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Lactogenesis


Lactogensis -
Initiation of
Lactation

Piglets suckling a sow.

Lactogensis is the term meaning the initiation of lactation. This it the process of functional differentiation which mammary tissue undergoes when changing from a nonlactating to a lactating state. This process is normally associated with the end of pregnancy and around the time of parturition. Because lactogenesis is particularly dependent upon a specific set of hormones (called the Lactogenic Complex of hormones), mammary tissue from most states of the nonlactating mammary gland also can be made to undergo some degree of lactogenesis by administration of high amounts of those hormones, even in nonpregnant animals.


Defining Principles of Lactogenesis

Lactogenesis is a series of cellular changes whereby mammary epithelial cells are converted from a nonsecretory state to a secretory state.

Lactogenesis is a two stage process :

1. Cytologic and enzymatic differentiation of alveolar epithelial cells. This coincides with very limited milk synthesis and secretion before parturition. Cytological changes associated with stage 1 of lactogenesis are described below. Enzymatic changes include increased synthesis of acetyl CoA carboxylase, fatty acid synthetase, and other enzymes associated with lactation, and increases in uptake transport systems for amino acids, glucose, and other substrates for milk synthesis. Note that synthesis of a-lactalbumin, and therefore, lactose synthesis does not begin until stage 2 of lactogenesis. Stage 1 of lactogenesis coincides with the formation of colostrum and immunoglobulin uptake (see The Neonate and Colostrum sections).

2. Copius secretion of all milk components. In the cow this begins about 0-4 days before parturition and extends through a few days postpartum. It is not until the release of the inhibitory effects of progesterone on lactogenesis (about 2 days prepartum in many mammals) and the stimulation by the very high blood concentrations of prolactin and glucocorticoids associated with parturition, that copious milk secretion begins (stage 2 of lactogenesis). One exception to this timing is in women. The drop in blood progesterone concentrations does not occur in women until parturition, so that the full impact of stage 2 of lactogenesis sometimes does not occur until about 2 days postpartum. In pigs and mice, stage 2 of lactogenesis is occurring immediately prior to and at the time of parturition. It is difficult to get any mammary secretion out of a sow until parturition, whereas in the cow, substantial mammary secretion volume can be collected up to several days prepartum.


Points to consider :

  • During late pregnancy the mammary gland develops the capacity to make milk, but copius milk secretion does not take place until near parturition.
  • The mammary gland seems to be locked into a state where milk secreting cells are present in late pregnancy, but copius milk secretion cannot take place. In fact, low levels of caseins and ß-lactoglobulin are synthesized in the mammary cells during roughly the last third of pregnancy. Milk fat accumulates in the cells and alveoli during the final few days of pregnancy. However, there is little synthesis of a-lactalbumin until the immediate peripartum period.

** Lactose synthesis is the key to milk secretion. **

Hormones associated with parturition (decreasing progesterone and increasing in glucocorticoid and prolactin) lead to transcription of the a-lactalbumin gene (see diagram below). The a-lactalbumin mRNA is translated at the RER and the a-lactalbumin protein interacts with galactosyltransferase in the Golgi apparatus in synthesis of lactose (see Lactose Synthesis section in the Milk Synthesis - Independent Study Module). Synthesis of lactose osmotically draws water into the Golgi and secretory vesicles. This process allows for secretion of large amounts of milk and is the most obvious manifestation of stage 2 of lactogenesis. At the same time, synthesis of other milk components is increased.

Diagram of Lactogenesis.


 
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