Embryonic Placental Fetal

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. Non-bolded values are in the oviduct. Bold values in the shaded area are in the uterus; (-) = no data

Species 2-cell 4-cell 8-cell Morula Blastocyst Hatching Gestation

Length (mo)

Bitch 3-7 d - - - - 13-15 d 2

Cow 24 h 1.5 d 3 d 4-7 d 4-10 d 9-11 d 9

Ewe 24 h 1.3 d 2.5 d 3-4 d 4-10 d 7-8 d 5

Mare 24 h 1.5 d 3 d 4-5 d 6-8 d 7-8 d 11

Queen - - - 5 d 8 d 10-12 d 2

Sow 14-16 h 1.0 d 2 d 3.5 d 4-5 d 6 d 3.8

Woman 24 h 2 d 3 d 4 d 5 d 5-6 d 9

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 2

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 Organs

Ectoderm Central nervous system Sense organs Mammary glands

Sweat glands Skin and hair Hooves

Mesoderm Circulatory system Skeletal system Muscle

Reproductive organs Kidneys Urinary tracts

Endoderm Digestive system Liver Lungs

Pancreas Thyroid gland Most other glands

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.


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.

A B

C D

E


5. 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.

6. Placental Attachment

Attachment or fusion of the placenta to the endometrium of the uterus. Species Day of Gestation Cow 30 - 35 Ewe 18 - 20 Mare 50 - 60 Sow 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.


Figure 2a. Different types of placental attachment and invasion


Figure 3. Cotyledonary placenta, characterized by the large number of discrete button-like 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


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).

In the bitch and queen, there is a central zone around the chorion that partially invades into the uterine endometrium (Figure 5).

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 5. A zonary placenta, observed in the bitch and queen, is characterized by the band-like zone of chorionic villi.

Figure 4A.


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).

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 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.

Credits: Dr. Kurt Benirschke Web page: Comparative Placentation (http://placentation.ucsd.edu/)

Figure 8. Depiction of absolute and relative growth rate curves in the bovine fetus.

Figure 7. Human placenta and embryo at 8 weeks of gestation. Credits (http://pregnancyknowledge.blogspot.com/)


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


Additional Notes:

Documents

Embryonic Placental Fetal