Prenatal development


Commercial importance

If the start of growth is measured from the time of conception rather than from the time of birth or of hatching, we find that a considerable proportion of the time taken to reach market weight is spent in the uterus or in the egg.

Beef 33% Lamb 43% Pork 42% Chicken 31%

Obviously, these data are approximate, because of variability in the duration of gestation between breeds and the even greater flexibility of animal age at slaughter, but the figures are close to those of many commercial operations. Bearing in mind that breeding stock must be fed, housed, tended, and paid for this period, these figures have a profound impact on the economics of animal production, not forgetting the money that must be spent to get the stock to the age at which they can reproduce, as well as the time taken to achieve conception!

The control that can be exercised over the prenatal growth of farm animals is relatively slight, because the environment created for the fetus by the uterus or cleidoic egg is buffered against changes in the mother's environment.

Mammals versus Poultry

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 the chicken's egg. This contrast to the situation in mammals, where the developing fetus receives a continuous external supply of nutrients from the mother's uterus.

A mammalian egg has virtually no yolk tissue (oligolecithal) whereas an avian egg contains a lot of yolk (telolecithal). Mammals evolved from reptiles with telolecithal eggs, and so they retain features that indicate that their early embryos once had to develop on a large inert yolk mass. The yolk mass of the poultry egg creates problems because it is thick and viscous, and cannot split into subunits as fast as the small cells of the developing embryo.

Phenoypes versus Genotypes

The selective breeding of meat animals should be based on the selection of animals bearing phenotypic characters (those seen in the animal) that are related to the economic yield of meat. The biological linkages between genotypes and phenotypes pass through an epigenetic space in which gene products react among themselves and with environmental factors. The major epigenetic interactions involved in the formation of a new animal take place in the prenatal period so that prenatal development may be the most important, but least accessible phase of meat animal development.

Embryo versus Foetus

While the newly formed animal is developing its various types of tissues it is called an embryo but, after these tissues are acquired and until birth or hatching, it is called a foetus.

Stages of Embryology

In a telolecithal egg, cell division starts as a small disk and does not spread very far into the viscous yolk mass on which the disk is located.

Cleavage is the process by which a zygote or fertilized ovum subdivides into smaller cells called blastomeres. 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.

The morula starts to form a new animal from a clump cells called the inner call mass(A). Then the morula forms itself into a layer of cells called the trophoblast(B) surrounding a fluid filled space - the blastocoele(C) .

The trophoblast contributes to the placenta and is lost at birth. The inner cell mass together with the trophoblast form a blasotocyst. The embryo developing from the inner cell mass becomes roofed-over by amniotic folds that later fuse to form a complete layer - the amnion.

Endoderm cells (F; which eventually form the gut and its associated organs like the liver) spread from the inner cell mass over the inner surface of the trophoblast and, at this stage, the blastocyst is said to be bilaminar or two- layered.

The blastocyst becomes trilaminar or three-layered when mesoderm cells (E; which eventually will form muscles, bones, and fat) migrate from the inner cell mass spreads between the outer trophoblast layer and the inner endoderm layer.

The mesoderm layer then splits internally, and a cavity expands within the mesoderm to become the extra-embryonic coelom(G).

Expansion of the extra-embryonic coelom cavity allows the yolk sac and allantois to expand into the coelom cavity from the gut of the embryo. In poultry, the rate of growth of the extra-embryonic membranes may be an important factor affecting overall growth so that, in genetic strains of poultry with a high growth rate, the growth of the nutrient-obtaining extra-embryonic membranes is faster than normal. Thus, paradoxically, although overall embryonic growth may be slower in the strain with a high growth rate, this allows growth to proceed at a faster rate than normal because nutrients can be absorbed from the yolk at a faster rate.

This is a horribly generalized view of how things fit inside each other:A = embry, B = coelom, C = yolk sac, D = allantois, E = two layers forming the chorion, F = four layers forming the allantochorion, G = two layers of mesoderm, H = endoderm, and I = ectodermal trophoblast.

 In chicks, the yolk sac contains the yolk whereas in cattle, sheep and pigs the yolk sac is rudimentary (because the developing mammal gets most of its nutrients from the mother's uterus rather than from stored yolk in a cleidoic egg).

In chicks, the allantois is used for respiration and storage of waste products whereas in cattle, sheep and pigs, the allantois is developed to provide vascular communication between the developing animal and its mother.

The double layer of cells formed from mesodermal cells pushed up against the ectodermal trophoblast is called the chorion. Likewise, the allantoic membrane is pushed against the chorion to form the allantochorion.

Membranes around a fetal piglet: A = allantoic cavity, B = amniotic cavity, C = coelom, D = allantois, and E = chorion.

 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 to form a diffuse placenta whereas, in cattle and sheep, villi are grouped into about 100 separate mushroom-like structures called cotyledons.

The space within an avian egg is restricted so that the growth of the fetus and allantois must be balanced against the shrinkage of the yolk sac as its nutrients are used for the developing fetus. The mammalian uterus, however, is capable of considerable expansion, which allows the allantoic cavity to expand enormously.

Measurement of Prenatal Growth

The most widely used measurement of embryonic and fetal growth is called the crown-rump or CR length. This measurement is a straight line taken from the crown of the head to the base of the tail. The crown is the point midway between the eye orbits. Growth in weight is usually more variable than CR length, so CR length provides is more reliable estimate than weight when attempting to estimate the age of a fetus.

Head to Tail Development

Embryonic growth and development proceed in an anterior to posterior sequence (from head to tail) so that the head reaches a relatively large size early in development. During fetal 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. This pattern of differential growth is called allometric growth (change in body proportions).

Compare this 20 to 25 mm pig embryo with this 45 to 50 mm embryo. looking at the differences in relatives sizes of the head, body, and limbs.



Now compare this 9-day chick  with this 12 day chick.using the scale to correct for differences in magnification. See any feathers developing?

Slow Growth

Severly inadequate maternal nutrition or competition between fetuses may inhibit skeletomuscular growth_ In such competitive situations, males sustain "less inhibition of growth than females. Differences in body weight between twin and triplet lambs are related to differences in degree of placental development and position in the uterus. In prolific ewes, high fetal losses early in development are associated with the suboptimal growth of surviving fetuses.

Sows Have Litters

Sows are multiparous (they normally have a litter of ten or more rather than one or two fetuses). Wild pigs have a longer gestation period (130 days) and fewer offspring per litter than modern commercial pigs, so that selection for larger litters may have decreased the length of gestation in pigs. Pig breeds with few piglets per litter have embryos with low embryonic growth rate and fewer cells. Fetal piglets compete with each other for the nutrients made available by the sow's uterus, and the success of a fetus is related to its position within the uterus. Selection for muscle growth in pigs may have resulted in piglets that are heavier but less mature at birth.

Longitudinal growth of bones depends more on nutritional status and fetal body weight than on the age of the fetus. Maturation of the epiphyses, however, depends more on age than on nutritional status. The muscles of runt pigs are smaller than those of their more successful littermates. Runt pigs appear to have fewer muscle fibers, particularly white fibers, most of which are formed in the late fetal phase of fiber mass production. The smaller pigs of a litter, if they are able to compete successfully for milk after birth, may be able to catch up in their growth by the time of weaning. Below a birth weight of about 1 kg, however, runt piglets are doomed to produce carcasses with less muscle and extra fat.

Splayleg Piglets

Sows sometimes sit on their unwary offspring and kill them, and any degree of locomotor impairment in baby pigs increases this risk. Splayleg is a condition in which the hindlimbs and sometimes the forelimbs of a newborn pig temporarily are unable to support the body. If affected pigs are nursed through the period of high risk just after farrowing, they may compensate by enhanced synthesis of muscle proteins and then may grow normally. Slippery floors make the splayleg problem worse. Affected pigs may be helped with a loose coupling between the hindlimbs to prevent the limbs splaying outwards.

Poultry Embryology

In poultry, embryology is different to that in mammals because the developing embryo is flattened into a disk on top of the yolk.

Cleavage occurs as the zygote is passing along the oviduct before the egg is laid. The cytoplasm is concentrated in the blastodisc at the opposite pole to the yolk mass.


The blastodisc (sectioned in the diagram above)is a pale area several millimeters across. A flat cavity called either the blastocoele (2) or the cleft space develops within the depth of the blastodisc to produce superficial (1 = epiblast) and deep (3 = hypoblast) layers of cells. The presence of a subgerminal cavity gives the central part of the blastodisc a semitransparent appearance so that this area is called the area pellucida(6). The surrounding area (off the diagram) where the edges of the blastodisc are in contact with the yolk is called the area opaca.

All the cells for the future chick embryo are recruited from the two layers of cells: the epiblast (1) and the hypoblast (3) in the area pellucida (6). The roles of cells from various regions of the epiblast and hypoblast in the developing embryo may be determined experimentally by marking the cells with nontoxic dyes.

Gastrulation is relatively simple in oligolecithal eggs, but in the flat blastodisc of poultry eggs gastrulation is more complex and the cellular intrusion characteristic of gastrulation occurs along a slit called the primitive groove . The cells (1) that will sink downwards and migrate sideways are located along a thick line of actively growing cells called the primitive streak(2) located between the epiblast (3) and the hypoblast (4).

 Top is a section, the two bottom diagrams are plan views showing the area pellucida (5) starting to develop a primitive streak (6). At 24 hours, as shown below, the primitive streak is clearly visible.

The primitive streak develops after the egg is laid and by about 16 hours of incubation is the most conspicuous feature of the developing embryo. The primitive streak indicates the location of the future vertebral axis. The mesoderm develops from a middle layer of cells that accumulates and spreads sideways below the primitive groove.

After the longitudinal axis of the chcik is established, primitive streak activity is concluded. The embryo is lifted off the yolk by the development of infoldings below the future head and tail regions of the embryo. These infoldings unite along the sides of the embryo so that its connection to the yolk is progressively restricted.

The amnion grows up and over, all around the embryo, and the amniotic folds then fuse to form a tent-like roof over the embryo.

Sectioned chick embryos showing how the neural tube rolls up and sinks downwards: 1 = neural tube, 2 = epidermis, 3 = somite, 4 = intermediate mesoderm, 5 = somatic mesoderm, 6 = coelom, 7 = splanchnic mesoderm, and 8 = notochord.

 The notochord establishes the future axis of the vertebral column and is formed from some of the mesodermal cells in a middle layer between the ectoderm and the endoderm along the primitive streak. Formation of the notochord starts anteriorly and then posterior epiblast cells are recruited as the notochord grows posteriorly.

Not all of the mesoderm within the primitive streak is used for notochord formation and along each side of the notochord there remains a long strip of mesoderm that develops into somitic and intermediate mesoderm. Laterally, the remaining mesoderm splits into outer somatic mesoderm and inner splanchnic mesoderm.

By this time, the nerve cord has developed from a sunken roll of ectoderm sunken located above the notochord.

Embryonic Induction

The phenomenon of embryonic induction is still one of the great mysteries of animal development, although very rapid advances are being made in this field as the various genes that control develop are discovered. For example, one recent discovery is the gene that controls asyymetric development between left and right sides of the body (for example, why the heart is normally on the left side of the body while the appendix is on the right side). Most classical scientific names are rather difficult and boring, but in trendy new areas of science like particle physics and genetic engineering things are a little different: the newly discovered gene controlling asymmetric development is called "Sonic Hedgehog".

Understanding induction eventually may enable scientists to manipulate muscle development in the meat animals but, so far, attempts to identify the biochemicals responsible for embryonic induction have failed because of the infinitesimally small volumes involved. At present, we should be cautious in assuming that transmissible substances are solely responsible, because the messages that cause induction may be transmitted or faciliated by other means of cellular communication. However, the messengers of embryonic induction are most likely to be ribonucleoproteins.


Genes were first transferred experimentally into new locations by techniques such as chromosome assimilation and cell fusion. Genetic engineering of mammalian development by such techniques has only just started and, at the present time, it depends heavily on the use of relatively simple and extensively studied genes. Apart from important exceptions such as double- muscling in 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.

Animal growth rates may be increased dramatically by introducing genes that lead to increased levels of growth hormone but 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 the meat. However, there is money to be made doing this so, probably, it will all happen and butchers one day will be cutting and selling meat cuts from designer animals (if we have not all been replaced by robots).