Chapter 8 Gestation
A. Definition: The period of pregnancy: Fertilization ----> Parturition
B. Gestation length:
a. Ave. gestation length (days) in farm animals: See TABLE 8-1 for breed difference
b. Mean length of gestation in some mammals: See the following table
c. Variation in gestation: 1) Breed difference
2) Individual difference
3) A little longer in cows carring a male
4) Shorter in cows with twins
5) Longer in animals with embryonic diapause
* Mean length of gestation in some mammals (days)
* Includes a period of embryonic diapause.
* Mean length of pseudopregnancy in some mammals (days)
C. Nutrition of conceptus:
a. Before implantation: Nutrients from cytoplasm of embryo and uterine milk
b. After implantation: Nutrients and waste products through maternal blood
D. Time when placentation starts:
Days after fertilization
E. Placentation process in cows:
Days after fertilization
F. Spacing of embryos:
a. Sow: 1) Embryo migration from one side to the other by 12 days
2) Equal distribution of embryos in both horns
b. Mare: Free migration of embryos into both sides
c. Cow and ewe: Less frequent transuterine migration
G. Development stage of conceptus:
a. Cleavage: 난할기 -----------> 2 細胞期 부터 이미 受精卵이 아니고 胚 즉
b. Differentiation: 분화기 Embryo 이므로 이 기간을 卵期 즉 Stage of
c. Growth: 성장기 ovum 이라고 칭하고, b.를 胚兒期로 칭하는 것은 옳지 않다.
A. Definition: Process of cell division without growth & differentiation From zygote ---> 2-, 4-, 8-,
16-cell, morula, and to blastocyst before hatching.
B. Process of cleavage: See FIGURE 8-1
C. Cleavage time comparisons by species: See TABLE 8-2
a. Similar pattern for different farm aniamls
b. Whole period of cleavage:
proportional to length of gestation
* Definition: True period of embryo: The process of forming specific organs: From hatched
blastocyst→ germ layers→ extraembryonic membranes → to organs
8-2-1 Germ layers: See FIGURE 8-2
* Three layers from inner cell mass of blastocyst
A. Endoderm → Digestive & respiratory system,liver & other internal organs
B. Mesoderm → Circulatory, skeletal & urogenital system & muscle
C. Ectoderm → Nervous, sensory & mammary system, skin, hair & hooves
8-2-2 Extraembryonic membranes: See FIGURE 8-3
* Soon after appearance of germ layers
A. Yolk sac : From endoderm Provide nutrirents in very early stage. Regress as the yolk is depleted.
B. Amnion : From trophoderm (by fusion of ectoderm and mesoderm) folding around the embryo Form
amnionic cavity containing amnionic fluid Amnionic fluid is turgid at early stage, suspends and
protects embryo and fetus.
C. Allantois: From endoderm and mesoderm: connects to embryonic bladder Form allantois cavity containing fluids high in waste products.
D. Chorion : From trophoderm : outer layer of extraembryonic membranes Form allanto-chorion by fusion with allantois
Allanto-chorion: attached to endometrium forming placenta
8-2-3 Organ formation
A. Formation of major organs: See TABLE 8-4
a. Open neural tube ---> central nervous system: brain and spinal cord
b. Circulatory system: embryonic heartbeat: detected at 16d in sow;22d in cow
c. Liver; pancreas; lungs; digestive organs; limb buds-->legs; tail buds, etc
B. Similar appearance of early embryos in various species: See FIGURE 8-4
C. Formation of male and female reproductive organs (Sex development) Male ←Undifferentiated stages→Female
a. Undifferentiated gonads as genital ridge
Cortex →XX→Ovary (Secondary sex cords)
Testis←XY←Medulla(Primary sex cords)
b. Urogenital ducts
Epididymis ←T―+ Mullerian duct→+→ Oviduct
Vas deferens ←T←(+MIF) (-MIF-T/DT)+--| ) | > Uterus
Vesicular gland <----T----+-- Wolffian duct +------------> Anterior vagina Prostate gl. <---DT--+- c. Urogenital sinus ---+------------> Low vagina
Cowper's gl. <---DT--+ +------------> Urethra
Urethra <-----DT-----+ Penis <------DT-------- d. Genital tubercle ----------------> Clitoris Scrotum <-----DT------- e. Labio-scrotal swelling ----------> Labia major
D. Period of differentaition: See TABLE 8-4
a. Sow: Day 7-28 Completely differentiated bovine embryo:
b. Ewe: Day 11-45 See FIGURE 8-5
C. Cow: Day 13-45
8-3 Fetal growth
A. Definition of fetus: Completely differentiated conceptus
Development of fetus: Growth only
B. Two concepts for growth rate:
a. (Daily, Weekly, Monthly) growth rate: Weight gain/ Given period: increase as sigmoid curve:
growth curve Over 1/2 of calf birth wt: gained during the last 2 months
b. Relative growth rate: Present wt./ Previous wt.: increse then decrease
C. Growth of bovine fetus: See TABLE 8-5
* Some landmarks:
a. Day 70: Calcification of bone matrix starts.
b. Day 110: Tooth formation begins.
c. Day 150: Hair can be seen around eyes and muzzle.
d. Day 230: Hair covers entire body.
D. Growth of ovine fetus: See TABLE 8-6
a. Growth rate for twins in last month of gestation: affected by nutrition
E. Weight change of fetal membrane, fetal fluids and uterus:
a. Inc. as gestation progresses.
b. Inc. in fetal weight lags behind expansion of fetal membrane.
c. Weights at calving:
Fetus : 25-40kg
Fetal fluid : 15.5kg
Fetal membranes: 3.8kg
Uterus : 10kg (1.0kg before conception)
A. Twinning rates in monotocous species:
a. Ewe: 20-40%
b. Cow: 0.5-4% -- Higher in dairy breeds especially in Holstines:
Problems: 1) High incidence in retained placenta: prevented by PGF2α injection just after calving
2) Weaker claves: more difficult to raise
3) Reduced milk production after twinning
c. Mare: Undesirable: double ovulations: 25% -- Abortion occurs with twins.
B. Genetics of twinning
a. Cattle: low heritalbe
b. Ewe: Litter size : Merino: 1.0; Finnsheep: 2.7 Booroola line of Merino: higher twinning from more ovulations--controlled by a few dominant genes
c. No twins in primiparous females, inc. with age and then dec.
d. Monozytotic twins: See FIGURE 8-6
Of twin births, 8-10% in cows and 30% in humans
8-5 Hormones important to gestation: See FIGURE 8-7
A. Progesterone has a dominant role in pregnancy maintenance Dec. tone of myometrium and inhibit
uterine contraction <--down-regulation of receptor sites for E2 and oxytocin
B. Maternal recognition of pregnancy: at day 13 in ewe; day 15-17 in cow If pregnant this time,
preventing PGF2@ release and luteolysis and stimulating production of antiluteolysin (PGE2)
Soften the connective tissue to stretch uterine muscle Cause the pelvic canal to expand during late
D. Estrogen in late gestation: from placenta Synergistic action with P4 for mammary development
E. Placental lactogen Mammary development and fetal growth