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The mammary gland is an organ unique to mammals that produces milk for feeding their offspring. In humans, the mammary glands are also called breasts.

The maternal to zygotic transition occurs in the zygote during the first days after fertilization but before implantation into the uterine wall. Prior to this transition, all messenger RNAs are supplied from the egg that came from the mother. After the transition, maternal messages are destroyed, and new messages from the zygote are generated.

Meiosis is a type of cell division that produces gametes (sperm and egg cells). Unlike regular somatic cells, which contain 46 chromosomes, gametes contain only 23 chromosomes.

Melatonin is a chemical found in the human body and is part of the biological machinery that regulates the sleep-wake cycle of an individual over the course of a day. It is also found in other living species, including animals, plants, and microorganisms. In humans, high levels of melatonin are secreted by the pineal gland at night, which causes drowsiness. Medications containing melatonin have been used to treat jet lag and insomnia, although efficacy is still debatable.

Menarche is a woman’s first menstruation and occurs at an average age of 13 in the United States.

Menopause is the phase that marks the end of a woman’s reproductive cycle and occurs at an average age of 52 in the United States. Menstrual cycles permanently cease to occur, and ovarian follicles no longer mature. Clinically, menopause is marked by high levels of the hormone FSH and is associated with hot flashes, osteoporosis, and heart disease.

Menorrhagia refers to abnormally heavy bleeding and/or cramping during the menstrual period.

Menses is the period of time in which female menstruation occurs.

The menstrual cycle is the recurring cycle of physiological changes that occur in females and other primates of reproductive age. The menstrual cycle results from the coordinated actions of hormones made by the brain and pituitary gland acting on the ovaries and uterus in order to coordinate ovulation and fertilization. Hormones like FSH, LH, and activin made in the pituitary gland respond to and control the release of estrogen, progesterone, and inhibin by the ovary. These hormones then control and prepare the uterus for an egg, if fertilized. If the egg that is released each month is not fertilized, the uterine wall is shed. The shedding of the uterine wall is known as menstruation and is the more obvious external evidence of a functional reproductive cycle.

Menstruation is bleeding that occurs when the endometrial lining is shed from the uterus.

Mesoderm is the layer of cells in a eukaryote that lies between the ectoderm and the endoderm. It is also the layer of cells that lies between the ectoderm and the endoderm in the developing embryo once it has reached the gastrula stage. This cell layer ultimately gives rise to the muscles, cartilage, bones, blood, and lymph, forming tissues and vessels, spleen, kidney, gonads, genital tract, linings of the body cavities, and adrenal cortex.

Metaphase is the stage of cell division during which chromosomes are aligned at the equator of the spindle. The cell is stalled in this stage until all chromosomes are properly aligned. This is known as the metaphase to anaphase checkpoint. Mistakes in aligning the chromosomes (e.g. lagging chromosomes) or in the metaphase to anaphase checkpoint can result in aneuploidy.

A microtome is a machine designed for cutting uniform and often very thin sections of tissue following fixation and embedding. These thin sections obtained from the microtome can be used for downstream applications, such as hematoxylin and eosin staining.

The sperm mid-piece is the part of the sperm tail closest to the head where the mitochondria are located. The mitochondria provide the sperm with energy as they travel through the female reproductive tract.

A miscarriage is a spontaneous termination or abortion of pregnancy prior to 20 weeks of fetal development. Miscarriages are the most common complication during the first trimester, and they can occur due to hormonal, genetic, uterine, or reproductive abnormalities.

Mitochondria are organelles responsible for producing ATP, which is the cellular energy source. Additionally, they can synthesize steroids. Mitochondria are considered to be a semi-autonomous organelle because they replicate by self-division, have their own protein synthesis machinery, and have their own genome. Unlike the genome of the cell, which is inherited maternally and paternally, the mitochondrial genome is inherited only from the mother through mitochondria present in the egg at fertilization.

Mitosis is the process of cell division in which daughter cells receive the exact chromosomal and genetic makeup of the parent cell. This occurs during growth and repair.

Monosomy is a form of aneuploidy and occurs when only one chromosome in a pair is present. All instances of monosomy are lethal except for Turner Syndrome.

Mons pubis is the tissue that covers a woman’s pubic area. The skin covering the mons pubis contains hair and lies between the abdominal wall and labia majora.

Morphology refers to the shape and structure of a cell, tissue, organ, or organism.

The morula is an embryonic mass of blastomeres formed at the early stages of embryonic development before formation of the blastula and resulting from cleavage of the zygote.

Mosaicism exists when cells undergo changes during development such that one group of cells differs from a neighboring group. It can occur in both somatic and germ cells. Mosaicism can be caused by spontaneous DNA mutations (in either nuclear or mitochondrial DNA), spontaneous reversion of an existing DNA mutation, epigenetic changes in chromosomal DNA, and chromosomal abnormalities. Symptoms that occur as a result of mosaicism depends on the extent of the mosaic cell population. Mosaicism is important in terms of human disease. It also provides variations at the molecular level among humans, including between identical twins.

mRNA, also known as messenger RNA, is transported from the DNA in the nucleus to the ribosome in the cytoplasm containing the genetic code. It will be translated into its corresponding amino acid sequence at the ribosome. 

A mucus plug is present during pregnancy and is located at the cervix to block bacteria from passing into the uterus.

The Müllerian ducts are paired, mesodermal ducts present in the early embryo that will develop into the fallopian tubes, uterus, cervix, and upper vagina in females. In males, they are lost during the early stages of development.

Mutagenesis is the process by which genetic mutations occur.

A mutation is a change in the DNA sequence that can alter amount, structure, or function of a protein. Mutations can arise spontaneously, be inherited from a parent, or be chemically induced.

The myometrium is the middle layer of the uterine wall, consisting mostly of smooth muscle cells, with some vascular and stromal tissue. The myometrium contracts during menses and labor.