Lanugo

Lanugo is a fine, soft, and unpigmented type of body hair that first develops during the early stages of fetal development. It serves as the initial hair covering of the fetus and typically appears around the fourth or fifth month of gestation. Lanugo is much thinner and softer than other types of hair, such as vellus or terminal hair, and it lacks pigment, giving it a light, almost translucent appearance. This hair helps protect the skin during development but is usually shed before or shortly after birth.

Location

Lanugo covers nearly the entire body of the fetus, including the face, shoulders, back, and limbs. It is particularly concentrated on areas like the forehead, cheeks, and upper arms. While it is normally shed around the seventh or eighth month of pregnancy, some newborns, especially preterm infants, may still have lanugo at birth.

Structure and Anatomy

Lanugo is a specialized type of hair that forms during early fetal development. While it shares basic characteristics with other types of hair found on the body, it has distinct features in terms of its structure and function. Below is a detailed breakdown of the anatomy of lanugo.

Hair Shaft Structure

• Fine and Soft Texture: Lanugo is characterized by its fine and soft texture. Unlike terminal hair (such as scalp hair or body hair after puberty), lanugo lacks the coarseness and thickness of adult hair. Its diameter is significantly smaller, making it almost imperceptible to touch.
• Lack of Pigment: Lanugo is unpigmented or very lightly pigmented, meaning it is usually pale, almost translucent. This is due to the absence of melanin production in the hair follicle during the early stages of development, giving lanugo a very light appearance.
• Lack of Medulla: Lanugo lacks a medulla, which is the innermost core of mature hair strands. The medulla is present in terminal hair and some vellus hair but is absent in lanugo, contributing to its extremely fine and soft nature.

Hair Follicle

• Shallow Follicular Root: Lanugo grows from hair follicles, similar to other types of hair, but the follicles are shallow and less developed than those of terminal hair. These follicles are embedded in the superficial layers of the skin, primarily in the epidermis and upper dermis.
• Simple Follicular Structure: The hair follicle for lanugo is simpler in structure compared to mature hair follicles. It lacks some of the complex features seen in terminal hair follicles, such as a well-defined bulb or a robust sebaceous gland attached to it. Lanugo follicles are smaller, and their attachment to other skin structures is less complex.

Growth Cycle

• Rapid Growth Phase (Anagen): Lanugo enters the anagen phase (growth phase) early in fetal development, around the fourth or fifth month of gestation. The anagen phase is when the hair actively grows, and in the case of lanugo, this phase happens rapidly and uniformly across the body.
• Short Lifespan: The lifespan of lanugo is relatively short, lasting only a few months during gestation. By the seventh or eighth month of pregnancy, lanugo typically enters the catagen phase (transition phase) and telogen phase (resting phase), where the hair becomes detached from the follicle and is eventually shed.
• Shedding and Replacement: Most lanugo is shed in the womb before birth and is replaced by vellus hair or fine terminal hair in some areas. The shedding of lanugo occurs in the amniotic fluid, where it is ingested by the fetus as part of its development. Some newborns, particularly those born prematurely, may still have lanugo covering parts of their body at birth.

Length and Distribution

• Longer than Vellus Hair: Lanugo is typically longer than vellus hair, which is the fine hair found on the body after birth. Lanugo can grow up to 2-3 centimeters in length, whereas vellus hair is generally much shorter. Despite its fine texture, the relative length of lanugo makes it distinctive.
• Uniform Distribution: Unlike terminal or vellus hair, which tends to grow in specific regions of the body (scalp, eyebrows, eyelashes, etc.), lanugo covers almost the entire surface of the fetus’s body. It is present on areas such as the forehead, cheeks, shoulders, back, upper arms, and legs.

Lack of Association with Sebaceous Glands

Minimal Sebaceous Activity: Unlike terminal hair, which is often associated with sebaceous glands (glands that secrete sebum to lubricate the hair and skin), lanugo has very little or no association with sebaceous glands. This is due to the fact that lanugo is temporary and does not require the lubrication needed by more permanent hair types like scalp hair or body hair.

Anchoring to the Skin

• Superficial Root Anchoring: The roots of lanugo hairs are more superficially anchored in the skin compared to terminal or vellus hairs, which have deeper follicles that extend into the dermis and subcutaneous layers. Because of this shallow anchoring, lanugo hairs are more easily shed as they grow out of their short anagen phase.
• Lack of Deep Follicular Structure: The hair follicles of lanugo do not develop deep roots or bulbs like those of terminal hair. This shallower attachment to the skin allows for the easy shedding of the hair after its short growth cycle.

Transition to Other Hair Types

• Shedding and Replacement with Vellus Hair: After lanugo is shed, it is typically replaced by vellus hair, which is also fine and light but shorter and less noticeable. In some cases, lanugo is replaced by fine terminal hair in areas such as the scalp and eyebrows, where more permanent hair growth is required.
• Transition at Birth: By the time of birth, most fetuses have already shed their lanugo, and vellus hair has taken its place. In premature infants, lanugo may still be present at birth but will typically shed in the weeks following delivery as the infant develops.

Lack of Pigmentation

• Absence of Melanin: Lanugo hair lacks melanin, the pigment responsible for coloring hair, skin, and eyes. As a result, lanugo is almost always colorless or very pale in appearance. This absence of pigmentation is due to the fact that melanin production in the hair follicles has not fully developed during the early stages of gestation when lanugo forms.
• Developmental Transition to Pigmented Hair: After the shedding of lanugo, the body replaces it with vellus or terminal hair, which may develop pigmentation depending on genetics and the presence of melanocytes in the hair follicles. Vellus hair remains light, but terminal hair can become darker and more pigmented as the child grows.

Presence in Premature Infants

• Retention at Birth: In premature infants, lanugo is often still present at birth, especially if the baby is born before the full term. This fine hair may cover large areas of the body, particularly the back, forearms, shoulders, and face. As the infant matures, this hair will typically shed within a few weeks.
• Absence in Full-Term Infants: Most full-term infants have already shed their lanugo by the time they are born, and the body is instead covered with vellus hair. If any lanugo remains in full-term newborns, it is typically limited to small patches and is quickly replaced post-birth.

Function

Lanugo plays several important roles during fetal development, particularly in relation to the skin, protection, and physiological processes. Although it is temporary and typically shed before or shortly after birth, it serves crucial functions during the early stages of development. Below is a detailed breakdown of the functions of lanugo:

Protection of Fetal Skin

• Barrier Between Fetal Skin and Amniotic Fluid: One of the primary functions of lanugo is to create a protective barrier between the delicate fetal skin and the amniotic fluid. During gestation, the fetus is surrounded by amniotic fluid, which can potentially cause irritation or damage to the developing skin. Lanugo acts as a soft covering that minimizes direct contact with the fluid, preserving the integrity of the skin.
• Prevention of Skin Maceration: Without lanugo, the skin of the fetus could become macerated or softened by prolonged exposure to the amniotic fluid, leading to potential skin damage. Lanugo helps to minimize this risk by creating a buffer, particularly during the early stages of development when the skin is thin and vulnerable.

Anchoring of Vernix Caseosa

• Holding Vernix Caseosa in Place: Lanugo plays a crucial role in helping to anchor the vernix caseosa, a waxy, white substance that forms on the skin of the fetus during the second trimester. Vernix is composed of lipids, proteins, and water, and serves to protect the fetal skin from dehydration and external factors. Lanugo helps trap this vernix on the skin, ensuring an even coating.
• Enhancing Vernix’s Protective Effects: By holding the vernix caseosa in place, lanugo ensures that the fetal skin is thoroughly coated, maximizing the protective benefits of the vernix. This combination of lanugo and vernix creates a barrier that guards against infection, mechanical damage, and the drying effects of the amniotic fluid.

Thermoregulation

• Insulation for the Fetus: Lanugo provides a layer of insulation that helps the fetus regulate its body temperature. This is particularly important because the fetus is unable to generate or retain heat as efficiently as a newborn. Lanugo traps a layer of air close to the skin, providing warmth and acting as a thermal barrier during development.
• Maintaining Body Temperature: The fetus’s underdeveloped thermoregulatory system requires additional support to maintain an appropriate body temperature. Lanugo, in conjunction with the vernix caseosa, helps prevent heat loss by creating a physical barrier that insulates the fetus while in the womb.

Facilitating Skin Development

• Promoting Skin Maturation: Lanugo contributes to the maturation of the skin during fetal development. By providing a protective layer and holding the vernix caseosa in place, lanugo helps shield the skin from environmental stressors in the womb. This protection allows the skin to develop properly and prepare for the transition to the external environment after birth.
• Preventing Skin Dryness: Lanugo works in combination with the vernix caseosa to prevent the fetal skin from becoming dry and flaky. By trapping the vernix and reducing water loss, lanugo ensures that the skin remains hydrated and supple as it matures.

Shedding and Contribution to Meconium

• Shedding into Amniotic Fluid: As the fetus develops, lanugo begins to shed around the seventh or eighth month of gestation. Once shed, it floats in the amniotic fluid, where it is ingested by the fetus. This ingestion contributes to the formation of meconium, the first bowel movement of the newborn, which is composed of ingested materials such as amniotic fluid, bile, and shed lanugo.
• Part of the Fetus’s Digestive Development: The shedding and ingestion of lanugo play a small role in the development of the fetal digestive system. Ingesting lanugo along with amniotic fluid helps prepare the fetus’s intestines for processing waste materials, contributing to the formation of the first stool passed after birth.

Sensory Function and Tactile Stimulation

• Tactile Sensation in the Womb: Lanugo may also serve a sensory function, providing the fetus with tactile stimulation while in the womb. The fine, soft hairs are thought to be sensitive to the movements of the fetus within the amniotic fluid, potentially aiding in the development of sensory perception and the nervous system.
• Early Sensory Development: By responding to movements in the fluid and changes in the environment, lanugo may help the fetus develop an early sense of touch, which is crucial for neurological development and for the fetus to start recognizing its own body.

Transition to Other Hair Types

• Role in Hair Growth Cycle: Lanugo is the first hair type to form during fetal development, marking the beginning of the hair growth cycle. Although it is temporary, the development and shedding of lanugo set the stage for the transition to vellus hair or fine terminal hair, which will later cover different parts of the body.
• Preparation for Postnatal Hair Growth: After lanugo is shed, the skin and hair follicles prepare for the growth of more permanent hair types. This transition is important for the proper growth of scalp hair, eyebrows, eyelashes, and other body hair types that will serve functional and aesthetic roles after birth.

Retention of Lanugo in Premature Infants

• Presence in Preterm Babies: In premature infants, lanugo may still be present at birth, covering parts of the body such as the back, shoulders, forearms, and face. While it doesn’t play a significant role after birth, in preterm infants, lanugo may continue to offer protective functions similar to those it served in utero, particularly in regulating temperature and protecting the skin.
• Gradual Shedding After Birth: In premature infants, lanugo will typically shed within a few weeks after birth as the baby matures. This process is normal and signals the transition to a more mature skin and hair structure.

Evolutionary Function

• Evolutionary Remnant: Lanugo is considered an evolutionary remnant from earlier stages in human evolution when hair may have played a larger role in thermoregulation and protection. The presence of lanugo during fetal development suggests that hair served protective functions in the early stages of life, long before human beings evolved less body hair as a characteristic of modern species.
• Connection to Other Mammals: Many mammals develop a similar covering of fine hair during fetal development, suggesting that lanugo serves a protective and adaptive role in mammals that is retained in human fetuses.

Retention of Heat in Utero

Heat Conservation: While in the womb, the fetus is surrounded by amniotic fluid, which can absorb and dissipate heat. Lanugo, along with the vernix caseosa, helps the fetus retain body heat by creating an insulating layer that traps warmth close to the skin. This is especially important in maintaining an optimal temperature for development.

Clinical Significance

Lanugo, though typically shed before birth, can have clinical significance in both infants and adults. In premature infants, lanugo may still be present at birth, covering areas such as the back, shoulders, and face. While it usually sheds within a few weeks as the infant matures, its presence in preterm babies can indicate developmental immaturity.

In adults, the appearance of lanugo-like hair can be associated with certain medical conditions, particularly eating disorders like anorexia nervosa. In these cases, the body grows fine, soft hair in response to extreme weight loss and low body fat as a means of conserving body heat. Lanugo in adults is a sign of severe malnutrition and requires medical attention. Additionally, lanugo can appear in individuals with certain hormonal imbalances or endocrine disorders.