Oogenesis and Spermatogenesis
The process of fertilisation involves the union of female and male gametes, the ovum and spermatozoon, respectively. In humans there is a ready supply of spermatozoa constantly available from the normal healthy male after the age of puberty. By contrast, the normal healthy female will bring only one ovum to maturity and ovulation in each 28-day cycle.
Oogenesis
During fetal life the developing ovaries become populated with primordial germ cells (gamete precursors) which differentiate into oogonia and continue to divide by mitosis until a few weeks before birth. After this time, no new oocytes are produced, and the female is born with all the oocytes she will ever have (approximately 1,000,000), which are not replaced. From early in gestation, fetal oogonia enter meiosis, reaching the first prophase stage, whereupon they become arrested and remain so beyond birth and until just before ovulation. During this arrest, the single diplotene oocyte becomes surrounded with a monolayer of flattened granulosa cells and becomes known as a primordial follicle . These primordial follicles are scattered throughout the cortex of the ovaries, surrounded by interstitial connective tissue. The primordial follicles can remain in this arrested state for up to 50 years whilst awaiting signalling to resume development. The majority of ovarian oocytes become atretic by puberty, leaving only approximately 250,000 available in the reproductive phase of life. Of these, only approximately 400 will be ovulated.
In the ovary there is continual recruitment of small numbers of primordial follicles to start folliculogenesis, which is a lengthy process taking 6 months or longer. This recruitment continues until the supply of primordial follicles is exhausted, around the time of the menopause. Folliculogenesis encompasses recruitment of a cohort of primordial follicles from the resting pool, initiation of follicle and oocyte growth; this is followed by final selection and maturation of a single preovulatory follicle, with the remaining follicles being eliminated by atresia. During this time, the oocyte grows from 35 to 120 μm in diameter, undergoes meiosis to produce a haploid gamete, produces large amounts of stable RNA to support early embryonic development and acquires the nuclear and cytoplasmic maturity to undergo fertilisation and embryogenesis.
Following recruitment, the granulosa cells of the primordial follicle become cuboidal in shape and undergo cell division. When the follicle reaches the secondary stage, with two layers of granulosa cells, a layer of theca cells develops around the follicle. The theca and granulosa cells of the follicle, which are epithelial in nature, create a specialised microenvironment for the developing oocyte. At the same time, the granulosa cells secrete a glycoprotein coat around the oocyte, which forms a translucent acellular layer separating the oocyte from the surrounding granulosa cells, known as the zona pellucida . Later on, this will provide species-specific sperm receptors at fertilisation and protect the embryo before implantation. Microvilli extend from the granulosa cells through the zona pellucida to the plasma membrane of the oocyte and are intimately involved in the transfer of nutrients and signalling molecules between the two.
When there are several layers of granulosa cells and the oocyte is fully grown, a fluid-filled cavity (the antrum) appears and starts expanding. The oocyte itself becomes suspended in this fluid and is pushed to one side and surrounded by two or three layers of tightly knit granulosa cells, the corona radiata. The oocyte and surrounding corona radiata are attached to the rim of peripheral granulosa cells by a thin ‘stalk’ of cells. From now until ovulation, follicular development is subject to endocrine control, predominantly by follicle-stimulating hormone (FSH). At the beginning of each menstrual cycle, there is a group of approximately 20 small antral follicles, only one of which will ovulate 2 weeks later. The rest of the group undergo atresia and die by apoptosis.
After antrum formation, the rate of cell division in the granulosa cell population slows down, and these cells differentiate and become steroidogenic, using theca-derived androgen to produce increasing amounts of oestradiol. In the midfollicular phase, a dominant follicle emerges, and its secretion is responsible for approximately 95% of circulating oestradiol levels in the late follicular phase. During the final maturation of the follicle, the corona cells become columnar and less tightly packed. The primary oocyte resumes meiotic maturation in response to the onset of the midcycle surge of luteinising hormone (LH). The germinal vesicle breaks down and the first meiotic division is completed in which half the chromosomes (one of each homologous pair, 23 in total made up of two chromatids) and almost all the cytoplasm go to one cell, which is now known as the secondary oocyte. The remaining chromosomes and a minute amount of cytoplasm are extruded in the first polar body. The oocyte proceeds at this time through the second meiotic division, where it arrests again at metaphase II, and is stimulated to complete meiosis only at fertilisation. The ovarian follicle now undergoes further growth, which culminates in ovulation. As the follicle continues to grow, it begins to bulge from the ovarian surface, and a slightly raised nipple on the thinning follicle wall (known as the stigma) breaks down and allows release of the secondary oocyte from this site. The oocyte oozes out in a sticky envelope of cumulus cells loosely packed around it, with the innermost layer of these cells known as the corona radiata. The collapsed follicle transforms after ovulation into the corpus luteum.
Spermatogenesis
By comparison with the mature ovum, the mature spermatozoon is very small, the headpiece measuring only 4 to 5 μm in length. Maturation of an ovum is a prolonged process starting in fetal life and involving two substantial resting phases before producing the definitive cell in the adult female. By contrast, the spermatozoon is produced in 70 to 80 days in a continuous process of development and maturation, which occurs only after puberty in the male. Spermatozoa develop from the basic germ cells of the male, the spermatogonia, which line the basal lamina of the seminiferous tubules interspersed with Sertoli cells. Spermatogenesis, the process by which spermatozoa are formed, depends on the hormonal drive of two principal gonadotrophins from the pituitary gland: FSH, which provides the impetus for the early development stages, and LH, which provokes the Leydig cells to produce testosterone, thus stimulating spermatogenesis. FSH acts both independently and in synergy with testosterone, for the proliferation, maturation and function of the supporting Sertoli cells that produce regulatory signals and nutrients for the maintenance of developing germ cells.
At the onset of puberty, spermatogonia are reactivated to constantly divide by mitosis, providing an endless supply of spermatogonial stem cells. From this pool of self-regenerating stem cells, some distinct spermatogonia emerge at intervals, which increase in size and develop into primary spermatocytes, each containing 46 chromosomes. Like the primary oocytes, these primary spermatocytes undergo a reduction division, known as the first meiotic division, in which the two daughter haploid cells receive 23 chromosomes (each consisting of a pair of duplicate chromatids) and are known as secondary spermatocytes. The first meiotic division of the oocyte produces one secondary oocyte and one polar body which is expelled, whereas the same division in the male produces two equal secondary spermatocytes of the same size and cytoplasmic content. Each of the secondary spermatocytes undergoes a further meiotic division to form two equal spermatids, each with 23 chromosomes. As the spermatozoa develop through the phases of primary spermatocyte, secondary spermatocyte and spermatid, they progress towards the lumen of the seminiferous tubule. The various generations of spermatogonia, spermatocytes and spermatids are linked in small groups by cytoplasmic bridges, possibly as an aid to nutrition and also to ensure synchronous development. The occasional occurrence of twinned mature sperm may represent failure of separation of these bridges.
The individual spermatids undergo substantial metamorphosis known as spermiogenesis to produce mature spermatozoa. Until now, the spermatids have been radially symmetric and round but now develop polarity to become elongated. The nuclear material migrates to form the dense sperm head and the cytoplasm is gradually reduced, leaving the head piece almost totally full of nuclear material. The sperm head becomes covered by the acrosomal cap ( Fig. 3.1 ) which is developed from vacuoles in the Golgi apparatus that fuse to form the acrosomal vesicle, which spreads out over the nucleus. The very important function of the acrosomal contents in penetrating the ovum is considered under the section Fertilisation. Meanwhile, the centriole which lies at the opposite pole of the nuclear membrane divides into two, and it is from here that the axial filament or flagellum develops. Most of the mitochondria form a sheath for the proximal part of the middle piece of the spermatozoon, whereas the tail piece develops a thin fibrous sheath. The centrioles link the midpiece and tail to the headpiece of the spermatozoon. The mature spermatozoon thus consists of a head piece covered by an acrosomal membrane, and a tail divided into four sections; the neck, midpiece, principal piece and endpiece. The DNA is confined to the nucleus in the head piece, and this alone penetrates the ovum at fertilisation. The remainder of the spermatozoon is responsible for its movement.
The ripe spermatozoa are released into the lumen of the seminiferous tubules of the testis and through to the epididymis together with the residual fragments of cytoplasm, mitochondria and Golgi apparatus, which separate from the sperm and eventually degenerate. Taken from this source they are known to have the capacity for fertilisation in vivo and in vitro. During ejaculation the spermatozoa are ejected through the vas deferens and prostatic urethra, where they combine with local secretions to form the seminal fluid.
Early Embryogenesis
Fertilisation
The complicated process of fertilisation implies the union of the mature germ cells, the ovum and spermatozoon. In humans there is a ready supply of spermatozoa constantly available from the normal healthy male after the age of puberty. An average ejaculate will consist of 2 to 5 mL of seminal fluid with an average sperm density of 60 × 10 6 /mL.
By contrast, the normal healthy female will bring only one ovum to maturity and ovulation in each 28-day cycle. Other follicles do develop partially in the same cycle but rarely will more than one reach full maturity. The timing of ovulation is regulated by the cyclical release of gonadotrophins from the pituitary, and, at this time, the fimbrial end of the ipsilateral fallopian tube gently folds over the ovary and comes to rest over the stigma from where the ovum is released so that it can be taken up into the tube directly. Although this is the normal pattern, it is also possible for the ovum to move over the peritoneal surface of the pelvis behind the uterus to reach the fimbrial end of the contralateral tube.
Once inside the tube, the ovum is wafted medially by the rhythmic action of the cilia, which line the lumen. This movement is augmented by the finely tuned muscular activity of the fallopian tube, which by a combination of peristalsis and shunting squeezes the contents towards the uterus. This process is then temporarily halted for up to 38 hours, when the ovum reaches the ampulla of the tube. There appears to be a physiologic valve mechanism which prevents further passage of the ovum and is possibly only released by the rising concentration of the progesterone from the newly formed corpus luteum. When the valve is released, the ovum is moved on once again by the combination of cilial and muscular activity.
This temporary hold-up of the ovum in the ampulla allows additional time for fertilisation and means that sexual intercourse need not coincide precisely with ovulation. Furthermore, spermatozoa have the capacity to retain their potency in the tube for at least 48 hours after ejaculation, with the implication that, providing coitus occurs within 2 days before or after ovulation, fertilisation of the ovum is possible. Because the ovum is temporarily held up at the ampulla, the majority of fertilisations occur at that site. Experimental work in which the fallopian tubes have been cut into sections after insemination have defined the section of the tubes in which most newly fertilised ova are found.
Sexual intercourse occurs at random in humans, although the female may be more responsive at ovulation time, when the cervical glands produce a copious watery secretion which not only serves to lubricate the vagina but also assists the ascent of the spermatozoa. Normal ejaculation will occur into the upper vagina, where the semen forms a coagulum for approximately 20 minutes before liquefying. The coagulum prevents immediate loss of fluid from the vagina after sexual intercourse. The surface cells of the vagina are rich in glycogen, especially when under the influence of oestrogen in the follicular phase of the menstrual cycle. Döderlein bacilli convert glycogen to lactic acid with the result that the vagina becomes weakly acidic and, as such, is hostile to spermatozoa. However, the seminal fluid is alkaline and acts as a buffer for the sperm until they can reach the cervical fluid, which is also alkaline. At midcycle the flow of cervical mucus will increase the pH of the upper vagina and facilitate the activity of the sperm. The early progress of the spermatozoa is dependent on the propulsive effect of the tail piece, which acts as a flagellum; thus poor motility of the sperm in the seminal sample is an important cause of male infertility. In addition, the passage of the spermatozoa is aided by low-grade contractions of the uterus, which produce a slight negative pressure in the cavity serving to draw the sperm upwards. Spermatozoa have the ability to pass through the uterus and fallopian tubes with amazing rapidity. It is possible to aspirate viable sperm from the pouch of Douglas within 30 minutes of artificial insemination in the upper vagina.
To be able to penetrate and fertilise an egg, spermatozoa must undergo physiologic maturation changes in a process known as capacitation. This process occurs during the first 6 hours of the sperm being in the female genital tract, and capacitated sperm are distinguished by the development of hyperactivated motility, a change in their surface properties which allows them to be responsive to signals near the oocyte and the ability to bind to the zona pellucida of the ovum. Following capacitation, when a spermatozoon reaches the cumulus around the ovum, the spermatozoon plasma membrane initially attaches loosely to it and then more firmly to specific sperm receptors on the zona pellucida. Following initial binding a quite definite change occurs in the acrosomal cap and the sperm undergoes an irreversible form of lysosome exocytosis. The outer acrosomal membrane fuses with the spermatozoon plasma membrane and, as they coalesce, fine pores open up to release various lytic enzymes which have the ability to break up the oocyte cumulus cells and corona radiata to penetrate the zona pellucida, through a narrow channel. The first spermatozoon to reach the cell membrane of the ovum fuses with it, and the head piece containing the nucleus passes into the cytoplasm of the oocyte, where it appears as the male pronucleus. It is easily discernible by light microscopy next to the nucleus of the oocyte, which forms the female pronucleus. The tail piece of the spermatozoon is left behind outside the cell membrane of the oocyte.
As soon as the head piece has penetrated the oocyte, cortical granules release their contents into the space between the egg and the zona pellucida, changing the cell membrane and preventing further penetration by any other spermatozoa, thus establishing a block to polyspermy. Although only one spermatozoon out of many millions produced in a single ejaculation is needed for fertilisation, hundreds of spermatozoa will undergo the acrosomal reaction to help degrade the cumulus and zona pellucida, thereby allowing one sperm to reach the cell membrane of the oocyte and fuse with it. Hence low-density semen of less than 20 million/mL is associated with relative infertility. In vitro, however, a much lower sperm density is compatible with fertilisation, providing that the motility and morphology are normal.
In the majority of cases a woman will release a single egg during each ovulation cycle. However, if two ova are released and fertilised independently by two sperm, this leads to two zygotes forming and developing, resulting in dizygotic (nonidentical) twins. Much less commonly, if a single zygote splits into two embryos this results in monozygotic (identical) twins.
Postfertilisation Transportation and Implantation
Following fertilisation, the ovum continues to move towards the uterus aided as before by the muscle activity of the tube and to a lesser extent by the cilia, which are sparser at the medial end of the tubes where the glandular secretory cells are more numerous. The early development of the fertilised ovum depends on the nutrients derived from the secretions from these cells.
The first 4 or 5 days after fertilisation produce the most remarkable series of changes in the oocyte, all of which have now been followed clearly during in vitro experiments. Following fertilisation the second meiotic division of the oocyte is completed, at which time the pairs of chromatids separate, with 23 being retained in the oocyte and 23 being expelled in the second polar body (see Oogenesis earlier). Thereafter a nuclear membrane re-forms around the ovum’s set of haploid chromosomes, as well as those from the sperm, resulting in the formation of female and male pronuclei, which each contain 23 chromosomes. The pronuclei migrate towards each other, but it is not until the time of the first cleavage division that the maternally derived and paternally derived chromosomes finally come together on the first mitotic spindle. The resulting diploid zygote now has 23 pairs of chromosomes, with each pair consisting of one chromosome from the mother and one from the father. The genetic features of the offspring are thus ordained with all of the genetic material required for development.
Following fertilisation, the zygote traverses the fallopian tube over the course of 4 days to reach the uterine cavity. Within 30 hours of fertilisation, the first mitotic division occurs, in which the fertilised ovum splits equally into two separate cells ( Fig. 3.2 ). This process is known as ‘cleavage’; each of the daughter cells, or blastomeres, contains a nucleus with a full complement of 46 chromosomes. Within 12 hours, a second cleavage occurs when each of the daughter cells divides into two again by mitotic division. Subsequent cleavage of successive generations of cells follows in quick succession and not always synchronously, so that at any particular time there may be an uneven number of cells. During the preimplantation period there is no growth; blastomeres cleave to form successively smaller daughter cells until, just before implantation, they attain the size of adult somatic cells. During early cleavage the cells are spherical, loosely attached to each other, and totipotent (i.e. able to contribute to any embryonic or extraembryonic lineage). During the first few cleavage divisions the conceptus is solely dependent on maternal stores of RNA laid down during oogenesis because the genetic material brought in by the sperm is not active. Between the 4- and 8-cell stages the ‘embryonic’ genome is activated.
When the conceptus has between 16 and 32 cells, after the fourth cleavage division, it undergoes a process known as compaction. The cells maximise their intercellular contacts with each other, and flatten onto each other. It becomes impossible to discern the cell outlines, and the embryo becomes known as a morula ( Fig. 3.3 ), Latin for a mulberry, which it resembles. At the morula stage, the embryo moves from the fallopian tube to the uterine cavity, at which stage a fluid-filled cavity (the blastocoele) develops between the cells and a blastocyst is formed (see Fig. 3.22 ).
After morula formation, the cells differentiate for the first time, when the embryo has approximately 32 cells. The outer cells (adjacent to the zona pellucida) become polarised and epithelial, forming zonular tight junctions with each other to make a watertight seal. Sodium is actively pumped into the interstitial spaces inside the conceptus, which in turn draws in water through the cells by osmosis, with the formation of a blastocoele cavity. This outer epithelial layer is known as the trophectoderm, which gives rise mainly to the extraembryonic membranes and the fetal portion of the placenta. The inner cells remain totipotent and form an eccentrically positioned clump of cells which are fated to become the embryo.
In vitro studies of human preimplantation embryo development have shown that human embryos have variable morphology and developmental potential. Approximately 75% of embryos have varying numbers of cytoplasmic membrane-bound fragments, and blastomeres are frequently uneven in size. Only approximately 50% of embryos cultured in vitro will reach the blastocyst stage, with the remaining embryos arresting development mainly between the 4-cell and morula stages. The reasons for this embryonic arrest are unclear but may reflect a combination of suboptimal culture conditions, chromosomal abnormalities or inadequate oocyte maturation. It is becoming apparent that a large proportion (approximately 20%) of human embryos have gross chromosomal abnormalities, and nearly 70% of embryos have one or more blastomeres with two or more nuclei. These factors, combined with a sensitivity to the environment, may contribute to the low rates of implantation (approximately 25%) following in vitro fertilisation and transfer of embryos at the 2- to 4-cell stage. High levels of embryonic arrest, coupled with low implantation rates, suggest that in the human there are very high levels of embryonic loss during the first 2 weeks following fertilisation.
The blastocyst implants into the secretory endometrium of the uterus approximately 6 days after fertilisation. The trophoblast cells produce a proteolytic enzyme which degrades the zona pellucida, thus allowing the blastocyst to break free in a process known as ‘hatching’. At this time, the trophoblast divides and proliferates rapidly, and a more superficial syncytium of cells, the syncytiotrophoblast (ST) (outer layer), is produced along with the basal (inner layer) cytotrophoblast (CT) (see Fig. 3.17 ). The ST invades the uterine wall and interlocks into the spongy network of the endometrium so that the blastocyst is secured firmly to it, thus establishing the maternal and embryonic interface. By the end of 10 days, the early embryo has burrowed into the endometrium to such an extent that it is completely covered. It extracts nutrients from the endometrial secretions and is already producing human chorionic gonadotropin (hCG), which may be measured in maternal serum or urine. The trophoblast cells go on to form the placenta, which is described later in this chapter.
Early Development of the Embryo
The cells known as the inner cell mass which give rise to the embryo and contribute to the formation of the yolk sac and amnion start a rapid development from the 10th day following conception. They are heaped up on one wall and thus remain in contact with the CT on the inner aspect of the blastocyst wall. At this time, the inner cell mass arranges itself into a bilaminar disc, the outer, which is in contact with the CT, forming the primitive embryonic ectoderm (epiblast) and the inner layer (i.e. that facing the blastocyst cavity) forming the primitive embryonic endoderm (hypoblast). By the 12th postovulatory day, a slit-like space opens up between the embryonic ectoderm and the adjacent CT, which enlarges to form a small cavity (the amniotic vesicle, later becoming the amniotic sac) the base of which is formed by embryonic ectoderm and the walls and roof of which are formed of CT. At the same time, hypoblast cells migrate out from the deeper layer of the embryonic disc and migrate along the inner surface of the CT to create a cavity in the lower half of the blastocyst, which is called the endocervical vesicle, later becoming the yolk sac (see Fig. 3.23 ).
Only two layers of cells lie between the two fluid cavities of the amniotic sac and yolk sac – the epiblast and hypoblast. Between the epiblast and hypoblast a third layer of cells grows in the third week of development in a process known as gastrulation. Gastrulation is initiated with the formation of the primitive streak, with epiblast cell migration and invagination, displacing the hypoblast to form two cells layers, the mesoderm (which lies between the epiblast and hypoblast) and endoderm (which displaces much of the hypoblast tissue). The remaining epiblast develops into ectoderm, forming the trilaminar germ disc.
The ectoderm cells adjacent to the amniotic sac are tall columnar cells from which all the ectodermal tissues of the fetus develop (i.e. the skin and all its appendages) and also the neural tube and its derivatives (the brain, spinal cord, nerves, autonomic ganglia and adrenal medulla).
The endodermal cells adjacent to the yolk sac form all the endodermal tissues of the developing fetus (i.e. the lining of the gut and the epithelial cells of the gut derivatives – the thyroid, parathyroid, trachea, lungs, liver and pancreas).
The middle layer forms the intraembryonic mesoderm, and, from it, all the mesodermal tissues of the fetus develop (i.e. the bones, muscles, cartilage and subcutaneous tissues of the skin, heart, blood vessels and kidneys). Extraembryonic mesoderm cells migrate between the CT and yolk sac and amniotic sac largely separating the blastocyst from the trophoblast except at the connecting stalk (primitive body stalk), which is the primordium of the umbilical cord.
Organogenesis
Development of the Germ Layers
The three layers of ectoderm, mesoderm and endoderm initially take the form of a flat circular sandwich, but later there is a disproportionate growth of the ectoderm at opposite poles so that the embryonic plate elongates into an oval, each end of which curves towards the yolk sac, thus forming a C-shape with a head fold and tail fold. Intraembryonic mesoderm lies between the ectoderm and endoderm except at two locations – the prochordal plate (buccopharyngeal membrane), which breaks down in the fourth week, and the cloacal plate membrane, which breaks down at the seventh week.
On the dorsal or amniotic surface of the ectoderm, specialised neuroectodermal tissue thickens into the neural plate. A groove develops in the middle of the neural plate from the head to the tail of the embryo, and tissues on either side fold upwards into a neural fold. The edges grow over and eventually unite and close to change the groove into a tube – the neural tube – from which the nervous system will develop ( Fig. 3.4 ). Meanwhile, the mesodermal layer, which is now adjacent to the neural tube, is growing laterally, the part nearest the midline becoming the paraxial mesoderm, the part further out becoming the intermediate mesoderm (intermediate cell mass) and the part that is most lateral becoming the lateral plate mesoderm (see Fig. 3.4 ). The amniotic sac, which was sitting on top of the bilaminar flat embryo, enlarges as the embryo grows and folds until it completely surrounds the developing embryo and yolk sac.
Growth of the endoderm is at first lateral and then ventral, eventually folding round to envelop a portion of the yolk sac and creating a tube, the primitive gut. The gut tube has three distinct sections: foregut, midgut and hindgut. At first, the midgut is in direct continuity with the diminishing yolk sac, but as the lateral folds of the embryo grow round, they constrict the opening to the yolk sac, which eventually becomes separated from the gut altogether and forms a tubular stalk, the vitellointestinal duct. Occasionally, the connection with the gut may persist as Meckel diverticulum.
The lateral plate of the mesoderm divides into the somatopleure, which remains adjacent to the ectoderm, and the splanchnopleure, which grows round the developing gut. The space between the somatopleure and splanchnopleure forms the coelomic cavity ( Fig. 3.5 ), later the pleural and peritoneal cavities.
The paraxial mesoderm becomes segmented into discrete masses of cells, or somites, which appear on either side of the neural tube under the surface of the ectoderm on the dorsal aspect of the embryo progressively along its length from the base of skull to the tail. The paraxial mesoderm somites develop into the vertebrae, dura mater, muscles of the body wall and part of the dermis of the neck and trunk. The intermediate mesoderm develops in a ventral direction towards the coelomic cavity and forms the origins of the urogenital system. The limb buds develop from the lateral plate mesoderm, pushing out a covering of ectoderm. The nerve supply to the limb buds comes off the neural tube at the level at which they originate.
Much of the early development of the embryo is at the head end, where the coverings of the neural tube develop with the brain. In addition, a condensation of mesoderm occurs at the cranial end of the coelomic cavity, and this forms the pericardial cavity and the primitive heart tubes. A further accumulation of mesoderm caudal to the developing heart is called the septum transversum and is destined to become the centre of the diaphragm.
As the head fold grows more quickly on the dorsal surface than on the ventral surface, it begins to curl round the developing heart and diaphragm ( Fig. 3.6 ). The foregut also curves round behind the pericardium and reaches the surface at the pit between the forebrain and pericardium, known as the stomatodeum (see Fig. 3.6 ). The thin buccopharyngeal membrane at this point breaks down at the fourth week of embryonic life, leaving a continuous channel between mouth, lined with ectoderm, and foregut or pharynx, lined with endoderm. A small outpouch in the roof of the mouth grows up into the developing brain. This is the Rathke pouch, which develops into the anterior lobe of the pituitary gland.
