Description

This assignment deals asks you to consider a novel cell biology situation.  Throughout the assignment you will find pertinent background information about this scenario.  You are expected to use the provided information AND apply the knowledge you have acquired throughout our semester studying cellular biology to answer the questions posed.  You must answer all of the questions below. Total = 60 Marks

Melanocytes are a specialized cell type that gives rise to the colour in the skin, hair, and eyes of many organisms including humans.  A melanocyte found in the skin has an ovoid cell body with dendritic processes that make contact with 30-40 keratinocytes, the major cell type of the skin (Figure 1). 

Melanocytes contain a membrane-bound, lysosome-related organelle known as a melanosome.  Similar to lysosomes, melanosome use proton pumps to actively maintain an acidic internal pH.  The acidic environment is required for the synthesis and storage of melanins, the most common family of light-absorbing pigments in the animal kingdom.  Maturation of melanosomes occurs in stages (I to IV), characterized by progressive melanin synthesis and transport of the organelle toward the cell periphery. Once fully matured (stage IV), melanosomes are transferred to keratinocytes (see the zoomed-in region of figure 1). It is the melanin contained within the keratinocytes of the skin or hair that ultimately determine their colouration.

Within melanocytes, mitochondria-melanosome tethering via 20-30 nm long fibrillar bridges has been observed and is depicted in the figure below.  This tethering is more frequently observed in immature (stage I and II) melanosomes and evidence suggests that it helps to ensure a local and timely supply of ATP to immature melanosomes which have higher energy requirements associated with organelle transport, melanin synthesis, or maintenance of intramelanosomal pH.

1. You have identified an individual with a genetic mutation that prevents the actin protein from polymerizing in their melanocytes.  Would you expect the structure of their melanocytes to differ from individuals who do not have this mutation?  If so, describe how they would differ. (2 marks)

2. Considering what you know about transport across the cell membrane, describe all of the types of transport used in the transfer of mature melanosomes from a melanocyte to a keratinocyte as depicted in the zoomed-in region of figure 1. (2 marks)

3. What type of microscope would you use to study and observe mitochondria-melanosome tethering? Using no more than two sentences, explain your reasoning. (2 marks)

4. You identify an individual who does not express a functional microtubule-associated motor protein, dynein, within their melanocytes.  Would you expect to find mature melanosomes in the keratinocytes of this individual? Using no more than two sentences, explain your reasoning. (2 marks)