14 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
tail provides the motility of the sperm, assisting with its transport to the site of fertilization in the ampulla of the uterine tube. The tail of the sperm consists of three parts:
middle piece, principal piece, and end piece. The middle piece contains the energy-producing mitochondria, which fuel the lashing movements of the tail. Hox genes influ- ence microtube dynamics at the molecular level in shaping the head of the sperm and in the formation of the tail.
Oogenesis
Oogenesis refers to the sequence of events by which oogonia (primordial oocytes) are transformed into primary oocytes. The maturation process begins during the fetal period; however, is not completed until after puberty—16 years. During early fetal life, oogonia pro- liferate by mitosis and enlarge to form primary oocytes (see Fig. 2-5). At birth, all primary oocytes have com- pleted prophase (first stage of mitosis) of the first meiotic division (see Fig. 2-6). The oocytes remain in prophase until puberty. Shortly before ovulation, a primary oocyte completes the first meiotic division. Unlike the corre- sponding stage of spermatogenesis, the division of cyto- plasm is unequal (see Fig. 2-5). The secondary oocyte receives almost all the cytoplasm, whereas the first polar body receives very little, causing it to degenerate after a short time. At ovulation (release of oocyte), the nucleus of the secondary oocyte begins the second meiotic divi- sion, but progresses only to metaphase.
If the secondary oocyte is fertilized by a sperm, the second meiotic division is completed and a second polar body is formed (see Fig. 2-5). The secondary oocyte released at ovulation is surrounded by a covering of amorphous material—the zona pellucida—and a layer of follicular cells—the corona radiata—(see Fig. 2-4C).
The secondary oocyte is large, being just visible to the unaided eye.
Up to 2 million primary oocytes are usually present in the ovaries of a neonate. Most of these oocytes regress during childhood so that, by puberty, no more than 40,000 remain. Of these, only approximately 400 oocytes mature into secondary oocytes and are expelled at ovula- tion (see Fig. 2-5).
Comparison of Male and Female Gametes
Compared with sperms, the oocytes are massive, are immotile, and have an abundance of cytoplasm (see Fig 2-4B and C). In terms of sex chromosome constitution, there are two kinds of sperms (see Fig. 2-5): 22 autosomes plus either an X sex chromosome (i.e., 23,X) or a Y sex chromosome (i.e., 23,Y). There is only one kind of sec- ondary oocyte: 22 autosomes plus an X sex chromosome (i.e., 23,X). The difference in sex chromosome comple- ment forms the basis of primary sex determination.
C H A P T E R 2 HuMAN REPRODuCTION 15
Figure 2–7 Abnormal gametogenesis. The illustrations show how nondisjunction, an error in cell division, results in an abnormal chromosome distribution in gametes. Although nondis- junction of sex chromosomes is illustrated, a similar defect may occur during the division of autosomes (any chromosomes other than sex chromosomes). When nondisjunction occurs during the first meiotic division of spermatogenesis, one secondary spermatocyte contains 22 autosomes plus an X and a Y chromosome, whereas the other one contains 22 autosomes and no sex chromosome. Similarly, nondisjunction during oogenesis may give rise to an oocyte with 22 autosomes and two X chromosomes (as shown) or one with 22 autosomes and no sex chromosome.
Ovary
Second meiotic division First meiotic division
Second meiotic division completed
First meiotic division completed Testis
SPERMATOGENESIS
Spermatogonium 46,XY
Primary spermatocyte 46,XY
Abnormal secondary spermatocytes
Spermatids
Abnormal sperms SPERMIOGENESIS 24,XY
24,XY 24,XY 22,0 22,0
24,XY 24,XY 22,0 22,0
22,0
ABNORMAL GAMETOGENESIS
OOGENESIS
Primary oocyte 46,XX
Primary oocyte 46,XX Follicular cells
Nondisjunction
Zona pellucida
Antrum
Sperm
Fertilized abnormal oocyte (24,XX)
Second polar body 22,0
Corona radiata
First polar body 22,0
Primary oocyte 46,XX
Abnormal secondary oocyte 24,XX Nondisjunction
16 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
Figure 2–8 Illustrations of the interrelationships among the hypothalamus, pituitary gland, ovaries, and endome- trium. One complete menstrual cycle and the beginning of another are shown. FSH, Folli- cle-stimulating hormone; LH, luteinizing hormone.
Pituitary gland
Primary follicle
Growing follicle
Mature
follicle Ovulation Developing corpus luteum
Estrogen
Theca folliculi Primary
oocyte Progesterone
estrogenand
Degenerating corpora lutea
Secondary oocyte
Hypothalamus
Gonadotropin-releasing hormone
LH Gonadotropic hormones FSH
Menstrual
phase Ischemia Menstrual
phase
1 5
Days 14 27 28
Proliferative phase Luteal phase
Figure 2–9 Photomicrographs of sections from adult human ovaries. A, Light micrograph of the ovarian cortex demonstrating primordial follicles (P), which are primary oocytes sur- rounded by follicular cells (×270). B, Light micrograph of a secondary follicle. Observe the primary oocyte and antrum containing the follicular fluid (×132). (From Gartner LP, Hiatt JL:
Color Textbook of Histology, 2nd ed. Philadelphia, Saunders, 2001.) P
P
A B
Antrum Primary oocyte
C H A P T E R 2 HuMAN REPRODuCTION 17
follicle. The primary oocyte is surrounded by follicular cells—the cumulus oophorus—that project into the enlarged antrum. The follicle continues to enlarge and soon forms a bulge on the surface of the ovary. A small oval, avascular spot, the stigma, soon appears on this bulge (see Fig. 2-10A). Before ovulation, the secondary oocyte and some cells of the cumulus oophorus detach from the interior of the distended follicle (see Fig. 2-10B).
Ovulation follows within 24 hours of a surge of LH production, which appears to be the result of signaling molecules from the granulosa cells. This surge, elicited by a high estrogen level in blood (Fig. 2-11), appears to cause the stigma to rupture, expelling the secondary oocyte along with the follicular fluid (see Fig. 2-10D).
Plasmins and matrix metalloproteinases also appear to have some control over stigma rupture.
The expelled secondary oocyte is surrounded by the zona pellucida, an acellular glycoprotein coat, and one or more layers of follicular cells, which are radially arranged to form the corona radiata and cumulus oophorus (see Fig. 2-4C).
Figure 2–10 Diagrams (A–D) illustrating ovulation. When the stigma ruptures, the secondary oocyte is expelled from the ovarian follicle with the follicular fluid. After ovulation, the wall of the follicle collapses.
Fimbriae of uterine tube Surface of ovary
Stigma Wall of uterine tube
Mucosal lining
Infundibulum of tube
Follicular fluid
Ampulla of tube
Corona radiata
Developing corpus luteum Peritoneal cavity
Secondary oocyte
A
B
C
D
Ovulation
The follicular cells divide actively, producing a stratified layer around the oocyte (see Fig. 2-9A and B). Subse- quently, fluid-filled spaces appear around the follicular cells, which coalesce to form a single cavity, the antrum, containing follicular fluid (see Fig. 2-9B). When the antrum forms, the ovarian follicle is called a secondary
A variable amount of abdominal pain—mittelschmerz—
accompanies ovulation in some women. Mittelschmerz may be used as a secondary sign of ovulation; however, there are better primary indicators, including slight elevation of basal body temperature, fertile cervical mucus, and change in the cervical position.