The document is a histology assignment discussing types of intercellular junctions (tight junctions, desmosomes, and gap junctions) and their roles in cell communication and tissue integrity. It also details the structural differences between the dorsal and ventral sides of the spinal cord, the characteristics of nerve cells, smooth muscle fibers, reticular fibers, mechanisms of cartilage growth, and the processes of intramembranous versus endochondral ossification. This comprehensive overview is aimed at students in the veterinary medicine field, providing essential histological information.

1. Histology assignment by Kula Jilo
Jimma university school of veterinary medicine, December, 2015[Type text] Page 1
1. What are intercellular junctions are commonly found in epithelia?
 Communication between cells can be carried out through three types of junctions:
I. Tight junction
II. Desmosomes
III. Gap junctions
I. Tight junction
 Tight junction is a watertight seal between two adjacent animal cells. The cells
are held tightly against each other by proteins (predominantly two proteins called
claudins and occludins). This tight adherence prevents materials from leaking
between the cells. These junctions are typically found in epithelial tissues that line
internal organs and cavities and comprise most of the skin. For example, the tight
junctions of the epithelial cells lining your urinary bladder prevent urine from
leaking out into the extracellular space.
II. Desmosomes
 Desmosomes act like spot welds between adjacent epithelial cells, connecting
them. Short proteins called cadherins in the plasma membrane connect to
intermediate filaments to create desmosomes. The cadherins join two adjacent
cells together and maintain the cells in a sheet-like formation in organs and tissues
that stretch, such as the skin, heart, and muscles.
III. Gap junctions
 Gap junctions are the third type of direct junction found within animal cells.
These junctions are channels between adjacent cells that allow for the transport of
ions, nutrients, and other substances that enable cells to communicate.
Structurally, however, gap junctions and plasmodesmata differ. Gap junctions
develop when a set of six proteins (called connexins) in the plasma membrane
arrange themselves in an elongated doughnut-like configuration called a
connexon. Gap junctions are particularly important in cardiac muscle. The
electrical signal for the muscle to contract is passed efficiently through gap
junctions, which allows the heart muscle cells to contract in tandem.

2. Histology assignment by Kula Jilo
Jimma university school of veterinary medicine, December, 2015[Type text] Page 2
2. How the structures of the dorsal side of spinal cord different from the
ventral side?
 Dorsal Side
 Information from the skin, skeletal muscle and joints is relayed to the spinal cord by
sensory cells located in the dorsal root ganglia. The dorsal root fibers are the axons
originated from the primary sensory dorsal root ganglion cells. Each ascending dorsal
root axon, before reaching the spinal cord, bifurcates into ascending and descending
branches entering several segments below and above their own segment. The ascending
dorsal root fibers and the descending ventral root fibers from and to discrete body areas
form a spinal nerve (Figure 3.10). There are 31 paired spinal nerves. The dorsal root
fibers segregate into lateral and medial divisions. The lateral division contains most of the
unmyelinated and small myelinated axons carrying pain and temperature information to
be terminated in the Rexed laminae I, II, and IV of the gray matter. The medial division
of dorsal root fibers consists mainly of myelinated axons conducting sensory fibers from
skin, muscles and joints; it enters the dorsal/posterior column/funiculus and ascend in the
dorsal column to be terminated in the ipsilateral nucleus gracilis or nucleus cuneatus at
the medulla oblongata region, i.e., the axons of the first-order (1°) sensory neurons
synapse in the medulla oblongata on the second order (2°) neurons (in nucleus gracilis or
nucleus cuneatus). In entering the spinal cord, all fibers send collaterals to different
Rexed lamina.
 Axons entering the cord in the sacral region are found in the dorsal column near the
midline and comprise the fasciculus gracilis, whereas axons that enter at higher levels are
added in lateral positions and comprise the fasciculus cuneatus
Ventral roots
 Ventral root fibers are the axons of motor and visceral efferent fibers and emerge from poorly
defined ventral lateral sulcus as ventral rootlets. The ventral rootlets from discrete spinal cord
section unite and form the ventral root, which contain motor nerve axons from motor and visceral
motor neurons. The α motor nerve axons innervate the extrafusal muscle fibers while the small γ
motor neuron axons innervate the intrafusal muscle fibers located within the muscle spindles. The
visceral neurons send preganglionic fibers to innervate the visceral organs. All these fibers join
the dorsal root fibers distal to the dorsal root ganglion to form the spinal nerv
3. What are cells within the nerve whose nuclei are stained?
 In the longitudinal section, note the argyrophilic axons, many of which are surrounded by
an "unstained" myelin sheath.The neurilemmal sheath (outer layer of Schwann cell
cytoplasm) is visible. Find some nodes of Ranvier.
 In the cross section note the axon (black), which is surrounded in turn by a myelin sheath
and its Schwann cell neurilemma(brown). Locate lightly myelinated and unmyelinated
fibers. Speed of conduction is related to the diameter (including myelin sheath) of a nerve
fiber.

3. Histology assignment by Kula Jilo
Jimma university school of veterinary medicine, December, 2015[Type text] Page 3
4. Why do smooth muscle fibers in cross section have different diameters and
why do some of these fail to show nuclei?

Smooth muscle cells have tapered ends. Since the cells interdigitate different diameters
would be revealed in a particular plane of section and the plane of section does not
always go through the nucleus.
5. Are reticular fibers distinguishable in tissue stained with H&E?
 In H & E stained sections reticular fibers cannot be distinguished from other types of
collagen. But they are usually stained black by methods such as Gomori's or Foot-
Hortega stains (which contain silver salts).
6. What is the mechanism of cartilage growth?
 Growth of cartilage is attributable to two processes:
1. Interstitial growth which includes:
o Cell division of the chondrocytes
o Synthesis of the extracellular matrix
o Expansion of the cartilage matrix from within
2. Appositional growth which includes:
o Differentiation of the chondroblasts or perichondrial cells
o Synthesis of the extracellular matrix
o Expansion of the girth of the cartilage
7. What are difference between intramembranous and andochonral
ossification?
Intramembranous Ossification:
 Forms the flat bones of the skull, face, jaw, and center of clavicle.
 Bone is formed in sheet-like layers that resemble a membrane.
Endochondral Ossification:
 Forms most bones in the body, mostly long bones, and replace cartilage with bone.