Ectoderm extending from the axillary to the inguinal regions (the “milk line”) form called mammary crests



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Mammary glands are a modified and highly specialized type of sweat glands
About the fourth week, two thickened strips of ectoderm extending from the axillary to the inguinal regions (the “milk line”) form called mammary crests. In humans, the crests persist only in the pectoral region. In week six, the mesenchyme beneath these crests induces the epidermis to grow downward into the mesenchyme forming mammary buds (primary bud). Each bud gives rise to several secondary mammary buds. Placental sex hormones entering the fetal circulation induce canalization of these buds, transforming them into lactiferous ducts and their branches, until 15-20 ducts are formed. The fibrous connective tissue and fat of the mammary gland develop from the surrounding mesenchyme.

In late fetal period, the epidermis above the mammary gland depresses forming a mammary pit. After birth, the nipples rise from the pit due to proliferation of the surrounding connective tissue of the areola. The surrounding mesenchyme cells develop into the smooth muscle fibers of the nipple and areola. At birth, both the male and female rudimentary mammary glands are identical, and only the main lactiferous ducts are fully formed.

At puberty in females, estrogens induce the growth of the duct system. As the mammary glands develop further and fat accumulates around them, the breasts increase in size. (Progesterone, prolactin, corticoids, and growth hormone also play a role)

If pregnancy occurs, the mammary glands complete their development due to the raised estrogen levels and the substantial increase in progesterone levels. The intralobular ducts undergo rapid development, forming buds that become alveoli.


Gynecomastia = excessive development of the male mammary tissue. It is often present at birth since maternal sex hormones stimulate the glandular tissue (goes away in 2weeks). Often due to decreased testosterone to estradiol ratios.
Breast Aplasia = difference in breasts’ size. The breasts of a postpubertal female often differ somewhat in size. Substantial differences are regarded as anomalies because both breasts should have been exposed to the same levels of hormones. Often in BA there is an associated rudimentary development of the muscles, particularly the pectoralis major.
Polythelia = extra nipple, usually below inferior to normal breast. The nipples develop from extra mammary buds that developed along the mammary crests. Usually become obvious during pregnancy because the elevated levels of hormones cause the glands to develop.
Inverted Nipples = failure of nipples to elevate after birth (cause not given).

fibrous connective tissue and fat of the mammary gland

Mesenchyme surrounding bud

smooth muscles of nipple and areola
Odontogenesis: formation of Teeth
Is a continuous process, but divided into stages as not all teeth begin to develop at the same time.

The first tooth buds appear in the anterior mandibular region; later development occurs in the anterior maxillary region; last place is posteriorly in both jaws.


Two sets of teeth normally develop:

Deciduous teeth = primary dentition, shed during childhood

  • 20 total

  • buds develop around sixth week of devo

  • eruption between sixth and twenty-fourth month after birth

Permanent teeth = secondary detention

  • 32 total

  • buds develop around tenth week of devo if have deciduous predecessor; the buds for the first molars with no predacessors form in fetal period; second and third permanent molars form after birth

  • erupt in sixth year and continue to into adulthood

Teeth precursors:


outer enamel epithelium  epithelial root sheath

Oral ectoderm  Enamel organ



inner enamel epithelium  ameloblasts  enamel
dental papilla  odontoblasts  dentin

Mesoderm + Neural crest mesenchyme



Dental Sac  cementoblasts + periodontal ligament
Bud Stage

Dental Laminae are oral epithelium, a derivative of surface ectoderm, patterned in U-shaped bands following the curves of the primitive jaws (basically the naked gums). Neural crest cells, imprinted with morphogenetic information, migrate from the neural crest. They induce the ectoderm to migrate into the underlying mesenchyme in ten centers of proliferation forming swellings or Tooth buds. These buds develop into the deciduous teeth. Tooth buds for permanent teeth with deciduous precursors form to the tongue-side of the deciduous teeth in the tenth, or along the dental laminae at a later time if there is no precursor, such as in molars.
Cap Stage

Mesenchyme invaginates upward into the tooth bud, making the bud appear cap-shaped. This internalized mesenchyme becomes the dental papilla while the ectodermal part becomes the enamel organ. Together they are called the tooth germ. The enamel organ splits into an outer enamel epithelium and an inner enamel epithelium and is filled in the middle by loosely arranged cells forming the enamel (stellate) reticulum.

While all this is occurring, the mesenchyme surrounding the tooth germ condenses forming a vascularized capsular structure called the dental sac.
Bell Stage

Crown formation: The dental papilla cells adjacent to the inner enamel epithelium differentiate into odontoblasts, which produce pre-dentin. The pre-dentin accumulates between the papilla and the inner enamel epithelium and later calcifies into dentin. As the dentin thickens, the odontoblasts regress to the center of the dental papilla, but leave fingerlike odontoblastic processes embedded in the dentin. Meanwhile, the inner enamel epithelium differentiates into ameloblasts and produce enamel. The ameloblasts regress towards the outer epithelium. Enamel (2nd hardest tissue in body) grows around the brittle dentin from the tip to the root (1st hardest tissue in body) and protects it.

Root formation: After the crown is formed, the inner and outer enamel epithelia fuse at the neck of the tooth forming a fold, the epithelial root sheath. This grows into the mesenchyme and initiates the nearby odontoblasts to produce dentin, continuous with the crown. The accumulating dentin forms a narrow root cavity that allows passage of vessels and nerves. The dental sac then differentiates into cementoblasts which produce cement, the bonelike rigid connective tissue covering the root of the tooth. As the jaws ossify, the dental sac also differentiates intothe periodontal ligament, the fibrous connective tissue surrounding the root of the tooth and anchor it to the alveolar bone.
Tooth eruption

As tooth develops, it slowly moves to the surface of the oral cavity. The mandibular teeth usually erupt before the maxillary. The central incisors usually erupt between six and eight months, and all 20 deciduous should be out by 2 years. The gingiva forms from the oral mucosa around the erupted crown.

With permanent teeth, they grow in a similar fashion to the deciduous. As the permanent grows, osteoclasts gradually reabsorb the roots of the deciduous tooth so when it is shed, just the crown and barely any root is removed.

***The shape of the face is affected by the growth of the maxilla and mandible to accommodate the teeth. Lengthening of alveolar processes increase the depth of face during childhood.


Natal Teeth = teeth erupted at birth, may produce maternal discomfort during breastfeeding . The infants tongue may be lacerated or the teeth may detach and be aspirated.
Enamel Hypoplasia = defective enamel formation causing pits and/or fissures in the enamel, from temporary disturbances in ameoblast differentiation or enamel formation. **Rickets causes it.
Variation in

  1. shape = aberrant groups of ameoblasts forming enamel pearls. Syphillis gives arise to screwdriver shaped incisors.

  2. number = supernumerary teeth usually in maxillary incisor area, disrupt position of normals. Partial adonotia is one or more teeth missing, familial trait. Total adonotia is rare.

  3. size = disturbances in differentiation.

  4. fusion = tooth bud divides or two buds partially fuse to form fused teeth. Some cases permanent tooth doesn’t form, so maybe the deciduous bud fused with the permanent bud.


Amerlogenesis Imperfectaa complex group of clinical entities that involve developmental aberration in enamel formation in the absence of any systemic disorder. Enamel is soft and friable because of hypocalcification, teeth are yellow because can see dentin through enamel.

Dentinogenesis Imperfecta – odontontoblasts fail to develop, get poorly calcified dentin. Teeth are gray-blue to brown. Enamel usually falls off. Common in white children.
Discolored teeth – foreign substances are incorporated into the developing enamel and dentin and discolor teeth. Tetracyclines are extensively incorporated into the enamel and dentin, producing brownish yellow discoloration and hypoplasia of enamel.


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