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Mammary Gland Development
Pregnancy


Mammary Gland
Development During
Pregnancy

Cross section of a beef heifer's udder.

See references from Tucker (1987) and from Forsyth (1986) in the reference list above for review of hormonal differences among species.

Mammary growth (of the mother) accelerates throughout pregnancy. This is fastest during the later stages of pregnancy, which coincides with the most rapid period of fetal growth. Pregnancy is often considered to be the period of most extensive mammary growth.

Extensive lobuloalveolar development occurs only during pregnancy ! [For a review of lobules and alveoli, see Mammary Gland Structure.]

The milk secretory cells develop only during pregnancy, therefore this period is extremely important in determining the number of secretory cells in the lactating gland and the subsequent production of milk.

Correlations between the total DNA in the lactating gland and milk yield for various species range between 0.50 and 0.85. The correlation coefficient between the number of mammary cells estimated in the lactating glands of rats and milk yield is 0.85, indicating a close relationship between cell number of the lactating mammary gland and milk yield (Tucker, 1966; Tucker, 1987). Some other species do not have as high a correlation, probably because their mammary tissue contains more non-epithelial components that make estimating mammary DNA more variable.


Hormonal Regulation of Mammary Development During Pregnancy

Estrogen and Progesterone

Optimal mammary growth requires both estrogen and progesterone. During pregnancy, the mammary tissue has estrogen receptors and progesterone receptors. During lactation the mammary gland has estrogen receptors, but not progesterone receptors. Concurrently elevated estrogen and progesterone, such as found during pregnancy, establish the conditions needed for geometric cell multiplication to occur. For example, from one original cell, 8 cell divisions will yield 128 cells. Concurrently elevated estrogen and progesterone also results in lobuloalveolar growth, which is characteristic of the type of mammary tissue development that occcurs during pregnancy. In the cow, progesterone is elevated throughout gestation (required for maintenance of pregnancy), while estrogen is particularly elevated during the second half of gestation. Consequently, most of the mammary growth during the first half of gestation is mainly ductal growth and lobular formation. In the second half of gestation, ductal growth continues, but most growth is lobuloalveolar.

In hypophysectomized- ovariectomized goats : administration of E, P4, PRL, GH, and ACTH (stimulates glucocorticoid secretion by the adrenal gland) are required for alveolar development comparable to that in mid-pregnancy. This suggests that all of these hormones are involved in mammary development during pregnancy. The roles of some of these and other hormones is discussed below.


Prolactin (PRL) and Growth Hormone (GH)

In virgin rats, transplantation of the pituitary to the kidney capsule (which releases the pituitary from inhibition of PRL secretion) results in marked stimulation of the mammary.

In the goat, (see Buttle et al. 1979 J. Endocr. 80:343-351) hypophysectomy reduces lobuloalveolar development. Administration of bromocriptine (an ergot alkaloid which inhibits prolactin secretion) also inhibits lobuloalveolar development.

Group

Lobuloalveolar tissue

Control

408 g

Hypophysectomy

234 g

Bromocriptine

197 g


However - blood levels of both PRL and GH are normally suppressed during gestation in most species. Probably both PRL and GH are required for mammary development, but their blood levels are not normally limiting. Remember that GH was implicated in mammary growth during the fetal stage and prepubertal stage of mammary development (see above)

An exception to this may be the rabbit where PRL alone seems to be sufficient for mammary development. Rabbit is also one of the few species that does not have a placental lactogen.


Placental Lactogen (PL)

[For review of PLs of ruminant species see Byatt et al., 1992, J. Anim. Sci. 70:2911.]

Placental lactogens are synthesized and secreted from the placenta. Generally they have both PRL- and GH-like activities. However there is great variation between species. Pigs and rabbits are two species which do not seem to have a placental lactogen.

In the rat, during the first half of pregnancy, PRL and GH are the primary hormones involved in mammary development. Significant PL is not secreted until day 12 of gestation. Hypophysectomy of a rat at day 12 of pregnancy or later, does not affect mammary cell number or maintenance of pregnancy.

In goats, the level of placental lactogen in the maternal blood is closely correlated with the number of fetuses the nanny is carrying (Hayden et al. 1980). One interpretation of this might be that the level of total lactogenic hormone activity in the maternal blood (PRL plus PL or other hormones) regulates the extent of mammary development during late pregnancy. However, in the cow, the level of placental lactogen in the maternal blood is very low.

Placental Lactogen probably synergizes with E, P4, PRL and GH in mammary development. It must remembered that the placenta also secretes estrogens and progesterone (these are highly species dependent).


Relaxin

These images represent normal mammary development in the pregnant gilt. Relaxin is required to achieve this level of mammary development in this species during pregnancy.

Cross-section of a mammary gland of a gilt at 80 days of pregnancy.

Cross-section of a gilt's mammary gland at 80 days of pregnancy.
Cross-section of a mammary gland of a gilt at 110 days of pregnancy. Cross-section of a gilt's mammary gland at 110 days of pregnancy.
Cross-section of a mammary gland of a sow during lactation. Mammary growth during lactation does not require relaxin. Cross-section of a sow's mammary gland during lactation.

Relaxin is a hormone secreted during pregnancy. It's role is usually associated with preparing the late pregnant reproductive tract for parturition. For example, it is involved in softening of the cervix prior to parturition in a number of species (O'Day et al., 1989). Most effects of relaxin require the presence of estrogen stimulation, too.

Relaxin also has a major mammogenic role in the sow (Hurley et al., 1991). Removal of the ovary from late pregnant gilts ( at day 80 or 100 of pregnancy; exogenous progesterone must always be administered to maintain pregnancy) also removes the source of relaxin in the pregnant pig. This results in substantially reduced mammary development during the last weeks of pregnancy. Remember that estrogen is secreted by the pig placenta.

Relaxin has been shown to stimulate mammary development in virgin gilts, too (Winn et al., 1994). Here, cycling, unbred gilts were ovariectomized and injected with relaxin, estrogen, or relaxin plus estrogen. Relaxin administration alone had minimal effect on mammary growth, while estrogen administration alone did cause some mammary growth. However, the greatest level of mammary growth was achieved with the combination of relaxin plus estrogen.

It is interesting that, while the pig does not have a placental lactogen, it does have relaxin as a major mammogenic factor during pregnancy. The level of relaxin in the mother's blood is correlated with the number of corpora lutea on her ovaries (Thomford et al., 1984), the number of corpora lutea being directly related to the number of ovulated oocytes and related to the number of fetuses. Thus, even in the pig there is a relationship between the number of fetuses and the level of mammogenic stimulus on the developing mammary gland.


Insulin

Required to maintain mammary tissue function in vitro. Mammary cells are resistant to insulin before conception, then they become sensitive to insulin during gestation and lactation, and become insensitive again during involution. Insulin stimulates mitosis of mammary cells in vitro, but is not absolutely essential in vivo. Severely diabetic mice given E and P4 will develop extensive lobuloalveolar structures. Still, insulin will synergize with E and P4 to increase mammary development. Insulin decreases during gestation (as does PRL and GH), so it is probably not rate limiting for normal mammary development.


Thyroid Hormones

Thyroid hormones are involved in the overall metabolic rate and oxygen consumption of the body. Their effect on mammary development is probably indirect or via the normal requirements of cell maintenance. Hypothyroidism retards ductal and lobuloalveolar growth. Administration of thyroid hormones restores the normal developmental pattern.


 
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