Histology and Embryology 1941 - Histology 2

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Nonidez JF. Histology and Embryology. (1941) Oxford University Press, London.

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This historic 1941 textbook by Nonidez describes both embryology and histology.

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   Histology and Embryology 1941: Histology - 1 The Cell | 2 The Tissues | 3 The Organs     Embryology - 1 General Development | 2 Organogenesis | Bibliography

Part Two - The Tissues

The association or grouping of cells which have the same origin and perform similar functions is called a tissue. The tissues of the mammalian body are: I. Epithelium; II. Blood and lymph; III. The supporting tissues (connective tissue; reticulo-endothelium; cartilage; bone); IV. Muscular tissue; V. Nervous tissue.


In epithelium the cells are cemented together by a small amount of intercellular substance; they are of uniform type or differ in shape and function. Epithelium is usually separated from the underlying tissue by a basement membrane. It lacks vessels of its own.

The three blastodermic layers of the embryo give rise to epithelium. Some varieties cover the surface of the body and line closed cavities, or cavities which communicate with the outside (covering epithelium). Others consist of secretory cells arranged in various ways (glandular epithelium). Finally, the epithelial cells may receive stimuli which are taken up by sensory nerve endings (neuroepithelium) .

I. Covering Epithelium

The cells form a single layer or they are arranged into several layers. Transitions also occur.

A. Single-layered epithelia:

1. Squamous or pavement epithelium. Flattened cells with irregular or wavy outlines. Nucleus flat, oval or round in outline.

a. Endothelium. Lines the cavities of the heart and the lumina of the blood and lymph vessels.

b. Mesothelium. Similar to the preceding except that it lines the serous (closed) cavities of the body (pericardium, pleura and abdominal cavity).

2. Cuboidal. In vertical section the cells appear as squares; on surface view they have polygonal shape due to mutual pressure.

3. Columnar or prismatic. The cells are tall prisms due to mutual pressure but in a vertical section appear as rectangles. The basal end of the cell - in contact with the basement membrane — is often pointed or even branched (intestine).

4. Ciliated. The cells bear cilia on their free surfaces. The cilia are either motile (bronchi, oviduct) or non-motile, resembling a brush border.

B. Stratified (many-layered) epithelium. The shape of the cells varies in the different layers, which are produced through mitoses of the cells resting on the basement membrane (basal layer). The most superficial cells are, therefore, the oldest.

1. Stratified squamous. The cells on the surface are flat; below they gradually change into irregular polyhedral cells. The basal layer is formed of cuboidal or low columnar cells.

a. Production of keratin (cornification) results in the disappearance of the nuclei of the superficial cells (skin).

b. In the absence of cornification the superficial cells retain their nuclei and are not so flat (mouth cavity, oesophagus, cornea, vagina, etc.).

2. Stratified columnar. The superficial cells are columnar but fail to reach the basement membrane (cavernous urethra).

3. Pseudostratified. It contains columnar cells extending from the basement membrane to the surface of the epithelium (i.e. a single layer of tall cells) whereas the other cells fall short of the surface and are crowded among the bases of the columnar cells, forming two or more layers.

a. Columnar (female urethra).

b. Ciliated. The tall cells have motile cilia (respiratory passages) .

c. Stereociliated. The cilia are non-motile (epididymis).

C. Transitional epithelium. The number of cell layers varies according to the degree of contraction of the organs in which it occurs (ureter, bladder).

1. When contracted it consists of several layers and resembles closely the stratified epithelium, but the superficial cells are larger and have a convex free surface.

2. If stretched there are usually two layers: a deep layer of more or less cuboidal cells, and a superficial layer of large flattened cells.

II. Glandular Epithelium

Composed of secretory cells which show a definite polarity: the lower (basal) half contains the nucleus, the upper (apical) half is filled with granules or droplets of secretion.

The secretion may leave the cell as a fluid which crosses the permeable cell membrane: merocrine type (the most widespread); or in leaving it may destroy the membrane and surface protoplasm: apocrine type (mammary gland and armpit glands) ; or lastly, the secretion is not released until the cell dies and disintegrates: holocrine type (sebaceous glands). In the latter type there is constant replacement of the dead cells by new ones.

A. Exocrine (external secreting) glands. The secretion flows out through a duct which may open on the body surface (glands of the skin, mammary gland) or in cavities which communicate with the outside (glands of the alimentary canal, kidney, etc.).

1. Unicellular glands. Cells scattered among the elements of an epithelium; they usually secrete mucus.

a. Typical is the goblet cell of the intestine, respiratory mucosa, etc.

2. Multicellular glands. The epithelial secretory cells are arranged in various ways, as for instance, straight or coiled tubules, branched tubules, or in small vesicles (acini or alveoli).

a. Simple. The secretory units open directly on the surface of an epithelium (intestinal and sweat glands) or they open into a single simple duct (glands of the stomach, uterus, etc.).

b. Compound. The secretory units are much more numerous and are grouped into lobules more or less completely separated by connective tissue septa. The ducts of each lobule converge into larger ducts and the latter finally open into a main duct (salivary glands, lacrimal gland, etc.).

B. Endocrine (internal secreting) glands. These lack ducts; their secretions (called hormones) enter the blood stream after crossing the walls of the capillaries of the gland.

1. The glandular epithelium is arranged into vesicles (thyroid) or it forms irregular anastomosing strands (parathyroids, anterior pituitary, suprarenals) .

C. Exo-endocrine glands. Compound glands with external and internal secretions. The endocrine portion may be represented by cell groups (islands of Langerhans of pancreas) ; or the gland cells may produce an external secretion conveyed by ducts and an internal passing directly into the blood (liver).

III. Neuroepithelium

Formed by cells adapted to receive stimuli which they transmit to sensory nerve endings. The free surface of the cells may have stiff cilia (cells of organ of Corti, maculae and cristae of the inner ear; gustatory cells). The epithelioid cells of certain chemoreceptors (carotid glomus, aortic bodies, p. 29) are probably neuroepithelium. (The cones and rods of the retina and the olfactory cells are modified nerve cells.)

Blood and Lymph

I. Blood

A tissue with fluid intercellular substance (blood plasma); the cell elements are: erythrocytes (red blood corpuscles) and leucocytes (white blood corpuscles). There are also non-nucleated corpuscles, the platelets.

A. Erythrocytes. In mammals they are biconcave discs measuring from 7.5 to 7.7P. The central, thin area was formerly occupied by the nucleus. Remnants of the latter are occasionally seen (HowellJolly bodies). Contain hemoglobin. There are normally 4% to 5 millions per cubic millimeter.

B. Leucocytes. Nucleated cells, much less numerous (5000 to 9000 per c. mm. in the human adult). Some lack granules in their cytoplasm (agranulocytes), while others have abundant granulations (granular leucocytes or granulocytes).

1. Agranulocytes.

a. Lymphocytes. Their size varies between 6 \i (small) and 8-9P (large lymphocytes). Nucleus round or slightly irregular, with heavy chromatin blocks. Ba sophilic cytoplas m. Ameboid but non-phagocytic. 20-25% total leucocyte count.

b. Monocytes (large mononuclears). Larger than the preceding (9 to i2p). Nucleus often kidney-shaped. Abundant, slightly basophilic cytoplasm with occasional granules. Non-phagocytic while in the blood stream. 3-8% of leucocyte count.

2. Granulocytes.

a. Neutrophils (also called polymorphonuclears or simply ‘polys’). Ameboid cells (10-12P) with very irregular, deeply staining nucleus and cytoplasm containing pale violet or lilac granules. Most abundant of the leucocytes (65 to 75%); they ingest small foreign particles and bacteria.

b. Acidophils or eosinophils. Rarer than the preceding (2 to 5%). Nucleus not so irregular, often bilobed. Cytoplasm filled with granules stained with acid dyes.

c. Basophils. Rarest of all (0.5%). Granules are irregular, strongly basophilic. Nucleus irregular, lightly stained. Regarded by some as degenerative forms.

C. Platelets. Small (about 3P), flattened non-nucleated bodies with a central granular area (endoplasm) and a rim without granules (ectoplasm). They appear as clusters in smears and are supposed to play a part in blood clotting.

D. Formation of the blood (hemopoiesis). In the embryo blood arises first in the wall of the yolk sac, then in the liver and spleen and, finally, in the bone marrow. Most blood cells in the yolk sac and liver are erythrocytes. Blood is formed in the adult in the bone marrow but most lymphocytes arise elsewhere (see lymph).

1. Erythropoiesis: formation of erythrocytes. The chief stages ^re:

a. Basophilic erythroblasts. They are the source of the erythrocytes in the adult. Cells with basophilic cytoplasm and heavy chromatin blocks in the nucleus (checker-board appearance). Divide.

b. Normoblasts. Smaller than the preceding and slightly larger than the erythrocytes. Small, deeply stained nucleus. Hemoglobin present in the cytoplasm. The younger normoblasts are still capable of division.

c. Erythrocytes arise through loss of the nuclei of the normoblasts. The dense, pyknotic nucleus may be extruded or it breaks up into particles which gradually disappear.

2. Leucopoiesis: production of leucocytes. The granulocytes pass through the following stages:

a. Myeloblasts. Basophilic cells with large nucleus containing a chromatin network and one or more large, compound nucleoli. Abound in fetal and young marrow; rarer in the adult. Divide by mitosis.

b. Myelocytes. Cells with roundish nucleus and a larger amount of cytoplasm containing granules which may be eosinophilic, basophilic or neutrophilic. Divide by mitosis; present in large numbers in adult marrow.

c. Metamyelocytes. Immature granulocytes with horseshoeshaped nucleus, occasionally seen in the blood, especially in infections. They change into granulocytes.

3. Formation of the monocytes. This is a much debated question.

a. Some authors suppose that they are derived from specific cells (monoblasts) in the bone marrow.

b. Others see their origin in endothelium or in mesenchyme.

c. A third view is that they arise from lymphocytes in venous sinuses in the spleen, bone marrow, liver, etc.

4. Formation of the platelets. These arise through fragmentation of giant cells called megakaryocytes (see bone marrow; p. 18).

II. Lymph

A. Composition. A colorless fluid collected from all over the body and normally containing variable numbers of small lymphocytes. A few granulocytes (mostly eosinophils) may occur. The lymph from the small intestine contains much fat and has a milky appearance^ during digestion; it is then known as chyle.

B. Formation of the lymphocytes (lymphopoiesis). Lymphocytes arise in large numbers in the lymph nodes, spleen, thymus, and in patches of lymphoid elements in diverse organs. The following stages are recognizable:

1. Medium-sized lymphocytes. Cells with relatively large nucleus, containing one or more nucleoli. Basophilic cytoplasm. Divide by mitosis giving rise to small lymphocytes.

2. Large lymphocytes (lymphoblasts). Regarded generally as hypertrophied small lymphocytes; produce lymphocytes of smaller sizes through mitotic division. They may be scattered or grouped into - germinal centers - (p. 41).

The Supporting Tissues

They constitute the framework of the body. They are: connective tissue, reticulo-endothelium, cartilage and bone. They are all of mesodermic origin.

I. Connective Tissue

Present in all organs. Its characteristic cells are fibroblasts (or fibrocytes) separated by variable amounts of intercellular substance containing fibers. The latter may be moderately abundant, loose connective tissue; or they may predominate, fibrous connective tissue.

A. Loose connective tissue. Fibroblasts and other cell types present.

1. Fibroblasts. Rather fixed, flattened, irregularly branched cells with large nucleus. Non-phagocytic.

2. Fibers. Produced through activity of the fibroblasts.

a. Collagenous. Bundles of fine fibrils held together by a cementing substance. They vary in size and anastomose freely. They yield gelatin when boiled with water.

b. Argyrophil. An extension of the preceding in areas in which cells are closely placed. They are impregnated with silver, hence their name. May change into collagenous fibers.

c. Elastic. Homogeneous, branched fibers anastomosed into loose networks. They are highly refractive and thinner than the collagenous.

3. Ground substance. Believed to be a system of thin membranes delimiting irregular spaces filled with fluid which increases in edema.

4. Other cell types.

a. Leucocytes migrated from the blood.

b. Resting and wandering histiocytes. When resting they have ragged outlines; nucleus smaller than in the fibroblast. Become ameboid and phagocytic in inflammation and engulf foreign matter (macrophages).

c. Mast cells. Irregular elements with small nucleus and cytoplasm loaded with strong basophilic granules. Normally rare.

d. Plasma cells (plasmocytes). Also rare except in certain locations and in chronic inflammations. One or two nuclei closely resembling the nucleus of the lymphocyte, from which they may arise. Basophilic cytoplasm. Non-phagocytic.

e. Fat cells. Fat may appear as independent droplets or the latter merge into a single large drop which fills the cell. When very abundant they form adipose tissue.

B. Fibrous connective tissue. The cells present are mosdy fibroblasts. The fibers are very abundant, the intercellular spaces very much reduced. The collagen or the elastic fibers may predominate.

1. Mostly collagenous. The bundles may be arranged regularly (tendons, ligaments, aponeuroses and fasciae) or irregularly (dense connective tissue). Whitish aspect due to abundance of collagen.

a. Tendons and ligaments. Thick, closely placed and parallel collagenous bundles. The fibroblasts appear as rows between the bundles and are very much compressed. The arrangement of the bundles is less regular in the ligaments.

b. Aponeuroses and fasciae. Bundles woven into a very compact meshwork containing also elastic fibers. Fibroblasts and other connective tissue cells present (derma of skin, sclera of the eye, periosteum, submucosa of alimentary canal, etc.).

2. Mostly elastic. Less frequent than preceding. Has yellowish color. The fibers form parallel strands with frequent anastomoses (ligamenta flava of the vertebrae, stylohyoid ligament, ligamentum nuchae, etc.).

II. Reticulo-endothelium

It consists of two closely associated parts: a meshwork of argyrophil fibers (reticulum) and fixed and wandering histiocytes. The latter are often flattened and contribute to the lining of the lymph and blood channels, thus resembling endothelial cells. Since endothelium is not phagocytic the term given to this close association of elements is a misnomer. Reticulo-endothelium forms the framework of the lymph nodes, spleen, bone marrow and liver; it also occurs in some endocrines (pituitary, adrenal), the uterine mucosa, skin, etc.

A. Reticulum. The argyrophil fibers vary in diameter and are repeatedly anastomosed. They are continuous with the collagen fibers.

B. Histiocytes (clasmatocytes, macrophages). They are identical with the resting and wandering histiocytes of loose connective tissue. The resting cells may become ameboid and ingest bacteria, foreign particles and degenerated cells.

1. Resting histiocytes. They are either anchored to the reticulum (which they may partly envelop with their prolongations), or they lie among the argyrophil fibers. According to their location they have received special names:

a. Reticular cells. Occur in the lymph nodes.

b. Splenocytes. Present in large numbers in the red pulp of the spleen.

c. Kupffer cells. Scattered among the endothelial cells lining the liver sinusoids.

d. Adventitial cells. In the externa (or adventitia) of the smaller vessels of the body.

2. Wandering histiocytes. Ameboid cells resembling the blood monocytes; their nuclei are often kidney-shaped. They may phagocytose whole cells (erythrocytes, lymphocytes, etc.).

C. Other cell types. Undifferentiated (mesenchyme) cells and fibroblasts also occur associated with the reticulum.

D. Functions. Reticulo-endothelium is an important part of the defenses of the body. Besides being phagocytic its cells are supposed to produce antibodies. The latter activity is said to be diminished when the cells are loaded with ingested particles (India ink, lithium carmine, etc.). This is called ‘blockage’ of the reticulo-endothelial system.

III. Cartilage

Consists of roundish cells (chondrocytes) separated by a solid intercellular substance which may become calcified in the adult. Cartilage is surrounded by a layer of dense connective tissue (perichondrium) and transitions between its fibroblasts and the superficial chondrocytes are found. It lacks vessels of its own, as well as nerves.

The ground substance has a homogeneous, glassy aspect (hyaline cartilage) or is occupied by elastic networks (elastic cartilage). In a third variety the cells occur as elongated groups separated by collagenous bundles (flbrocartilage).

A. Hyaline cartilage. It has a distinct whitish color when fresh. When boiled with water yields gelatin.

1. Cells. Although usually spherical they may be flattened through mutual pressure. The cells in the surface are also flattened. The nucleus is rather large and the cytoplasm contains glycogen, fat droplets and vacuoles.

2. Ground substance. It has cavities or lacunae occupied by the chondrocytes, and numerous fibrils.

a. Lacunae. Each lacuna is normally occupied by a single chondrocyte. In the adult the lacunae usually occur in groups corresponding to cells derived by division of a single chondrocyte (isogenic groups).

b. Capsule. This is distinct layer around each lacuna; it differs from the rest of the ground substance in that it is more refractile and stains more deeply.

c. Fibrils. Although the ground substance is apparently homogeneous it contains a close meshwork of fine collagenous fibrils visible only after digestion with trypsin or impregnation with certain silver techniques.

3. Distribution. Hyaline cartilage is the most typical and widespread. It forms most of the embryonic skeleton and is later replaced by bone except in certain places (ventral end of the ribs, articular surfaces of all bones). It also occurs in the nose, larynx, trachea and bronchial tree.

B. Elastic cartilage.

1. Cells. They do not differ from those in the preceding variety.

2. Ground substance. It also has lacunae bounded by capsules. It contains elastic networks which may be very dense and are continuous with the elastic fibers of the perichondrium.

a. The abundance of elastic fibers imparts a yellowish color to this variety of cartilage.

3. Distribution. Elastic cartilage is not so common as the hyaline variety. It is found in the pinna of the ear, external auditory meatus and eustachian tubes, epiglottis, certain cartilages of the larynx, etc.

C. Fibrocartilage. It is a transition between the dense connective tissue of tendons and ligaments and the hyaline cartilage. Accordingly its cells tend to form rows. It lacks a true perichondrium.

1. Cells. They are spherical or slightly elongated.

2. Ground substance. The poorly developed, hyaline ground substance may be condensed as a capsule around each cell. The cell groups are separated from each other by collagenous bundles continuous with those of the tendon or ligament.

3. Distribution. It is found in the interarticular cartilages (lower jaw, knee, clavicle), intervertebral disks, symphysis pubis and some other locations.

IV. Bone

Bone is a tissue consisting of branched cells (osteocytes) separated by a fibrillar intercellular substance hardened by calcium salts. It forms the greater part of the mammalian skeleton.

A. Structure.

1. Osteocytes. Cells with rather large, deeply stained nucleus and faintly basophilic cytoplasm continued into branched, freely anastomosing processes.

2. Lacunae. Each osteocyte is contained within a lacuna, which it fills completely. From the lacuna arise fine canals (bone canalicules) which contain the processes of the osteocyte. Around each lacuna there is a thin capsule.

3. Interstitial substance. Apparently homogeneous but actually formed by thin bundles of fibers (osteocollagenous fibers) which yield gelatin when boiled with water. They can be demonstrated by silver impregnation.

a. The osteocollagenous fibers are cemented together by an amorphous substance.

b. Removal of the calcium salts by weak acid solutions (decalcification) leaves the organic portion intact.

4. Mineral substance. According to most authors the calcium deposits occur only in the amorphous substance binding the osteocollagenous fibers. The calcium is present mostly as dahllite (Ca Co3.2Ca 3 (P0 4 )2). The abundance of calcium salts causes the bone to retain its shape and structure after maceration. In thin plates of dry bone the empty lacunae and the canalicules can be readily recognized.

B. Types of bone. Bone may have a spongy or a compact appearance. In long bones the shaft or diaphysis consists chiefly of compact bone inclosing a large marrow cavity, while the epiphyses are made of spongy bone with a thin, peripheral layer of compact bone. Short bones closely resemble the epiphyses.

1. Spongy (cancellous) bone. Bone is first laid down as a framework of anastomosing trabeculae. The intertrabecular spaces are occupied by marrow. Bone-forming cells (osteoblasts) and bonedestroying elements (osteoclasts) are often seen on the surface of the trabeculae.

2. Compact bone. The interstitial substance is divided into regularly arranged, thin bony plates or lamellae; this arrangement is closely connected with the distribution of the blood vessels which supply the bone.

a. Haversian systems. An Haversian system contains a small central canal occupied by small blood vessels and lymphatics.

(1) The canai is surrounded by a number of concentrically arranged, thin lamellae (4 to 20 or more) appearing as rings in cross section.

(2) The direction of the osteocollagenous fibrils is different in each lamella since they run spirally to the axis of the Haversian canal, each spiral being independent from those in adjacent lamellae.

(3) In and between the lamellae there are osteocytes, and their anastomosing canalicules open into the canal. The canals branch and anastomose with each other.

b. Interstitial lamellae. They fill the spaces between the Haversian systems. Most of them are the remnants of these systems after partial destruction during bone formation.

c. Circumferential lamellae. They occur on the external surface of compact bone and on the wall of the marrow cavity of long bones. They also contain osteocytes enclosed within lacunae.

d. Sharpey’s (perforating) fibers. These are collagenous bundles of varying thickness which enter the compact bone from the periosteum.

e. Volkmann's canals. Vascular branches from the periosteum enter through these thin tunnels, which are not surrounded by concentric lamellae. They connect with the Haversian canals.

C. The bones as organs. Closely associated with the bone tissue proper are:

1. Periosteum. A dense connective tissue envelope firmly attached to the surface of the bone. It is absent in the articular surfaces and the inner surface of the cranial bones. It consists of two layers :

a. An outer (vascular) layer of dense connective tissue rich in blood vessels and containing also lymphatics and nerves. Branches of these vessels enter the bone through Volkmann’s canals.

b. An inner (fibrous) layer of collagenous and elastic fibers. The cells in this layer are flattened and under certain conditions (i.e. after fractures) they give rise to osteoblasts and osteoclasts. The former abound in the embryo and young in which they may form a third (osteoblastic) layer.

2 . Endosteum. A thin connective tissue layer lining the larger bone cavities, especially the central (marrow) cavity in long bones.

3. Bone marrow. Fills the cavities of the bones. It consists of a reticulo-endothelial framework continuous with the endosteum. The meshes of the framework contain:

a. Numerous thin- walled vessels.

b. Megakaryocytes. Giant cells with very irregular, often ringshaped, large nucleus which stains deeply. Cytoplasm with a central granular portion (endoplasm) and a non-granular superficial layer (ectoplasm). Megakaryocytes are the source of the blood platelets (p. io).

c. Intermediate stages in the formation of the blood cells (p. io ) d. Fat cells. Abound in the adult, especially in the shaft of the long bones where the marrow is really adipose tissue (yellow marrow). The blood-producing (red) marrow remains throughout life in the epiphyses of the long bones and in the vertebrae, ribs, sternum and base of the skull.

D. Formation of bone (osteogenesis). Bone tissue is laid down relatively late in fetal life. Previous to this the skeleton consists of hyaline cartilage. The latter is then replaced by bone (endochondral ossification) but certain bones may arise directly from mesenchymal tissue (intramembranous ossification).

1. Intramembranous ossification. The bones of the vault of the cranium, the flat bones of the face, and the jaw (membrane bones) arise by this method.

a. Formation of the osteoblasts. Mesenchymal cells are changed into these without losing their processes, but their cytoplasm becomes decidedly basophilic and the nucleus excentric. Between the modified mesenchyme cells there are delicate bundles of collagenous fibrils.

b. Deposition of intercellular substance. The osteoblasts deposit fibrillar bone in the form of anastomosing trabeculae, which increase in size through the activity of newly formed osteoblasts and division of preexisting ones.

c. Formation of the osteocytes. Numbers of osteoblasts become imprisoned within the trabeculae to form the osteocytes of mature bone.

d. Calcification. The newly formed trabeculae soon become calcified.

2. Endochondral (intracartilaginous) ossification. This implies the removal of the preexisting cartilage and its replacement by bone. Accordingly, the first stages (a-d) are those of degeneration of the cartilage.

a. Hypertrophy of the chondrocytes. The cartilage cells (chondrocytes) and the lacunae become much larger, with a corresponding decrease in the amount of intercellular substance. Many of the enlarged lacunae become confluent.

b. Degeneration of the chondrocytes. After their initial hypertrophy the chondrocytes shrink and finally disappear.

c. Calcification of the remaining ground substance. The strips of ground substance between the groups of confluent lacunae — which form parallel rows in the diaphyses of the long bones — become calcified.

d. Formation of the primordial marrow cavities. They arise through further merging of the enlarged lacunae and will be occupied by the marrow and numerous blood vessels.

e. Penetration of the osteomyelogenic mesenchyme. During the preceding stage the innermost of the layers of the fetal periosteum forms buds which corrode the peripheral portion of the cartilage, bringing into the primordial marrow cavities mesenchymal elements which will give rise to osteoblasts and cells of the primitive bone marrow, respectively.

f. Penetration of the blood vessels. Vascular loops and buds from the periosteal vessels also grow rapidly within the primordial marrow cavities. In long bones they advance toward the ephiphyses which are ossified independently from the diaphysis.

g. Formation of the osteoblasts. The mesenchyme cells near the surface of the bars of calcified cartilage become osteoblasts, which are cells of various shapes with basophilic cytoplasm and excentric nucleus. Slender prolongations may arise from their cytoplasm.

h. Deposition of osteoid. The osteoblasts arrange themselves into an irregular layer on the surface of the strips of calcified cartilage and deposit soft bone. Those which remain imprisoned within the latter become osteocytes. The osteoid or pre-osteal substance soon becomes calcified.

i. Absorption of the newly-formed bone. During fetal life and also in early post-natal life growth of the bones is continuous. To avoid massiveness and permit enlargement of the primordial marrow cavities there is a process of destruction and reconstruction of the bone. Destruction is effected by the osteoclasts which are cells with acidophilic cytoplasm containing several nuclei (up to 20 or more). They are seen in contact with the surface of the bony trabeculae.

j. Periosteal ossification. Osteoid is laid down in the periphery by the osteoblasts of the inner layer of the periosteum. In long bones a collar or cylinder of osteoid is formed around the diaphysis even before the buds of osteomyelogenic mesenchyme have penetrated the degenerated cartilage. Most of the compact bone is formed this way.

k. Epiphyseal ossification. The epiphyseal ossification does not begin until about the time of birth. The osteomyelogenic mesenchyme reaches the center of the epiphyseal cartilage from the diaphysis. Spongy bone is formed by the osteoblasts. Between the epiphysis and the diaphysis there is a line of actively growing cartilage (epiphyseal line or synchondrosis) which maintains the growth of the bone in length until it is terminated in adult life.

Muscular Tissue

Muscular tissue consists of much elongated cells (muscle fibers) differentiated for contractility. The latter is practically limited to one direction, the long axis of the cell. The contractile elements are special fibrils (myofibrils) which develop within the embryonic muscle cells or myoblasts.

Three varieties occur in the adult: Smooth, striated (skeletal) and cardiac muscle, respectively. With few exceptions (iris of the eye, sweat glands) muscular tissue arises from mesoderm.

I. Smooth Muscle

Smooth muscle fibers are long spindles (20 to 500P) with centrally placed nuclei. They abound in the arteries, alimentary canal, bladder, uterus, etc.

A. Structure.

1. Cell membrane. It is indistinct. Intercellular bridges connecting

the cells are known to occur.

2 . Sarcoplasm. The cytoplasm or sarcoplasm is occupied by numerous myofibrils. The latter are long, slender filaments of a homogeneous substance; those in the periphery are coarser (border or external fibrils). The myofibrils are barely visible in routine preparations.

3 . Nucleus. Its shape varies according to the degree of contraction of the fiber. If the latter is relaxed the nucleus is almost cylindrical. In much contracted fibers it appears lobulated or even twisted. It has a pale chromatin network and one or more nucleoli.

B. Appearance in sections. Smooth muscle fibers are either scattered or they form definite bundles and layers. The direction of the fibers is the same in each bundle or layer. Since the thick middle portions of the fibers (containing the nucleus) do not lie at the same level, a cross section of a bundle will show only a few nuclei and many non-nucleated portions, i.e. sections of the tapering portions of adjacent fibers.

C. Development. Smooth muscle arises from myoblasts in the visceral or splanchnic layer of the mesoderm (p. 109). The early myoblast is a short spindle-shaped cell containing cytoplasmic granules which gradually merge to form the myofibrils.

II. Striated (Skeletal) Muscle

The fibers are long (1 to 40 mm.), contain many peripherally placed nuclei and numerous myofibrils composed of alternating light and dark disks; the latter cause the cross-striated appearance.

A. Structure.

1. Cell membrane (sarcolemma). A very thin, structureless membrane which envelops the fiber.

2. Sarcoplasm. It fills the spaces between the numerous myofibrils. It is best seen around the nuclei. According to the amount of sarcoplasm two varieties of fibers (mixed in the human muscles) can be differentiated :

a. Red fibers. These are rich in sarcoplasm and relatively poor in myofibrils. Their color is due to the presence of a diffuse, red pigment. They are less easily fatigued than the following.

b. White fibers. They contain less sarcoplasm; the myofibrils are more numerous and thinner. They predominate in the least active muscles.

3 . Myofibrils. They are clearly visible, especially in cross sections of the fibers in which they appear as dots, evenly distributed or grouped into polygonal areas (fields of Conheim) separated from each other by tracts of sarcoplasm. They consist of alternating dark and light disks.

a. Dark or anisotropic disk. Made of double-refractive substance which is bisected by a narrow light disk (disk of Hensen).

b. Light, single-refractive (isotropic) disk. It is likewise bisected by a thin, dark band: the membrane of Krause or telophragm. Krause’s membrane, however, forms a transverse partition across the whole fiber and is united with the sarcolemma peripherally.

c. Sarcomeres. These are the portions of the myofibrils between two membranes of Krause.

4. Nuclei. Their number depends roughly on the length of the fiber and may reach several hundred. In fully developed fibers they are always found in the periphery, under the sarcolemma; in the muscle fibers of infants and in some red fibers a few centrally placed nuclei may be seen. The nuclei are oval or elliptical and have one or more nucleoli.

5. Motor plates. These correspond to the areas of termination of motor nerve fibers on the muscle. Normally there is one plate for each fiber, placed midway between the ends or closer to one end (tongue). The motor plate causes a swelling on the surface of the fiber and is partially surrounded by nuclei (p. 28) .

B. Development. Striated muscles arise from the embryonic myotomes (see p. 109). The latter consist of spindle-shaped myoblasts, each of which has at first a single nucleus.

1. Formation of the myofibrils. They appear first as homogeneous threads, but they soon show thickenings, the forerunners of the anisotropic disks. They increase in numbers through splitting.

2. Nuclear multiplication. Through repeated divisions numerous nuclei are formed. They appear as a row in the center of the fiber, while the periphery is occupied by the myofibrils. In the meantime the fiber becomes elongated.

3. Migration of the nuclei. The nuclei finally migrate toward the periphery while the myofibrils move toward the center of the fiber.

4. Fusion of fibers. According to some authors a striated muscle fiber is not a single, elongated cell but a syncytium arising through fusion of several myoblasts. In this way greater length is attained.

C. The skeletal muscles as organs. Muscles are formed of large numbers of parallel muscle fibers bound together by connective tissue. The fibers are grouped into primary bundles, several of which form secondary bundles, etc.

1. Epimysium. This is a sheath of connective tissue (also known as the external perimysium) enveloping the muscle.

2. Perimysium. Partitions arising from the epimysium enter the body of the muscle.

3. Endomysium. Represented by the connective tissue which separates the muscle fibers within the primary bundles.

4. Muscle-spindles. These are groups of poorly differentiated muscle fibers enclosed within a connective tissue sheath. The latter is perforated by one or more sensory nerve fibers which end around the muscle fibers (p. 29).

5. Muscle-tendon junction. Many histologists believe that there is no transition between the muscle fibers and the collagenous bundles of the tendon; the former end abruptly, their rounded ends being covered by the sarcolemma to which are attached the collagenous fibers. This view is not held by others.

III. Cardiac Muscle

The characteristic muscle of the heart is striated but it differs from skeletal muscle in that it is formed by repeatedly anastomosed fibers (really a syncytium). The nuclei are centrally placed in the syncytial strands.

A. Structure.

1. Sarcolemma. It is a delicate membrane best seen in longitudinal sections of contracted fibers.

2. Sarcoplasm. It fills the spaces between the myofibrils; it is more abundant near the poles of the nucleus where it usually has granules of pale yellow pigment increasing with age.

3. Myofibrils. They are coarser than in skeletal muscle and have the same structure. Krause’s membrane is also present.

4. Nuclei. The nuclei are oval, but may appear irregular in cases of extreme contraction.

5. Intercalated disks. Either straight bands, or V-shaped stripes or a group of disks arranged like steps, highly characteristic of cardiac muscle. They are usually indistinct in routine slides, but can be brought out with certain dyes.

a. They were formerly regarded as the thickened membranes at the end of the fibers; however several intercalated disks may occur in close proximity, without nuclei in between.

b. Their function is unknown.

B. Development. Cardiac muscle tissue is derived from the splanchnic mesoderm in contact with the cardiac tubes (p. 133). The myoblasts are at first independent but they soon merge into a syncytium; the nuclei multiply and the myofibrils arise as in skeletal muscle. A lesser degree of differentiation is characteristic of the muscular portions which become the conductive system of the heart (p. 39).

Nervous Tissue

Nerve cells are built for the reception of stimuli and their transmission to other nerve cells or to diverse structures (musculature, glands, etc.). They are the most highly specialized cells of the body. Nervous tissue consists of two different elements: the nerve cells or neurones, and the supporting tissue or neuroglia, respectively.

I. Nerve Cells (Neurones)

A typical nerve cell has branched processes arising from the cell body or perikaryon. One of these, called the axon, carries impulses away from the cell body (i.e. centrifugally) while the others, known as dendrites, generally receive impulses which they transmit in the opposite direction (i.e. centripetally).

A. Shape. The shape of the nerve cell is highly variable, as well as its size. According to the number and position of the processes three main types are recognized:

1 . Multipolar. Has one axon and several dendrites.

2 . Bipolar. Only two processes, one of which is a dendrite, the other the axon (retina; ganglia of the inner ear).

3 . Monopolar. The single process soon bifurcates: one branch corresponds to the dendrite (i.e. it transmits centripetally), the other is the axon (most cells of the cerebrospinal ganglia except those noted above).

4 . Transitions between 2 and 3 are of common occurrence in the cerebrospinal ganglia.

5 . Dendrites. They may be poorly or profusely branched. In manycells they are beset with very small spiny-like processes (gemmules).

6. Axon. The axon or axis-cylinder is a thin, smooth process of variable length. It gives of? branches roughly at right angles to its axis (collaterals) and ends in a tuft of branches (terminal arborization).

a. Short axon type. The axon breaks up into numerous branches near its emergence from the cell body.

b. Long axon type. The terminal arborization occurs at a variable distance from the cell body (a few millimeters to many inches).

7. Neurone theory. A nerve cell, including its dendrites and axon, is regarded by most histologists as a unit or entity, termed neurone.

a. The neurone theory claims a lack of continuity between the processes of one neurone and those of another.

b. According to this view the nervous system consists of chains of independent neurones associated with each other in various ways.

B. Structure.

1. Cytoplasm (neuroplasm). A homogeneous, semiliquid and highly viscid substance which occupies the entire neurone. In the axon it is called axoplasm. A cell membrane is absent.

2. Neurofibrils. Threads of variable thickness (seen in the living) which run in every direction and extend into the dendrites and axon to the finest terminal twigs. They anastomose frequently to form an elaborate meshwork within the cell.

a. Neurofibrils are impregnated with silver techniques (neurofibrillar methods).

b. They have been regarded by some as the substratum for transmission of nervous impulses, while others consider them as a supporting framework of the neurone.

3. Chromophilic (tigroid) substance. Occurs in large and medium-sized (chromophilic) neurones.

a. Structure. It consists of granules clumped together into masses of various sizes (Nissl bodies). The granules stain deeply with basic aniline dyes. They are, most probably, preexisting structures, not artifacts, which occupy the meshes of the neurofibrillar framework.

b. Distribution. When present, the Nissl bodies occur not only in the cell body but also in the dendrites; they are absent in the axon and the point of its implantation on the cell body (axon hillock).

(i) The form, size and distribution of the Nissl bodies is extremely variable in different cells and may be related to variations in the density of the neurofibrillar meshwork.

c. Changes. Nissl bodies decrease in numbers and even disappear in fatigued nerve cells and in certain pathological processes (chromatolysis). They are similarly affected after injury to the axon.

4. Nucleus. The nucleus is roughly spherical and relatively large, specially in the motor neurones of the spinal cord. Its structure varies according to whether the neurone is chromophilic or chromophobic (i.e. lacks Nissl bodies).

a. Chromophilic neurones. The large nucleus contains a linin meshwork (p. 2), abundant nuclear sap and a large nucleolus (rarely two or more). The chromatin is scanty, being represented by one or two dense, small masses near the nucleolus.

b. Chromophobic neurones. In small and many medium-sized neurones the chromatin is scattered over the linin meshwork or forms large knots.

5. Organoids and inclusions. Nerve cells possess a well-developed Golgi apparatus and mitochondria. Inclusions are represented chiefly by a yellowish pigment (lipochrome), to which the color of the gray matter is due. This type of pigment increases with age. Melanin occurs only in certain areas of the central nervous system and in the neurones of the cerebrospinal and sympathetic ganglia.

6. Cytocentrum. Although adult nerve cells do not divide a cytocentrum containing two centrioles of unequal size and shape has been described.

C. Nerve fibers. Since axons are the chief constituents of the peripheral nerves they are called nerve fibers, a term also used for the long axons within the central nervous system. In the peripheral nerves axons are always enclosed within sheaths; in the central nervous system there are both sheathed and naked axons.

1. Structure of the axon. It contains numerous neurofibrils placed so closely that they cannot be identified as such except in thick axons. Thin axons always show a compact structure. The neurofibrils in either case may be seen in the branches of termination.

2. Sheath of Schwann or neurilemma. Present in the peripheral nerves. It is a delicate membrane consisting of a single layer of flattened, branched cells (cells of Schwann); the branches wrap themselves around the axon.

a. They are not of connective tissue nature but probably belong to neuroglia (see p. 33).

b. They are important as nutritive elements for the axon and also play an active part during nerve regeneration.

3. Myelin sheath. Occurs in the peripheral nerves and also in the central nervous system. It is an homogeneous envelope made up of a mixture of various lipoids (of which cholesterol is the most important), certain cerebrosides, phospholipins and fatty acids.

a. Nodes of Ranvier. In the central nervous system the myelin sheath is practically continuous. In peripheral nerves it is divided into segments by constrictions, called nodes of Ranvier. The thicker the axon the longer the segments.

b. Clefts of Schmidt-Lantermann. The myelin of each segment is practically interrupted by oblique clefts; the segment is thus divided into funnel-shaped sections.

c. Relation of the Schwann cells to the myelin segments. Each myelin segment is surrounded by a single cell of Schwann. In thick fibers the nucleus of the Schwann cell usually appears within a shallow depression of the myelin segment.

4. Myelinated and unmyelinated fibers. The thickness of the myelin sheath is in direct ratio to the diameter of the axon. In very thin myelinated fibers the sheath is not visible unless it is stained. Unmyelinated fibers are grouped into small bundles enclosed within a syncytium of Schwann cells.

5. The nerves as organs. One or more bundles of nerve fibers enclosed within a connective tissue sheath is called a nerve. The bundles and the nerve fibers within them are bound together by connective tissue which carries the vessels for the nerve.

a. Epineurium. This is the outermost connective tissue sheath.

b. Perineurium. The sheath around each individual fascicle. When several fascicles occur (i.e. in the sciatic nerve) the space between the perineurium and the epineurium contains connective tissue and fat cells.

c. Endoneurium. Septa from the preceding, dividing the nerve fascicle into compartments, are known collectively as the endoneurium. The compartments are occupied by numerous nerve fibers.

d. Sheath of Key-Retzius. The nerve fibers are separated from each other by fine collagenous bundles which collectively form this sheath. A still finer sheath composed of delicate argyrophil fibers occurs in contact with the cells of Schwann.

D. Nerve endings. These are the terminations of the axons and their branches (collaterals). The endings may be located within the central nervous system and ganglia (see synapse) or in various tissues in diverse parts of the body. The latter will be considered here.

1. Efferent. They discharge nervous impulses on the muscle fibers, ganglia, and the cells of various glands.

a. Motor plates. Their position has already been indicated (p. 22). The nerve fiber ending within the plate loses its myelin before it crosses the sarcolemma. The (hypolemmal) ending has short, irregularly dilated branches; nuclei occur among them.

(1) The boundary between the plate and the sarcoplasm (the “sole”) has nuclei belonging to the muscle fiber.

b. Diffuse motor endings. Found in the smooth musculature of various organs and in cardiac muscle. The terminal arborizations have long branches which touch the surface of the muscle fibers (epilemmal endings).

c. Secretory endings. Occur in contact with the secretory cells of certain glands (salivary and lacrimal gland, pancreas, etc.).

d. Preganglionic endings. Are located within the ganglia of the sympathetic and parasympathetic divisions of the autonomic system. They are the terminations of axons of neurones which reside in the spinal cord, medulla oblongata, and midbrain (p- 32)

2. Afferent. Afferent nerve endings receive impulses originating in various parts of the body, which they transmit to the central nervous system. They may be free (a-c) or encapsulated (d-h).

a. Intraepithelial endings. The terminal branches occur among the epithelial cells, usually in stratified (skin, sheath of hair follicle, cornea) and pseudostratified (respiratory) epithelia.

The terminal swellings may be very close to the epithelial surface.

b. Connective tissue endings. They occur mostly under epithelia (skin, cornea). They are usually diffuse.

c. Vascular endings. The best known are the pressoreceptors of the carotid sinus, arch of the aorta and base of the right subclavia; they are stimulated mechanically by changes in the blood pressure which stretch the vessel wall. Appear as elaborate arborizations with reticulated swellings in their branches. Also present in the large veins entering the heart.

d. End bulbs. Spherical or oval, with a thin connective tissue capsule which contains the twisted terminal arborizations of one or more sensory fibers. They occur in diverse areas of the body (conjunctiva of eye, lips and buccal mucosa, nasal cavities, external genitals, etc.) .

e. Tactile corpuscles (of Meissner). More complex than the preceding. Oval bodies divided into compartments by horizontal septa attached to the capsule. The compartments contain the much twisted terminal branches of one or more sensory fibers.

(i) They occur in the skin, especially in the finger tips, palm of the hand, and sole of the foot.

f. Pacinian corpuscles. Usually large, laminated, elliptical structures. The capsule is formed by many concentrical lamellae of connective tissue nature. The axis is occupied by a cavity where a single nerve fiber ends without branching.

(1) Occur in the subcutaneous tissue in various parts of the body and in the mesenteries.

(2) They are stimulated by deep, or heavy, pressure.

g. Neuromuscular spindles. Already mentioned (p. 23). The sensory fibers branch profusely around the poorly differentiated muscle fibers, which they touch in many places.

(1) They are affected by stretching and also by extreme contraction of the muscle.

h. Muscle-tendon spindles. Occur at or near the junction of the muscle with the tendon. The spindle consists of several tendon bundles covered by a thin capsule. One or more sensory fibers enter the spindle, where they break into complicated arborizations. Function as the preceding.

i. Chemoreceptor endings. Occur in small organs formerly regarded as paraganglia (carotid and aortic glomus, supracardial paraganglia) . The nerve terminals are in close contact with cells (probably neuroepithelial) .

(i) They register changes in the oxygen content of the blood, which elicit respiratory reflexes.

E. Distribution of the neurones; their relations (synapse). Nerve cells are not scattered throughout the nervous system but occupy definite areas in the brain and spinal cord and outside these regions (i.e. in the ganglia).

1. Gray matter. Areas occupied by neurones are distinguished from other parts of the nervous system in that they have a distinct grayish color. This is due to the presence of pigment within the neurones (p. 26). The neurones may occur in groups or be arranged in layers. The gray matter contains not only the cell bodies but also the dendrites and a variable portion of the axons of the neurones.

a. Brain. In the cerebral hemispheres and the cerebellum the gray matter occurs in the periphery while the more central portions contain white matter with a number of gray masses or nuclei (basal nuclei of the brain; nuclei of cerebellum).

b. Spinal cord. The relations of the gray and white matter are reversed, i.e. the latter occurs in the periphery, the gray matter in the center. In cross section the gray matter roughly simulates an H.

2. White matter. It is formed by the axons of the neurones, which are myelinated, for the most part. Hence its color. The sheath of Schwann is absent but the axons are enveloped by the processes of the neuroglia cells, especially the oligodendroglia (p. 33).

3. Synapse. The neurones of the central nervous system are arranged in such a way that the termination of the axon of a neurone touches the cell body and dendrites of one or more neurones. This contact is called synapse. Synapses occur only in the gray matter.

a. Terminal buttons (‘boutons terminaux’). They are thickenings of the finest axonic twigs in contact with the neurone.

b. Axosomatic synapses. The branches of the axon end in contact with the cell body of the neurone.

c. Axodendritic synapses. Contact is effected with the dendrites (climbing and mossy fibers of the cerebellum).

d. Mixed synapses. A neurone may receive impulses from several sources, some axons ending in contact with the cell body, others in contact with the dendrites (motor neurones of the spinal cord).

F. Structure of the ganglia. A ganglion is a collection of neurones in the path of a nerve. The neurones may receive impulses through a peripheral prolongation ending somewhere in the body (sensory or afferent neurones) or they may originate impulses which are projected on smooth and cardiac muscle, glands, etc. (autonomic neurones).

1. Cerebrospinal ganglia. The ganglia of some of the cranial nerves (trigeminus, facial, auditory, glossopharyngeus and vagus) and the spinal ganglia are sensory and have the same structure. They consist of monopolar neurones, except the cochlear and vestibular ganglia of the auditory nerve in which the neurones are bipolar. A few of the latter may occur in the other ganglia.

a. Distribution of the neurones. They form a continuous layer in the periphery of the ganglion; toward the center they are grouped into irregular strands separated by bundles of nerve fibers. The neurones vary considerably in size.

b. Structure. Sensory neurones have a neurofibrillar meshwork and Nissl bodies; the latter are well developed in the large neurones which have a vesicular nucleus, poor in chromatin ( P . 26).

c. Processes.

(1) Bipolar cells. The peripheral process is usually thicker than the central process which enters the spinal cord. Both are myelinated, and in some cases the myelin sheath may also enclose the cell body.

(2) Monopolar cells. They have a single process which soon acquires a myelin sheath. It bifurcates into a peripheral branch and a thinner central branch entering the cord; both are myelinated in the larger cells.

d. Capsule. Each neurone is surrounded by a connective tissue capsule (continuous with the endoneurium) which is lined internally by flattened cells.

e. Satellite cells (amphicytes). These are branched elements residing within the capsule, in contact with the surface of the neurone which they may corrode in old subjects. They are probably neuroglia cells.

f. Absence of synapses. Since sensory neurones do not receive impulses from other nerve cells there are no synapses in cerebrospinal ganglia.

2. Ganglia of the autonomic system. The sympathetic and parasympathetic ganglia have a similar structure. They consist of multipolar neurones of various sizes and shapes. The smallest of the parasympathetic ganglia may have only a few neurones.

a. Distribution of the neurones. Similar to that mentioned for the cerebrospinal ganglia.

b. Structure. Neurofibrillar meshwork and Nissl bodies are present. The latter are usually smaller and more diffuse than in the cerebrospinal ganglion cells. Binucleated neurones may occur.

c. Dendritic processes. The dendrites vary enormously in number, shape and length, as well as in the degrees of branching. In many ganglia the dendritic processes of several cells form bundles (protoplasmic tracts). Short (intracapsular) dendrites occur in the apes and in man.

d. Axon. Resembles closely a dendrite and does not give off collaterals. The thinner axons are unmyelinated (fibers of Remak). The myelin sheath around the thicker axons may be interrupted in stretches.

e. Capsule. Similar to the capsule of the cerebrospinal neurones but extending somewhat along the larger dendrites. In some small parasympathetic ganglia (plexuses of the intestine) the capsule is normally absent.

f. Satellite cells. Present in variable numbers.

g. Synapses. Since the autonomic ganglia receive impulses through preganglionic fibers they are the site of numerous synapses.

(1) The terminal twigs of the preganglionics end as buttons of variable size and, in many cases, as minute rings.

( 2 ) Synaptic contact may take place with the dendrites or with the cell body; in the latter case the terminal branches, after crossing the capsule, wrap themselves around the cell (pericellular baskets) .

II. Neuroglia

The supporting tissue of the central nervous system is known as neuroglia. It fills the spaces between the neurones and their dendrites, and also occurs between the axons; i.e. is present in the gray as well as in the white matter. Several varieties are recognized.

A. Ependyma. It has the appearance of a columnar epithelium. It lines the central canal of the spinal cord and the ventricles of the brain. The ependymal cells have basal processes which branch and penetrate more or less deeply in the surrounding (nervous) tissue. They contain specific (neuroglia) fibers, and are to be considered as neuroglia cells which have retained their embryonic shape and position (see p. 157).

B. Neuroglia proper. The cells are branched. Three varieties are recognized :

1. Astroglia. Made up of star-shaped cells (astrocytes) with numerous processes, some of which end on the walls of capillaries by means of conical or flattened swellings (vascular feet).

a. Protoplasmic astrocytes. Their processes are relatively short and profusely divided. Ovoid nucleus with scant chromatin. The cell body and processes contain small, roundish granules called gliosomes.

(1) Occur chiefly in the gray matter of the brain and cord.

b. Fibrous astrocytes. The processes are much longer; they contain fibers (‘glia’ fibers) which extend into them for considerable distances. They also have gliosomes.

(1) Found chiefly in the white matter.

2. Oligodendroglia. Smaller than the astrocytes, with relatively few processes which wrap themselves around the myelin sheaths of the axons. The nuclei are round and have small chromatin blocks. More abundant in the white than in the gray matter.

a. The cells of the sheath of Schwann (p. 27) and the satellite cells of the ganglia are probably oligodendroglia.

3. Microglia. The smallest of the neuroglia. Their function is similar to that of the histiocytes of connective tissue. Evenly scattered in the white and gray matter, but they predominate in the latter.

a. Resting stage. The nucleus is small, often irregular and deeply stained. The cell processes are delicate, rather short and finely divided.

b. Wandering stage. The microglia cells are mobilized in trauma and other destructive lesions; their processes are withdrawn, the cells as a whole swell, become ameboid and ingest debris and fat droplets liberated during degeneration of the myelin.

C. Origin. The astroglia and oligodendroglia are ectodermic and arise within the embryonic neural tube from the ependymal layer (p. 155). The microglia is of mesodermic origin; it enters the central nervous system long before birth.

Cite this page: Hill, M.A. (2021, April 11) Embryology Histology and Embryology 1941 - Histology 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Histology_and_Embryology_1941_-_Histology_2

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