Download Document

1
Summary
Primordial germ cells appear in the wall of the yolk sac in the fourth
week and migrate to the indifferent gonad (Fig. 1.1), where they arrive
at the end of the fifth week. In preparation for fertilization, both
male and female germ cells undergo gametogenesis, which includes meiosis
and cytodifferentiation. During meiosis I, homologous chromosomes
pair and exchange genetic material; during meiosis II, cells fail to replicate
DNA, and each cell is thus provided with a haploid number of chromosomes
and half the amount of DNA of a normal somatic cell (Fig. 1.3). Hence, mature
male and female gametes have, respectively, 22 plus X or 22 plus Y
chromosomes.
Birth defects may arise through abnormalities in chromosome number
or structure and from single gene mutations. Approximately 7% of major
Chapter 1: Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes 29
birth defects are a result of chromosome abnormalities, and 8%, are a result
of gene mutations. Trisomies (an extra chromosome) and monosomies
(loss of a chromosome) arise during mitosis or meiosis. During meiosis, homologous
chromosomes normally pair and then separate. However, if separation
fails (nondisjunction), one cell receives too many chromosomes and
one receives too few (Fig. 1.5). The incidence of abnormalities of chromosome
number increases with age of the mother, particularly with mothers
aged 35 years and older. Structural abnormalities of chromosomes include
large deletions (cri-du-chat syndrome) and microdeletions. Microdeletions
involve contiguous genes that may result in defects such as Angelman syndrome
(maternal deletion, chromosome 15q11–15q13) or Prader-Willi syndrome
(paternal deletion, 15q11–15q13). Because these syndromes depend
on whether the affected genetic material is inherited from the mother or the
father, they also are an example of imprinting. Gene mutations may be dominant
(only one gene of an allelic pair has to be affected to produce an alteration)
or recessive (both allelic gene pairs must be mutated). Mutations responsible
for many birth defects affect genes involved in normal embryological
development.
In the female, maturation fromprimitive germ cell to mature gamete, which
is called oogenesis, begins before birth; in the male, it is called spermatogenesis,
and it begins at puberty. In the female, primordial germ cells form
oogonia. After repeated mitotic divisions, some of these arrest in prophase of
meiosis I to form primary oocytes. By the seventh month, nearly all oogonia
have become atretic, and only primary oocytes remain surrounded by
a layer of follicular cells derived from the surface epithelium of the ovary
(Fig. 1.17). Together, they form the primordial follicle. At puberty, a pool of
growing follicles is recruited and maintained from the finite supply of primordial
follicles. Thus, everyday 15 to 20 follicles begin to grow, and as they mature,
they pass through three stages: 1) primary or preantral; 2) secondary
or antral (vesicular, Graafian); and 3) preovulatory. The primary oocyte remains
in prophase of the first meiotic division until the secondary follicle is
mature. At this point, a surge in luteinizing hormone (LH) stimulates preovulatory
growth: meiosis I is completed and a secondary oocyte and polar
body are formed. Then, the secondary oocyte is arrested in metaphase of
meiosis II approximately 3 hours before ovulation and will not complete this
cell division until fertilization. In the male, primordial cells remain dormant
until puberty, and only then do they differentiate into spermatogonia. These
stem cells give rise to primary spermatocytes, which through two successive
meiotic divisions produce four spermatids (Fig. 1.4). Spermatids go through
a series of changes (spermiogenesis) (Fig. 1.25) including (a) formation of
the acrosome, (b) condensation of the nucleus, (c) formation of neck, middle
piece, and tail, and (d) shedding of most of the cytoplasm. The time required
for a spermatogonium to become a mature spermatozoon is approximately
64 days.
2
Summary
With each ovarian cycle, a number of primary follicles begin to grow, but
usually only one reaches full maturity, and only one oocyte is discharged
at ovulation. At ovulation, the oocyte is in metaphase of the second
meiotic division and is surrounded by the zona pellucida and some granulosa
cells (Fig. 2.4). Sweeping action of tubal fimbriae carries the oocyte into the
uterine tube.
48 Part One: General Embryology
Before spermatozoa can fertilize the oocyte, they must undergo (a) capacitation,
during which time a glycoprotein coat and seminal plasma proteins
are removed from the spermatozoon head, and (b) the acrosome reaction,
during which acrosin and trypsin-like substances are released to penetrate the
zona pellucida. During fertilization, the spermatozoon must penetrate (a) the
corona radiata, (b) the zona pellucida, and (c) the oocyte cell membrane
(Fig. 2.5). As soon as the spermatocyte has entered the oocyte, (a) the oocyte
finishes its second meiotic division and forms the female pronucleus; (b) the
zona pellucida becomes impenetrable to other spermatozoa; and (c) the head
of the sperm separates from the tail, swells, and forms the male pronucleus
(Figs. 2.6 and 2.7). After both pronuclei have replicated their DNA, paternal
and maternal chromosomes intermingle, split longitudinally, and go through a
mitotic division, giving rise to the two-cell stage. The results of fertilization are
(a) restoration of the diploid number of chromosomes, (b) determination of
chromosomal sex, and (c) initiation of cleavage.
Cleavage is a series of mitotic divisions that results in an increase in cells,
blastomeres, which become smaller with each division. After three divisions,
blastomeres undergo compaction to become a tightly grouped ball of cells with
inner and outer layers. Compacted blastomeres divide to form a 16-cell morula.
As the morula enters the uterus on the third or fourth day after fertilization, a
cavity begins to appear, and the blastocyst forms. The inner cell mass, which
is formed at the time of compaction and will develop into the embryo proper,
is at one pole of the blastocyst. The outer cell mass, which surrounds the inner
cells and the blastocyst cavity, will form the trophoblast.
The uterus at the time of implantation is in the secretory phase, and the
blastocyst implants in the endometrium along the anterior or posterior wall. If
fertilization does not occur, then the menstrual phase begins and the spongy
and compact endometrial layers are shed. The basal layer remains to regenerate
the other layers during the next cycle.
3
Summary
At the beginning of the second week, the blastocyst is partially embedded
in the endometrial stroma. The trophoblast differentiates into (a)
an inner, actively proliferating layer, the cytotrophoblast, and (b) an
outer layer, the syncytiotrophoblast, which erodes maternal tissues (Fig. 3.1).
By day 9, lacunae develop in the syncytiotrophoblast. Subsequently, maternal
sinusoids are eroded by the syncytiotrophoblast, maternal blood enters the
lacunar network, and by the end of the second week, a primitive uteroplacental
circulation begins (Fig. 3.6). The cytotrophoblast, meanwhile, forms
cellular columns penetrating into and surrounded by the syncytium. These
columns are primary villi. By the end of the second week, the blastocyst
is completely embedded, and the surface defect in the mucosa has healed
(Fig. 3.6).
The inner cell mass or embryoblast, meanwhile, differentiates into (a) the
epiblast and (b) the hypoblast, together forming a bilaminar disc (Fig. 3.1).
Epiblast cells give rise to amnioblasts that line the amniotic cavity superior
to the epiblast layer. Endoderm cells are continuous with the exocoelomic
membrane, and together they surround the primitive yolk sac (Fig. 3.4). By
the end of the second week, extraembryonic mesoderm fills the space between
the trophoblast and the amnion and exocoelomic membrane internally. When
vacuoles develop in this tissue, the extraembryonic coelom or chorionic cavity
forms (Fig. 3.6). Extraembryonic mesoderm lining the cytotrophoblast and
62 Part One: General Embryology
amnion is extraembryonic somatopleuric mesoderm; the lining surrounding
the yolk sac is extraembryonic splanchnopleuric mesoderm (Fig. 3.6).
The second week of development is known as the week of twos: The
trophoblast differentiates into two layers, the cytotrophoblast and syncytiotrophoblast.
The embryoblast forms two layers, the epiblast and hypoblast.
The extraembryonic mesoderm splits into two layers, the somatopleure and
splanchnopleure. And two cavities, the amniotic and yolk sac cavities, form.
Implantation occurs at the end of the first week. Trophoblast cells invade the
epithelium and underlying endometrial stroma with the help of proteolytic enzymes.
Implantationmay also occur outside the uterus, such as in the rectouterine
pouch, on the mesentery, in the uterine tube, or in the ovary (ectopic pregnancies).
4
Summary
The most characteristic event occurring during the third week is gastrulation,
which begins with the appearance of the primitive streak,
which has at its cephalic end the primitive node. In the region of the
node and streak, epiblast cells move inward (invaginate) to form new cell layers,
endoderm and mesoderm. Hence, epiblast gives rise to all three germ
layers in the embryo. Cells of the intraembryonic mesodermal germ layer
migrate between the two other germ layers until they establish contact with
the extraembryonic mesoderm covering the yolk sac and amnion (Figs. 4.3
and 4.4).
Prenotochordal cells invaginating in the primitive pit move forward until
they reach the prechordal plate. They intercalate in the endoderm as the notochordal
plate (Fig. 4.4). With further development, the plate detaches from the
endoderm, and a solid cord, the notochord, is formed. It forms a midline axis,
84 Part One: General Embryology
Figure 4.18 Stem villi (SV) extend from the chorionic plate (CP) to the basal plate (BP).
Terminal villi (arrows) are represented by fine branches from stem villi.
which will serve as the basis of the axial skeleton (Fig. 4.4). Cephalic and caudal
ends of the embryo are established before the primitive streak is formed.
Thus, cells in the hypoblast (endoderm) at the cephalic margin of the disc form
the anterior visceral endoderm that expresses head-forming genes, including
OTX2, LIM1, and HESX1 and the secreted factor cerberus. Nodal, a member
of the TGF-β family of genes, is then activated and initiates and maintains the
integrity of the node and streak. BMP-4, in the presence of FGF, ventralizes
mesoderm during gastrulation so that it forms intermediate and lateral plate
mesoderm. Chordin, noggin, and follistatin antagonize BMP-4 activity and
dorsalize mesoderm to form the notochord and somitomeres in the head region.
Formation of these structures in more caudal regions is regulated by the
Brachyury (T) gene. Left-right asymmetry is regulated by a cascade of genes;
first, FGF-8, secreted by cells in the node and streak, induces Nodal and Lefty-2
expression on the left side. These genes upregulate PITX2, a transcription factor
responsible for left sidedness.
Epiblast cells moving through the node and streak are predetermined by
their position to become specific types of mesoderm and endoderm. Thus, it is
possible to construct a fate map of the epiblast showing this pattern (Fig. 4.11).
Chapter 4: Third Week of Development: Trilaminar Germ Disc 85
By the end of the third week, three basic germ layers, consisting of ectoderm,
mesoderm, and endoderm, are established in the head region, and
the process continues to produce these germ layers for more caudal areas of
the embryo until the end of the 4th week. Tissue and organ differentiation has
begun, and it occurs in a cephalocaudal direction as gastrulation continues.
In the meantime, the trophoblast progresses rapidly. Primary villi obtain
a mesenchymal core in which small capillaries arise (Fig. 4.17). When these
villous capillaries make contact with capillaries in the chorionic plate and connecting
stalk, the villous system is ready to supply the embryo with its nutrients
and oxygen (Fig. 4.17).
5
Summary
The embryonic period, which extends fromthe third to the eighthweeks
of development, is the period during which each of the three germ layers,
ectoderm, mesoderm, and endoderm, gives rise to its own tissues and
organ systems. As a result of organ formation, major features of body form are
established (Table 5.4).
The ectodermal germ layer gives rise to the organs and structures that
maintain contact with the outside world: (a) central nervous system; (b) peripheral
nervous system; (c) sensory epithelium of ear, nose, and eye; (d)
skin, including hair and nails; and (e) pituitary, mammary, and sweat glands
and enamel of the teeth. Induction of the neural plate is regulated by inactivation
of the growth factor BMP-4. In the cranial region, inactivation is caused by
noggin, chordin, and follistatin secreted by the node, notochord, and prechordal
mesoderm. Inactivation of BMP-4 in the hindbrain and spinal cord regions is
effected by WNT3a and FGF. In the absence of inactivation, BMP-4 causes ectoderm
to become epidermis and mesoderm to ventralize to form intermediate
and lateral plate mesoderm.
Important components of the mesodermal germ layer are paraxial, intermediate,
and lateral plate mesoderm. Paraxial mesoderm forms somitomeres,
which give rise to mesenchyme of the head and organize into somites
in occipital and caudal segments. Somites give rise to themyotome (muscle tissue),
sclerotome (cartilage and bone), and dermatome (subcutaneous tissue of
the skin), which are all supporting tissues of the body. Signals for somite differentiation
are derived from surrounding structures, including the notochord,
neural tube, and epidermis. The notochord and floor plate of the neural tube
secrete Sonic hedgehog, which induces the sclerotome. WNT proteins from
the dorsal neural tube cause the dorsomedial portion of the somite to form
epaxial musculature, while BMP-4, FGFs from the lateral plate mesoderm, and
WNTs from the epidermis cause the dorsolateral portion to form limb and body
wall musculature. The dorsal midportion of the somite becomes dermis under
the influence of neurotrophin 3, secreted by the dorsal neural tube (Fig. 5.12).
Mesoderm also gives rise to the vascular system, that is, the heart, arteries,
veins, lymph vessels, and all blood and lymph cells. Furthermore, it gives rise
to the urogenital system: kidneys, gonads, and their ducts (but not the bladder).
Finally, the spleen and cortex of the suprarenal glands are mesodermal
derivatives.
The endodermal germ layer provides the epithelial lining of the gastrointestinal
tract, respiratory tract, and urinary bladder. It also forms the
parenchyma of the thyroid, parathyroids, liver, and pancreas. Finally, the
Chapter 5: Third to Eighth Week: The Embryonic Period 113
TABLE 5.4 Summary of Key Events During the Embryonic
Days Somites Length (mm) Figure Characteristic Features
14–15 0 0.2 5.1A Appearance of primitive streak
16–18 0 0.4 5.1B Notochordal process appears;
hemopoietic cells in yolk sac
19–20 0 1.0–2.0 5.2A Intraembryonic mesoderm spread
under cranial ectoderm; primitive
streak continues; umbilical vessels
and cranial neural folds beginning
to form
20–21 1–4 2.0–3.0 5.2B,C Cranial neural folds elevated, and
deep neural groove established;
embryo beginning to bend
22–23 5–12 3.0–3.5 5.5A, B; Fusion of neural folds begins in
5.6; 5.7 cervical region; cranial and caudal
Period
neuropores open widely; visceral
arches 1 and 2 present; heart tube
beginning to fold
24–25 13–20 3.0–4.5 5.8A Cephalocaudal folding under way;
cranial neuropore closing or closed;
optic vesicles formed; otic placodes
appear
26–27 21–29 3.5–5.0 5.8B; Caudal neuropore closing or closed;
5.20A, B upper limb buds appear; 3 pairs of
visceral arches
28–30 30–35 4.0–6.0 5.8B Fourth visceral arch formed; hindlimb
buds appear; otic vesicle and lens
placode
31–35 7.0–10.0 5.19 Forelimbs paddle-shaped; nasal pits
formed; embryo tightly C-shaped
36–42 9.0–14.0 5.21 Digital rays in hand and footplates;
brain vesicles prominent; external
auricle forming from auricular
hillocks; umbilical herniation
initiated
43–49 13.0–22.0 5.23 Pigmentation of retina visible; digital
rays separating; nipples and eyelids
formed; maxillary swellings fuse
with medial nasal swellings as
upper lip forms; prominent
umbilical herniation
50–56 21.0–31.0 5.24 Limbs long, bent at elbows, knees;
fingers, toes free; face more
human-like; tail disappears;
umbilical herniation persists to end
of third month
114 Part One: General Embryology
epithelial lining of the tympanic cavity and auditory tube originate in the
endodermal germ layer.
Craniocaudal patterning of the embryonic axis is controlled by homeobox
genes. These genes, conserved from Drosophila, are arranged in four clusters,
HOXA, HOXB, HOXC, and HOXD, on four different chromosomes. Genes toward
the 3_ end of the chromosome control development of more cranial structures;
those more toward the 5_ end regulate differentiation of more posterior
structures. Together, they regulate patterning of the hindbrain and axis of the
embryo (Fig. 5.22).
As a result of formation of organ systems and rapid growth of the central
nervous system, the initial flat embryonic disc begins to fold cephalocaudally,
establishing the head and tail folds. The disc also folds transversely
(lateral folds), establishing the rounded body form. Connection with the yolk
sac and placenta is maintained through the vitelline duct and umbilical cord,
respectively.
6
Summary
The fetal period extends from the ninth week of gestation until birth
and is characterized by rapid growth of the body and maturation of
organ systems. Growth in length is particularly striking during the third,
fourth, and fifth months (approximately 5 cm per month), while increase in
weight is most striking during the last 2 months of gestation (approximately
700 g per month) (Table 6.1; p. 118).
A striking change is the relative slowdown in the growth of the head. In the
third month, it is about half of CRL. By the fifth month the size of the head is
about one-third of CHL, and at birth it is one-fourth of CHL (Fig. 6.2).
During the fifth month, fetal movements are clearly recognized by the
mother, and the fetus is covered with fine, small hair.
A fetus born during the sixth or the beginning of the seventh month has
difficulty surviving, mainly because the respiratory and central nervous systems
have not differentiated sufficiently.
In general, the length of pregnancy for a full-term fetus is considered to
be 280 days, or 40 weeks after onset of the last menstruation or, more
accurately, 266 days or 38 weeks after fertilization.
The placenta consists of two components: (a) a fetal portion, derived from
the chorion frondosum or villous chorion, and (b) a maternal portion, derived
from the decidua basalis. The space between the chorionic and decidual plates
is filled with intervillous lakes of maternal blood. Villous trees (fetal tissue)
grow into the maternal blood lakes and are bathed in them. The fetal circulation
is at all times separated from the maternal circulation by (a) a syncytial
membrane (a chorion derivative) and (b) endothelial cells from fetal capillaries.
Hence the human placenta is of the hemochorial type.
Intervillous lakes of the fully grown placenta contain approximately 150 ml
of maternal blood, which is renewed 3 or 4 times per minute. The villous area
varies from 4 to 14 m2, facilitating exchange between mother and child.
Main functions of the placenta are (a) exchange of gases; (b) exchange of
nutrients and electrolytes; (c) transmission of maternal antibodies, providing
the fetus with passive immunity; (d) production of hormones, such as progesterone,
estradiol, and estrogen (in addition, it produces hCG and somatomammotropin);
and (e) detoxification of some drugs.
The amnion is a large sac containing amniotic fluid in which the fetus
is suspended by its umbilical cord. The fluid (a) absorbs jolts, (b) allows for
fetal movements, and (c) prevents adherence of the embryo to surrounding
Chapter 6: Third Month to Birth: The Fetus and Placenta 147
tissues. The fetus swallows amniotic fluid, which is absorbed through its gut
and cleared by the placenta. The fetus adds urine to the amniotic fluid, but
this is mostly water. An excessive amount of amniotic fluid (hydramnios) is
associated with anencephaly and esophageal atresia, whereas an insufficient
amount (oligohydramnios) is related to renal agenesis.
The umbilical cord, surrounded by the amnion, contains (a) two umbilical
arteries, (b) one umbilical vein, and (c) Wharton’s jelly, which serves as a
protective cushion for the vessels.
Fetal membranes in twins vary according to their origin and time of formation.
Two-thirds of twins are dizygotic, or fraternal; they have two amnions,
two chorions, and two placentas, which sometimes are fused. Monozygotic
twins usually have two amnions, one chorion, and one placenta. In cases of
conjoined twins, in which the fetuses are not entirely split from each other,
there is one amnion, one chorion, and one placenta.
Signals initiating parturition (birth) are not clear, but preparation for labor
usually begins between 34 and 38 weeks. Labor itself consists of three stages: 1)
effacement and dilatation of the cervix; 2) delivery of the fetus; and 3) delivery
of the placenta and fetal membranes.
.. ‫هللا يوفقكم‬
‫أبودخيّل‬