The document describes the structural adaptations of melanosomes and neuroendocrine cells that allow them to successfully produce melanin and hormones, respectively. Melanosomes are specialized organelles that produce melanin pigment and have a characteristic shape and internal membrane structures that change as melanin is deposited. Their size and shape allow them to transport melanin within cells and between cells. Neuroendocrine cells connect the nervous and endocrine systems and have a flask-like shape with areas to receive signals and release hormones into the bloodstream, adapting them for their signaling and secretory functions.

1. 1
Describing the morphological/ structural adaptations
observed in the melanosomes and neuroendocrine cells
which make them successful in expression of melanin
and hormones respectively.”
BY: MUUNDA MUDENDA, MSc Molecular Biology and Biotechnology
Email: muundamudenda@gmail.com
MELANOSOMES
Melanosomes are part of the Lysosome Related Organelles (LRO) that are tissue
specific and are responsible for pigmentation in animals (Curie et al., 2009). They are
synthesized in melanocytes (or melanoblasts) of mammalian skin, choroid and retinal epithelial
cells, and melanophores in lower vertebrates. Melanosomes produce melanins which is also a
characteristic of keratinocytes functioning in photo protection. Melanins are composed of
copolymers of black and brown eumelanins and red and yellow pheomelanins. Melanins
provided the skin, hair and eyes of mammals with color and photo protection against ionizing
radiation (Setaluri, 2003). These pigments also function in the development of the optic
nervous system and regulate retinal function.
Structure wise, melanosomes are said to be about 500nm in diameter, bound by bilipid
membranes, and have a constant shape from cell type to cell type (Wasmeier et al., 2008). The
shape can be cigar-like (hollow) or rounded. Apart from that, the visibility of melanosomes
under the microscope makes them useful as representative models for studying biogenesis and
transfer of LRO. The diagram below illustrates the melanocytes in the basal of epidermis in the
skin and how melanocytes translate to melanosomes and ultimately the melanin which is seen
by how it affects skin color.

2. 2
Source of Image: courses.lumenlearning.com
Distinct Morphological Stages of Melanosome Maturation.
The following stages are excerpts from (Setaluri, 2003)
 Stage 1: Membrane vesicles containing no visible pigment but irregular internal
membrane structures are called premelanosomes.
 Stage 2: Premelanosomes of stage 1 results in elongation of the vesicles, and ordering
of the internal membranes into parallel structures.
 Stage 3: Melanosomes can be distinguished by the presence of ordered deposition of
melanin on the internal fibers.
 Stage 4: Melanosomes filled with melanin pigment no luminal structures are
distinguishable.
The diagram below illustrates the morphological stages of melanosomes. The diagram also
illustrates the morphological differences between eumelanosomes and pheomelanosomes.
Source of Image: (D’Alba & Shawkey, 2019)
Structural adaption of Melanosomes
In essence, melanin is synthesized in the melanocytes and the melanosomes are the
vehicles by which melanin is transported from the melanocytes of basal epidermis to the lower
and upper keratinocytes. As such, the size and shape of the melanocytes play a major role when
it comes to the transportation of melanocytes. To begin with, the fairly large size of
melanosomes helps to make readily available enough melanin pigments where they are needed.
According to D’Alba & Shawkey, (2019), melanin is composed of 30–50 nm Nano-
particles, in which melanin monomers are cross-linked together and form stacking structures
by pi-pi interactions. This implies that the shape of melanosomes is well adapted to

3. 3
accommodate the stacking structure organization made by melanin during its transportation to
the action regions of the body. Moreover, melanosomes with ciger-like structures easily
navigate the cell matrix and its components to the regions where they are needed to deliver
melanin pigment. The different biological molecules such as proteins and lipids, found in
melanosomes and the physical and chemical properties of their surfaces determine the
properties and biological functions of melanins (Simon et al., 2008).
NEUROENDOCRINE CELLS
Neuroendocrine cells are the connecting link between the nervous system and the
endocrine system. The two systems, together, form what is called the neuroendocrine system.
The neuroendocrine cells are sometimes referred to as Neurosecretory Cells (NSC) because of
attributes like membrane excitability which is similar to neurons. However, unlink neurons
which release neurotransmitters into the synaptic clefts, neuroendocrine cells secret hormones
(Volume et al., 2009). How the neuroendocrine system works is that, the nervous system
release signals (neurotransmitters) which are sent to the neuroendocrine cells and these release
(secret) neurohormones (peptides, proteins) into the blood leading to action to the target site.
Neuroendocrine cells are located in multiple organs (Figure 1) of the body which enables them
to control different cellular and physiological processes in the body. Because of their scattered
or dispersed kind of organization, these are often called dispersed or diffused endocrine cells.
A classic example of neuroendocrine cells is the Adrenal Medulla cells in the inner part
of the Adrenal glands. These release the hormone adrenaline to the blood. Other hormones
related to the neuroendocrine system include; gastrin, serotonin, epinephrine, insulin, growth
hormone, vasopressin, and oxytocin among several others.
Figure 1.1: Distribution of neuroendocrine cells in the neuroendocrine system.
Source of Image: neuroendocrinecancer.org.uk

4. 4
Cancer.ca highlights some functions of neuroendocrine cells as follows;
 Regulation of air and blood flow through the lungs.
 Regulating blood pressure and heart rates.
 Release of digestive enzymes to break down food.
 Controlling movement of food in the Gastro Intestinal Track.
 Controlling bone and muscle growth and development.
Structure and Function
Following the diagram by (Hartenstein, 2006) below, the endocrine cells are seen to
have long axons and flask-like shaped with two ends. The first end, called the neuronal input
site, is where the neurotransmitter signals are received by the neuroendocrine cells. The second
end is termed the neurohemal release site. It is the part linked to blood vessels. According to
this structural description it can be noted that neuroendocrine cells are well adapted for their
function as they connect the source of signals (nervous system), produce the hormones, and
they connect to the endocrine system through the blood vessels and endocrine cells. At the
neurohemal terminal, the neuroendocrine cells are able to release the neurohormones going to
endocrine cells in endocrine glands. It should be noted that the diagram below illustrates the
neuroendocrine cells in the normal prostate.
Source of Image: (Hartenstein, 2006)

5. 5
REFERENCES
Curie, I., Recherche, C. De, & France, F.-. (2009). Melanosomes – dark organelles enlighten endosomal mem. Nature
Molecular Cell Biology, 8(10), 786–797. https://doi.org/10.1038/nrm2258.Melanosomes
D’Alba, L., & Shawkey, M. D. (2019). Melanosomes: Biogenesis, properties, and evolution of an ancient organelle.
Physiological Reviews, 99(1), 1–19. https://doi.org/10.1152/physrev.00059.2017
Hartenstein, V. (2006). The neuroendocrine system of invertebrates: A developmental and evolutionary perspective. Journal
of Endocrinology, 190(3), 555–570. https://doi.org/10.1677/joe.1.06964
Setaluri, V. (2003). The Melanosome: Dark Pigment Granule Shines Bright Light on Vesicle Biogenesis and More. Journal
of Investigative Dermatology, 121(4), 650–660. https://doi.org/10.1046/j.1523-1747.2003.12500.x
Simon, J. D., Hong, L., & Peles, D. N. (2008). Insights into melanosomes and melanin from some interesting spatial and
temporal properties. Journal of Physical Chemistry B, 112(42), 13201–13217. https://doi.org/10.1021/jp804248h
Volume, I., Sirovich, L., & Marsden, J. E. (2009). Mathematical physiology: v.1: Cellular physiology. In Choice Reviews
Online (Vol. 46, Issue 10). https://doi.org/10.5860/choice.46-5592
Wasmeier, C., Hume, A. N., Bolasco, G., & Seabra, M. C. (2008). Melanosomes at a glance. Journal of Cell Science,
121(24), 3995–3999. https://doi.org/10.1242/jcs.040667