Histology and Embryology 1941 - Embryology 1

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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 One - General Development

For the formation and structure of the germ cells see pp. 77, 83.

Fertilization, Cleavage and the Origin of the Germ Layers

The processes of fertilization and cleavage of the human ovum have never been observed; in fact the earliest human embryos known are nearly two weeks old and have the three germ layers already formed. What follows is based on observations in mammals.

I. Fertilization

It consists essentially in the fusion of the gametes (spermatozoon and ovum), each of which carries one half the number of chromosomes characteristic of the species; the original number is thus restored.

A. Site of fertilization. In man it occurs in the upper third of the oviduct. As proof of this can be cited the fact that the embryo may be implanted in the oviduct (tubal pregnancy).

B. The process of fertilization.

1. Penetration of the spermatozoon. It usually takes place after the second maturation mitosis has been completed. The spermatozoon crosses the zona pellucida of the ovum (p. 82) and enters the cytoplasm, where it loses its tail. Its middle piece furnishes the centrosomes and spindle for the dividing ovum.

2. Formation of the pronuclei. After the second maturation mitosis the nucleus of the ovum is termed the female pronucleus.

The compact nucleus (head) of the spermatozoon becomes vesicular and shows a distinct chromatin network (male pronucleus).

3. Fusion of the pronuclei. The male pronucleus moves toward the female pronucleus; when the two finally come into contact they merge into a single (segmentation) nucleus.

4. First cleavage mitosis. Immediately after fusion of the pronuclei the segmentation nucleus enters the prophase of the first cleavage mitosis. The centrosomes contributed by the spermatozoon form a spindle.

II. Cleavage (Segmentation) of the Mammalian Ovum

Cleavage is greatly influenced by the amount of yolk present in the ovum. If the yolk is very abundant it does not affect the whole ovum (meroblastic cleavage of fishes, reptiles and birds). If it is scarce or present in moderate amounts the whole ovum will divide (holoblastic cleavage of the amphibia and mammals). Although the microscopic mammalian ovum contains little yolk its cleavage does not correspond to the segmentation of similar ova in other forms (i.e. Amphioxus) because it does not give rise to a typical blastula.

A. Early cleavage. The fertilized egg divides into two blastomeres; each of the latter divides in turn and a total of four is produced. The first two divisions are meridional. The third is equatorial and gives rise to eight blastomeres which upon further division become sixteen, etc.

B. Morula. When this stage is reached the cell aggregation shows two types of blastomeres: one light, the other dark (i.e. more granular). The light blastomeres arrange themselves as a capsule around the dark cells; the latter are termed the inner cell mass while the layer of light blastomeres is known as the trophoblast (or trophectoderm).

C. Formation of the blastocyst. Fluid collects between the inner cell mass and the trophoblast; as a result of this the morula is changed into a hollow vesicle in which the inner cell mass remains in contact with the trophoblast at one of the poles.

D. Significance of the inner cell mass. It corresponds to the blastoderm of the chick since it will give rise to the embryo, while the trophoblast becomes associated intimately with the uterine mucosa and is concerned with the nutrition, respiration and excretion of the embryo.

III. Gastrulation

Gastrulation in mammals is much more similar to the corresponding process of reptiles and birds than to the typical gastrulation observed in Amphioxus and the amphibians.

A. Formation of the endoderm. It does not arise by invagination but it is constituted by the arrangement of cells in a layer on the under surface of the inner cell mass, which is now the ectoderm.

B. Extension of the endoderm. In most mammals the endoderm spreads rapidly on the inner surface of the blastocyst, which it lines completely.

IV. Formation of the Mesoderm

The formation of the mammalian mesoderm closely resembles the production of the same layer in birds.

A. Primitive streak. It appears in the pear-shaped blastoderm and marks the axis of the embryo. It ends anteriorly in a primitive knot (of Hensen). In vertical section is seen to be a thickened band continuous with the ectoderm; its under-surface produces mesodermic cells which spread laterally and caudally between the ectoderm and endoderm.

B. Formation of the head process. This arises as a forward extension of Hensen’s knot and in man and many mammals it has a cavity (notochordal canal) which opens at the primitive pit. Its floor fuses with the endoderm, after which both disappear in the area of fusion. The roof (notochordal plate) becomes the notochord.

C. Formation of the coelom. The mammalian mesoderm grows rapidly between the ecto- and endoderm. At first it is a single sheet but it soon splits into two layers, one associated with the ectoderm (somatic mesoderm) and the other with the endoderm (splanchnic mesoderm). The cavity between the two mesodermic layers is the coelom or body cavity.

D. Extra-embryonic coelom. Since the mesoderm spreads between the ecto- and endoderm throughout the blastodermic vesicle its splitting will extend beyond the embryonic area. An extra-embryonic coelom is thus formed.

Early Differentiations of the Germ Layers; Development of the External Form

After gastrulation and the formation of the mesoderm the three germ layers of the embryo begin to differentiate and produce the primordia of the most important organic systems.

I. Early Differentiations of the Germ Layers

A. Formation of the neural tube. The neural tube is the primordium of the central nervous system; its anterior (or cephalic) portion gives rise to the brain, the rest to the spinal cord. The first indication of the formation of the neural tube is the differentiation of the:

1. Neural (medullary) plate. This is a thickening of the ectoderm along the longer axis of the blastoderm. It begins in Hensen’s node and extends anteriorly (i.e. cephalad).

2. Neural (medullary) folds. The edges of the medullary plate are converted into folds, which diverge posteriorly enclosing Hensen’s node. The neural plate is thus changed into a groove.

3. Closure of the neural groove. The neural folds increase in height and curve toward each other, finally meeting in the midline at a point which corresponds to the neck region of the adult. This changes the neural groove into a tube, the lumen of which persists throughout adult life (p. 158) .

4 . The neural crest. A longitudinal band of cells between the ectoderm and the edge of the plate sinks into the neural fold and becomes this structure.

5. Anterior and posterior neuropores. The closure of the neural groove does not take place simultaneously along its entire length but proceeds slowly cephalad and caudad. For a considerable period there are two openings in the ends of the tube, the anterior and posterior neuropores, respectively.

6. Primary brain vesicles. The anterior, expanded region of the neural tube is divided through constrictions into the three primary brain vesicles: the fore-, mid-, and hindbrain.

B. Formation of the notochord. The roof of the head process (p. 107) is known as the notochordal plate. It loses all connection with both endoderm and mesoderm and becomes a rod, the notochord, which extends beneath the neural tube and ends anteriorly under the midbrain. The notochord is the axis around which the vertebral column will develop. Remnants are occasionally found as the pulpy nuclei of the intervertebral disks.

C. Differentiation of the mesoderm. The splitting of the mesoderm into somatic and splanchnic layers has been mentioned (p. 107). The two layers of each side are continuous near the midline, i.e. lateral to the notochordal plate.

1. Formation of the somites. At the junction of somatic and splanchnic layers the mesoderm is much thicker. This dorsal or median mesoderm becomes divided transversely into a number of more or less cuboidal, usually solid masses, the somites.

a. Order of appearance. The first somite appears behind the future occipital region of the adult. The segmentation of the dorsal mesoderm proceeds caudad until 38 pairs have developed in the neck and trunk regions of the body, in addition to those that are developed in the occipital region of the head (probably four).

b. Differentiation. Each somite becomes differentiated into three distinct portions:

(1) Sclerotome. The cells of that portion of the somite next to the notochord grow inward toward the midline to surround the notochord and lateral walls of the neural tube. They will form the body of the vertebrae and the vertebral arches.

(2) Myotome. The middle portion of the somite will give rise to a part of the skeletal (voluntary) musculature of the body.

(3) Dermatome. The outer or lateral portion of the somite is believed to be transformed into the derma of the skin (p. 46). Its existence in mammals and man has been denied, however.

2. The ventral (lateral) mesoderm. It never becomes segmented in the neck and trunk regions. The coelom occurs between its somatic and splanchnic layers.

a. Somatic layer. It is continuous laterally with the mesodermic layer which lines the outer surface of the amnion (p. 115). Mesially it passes into the:

b. Splanchnic layer, which is applied closely to the endoderm of the:

(1) Digestive tract, derived from the dorsal portion of the yolk sac (p. 114). The splanchnic mesoderm becomes converted into mesenchyme out of which the muscular coats will develop.

(2) Yolk sac. The splanchnic layer surrounding the yolk sac is the source of the first blood vessels and blood of the embryo and corresponds to the vascular area of the chick blastoderm.

3. The intermediate cell mass (nephrotome). This is a narrow area underlying the original longitudinal groove which separates the somite area from the ventral mesoderm. It produces the pronephroi, Wolffian bodies (mesonephroi) and the mesonephric ducts (p. 141).

II. Development of the External Form

The early appearance of the embryo differs greatly from that of the child and has features in common with the other vertebrates: this is the embryonic period, which lasts two months. After this the resemblance with the child is more striking (fetal period).

A. Embryonic period.

1. Separation of the embryo from the yolk sac. At the end of the

formation of the mesoderm the embryonic disk (blastoderm) forms the roof of the yolk sac, the whole being connected with the chorion by the body stalk, occupied by the rudimentary allantois.

a. Folding of the blastoderm. A groove which appears at the periphery of the blastoderm, between the latter and the sac, marks the beginning of the separation of these two portions. As it deepens the flat blastoderm is gradually transformed into a cylinder.

b. Formation of the digestive tract. The endoderm of the roof of the yolk sac is incorporated into the embryo and constitutes the primordium of the digestive tract.

c. Formation of the yolk stalk. With the deepening of the groove the connection between the yolk sac and the embryo is very much reduced and becomes the yolk stalk.

2. Formation of the cephalic and caudal folds. These are due to rapid elongation of the body of the embryo.

a. The cephalic fold. With the closure of the neural groove the anterior end of the embryo rises and projects beyond the yolk sac. The primitive brain vesicles become distinct.

b. The fore-gut. As the cephalic fold rises it carries along the endoderm, which forms a blind diverticulum, the fore-gut. Beneath the fore-gut the heart is developing.

c. The caudal fold and hind-gut. A less developed fold is formed at the posterior end of the embryo; it also contains an endodermic diverticulum, the hind-gut.

3. Establishment of the primary flexures. Continued elongation of the embryo during the 5th week causes the bending of the body in three different regions:

a. Cephalic flexure. This is a sharp bend at the level of the midbrain.

b. Cervical flexure. A second more prominent flexure occuring more posteriorly, in the region of the future neck.

c. Caudal (sacral) flexure. Is present toward the posterior end of the body, which ends in a short, pointed tail.

4. Appearance of the limb buds. These are already present at the end of the fifth week. The bud for the arm is seen on each side of the body a little posterior to the cervical flexure. The lower limb buds, located in the region of the caudal flexure, are slightly smaller.

5. The branchial arches. In common with other vertebrates the human embryo shows on each side of the region of the future pharynx four grooves which separate five branchial arches. Their formation will be considered later (p. 123).

a. Mandibular arch. The first arch consists of a main portion which gives rise to the jaw, and a maxillary process appearing as a wedge between the eye and the mandibular process. Retardation of the development of the mandibular process causes an abnormally small jaw (micrognathus) ; its complete absence is also possible (agnathus).

b. Hyoid arch. It is separated from the mandibular by the first branchial groove which sometimes is actually a cleft.

c. Other arches. They are less developed, especially the fifth, placed behind the fourth groove and poorly defined posteriorly.

d. Cervical sinus. After the sixth week the first two arches overlap the other three, which sink into a triangular depression called the cervical sinus. Later the posterior edge of the hyoid arch fuses with the thoracic wall and the sinus is cut off from the outside.

6. Development of the neck. The neck develops in the area occupied by the branchial arches whose mesoderm gives rise to muscles, bones and blood vessels. From the lining of the endodermic pouches 'several important organs arise (p. 124). When this differentiation has been completed the embryo has acquired a neck, not present in earlier stages in which the mandibular arch rests on the thorax. The neck results from an elongation of the region between the mandibular arch and the pericardium. Incomplete closure of the branchial clefts causes cervical fistulae.

7 . The face. The formation of the face is a complex process. It takes place chiefly between the 5th and 8th weeks.

a. The olfactory pits. They are first represented by ectodermic thickenings (olfactory placodes) on the ventrolateral aspect of the head. The placodes sink and become converted into shallow pits.

b. Fronto-nasal process. This is the region of the head between the olfactory pits in early embryos. In later stages the olfactory pits subdivide the fronto-nasal process into:

(1) Lateral nasal processes, which with the

(2) Median nasal processes, bound the nostrils externally.

c. Fusion of the median nasal processes with the maxillary processes. This fusion forms the upper jaw. When incomplete it causes hare-lip.

d. Fusion of the lateral nasal processes with the maxillary processes. It obliterates the naso-lacrimal groove, which extends between the maxillary process and the lateral nasal process and is thus changed into the naso-lacrimal canal of the adult. This fusion also forms the wings (alae) of the nose and the cheek region. Its failure causes oblique facial cleft.

e. The nose. When first formed the nose is broad and flat with the nostrils set far apart. In later fetal months the bridge of the nose rises and the nostrils are approximated.

f. The mouth. The mouth is also very wide in early stages but during later development it is much reduced in width.

8. The external ear. This is formed around the first branchial groove (between the mandibular and hyoid arches) by the fusion of several small tubercles in the two arches. The groove becomes the external auditory meatus.

9. The eye. During the embryonic period the eye lacks eyelids. The iris and pupil are clearly seen. The eyelids develop during the second month.

10. The limbs. At the earliest stage of their development (5th week) the limbs are lateral swellings (limb buds), the location of which has already been given (p. 111). The upper limb buds arise first.

a. Division into proximal and distal regions. The distal end of the limb bud flattens and a constriction separates this portion from a more proximal, cylindrical segment. The flattened portion becomes the hand and the foot, respectively. Radial ridges indicate the formation of the digits.

b. Subdivision of the proximal region. A second constriction separates the proximal region into two segments: the arm and forearm, and the thigh and leg, respectively.

c. Rotation of the limbs. During their development the limbs undergo changes in position. At the beginning they point caudad, but soon project outwards at right angles to the body wall. Later the palmar and plantar surfaces face the body. Further rotation brings them into the position of the adult.

d. Anomalies. They are frequent and range from complete or almost complete absence of the limbs (amelus) to a partial duplication of the hand or foot (dichirus).

(1) The distal portion may resemble a stump (hemimelus) or the proximal segments may be missing so that the hand or foot spring directly from the body (phocomelus).

(2) The digits may be fused (syndactyly), or be excessively short (brachydactyly) or they may have more phalanges than usual (hyperphalangism).

(3) More than the normal number of digits may also occur (polydactyly).

(4) Clubhand and clubfoot also result from primary defects in the development of the limb buds.

B. Fetal period. The fetus definitely resembles the child, but at the beginning of the period (3rd lunar month) the head is still disproportionately large; the embryonic flexures have disappeared. The sex can be distinguished readily.

1. Appearance of lanugo. This fine, silky hair covers the face and body during the 5th month, and begins to be shed before birth (10th lunar month) except on the face.

2. Growth of eyebrows and lashes. The eyelids fuse during the 3rd month and remain so until the 7th month. During the 6th month the eyebrows and lashes grow.

3. Growth of the nails. They begin to form during the 3rd month and project at the finger tips during the 9th.

4 . Descent of the testes into the scrotum. This takes place during the 8th month.

Human Fetal Membranes, Placentation and Deciduae

I. Fetal Appendages and Membranes

The human fetal appendages and membranes are: the yolk sac, chorion, amnion, allantois and umbilical cord. They all appear early since they are concerned with important protective and nutritional functions.

A. The yolk sac. Although it does not contain yolk it is an important structure whose roof will provide the endodermic epithelium of the whole digestive system (p. 121).

1. Vitelline (omphalomesenteric) vessels. The splanchnic mesoderm which invests the outer surface of the sac is a source of blood for the embryo since the vitelline vessels arise in it.

2. Yolk stalk. This is a constriction between the sac proper and the body of the embryo. It becomes thinner as development proceeds and is located within the umbilical cord.

3. Fate of the sac. It shrinks somewhat and becomes a small, solid structure containing detritus, usually seen near the umbilical cord in the after-birth.

4 . Meckel’s diverticulum. This is the persisting proximal end of the yolk stalk. When it opens at the umbilicus (navel) is called an umbilical fistula.

B. The chorion. It is the continuation of the trophoblast of the blastocyst to which is added an inner lining of extra-embryonic (somatic) mesoderm. The human chorion is studded with villi, each of which contains a mesodermic core. Blood vessels enter the latter, especially in those villi next to the uterine wall, which are incorporated into the placenta (p. 117).

C. The amnion. While in most mammals this membrane arises by a process of folding, in man and certain mammals (bat, guinea pig, apes) the amnion is formed by a different method at a very early stage of development.

1. Origin. A cavity appears in the solid ectodermal cell mass which remains after formation of the endoderm (p. 106). The roof and sides of the cavity become the amnion through a process of thinning, while the floor remains as the dorsal ectoderm of the embryo.

2. Structure. It is a thin, transparent, non-vascular membrane covered externally by mesenchyme; its interior is lined by a low cuboidal, ectodermic epithelium. It contains the amniotic fluid in which is suspended the embryo; the amount of fluid at birth is about one liter.

3. Fate of the amnion. After the 2nd month of pregnancy the amnion is loosely fused with the chorionic wall, with the resulting obliteration of the extra-embryonic coelom. It breaks during the early stages of childbirth and the fluid escapes as the “waters.” It is expelled attached to the after-birth (p. 120).

D. The allantois. It is rudimentary in man, whereas in other mammals (i.e. the pig) it attains great development. In man, however, it is important in that it conveys the umbilical vessels to the chorion.

1. Origin. It appears very early, even before the gut begins to assume a tubular form. Due to this it cannot be properly regarded as an evagination of the hind-gut, as in other mammals. It occupies the body stalk, a mesodermic bridge connecting the embryo to the chorion, which it reaches.

2. Fate. Growth of the allantois soon ceases, and it becomes obliterated. Remnants are still discernible in the proximal part of the umbilical cord in early pregnancy.

E. The umbilical cord. The human cord is fully formed during the 6th week of pregnancy through the wrapping of the amnion around the body stalk, yolk stalk and sac.

1. Contents. The young cord contains the body stalk (with the enclosed allantois) and a prolongation of the coelom occupied by an intestinal loop to which is attached the yolk sac. The yolk stalk carries the vitelline vessels. In addition there are the umbilical vessels (two arteries and a vein).

2. Structure at term. The walls of the cord develop a peculiar form of connective tissue (‘mucous,’ or jelly of Wharton) which causes the obliteration of the coelomic prolongation within the cord after the intestinal loop has been withdrawn into the body. Externally it is covered with simple cuboidal epithelium. Remnants of the allantois and yolk stalk may still be seen.

3. Umbilical hernia. It is caused by failure of the intestinal loop to be withdrawn into the body cavity.

II. Placentation

This includes the implantation of the early embryo in the uterine wall and the formation of the placenta.

A. Preparation of the uterus for the embryo. It is accomplished during the premenstrual (progravid) stage of the uterine cycle (p. 86), during which the endometrium undergoes changes resulting in increased vascularity, increased glandular secretions rich in glycogen, and a general loosening of the tissues of the endometrium.

B. Passage of the segmenting ovum into the uterus. When the ovum enters the uterus it is probably in the morula stage. Several days elapse before the blastocyst becomes embedded in the uterine wall. The period between fertilization and implantation is estimated at nine or ten days.

C. Implantation. The blastocyst becomes embedded in the endometrium. How this is accomplished in the human is not exactly known, but the whole process is supposed to take no more than a day. Implantation has been studied in detail in the guinea pig.

1. Destruction of the epithelium. Contact of the trophoblast with the uterine epithelium causes destruction of the latter, probably under the influence of trophoblastic enzymes.

2. Invasion of the tunica propria. After destruction of the epithelium trophoblastic processes grow into the tunica propria. The blastocyst gradually sinks into the endometrium. The orifice of entrance is closed later.

3. Site of implantation. It varies somewhat but it is usually at or near the fundus of the uterus on either the anterior or the posterior wall.

D. Establishment of the embryo in the endometrium.

1. Formation of embryotroph. As a result of the enzymatic activity of the trophoblast spaces are formed which contain cellular debris and some blood. This material (embryotroph) is absorbed by the trophoblast and serves as food for the embryo.

2. Hemotrophic nutrition. The rapid growth of the blastocyst after implantation results in additional destruction of the surrounding tissues. Dissolution of the walls of the capillaries and small veins leads to the formation of blood sinuses; a hemotrophic type of nutrition is thus established.

3. Primary villi. The enzymatic activities of the trophoblast are exerted only by the more superficial cells of this layer, which form syncytial projections (primary villi).

4. Secondary villi. The chorionic villi (secondary villi) are lined by the more deeply placed trophoblastic cells which are concerned with absorption of maternal nutritive substances from the blood sinuses. Hemotrophic nutrition is definitely established when branches of the umbilical vessels enter the chorion; this happens about three weeks after fertilization.

a. Differentiation of the chorionic villi. Since the umbilical vessels reach the chorion by way of the body stalk, the villi which are remote from the attachment of the latter to the chorion do not receive an abundant blood supply and gradually atrophy. The region in which they formerly occurred is the chorion laeve. The other villi continue their growth and become profusely branched (chorion frondosum).

b. Structure. Each young villus consists of a core of mesoderm containing blood vessels, enclosed within a double-layered epithelium.

(1) Syncytium. This is the outer epithelial covering consisting of a continuous layer of cytoplasm with evenly distributed nuclei. In the older villi and in the placenta at term it is represented by scattered cytoplasmic clumps containing many nuclei (syncytial knots).

(2) Langhans layer. The inner layer consists of epithelial cells (of Langhans) with definite outlines. They gradually atrophy as the villus grows, persisting only in small scattered areas.

(3) Blood vessels. The mesodermic core contains usually two arterioles and two somewhat larger venules connected by capillaries situated mainly at the tips of the villus.

( 4 ) Canalized fibrin. After degeneration of portions of the syncytium and of the Langhans layer the vacant spaces are occupied by depositions of fibrin. These areas of canalized fibrin increase in extent in the older placentae.

III. The Deciduae

Before describing the placenta it will be necessary to consider the changes which take place in the endometrium following implantation of the embryo. Since the endometrium is cast off after birth of the fetus it is known as the decidua. Three different regions are distinguished: The decidua vera, the decidua capsularis and the decidua basalis.

A. Decidua vera (parietalis). This is the general lining of the pregnant uterus exclusive of the region of the embryo and the cervix. Two layers are distinguished.

1. Compact layer. Contains the straight, dilated segments of the uterine glands. Its surface epithelium disappears by the end of the third (lunar) month due to contact with the decidua capsularis.

2. Spongy layer. In early pregnancy it is characterized by the greatly enlarged and tortuous portions of the uterine glands, with their long axes perpendicular to the surface of the endometrium. After the second month stretching of the decidua reduces the glands to elongated clefts parallel to the uterine surface.

3. Decidual cells. They are large, polygonal elements with one or more nuclei. Decidual cells arise from reticulo-endothelial elements which abound in the tunica propria (p. 86 ). They contain glycogen and are highly characteristic of pregnancy. Their function is not clearly understood.

4. Regression. The period of growth of the decidua vera is limited to the first three or four months of pregnancy; later it becomes thinner, less vascular and shows regressive changes. The decidual cells become smaller and many degenerate.

B. Decidua capsularis (reflexa). This is the endometrial portion which covers the area of implantation of the embryo. In the early stages of pregnancy it shows some of the endometrial characteristics and is covered by columnar epithelium. As the chorionic sac expands it becomes thin and atrophic. At the end of the third month it fuses with the decidua vera and then degenerates, allowing the chorion laeve to adhere to the decidua vera.

C. Decidua basalis. Since the blastocyst is implanted superficially in the endometrium the deeper part of the compact and the spongy layer, respectively, remain intact. The two together become the basalis, which is an important part of the placenta. Decidual cells also occur in this layer.

IV. The Placenta

The placenta is a structure, part of which (chorion frondosum) is derived from the embryo (fetal placenta), the other (decidua basalis) from the mother (maternal placenta). The two portions are intimately associated into a solid, disk-shaped organ to which is attached the umbilical cord.

A. Fetal placenta. The structure of the villi of the chorion frondosum has already been described (p. 117).

1. Cotyledons. The villi of the frondosum are evenly distributed at first but in the older placentae become separated into 15 to 20 groups or cotyledons by the growth of trabeculae (placental septa) from the walls of the uterus.

2. Fixation (anchoring) villi. These are attached to the decidual wall and to the placental septa.

3. Free villi. In the older villous trees there are many villi which float in the cavity of the blood sinuses.

4. Chorionic plate. This is the portion of the chorion between the bases of the villi. It contains the larger chorionic vessels which converge in the center, where they enter the umbilical cord.

a. Epithelium. The plate is lined externally by a layer of trophoblastic epithelium which rests on a layer of mesoderm.

b. Fusion with the amnion. By the end of the second month the amnion is brought into contact with the chorion. In the placental area it fuses with the chorionic plate.

c. Production of fibrin. During the last half of pregnancy the epithelium is replaced by canalized fibrin.

B. Maternal placenta. This is represented by the decidua basalis.

1. Glands of the spongy layer. They become stretched into clefts by the third month.

2. Basal plate. This is what remains of the compact layer of the endometrium; it is incorporated into the placenta. It consists of a connective tissue stroma containing decidual cells, fibrinoid material and portions of the trophoblast.

3. Intervillous spaces. They arise through fusion of the blood sinuses (p. 117) during the early stages of development, following implantation. Arteries and veins open into them. The ends of the free villi float in these large spaces which are supposed to contain circulating maternal blood. This point, however, has been recently questioned.

4. Placental septa (or trabeculae). They are the septa which separate the villi of the chorion frondosum into cotyledons.

C. The placenta at term. It is convex on the uterine surface (covered by the decidua basalis) and concave on the fetal side (covered by the amnion), but after it is expelled these relations are reversed. Its margin is continuous with a membrane produced through fusion of:

1. The decidua vera.

2. The decidua capsularis.

3. The chorion laeve.

4. The amnion.

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

Cite this page: Hill, M.A. (2021, April 15) Embryology Histology and Embryology 1941 - Embryology 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Histology_and_Embryology_1941_-_Embryology_1

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© Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G