspective. Dev. Genetics 16:
Hecht, N.B. 1998. Molecular mechanisms of male germ cell differentiation. Bioessays