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On this page, the information will explain the development, structure, function, and endocrinology of the mammary glands.
Image (above): The development of the mammary glands throughout the different periods of a female's life.
Mammary Gland Development & Endocrinology
Mammary Gland Development & Endocrinology
Prenatal:
Mammary Ridge: Epidermal tissue, that is thickened, gives rise to the mammary gland.
Primary Mammary Bud: Thickened area develops inward and pushes into the underlying dermis.
Secondary Mammary Bud: Branches form outwards and spread into mesenchyme dermis layer.
Canalization: Canals will form with myoepithelial around the terminal ends of the canals and these form the lactiferous ducts.
Pre-pubertal: Isometric growth or growth that happens at the same rate as the rest of the body.
At Puberty: Allometric growth starts during this period and will develop the alveolar framework. Allometric growth is growth that happens at a faster rate than the rest of the body. During estrous cycles, estradiol causes ducts to branch more and increase in diameter. When progesterone increases, it causes terminal ends to develop into the beginnings of the alveoli and form secretory elements. Prolactin and growth hormone (somatotrophin) cause rapid duct development.
During Pregnancy: Alveolar development will finish in the last trimester of pregnancy. Terminal alveoli form bunches called lobules and these lobulo-alveolar structures will continue to grow until 90% of mammary mass under the influence of prolactin, adrenal hormones, and placental lactogen stimulate growth.
During Lactation: Alveoli develop in order to secrete milk. They also expand due to the milk synthesis process starting. Prolactin, adrenal hormones, and placental lactogen stimulate the synthesis of milk as well.
Duration: about 4 weeks but can last up to 8 weeks after queening (1)
During Involution: Cellular death of the alveoli occurs along with cellular remodeling which follows with a period of rest for the mammary glands. Cellular death is caused by a buildup in milk causing pressure within the alveoli.
*all information was from source 2 unless otherwise stated
In cats, they develop along two lateral lines on ventral surface which extends from axillary region to inguinal region. Most cats will 8 teats with anywhere from 7-16 ducts openings. It is believed that kittens are able to find the same teat each time they nurse due to difference in scents between each teat. The mammary glands have an extensive blood supply (4). Alveoli or secretory tissue is where milk is synthesized. It is released into larger and larger ducts (gland sinus, teat sinus, and teat canal). It exists the mammary glands through openings called teat orifices (2).
Image (above): The internal structure of a mammary gland.
Mammary glands are sophisticated sweat glands that synthesize milk. Because of the kitten's inability to depend on itself for food, water, and nutrition, the queen provides it to the kitten through this milk.
Contains:
water
proteins (opioid peptides, antihypertensive compounds, antithrombotic compounds)
fats
hormones (estrogen, progesterone, lipophilic, GnRH, Prolactin, growth hormones, etc.)
lactose
Colostrum is the first milk an animal receives from its dam. This should be given to the kittens as soon as possible. Colostrum contains immunoglobulins that help the offspring gain passive immunity from its dam. Otherwise, the offspring would have a an underdeveloped and underexposed immune system. It would be extremely susceptible to disease. Colostrum also contains higher levels of growth hormones that help promote neonatal growth and development. These growth hormones include: Insulin Like Growth Factors 1 & 2, Epidermal Growth Factor, and Transforming Growth Factor a and b (2).
*all information was from source 3 unless otherwise stated
Image (above): The key components of the process of milk ejection.
Image Source: Lactation (Source 3) slide 18
Milk Ejection
Milk Ejection
Milk secretion is when milk collects in the alveolar lumina after being synthesized within the alveolar cell. Milk ejection is the movement of milk from the alveolar lumina to exit through the teat. It requires both sensory stimulation and endocrine regulation.
The process begins with already synthesized milk stored within the alveolar lumina and fine ducts connected to them. Tactile stimulation of the teat as well as auditory and visual triggers initiate milk ejection. These triggers send impulses to the hypothalamus. The hypothalamus signals the posterior pituitary gland to release oxytocin. Oxytocin travels through the blood to the myoepithelial cell surrounding the alveolus within the mammary glands. Oxytocin causes contractions within the myoepithelial cells. These contractions cause milk to move from the alveolar lumina, to small ducts, then larger ducts, until it exits through the teat.
References for Mammary Glands Development, Structure, and Function
Little, S. E. (2012). Female Reproduction. The Cat, 1195–1227. https://doi.org/10.1016/B978-1-4377-0660-4.00040-5
Senger, P.L. (2012) Pathways to Pregnancy and Parturition. 3rd ed. Current Conceptions, Inc.
VanKlompenberg, M. (2023) Lactation
Wells, V. (2015, August 2). Structure and function of the mammary system in cats. Pet Place. https://www.petplace.com/article/cats/pet-health/structure-and-function-of-the-mammary-system-in-cats#:~:text=Mammary%20glands%20are%20composed%20of,to%2050%20days%20after%20weaning.
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