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BIO 03048 ANATOMY; WEEK 1: THE TYPICAL BODY SEGMENT; BACK, EPITHELIUM, SKIN

8/31/99 W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, www.hvcc.edu/academ/faculty/crone/index.html)

Readings: Moore and Dalley, Introduction, Ch. 4; Cormack, Chs. 1 and 2

possible web sites: http://www.dccc.edu/virtualmicroscope/VMPage/very1st.htm

(The Virtual Microscope: major tissues and organs, with a practice test too)

http://www.kumc.edu/instruction/medicine/anatomy/histoweb/contents.htm

(U. of Kansas med school histology resources)

THE TYPICAL BODY SEGMENT: During development, the embryo has three germ layers of ectoderm, mesoderm, and endoderm that will set up adult features. The mesoderm nearest the notochord (primitive chordate stiffening rod) is segmented into somites around the fourth week. These somites develop into vertebrae, the striated musculature innervated by spinal nerves moving them, and the dermis.² In contrast, unsegmented splanchnic (visceral) and somatic (parietal) mesoderm surround the coelom (body cavity), and have a very different developmental pattern. Smooth and cardiac muscles, and glands produced by this"coelom-wall" mesoderm are not controlled voluntarily, but by the autonomic nervous system.²

The innermost part of the"coelom-wall" mesoderm forms the lining of the coelom, which because of partitions that occur during development, we know by different names: pleura, serous pericardium, and peritoneum. A double fold of this lining suspends the gut and contains the gut's blood vessels and nerves--the dorsal mesentery. Other parts of the coelom-wall mesoderm condense around endodermal tissues like the gut, lungs, liver, and pancreas to form the connective tissues, the blood vessels, etc. The blood vessels that the coelom-wall mesoderm creates will penetrate and nourish the somites as well, so that the muscle in the blood vessels is involuntarily controlled as well (by the sympathetic, not parasympathetic system). An intermediate zone of mesoderm, dorsal to the coelom-wall mesoderm, but still ventral to the somites, forms the beginnings of the genitourinary system.

Returning to the somites, the vertebral column splits the skeletal muscle into epaxial vs. hypaxial muscle. the epaxial muscles are the deep muscles of the back (erector spinae, transversospinalis, interspinalis). Some hypaxial muscles also stay close to the vertebrae, but in contrast, many hypaxial muscles spread out grow around the viscera to form a muscular body wall. Into this body wall grow costal processes (ribs). The hypaxial mesoderm gives rise to the limbs as well, but with several somites contributing, hence plexuses, e.g, brachial, that represent the nerve input.

Nervous system of two parts: central nervous system and peripheral nervous system. In a lamprey (jawless fish), the setup is clear--ventral motor neuron bodies in nerve cord, and peripheral sensory fibers forming bundles entering the dorsal part of the cord; hence, ventral roots and dorsal roots.² The dorsal and ventral roots that come out the brain are the cranial nerves, and the ones coming from the spinal cord, spinal nerves. What about vertebral gut musculature? The parasympathetic nervous system originates from cranial and caudal sources. The sympathetic nervous system is supplied by spinal nerves in between, and over evolution, the two systems have invaded each other's"turf." Many nerve cell bodies are found in clusters called ganglia peripheral to the central nervous system, formed by migrating neural crest cells.

Every body segment contains sympathetic ganglia from which postganglionic axes can spread. Sympathetic motor fibers running out of the ventral ramus aim for the pinkish grey lumps of paravertebral ganglia, which connected together form the sympathetic trunk.

Spinal nerves (31 pairs) formed by motor ventral root fusing with dorsal sensory root (in contrast to the lamprey). As this spinal nerve passes out of the intervertebral foramen, it divides into a:

dorsal ramus: epaxial muscles and overlying skin

ventral ramus: all the rest of that body segment

The ventral ramus of the mixed spinal nerve gives off a lateral cutaneous branch halfway around and an anterior cutaneous branch near the ventral midline. A dermatome is a band of skin innervated by the cutaneous branches of a spinal nerve, e.g., T4-nipples, T10-umbilicus. Shingles: herpes zoster affecting a dermatome.⁶

SPINAL CORD: a cross section of the spinal cord includes outer white matter (site of multiple tracts to be discussed under neuroanatomy) and a central mass of gray matter (home to neuron cell bodies, both motor and interneuron). There are three connective tissue membranes or meninges that cover the CNS, from superficial to deep:

a) dura mater: outermost, tough, fibrous

--a small (mostly potential) subdural space

b) arachnoid mater: thin, delicate, transparent

--subarachnoid space with cerebrospinal fluid (CSF)

c) pia mater: more delicate than the arachnoid, and is close up against the spinal cord

At each spinal nerve, the dura and arachnoid fade out as the nerve extends out. The dura and arachnoid reach as low as S2, but the spinal cord reaches only to L1-L2, so that the collection of long nerve roots (cauda equina) of lumbar, sacral, and coccygeal nerves are still surrounded. Hence, an useful level for lumbar puncture (spinal tap) is L3-4 (or L4-5).⁶

THE VERTEBRAL COLUMN: Five regions of vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, 4 coccygeal. Formation of the vertebral body, the vertebral arch, and the transverse and spinous processes via parts of the somites. Also, articular processes (upper and lower) on the arch and the thicker pedicles of the arch and the thinner"roofs" or laminae, e.g., laminectomy to expose the spinal cord. Failure of the vertebral arch to fuse results in spina bifida, especially seen in the lumbar region.⁶ The intervertebral discs support most of the weight of the vertebral column. To help limit the motion of these joints are anterior and posterior longitudinal ligaments of the spine, as well as interspinous, supraspinous ligaments and ligamenta flava.

CERVICAL: There are 7 cervical vertebrae that form a flexible framework for the neck and support the skull. C1 and C2 are more specialized for increased movement. C1 is the atlas, which articulates with occipital condyles of the skull via its superior articular surfaces: the"yes" nod. C2 is the axis, with the projecting dens or odontoid process. This is a pivot for the atlas, allowing the"no" movement.

Whiplash (flexion-extension syndrome) is a common general term for injury, usually from hyperextension, to the cervical region, as in a"rear-ender." Minor whiplash with muscle (e.g., sternocleidomastoid m.) and ligament strains, but severe whiplash with spinal cord compression is serious. C4-C5 and C5-C6 are very mobile joints, and so are prone to dislocation. A bilateral dislocation runs the risk of spinal cord disruption. Otherwise, pinched spinal nerves may result from forced/rebound neck flexion.^5,6

THORACIC: considerations of kyphosis (abnormal increase in anterior curvature) and scoliosis (abnormal increase in lateral curvature, especially in pubertal girls).

LUMBAR: the five lumbar vertebrae are massive, with thick processes that support many muscles. With weight bearing an important function here, the intervertebral discs take on special consideration. They consist of an anulus fibrosus connected to the vertebral bodies above and below, and a central nucleus pulposus within. The discs make the movement between vertebral bodies possible, but they degenerate, and a tear or a weak spot in the anulus can lead to a protruded disc that can compress on spinal nerve roots that extend through the intervertebral foramen between two vertebrae.^5,6

The deep (epaxial) muscles of the back assist in support, with major groups of muscles:

Annual incidence of low back pain in USA is 15-20%, but exact cause(s) are rarely pinpointed. Commonly, lumbosacral strain in paraspinal muscles, e.g., around L3-L4, or at the origin of the erector spinae mm. group above the sacroiliac joint. Sciatica or lumbar radiculopathy with herniated nucleus pulposus or other impingement, lateral shooting pains L4-L5, and posterior pains L5-S1.¹

HISTOLOGY: Study of tissues. 4 major animal tissue types: epithelial, connective, muscle, nervous

EPITHELIAL TISSUE: MAJOR CHARACTERISTICS

epithelial cells as linings and coverings

basement membrane (lamina)

intercellular junctions

polarity of epithelial cells

specializations on luminal surface

glands as epithelia that penetrate into connective tissue and form secretory units

SIMPLE SQUAMOUS EPITHELIUM

Lining of vascular system, body cavities, respiratory spaces in lung; exchange.

SIMPLE CUBOIDAL EPITHELIUM

Small ducts of exocrine glands, kidney tubules; absorption.

SIMPLE COLUMNAR EPITHELIUM

Lining of stomach, small intestine, colon, gall bladder; secretion and absorption.

PSEUDOSTRATIFIED COLUMNAR EPITHELIUM

Nuclei appear at different levels, as not all cells reach the surface; ciliated: lining of trachea; secretion, conduit.

TRANSITIONAL EPITHELIUM

Several layers of rounded cells with the ability to flatten out; renal system from calyces to the urethra.

STRATIFIED SQUAMOUS EPITHELIUM

Keratinized: epidermis; nonkeratinized (moist): mouth, esophagus, vagina; barrier, protection.

SKIN

Layers of epidermis (deep to superficial) and activities in each:^3,4

layers of dermis (deep to superficial):

Subcutaneous tissue (superficial fascia) or hypodermis--we will see separate fatty and (deeper) membranous layers of superficial fascia come up in the abdominal wall. Deep fascia envelopes muscles.

Burns: first degree damage the epidermis, second degree involve the dermis and third degree burns destroy both.^3,5 Body surface area (BSA):"rule of nines" to evaluate burns, e.g., head, arms at 9^o, legs and ant., post. trunk @ 18^o, groin at 1^o5

1. BC Anderson, Office Orthopedics for Primary Care (WB Saunders, Philadelphia, 1995), pp. 61, 63, 64.
2. M Cartmill et al., Human Structure (Harvard U Press, Cambridge, MA, 1987), pp. 15-26.
3. DH Cormack, Clinically Integrated Histology (Lippincott-Raven, Philadelphia, 1998), pp. 13, 23, 24, 25.
4. LP Gartner et al., Board Review Series: Cell Biology and Histology, 2^nd ed. (Harwal Publishing, Philadelphia, 1993), pp. 209, 210.
5. BE Gould, Pathophysiology for the Health-Related Professions (WB Saunders, Philadelphia, 1997), pp. 18-20, 357-361.
6. KL Moore, AF Dalley, Clinically Oriented Anatomy, 4^th ed. (Lippincott Williams & Wilkins, Philadelphia, 1999), pp. 87, 448, 449, 483.

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