AP® Biology - Endocrine and Development Part 4

In the epididymis, sperm become fully mature and motile. This structure also stores sperm after they are made.

The prostate gland secretes a basic solution that protects the sperm from acidic conditions. The bulbourethral or Cowper's glands produce a lubricating fluid prior to ejaculation.

A polar body is a haploid cell produced by meiosis I or meiosis II of oogenesis. Though they possess normal amounts of genetic material, polar bodies contain little cytosol and few organelles.

Oogenesis is thus characterized by uneven division of cytoplasm. Most of the cytoplasm is allocated to the functioning oocyte, while little is given to the polar body.

28 days

The menstrual cycle involves the ovarian cycle and the uterine cycle.

In general, the ovarian cycle relates to the development of the follicle within the ovary. The uterine cycle describes the buildup and degradation of the endometrial lining.

The ovarian cycle contains three phases or events: the follicular phase, ovulation, and the luteal phase.

The follicular phase roughly corresponds to the first half of the menstrual cycle, with the luteal phase comprising the other half. Ovulation occurs between these two phases.

Both LH and FSH are released at fairly high levels, stimulating the development of the follicle. The follicle itself starts to secrete estrogen.

Remember, both LH (luteinizing hormone) and FSH (follicle-stimulating hormone) are peptides released from the anterior pituitary.

Follicles are multicellular structures located within the ovaries. Each follicle surrounds and nourishes a single oocyte.

During ovulation, the follicle bursts and develops into the corpus luteum.

The release of an oocyte from the ovary is called ovulation.

Estrogen levels increase due to its release from the follicle. At this time only, estrogen exerts a positive feedback effect on LH secretion, causing a spike known as the LH surge. The follicle then ruptures, releasing the oocyte.

Ovulation would be inhibited.

The presence of both estrogen and progesterone mimics the condition of pregnancy, in which FSH and LH are inhibited. Without these hormones, no LH surge can be initiated and ovulation cannot occur. In pregnancy, this effect is important to prevent multiple pregnancies from simultaneously occurring.

The follicle, now called the corpus luteum, begins to secrete progesterone. LH and FSH fall to very low levels.

If fertilization occurs, implantation of the zygote would occur during this phase. Menstrual cycles, then, would temporarily cease due to pregnancy.

The corpus luteum is the structure that arises from the burst follicle after ovulation. If the ovum is not fertilized, it is degraded; otherwise, it persists during early pregnancy.

Since the corpus luteum secretes progesterone, it technically functions as an endocrine gland.

The first phase, menstruation, is marked by the degeneration of the endometrial lining.

The second or proliferative phase involves regrowth of the endometrium, promoted by estrogen.

The final or secretory phase is affected by progesterone from the corpus luteum. This causes the uterine lining to release nutrients.

The follicular phase encompasses both menstruation and the proliferative phase, while the luteal phase aligns perfectly with the secretory phase.

Estrogen exhibits positive feedback, triggering the LH surge. At other times, however, estrogen negatively feeds back on LH and FSH production.

Estrogen's activity prior to ovulation is one of the few examples of positive feedback that you may see on the AP Biology exam.

Menopause

Specifically, menopause is the period when menstrual cycles stop occurring and a female can no longer become pregnant.

One sperm fertilizes each ovum, assuming fertilization occurs at all.

Ova use specific mechanisms to avoid polyspermy, or fertilization by multiple sperm. The details of these processes are unlikely to be tested on the AP Biology exam.

A zygote's mitochondria are contributed by the ovum.

While sperm contain mitochondria, they are destroyed by the ovum shortly after fertilization. As a result, mitochondrial diseases are inherited maternally.

The zygote is the diploid cell created when a sperm cell fertilizes an ovum.

Note that a zygote is single-celled; once it undergoes a single division, it will be called an embryo.

The early divisions of the embryo are referred to as cleavage. In this process, the cells divide very quickly without growing between divisions.

The regular cell cycle includes two rest or "gap" phases, G1 and G2. During cleavage, dividing cells skip these phases.

Fertilization occurs in the Fallopian tubes. From there, the zygote travels directly to the uterus and implants in the endometrium, or uterine lining.

The morula is the name given to the embryo after multiple cleavage divisions. This structure is extremely dense.

The morula later develops into the blastula.

A blastula is a hollow cell arrangement that develops from the morula. A blastula contains a central blastocoel, a dense inner cell mass, and a surrounding trophoblast.

Gastrulation is the differentiation of the blastula into three distinct layers: the ectoderm, the mesoderm, and the endoderm.

The endoderm is the innermost germ layer formed during development.

Structures arising from the endoderm include internal organs such as the pancreas and urinary bladder. The endoderm also gives rise to the linings of the digestive tract and lungs.

The ectoderm is the outermost germ layer formed during development.

Structures arising from the ectoderm include the eyes, skin, hair, nails, and nervous system.

The mesoderm is the middle germ layer formed during development.

Structures arising from the mesoderm include blood vessels, gonads, and the muscular and skeletal systems. Note that the kidneys also derive from mesoderm, not endoderm.

The student was wrong about the kidneys, which develop from the mesoderm.

The remaining organs (pancreas, lungs, and urinary bladder) do arise from the endoderm.

This differentiation process is called neurulation.

The final product of neurulation is the neural tube, which develops into the central nervous system. Peripheral nervous structures arise from cells of the neural crest.

The notochord, a long, cylindrical structure related to the backbone, forms.

The neural plate (ectoderm lying above the notochord) begins pinching inward to create neural folds.

The neural folds meet, forming the neural tube.

Neural crest cells move away to form peripheral structures.

The zygote is considered an embryo after its first cell division. The embryo is classified as a fetus during the first trimester, specifically after the eighth week following fertilization.

Human chorionic gonadotropin (hCG), a classic sign of pregnancy, maintains the corpus luteum.

Note that hCG is a peptide hormone.

Prolactin, a peptide hormone, promotes lactation.

Estrogen, progesterone, and hCG exist at high levels during pregnancy.

In early pregnancy, progesterone and some estrogen are secreted by the corpus luteum. Within the third month, the placenta becomes the main source of these hormones, as well as hCG.

The placenta is the organ through which a fetus obtains nutrients and performs gas exchange.

The placenta also performs endocrine functions, secreting estrogen, progesterone, and hCG.