In addition to the production of eggs, the female reproductive system also produces hormones that aid in the control of body functions. These include androgen, oestrogen and progesterone. Androgen causes comb growth and condition, and has a function in the formation of albumen. Oestrogen causes the growth of the female plumage, mating and nesting behaviour, oviduct development together with the nutrient supply to the ovary/oviduct for egg formation. Progesterone, with androgen is involved in the production of albumen and the carriage of the message to the pituitary gland to release luteinising hormone.
The female reproductive system remains dormant in the young chicken and growing pullet until she reaches the age when these organs start to prepare for the normal production of eggs. One of the first signs of her developing maturity is the change in the comb development. This organ starts to grow and to take on a vivid red hue as the hormones produced by the now awakening ovary start to have an effect.
The formation of the hen's egg
The normal egg consists of the following major parts:
- Yolk carrying the ovum - produced by the ovary
- Albumen or white – produced mainly in the magnum
- Shell membranes – produced in the isthmus
- Shell – produced in the uterus or shell gland
The ovary and yolk formation
The ovary is attached to the abdominal cavity wall by the meso-ovarian ligament. It carries anything from 2000 to 12000 small ova in miniature follicles on its surface, plus hormone producing cells in its body. Not all of the ova found on the immature ovary develop and only approximately 200 to 350 reach maturity under normal modern commercial practice. Each yolk or ova takes about 10 days to grow and reach maturity when it is approximately 31% of the weight of the egg.
The composition of the yolk material is as follows:
| Component | % |
| Water | 48.0 |
| Protein | 17.5 |
| Fat | 32.5 |
| Carbohydrate | 1.0 |
| Other compounds | 1.0 |
The yolk is laid down in concentric rings of darker and lighter coloured material, the colour being produced by xanthophylls that are yellow/orange/red pigments occurring in many plants, plant products and other naturally occurring materials. The bulk of the yolk material provides a source of food for the developing embryo that originates by the fertilising of the germ disc or blastoderm usually located on the upper surface of the yolk of the broken out egg. It lies in the surface segment of the latebra which is a vase shaped segment of different yolk with it’s base in the centre of the yolk, the lips on the surface and the stem joining the base to the lips.
The yolk is contained in a very thin, transparent membrane called the vitelline membrane. As an egg becomes stale, the vitelline membrane becomes significantly weakened and often breaks to release the yolk contents when the stale egg is broken out. On ovulation the yolk is released and enters the oviduct where, as it passes along that organ, fertilization occurs and the remaining parts of the egg are added around it.The yolk is located on the ovary held in a sack called the follicle. The follicle, while being quite thin walled, is extremely well supplied with blood vessels. These are necessary to carry the materials constituting the yolk that have been formed in the liver.
Yolk development in the maturing pullet is initiated by follicle stimulating hormone (FSH) produced by the anterior lobe of the pituitary gland. The compounds in the yolk material are formed in the liver and, on the appropriate signal, are transported by the blood stream to the target follicle and into the yolk. The appropriate signal for this development comes from hormones - oestrogen, progesterone and testosterone produced by the ovary after receiving the signal of the Follicle Stimulating Hormone. These ovarian hormones also provide the stimulus for the formation of the development of the oviduct.
