Mammary gland

Animals make milk to feed their young.

This milk helps babies grow strong.

Many animals have parts to make milk. Cows and goats have udders. Humans and chimps have breasts.

Dogs and cats have different parts too.

Inside the body, tiny parts make the milk. These parts are like small, hollow rooms. They join together to form tubes.

When a baby starts to drink, it sends a signal. This signal tells the body to push the milk out. The milk flows through the tubes to the nipple.

This special way of feeding is found in all mammals.

Mammals make milk to feed their young.

This milk helps babies grow. Different animals have different parts for milk. Humans and chimps have breasts. Cows and goats have udders. Dogs, cats, and pigs have parts called dugs.

Inside the body, there are tiny hollow spaces. We call these alveoli. These small parts make the milk. Many alveoli join to form groups called lobules. Each lobule has a lactiferous duct. This is a tube that carries milk to the nipple.

Special cells surround the alveoli. We call these myoepithelial cells. They help push the milk out. When a baby starts to suck, a hormone called oxytocin helps. This causes a "let down reflex." This means the milk is pushed out of the gland. The baby does not suck the milk out of the gland. Instead, the gland lets the milk out into the baby's mouth. Most mammals only make milk when they have had babies. This happens because of hormones in the body.

Mammals use mammary glands to make milk for their young. The word mammal comes from the Latin word mamma, which means breast.

These glands are arranged in different ways depending on the animal. Primates like humans and chimpanzees have breasts. Ruminants like cows and goats have udders. Other animals like dogs and cats have parts called dugs. Most mammals only produce enough milk for nursing during certain times. This usually happens in females that have recently had babies. Hormones in the body guide this important process.

Inside the gland, milk is made in tiny hollow spaces called alveoli.

These small cavities are lined with cells that secrete milk. Many alveoli join together to form groups called lobules. Each lobule has a lactiferous duct, which is a tube that carries milk. These tubes drain into openings in the nipple. Special cells called myoepithelial cells surround the alveoli. When a hormone called oxytocin is released, these cells contract. This causes a let down reflex. This reflex pushes the milk out of the gland and into the infant's mouth.

These glands develop through many different growth cycles. They exist in both sexes while an embryo is growing. During this early stage, the glands form a basic tree of ducts. As a mammal grows, the ducts grow longer and reach into a fat pad. At puberty, the ducts grow even more. In females, estrogen helps the ducts branch out and grow. Growth hormone and a factor called IGF-1 also help this happen. These changes prepare the body for the possibility of nursing later.

During pregnancy, the glands change very quickly to get ready. The structures called alveoli form within the branches of the ducts. This is when the gland is most ready to produce milk. After a baby is weaned, a process called involution happens. This is when the mammary gland turns back toward its earlier state. In humans, there are normally two complex mammary glands. Each one is made of 10 to 20 simple glands. Some people may have more than two nipples, which is called polythelia.

It is helpful to think of the mammary gland like a tree. The nipple is like the trunk of the tree. The lactiferous ducts are like the branches growing out from it. The alveoli are like the tiny leaves at the very ends of the branches. Just as a tree needs a strong structure to grow, the gland needs a support system called the stroma. This stroma includes fat cells and connective tissue. This system helps the gland stay organized as it develops and works.

A mammary gland is a specialized exocrine gland found in all mammals. The term comes from the Latin word *mamma*, meaning breast. These glands are essential because they produce milk to nourish young offspring. While humans and primates have breasts, other animals have different arrangements. Ruminants, such as cows and goats, possess udders. Dogs, cats, and rabbits have structures called dugs. In most species, lactation—the production of milk for nursing—occurs only in females who have recently gestated. However, some species like certain bats allow for different patterns, and in rare cases, male lactation can occur.

The internal structure of a mature gland is a complex system of tubes and cavities. The primary units are called alveoli. These are tiny, hollow cavities only a few millimeters in size. Alveoli are lined with milk-secreting cuboidal cells. Surrounding these cavities are myoepithelial cells. These cells play a vital role in milk delivery. Multiple alveoli group together to form structures called lobules. Each lobule contains a lactiferous duct, which is a tube that drains into the nipple. In humans, we have two complex mammary glands. Each complex gland is made of 10 to 20 simple glands.

Milk production and delivery follow a specific biological sequence. First, the alveolar cells secrete milk into the lumen, or the hollow center, of the alveoli. To move this milk, the body uses a hormone called oxytocin. When an infant begins to suck, the oxytocin triggers the myoepithelial cells to contract. This contraction causes the "let down reflex." This reflex does not suck the milk out; instead, it pushes the milk from the alveoli through the ducts toward the nipple. The milk is then secreted into the infant's mouth. This process relies on a supportive environment called the mammary stroma. The stroma includes adipocytes, fibroblasts, and an extracellular matrix (ECM) that maintains the gland's shape.

Mammary gland development is a lifelong process divided into distinct stages. It begins during the embryonic stage in both sexes. Around embryonic day 10.5, milk lines form along the sides of the body. By day 11.5, placodes form, which will eventually become the nipple. By day 12.5, cells invaginate into the mesenchyme to create the mammary anlage. After birth, the ducts elongate into a mammary fat pad. During puberty, the ducts undergo branching morphogenesis. This creates an arboreal, or tree-like, network of ducts. In females, this growth is driven by the synergy between estrogen and growth hormone (GH).

Chemical signals guide how these glands grow and change. Growth hormone works largely by inducing the secretion of insulin-like growth factor 1 (IGF-1). While GH is helpful, IGF-1 can induce development even without it when combined with estrogen. During pregnancy, the gland undergoes rapid changes to prepare for nursing. Progesterone and prolactin drive the development of the lobuloalveolar structures. Once the infant is weaned, the gland undergoes involution. This is a controlled process where mammary epithelial cells undergo apoptosis, or programmed cell death. This allows the gland to revert to a pubertal state.

Evolutionary history shows that these glands are highly modified. Many scientists believe mammary glands evolved from modified sweat glands. In monotremes, such as the platypus, the glands are modified sebaceous glands. These animals lack nipples entirely. In most other mammals, the glands are organized into breasts or udders. An udder is a single mass containing multiple mammary glands. For example, a cow's udder has two pairs of glands and four teats. A sheep's udder has one pair of glands and two teats. These variations show how the gland adapts to the specific needs of different species.

There are also rare biological variations in gland structure. Some individuals exhibit polythelia, which is the presence of more than two nipples. Others may have polymastia, meaning they have more than two complex mammary glands. The development of these glands is a delicate balance of hormones. For instance, androgens like testosterone can actually inhibit mammary development. They do this by reducing the expression of estrogen receptors in the tissue. This is why most males do not develop functional mammary glands. The entire system is a precise interaction between local cells and systemic hormones.