Download Early Embryonic, Placental, and Fetal Development

ANS 3319C Reproductive Physiology & Endocrinology Lab
Early Embryonic, Placental, and Fetal Development
Objectives
1) To develop an understanding of fertilization and early embryonic development in mammals.
2) To introduce the anatomy of the early developing embryo and fetal/placental unit by dissecting
pregnant reproductive tracts.
3) To discuss the developmental and functional significance of the fetal /placental unit of the
pregnancy in farm animals.
Early Embryonic Development (Figures 1 &2)
1.
Cleavage - Mitotic cell division of zygote without an increase in cell mass.
Initial nutrients come from the cytoplasm with continued development being aided by oviductal
and uterine secretions known as uterine milk.
After the first cleavage, the cells are referred to as blastomeres.
The blastomeres of the early developing zygote are totipotent, meaning that they are capable
of giving rise to an intact embryo.
Once the zygotes reach the eight to sixteen cell stage, they are called morulas.
Table 13-1 Timing of pre-attachment embryogenesis relative to ovulation within females of various species. Nonbolded values are in the oviduct. Bold values in the shaded area are in the uterus; (-) = no data
Blastocyst
Hatching
Gestation
Length (mo)
-
-
13-15 d
2
4-7 d
4-10 d
9-11 d
9
3-4 d
4-10 d
7-8 d
5
3d
4-5 d
6-8 d
7-8 d
11
-
5d
8d
10-12 d
2
1.0 d
2d
3.5 d
4-5 d
6d
3.8
2d
3d
4d
5d
5-6 d
9
Species
2-cell
Bitch
3-7 d
-
-
Cow
24 h
1.5 d
3d
Ewe
24 h
1.3 d
2.5 d
Mare
24 h
1.5 d
Queen
-
-
14-16 h
24 h
Sow
Woman
4-cell
8-cell
Morula
2.
Blastocyst Formation
Embryo starts to develop into two distinct cell populations:
Inner cell mass gives rise to the embryo proper.
Trophoblast forms the chorion of the placenta.
Blastocyst expansion is a result of cellular hyperplasia and fluid accumulation.
Results in the formation of the blastocoele (fluid-filled cavity).
Zona hatching - release of blastocyst from the zona pellucida. Day 8 to11 of gestation.
3.
Blastocyst Elongation
Rapid growth of the conceptus occurs during the second week of gestation.
Elongation is logarithmic and filamentous.
By day 18 of gestation, the blastocyst has extended into the contralateral uterine horn.
Cattle and sheep - elongation is slow and takes days to finish.
Pig - elongation is very rapid and occurs within a couple of hours.
o Pig embryos can become as long a meter in length.
Horse - does not elongate but increases in diameter 2 to 3 mm per day.
o Embryo become large and spherical like a baseball
ANS 3319C Reproductive Physiology and Endocrinology – Early Embryonic, Fetal, and Placental Development
Figure 1. Development of a pre-implantation embryo within the zona pellucida. Male and female pronuclei, along
with the first and second polar bodies are present in ootid. Fusion of male and female pronuclei into a single diploid
nucleus constitutes syngamy. The single-celled embryo (zygote) undergoes cleavage (mitotic division) to give rise to
two daughter cells called blastomeres. Mitotic divisions continue until a morula is formed. The morula develops into a
blastocyst consisting of an inner cell mass (ICM), a blastocoele cavity and a trophoblast. Finally, the rapidly growing
blastocyst “hatches” from the zona.
4.
Differentiation - The period of development where the embryo forms specific tissue layers that
eventually form the extraembryonic membranes and organs in the body (Figure 2).
Germ Layer
Ectoderm
Mesoderm
Endoderm
Organs
Central nervous system
Sense organs
Mammary glands
Circulatory system
Skeletal system
Muscle
Digestive system
Liver
Lungs
Sweat glands
Skin and hair
Hooves
Reproductive organs
Kidneys
Urinary tracts
Pancreas
Thyroid gland
Most other glands
2
ANS 3319C Reproductive Physiology and Endocrinology – Early Embryonic, Fetal, and Placental Development
A
B
D
C
E
Figure 2. Schematic illustration of the general developmental course of the extraembryonic membranes in domestic
animals. The sequence shown occurs between about day 10 and day 20 after ovulation. A) The primitive endoderm
forms beneath the inner cell mass and begins to grow downward (arrows). B) As the primitive endoderm grows, an
evagination in the ventral inner cell mass forms the yolk sac. C) The newly formed primitive endoderm fuses with the
trophoblast to form a double membrane called the chorion. The chorion pushes upward and begins to surround the
embryo. At the same time a new sac, called the allantois (A), begins to form the primitive gut. D) The Yolk Sac (YS)
regresses and the allantois expands. The chorion nearly surrounds the embryo. E) When the leading edges of the
chorion fuse, a complete sac, called the amnion, surrounds the embryo and forms the amnionic cavity. The yolk sac
continues to regress while the allantois expands, making contact with the chorion. The allantois and chorion
eventually fuse, forming the chorioallantoic membrane.
3
ANS 3319C Reproductive Physiology and Endocrinology – Early Embryonic, Fetal, and Placental Development
5.
6.
4
Placental Anatomy and Function
The primary function of the placenta is to accommodate the fetus throughout gestation and to
allow for nutrient transfer from the maternal circulation to the fetal circulation so the fetus can
grow and develop. It is important to remember that the maternal and fetal circulatory systems
never mix.
Additionally, the individual components of the placenta have specific functions.
Yolk Sac
Nutrient supply for the early developing embryo. Becomes vestigal as gestation
progresses.
Amnion
Protects fetus from injury, provides lubrication for parturition, and serves as a
reservoir for urine and waste.
Allantois
Fuses with chorion (chorio-allantoic placenta), carries blood vessels of umbilical
cord, which attaches fetus to allantois, and is a reservoir for nutrients and waste.
Chorion
Attaches to uterus, absorbs nutrients from the uterus, allows maternal/fetal gas
exchange. Produces hormones.
Placental Attachment
Attachment or fusion of the placenta to the endometrium of the uterus.
Species
Cow
Ewe
Mare
Sow
Day of Gestation
30 - 35
18 - 20
50 - 60
12 - 20
Implantation is the invasion of the embryo into the endometrium where the embryo and
placenta continue to develop. This type of placentation is observed in humans, primates and
rodents. Dogs and cats have a semi-invasive placentation.
7.
Types of Placental Attachment (Based on chorionic villous pattern & maternal-fetal barrier)
The type of placenta is determined by distribution of chorionic villi over the surface of placenta.
Whereas the degree of placental invasion is best described by the maternal-fetal barrier, See
Figure 2a for descriptions of different placental attachment and invasions.
Species
Chorionic villous pattern
Maternal-fetal barrier
Pig, whale
Diffuse
Epitheliochorial
Mare
Diffuse & Microcotylendons
Epitheliochorial
Cow, sheep, goat
Cotyledonary
Epitheliochorial or Synepitheliochorial
Dog, cat, elephant
Zonary
Endotheliochorial
Human, most primates, rabbit
Discoid
Hemochorial
Rat, mouse
Discoid
Hemotrichorial
Chorionic-villi function to increase the surface area of placenta to increase nutrient exchange.
In ruminants (Figure 3), the connection is characterized by the presence of placentomes
consisting of:
Cotyledon – fetal attachment coming from the fetus.
Caruncle – maternal attachment coming from the uterine mucosa.
Sheep: 90 - 100 placentomes are distributed evenly throughout uterine horns.
Cattle: 70 - 120 placentomes are usually developed around fetus.
ANS 3319C Reproductive Physiology and Endocrinology – Early Embryonic, Fetal, and Placental Development
Figure 2a. Different types of placental attachment and invasion
5
ANS 3319C Reproductive Physiology and Endocrinology – Early Embryonic, Fetal, and Placental Development
6
Figure 3. Cotyledonary
placenta, characterized by the
large number of discrete buttonlike structures called
cotyledons. Picture A below is
an 80 day bovine pregnancy
showing the fetus and
chorioallantois membrane,
which contains the fetal
cotyledons. Picture B is the
corresponding uterus with the
maternal caruncles being
exposed.
A
B
ANS 3319C Reproductive Physiology and Endocrinology – Early Embryonic, Fetal, and Placental Development
7
The chorionic sacs of adjacent porcine conceptuses are in apposition and attachment of
adjacent chorions is frequently observed between fetuses.
In the pig and mare, the complex folding of the placental membrane and the endometrial
epithelia give rise to microcotyledon to increase surface area for nutrient transfer (Figure 4).
Figure 4. Examples of diffuse placentas, characterized by the uniform distribution of chorionic villi, which cover the
entire surface of the chorion. A pig conceptus is shown below in Figure 4A. Note the absence of fetal cotyledons
on the surface of the chorio-allantoic membrane.
Figure 4A.
In the bitch and queen, there is a central zone around the chorion that partially invades into the
uterine endometrium (Figure 5).
Figure 5. A zonary placenta,
observed in the bitch and queen,
is characterized by the band-like
zone of chorionic villi.
ANS 3319C Reproductive Physiology and Endocrinology – Early Embryonic, Fetal, and Placental Development
8
In the primate and human, the placentation is composed of a small circular area of the
placenta which invades and implants into the maternal endometrium (Figures 6, 7).
Figure 6. An example of a discoid placenta observed in
primates and humans. It is characterized by the regionalized
chorionic disk. This picture is an example of a liontailed macac
fetus in utero. Note the umbilical cord wrapped around its neck.
Figure 7. Human placenta and embryo at 8 weeks of
gestation. Credits (http://pregnancyknowledge.blogspot.com/)
Credits: Dr. Kurt Benirschke Web page: Comparative
Placentation (http://placentation.ucsd.edu/)
7.
Fetal Growth (i.e., bovine)
Relative growth rate, or the percentage increase in weight and size per unit of time, is most
rapid in early stages of development, and declines as gestation advances (Figure 7).
Absolute growth rate, or the absolute increase per unit time, reaches it maximum late in
gestation on approximately day 230 of gestation (Figure 8).
In cattle, over half the increase in fetal weight occurs during the last two months of gestation.
Figure 8. Depiction of absolute and relative growth rate curves in the bovine fetus.
ANS 3319C Reproductive Physiology and Endocrinology – Early Embryonic, Fetal, and Placental Development
Estimating fetal age by fetal crown-to-rump measurements in pigs, cattle and sheep:
Crown to Rump Length (cm)
Day of Gestation
Pigs
20
.9
30
2.5
40
4.8
50
8.2
60
11.9
70
15.8
80
17.6
90
19.4
100
22.6
110
23.9
Crown to Rump Length (cm)
Day of Gestation
Cattle
Sheep
20 – 39
.9
1.5
40 – 59
2.7
4.8
60 – 79
8.3
14.5
80 – 99
15.2
21.3
100 – 119
20.8
29.9
120 – 139
28.0
29.9
140 – 159
39.0
160 – 179
44.0
180 – 199
52.0
200 – 219
68.0
220 – 239
74.0
240 – 259
85.3
9
ANS 3319C Reproductive Physiology and Endocrinology – Early Embryonic, Fetal, and Placental Development
Additional Notes:
10