Biology 241 – Lab
LAB #6
(6th/21 Lab Sessions for Fall Quarter, 2008)
TOPICS TO BE COVERED:
»Overview of Nervous Tissue
»Overview of the Skin
»Microscopic Examination of Ventral Gray Horn of the Spinal Cord
»Microscopic Examination of Pigmented and Non-Pigmented Skin
DESIRED OUTCOMES:
After completing the activities described for this lab session, students should:
»Recognize and be able to describe two basic types of cells found in nervous tissue.
»Be able to identify a neuron under a microscope and describe its basic structure.
»Understand the CNS and PNS have different types of neuroglial cells and that each
neuroglial cell type performs specific functions.
»Be able to identify anatomical structures of the integumentary system on models/diagrams.
»Recognize and be able to describe histologic structures of the skin on prepared slides.
MATERIALS NEEDED:
»Anatomical models: Neuron, Synapse, and Axonal End Bulb;
Oversized 3-D Model of Thin Skin / Thick Skin
»Blue Box of Histology Slides, specifically Slide #20: Giant Multipolar Neuron and
Slide #16: The Skin (aka Cutaneous Membrane)
»Wall Charts: Histomicrographs of Nervous Tissue; Skin (Pigmented and Non-Pigmented)
»Photographic Atlas, Ch. 3; Ch. 4; Ch. 9 (Figures 9.2 and 9.3)
Activity #1: Overview of Nervous Tissue
The Nervous System is one of the major homeostatic systems in the body. It regulates cellular activities
rapidly by generating and propagating (sending) nerve impulses along specialized cells called neurons. Thus,
neurons constitute the functional cells of nervous tissue. Neurons are, in turn, attended to by smaller cells called
neuroglial cells, which are cells derived from connective tissue but that now function as the supporting cells of
nervous tissue. Together these two cell types: the functional neurons and the supporting neuroglial cells make
up nervous tissue.
Neurons are very large cells that generate and transmit messages in the form of nerve impulses. They
vary widely in size and structure, but all have the following features in common: one cell body, one axon, and
one or more dendrite(s). (Together, axons and dendrites are referred to as nerve fibers.)
(1) Cell body (aka Soma): Since this is the biosynthetic center of the neuron, it contains the large, darkly staining, centrally located nucleus which is surrounded by a light staining “halo” called the perikaryon region. Just
peripheral to the perikaryon region, most of the organelles are located. Throughout the neuroplasm, there is a
visible cytoskeleton with densely packed microtubules and neurofibrils, which serve to compartmentalize the
rough endoplasmic reticulum into dark-staining structures called Nissl bodies. Neuron cell bodies are often
found in clusters. (In the Central Nervous System (CNS), these clusters of neuron cell bodies are called nuclei;
and in the Peripheral Nervous System (PNS), such clusters are called ganglia.)
(2) Axon: A single axon exits the neuron cell body to transmit messages to other neurons or to effectors (muscles/glands). The single axon originates at a cone-shaped elevation of neuroplasm at one side of the soma, a region called the axonal hillock. The part of the axon closest to this axonal hillock is the initial segment. In most
neurons, nerve impulses arise at the junction of the hillock and the initial segment, an area called the trigger
zone, from which they travel along the axon to their destination. An axon contains mitochondria, microtubules,
and neurofibrils. (Because rough endoplasmic reticulum is not present, protein sysnthesis does not occur in the axon.)
The cytoplasm of an axon, called axoplasm, is surrounded by a plasma membrane known as the axolemma.
Along the length of an axon, side branches called axon collaterals may branch off, typically at a right angle to
Biology 241 – LAB #6 – continued
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the axon. The axon and its collaterals end by dividing into many fine processes called axon terminals (aka telodendria) which have at their distal-most tips swollen, bulb-shaped structures called synaptic end bulbs. Synaptic end bulbs contain many tiny membrane-enclosed sacs called synaptic vesicles that store a chemical neurotransmitter. Many neurons contain two or even three types of neurotransmitters, each with different effects on
the postsynaptic cell. When neurotransmitter molecules are released from synaptic vesicles, they excite or inhibit other neurons, or the effector cell(s).
(3) Dendrites: These are the receiving, or input, portions of a neuron that convey messages from sensory
receptors or other neurons to the neuron cell body. They usually are short, tapering, and highly branched. In
many neurons, the dendrites form a tree-shaped array of processes extending from the soma.
Their cytoplasm contains Nissl bodies, mitochondria, and other organelles.
( Remember, a nerve fiber is a general term for ANY neuronal process or extension that emerges from the soma
of a neuron. The two kinds of nerve fibers from any one neuron are an axon and one or more dendrite(s). )
What is a Synapse? A synapse is an area of chemical (NOT physical) interaction between a neuron and another
neuron or between a neuron and an effector cell. The synapse has three parts:
a. Presynaptic unit: always a neuron that is sending the message.
b. Synaptic cleft: small space through which the neurotransmitter diffuses when released from the synaptic vesicles.
c. Postsynaptic unit: whatever type of cell is receiving the message (another neuron, a myofiber, or a gland.) The
surface of this cell has receptors for the specific neurotransmitter substance released by the pre-synaptic unit;
and when neurotransmitter binds, it causes a change in the membrane potential of the postsynaptic cell, either
triggering or inhibiting the resulting activity.
We will study the physiology of synapses in more detail in Lecture (Biology 240).
Neuroglial cells are much smaller than neurons, and they outnumber neurons about 50 to 1 – resulting in
quite an impressive number considering that the Nervous System contains about a trillion (1012) neurons. Additionally, they make up about half the volume of the CNS. (Early histologists thought they were the “glue” that
held nervous tissue together, hence the name!) It is known now, however, that neuroglial cells are not merely
passive bystanders, but rather play a variety of very important roles including actively growing and undergoing
cell division to multiply their numbers; in fact, they multiply to fill in the spaces formerly occupied by neurons.
They do not, however, generate or propagate action potential.
Of the six types of neuroglial cells, four of them: astrocytes, oligodendrocytes, microglia, and
ependymal cells are found only in the CNS. The remaining two types: Schwann cells and satellite cells are
present only in the PNS. Please review the structural and functional differences of these six types of neuroglia
in Chapter 12 of your textbook.
Just a quick word about the structure called the myelin sheath. The myelin sheath, which covers the
nerve fibers of certain neurons, is actually the cell membrane of Schwann cells (in the PNS) and oligodendrocytes (in the CNS). The myelin sheath functions to protect and insulate the nerve fibers and speed up conduction of nerve impulses. Because the sheath is made up of individual neuroglial cells, there are small gaps between the cells where the cell membrane of the nerve fiber is exposed. These gaps are called Nodes of Ranvier,
and the myelin-covered segments between the Nodes are called internodes.
Activity #2: Microscopic Examination of Slide #20: Ventral Gray Horn of the Spinal Cord
Examine Slide #20, draw several giant, multipolar neurons and their surrounding neuroglial cells. Label
all microscopic features possible, understanding that you may have to “merge” the views of several neurons to
get one “generic neuron” with all its significant anatomy.
Biology 241 – LAB #6 – continued
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Activity #3: Overview of the Skin
Thus far in our Lab sessions, we have looked at tissues: groups of identical or very similar cells, and
their intercellular substance where present, joined together to perform one or more specific functions. Now, we
will examine an organ: a structure composed of two (or more) different kinds of tissues with a specific function
and a recognizeable form. The simplest organ is a true membrane (aka epithelial membrane):
a combination of an epithelial tissue layer and an underlying connective tissue layer. The three types of true
membranes are 1) mucous membranes (pliable “sheets” that line cavities open to the outside), 2) serous membranes
(pliable “sheets” that line ventral cavities closed to the outside and cover the organs contained within those cavities ), and
3) the cutaneous membrane (aka the skin), a pliable “sheet” that covers the exterior surface of the body. (What
of another type of membrane called a synovial membrane? you might ask. Synovial membranes (pliable “sheets”
that line joints and bursae) are NOT true membranes, because they consist of connective tissue only, no epithelial tissue.
So, now we have “made the leap” from tissues to organs; at least, one specific organ: the Skin. However, according to the definition of a system: an association of two or more organs that have one or more common functions, the Skin is ALSO a system, because it consists of the largest organ (the cutaneous membrane) as
well as associated organs such as hair, glands, and nails. This system is given the name Integumentary System,
and, indeed, consists of the skin and the above named accessory organs.
As mentioned before, the skin is the largest organ in the body in both surface area and weight. (In adults,
the skin covers an area of 2 square meters (≈ 22 square feet) and weights 4.5 – 5 kg (≈10-11 lb). It constitutes about 16%
of total body weight.) Structurally, it is composed of two layers: the epidermis and the dermis, each of which are
subdivided (for the purpose of study) into thinner layers. The tissue deep to the dermis, called by various names
including: hypodermis, subcutaneous layer (aka “sub-Q”), and superficial fascia, connects the skin to the underlying deep fascial layers. Technically, this underlying tissue is not considered to be part of the integument itself;
however, it is so closely associated with it, and even shares part of the name, that it is best to think of it as connected. (The three layers go together so naturally: epidermis, dermis, and hypodermis.)
What follows is a brief discussion of the thinner zones of which the epidermis is composed (epidermal
strata) and of which the dermis is composed (dermal regions). These are the anatomical structures and features
of each of these important parts of this organ.
Please refer to Ch. 4 of your Photographic Atlas: The epidermis consists of keratinizing, stratified squamous
epithelium. From deep to superficial, the strata are as follows:
(1) Stratum basale: a single row of roughly cuboidal-shaped cells; all of whose basal surfaces are attached to the
Basement Membrane. Also called the stratum germinativum, because cells of this single row are the only ones
in this tissue that can undergo somatic cell division. (Once cells lose their attachment to the B.M., they are “destined” to manufacture keratin, become keratinized, die, and slough off.) Mitotic figures may be seen by careful examination of the cells of this stratum. (Melanocytes and Merkel’s cells also “reside” amongst the cells of this single row,
although these are not differentially stained and usually cannot be discerned from the far more numerous epitheliocytes.)
(2) Stratum spinosum: 8 – 10 rows of polygon-shaped (many-sided) cells which are actively metabolizing cells.
They are growing and changing shape; and because there are so many tight junctions binding one neighbor cell
to another, the plasmalemma of each cell seems to be in a “push-me-pull-you” state of “prickle-shape” – hence
the name.
(3) Stratum granulosum: There are about 3 – 5 rows of cells in this stratum and the superficial cells are dead,
but the deeper cells are still alive. This stratum is named for the cell’s visible cytoplasmic granules, some of
which are which keratohyalin granules (the protein needed to produce keratin) and some are lamellar granules
which release an oily, acidic, water-proofing substance. Note that the cells in this stratum have assumed their
definitive squamous shape.
(4) Stratum lucidum: This layer consists of 3 – 5 rows of very flattened, translucent, dead cells. This layer is
found only in the skin of the fingertips, palms of the hands and soles of the feet.
(5) Stratum corneum: This layer consists of 25+ rows of very flattened, dead keratinocytes
Biology 241 – LAB #6 – continued
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The second, deeper part of the skin, the dermis, is composed mainly of connective tissue. Blood vessels, nerves,
glands, and hair follicles are embedded in this dermal tissue which lies immediately deep to the Basement Membrane of the epidermis. Based on its tissue structure, the dermis can be divided into two regions: 1) a papillary
region, and 2) a reticular region.
1. The papillary region is more superficial and is composed of loose areolar connective tissue. It contains fingerlike projections called dermal papillae that project into the epidermis. Theses dermal papillae contain touch receptors called Meissner’s corpuscles (nerve endings that are sensitive to touch), as
well as free nerve endings that initiate signals giving rise to sensations of warmth, coolness, pain,
tickle, and itch. Also, dermal papillae contain capillary loops (blood capillaries) that provide blood
supply to the avascular epidermis.
Note: It is the particular arrangement of these dermal papillae (both primary and secondary ones) and
how “high” and “low” they are that forms the basis of fingerprints. No two people have the
same pattern of papillae formation.
2. The reticular region is deep to the papillary region and is attached to the hypodermis. It consists of
dense irregular connective tissue containing fibroblasts, bundles of collagen, and some coarse elastic
fibers/bundles. The collagen fibers in the reticular region interlace in a netlike manner. (It is the
combination of collagen and elastic fibers in the reticular region that account for the skin’s amazing
degree of extensibility (ability to stretch) and elasticity (ability to return to original shape after stretching).
Of course, with the aging process comes a marked decrease in the rate of renewal of these protein fibers once they “wear out”…and thus, with loss of elasticity, “wrinkles” occur!
Also, few adipocytes, hair follicles, nerves, sebaceous glands, and sudoriferous glands occupy the
spaces between the protein fibers of this reticular region.
During today’s lab session, you will examine and generate two labeled drawings of the skin:
One drawing at TM = 40X showing all three layers: epidermis, dermis, and hypodermis and the relative proportion each of these constitutes of the skin. Relatively little detail will show up at this magnification; but certainly the interface between the epidermis and dermis will be seen for its “ridged” and “uneven” appearance
because of the dermal papillae; and the hypodermis will be readily identifiable by the presence of adipose connective tissue.
The second drawing at TM = 400X showing detail of the strata of the epidermis. Be discerning to choose an
area that shows several dermal papillae, a distinct Basement Membrane, and crisp detail of the arrangement of
keratinocytes in the various strata. Also, if you are drawing pigmented skin – you should see the dispersal of the
melanin granules (melanosomes) to the surface of the epidermis.
Your field of view will be markedly smaller at the higher power magnification; so be careful in selecting the
exact place on your specimen to spend your time drawing. Ask your Instructor to “okay” your selected spot before investing lots of time drawing.