Ontogeny

Ontogeny (ontogenesis or morphogenesis) describes the development of an organism from the fertilized egg to its adult form. Here we focus primarily on the ontogeny of the vomeronasal organ (VNO), the sensory organ necessary for receiving olfactory cues relating to mate attraction (pheromones) in Mus musculus.

 

Figure 1. This figure displays the locations of various olfactory structures in a mouse.
[Source: http://neuro.fsu.edu/~mmered/index.htm]

 

Anatomy of the VNO

One of the most notable differences between the VNO and the MOS is that the MOS epithelium lines the nasal cavity, becoming directly accessible to odorant molecules in the nasal passage.  The VNO, however, is contained within a bony structure posterior to the MOS and, depending on the species, opens either into the nasal or oral cavity [Dulac & Torello, 2003].  The VNO neurons combine to form a single, unbranched axon through the skull leading to the accessory olfactory bulb, differing from the multiple branching sites of MOB neurons.  This axon leading to the accessory olfactory bulb is then connected to the neuroendocrine hypothalamus, which regulates many hormones critical for reproduction.  The impact of the VNO upon the hypothalamus is supported by studies conducted in female mice displaying the onset of puberty and induction of oestrus after the detection of male pheromones (this is known as the Whitten Effect) [Dulac & Torello, 2003].  Other effects of pheromone detection in regards to changes in reproductive capability or readiness include the Vendenbergh Effect and the well-known Bruce Effect [Halpern, 1987].  This dichotomous anatomy between the VNO and the MOS has led to debate about the developmental process of the two organs.  It is still uncertain whether the MOS or the VNO develops first in mice [Halpern, 1987].

 

Development of the VNO

Current Knowledge:

  • During gestation, the epithelium of the medial wall in the olfactory pit invaginates and fuses, creating a primordial VNO.[Halpern, 1987]
  • The sensory epithelium, constituted mainly of undifferentiated cells during fetal life, begins to acquire microvilli in the sensory epithelium around 8-10 days after birth.  [Halpern, 1987]
  • Around 1-4 months of age, neurosensory cells in the VNO increase at a rate of 43%.
  • During adult life, neurosensory cells in the VNO decrease to 21%. [Halpern, 1987]
  • VNO appears to develop after the main olfactory epithelium. [Halpern, 1987]
  • Neurogenesis of the accessory olfactory bulb (which the VNO immediately interacts with) precedes the main olfactory bulb neurogenesis in mice. [Halpern, 1987]

Unanswered Questions:

  • The time in which the VNO is developed [Halpern, 1987]
  • The time in which the MOS is developed [Halpern, 1987]
  • The relationship between VNO and MOS timing [Halpern, 1987]
  • The developmental stage at which the systems are functional [Halpern, 1987]
  • The reason of the decrease in neurosensory cells in the VNO during adult life [Halpern, 1987]