LEC3

3 Embryology of Farm Mammals

3.1 Duration of prenatal life

To keep things simple, we are going to call hatching - birth. This will enable us to use prenatal to indicate events occurring before birth in mammals and before hatching in poultry, and postnatal for events after birth or hatching. The duration of the prenatal period is very important. Obviously, the producer needs a good idea of when new animals will be born. A more subtle point is the length of the prenatal period determines the cost of final products. To produce a new calf takes a certain length of time - a time during which all the costs of production must be paid by the producer. Most of the animal's live weight is obtained by postnatal growth. Thus, veal must be more expensive per kilogram than beef - the prenatal period is the same for both, but beef gets a magnifier effect from longer postnatal growth. Lamb must be more expensive per kilogram than mutton.

Stage of development	Beef	Lamb	Pork	Chicken
Forelimb bud	24	20	16-17	2.2
Hind limb bud	26	21	17-18	2.2
Foetus	45	21	20	5
Birth	285	150	112	22
Slaughter	850	350	270	70
Prenatal duration %	33	43	42	31

The data in the table are days approximately. Gestation varies between breeds and slaughter animals vary in age, depending on market conditions. But the percentages in the bottom line are close to commercial approximations.
Our control over prenatal growth is minimal because the environment created for the foetus by the uterus or cleidoic egg is buffered against the farm environment.
A cleidoic egg is one that, except for temperature and the exchange of gases, is isolated from its environment by membranes and a shell, as in poultry.

3.2 Embryology

Oligolecithal versus telolecithal eggs. Mammalian eggs are oligolecithal - they contain very little yolk. Poultry eggs are telolecithal - they contain lots of yolk. In a telolecithal egg, cell division in the relatively small amount of protoplasm does not spread very far into the viscous yolk mass. The future chick develops as a flat disk of cells embedded on the surface of undivided yolk. Mammals have very little yolk but have evolved from reptiles with telolecithal eggs and, thus, they retain features to cope with the problem of a large inert yolk mass.
Cleavage. Cleavage is the process by which a zygote (fertilized ovum) is subdivided into smaller cells called blastomeres.
Morula. When a new embryo has subdivided into a ball of eight or more blastomeres, but has not yet formed layers of blastomeres, it is called a morula.
Inner cell mass versus trophoblast. The morula starts to form a new animal from the inner cell mass in the morula. The morula also forms a fluid‑filled sphere of cells called the trophoblast. The fluid-filled cavity is called the blastocoel. The trophoblast contributes to the placenta and is lost at birth. The inner cell mass together with the trophoblast is called a blasotocyst.

The embryo will develop from the inner cell mass roofed‑over by amniotic folds - these later fuse. Endoderm from the inner cell mass spreads over the inner surface of the trophoblast and, at this stage, the blastocyst is said to be bilaminar. The blastocyst becomes trilaminar when mesoderm from the inner cell mass spreads between the outer trophoblast layer and the inner endoderm layer. The mesoderm layer then splits within itself to form a cavity within the mesoderm, the extra-embryonic coelom.

Expansion of the extra‑embryonic coelom allows the yolk sac and allantois to expand into the coelom from the gut of the embryo. The rate of growth of the extra‑embryonic membranes may be an important factor affecting overall growth. Thus, in animals with a high growth rate, growth of the nutrient‑obtaining extra‑embryonic membranes is faster than normal. Initially, embryonic growth is slow in animals with a high growth rate, but later it proceeds faster than normal as nutrients are assimilated at a faster rate.
In chicks, the yolk sac contains the yolk whereas in cattle, sheep and pigs the yolk sac is rudimentary.
In chicks, the allantois is used for respiration and storage of waste products whereas in cattle, sheep and pigs, the allantois enables vascular communication between the developing animal and its mother.
The double layer of cells formed by the mesoderm and the ectodermal trophoblast is called the chorion.
The allantoic membrane is pushed against the chorion to form the allantochorion.
The interface between the allantochorion and the mammalian uterus is increased in surface area by the formation of chorionic villi.
In pigs, villi are scattered over the chorion (diffuse placenta) whereas, in cattle and sheep, villi are grouped into about 100 patches called cotyledons.

The space within an avian egg is limited, and the growth of the foetus and allantois is balanced by shrinkage of the yolk sac. The mammalian uterus, however, is capable of considerable expansion, and the allantoic cavity grows quite large.

3.3 Cattle

The most widely used measure of embryonic and ofetal growth is crown-rump (CR) length, a straight line measured from the crown of the head to the base of the tail. The crown may be taken as a point midway between the eye orbits on a line traversing the approximate position of the frontal eminence.
Growth in weight of the calf is usually more variable than CR length. The weight of the foetus does not usually overtake the weight of the foetal membranes until approximately three months after conception.
The relative water content of muscles decreases during foetal development.
The concentration of DNA and RNA remains high until approximately 6 to 7 months gestation: after this time, the relative DNA content sharply declines and there is an increase in dry matter and total nitrogen.

3.4 Sheep

In foetuses of unknown age, CR length may be used to determine the age of the foetus from 50 to 100 days gestation.
The age of foetal lambs also may be estimated from the state of development of the hair follicles in the skin.
Undernutrition of the ewe during the last six weeks of gestation causes a severe decrease in the birth weight of lambs.
Embryonic growth and development proceed in an anterior to posterior sequence with the head reaching a relatively large size early in development. During foetal development, the remainder of the body catches up to, and overtakes the growth of the head. The head to body ratio, however, is rather variable between animals.
The feet and the tail develop last.
Birth weight has a strong effect on the early postnatal growth rate of lambs which slowly diminishes with age.
Competition between twin and triplet foetuses may inhibit growth. In competitive situations, males sustain less inhibition of growth than females.

3.5 Pigs

Sows are multiparous and usually have ten or more offspring in each litter.
Wild pigs have a longer gestation period (130 days) and fewer offspring per litter than modern commercial pigs, so selection for larger litters may have decreased the length of gestation.
Breeds with few piglets per litter have embryos with low embryonic growth rate and fewer cells.
Foetuses compete with each other for the nutrients made available by the uterus, and the success of a foetus is related to its position within the uterus.
Selection for muscle growth in pigs may have resulted in piglets being heavier but less mature at birth.
Longitudinal growth of bones depends more on nutritional status and foetal body weight than on the age of the foetus.
Runt pigs (the smallest in a litter) have fewer myofibres than normal.
If runts are able to compete successfully for milk after birth, may be able to catch up in their growth by the time of weaning.
But, below a birth weight of about 1 kg, runt piglets are doomed to produce carcasses with less muscle and extra fat than normal.
Splayleg is a condition in which newborn piglets cannot bring their hindlimbs together normally. Thus, their locomotion is impaired and they may be crushed by the sow.

3.6 Genetic engineering

The experimental manipulation of prenatal development in farm animals has resulted in commercial methods such as embryo transfer. This early stage of animal development is also the most amenable stage for genetic engineering. Genes were first transferred experimentally into new locations by techniques such as chromosome assimilation and cell fusion. To show the transfer of genes actually had taken place, the recipient cell was placed in an environment in which the transferred genes would reveal themselves. For example, the recipient cells might suddenly acquire the ability to survive in a medium that lacked a hitherto essential nutrient. Other techniques also were developed for the transfer of isolated DNA: (1) direct injection into the nucleus of a recipient cell using a micropipette or high-speed impact, (2) the use of a virus to carry the DNA into the recipient cell, and (3) precipitation of the DNA with calcium phosphate to make it attractive for uptake by the recipient cell.

The genetic engineering of mammalian development by such techniques has only just started and, at the present time, depends heavily on the use of relatively simple and extensively studied genes. Apart from important exceptions such as myofibre hyperplasia in double-muscled cattle, most of the commercially important traits of meat animals are regulated by large numbers of genes and are difficult to manipulate by genetic engineering. In laboratory animals, growth rates may be increased dramatically by introducing genes that lead to increased levels of somatotropic hormone (STH). Whether or not genetic engineering at the molecular level will enable us to develop superior types of meat animals remains to be seen. The challenge is to make meat animals more efficient, not merely larger, and to enhance the quality of their meat - not to scare the meat-eating public towards vegetarianism.

Further information

Structure and Development of Meat Animals and Poultry, Chapter 8.